Molecular and Laser Spectroscopy: Advances and Applications: Volume 2 [2, 1 ed.] 0128188707, 9780128188705

Table of contents :
Cover
Molecular and Laser
Spectroscopy:
Advances and Applications: Volume 2
Copyright
Dedication
Contributors
Preface
1-
Introduction and overview
1 - Introduction
1.1 - Significance of spectroscopic studies
1.2 - Spectroscopic techniques
1.2.1 - Infrared spectroscopy
1.2.2 - Raman spectroscopy
1.2.3 - Electronic spectroscopy
1.2.4 - Other techniques
2 - Background information and overview
2.1 - Vibrational optical activity spectroscopy
2.2 - Cavity ring-down spectroscopy
2.3 - Terahertz time-domain spectroscopy (THz-TDS)
2.4 - Matrix isolation studies
2.5 - Optogalvanic spectroscopy
2.6 - Far- and deep-ultraviolet spectroscopy for inorganic semiconductor
2.7 - Hyper-Rayleigh scattering (HRS)
2.8 - Vibrational sum frequency generation (VSFG) spectroscopy
2.9 - Surface-enhanced Raman spectroscopy
2.10 - Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)
2.11 - Stimulated Raman scattering (SRS)
2.12 - Synchrotron-based UV resonance Raman scattering (SR-UVRR)
2.13 - Stand-off Raman spectroscopy
2.14 - Ultrafast time-resolved molecular spectroscopy techniques
2.14.1 - Overview of time-resolved electronic spectroscopic studies
2.14.2 - Overview of ultrafast time-resolved molecular spectroscopy studies
2.15 - Infrared and Raman imaging and microscopy in medical applications
2.15.1 - Overview of infrared imaging and microscopy
2.15.2 - Overview of Raman imaging and microscopy
References
2 -
Vibrational optical activity spectroscopy
Chapter outline
1 - Introduction
2 - Principles of vibrational optical activity spectroscopy
2.1 Raman optical activity
2.2 - Nonresonance Raman optical activity
2.3 - Resonance effect in Raman optical activity
2.4 - Vibrational circular dichroism
3 - Instrumentation of vibrational optical activity spectroscopy
3.1 - Instrumentation of Raman optical activity
3.2 - Instrumentation of vibrational circular dichroism
4 - Spectral analysis in vibrational optical activity spectroscopy
4.1 - Quantum chemical calculations of vibrational optical activity spectra
4.2 - Effects of solvent and conformational averaging
4.3 - Applications to large systems
5 - Selected applications of Raman optical activity and vibrational circular dichroism spectroscopy
5.1 - Raman optical activity
5.1.1 - Absolute configuration of small molecules
5.1.2 - Peptides and proteins
5.1.3 - Carbohydrates
5.1.4 - Chromophoric proteins
5.1.5 - Resonance Raman optical activity
5.2 - Vibrational circular dichroism
5.2.1 - Determination of absolute configuration
5.2.2 - Determination of absolute configuration by vibrational circular dichroism exciton coupling
5.2.3 - Structural analysis of biopolymers
5.2.4 - Supramolecules
6 - Summary
References
3 -
Cavity ring-down spectroscopy: recent technological advances and applications
Chapter outline
1 - Introduction
2 - Principle of CRDS operation
2.1 - Sensitivity of CRDS
3 - Mode Structure of an optical cavity
4 - Brief historical overview
5 - Continuous wave (cw) cavity ring-down spectroscopy
6 - Recent technological advances
6.1 - Rapidly swept cw-CRD spectroscopy
6.2 - Cavity-enhanced (CE) optical frequency comb (OFC) spectroscopy
6.3 - Optical feedback (OF) cavity-enhanced spectroscopy
6.4 - Pound–Drever–Hall (PDH) locking and frequency stabilized (FS) cavity ring-down spectroscopy
6.5 - Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS)
6.6 - Frequency-agile, rapid-scanning (FARS) cavity ring-down spectroscopy
6.7 - Quantum cascade laser (QCL) coupled cavity ring-down spectroscopy
7 - Applications of cavity ring-down spectroscopy
7.1 - High-resolution fundamental spectroscopy
7.2 - Environmental monitoring
7.3 - Dissolved trace gas monitoring
7.4 - Bio-medical diagnostics
7.5 - Plasma diagnostics
7.6 - Liquid-phase CRDS
8 - Conclusion and future perspectives
References
4 -
Terahertz time-domain spectroscopy: advanced techniques
Chapter outline
1 - Introduction
2 - Basic concepts of THz time-domain spectroscopy
2.1 - Generation and detection of picosecond electromagnetic bursts
2.1.1 - Photo-conducting antennas
2.1.2 - Electrooptic (EO) antennas
2.2 - THz-TDS systems and THz-TDS signals
2.2.1 - THz-TDS setups
2.2.2 - THz-TDS signals
2.2.3 - Noise and errors
2.3 - Extraction of the THz parameters of samples
2.3.1 - THz-TDS in transmission
2.3.2 - THz-TDS in reflection
2.3.3 - Precision of the extraction
2.4 - Comparison with other far-infrared characterization techniques
2.4.1 - CW optoelectronic systems
2.4.2 - CW VNA systems
2.4.3 - Comparison with FTIR
3 - Dedicated measurements
3.1 - Characterization of thin films
3.2 - Characterization of liquids
3.3 - Ellipsometry
3.4 - Characterization of anisotropic materials and magnetic materials
3.5 - Characterization of scattering materials
3.6 - Determination of the sample thickness
4 - THz-TDS time-resolved studies: from pump-and-probe to THz spectro-chronography techniques
4.1 - Pump-and-probe THz-TDS
4.2 - THz spectro-chronography: the windowed Fourier transform procedure
5 - Generation of THz waves in gases
5.1 - Generalities and historical overview
5.2 - Photo-induced ionization of a gas
5.2.1 - Ponderomotive force
5.3 - Laser wakefield-accelerated electron bunch transition radiation
5.4 - Generation in the presence of an electrical bias
5.5 - THz generation in gases excited by the fundamental and second harmonic frequencies of the laser beam
5.5.1 - THz generation by four-wave mixing rectification
5.5.2 - Optical and photocurrent asymmetry at the plasma
5.6 - Estimation of the THz pulse electric field using air-based photonics
5.7 - THz-TDS with air-plasma sources
6 - Conclusion
References
5 - Spectroscopy of molecules confined in solid para-hydrogen
Chapter outline
1 - Introduction
2 - Properties of H2
2.1 - Ortho and para species
2.2 - Properties of solid p-H2
2.3 - Quantum solid
3 - Instrumentation: preparation of p-H2 and matrix-isolation spectroscopy
3.1 - Ortho-to-para converter
3.2 - Matrix-isolation spectrometry
3.3 - Ortho-H2 mixing ratio in solid para-H2
3.4 - Estimation of mixing ratio from IR spectra
3.5 - Estimation of sample temperature
4 - Spectroscopy of stable molecules
4.1 - High-resolution infrared spectroscopy
4.1.1 - Methane [12,43,44]
4.1.2 - Propene [46]
4.2 - Large amplitude motion: methyl internal rotation
4.3 - Interaction between guest molecules and o-H2 impurity
4.4 - Electronic spectroscopy
5 - Spectroscopy of free radicals and ions
5.1 - Diminished cage effect and spectroscopy of radicals
5.1.1 - Photolysis in situ
5.1.2 - Bimolecular reactions: reaction of Cl atom with unsaturated hydrocarbon molecules
5.2 - Protonated species
5.2.1 - Infrared spectroscopy of protonated species
5.2.2 - Electron bombardment during p-H2 matrix deposition
5.2.3 - Application to polycyclic aromatic hydrocarbons (PAH)
5.2.4 - Application to small molecules
5.2.5 - Identification of proton-bound dimers
5.3 - Hydrogen atoms and hydrogen reaction in solid p-H2
5.3.1 - Generation of hydrogen atoms in solid p-H2
5.3.2 - Spectroscopy of hydrogenated species
5.3.3 - Hydrogen abstraction reaction
6 - Future perspective
Acknowledgments
References
6 - Optogalvanic spectroscopy and its applications
Chapter outline
1 - Introduction
2 - Physics of optogalvanic spectroscopy
3 - Experimental systems
3.1 - Laser optogalvanic spectroscopy of dc discharge
3.2 - Laser optogalvanic spectroscopy with hollow cathode discharge
3.3 - Laser optogalvanic spectroscopy of radio frequency and microwave discharges
3.4 - Laser optogalvanic spectroscopy in flames
4 - Applications of optogalvanic spectroscopy
4.1 - Optogalvanic spectroscopy of rare gases
4.2 - Optogalvanic spectroscopy of molecules
4.3 - Mobility measurements of ions and small particles in flames
4.4 - Electron-photodetachment studies by optogalvanic spectroscopy
4.4.1 - Photodetachment threshold
4.5 - Intracavity optogalvanic spectroscopy
4.6 - Wavelength calibration
4.7 - Laser frequency and power stabilization
4.8 - Rydberg states of atoms
4.9 - Understanding the physics of OGS
5 - Conclusion
Acknowledgments
References
7 - Far- and deep-ultraviolet spectroscopy for inorganic semiconductor materials
Chapter outline
1 - Introduction
2 - Study of optical properties of TiO2 using radiation spectroscopy and theoretical simulation
3 - ATR spectroscopy for semiconductor materials
3.1 - ATR-FUV instrument
3.2 - ATR-FUV measurements of semiconductor powders
3.3 - Photon-induced spectral changes of TiO2
4 - DUV Rayleigh scattering spectroscopy for individual TiO2 nanocrystals
5 - Applications of UV Raman spectroscopy for semiconductor nanocrystals
5.1 - Study of TiO2 phase transformation using UV Raman spectroscopy
5.2 - UV Raman spectroscopy of zirconia nanocrystals
6 - Summary and future outlook
References
8 -
First-order hyperpolarizability of organic molecules: hyper-Rayleigh scattering and applications
Chapter outline
1 - Introduction
2 - Microscopic description of the nonlinear optical response: Electronic first-order hyperpolarizability
3 - Hyper-Rayleigh scattering technique
4 - Theoretical calculation
4.1 - Importance of symmetry on the second-order NLO responses
4.1.1 - Intrinsic symmetry of permutation
4.1.2 - Kleinman’s symmetry
4.1.3 - Molecular symmetry
4.2 - Molecular first-order hyperpolarizability by hyper-Rayleigh scattering experiment
4.3 - Schemes for the determination of the molecular hyperpolarizabilities
5 - First-order hyperpolarizability in push-pull octupolar molecules
5.1 - Enhancing the electronic first-order hyperpolarizability
5.2 - Comparison between experimental and theoretical data for dynamic first-order hyperpolarizability
5.3 - Molecular branching effect on the dynamic first-order hyperpolarizability
5.4 - Quantifying molecular interaction via HRS signal
6 - Discussing HRS results based on quantum chemical results
7 - Final Remarks
Acknowledgments
References
9 - Heterodyne-detected chiral vibrational sum frequency generation spectroscopy of bulk and interfacial samples
Chapter outline
1 - Introduction
2 - Principles of chiral VSFG spectroscopy
2.1 - What is VSFG spectroscopy?
2.2 - VSFG susceptibility and its symmetric properties
2.3 - VSFG susceptibility and molecular hyperpolarizability
2.4 - Relation between chiral VSFG susceptibility and the symmetry of Raman tensor
2.5 - Polarization combinations for chiral and achiral SFG measurements
2.6 - Modes of SFG signal measurement
2.6.1 - Narrowband IR scheme and multiplex scheme of SFG spectral measurement
2.6.2 - Intensity measurement and phase-sensitive measurement of SFG signals
3 - Experimental setup and the analysis of observed data in HD chiral VSFG
3.1 - Multiplex HD VSFG spectrometer
3.2 - Method for analyzing raw data to calculate the susceptibility of a sample
4 - Applications of HD chiral VSFG spectroscopy
4.1 - Neat liquid limonene
4.2 - Vibrationally-electronically doubly-resonant chiral SFG of chiral solutions
4.3 - Vibrationally-electronically doubly-resonant chiral SFG of chiral monolayers – electronic excitation profiles of comp...
4.4 - Polymer thin films – bulk-or-interface assignment by polarization dependence
4.5 - Air/protein solution interfaces
5 - Concluding remarks
References
Appendix A Fresnel factors
10-
Surface-enhanced Raman scattering (SERS) and applications
Chapter outline
1 - SERS and its mechanisms: a brief introduction
2 - SERS-active substrates
2.1 - Noble metals
2.2 - Transition metals
2.3 - Semiconductors
2.3.1 - Metal oxides
2.4 - Semiconductor-metal heterostructures
3 - Mechanism of SERS on semiconductor nanomaterials
3.1 - Plasmon resonance
3.2 - Mie resonance
3.3 - CT resonance
3.4 - Exciton resonance
3.5 - Key points of SERS on pure semiconductor nanomaterials
4 - Applications
4.1 - Probing CT in dye-sensitized solar cells
4.1.1 - ZnO-TiO2/N3/Ag
4.1.2 - Au@Ag/N3/TiO2
4.2 - Chemical and biological sensing
4.2.1 - Small ions and toxic molecules
4.2.2 - Protein biomarkers
4.2.3 - Cell viability and apoptosis assays
4.3 - Probing intermolecular interactions
4.3.1 - The effect of hydrogen bonds on CT
4.3.2 - Enantioselective discrimination by hydrogen binding
4.3.3 - ET between redox proteins
5 - Conclusions and outlook
References
11- Shell-isolated nanoparticle-enhanced Raman spectroscopy: a review
1 - Introduction
2 - Interaction of light with the plasmonic nanoparticles
3 - Synthesis of plasmonic cores for SHINERS nanoresonators
4 - Formation of the protecting layer
5 - Example applications of SHINERS spectroscopy
6 - Summary
Acknowledgments
References
12 - Novel application of stimulated Raman scattering for high-resolution spectroscopic imaging utilizing its phase info...
Chapter outline
1 - The brief history and the principle of stimulated Raman scattering microscopy
1.1 - Principles of spontaneous and coherent Raman scattering
1.2 - Application of coherent Raman scattering to microscopic imaging
1.3 - Difficulties in conventional stimulated Raman scattering microscopy and possible solutions
2 - Interferometric approach for obtaining the phase information from the stimulated Raman scattering signal
2.1 - Principle of stimulated Raman scattering interferometry
2.2 - Instrumental setup
2.3 - Results and discussion
3 - Differential interference contrast stimulated Raman scattering microscopy
3.1 - Principle of differential interference contrast–stimulated Raman scattering microscopy
3.2 - Instrumental setup
3.3 - Results and discussion
4 - Near–infrared stimulated Raman scattering photoacoustic spectroscopy
4.1 - Principle of near–infrared stimulated Raman scattering photoacoustic spectroscopy
4.2 - Instrumental setup
4.3 - Results and discussion
5 - Future plans: introduction of wave-front modulation technique
5.1 - Improvement of the lateral resolution by spot shaping based on Fourier optics
5.2 - Acceleration of imaging by multi-focus stimulated Raman scattering microscopy
5.3 - Image correction by the technique based on adaptive optics
6 - Conclusion
Acknowledgments
References
13 - Synchrotron-based ultraviolet resonance Raman scattering for material science
Chapter outline
1 - Introduction to resonance Raman spectroscopy
1.1 - Light scattering and Raman effect
1.2 - Resonance Raman scattering
1.3 - Advantages and limitations of resonance Raman spectroscopy
2 - Synchrotron-based ultraviolet resonance Raman setup at Elettra
3 - Ultraviolet resonance Raman for investigation of structure and dynamics of peptides and proteins
3.1 - Aqueous solvation of peptides
3.2 - Isotope-labeling for monitoring structural conformations in peptides
3.3 - Selectivity of synchrotron radiation-based ultraviolet resonance Raman for proteins
4 - Ultraviolet resonance Raman study of deoxyribonucleic acid and their assemblies
4.1 - Selectivity of ultraviolet resonance Raman on nucleobases
4.2 - Conformational stability of deoxyribonucleic acid in aqueous solution
4.3 - Thermal stability of deoxyribonucleic acid G-quadruplexes complexed with anticancer drug
4.4 - Complementarity of ultraviolet resonance Raman and infrared spectroscopies for investigation of deoxyribonucleic acid
5 - Final remarks and perspectives
References
14 - Concept and applications of standoff Raman spectroscopy techniques
Chapter outline
1 - Concept of standoff spectroscopy
2 - Standoff spectroscopic techniques
2.1 - Standoff Raman spectroscopy
2.1.1 - Experimental methods
2.2 - Time-resolved standoff Raman spectroscopy
2.3 - Standoff resonance Raman spectroscopy
2.4 - Standoff spatially offset Raman spectroscopy
2.5 - Raman–laser-induced breakdown spectroscopy
2.6 - Raman–light detection and ranging spectroscopy
3 - Applications
3.1 - Chemical and mineral detection
3.2 - Explosives detection
3.3 - Atmospheric applications
3.4 - Art and archeology
4 - Future scope
References
15 - The role of excited states in deciphering molecules and materials: time-resolved electronic spectroscopic studies
Chapter outline
1 - Introduction
2 - Probing microheterogeneity of a medium by monitoring the spectral properties
2.1 - Effect of solvent polarity
2.2 - Photoisomerization
2.3 - Excited-state proton transfer
3 - Experimental techniques for monitoring excited-state properties
3.1 - Time-correlated single-photon counting
3.1.1 - Time-resolved area normalized emission spectroscopic analysis
3.2 - Transient absorption spectroscopy
3.2.1 - Global analysis of transient absorption spectroscopy data
3.2.2 - A note on time-resolved emission spectra and decay-associated spectra
4 - Applications
4.1 - The microenvironment in Nafion probed by ultrafast fluorescence spectroscopy
4.2 - The effect of protein binding on the dynamics of DNA probed by fluorescence spectroscopy
4.3 - Early intramolecular events probed by transient absorption spectroscopy
4.4 - Understanding microenvironment inside thermophilic rhodopsin using transient absorption spectroscopy
4.5 - Investigating intermolecular charge separation in light-harvesting units using transient absorption spectroscopy
5 - Conclusion
References
16 - Ultrafast time-resolved molecular spectroscopy
Chapter outline
1 - Introduction
2 - Transient absorption spectroscopy (flash photolysis)
3 - Time-resolved fluorescence spectroscopy
3.1 - Fluorescence lifetime imaging microscopy
3.2 - Time-resolved fluorescence resonance energy transfer
4 - Time-resolved linear vibrational spectroscopy
4.1 - Time-resolved infrared spectroscopy
4.2 - Time-resolved resonance Raman spectroscopy
5 - Time-resolved nonlinear vibrational spectroscopy
5.1 - Time-resolved sum-frequency generation vibrational spectroscopy
5.1.1 - Steady-state sum-frequency generation vibrational spectroscopy
5.1.2 - Time-resolved infrared-visible sum-frequency generation vibrational spectroscopy
5.1.3 - Time-resolved pump/sum-frequency generation-probe spectroscopy
5.2 - Time-resolved coherent anti-Stokes Raman scattering spectroscopy
5.3 - Time-resolved femtosecond stimulated Raman spectroscopy
5.4 - Time-resolved femtosecond Raman-induced Kerr-effect spectroscopy
6 - Conclusion
Acknowledgments
References
17 - Infrared spectroscopic imaging: a case study for digital molecular histopathology
Chapter outline
1 - Introduction
2 - Fundamentals of infrared imaging and considerations for use
2.1 - Michelson interferometer
2.2 - Interferogram
2.3 - Resolution
2.3.1 - Spectral resolution
2.3.2 - Spatial resolution
2.4 - Computational processing
2.4.1 - Apodization
2.4.2 - Baseline correction
2.4.3 - Noise reduction
3 - Infrared microscope design and influence of optics and sample properties on the data recorded
3.1 - Imaging setups
3.2 - Scattering effects
4 - Applications: a case study of breast cancer
4.1 - Overview of medical diagnosis
4.2 - Tumor detection and associated microenvironment
4.3 - Molecular content in infrared imaging
5 - Prospects and outlook
References
18 - Emerging trends in biomedical imaging and disease diagnosis using Raman spectroscopy
Chapter outline
1 - Introduction: biomedical Raman spectroscopy
2 - Biomedical Raman instrumentation: state-of-the-art
2.1 - Excitation sources
2.2 - Collection optics and detection system
2.3 - Integration with microscopes
2.4 - Fiber optic Raman probes for delivery and collection of light
3 - Clinical applications of Raman spectroscopy
3.1 - Ex vivo analysis for disease detection and bioanalyte monitoring
3.1.1 - Biofluids
3.1.2 - Bone and mineralized tissues
3.1.3 - Solid tumors
3.2 - In vivo tissue analysis for disease detection
3.2.1 - Intraoperative margin assessment
3.2.2 - Endoscopic applications
4 - Preclinical applications beyond disease diagnosis
4.1 - Insights into metastatic progression of cancer
4.2 - Personalized cancer therapy and response monitoring
5 - Cellular analysis using linear and nonlinear Raman imaging
6 - Surface enhanced Raman spectroscopy
7 - Concluding remarks
References
Index
Back Cover

Citation preview

Molecular and Laser Spectroscopy Advances and Applications: Volume 2

Edited by

V.P. Gupta University of Lucknow, Lucknow, India University of Jammu, Jammu-Tawi, India Universite de Provence, Marseilles, France

Yukihiro Ozaki School of Science and Technology, Kwansei Gakuin University, Gakuen, Sanda, Hyogo, Japan

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/ permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-818870-5 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Susan Dennis Acquisitions Editor: Kathryn Eryilmaz Editorial Project Manager: Andrea Dulberger Production Project Manager: Debasish Ghosh Designer: Victoria Pearson Typeset by Thomson Digital

Fondly dedicated to the loving memory of my beloved parents Dr. D.P. Gupta and Smt. Ram Kali You have been an incredible driving force and anything good that has come to my life has been because of your example, guidance, and love.

Contributors Motohiro Banno  Department of Chemistry, Faculty of Science, Tokyo University of Science, Tokyo, Japan Ishan Barman  Department of Mechanical Engineering, Johns Hopkins University; Department of Oncology, The Johns Hopkins University School of Medicine; The Russell H. Morgan Department of Radiology and Radiological Science, The Johns Hopkins University School of Medicine, Baltimore, MD, United States Rohit Bhargava  Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign; Department of Bioengineering, University of Illinois at Urbana-Champaign; Departments of Mechanical Science and Engineering, Chemical and Biomolecular Engineering, Electrical and Computer Engineering, and Chemistry, University of Illinois at Urbana-Champaign; Cancer Center at Illinois, University of Illinois at Urbana-Champaign, Urbana, IL, United States Cettina Bottari  Elettra Sincrotrone Trieste, Trieste; Department of Physics, University of Trieste, Trieste, Italy Sara Catalini  European Laboratory for Non-Linear Spectroscopy (LENS), Firenze, Italy Jean-Louis Coutaz  IMEP-LAHC, University Savoie Mont Blanc, France Francesco D’Amico  Elettra Sincrotrone Trieste, Trieste, Italy Daniel L. da Silva  Department of Natural Science, Mathematics and Education, Federal University of São Carlos, Araras, SP, Brazil Leonardo De Boni  Physics Institute of São Carlos, University of São Paulo, São Carlos, SP, Brazil Yashashchandra Dwivedi  Department of Physics, National Institute of Technology Kurukshetra, Haryana, India Tomotsumi Fujisawa  Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga, Japan Alessandro Gessini  Elettra Sincrotrone Trieste, Trieste, Italy V.P. Gupta  University of Lucknow, Lucknow; University of Jammu, Jammu-Tawi, India; Université de Provence, Marseilles, France Xiaoxia Han  State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, P.R. China

xx Contributors

Eckart Hasselbrink  Department of Physical Chemistry, University of Duisburg-Essen, Essen, Germany Anna Higham  Carle Illinois College of Medicine, Mills Breast Cancer Institute, Urbana, IL, United States Taka-aki Ishibashi  University of Tsukuba, Tsukuba, Japan E. Siva Subramaniam Iyer  School of Chemical and Biological Sciences, Indian Institute of Technology Goa, Ponda, Goa, India Jan Krajczewski  Faculty of Chemistry, University of Warsaw, Poland Andrzej Kudelski  Faculty of Chemistry, University of Warsaw, Poland Vikas Kumar  Department of Physical Chemistry, University of Duisburg-Essen, Essen, Germany Yuan-Pern Lee  Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan; Center for Emergent Functional Matter Science, National Chiao Tung University, Hsinchu; Institute of Atomic and Molecular Sciences, Taipei, Taiwan Sanchi Maithani  S. N. Bose National Centre for Basic Sciences, Salt Lake, Kolkata, India Abhijit Maity  S. N. Bose National Centre for Basic Sciences, Salt Lake, Kolkata, India Claudio Masciovecchio  Elettra Sincrotrone Trieste, Trieste, Italy Cleber R. Mendonca  Physics Institute of São Carlos, University of São Paulo, São Carlos, SP, Brazil Anirudh Mittal  Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign; Department of Bioengineering, University of Illinois at Urbana-Champaign, Urbana, IL, United States Shachi Mittal  Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL, United States Masanari Okuno  The University of Tokyo, Tokyo, Japan Santosh Kumar Paidi  Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD, United States Rishikesh Pandey  CytoVeris Inc, Farmington; Department of Biomedical Engineering, University of Connecticut, Storrs, CT, United States Manik Pradhan  S. N. Bose National Centre for Basic Sciences, Salt Lake, Kolkata, India

Contributors

xxi

Barbara Rossi  Elettra Sincrotrone Trieste, Trieste, Italy Sebastian Schlücker  Department of Physical Chemistry, University of Duisburg-Essen, Essen, Germany Alexander P. Shkurinov  Department of Physics, Moscow State University, Moscow, Russia Ichiro Tanabe  Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan Surya N. Thakur  Department of Physics, Banaras Hindu University, Varanasi, India Masashi Tsuge  Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan; Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan Masashi Unno  Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga, Japan Marcelo G. Vivas  Optical Spectroscopy and Photonics Laboratory, Federal University of Alfenas, Poços de Caldas, MG, Brazil Kevin Yeh  Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL, United States Hiroharu Yui  Department of Chemistry, Faculty of Science, Tokyo University of Science; Water Frontier Science & Technology (W-FST) Research Center, Tokyo University of Science, Tokyo, Japan Bing Zhao  State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, P.R. China

Preface Molecular spectroscopy, spread out from vacuum ultraviolet to microwave region of the electromagnetic spectrum, is an essential tool in establishing the physical and chemical properties of materials through their relationship with molecular structure, the energy levels of molecules, ions or aggregates. Things have changed since the time the laser having remarkable properties of high intensity, monochromaticity, coherence, polarization, short pulse widths, and tunability, etc. was introduced into the field of molecular spectroscopy. This has dramatically altered the field of conventional optical spectroscopy, particularly Raman spectroscopy, and has led to the development of many powerful new spectroscopic techniques, both linear and nonlinear in nature. It is now possible to investigate even the unstable and transient species with many orders of magnitude more sensitivity and selectivity than was possible previously. Many experiments that could not be done before the application of lasers because of lack of intensity or insufficient resolution are readily performed with lasers. Laser molecular spectroscopy has become an inseparable part of modern molecular spectroscopy. The development of precisely controlled tunable narrow linewidth laser sources with appreciable power has provided powerful tools for the measurement and analysis of molecular spectra. Various sensitive spectroscopic techniques, including nonlinear techniques, allow a spectral resolution below the Doppler width. The unique properties of lasers have been exploited for accurate non-intrusive measurements with high spatial and temporal resolution and in advanced applications in industrial processing to monitoring, control engineering, stand-off explosive detection and security, disease diagnosis, medicine, and surgery. The advent of very short laser pulses such as picoand femtosecond pulses, has made it possible to conduct observations in time-scales shorter than vibrational oscillation periods and put to several practical uses not hitherto possible, for example, in understanding electron and proton transfer mechanisms, ring-opening and rearrangement reactions, etc. The development of IR and Raman imaging techniques, now a part of vibrational microspectroscopy which combine the functions of a spectrometer and a microscope, not only enables the focusing and probing of small specimens which cannot be resolved by the human eye but also allows for direct chemical imaging over the whole field of view on a 3D sample. The emergence of powerful lasers, sensitive detectors, and advancements in fiber optics spurred the development of infrared and Raman spectroscopy for applications in biology and medicine. These have been widely used in the characterization of cells, tissues, etc., and as potential diagnostic tools for histopathological studies in research and for human disease evaluation. Although research papers and review articles in different areas of molecular and laser spectroscopy regularly appear in scientific journals, it is often difficult to find from the many articles spread over many journals a coherent representation of the basic principles, and the most recent advances and applications. This motivated us to bring leading experts from Brazil, China, France, Germany, India, Italy, Japan,

xxiv Preface

Poland, Russia, Taiwan, and the United States of America in various areas of molecular and laser spectroscopy on a single platform to develop specialized topics in a book-like fashion and provide a competent view of the current state of knowledge in terms of techniques, applications, and future projections, to fill up the gap between the elementary text and advanced material found in research articles. Applications of laser spectroscopy constitute a vast field, and it is difficult to cover it comprehensively in a single review. Examples have therefore been chosen from a variety of fields to illustrate the new developments and the power of applied laser spectroscopy, its associated challenges, and remaining inadequacies. This has been a challenging task. Since it was not possible to include all the material in a single book, some of the topics were covered in an earlier volume of the book ‘Molecular and Laser Spectroscopy: Advances and Applications,’ while the others are being covered in the present volume. The present volume includes advances in several conventional as well as new and upcoming areas of molecular and laser spectroscopy and their applications in medical sciences, material science, standoff detection, defence and security, chemicals and pharmaceuticals, and environmental science, which were not covered in the previous volume. It has 18 chapters broadly divided into four areas namely, Conventional Molecular Spectroscopy (e.g., cavity ringdown, matrix isolation, THz time-domain, far- and deep-UV, optogalvanic, etc.), Linear and Nonlinear Laser Spectroscopy (Rayleigh & Raman Scattering), Ultrafast Time-resolved Spectroscopy, and Medical Applications of Infrared and Raman Imaging and Spectroscopy. We take it as our privilege to express our gratitude to all the authors for their excellent contributions and for meeting the timelines. One of the editors (VPG) would like to thank his wife, Madhu, who has been a perennial source of inspiration behind this endeavor. It was her continued encouragement, support, love, and understanding that facilitated the successful execution of this work. He would also like to acknowledge the constant motivation and emotional support provided by his children and grandchildren, Manjari, Vikas, Ashish, Nidhi, Pulkit, Divayum, and Shubhang during this interesting endeavor. The other editor (YO) would like to thank his wife, Hisako, for her continuous understanding of his very busy scientific activity. We hope that this second volume shall enhance the interest of the readers in the fascinating field of molecular and laser spectroscopy and shall get from them the same overwhelming response as the previous volume. V.P. Gupta Yukihiro Ozaki Editors

Introduction and overview V.P. Gupta University of Lucknow, Lucknow, India; University of Jammu, Jammu-Tawi, India; Universite de Provence, Marseilles, France

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Chapter outline 1 Introduction 1 1.1  Significance of spectroscopic studies  1 1.2  Spectroscopic techniques  2 1.2.1  Infrared spectroscopy  2 1.2.2  Raman spectroscopy  3 1.2.3  Electronic spectroscopy  5 1.2.4  Other techniques  5

2  Background information and overview  6 2.1  Vibrational optical activity spectroscopy  6 2.2  Cavity ring-down spectroscopy  8 2.3  Terahertz time-domain spectroscopy (THz-TDS)  10 2.4  Matrix isolation studies  12 2.5  Optogalvanic spectroscopy  14 2.6  Far- and deep-ultraviolet spectroscopy for inorganic semiconductor  16 2.7  Hyper-Rayleigh scattering (HRS)  17 2.8  Vibrational sum frequency generation (VSFG) spectroscopy  18 2.9  Surface-enhanced Raman spectroscopy  21 2.10  Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)  24 2.11  Stimulated Raman scattering (SRS)  25 2.12  Synchrotron-based UV resonance Raman scattering (SR-UVRR)  27 2.13  Stand-off Raman spectroscopy  28 2.14  Ultrafast time-resolved molecular spectroscopy techniques  31 2.14.1  Overview of time-resolved electronic spectroscopic studies  32 2.14.2  Overview of ultrafast time-resolved molecular spectroscopy studies  33 2.15  Infrared and Raman imaging and microscopy in medical applications  33 2.15.1  Overview of infrared imaging and microscopy  35 2.15.2  Overview of Raman imaging and microscopy  36

References  36

1 Introduction 1.1  Significance of spectroscopic studies Molecular spectroscopy based on the interaction of radiation and matter is undoubtedly one of the most important tools that have taught us the most about the nature of atoms and molecules. The field of molecular spectroscopy has continued to advance Molecular and Laser Spectroscopy. http://dx.doi.org/10.1016/B978-0-12-818870-5.00001-0 Copyright © 2020 Elsevier Inc. All rights reserved.

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rapidly with measurements in a variety of molecules with increasing resolution, which lead to the observation of new effects and continued improvements in the theoretical description of the spectra. The study of fundamental regularities in the electronic (absorption, fluorescence, and phosphorescence), vibrational (infrared, Raman, and terahertz), rotational (terahertz and microwave), and hyperfine spectra has established their relation with molecular structure. Much of the molecular-structure data has been obtained from spectroscopic measurements. Spectroscopy has traditionally been widely used as an important tool for the characterization, identification, and analysis of materials. The development of laser light sources from deep ultraviolet to far-infrared, from narrow-bandwidth lasers to ultra-broadband lamps, from continuous-wave lasers to femtosecond-pulsed lasers have revolutionized the field of molecular spectroscopy. The characteristic properties of high intensity, monochromaticity, coherence, polarization, short-pulse widths, and tunability of lasers have been exploited for accurate nonintrusive measurements with high-spatial and temporal resolution. The introduction of lasers has dramatically altered the field of molecular spectroscopy. It has not only renewed the conventional optical spectroscopic techniques but has also led to the development of many powerful new spectroscopic techniques, both linear and nonlinear in nature. Various sensitive absorption spectroscopy techniques, including nonlinear techniques, allow a spectral resolution below the Doppler width. Molecular spectroscopy has thus become an important tool for application in various areas for detection, analysis, and diagnostics. Laser spectroscopy provides the practical means of studying the spectra of short-lived molecules, such as transient molecules, free radicals and molecular ions that were impossible to be studied by conventional spectroscopy because of low-sensitivity and slow-scanning speed. The development of ultra-short laser pulses in the picosecond to femtosecond time domain has opened a wide area for investigations of molecular dynamics, intramolecular and intermolecular energy transfer, fast collision-induced relaxations, and detailed real-time studies of chemical reactions by observation of the forming and breaking of chemical bonds. Development of more rugged, easy-to-use and cheaper lasers allows real-world applications of laser spectroscopy in areas such as art and archeology, homeland security, explosives detection, and environmental studies, etc.

1.2  Spectroscopic techniques 1.2.1  Infrared spectroscopy Among the various spectroscopic techniques, vibrational spectroscopy enjoys increasing popularity. It is mainly duly to the reasons that: (1) vibrational spectroscopy has the potential for a fast, noninvasive, and simultaneous analysis of different chemical and physical parameters and can be used for both quantitative and qualitative analysis and (2) there have been fast advancements in its technical and theoretical methodologies. Vibrational spectroscopy techniques based on mid-infrared (MIR, 3000–8000 nm; 400–4000 cm−1), near-infrared (NIR, 750–1400 nm; 4000–12,500 cm−1) and Raman spectroscopy and more recently terahertz spectroscopy (also known as far-infrared spectroscopy), particularly lying in the region 0.2–20 THz (1 THz = 300,000 nm = 33.33 cm−1), have been widely used in analytical chemistry, chemical process monitoring, environmental

Introduction and overview

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studies, and industry and defense. It provides access to fingerprint spectra of organic and inorganic constituents and also quantitative chemical, structural, and compositional information on constituent molecules in the gas, liquid, and solid phases. MIR spectroscopy using the attenuated total reflection (ATR) technique, especially in combination with fiber optic probes, has been applied to many industrial applications. It is used as a competing method for process monitoring. Several such fiber-optic probes, fabricated mostly from extruded polycrystalline silver halide fibers are now commercially available. NIR spectroscopy is concerned both with vibrational spectroscopy and electronic spectroscopy as bands arising from electronic transitions as well as those due to overtones and combination tones appear in the NIR region. Till recently, the NIR region was regarded as having little potential for analytical work though now it has become one of the most promising techniques of molecular spectroscopy and has found important applications in several fields such as condensed matter physics, material science, agricultural science, chemicals, biochemistry, medical diagnostics, high-bandwidth short distance secure communication/data transfer, military-defense, and process monitoring. Rapid advancements have taken place in the field of instrumentation in infrared spectroscopy. The development of micro and nanofabricated optical components such as light sources [laser diodes, tunable quantum cascade lasers (QCL) and interband cascade lasers (ICL), etc.], waveguides, detectors, and the system-level integration strategies (e.g., on-chip photonics), optical fibers etc. have provided the infrared regimes wider applicability and molecular selectivity. The use of laser sources instead of the conventional sources has indeed led to steadily decreasing limits of detection in vibrational spectroscopy achieving trace and ultra-trace concentration levels. Synchrotron facility is a brilliant source of infrared radiation as IR photons are collimated into a narrow transverse area resulting in a much brighter light—thus, a higher brilliance. There have also been significant improvements in the nature and design of spectrometers. Thus, the replacement of dispersion techniques used in prism and grating spectrographs by the interferometric techniques and the Fourier transform analysis used in FT-infrared (FTIR) and FT-Raman spectrometers, have increased spectral resolution manifold (from 1 to 0.01 cm−1). The single-mode lasers further allow a much higher resolution than the conventional spectrometers due to their extremely small width. In recent years, the precisely controlled tunable narrow line-width laser sources with appreciable power have eased the measurement and analysis of molecular spectra with high accuracy. Miniaturization of spectrometers in size down to portable and handheld devices for all three vibrational modes (MIR, NIR, and Raman) has made them popular in outdoor applications for use by non-spectroscopists as well [1,2]. Thus, handheld instruments are used in forensic and other security applications, and art and archeology, while the battery-operated versions are used for environmental and geological field studies. Versatile small-size instruments based on tunable Fabry-Perot interferometry have recently been developed, which enable a broader spectral range to be scanned but have a rather low-spectral resolution.

1.2.2  Raman spectroscopy Raman spectroscopy, discovered by Indian scientist Sir C.V. Raman in 1928, has emerged as one of the most sensitive, delicate, and informative techniques for interrogating

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matter at the molecular level for a wide range of sampling media and environments. Raman spectroscopy uses the effect of inelastic scattering of monochromatic light and has emerged as a very promising noninvasive technique of analytical chemistry. A combined approach involving the frequency shifts of the spectral lines and change in their intensity and also sometimes the width and shape of the spectral lines may give information on the geometrical and electronic structure of molecules and on transient inter- and intra-molecular forces. Such forces are involved in various processes such as during chemical reactions, molecular rearrangements, interactions, and associations via strong and/or weak forces, and phase changes. Vibrational spectroscopy is able to detect minute quantities of samples in a mixture in the ppm and ppb ranges, and the respective analyses are now standard tools in science as well as industry. Before the advent of lasers, applications of Raman effect were rather limited owing to the weak intensity of the Raman lines. Lasers have, however, not only revived the conventional Raman spectroscopic techniques where the observation of weak Raman peaks has become routine but have made even gas phase Raman spectroscopy a more commonplace. The introduction of lasers, having remarkable properties of high intensity, monochromaticity, coherence, polarization, short-pulse widths, and tunability, etc., have dramatically altered the field of optical spectroscopy, and particularly Raman spectroscopy. Lasers have given rise to several linear and nonlinear techniques (including multiphoton) such as Resonance Raman spectroscopy (RRS), Stimulated Raman spectroscopy (SRS), Surface-enhanced Raman spectroscopy (SERS), Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS), Tip-enhanced Raman spectroscopy (TERS), Coherent Anti-stokes Raman spectroscopy (CARS), HyperRaman scattering, Vibrational Sum Frequency Generation Spectroscopy (VSFG), etc. They have made it possible to investigate the spectroscopy of even the unstable and transient species with many orders of magnitude more sensitivity and selectivity than was possible previously. The development of Raman microspectrometers which combine the functions of a spectrometer and a microscope not only enables the focusing and probing of small specimens which cannot be resolved by the human eye but also allows for direct chemical imaging over the whole field of view on a 3D sample. Raman microscopy is used in many varied applications, including the characterization of materials pharmaceutics, forensics, medical diagnostics and life sciences. Some other emerging Raman techniques include Stand-off Raman spectroscopy used for remote Raman detection, Raman imaging, and Heterodyne imaging, which enhance the Raman signal and remove the fluorescence effects. As in the case of infrared spectroscopy, advances in laser sources, instrumentation, detectors, spectrometers, and optical filter technology has brought major transformation in Raman spectroscopy from a specialist laboratory technique to a practical analytical tool. Spectroscopy has very recently been introduced into hospitals for biological, medical, and sensing applications. This has been possible due to the development of new techniques in the fields of Raman, infrared, and fluorescence spectroscopy which has helped to overcome their previous limitations, such as the diffraction limit of light or direct imaging through difficult media like plastic, cells tissues, and bone. Some new developments in the fields of time-resolved linear and nonlinear Raman spectroscopy using picosecond and femtosecond lasers have opened the field of molecular

Introduction and overview

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vibrational dynamics and chemical reaction rate studies. It may, however, be noted that different spectroscopic methods have to be used for getting information on different aspects of molecular structure and function in a biological system as no single method can provide all the information; the choice of technique depends on requirements namely, whether the study is to be carried out both in vivo and in vitro, or the timescales are to be seconds or pico- or femtoseconds, etc.

1.2.3  Electronic spectroscopy A lot of activity has been of late reported in the ultraviolet (UV, 120–400 nm) and visible (VIS, 400–800 nm) spectroscopy which is often called electronic spectroscopy as the absorption of ultraviolet or visible radiation by a molecule leads transitions among electronic energy levels of the molecule. The ultraviolet region is usually divided into three regions; the vacuum ultraviolet, far-ultraviolet and the deep ultraviolet regions. It is now more common to define the 10–120 nm region as the vacuum ultraviolet (VUV) region, 120–200 nm region as the far-ultraviolet (FUV) region and the 200–400 nm region as the deep ultraviolet (DUV) region. Due to practical difficulties involved in the study of the 120–200 nm region that needs a high vacuum, the full potential of the UV spectral region could not be earlier exploited. It is now realized that nitrogen gas purging is enough to study this region. FUV and DUV spectroscopy have the potential for applications in several fields such as characterization of materials in the liquid and solid phase and semiconductors as well as in industry. Several new areas of spectroscopy such as FUV attenuated total reflection (ATR/FUV), DUV surface-enhanced Raman scattering (DUV-SERS), and DUV tip-enhanced Raman scattering (DUV-TERS) have been developed in the FUV and DUV regions. While ATR/FUV has made it possible to study the condensed matter, DUV-SERS, and DUV-TERS has opened up new possibilities for studying electronic structure and transition, selective molecular imaging, high-resolution microscopy, as well as applications for electronic devices. Valuable structural proposals can be obtained by combining the information provided by electronic spectroscopy with the information provided by NMR and IR spectral data.

1.2.4  Other techniques A few other techniques such as Cavity ring-down spectroscopy, Optogalvanic spectroscopy, and Matrix isolation (MI) spectroscopy have also provided high-resolution spectra of neutral molecules and reactive species, such as radicals, and unstable and transient species. The Cavity ring-down spectroscopy (CRDS) offers a highly sensitive optical spectroscopic technique for the measurement of absolute optical extinction by samples that scatter and absorb light, while Optogalvanic spectroscopy based on change of impedance of gaseous discharge following the absorption of resonant laser light has proved to be a powerful spectroscopic tool to investigate all kinds of matter in vapor phase in discharge plasmas and flames. Matrix isolation allows for an extended lifetime of unstable species and radicals and provides sharp spectral linewidths, removal of spectral congestion, and highly resolved spectral features. The present book is devoted to the study of the latest advancements in some of the areas of infrared, Raman and electronic spectroscopy of molecules resulting from

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new developments in instrumentation and theoretical (e.g., chemometrics, 2D correlation and 2D correlation combined with chemometrics) and experimental techniques and methodologies, and their applications in a wide range of areas including medical applications which were not included in the previous volume of the book [3].

2  Background information and overview The chapters in the book have been broadly classified in the following four areas: 1. 2. 3. 4.

Conventional molecular spectroscopy Linear and nonlinear laser spectroscopy (Rayleigh & Raman Scattering) Time-resolved spectroscopy Medical applications of spectroscopy

Conventional molecular spectroscopy covers the topics of Vibrational Optical Activity (VOA) Spectroscopy, Cavity Ring-Down Spectroscopy (CRDS), Terahertz time-domain spectroscopy and Matrix isolation spectroscopy of molecules confined in solid para-hydrogen, Optogalvanic Spectroscopy, and Far and deep-ultraviolet spectroscopy for inorganic semiconductor materials. Linear and nonlinear Laser Spectroscopy covers topics like Hyper-Rayleigh Scattering, Chiral Vibrational Sum Frequency Generation (cVSFG) spectroscopy, Surface-enhanced Raman scattering (SERS), Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS), Stimulated Raman Scattering (SRS) for high-resolution spectroscopic imaging utilizing its phase information, Synchrotron-based UV Resonant Raman scattering, and Stand-Off Raman Spectroscopy. The area of time-resolved spectroscopy covers topics on time-resolved electronic spectroscopic studies and ultrafast time-resolved vibrational spectroscopy. Medical applications of infrared and Raman spectroscopy include areas of biomedical imaging, disease diagnosis, and digital molecular histopathology. In order to provide a concise overview of the book, the background information and highlights of the various chapters are given in greater detail in the following sub-sections.

2.1  Vibrational optical activity spectroscopy Vibrational optical activity (VOA), as the name implies, is the area of spectroscopy that results from the introduction of optical activity into the field of vibrational spectroscopy. VOA can be broadly defined as the difference in the interaction of left and right circularly polarized radiation with a molecule or molecular assembly undergoing a vibrational transition. In case of infrared spectroscopy it is known as vibrational circular dichroism, or VCD, which is defined as the difference in the absorbance of the left minus the right circularly polarized light for a molecule undergoing a vibrational transition. In the case of Raman spectroscopy it is known as vibrational Raman optical activity, or ROA, which is defined as the difference in Raman scattering intensity for left minus right circularly polarized incident and/or scattered radiation. For ROA or VCD to be non-zero, the molecule must be chiral or else be in a chiral molecular

Introduction and overview

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environment, such as a non-chiral molecule in a chiral molecular crystal or bound to a chiral molecule. Molecular chirality refers to the stereochemical details and is one of the most subtle and important characteristics of molecular structure in three dimensions. Since a nonplanar structure makes the molecule chiral, VOA, including ROA and VCD, can yield a wealth of information about its structural and conformational details. The differences in the conformations of molecules, typically the biological molecules, results in minor, nonspecific changes in frequency, intensity, and shape of bands in the IR and Raman spectra, which are detected in VCD or ROA spectra. Although VCD and ROA are closely related, occurring with the same initial and final states, they are highly complementary, nonredundant techniques. Due to their sensitivity to molecular conformations, they not only help to determine absolute stereochemistry but also help in conformational analysis in solutions. ROA is especially useful for the structural analysis of aqueous solution samples of peptides, proteins, nucleic acids, and carbohydrates as the Raman intensities of water are relatively weak. Experimental and theoretical studies in the area of VOA have reached a point where the phenomenon is now well understood and can be used to get information about the stereochemistry of molecules, including the complex biomolecules, and molecular structure determination in diverse environments. VOA has many unique advantages and capabilities and has been shown to be a sensitive, non-invasive diagnostic probe of chiral purity or enantiomeric separation. In many cases, information concerning conformational distribution as well as solute-solvent interactions is obtained by analyzing the observed VOA spectra. Recently VCD has been measured with sub-picosecond laser pulses raising the prospect that the limitation of VCD measurement with rapid time evolution may be overcome in the near future. Most problems of molecular structure are however best approached by a combination of complementary experimental techniques such as NMR and X-ray crystallography and quantum chemical calculations. ROA scattering process is typically three to five orders of magnitude lower than the parent Raman scattering. For this reason, ROA has the limitation that it cannot be used in all cases, such as low concentration or rapid timescales, where other methods, such as electronic circular dichroism or femtosecond spectroscopy, have been successfully used. This limitation has been overcome by using plasmonic resonance effects as in the case of SERS. The combination of ROA with SERS in the form of SurfaceEnhanced Raman Optical Activity (SEROA) [4] is a promising solution for widening the application of ROA for molecular stereochemistry and conformational dynamics to other biomolecules that are not easily accessible to other structural methods, such as unfolded proteins, carbohydrates, and viruses. Recently, Unno et al. [5] have developed a near-infrared excited ROA spectrometer and have reported the measurement of near-infrared ROA spectra of a light-driven proton pump, bacteriorhodopsin. In Chapter 2, Unno and Fujisawa provide a detailed account of VOA spectroscopy (a combination of ROA and VCD), its new and important developments, and highlight the latest applications of VOA spectroscopy. The chapter describes the general theory of ROA under the non-resonance and resonance conditions and VCD, basic designs of experimental setups used for ROA measurements. These include : (a) the most orthodox setup based on the incident circularly polarization (ICP) method, which uses the right and left circularly polarized incident lights and measure the difference of

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Raman scattering intensities to obtain the ROA spectrum, (b) the more popular scattered circularly polarization (SCP) method, where the unpolarized (or linearly polarized) incident light is used to excite the sample and the difference between the right and left circularly polarized components (IR and IL) of Raman scattering are measured, and (c) the two different forms of the dual circular polarization (DCP) method which measures ROA in a different manner than ICP and SCP. The instrumentation for VCD includes the dispersive and Fourier transform (FT) spectrophotometers. The chapter also discusses in detail the recent advancements in methods of computational analyses such as molecular mechanics (MM), molecular dynamics (MD), ab initio quantum mechanical (QM) methods such as SCF Hartree-Fock and DFT (density functional theory) and a combination of these for small, medium, and large molecules. The MD + QM/MM approach has been shown to be very powerful for simulating the VOA spectra, especially for the ROA spectra of aqueous solution samples, and the effects of solvent and conformational averaging. It goes on to discuss resonance ROA spectroscopy and the VCD exciton chirality method. The computational and the experimental ROA techniques have been applied to study the absolute configuration of small molecules and biologically relevant systems such as peptides, proteins and chromoproteins, and carbohydrates. VCD studies on absolute configuration by VCD Exciton Coupling and the structural analysis of biopolymers and supramolecules have been reviewed.

2.2  Cavity ring-down spectroscopy Cavity ring-down spectroscopy is a simple, highly sensitive direct-absorption technique that provides the absorption coefficient on an absolute scale. It is based on the principle that if a short-laser pulse is sent into an optical cavity (called “ring-down cavity”) consisting of two highly reflective mirrors (> 99.99% reflectance), the pulse gets reflected back and forth inside the cavity. Every time the pulse is reflected by one of the mirrors, a small fraction of the light is transmitted through that mirror resulting in a reduction of its intensity by a small percentage during each round trip. A measurement of the intensity of the transmitted light as a function of time by using a fast detector shall show an exponentially decaying intensity. The decay time, called the cavity ring-down time, represents the time required for light intensity exiting from the optical cavity to (1/e) value of its initial intensity in the absence of any sample. While in an empty cavity, the losses are only determined by the reflectivity of the cavity mirrors but on inserting an absorbing sample inside the cavity, a larger cavity loss results leading to a shorter ring-down time. Thus, by measuring the decay time after the cavity (instead of the total intensity), the rate of absorption can be determined. Provided the intensity decay is exponential, the cavity ring-down time τ is given by

τr (1.1) τ=  2 (1 − R ) + α ls   where τ r is the round-trip time of a light pulse in the cavity, (1−R) denotes the reflection loss for the cavity mirror of reflectivity R, and 2 α ls is the round-trip

Introduction and overview

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absorbance for a sample present in the cavity with absorption coefficient α and length ls . A plot of 1/ τ as a function of laser frequency gives the absolute absorption coefficient versus frequency, from which the sample’s number density (concentration) can be determined with knowledge of the sample’s absorption cross-section [6]. This directly provides the losses and the absorption coefficient on an absolute scale. The high sensitivity of the CRDS technique arises from two factors: (1) the very high effective absorption path length of up to several kilometers depending on the reflectivity of the mirrors; this allows the use of very small sample volumes), and (2) a sensitivity independent of intensity fluctuations of the light source, as the absorption is determined from the time behavior of the signal. The ring-down waveform decays exponentially so that the CRDS signal can be directly related to the absorption spectrum of the sample without a knowledge of the pulse characteristics. A simple setup can easily give a sensitivity of 10−6 cm−1 while better sensitivities up to 10 −14 cm−1 can be obtained by using different techniques. In the case of multi-mode excitation of the ring-down cavity with pulsed lasers and with CW lasers, the sensitivity is in the range of 10−6–10−9cm−1. Single-mode continuous-wave excitation offers the sensitivity of 10 −7 − 10 −12 cm −1 , and special variants can reach a sensitivity of 10 −14 cm−1. The CRDS technique can be made arbitrarily more sensitive by improvements in the cavity mirror reflectivity. The advantage of CRDS is that it can be applied to all spectral regions, as long as mirrors with sufficiently high reflectivity, detectors with a sufficiently fast time response, and tuneable light sources are available. Several schemes have been developed to use almost every kind of light source for CRDS, from deep ultraviolet to far-infrared, from narrow bandwidth lasers to ultra-broadband lamps, from continuous-wave lasers to femtosecond pulsed lasers. It has been applied at wavelengths between 197 nm and 3.2 mm [7,8]. The multiplexing CRDS techniques allow the use of CW lasers and broadband light sources (including arc lamps and white LEDs). In pulsed CRDS, the signal is initiated naturally by the arrival of the pulse in the optical cavity and decays away (or rings down) with a characteristic exponential time constant τ after the incident pulse terminates. By contrast, in CW CRDS it is necessary first to couple light into the cavity and then to interrupt the interaction between the CW light and the ringdown cavity; this is typically achieved by means of an active optical switch or modulator [9,10]. It is advantageous in many CRDS sensing applications to use CW tunable lasers for applications to combustion measurement and to atmospheric trace monitoring. As absorption bands in the condensed phase are relatively broad, broadband CRDS seems to be a natural choice for detecting molecules in liquids. Several variations of CRDS techniques have been developed for different types of applications. The sturdy, simple, and cheap variants which have a decent sensitivity can be used in field applications, whereas the delicate, complicated, and expensive variants that have spectacular sensitivity can be used for spectroscopic studies of transient molecules, like radicals, ions, and ionic complexes. The simplicity of CRDS has attracted its application in physical, atmospheric, environmental and analytical chemistry, and also in combustion science, physics, medical diagnostics, and biology. In environmental monitoring applications, however, the CRDS technique could not

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be widely used because of its susceptibility to vibrations, which can greatly reduce its sensitivity. A new device has now been developed [11,12] that can detect ultra-low concentrations of gases like nitrogen dioxide, methane, ammonia, and sulfur dioxide, etc., which are all important pollutants, accurately, and nearly instantaneously even when experiencing small vibration such as by the passing cars, near-by machinery, or by thermal changes or air currents. Attempts have been made at the miniaturization of the cavity as well as integrating with the optical fibers. Special techniques have been developed for use in hostile environments, combustion studies, trace gas detection, and life science applications. In Chapter 3 Pradhan and coworkers present the basic operating principle of CRDS technique followed by the limiting sensitivity of the technique and mode structure of a stable cavity. They discuss various experimental aspects associated with continuouswave (CW)-CRDS technique which has several advantages over its pulsed-laser counterpart and the recent technological advances in the field. These include topics such as: Rapidly swept CW-CRD spectroscopy, Cavity-Enhanced (CE) Optical Frequency Comb (OFC) Spectroscopy, Optical feedback (OF) Cavity-Enhanced Spectroscopy, Pound–Drever–Hall (PDH) locking and Frequency Stabilized (FS) cavity ring-down spectroscopy, Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE- OHMS), Frequency-agile, rapid-scanning (FARS) cavity ring-down spectroscopy, and Quantum Cascade Laser (QCL) coupled-cavity ring-down spectroscopy, etc. Various applications of the techniques to areas such as fundamental molecular spectroscopy, atmospheric sensing, dissolved trace gas monitoring, biomedical diagnostics such as exhaled breath analysis ranging from ppbv (10−9) to pptv (10−12), plasma diagnostics, that is, determination of the number density and spatial distribution of various chemical species in plasma plume, kinetics studies, liquid-phase CRDS to measure low concentration of analyte in solution phase with high sensitivity, and aerosol extinction and absorption monitoring, etc. have been reviewed. The chapter also highlights the limitations of CRDS techniques in detecting a host of interesting atmospheric radicals like peroxy radicals which are formed in the intermediate steps of the oxidation of VOCs. These still remain largely undetected in standard CRDS techniques due to their very weak transitions. It also highlights the limitations of the CRDS techniques in the terahertz regime and deep-UV regions and discusses the future perspectives in its applications.

2.3  Terahertz time-domain spectroscopy (THz-TDS) Until recently, the wide Terahertz (THz) frequency spectrum lying between 100 GHz and 20 THz (i.e., between the infrared and the microwaves) has been one of the least explored sections of the electromagnetic spectrum due to the lack of availability of THz sources and detectors. However, because of recent technological advances combining optical and electronic systems, THz systems have seen significant advancements. During the past two decades, a lot of development has been made in the area of Terahertz time-domain spectroscopy (THz-TDS) which has found potential applications in many real-life situations. The mechanism utilized in most of these developments involves the use of mode-locking femtosecond lasers in order to excite

Introduction and overview

11

photoconductive switches which generate THz radiation. The past 2–3 decades have seen the growth of high power THz sources like, for example, quantum cascade lasers (QCL), and equally sensitive THz detectors and new components for both pulsed and continuous THz signals. Simultaneously electronic devices, like resonant tunneling diodes, field-effect transistors, uni-traveling-carrier diodes, etc., have been strongly improved and are nowadays efficient in emission or in detection in the sub-THz range. The progress in THz optical components is very rapid resulting in the development of several new Metamaterial-based optical components. This progress has helped designers of spectrometers or imagers to redesign their bulky machines into compact ones, which are quite convenient to use. Terahertz spectroscopy is a dynamic new area of radiophysics and optics. The applications of THz radiation exploit the response of materials to fundamental physical processes such as rotational transitions in molecules, large-amplitude vibrational motions of organic compounds and lattice vibrations in solids. Explosives and narcotics have distinct absorption signatures in the THz region. This allows identification and distinguishing of chemicals, illicit drugs, and explosives from licit and benign compounds. Metallic substances are highly reflective at THz frequencies because of their high-electrical conductivity. This property is vital in detecting and tracking the exact shapes of concealed weapons, and sharp instruments such as knives. The terahertz spectral region has many applications in condensed matter physics systems, material science, biochemistry, chemicals, biological materials, medical diagnostics, high-bandwidth short-distance secure communication/data transfer, etc. This is mainly due to its nondestructive, nonionizing material evaluation properties unlike other radiation waves with higher or lower frequency compared to THz frequency region. Several of these and related developments, such as the basic principles and applications of Terahertz time-domain spectroscopy (THz-TDS), have been discussed by S.S. Prabhu in an earlier volume of this book [13]. The area of Terahertz timedomain spectroscopy (THz-TDS) is a powerful technique for material characterization and process control. It has been used to study molecules in the gas phase, liquids, and solutions, and in the solid-state. The goal of all of these applications is to measure the frequency-dependent complex refractive index of the sample as by using THz-TDS both the real and imaginary parts of the refractive index are measured simultaneously. While the imaginary part of the refractive index gives information about resonant absorption and conductivity, the real part is useful in layer thickness measurements, as in cases of quality control to measure the drying process and final thickness of paint coatings. THz-TDS has also been particularly important in the study of low-frequency modes in molecular crystals. In combination with density functional theory, THz-TDS has been used to study biochemical materials, especially the pharmaceutically important molecules, bacteria, and agricultural products and biomolecules. Mobile charge carriers can reflect and absorb THz radiation and as such in the field of materials science, THz-TDS is ideally suited to measure conductivity, topological insulators (TI), and superconductors, as well as phase transitions in these materials [14]. The wide variety of applications has resulted in quite a number of different configurations of THz spectrometers. The time-resolved measurements using THz signals fall under the domain of Time-Resolved THz Spectroscopy (TRTS) which is an optical pump/THz

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Molecular and Laser Spectroscopy

probe technique used to study systems in which a visible excitation initiates a change in far-infrared absorption properties on a sub-picosecond timescale. A lot of research interest has recently been generated in the area of Time-Resolved THz spectroscopy. Some of the more recent advances in the field of THz time-domain spectroscopy (THz-TDS) and advanced methods of characterization of the THz response of materials and devices, and the time-resolved THz techniques are described by J.L. Coutaz and A.P. Shkurinov in Chapter 4 “Terahertz time-domain spectroscopy: advanced techniques.” The chapter focuses on understanding the physical mechanisms of generation and detection of terahertz waves in different materials including gases, as well as important considerations for constructing terahertz time-domain spectrometers that exploit these effects. Most of the chapter is devoted to processing the THz-TDS signals, and to extract the electromagnetic parameters of studied samples at THz frequencies. The chapter is divided into two parts. The first part starts with the basics and principles of THz-TDS such as generation and detection of picosecond electromagnetic bursts with the use of photo-conducting and the electro-optic antennas, and THzTDS in transmission and reflection, etc. After a short introduction of the THz-TDS technique, it reviews dedicated special characterization methods and their applications to thin films, liquids, anisotropic materials, magnetic materials, and heterogeneous materials. Particular attention is paid to scattering materials and to the determination of the sample thickness. It also presents the study of the nonlinear response of materials at THz frequencies when subjected to very high-peak power THz pulses. The chapter also describes time-resolved THz spectroscopy (TR-TS) techniques such as “Pump-probe” (optical pump/THz probe techniques) and THz spectro-chronography techniques. The second part of the chapter deals with the matter response to highpower THz pulse excitation. It is devoted to THz photonics based on generation and detection of the THz signal through photo-induced sub-picosecond air-breaking plasmas. It gives a simple but complete description of the phenomena (current surge of free electrons, nonlinear response of neutral atoms) involved in the THz generation and also explains a smart technique for estimating the absolute THz pulse-field based on air-photonics. Recent ultra-broadband time-domain spectroscopy studies based on these air-photonics emitters and receivers are also listed.

2.4  Matrix isolation studies In the year 1954, Piemetal and coworkers [15] showed that reactive atoms and radicals in inert gas matrices could be stabilized by rapid cooling to liquid helium temperature. The study has helped matrix isolation (MI) spectroscopy to grow into a methodology for a variety of applications in which not only stable but also unstable molecules can be studied for understanding the physics and chemistry in the condensed phase. While the spectral lines in the condensed-phase matter are highly broadened as a result of homogeneous and inhomogeneous interactions, MI in inert gases provides narrower bandwidths, higher resolution and highly resolved spectral features due to removal of congestion. The initial MI spectroscopic studies were carried out in the IR region in inert gas matrices like Xenon, Argon, and Krypton and were followed by Raman spectroscopy. A complete review of the developments in the MI, IR, and Raman

Introduction and overview

13

techniques in inert gas matrixes, using both effusive and supersonic sources, and their applications have been reviewed by Vishwanathan et al. in the previous volume of this monograph [16]. Despite a better spectral resolution in MI, as compared to the condensed phase, it was still felt that high-resolution spectroscopy comparable to that in the gas phase was still not practicable. This conception has now been challenged with the development of solid parahydrogen matrices which provide spectroscopic resolution far exceeding the resolution attainable in conventional matrices making it possible to have detailed rotational structure analysis in favorable cases. Vibrationalrotational spectroscopy in solid parahydrogen has provided even narrower linewidths of the order of 0.0001 cm−1, exceeding the Doppler-limited linewidth of transitions in the gas phase. This has made the technique useful for the study of unstable molecules such as radicals and ions which have so far been studied mainly by electronic absorption and ESR spectroscopy. The special property of solid parahydrogen that makes it useful in matrix isolation work is the fact that it is a quantum solid and only the weak dispersion forces assemble the parahydrogen molecules in a van der Waals crystal of hexagonal close-packed (hcp) structure. Solid parahydrogen may be considered as a spacious and “soft” medium compared with ordinary rigid, non-quantum solids such as rare gases because the zero-point vibrational amplitude of solid hydrogen is about 18% of the lattice constant. This provides a soft environment to the subject under study, resulting in weak or diminished cage effects. Against this, in inert gases such as in solid Ar, the atoms are nearly localized to the lattice site. Solid parahydrogen is obtained from normal hydrogen gas which at room temperature consists of ortho (odd rotational states) and parahydrogen (even rotational states) in the ratio of three to one but, at liquid helium temperatures, it practically consists of all parahydrogen (J = 0) with less than 1% of orthohydrogen (J = 1). Using a catalyst (Iron (III) hydroxide) it has been possible to obtain parahydrogen contaminated with orthohydrogen at approx. 0.01% or less concentration [17]. While the parahydrogen has zero averaged permanent multipole moment, the orthohydrogen has a quadrupole moment and may, therefore, cause spectral line broadening when present in solid parahydrogen. The additional advantage of using a solid parahydrogen matrix is the feasibility of in situ photolysis due to subduction of the cage effect. This has made MI spectroscopy in solid parahydrogen a powerful technique for the study of photochemical reactions in detail and for the detection of chemical intermediates and unstable molecular products of photochemical reactions that are otherwise difficult to observe using other conventional spectroscopic techniques. The two advantages, that is, the feasibility of high-resolution spectroscopy and applicability of in situ photolytic technique, have paved a new avenue for MI spectroscopy of both stable and unstable molecules. Since solid parahydrogen is transparent up to VUV region, extension of the study to electronic spectroscopy should provide detailed information on potential energy surface and various dynamics of electronically excited molecules in solid hydrogen [18]. Developments in solid parahydrogen techniques have generated a lot of interest in several research areas such as the study of the vibrational and electronic spectra of stable molecules isolated in solid parahydrogen, spectroscopy of free radicals, spectroscopy of protonated species and hydrogenation reactions, etc. Of the various spectroscopic methods, IR, and Raman

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Molecular and Laser Spectroscopy

spectroscopy are widely used in the study of matrix-isolated species. Such MI studies are also being conducted using visible, UV, fluorescence, Mössbauer, magnetic circular dichroism, and electron spin resonance spectroscopic techniques. As mentioned earlier, solid parahydogen has emerged as a new host for matrix isolation spectroscopy because of some unique properties associated with this quantum solid. Lee and coworkers have described in Chapter 5, the details of parahydrogen matrix-isolation spectral studies related to vibrational and electronic transitions, and varied spectral applications to stable molecules, free radicals and ions, protonated species, and hydrogenated species. The chapter describes relevant properties of solid parahydrogen in connection with its use as the matrix, details of instrumentation, outlines of experimental procedures using solid parahydrogen as the matrix, examples of high-resolution spectroscopic studies, and examples of studies on photochemistry and cryochemistry. Some interesting applications using diminished cage effect like the in situ preparation of free radicals from photolysis, either directly or via bimolecular reactions, the production of protonated and hydrogenated species on electron bombardment during deposition at 3.2 K, etc., have been described. These methods have been applied to small molecules (e.g., N2, CO2, OCS), small aromatic molecules, and polycyclic aromatic hydrocarbons, proton-bound dimers (e.g., N2-H+-N2) and also polycyclic aromatic hydrocarbons. Hydrogen addition and abstraction reaction studies using photolytically produced H atoms and several reactions with nitrogen-containing molecules (e.g., HONO and formamide) that are important for understanding chemical evolution in space have also been taken up. The authors also provide a future perspective of matrix-isolation spectroscopy using solid parahydrogen.

2.5  Optogalvanic spectroscopy When a self-sustained gaseous discharge is illuminated by radiation resonant with an atomic or molecular transition of the elements within it, a change in its electrical properties occurs. This change is observed as an increase or decrease in the conductivity of the discharge and is known as the optogalvanic effect (OGE). Optogalvanic spectroscopy (OGS) works on the basis of the “Optogalvanic Effect,” sometimes also called “Joshi effect” by the name of an Indian scientist, who first reported it [19]. When the gaseous discharge is illuminated by a laser beam with the wave-length in resonance with two excited states, the steady-state distribution is changed. As a result, the ionization rates and the impedance of the discharge are changed. With the widespread availability of tuneable and short-pulsed lasers, optogalvanic spectroscopy has received a new impetus as they can be tuned precisely to resonate with the specific transitions of plasma species of interest. When used in conjunction with lasers as the excitation source, OGS is sometimes called Laser optogalvanic spectroscopy or LOGS. LOGS is a simple and inexpensive but powerful and very convenient detection technique for spectroscopic investigation of atoms, ions, molecules, and radicals. In addition, specific collisional state dynamics of a particular species can be investigated because its rate of ionization undergoes changes due to collisions within the discharge plasma [20]. LOGS is a generalized technique in that four different

Introduction and overview

15

types of sources such as flames, positive column DC discharges, hollow cathode DC discharges, and radio-frequency discharges [21–23] can be utilized for the production of plasma, and that lasers operating throughout the visible and infrared can be employed as radiation sources. Also, both resonant transitions and broadband nonresonant background signals are observed in the spectrum. This technique enables the detection of the weakly absorbing transitions which are generally eluded in absorption and fluorescence and, as such, it complements these techniques. The different spectroscopic methods developed for the traditional spectroscopy can also be applied to the LOGS, including the saturation methods for sub-Doppler resolution albeit with a smaller resolution and accuracy. No monochromator and no background filtering are needed in LOGS and the signal-to-noise ratios vary from 103 to 105 depending on the type of discharge excitation. Optogalvanic spectroscopy has found numerous practical applications. Some of these are: Study of Rydberg atoms and molecules (especially for studying Rydberg states corresponding to high principal quantum numbers, say n = 20–500.), combustion and plasma diagnostics, wavelength calibration, trace analysis and concentration measurement, material characterization, laser frequency and power stabilization, isotope analysis, ion detection and plasma diagnostics, and more recently, molecular spectroscopy, and dynamics. A new ultra-sensitive laser-based analytical technique, called “intracavity optogalvanic spectroscopy,” allows extremely high sensitivity for detection of 14C-labeled carbon dioxide and can quantify attomoles (10−18 moles) of 14C in sub-microgram samples with limits of detection of about 10−15 in the 14C:12C ratios [24]. LOGS measurements have several significant advantages as compared to ion beam and other spectrometric stable isotope measurement technologies. Likewise, LOGS can play a significant role in understanding the behavior of nanoplasmas [25–27]. The plasma conductance is also found to be dependent on the polarization of the irradiating laser beam as the LOGS signals induced by the linearly/circularly polarized light beam differ in amplitude as well as in their shape parameters in the time-resolved OG signals (TROGS). This has been used to study photoelectron emission (PE) processes. In Chapter 6, S.N. Thakur provides the historical development and the physics behind the Laser optogalvanic spectroscopy (LOGS) and discusses the different experimental setups for the technique. These include laser optogalvanic spectroscopy with d.c. discharge, hollow cathode discharge, radiofrequency and microwave discharges, and intracavity optogalvanic spectroscopy, etc. Applications of optogalvanic detection in moderate resolution spectroscopy, in Doppler-free spectroscopy and in analytic studies have been discussed. LOGS has been used to understand the spectra of rare gases, molecules, Rydberg states of atoms and the mobility of ions and small particles in flames. Studies on negatively charged ions and ionic molecules are important as they can provide very useful data on electron-photodetachment. Topics such as frequency calibration for tuneable dye lasers, electronic absorption from excited states of atoms and molecules, trace metal detection in flames, and isotope ratio measurements have been described in detail. Limitations of LOGS techniques and the need for further experimental and theoretical research has been emphasized due to the reason that electric discharge plasma involves complex atomic and molecular interactions with the optical radiation.

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2.6  Far- and deep-ultraviolet spectroscopy for inorganic semiconductor Semiconductor materials such as titanium-di-oxide (TiO2), zinc oxide (ZnO), and zinc sulfide (ZnS) have attracted much attention for their photocatalytic properties and the fascinating array of the novel physical phenomenon observed in them. They have great potential as next-generation solar-cell materials. Precise information on their electronic states is important not only for understanding their fundamental properties but also for designing newer and improved electronic devices. Several optical spectroscopic studies have therefore been carried out on semiconductor nanostructure systems in order to understand aspects of their electronic band structure and its possible consequences for practical applications. Since the semiconductor material shows absorption in the far-ultraviolet (FUV,120–200 nm) and deep-UV (DUV, 200–300 nm) regions, their spectra in this region can provide insight into their electronic states. FUV–DUV spectroscopy can, therefore, be profitably used for exploring electronic states of semiconductor materials. FUV-DUV has several advantages over the UV, visible, or near-IR Raman spectroscopy such as higher scattering cross-section due to their lower wavelength and the correspondingly higher intensity of the spectral lines, pre-resonance or resonance effects which can provide additional increase in Raman cross-sections by factors of up to several million times, fluorescence free spectra with solar-blind background and simplification of spectra. However, it also has a few major limitations such as complexity and high cost of the DUV Raman instrumentation, requirement of high vacuum due to absorption of FUV-DUV radiation by oxygen and air, large space, nonavailability of the DUV excitation laser source (as most lasers do not directly emit the DUV lines), nontransparency of ordinary glass optics in DUV region, and also instability of the sample in several cases. The DUV lines are typically obtained by frequency conversions such as Second Harmonic Generation (SHG) or higher-order harmonic generations using nonlinear processes. In order to overcome some of these issues, Ozaki [28] developed a novel FUV-DUV spectrometer based on attenuated total reflection (ATR) technique and successfully demonstrated [29] its ability to record spectra of semiconductor nanoparticles zinc oxide and zinc sulfide in the 140–300 nm region. The ATR/FUV–DUV system requires neither a high vacuum nor much space, and has generalized applicability for rapid and systematic investigation of the electronic states of various materials in the liquid and solid phase and semiconductors. It is realized that through a combined use of attenuated total reflectance (ATR) DUV–FUV spectroscopy, resonant DUV Rayleigh scattering spectroscopy, and DUV Raman spectroscopy, a deep understanding of the electronic states is possible which will significantly contribute to the material design of semiconductorbased materials, and lead to the further development of optical devices such as photocatalysts and solar cells. In Chapter 7, Ichiro Tanabe describes the attenuated total reflectance (ATR) system for measurements in the FUV and DUV regions and its applications to the study of water and ionic liquids and semiconductor materials like TiO2 and Au-, Pd-, and Pt-modified TiO2, and Zirconia (ZrO2). Semiconductor materials show strong absorbance in the DUV and FUV regions, and as such their properties, such as band-gap

Introduction and overview

17

energy and phase transitions could be determined by utilizing absorbance, Rayleigh scattering, and Raman scattering techniques. So far, the traditional methods of radiation spectroscopies were used for the optical investigation of semiconductor materials, but the new system of attenuated total reflectance (ATR) spectroscopy has gained importance as it enables easy and systematic FUV-DUV spectral measurements and provides rich information on the electronic states of the materials. The chapter reviews spectroscopic investigations of semiconductor materials utilizing the traditional methods such as absorbance, Rayleigh scattering, and Raman scattering techniques as well as by methods using FUV, DUV, and UV light. It also describes a recently developed system that enables spectral measurements under UV and visible light irradiation and the resonant Rayleigh scattering spectroscopy technique and applies it to the study the optical properties and the determination of the intrinsic bandgaps of single TiO2 nanocrystals, depending on their size. The phase transition process of semiconductor nanocrystals such as TiO2 and ZrO2 are analyzed combining UV and visible Raman scattering spectroscopy techniques, and the results comprehensively discussed. The chapter also reviews UV Raman scattering spectroscopy and its applications for semiconductor materials.

2.7  Hyper-Rayleigh scattering (HRS) HRS is a parametric optical effect in which two incident photons of frequency ν are annihilated to create a scattered photon of frequency 2ν. When a sample is illuminated by a giant pulse of frequency ν, the scattered radiation contains frequencies of 2ν (hyper-Rayleigh scattering) and 2ν ± νM (Stokes and anti-Stokes hyper-Raman scattering), where νM is the frequency of a normal vibration of the molecule. There are two main processes for scattering photons at twice the excitation frequency. In the first process, spontaneous, isotropic, dephased reemission of one photon takes place after two excitation photons have been absorbed. In this case, the process of absorption and reemission depends upon the parts of the dipole induced in the medium by a nonlinear interaction with the incident field. In the second process, an interaction between the electric field and its spatial derivative takes place resulting in emission which is highly anisotropic, coherent, and phase-coupled to the excitation. The former process is called hyper-Rayleigh scattering (HRS) while the latter process is called secondharmonic generation (SHG). HRS cannot arise in centrosymmetric materials. Even a weak octupole effect arising from the nonlinear polarizations also contributes to the hyper-Rayleigh intensity but this effect also disappears in centrosymmetric systems. Hyper-Rayleigh scattering technique has been used for the experimental determination of the first hyperpolarizability of molecules but it has now been extended as a means of determining the molecular structure of monomers and polymers in solution. The HRS technique has contributed to the establishment of a number of structureproperty relations and has provided essential structural information at a fraction of the cost of electron microscopy and magnetic resonance, making it a viable addition to the array of structural tools already available. HRS has been widely used to investigate novel materials with potential for the development of technologies, such as more efficient frequency doubling, high-resolution microscopy, fast electro-optic modulation,

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Molecular and Laser Spectroscopy

ultra-fast lasers and even to identify the interaction and affinity between biological molecules. It is widely used to understand nonlinear optical (NLO) properties of materials and in synthetically designing optimum NLO structures. Detailed information about molecular properties and the local environment can be obtained through simultaneous experimental and theoretical studies on hyper-Rayleigh scattering spectra. Two types of theoretical methods are generally employed to calculate molecular hyperpolarizabilities: those in which the electric field is explicitly included in the Hamiltonian, often labeled as Finite Field (FF), and those which use standard time-dependent perturbation theory, also known as Sum Over State (SOS) method. Accurate computations of electric, magnetic and mixed properties for small rigid systems in vacuum can be conducted on high-level post-HF models employing large basis sets, coupled with powerful response-theory expressions; these are, however, hardly feasible for technologically relevant systems (e.g., materials for multiphoton imaging, electro-optic polymers, etc.). In Chapter 8, Vivas and coworkers outline the fundamentals of the first-order hyperpolarizability in materials and present in detail the different experimental setups used to record HRS, control the irradiance and optimize the experiment time to obtain HRS curves without spurious effects from electronics or environment. Theoretical approaches applied to estimate the first-order hyperpolarizability are also presented. These approaches employ different quantum chemical methods combined with a numerical or analytical scheme. Three of the most commonly used schemes, namely the FF scheme, the SOS scheme, and the coupled-perturbed (CP) scheme, are briefly described. These approaches are used to aid in the understanding of the fundamental aspects that can be exploited at the molecular level to reach remarkable first-order hyperpolarizability (>10−27 cm5/esu). Finally, recent results about the first-order hyperpolarizability are described and discussed for organic molecules with distinct molecular structures, as well as the use of the HRS effect as a technique to quantify specific interactions of biological materials. The authors particularly focus on the study of the first-order hyperpolarizability of organic molecules through the experimental HRS technique and the application of quantum chemical calculations. They demonstrate a very effective molecular strategy to obtain high dynamic first-order hyperpolarizability (DFH) values for synthesizing organic molecules with push-pull octupolar structures. It is shown that HRS is a technique of choice to probe the symmetry of the molecules since it allows a fine decomposition of the multipolar contributions of the NLO responses.

2.8  Vibrational sum frequency generation (VSFG) spectroscopy Vibrational sum frequency generation spectroscopy (VSFG), a nonlinear laser spectroscopy technique developed by Shen et al. [30,31], is based on the principle that when two electromagnetic waves with frequencies w1 and w2 interact in a medium, a nonlinear polarization P(2)(ws = w1 + w2) = χ(2) : E1(w1)E2(w2) is induced, where χ(2) is the nonlinear susceptibility of the medium, and E1 and E2 are the input optical fields. Under the electric-dipole approximation, χ(2) is nonzero only if the medium has no inversion symmetry. Based on the principle that the second-order nonlinear optical

Introduction and overview

19

processes (or even-order nonlinear processes, in general) are forbidden in the medium having inversion symmetry, it follows that VSFG spectroscopy has very high interface selectivity and only detects molecules in the region where the molecules exhibit anisotropy such as specific orientations at liquid interfaces, which is typically as thin as a few molecular layers. This is also the case of chiral liquids with a net chirality. It is therefore used to characterize molecular structures and analyze surfaces and interfaces, probe the molecular chirality of monolayers, liquids, and films, etc., and has gained significant attention in surface science. The surface-specificity and chiral-selectivity make vibrational SFG spectroscopy a uniquely useful tool for probing biomacromolecules at interfaces in situ and in real-time. The method is particularly powerful for probing biomacromolecules at aqueous interfaces because achiral water structures do not contribute to the background. Soon after its invention, the technique was extended by Philippe Guyot-Sionnest [32] to obtain the first measurements of electronic and vibrational dynamics at surfaces. Another related development is chiral sum frequency generation (cVSFG) spectroscopy which has been used to study biomacromolecules at interfaces during the past decade. Chiral vibrational SFG spectroscopy (cVSFG) has already been applied to the study of DNA molecules and various proteins, yielding information about the structures, orientation, and kinetics of conformational changes. It can be extended to other biomacromolecules, such as RNA, synthetic biomimetics, and chiral polymers. It can provide a set of vibrational optical signatures for characterizing protein secondary structures at interfaces. Although previous applications have focused on the secondary structures of single-component systems, chiral SFG should also be applicable to the study of tertiary structures and higher-order structures as well as biomacromolecular interactions, such as protein − DNA and protein − protein interactions. In a typical VSFG setup, two laser beams, one at infrared (IR) frequency (w1) and the other at visible frequency (w2), spatially and temporally overlap at an interface and generate an output beam with a frequency equal to the sum of the two input frequencies, traveling in a direction given by the sum of the wave-vectors of incident beams. When w1 = w2, this leads to the second harmonic generation (SHG). Thus VSFG is an extension of SHG where the two incident beams have different frequencies. Being a second-order nonlinear optical effect, the sum-frequency generation requires highenergy pulses to generate sufficient signal for detection. There are two main types of sum-frequency spectrometers: scanning systems and broadband systems. In the scanning systems, one of the beams is a visible wavelength laser which is held at a constant frequency and the other is a tunable infrared laser. By tuning the IR laser, the system can scan over resonances and obtain the vibrational spectrum of the interfacial region in a piecewise fashion. The pico- and/or nanosecond laser sources are usually used in a scanning spectrometer. The scanning systems can generally provide good spectral resolution, high signal-to-noise ratio and wide tuning range such that vibrational spectra covering 1000–5000 cm−1 can be obtained in a single scan without extensive optical alignments. However, the frequency scanning makes it cumbersome to set up heterodyne detection that is desirable for providing phase information. In the case of broadband spectrometers, one laser (typically the visible laser) is kept at a fixed narrow wavelength, and the other laser (the infrared laser) produces a spectrally

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Molecular and Laser Spectroscopy

broad beam. The broad-bandwidth spectrometers utilize a femtosecond IR beam and a narrow-bandwidth picosecond visible beam. These laser beams, which again overlap at the interface being studied, can cover a wider range of resonances (spectral width 200–300 cm−1) at once, as compared to a spectrometer operating in scanning mode. Hence, in broad-bandwidth spectrometers, the spectra can be acquired much faster, facilitating time-resolved measurements with interfacial sensitivity. Recent developments have also enabled ultra-broadband VSFG studies with a spectral width of 600 cm−1. Spectral acquisition by this method allows kinetic studies to investigate changes in molecular population, composition, and orientation at interfaces. Advancements in laser technology have empowered the VSFG method to cover the entire mid-IR regions [33]. New developments are taking place in the field of VSFG spectroscopy. Efforts are in progress to improve the temporal resolution of cVSFG techniques so that the studies could be carried out in very short time periods. The pump − probe techniques applied to chiral SFG experiments can expand the time resolution to the nanosecond and femtosecond regimes and reveal interfacial ultrafast dynamics of biomacromolecules. Integration of two-dimensional techniques, similar to those applied to IR spectroscopy, further enhance the selectivity of cVSFG signals to study ultrafast structural dynamic details in biomacromolecular systems at interfaces. High-resolution VSFG using broad bandwidth spectrometers and strong and narrow-bandwidth picosecond visible input source may also enable study the dynamics and structures of biomacromolecules in fine vibrational details [34]. More recently, a new multidimensional spectroscopy technique, Triply Resonant Sum Frequency Spectroscopy (TR-SFS), has been developed. This is a four-wave mixing technique sharing advantages of both 2D-IR and resonance Raman experiments [35,36]. TR-SFS is a fully coherent mixed vibrational-electronic spectroscopic technique that is ideally suited for probing the vibrational-electronic couplings that become important in driving reactions. One of the important strengths of TRSF is its ability to selectively enhance weak vibrational features that are buried beneath strong solvent or co-solute absorption bands. As compared to conventional chiral vibrational spectroscopies, such as vibrational circular dichroism (VCD) and Raman optical activity (ROA), chiral vibrational sumfrequency generation spectroscopy (cVSFG) is a highly sensitive and background free technique. The background free feature is highly effective to detect chiral signals. cVSFG can be obtained directly in some chiral-specific polarization combinations. For the first time, Ishibashi and Okuno [37] have developed a heterodyne-detected (HD) chiral VSFG (HD-cVSFG) spectrometer combined with a multichannel detection that can measure the electric field of light with phase information instead of its intensity. The introduction of a heterodyne-detection technique permits detecting the phase of the VSFG signal field, from which enantiomeric samples can be distinguished. It has the advantage that it can provide more information and sensitivity than the conventional homodyne-detected chiral VSFG that measures the intensity of the SFG signal. The sensitivity can be further improved by conducting chiral VSFG under electronic resonant conditions. All the above and various other aspects of HD-cVSFG are described in Chapter 9 by Ishibashi and Okuno. The chapter introduces the basic theory of chiral VSFG

Introduction and overview

21

spectroscopy and describes in detail the multiplex HD-cVSFG spectrometer developed by the authors and the method for analyzing raw data to calculate the susceptibility of a sample. Narrowband IR scheme and multiplex scheme of SFG spectral measurements are described. Applications of vibrationally-electronically doublyresonant chiral VSFG to various bulk and interfacial samples such as chiral liquids, chiral solutions, chiral monolayers on water, chiral polymer films, and proteins at air-water interfaces have been discussed. The important issues and future challenges to the development of this new area of investigation, both in terms of theoretical and experimental capabilities, have been highlighted. On the theoretical side, it is felt that one of the most important issues to be addressed concerning cVSFG is the method for predicting cVSFG band amplitudes and phases for a chiral molecule or a chiraloriented molecular layer with a given absolute configuration.

2.9  Surface-enhanced Raman spectroscopy Raman spectroscopy, which has established itself as an important analytical tool for molecular study identification due to its molecular specificity and the ease of sampling, suffers from two shortcomings : (1) weak intensity of its spectral bands which make their detection difficult and (2) high probability of fluorescence which impair the spectral background and make the measurements less accurate and reliable. Both these shortcomings have been overcome by enhancing the Raman signal by factors of 1010–1011 by using the technique called Surface Enhanced Raman Spectroscopy (SERS). Discovered by Fleishmann et al. in 1974 [38], SERS is principally based on EM enhancement arising due to the collective movements of interacting particles called plasmons (such as electrons, which occur on the surface of the metals and are quantized), when an analyte is adsorbed to or is in the vicinity of the surface of certain noble metal structures like silver, gold, or Cu with nanoscale features. The collective oscillations of the plasmons are induced when the electron cloud of the metal interacts with the incoming electromagnetic radiation leading to its excitation. Another cause of the increase in the Raman signal strength is chemical enhancement (enhancement factor, EF 10–100) which occurs when the adsorbed analyte molecules have inherent charge transfer (CT) capability; thus, for example, molecules with a lone pair of electrons such as pyridine show the strongest SERS signals. A further possibility of enhanced Raman signal is offered by Surface-Enhanced Resonance Raman Scattering (SERRS) with enhancement factor 1014–1015 [39]. Raman intensities of molecular vibrations are greatly enhanced when the Raman excitation wavelength approaches or falls in the molecular electronic absorption region, called resonance Raman scattering. Surface-enhanced resonance Raman scattering (SERRS) is thus a form of SERS which combines surface enhancement provided by immobilizing an analyte onto a SERS-active substrate and a molecular resonance excited by an appropriate laser wavelength. SERRS has been widely used as a powerful tool for ultrasensitive chemical analysis down to a single-molecule level under favorable circumstances. The two techniques, SERS and SERRS, are often coupled to obtain similar but complementary information and are today some of the most popular

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areas of research in Raman spectroscopy. Also, time-resolved SERS and ultrafast SERS are the other current areas of interest; the latter has developed at the intersection of ultrafast vibrational spectroscopy and plasmonics. The range of SERS active nanomaterials has extended from noble metals and transition metals to semiconductor materials. Although initially discovered in noble metals like silver, gold, and copper belonging to group IB of the periodic table, SERS has now been found to occur in the nanostructured free-electron-like metals Pt, Ru, Rh, Pd, Fe, Co, and Ni in group VIIIB, transition metals and semiconductors—both organic and inorganic. The inorganic semiconductors are based on solid-state structures of group III, IV, and V elements and metal oxides/chalcogenides, the organic semiconductors are π-conjugated carbon-based molecular/polymeric structures that are bound by relatively weak intermolecular interactions. In the past few years, inorganic and organic semiconductors have gained extensive attention and have shown continuously improving SERS performances in terms of molecular sensitivity and quantitative detection. Single elemental semiconductors (Si, Ge, and graphene) were proved in recent years to be capable of displaying Raman enhancement based on the charge transfer (CT) mechanism. Various methods of synthesis of silver or gold nanoparticles (NPs) substrates, carbon-based SERS substrates, and composites, biosynthesized Ag nanoparticles, bimetallic nanoparticles, etc. [40] and the novel inorganic and organic semiconductor-based platforms for SERS applications [41] have been examined recently. Metal oxides are the most extensively investigated SERS semiconductors. In a number of studies SERS of adsorbed molecules and chemisorbed species on the crystal TiO2 (001) and NiO (110) surface have been investigated [42,43]. Graphene and Graphene Oxide (GO) also act as potential platforms as nanocomposites for SERS applications [44]. The present-day research in SERS lies at the intersection of Raman spectroscopy, photonics, and nanomaterials. In the field of Raman spectroscopy, an interesting feature that SERS offers is that while some Raman-active vibrational modes might not be present in a SERS spectrum, the others which are IR and/or Raman inactive can be observed. This phenomenon is due to the surface altering the symmetry of the adsorbed molecules, thus modifying the selection rules requirements for specific bonds. Due to these advantages, SERS has now become a promising analytical tool providing sensitive, selective and non-destructive bio-/chemical information, which is highly desirable in several fields such as medical diagnostic, environmental protection, food safety, and homeland security. Therefore, during the last two decades, there has been an increasing interest in improving the utilization of SERS in real-life applications. Active research is being conducted on the development of SERS- active substrates that may be used in various study fields of fundamental and practical applications. The intrinsic properties of the noble metals, transition metals, and semiconductors enable these materials to be utilized in various fields of study of fundamental and practical applications. Owing to the higher enhancement abilities than other SERS-active materials, noble metals (silver and gold) have more widespread applications in analytical chemistry including ultrasensitive sensors and biomedicine. SERS on transition metals is mainly applied in electrochemistry and surface catalysis. Semiconductors have been promising SERS-active

Introduction and overview

23

substrates with a considerably large Raman enhancement factor under optimized conditions, excellent chemical and thermal stability, spectral reproducibility and superior selectivity. Semiconductor-based SERS technique has been used for various applications such as biological analysis, photocatalysis, solar cells, sensing, and optoelectronic devices. Many questions regarding the effects of substrate variation and the details of the SERS enhancement mechanism still remain unanswered. As such, a lot of interest has also been generated in understanding the mechanism leading to surface enhancement. While in the case of noble metals, two mechanisms, namely, the electromagnetic enhancement and the chemical enhancement may be responsible for the enhancement of the Raman signal, the enhancement mechanism of SERS on semiconductors is still unclear. None-the-same, the emergence of novel theories such as electromagnetic (plasmonic resonance) and chemical enhancement (charge transfer resonance), Mie resonance, and exciton resonance enable a better understanding of SERS mechanisms. A necessary condition for enhanced scattering is that the plasmon oscillations should be in a perpendicular orientation to the SERS active surface. In the case of in-plane oscillation (along the surface), there would be no enhanced scattering, as only adsorption takes place. Therefore, surfaces having appropriate roughness are mostly needed for enhanced scattering. The SERS activity, therefore, depends on the shape and size of the substrates. Efforts have been made toward the implementation of the plasmonic nanoparticles having different shapes into SERSactive platforms. There are two significant drawbacks to the commonplace application of SERS in analytical chemistry: a lack of substrate/molecule generality and a lack of measurement reproducibility. To a large extent, these are overcome by Shell-isolated Nanoparticle-Enhanced Raman spectroscopy (SHINERS) discussed in the next section. X. Han and B. Zhao have reviewed some of the latest developments in SERS in Chapter 10. The variety of SERS-active substrates such as noble metals, transition metals, semiconductors, and semiconductor-metal heterostructures are outlined, and their characteristics are discussed. The semiconductor nanomaterials are reported to have unique optical and electrical properties and display remarkable charge transferinduced enhancement in comparison with noble metals and transition metals. The chapter also discusses the main mechanisms of SERS in a variety of SERS-active nanomaterials, in particular, those of semiconductor nanomaterials. These include Plasmon resonance, Mie resonance, CT resonance, and Exciton resonance. Owing to the ultra-high sensitivity and selectivity, SERS allows fast, non-invasive and in situ detection of target molecules, which enable numerous promising applications in nanomaterials and nanotechnology, chemical analysis, biophysical chemistry, and clinical medicine. Several such applications have been discussed in the chapter. The authors include applications such as probing charge transfer in dye-sensitized solar cells (ZnO-TiO2/N3/Ag, Au@Ag/N3/ TiO2), chemical and biological sensing (small ions and toxic molecules, protein biomarkers and cell viability and apoptosis assays) and probing intermolecular interactions such as the effect of hydrogen bonds on CT, enantioselective discrimination by hydrogen bonding and electron transfer between redox proteins.

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2.10  Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) As mentioned in the last section, the two significant drawbacks to the commonplace application of SERS in analytical chemistry are a lack of substrate/molecule generality and a lack of measurement reproducibility. The second limitation stems from an inability to manage hotspots, where most of the signal is generated. Though metal nanoparticles (e.g., Au, Ag, and Cu) have been extensively exploited as SERS active platforms and their utilization is simple and cost-effective, the poor controllability of their structures and limited formation of hot spots in the detection zone leads to discrepancy in the resulting SERS signals. The first limitation, namely, the lack of generality is intrinsic to the SERS phenomenon. It cannot be used to study the atomically flat single crystals that are of interest to the researchers in surface sciences. Also, certain probe/analyte molecules may be incompatible with certain SERS substrates with regard to sensitivity in single- or even several-molecule studies (probe/analyte specificity). This happens when their cross-sections are too small to yield a strong signal even when the maximum possible enhancement factor (EF) is offered by the substrate. This limitation of generality can be to some extent overcome by using Shellisolated Nanoparticle-Enhanced Raman spectroscopy (SHINERS) [45,46]. In SHINERS measurements the surface of the investigated sample is covered with the layer of P@D nanoparticles, where P denotes plasmonic material (Ag or Au) and D denotes dielectric transparent protecting layer (for example SiO2, MnO2, TiO2, or ZrO2). Plasmonic cores of nanoparticles act as electromagnetic resonators, significantly enhancing the electric field of the incident electromagnetic radiation and hence leading to a very large increase in the Raman signal. The ultrathin dielectric protecting coating separates nanoparticles from direct contact with the probed material and keeps them from agglomerating and allows the nanoparticles to conform to different contours of substrates, without damping significantly the surface electromagnetic enhancement. Direct interaction between the plasmonic metal structures and various biological molecules (e.g., peptides) may sometimes lead to a change of the structure of the analyzed biomolecules which may not be desirable in certain studies such as the study of the mechanism of interaction between the adsorbate and the adsorber. The dielectric layer overcomes this problem and also increases the stability of the plasmonic core. The approach adopted in SHINERS also offers a solution to the second problem of lack of measurement reproducibility due to the inability to manage hotspots. A shell or coating can hold surface atoms in place and prevent the degradation of precisely fabricated coinage metal nanostructures. In Chapter 11, Kudelski and coworkers discuss the shell-isolated nanoparticleenhanced Raman spectroscopy (SHINERS) developed for the chemical analysis of various buried interfaces, that is, surfaces of the solid samples in the liquid or the highpressure gas, such as interfaces of various biological samples in their natural environment, which may otherwise be damaged with the change in environment like creation of vacuum. The chapter provides an overview of the state of the art of SHINERS spectroscopy and discusses issues such as the mechanism of the large enhancement of the efficiency of generation of Raman signal by SHINERS nanoresonators, the

Introduction and overview

25

methods of synthesis of various plasmonic metal nanoparticles which are used as cores in SHINERS nanoresonators, and the methods of deposition of the protecting layers on plasmonic cores. It provides examples of various applications of SHINERS spectroscopy. Since the efficiency of Raman signal generation depends on the nature and structure of the plasmonic core (such as sharp structures or hollow nanostructures) and type and thickness of the dielectric coating, these can be controlled by methods of synthesis of plasmonic cores and the methods of deposition of the protecting layers. All these and related aspects are described in detail. SHINERS significantly expands the flexibility of SERS for useful applications in the materials and life sciences, as well as for food safety, drugs, and environmental pollutants. Detection and quantitative estimation of the concentration of various chemical compounds such as insecticides, pesticides, explosives, cyanide ions, and molecules of biochemical significance such as proteins (myoglobin), uric acid, glucose, etc., have been discussed. Other areas of application considered are the study of the mechanism of various surface processes, including electrochemical and catalytic reactions, adsorption and desorption processes, and investigation of various biological samples and clinical samples such as normal and pathologically changed cancer cells. These include normal breast tissue (NB), fibroadenoma (FD), atypical ductal hyperplasia (ADH), ductal carcinoma in situ (DCIS), and invasive ductal carcinoma (IDC) etcetera.

2.11  Stimulated Raman scattering (SRS) Recently, there has been an increased interest in label-free biomedical imaging based on vibrational spectroscopy. Methods for obtaining tomographic images have been extensively applied in the biomedical and material fields. In these fields, it is essential to measure the change of morphologies inside samples in a noninvasive way. As one of the tomographic techniques, optical coherence tomography (OCT) using NIR radiation has been widely applied. However, the application of NIR light allows spatial resolution of only a few micrometers due to the Abbe diffraction limit, in which the light cannot be focused to a diameter of less than the wavelength. In several systems such as in cellular biology, the structures in cells are much smaller than micrometers and in material fields, sometimes it is important to measure thicknesses of a few to hundreds of nanometers such as in multi-layered materials consisting of thin layers. In order to overcome this difficulty interferometry is considered as a solution. Analogous to the situation in optical interferometry, where interference between the reference beam and the light elastically scattered by the sample is observed, in Raman scattering interferometry the interference signal between the reference beam and the light inelastically scattered by the sample is observed. This may give an improved resolution less than the diffraction limit. The light generated by Raman scattering is incoherent, and it does not interfere with the reference light. However, an interference pattern between a coherent anti-stokes Raman (CARS) signal and a reference light has been successfully measured. The nonresonant background in the CARS signal, however, disturbs the detection of target species and often causes a distortion in the obtained spectrum. Interference is also observed between stimulated Raman scattering (SRS) signal, a variant of the Coherent Raman scattering (CRS) signal without a nonresonant

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background which arises from a third-order optical process, and the reference coherent light. This has led to the development of SRS microscopy having a spatial resolution of the order of nanometers when combined with a scanning probe microscope. An advantage of the combination of SRS and interferometry is that the chemical-contrast signal from the sample is detected in a heterodyne way. By applying the heterodyne detection, the signal intensity can be enhanced, and high-contrast imaging is enabled. Robles et al. [47] have reported heterodyne spectral multiplex detection of an SRS signal by applying the coherent feature of the SRS signal. Several advances have taken place recently in Coherent Raman Scattering (CRS) microscopy, including CARS and SRS microscopy, that have resulted in orders of magnitude higher sensitivity than conventional Raman microscopy. This has permitted in vivo imaging at video rates. Besides a better sensitivity than spontaneous Raman scattering, SRS microscopy has several other advantages over other methods. For example, it is insensitive to the nonresonant background present in CARS and has approximately an order of magnitude better spatial resolution than IR microscopy. In many ways, SRS spectra are identical to those of spontaneous Raman scattering and its signals are linear in concentration facilitating easy assignments on the basis of the Raman literature. In SRS the Raman signal is amplified by matching the frequency difference between two light fields to a particular molecular vibrational frequency, Ω. Under these conditions, the higher-energy field experiences a loss (stimulated Raman loss SRL), while the lower-energy field experiences a gain (Stimulated Raman Gain SRG). The SRL and SRG signals move in the same directions as the pump and the Stokes lights, respectively. Unlike spontaneous Raman scattering or CARS, where the signals of interest are emitted at a different wavelength than the input light, SRS relies on detecting small amplitude changes on the input fields (relative magnitude of 10−6–10−3). Different approaches have been adopted to detect the relatively low SRS signal in the presence of laser noise and improve the sensitivity of the SRS microscopy technique. In one such approach, high-frequency modulation scheme was adopted by Christian et al. [48]. In this scheme, either the pump or the Stokes beam is modulated at a high frequency and the modulation transfer to the other beam due to SRS of the sample is detected with phase-sensitive detection. A near shot-noise-limited sensitivity of ( ∆I/I ) 4Ts . For the observation of the decay curve, one must collect at least 10 independent time windows in the informative time interval T = 2/∆f , thus 10 ∆T > 2/∆f and f /∆f > 20 = N lower . In common measurements, we usually deal with a whole recording time T ≈ 80 ps and a sampling interval δt = 80 fs (therefore fmax = 8 THz) that lead to the sampling number kmax = 1024. The time interval T corresponds to a maximal spectral resolution δf = 10 GHz. Using the zero padding procedure, we extend the kmax up to kmax = 214 and accordingly the interval between spectral points becomes δf = 3 GHz. The calculated value S (Tn , f ) can be plotted as a 2D time-frequency distribution or as 1D dynamics of the frequency of interest S (Tn , f0 ) (see Fig. 4.9). In the example of Fig. 4.9, the excitation of SEW with the grating coupler occurs at ∼0.55 THz (first order of diffraction) and at 1.1 THz (2nd order). At the excitation time, coupled energy is missing in the reflected spectrum, and thus one sees black lines in the 2D plot up to ∼10 ps. At longer times, there is no more reflected THz signal as the incident THz pulse is over. However, the previously excited SEW is gradually coupled out by the

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147

Figure 4.9  The time-frequency distribution of the THz pulse reflected from the grating.

grating, and thus a bright line appears at the resonant frequencies up to ∼40 ps, this latter value corresponding to the SEW lifetime at the grating surface.

5  Generation of THz waves in gases The purpose of this part of the chapter is not to present the whole variety of methods and applications of THz photonics of gaseous media, but only to emphasize some important principles of this technology. As an example, we will only consider generation in air.

5.1  Generalities and historical overview The generation of THz waves in gases has emerged as one of the most promising techniques for performing studies with broadband and high-intensity THz pulses [1]. This has been accomplished through a number of different methods, either using 800nm pulses delivered by Ti:Sapphire fs lasers or in combination with 400-nm pulses obtained through second harmonic generation, or with DC fields to improve the efficiency. It has attracted much attention because it allows researchers to wholly utilize the potential of the high-intensity pulses produced by modern chirped-pulse amplified laser systems, which provide short (300 K; such a mixture is called normal H2 (n-H2). Because a transition (interconversion) between o-H2 and p-H2 is extremely slow under isolated conditions, the OPR remains 3 on cooling n-H2 to low temperature. To obtain a sample enriched in p-H2, one requires a catalyst to accelerate the conversion, as described in Section 3.1.

2.2  Properties of solid p-H2 The fundamental properties of H2 have been extensively investigated; an excellent review is available [17]. In this section, we focus on the properties of p-H2 as a matrix host. The electronic properties and crystal-related properties of p-H2, Ne, Ar, Kr, Xe, and N2 are summarized in Table 5.1. Electronic multipole moment: Most atoms and molecules used as matrix hosts (noble-gas atoms, dihydrogen, dinitrogen, and carbon dioxide) have no electric dipole moment because of a center of symmetry, but symmetric molecules might have electric quadrupole or hexadecapole moments. p-H2 has no quadrupole moment, but o-H2 has one, resulting in significant anisotropic interactions. o-H2, which invariably exists in a p-H2 sample in a small proportion, thus interacts strongly with guest molecules in solid p-H2 through, for instance, a dipole-quadrupole interaction; see Section 4.3 for details. The importance of quadrupole moments in the intermolecular interaction was realized during a study of the higher-energy isomer of formic acid, cis-HCOOH, in solid noble-gases and N2 [18]. cis-HCOOH is known to decay to trans–HCOOH via tunneling isomerization; the decay period in solid N2 is much greater than that in a solid noble gas. This difference has been attributed to dipole-quadrupole interactions in solid N2. Because the electric dipole moment of the cis isomer is greater than that in Table 5.1  Electronic and crystal-related properties of p-H2 and conventional matrix hosts (Ne, Ar, Kr, Xe, and N2).a Quadrupole Nearestmoment Dielectric Crystal Lattice neighbor Zero-point (Q)b constant (ε′)b structurec constant / Å distance / Å motiond p-H2 Ne Ar Kr Xe N2 a

0 (0.4)e 0 0 0 0 –1.4

1.28 (3 K) 1.25f 1.63 (20 K) 1.88 (20 K) 2.19 (20 K) 1.43 (4.2 K)g

hcp ccp ccp ccp ccp ccp

3.79 (4.2 K) 4.47 (15 K) 5.31 (15 K) 5.65 (15 K) 6.13 (15 K) 5.66 (15 K)

3.793 3.16 (4 K) 3.76 (4 K) 3.99 (4 K) 4.34 (4 K) 3.99 (4 K)

18% 9% 5% 3% 2% —

From Refs. [17,19,20] unless otherwise specified. Q in unit of 1026 esu cm2; ε′ in unit of F m–1. hcp, hexagonal close-packed; ccp, cubic close-packed, which has the same structure as fcc (face-centered cubic). d Root-mean-square longitudinal zero-point amplitude divided by the lattice constant, 1/2/R0; from Fig. 57 of Ref. [17]. e The quadrupole moment of o-H2 is listed within parentheses. f From Ref. [21]. g From Ref. [22]. b c

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the transition-state structure for cis-to-trans isomerization, an additional stabilization of the cis form is expected for the N2 environment. Dielectric constant: The effect of the environment on wavenumbers of IR lines of guest molecules in solution has been extensively studied and is interpreted in terms of specific solute-solvent dipolar interactions. Various theories have been developed to model the solvent-induced shifts; the relative permittivity (formerly dielectric constant) is generally a useful parameter to scale the shift. The shifts tend to be smaller in solvents with smaller relative permittivity. In principle, the same model is applicable to guest molecules in a low-temperature matrix, in which the solvent molecules surrounding a guest molecule are fixed in position, although the rigidity of the cage induces repulsive forces that should be taken into account. Matrix shifts induced in an Ar or Ne host have been demonstrated to be typically less than 1% from that in the gaseous phase [6,7]; consequently, from the relative permittivity of p-H2 that is as small as that of Ne, one can expect small matrix shifts for species in solid p-H2. Crystal structure: All noble-gas atoms crystallize in either a cubic close-packed (ccp) structure or a hexagonal close-packed (hcp) structure; the ccp structure is commonly described as a face-centered cubic (fcc) structure. These solids are generally polymorphic; that is, they crystallize in more than one structure. According to X-ray diffraction, solid Ar powder frozen from a liquid showed evidence for 1%–5% metastable hcp structure mixed with the ccp structure [23]. In contrast, the p-H2 crystal is dominated by an hcp structure. When solid p-H2 is prepared by direct deposition (see Section 3 for details) the solid might have mixed hcp and ccp structures, but annealing at ∼4.5 K converts the ccp structure to the more stable hcp structure. The presence of o-H2 and guest molecules in greater proportions might prevent the conversion to a pure hcp structure on annealing [24]. The enclosed-cell method allows one to prepare a crystal of hcp structure, despite disadvantages in embedding guest molecules (see Section 3.2) [12]. Zero-point motion: The amplitude of zero-point motion relative to the distance to the nearest neighbor can serve as a measure of the localization of particles in their equilibrium positions in a crystal; that is, the softness of the solid. This amplitude for solid p-H2 amounts to 18% of the lattice constant, whereas that of solid Ar is only 5%, indicating that Ar atoms are more nearly localized at the lattice site. Because such a large zero-point motion of p-H2 defies treatment with a classical theory of lattice dynamics, solid p-H2 is called a quantum solid, as also solids of 3He and 4He [12,25].

2.3  Quantum solid The Hamiltonian for the ground state of a solid is written as ˆ = − 1 λ 2 ∑ ∇ 2 + 1 ∑ v ( i, j ) H (5.3) i 2 2 i≠ j i in which λ =  / σ mε is the de Boer quantum parameter representing the relative contributions of kinetic versus potential energies approximated by a sum over all pairwise interactions [17]. The parameters σ and ε; are the core radius and the depth of the 12 6 potential well in the Lennard–Jones 6–12 potential of V (r ) = 4 ε (σ / r ) − (σ / r )  . Parameter λ is introduced to make the Hamiltonian dimensionless. The λ value

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increases for a light particle and a shallow potential (smaller ε); representative values are: λ2(3He) = 0.241, λ2(H2) = 0.076, λ2(Ne) = 0.0049, and λ2(Ar) = 0.0027. The greater is the value of λ, the softer the solid becomes. The softness of solid p-H2 is advantageous in, for example, producing free radicals via photolysis of guest molecules in situ (see Section 5.1). In addition to these structural features, the density of states in solid p-H2 is exceptionally sparse for a molecular solid because its rotational B parameter is 60.853 cm–1, the fundamental vibrational wavenumber is 4401.2 cm–1, and the first excited electronic energy is 91,700 cm–1, all exceeding the corresponding parameters of other molecules [12]. Furthermore, the Debye temperature, which characterizes the lattice motion in terms of temperature, is approximately 100 K (∼70 cm–1). All these features imply that the relaxation of excited molecules in solid p-H2 is extraordinarily slow; line broadening related to the lifetime of the excited state therefore decreases.

3  Instrumentation: preparation of p-H2 and matrixisolation spectroscopy 3.1  Ortho-to-para converter Because the conversion between o-H2 and p-H2 is slow, one requires an ortho-to-para converter to prepare para-enriched H2. To our knowledge, the first such convertor was reported by Bonhoeffer and Harteck in 1929 [26]; they cooled a tube filled with an activated charcoal catalyst and H2 in a vessel containing liquid nitrogen or liquid hydrogen to convert o-H2 to p-H2. A similar method has been used in the Oka group (University of Chicago) [27] and the Lester group (University of Virginia) [28]. A more convenient method was introduced by Tam and Fajardo, who employed a H2 flow system with a closed-cycle He cryostat [29]. This method has been described in detail elsewhere [29–31]; we introduce briefly a similar system used in National Chiao Tung University (NCTU), Taiwan. Fig. 5.1 shows a schematic diagram of an ortho-to-para converter at NCTU. A cryostat (10 K) is placed in a vacuum shroud, which is evacuated to ∼1 × 10–6 Torr with a turbo-molecular pump. A copper bobbin is attached to the cold head of the cryostat and a 1/8″ copper tube filled with catalyst (iron(III) oxide, hydrated, 30–50 mesh) is wound tightly onto it. The length of copper line inside the vacuum shroud was extended on making a few loops before and after the copper bobbin so as to avoid a flow of heat from outside the vacuum shroud. After the installation, the catalyst was activated on warming to about 200 °C under vacuum while passing Ar or H2. Normal hydrogen (n-H2, purity >99.9999%) is admitted to the tube; its flow rate is controlled with a needle valve. A typical flow rate of n-H2 is∼30 sccm, in which sccm indicates standard cm3 per minute; the conversion temperature is set to ∼13 K. The p-H2 gas is transferred to a vacuum manifold for gas mixing or direct matrix deposition via a copper tube. We employed vacuum components made of copper instead of stainless steel to avoid unexpected para-to-ortho conversion by a paramagnetic material. Methods to verify the o-H2 concentration in p-H2 are presented in Section 3.3.

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Figure 5.1  Schematic diagram of ortho-to-para converter at NCTU. Copper tubes (1/4″ and 1/8″) are represented with dotted and solid lines, respectively. BV, bellows valve; BG, Bourdon gauge; NV, needle valve.

A lower temperature allows a preparation of p-H2 with less o-H2, but the vapor pressure of p-H2 is also decreased at lower temperature. When using the conventional method of conversion (i.e., without flowing H2), the vapor pressure of H2 limits the purity of p-H2. Typically, if p-H2 at 500 Torr is required, the sublimation temperature should be above 19 K; the mixing ratio of o-H2 is ∼1140 ppm. On using the flow system, one can, however, use a lower conversion temperature to yield, for example, a mixing ratio of oH2  Ei In a stable discharge all ∆ni = 0 so that ∆V = 0. Let us suppose that the impact of monochromatic photons with the atoms in the discharge occurs at t = 0, then the stimulated transitions between Ei and Ej lead to following relations for the changes in the number density of atoms: ∆ni (0) = −Q(ni − n j ), ∆ni (0) = −∆n j (0) and ∆nk = 0 for all energy levels, where k ≠ i, j If τi and τj are the lifetimes of Ei and Ej, respectively, then the temporal variation in the changes of number densities of these levels are given by: ∆ni (t ) = ∆ni (0) exp(−t / τ i ) and ∆n j (t ) = ∆n j (0) exp(−t / τ j ) Thus in the light of Eq. (6.1), the temporal evolution of the voltage produced by the OGE is given by: ∆V(t) = − β Q( ni − n j )[ a j exp(− t / τ j ) − ai exp(−t / τ i )]. (6.2) The difference between the lifetimes of energy levels Ei and Ej generates two types of (positive and negative voltage) OG signals as discussed later. Suppose τi = τj = τ, then Eq. (6.2) reduces to: ∆V (t ) = − β Q(ni − n j )[ a j exp(−t / τ ) − ai exp(−t / τ )] = − β Q(ni − n j )(a j − ai ) exp(− t / τ ) = [ − ve]exp(− t / τ ) Since aj > ai, the OGE will generate a negative voltage (and current) change, after the impact of the pulsed monochromatic laser radiation, which would reach a minimum value and then grow with time to reach zero as shown in Fig. 6.7A. Suppose τi >> τj = τ, then it can be shown with the help of Eq. (6.2) that the term dependent on the lifetime of Ei becomes dominant with increasing time. Thus the initial signal is negative which passes through a minimum value and then grows to reach a maximum positive value before slowly decreasing to zero as shown in Fig. 6.7B.

Optogalvanic spectroscopy and its applications

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Figure 6.7  Schematic representation of the temporal evolution of negative (A) and positive (B) optogalvanic signals generated by a laser pulse.

The negative OG signal is observed when t  τi, so that we have: ∆V (t) = − β Q(ni − n j )[0 − ai exp(−t / τ i )] = [ + ve]exp(−t / τ i )

.

3  Experimental systems The experimental arrangement for recording the OG spectra consists of the atomic or molecular sample probed by a laser in some form of electrical discharge. The measured OG signal is related to an impedance change in the discharge. Flames have also been used in some experiments, to provide a means of ionization without a discharge to be probed by a laser.

3.1  Laser optogalvanic spectroscopy of dc discharge The positive column is the region of glow discharge with a characteristically uniform spatial intensity. An electrical discharge produced by a stable dc voltage is irradiated with a wavelength tunable laser beam. A resistance, which limits the current in the circuit, is connected in series with the discharge tube. The discharge current and gas pressure are adjusted to achieve a minimum noise level and the output is taken via a coupling capacitor which blocks the dc component. The experimental arrangement for recording the OG spectra is shown in Fig. 6.8. The dye laser beam is chopped at an appropriate frequency to get a good signal to noise ratio. This technique has been successfully used for gases and vapors at low pressure. The continuous electric discharge is characterized by the constant current and the constant voltage maintained between the two electrodes. This steady-state results from the stabilization of a number of coupled phenomena of which, the radiative and

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Figure 6.8  Experimental setup for optogalvanic spectroscopy of the positive column discharge.

electron impact processes involving atoms and molecules play the dominant role in the OGE. The plasma is composed of free electrons and neutral or ionized atoms with internal energies spread over all the possible excited states. During the steady-state condition, the atomic population density is stationary over the entire length of the discharge and gives rise to a total rate of created ions and a constant charge density. Thus, the resulting plasma has an internal constant impedance Z characterized by the nature of gas or vapor and the electric energy dissipated in the circuit that feeds the discharge. Illuminating the dc discharge with a laser light corresponding to an atomic transition perturbs the steady-state population of the atomic levels. Since the collisional ionization rates from different atomic levels are unequal, the ionization balance of the discharge is destroyed and a new equilibrium corresponding to a different steady-state current and voltage is set up during the period of irradiation. Bridges [7] was one of the first to systematically detect the OGE in dc discharge plasma containing cesium, uranium, mercury, hydrogen, neon, and argon. Using a mechanically chopped cw Rh6G laser, the effect was observed for incident laser wavelength corresponding to an allowed transition of the atomic species. The largest effect was found as a 30% voltage change in the operating voltage of the discharge. As shown in Fig. 6.8, in OGS the current through a gas discharge is monitored as the laser source is tuned through the frequencies (or wavelengths) of the allowed transitions for excited atoms in the discharge. Absorption of laser light by the atom excites it to a less bound state and thus, increases the probability of its ionization by discharge collisions leading to an increase in the discharge current. If, on the other hand, the probability of ionization is decreased from the new excited state reached by absorption, there is a decrease in the discharge current. This small change in discharge current is detected with great sensitivity by a lock-in amplifier due to periodic variation of discharge current at the chopping frequency of the laser beam. It is to be noted that in contrast to other spectroscopic techniques OGS does not require any conventional optical detector to obtain the atomic spectra. The gas discharge itself serves as a resonant photodetector. In the OG spectrum, the spectral intensity, in terms of change of discharge current, can be both positive and negative. If a tunable dye laser is pumped by a pulsed laser like Nd-YAG, with a repetition rate in the range of 5–30 Hz, transient OG signals are observed. The temporal profile of the OG signal is an indicator of the mechanism of return of the plasma discharge to its initial steady state before interacting with the resonant laser light. Metastable

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energy levels and “nl” Rydberg levels in atoms have a greater probability to ionize than to decay by spontaneous emission. In the case of Rydberg levels, the radiative lifetime increases as n3. It has been shown by Erez et al. [8] that if the excitation of atoms within the discharge to a higher level enhances the ionization, the OG signal profile appears as a negative voltage (and current) before slowly reaching the steady state as shown in Fig. 6.7A. On the other hand, the excitation from a metastable level gives rise to a temporal profile starting from a negative part due to enhanced ionization followed by a positive and slower return to steady state (see Fig. 6.7B).

3.2  Laser optogalvanic spectroscopy with hollow cathode discharge A hollow cathode discharge may be used as a reservoir of sputtered atoms for spectroscopic investigations. The negative glow from the opposite walls of the inner surface of hollow cathode merge to produce a high density of neutral and excited atoms as well as of ions at the center of the hollow cathode. The rare gas breaks down under the application of a potential of only a few 100 V between the two electrodes and creates a number of electron-ion pairs. The ions are accelerated in the high field of the cathode dark space and hit the cathode with sufficient energy to eject atoms from the lattice sites. The sputtered atoms rapidly lose their kinetic energy by elastic collisions with rare gas atoms and come into thermal equilibrium. In this process, sufficiently high steady state densities, of atoms in the ground and metastable states, are accumulated in the negative-glow region of the discharge for OGS to be performed. The experimental arrangement of Fig. 6.9 shows a Fe-Ne cathode lamp connected to a regulated

Figure 6.9  Experimental arrangement for recording the optogalvanic spectra of noble gases from hollow cathode discharge.

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Figure 6.10  Spectral profile of the dye laser output in the wavelength region 615–675 nm (top) and OG spectral lines from a Fe-Ne hollow cathode lamp for calibrating the observed spectra.

dc power supply through a current limiting resistance of 12 kΩ. The voltage across the lamp could be set between 170 and 190 V for normal glow discharge and the OG spectrum recorded using a 0.33 µF capacitor. The OG voltage signal could be seen on an oscilloscope and was processed by a boxcar averager before being fed to a chart recorder. Fig. 6.10 shows the wavelength calibration for the dye laser output in the wavelength region 615–675 nm, using the OG spectral lines [9].

3.3  Laser optogalvanic spectroscopy of radio frequency and microwave discharges When a low pressure gas or vapor cell is placed in a coil comprising part of a resonant circuit coupled to an RF oscillator, a uniform and noiseless discharge plasma is produced by electrical breakdown in the sample. Stanciulescu et al. [10] were the first to demonstrate the detection of optical resonant absorption in gases with radio frequency excitation, in an electrodeless cylindrical discharge tube. The electrodeless radio frequency discharge resembles, in many cases, the positive column of an equivalent dc discharge. The discharge plasma is almost neutral and diffusion controlled and

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Figure 6.11  Experimental arrangement for optical spectroscopy of radio frequency (RF) excited discharge.

in contrast to dc discharges, the plasma is spatially homogeneous. The electron temperatures of high frequency discharge plasmas are higher than those of dc discharges leading to significant population of excited states. A lower discharge power level minimizes the broadening of spectral lines and the noise caused by cathode sputtering in dc discharge is completely eliminated. OGS of electrodeless discharges is particularly suitable for corrosive gases. In the experimental arrangement of Fig. 6.11, a pickup coil around the discharge tube and the variable capacitor forms the tank circuit which has to be adjusted to be resonant with the applied RF field. The signal from the pickup coil is rectified and passed through an amplifier tuned to the modulation frequency. The reference from the chopper and the OG signal are processed by the lock-in amplifier and the spectrum recorded as a function of the tunable laser wavelength. Suzuki and co-workers [11–13] have used rf discharge for OGS of N2, NH3, and NO2 molecules in addition to argon gas in the wavelength region 580–630 nm, and of negative ions using an experimental set up similar to that shown in Fig. 6.11. The plasma in a high frequency electrodeless discharge is approximately neutral and diffusion controlled. Ions and electrons exhibit different motions in discharges maintained by radio frequency and microwave fields. In a glow discharge, electrons undergo many collisions in one period of oscillation in radio frequency field. But electrons undergo many oscillations in one period of collision in microwave field. The mechanism of OG effect where collisional ionization is the main process, the OG signals obtained from dc discharges, rf discharges, and microwave discharges provide valuable information about negative ions. The mechanism of OG effect of negative ions can be understood from large differences in the mobility of the negative ions and the photo detached electrons. This will be discussed in a later section where electron affinity of atomic and molecular species has been determined from the photodetachment threshold.

3.4  Laser optogalvanic spectroscopy in flames In contrast to electrically sustained discharges, the flame is a chemically sustained mild plasma. Any analytical flame can be used to investigate atoms, molecules, and

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Figure 6.12  Schematic of experimental setup for optogalvanic spectroscopy in flame with pulsed laser source.

free radicals using the OG technique. In the experimental setup of Fig. 6.12, a pair of tungsten wires are placed symmetrically around an analytical flame produced by a slot burner. The tungsten wires act as the cathode while the grounded slot burner head acts as the anode. The high voltage power supply maintains the cathode at a negative potential of about 1 kV with respect to the anode. The tunable dye laser beam is focused inside the flame between the two symmetrically placed electrodes. By changing the wavelength of the laser beam, the condition of resonance, with transitions in atomic or molecular species in the flame, is obtained. The induced transitions between a pair of levels perturb the thermal population and leads to the OGE, as described earlier. The pulsed OG signals are amplified before entering the boxcar averager, which gets the reference signal of the corresponding laser pulse from the combination of the beam splitter and the photodiode. The boxcar, averages the signal over 30 or more laser pulses to enhance the signal to noise ratio. Analytical flame spectrometry has been used in conjunction with the OGE to detect trace metallic elements [14,15]. Hastie and coworkers [16] have reviewed the importance of OGS of flames to emphasize its role as a probe of ionization effects and ion mobilities.

4  Applications of optogalvanic spectroscopy In OGS, current and voltage changes, produced upon UV-visible and IR irradiation of discharges and flames, are measured. These electrical signals are produced by absorption of photons whose energy corresponds to electronic transitions. Although OG signals result from optical absorption, the detection technique is superior to absorption spectroscopy and laser induced fluorescence (LIF) detection. Optical absorption techniques suffer from the disadvantage of detecting a small decrease in a large photon flux whereas LIF detection is hindered by scattered photons. In contrast, OG signals are detected against a much reduced background noise that is determined by the

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electrical stability of discharge. The simplicity of OG spectroscopy provides a convenient method of measuring optical resonances without the requirement of any optical detection. This method has been used in the laser absorption spectroscopy investigations of excited metastable levels that are inaccessible by direct radiative transitions. It has also proved useful in the studies on rare gas atoms that require vacuum ultraviolet radiation. The high sensitivity of the method is also used in trace element analysis in flames. Several review articles have been written on the variety of experimental techniques, theoretical models to explain the mechanism of OG effect and its applications in the investigations of atoms and molecules [17,18]. In the following sections we summarize some examples to illustrate the wide range of applications of OGS.

4.1  Optogalvanic spectroscopy of rare gases Smyth and his coworkers were the first to give a photon induced ionization model for OG signals associated with different groups of spectral lines of neon [19,20]. A semi-empirical theory for the pulse response characteristics of OG signals has been proposed by the research group of Erez [21–23]. Nestor [24] was the first to observe two-photon transitions in neon in the 600–640 nm region and Bickel & Innes [25] performed a very systematic study of two-photon transitions using the Racah notation [26] instead of the Paschen notations for the energy levels of neon. Fig. 6.13 presents a Rydberg series in two-photon OG spectrum of neon in the spectral region 500–548 nm (20,000–18,240 cm-1). The excited states of neon atom occur in pairs rather than multiplets and can be best described by the (j,l) coupling scheme of Racah [26] although its ground state is 1S0 resulting from the closed electronic configuration 1s22s22p6. The singly ionized neon with electronic configuration 1s22s22p5 gives rise to its lowest odd parity states 2Po3/2 and 2Po1/2 of which the former is of lower energy. The excited states of Ne shown in Fig. 6.14 are obtained by adding “s,”“p,”“d,” or “f” electrons to the lowest states of Ne+. Thus 1s22s22p53s corresponds to adding a “s” electron with j = ½

Figure 6.13  Two-photon OG transitions in neon originating from the metastable state 3s[3/2]o2 forming the “ns” and “nd” Rydberg series.

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to the 2Po3/2 and 2Po1/2 states leading to 3s[3/2]o2, 3s[3/2]o1 and 3s’[1/2]o0, 3s’[1/2]o1 states, respectively. The superscript “o” represents odd parity and the running electron orbital without prime corresponds to the ionization limit 2Po3/2 and that with a prime corresponds to 2Po1/2. It has been found that all members of the convergent series shown in Fig. 6.13 have a common lower level in the metastable 3s[3/2]o2 state [27]. Alternate members of the convergent series are single lines with upper levels identified as ns[3/2]o2 with n = 7 for the first member observed at 18245.5 cm-1. The upper states of series consisting of lines with fine structure have been identified as odd parity states belonging to “nd” electrons with n ≥ 6 The assignments shown in Fig. 6.13 are made in the light of known energy levels of neon [28]. A partial energy level diagram of Ne is shown in Fig. 6.14. Suzuki et al. [29] have used microwave discharge to record OG spectra of Ar and He with three different methods. It was found that the use of a double probe consisting of two parallel rods along the axis of the discharge tube resulted in providing the best spectra. The signal to noise ratio in this case was found to be two orders of magnitude larger than that obtained with a double probe consisting of two parallel rings that were set perpendicular to the axis of the discharge tube. This difference was attributed to the fact that the two parallel rods are in close contact with the discharge plasma and the laser beam can be passed between them. Thus the double probe of parallel rods is able to extract the OG signal with a much higher efficiency. They were able to measure the OG signals over a wide pressure range. Thus in the case of He discharge, stable discharge was maintained from 1 to 40 Torr at a microwave power of 20 W. For gas pressure greater than 10 Torr, OG signals around 590 nm, corresponding to He2 molecule were observed and the intensity was found to be maximum at 35 Torr. A portion of the laser-excited OG spectrum of Ar glow discharge is shown in Fig. 6.15 [30]. The strong OG spectral lines of Fig. 6.15A corresponding to one photon transitions are relatively broad compared to the sharp-looking two-photon transitions. In the spectra recorded after 2 µs of laser pulse, both one and two photon transitions are associated with a negative voltage signal (Fig. 6.15A middle and bottom) for linearly as well as circularly polarized light. However, for spectra recorded after 13 µs of the laser pulse, two-photon transitions are associated with negative voltage and one photon transitions are associated with positive voltage signals for the linearly polarized light [30]. The fact, that some of the signals exhibit negative OG effect at 2 µs but positive OG effect at 13 µs of the laser pulse absorption, indicates that these are associated with upper energy levels of shorter lifetime as explained in Section 2 and have been assigned as one photon transitions. The temporal profiles of one photon and two photon OG signals are shown in Fig. 6.16.

4.2  Optogalvanic spectroscopy of molecules Iodine molecule (I2) has acquired the same role in electronic spectroscopy of molecules as does the hydrogen atom in atomic spectroscopy. Rettner et al. [31] were the first to investigate the OG spectrum of I2 and found that the B-X electronic system closely resembles its fluorescence excitation spectrum except when the laser beam is coincident with the discharge axis. It was found that the OG signal changes sign

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Figure 6.14  Partial energy level diagram illustrating the odd Rydberg series in 2-photon OG spectrum of Ne. “ns” series (green) and “nd” series (red).

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Figure 6.15  (A) Laser optogalvanic spectrum of argon recorded 13 µs after the laser excitation (top) with linearly polarized light and after 2 µs of the laser pulses of linearly polarized light (middle) and of circularly polarized light (bottom). (B) Partial energy level diagram of argon with 2-photon transitions (red arrows) and one photon transitions (black arrows).

in the dc discharge tube when moving from the cathode to the anode, suggesting that more than one mechanism is responsible for OG effect in iodine. Webster et al. [32] used a pulsed dye laser to probe the discharge in iodine vapor to study the OG spectrum with a view to investigate electron photodetachment from I− ions. It is found that the first photodetachment threshold for the production of I (2P3/2) atoms occurs at 405.18 ± 0.02 nm leading to the electron affinity Ea(I) = 3.0591 ± 0.0001 eV. Rai et al [33] have studied the Doppler limited OG spectrum of the B-X system of I2 molecule in the wavelength region 570–630 nm. A weak continuum is found to be superposed over the discrete spectrum of the B-X system. When the applied voltage is increased, the OG signals corresponding to some of the discrete bands, in the lower wavelength region, change from positive to negative. The continuum OG signal in the entire region also changes from positive to negative but the remaining spectrum is much less affected by the change in applied voltage. These studies indicate that there are multiple mechanisms responsible for the OG effect in iodine molecule. Vasudev and Zare [34] have studied the OG spectrum of the A-X electronic system of HCO radical. Most of the levels of the excited state of HCO are diffuse and LIF is inapplicable to investigate the A-X system due to extremely low fluorescence quantum yield of the A state. The OG spectrum was recorded, in an electrodeless rf discharge through acetaldehyde to produce HCO, by using rhodamine 6G dye laser in

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Figure 6.16  Temporal profiles of one photon (left) and two-photon (right) OG signals (top) and the gate delays of 2 and 13 µs at which spectra have been recorded (bottom).

the 580–620 nm region. It is found that the OG effect is especially large when the laser excited transition involves a predissociated state. The analysis of the OG spectrum has been done by assuming a number of predissociation mechanisms. Using an experimental set up similar to that shown in Fig. 6.12, Schenck et al. [35] have investigated the absorption spectra of metal oxides using the OG technique. Pemixes H2/air flame was maintained at atmospheric pressure and aqueous solutions were aspirated into the flame. Sc2O3 was used to prepare scandium solution, yttrium solution was made from YCl3 and lanthanum solution from La(NO3)2.6H2O. At flame temperatures of about 2000 K, the only products are water, nitrogen, and excess hydrogen and the spectra are independent of the proportion in which the chemical elements combine. In LaO four excited electronic states have been detected, giving rise to 18 sequences in the 360–610 nm wavelength region. High-lying electronic states in ScO and YO have been detected in the shorter wavelength region of 360–450 nm. The spectral resolution is limited by the laser bandwidth with very good signal to noise ratio.

4.3  Mobility measurements of ions and small particles in flames Smyth and his group [36,37] have used a slightly modified experimental set up from that shown in Fig. 6.12 to measure the mobility of atomic ions and small particles.

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Figure 6.17  Schematic oscilloscope trace for irradiation of a C2H2/air flame containing Na and U atoms with a single laser pulse at 539.9 nm. Adapted from Ref. [36].

Laser enhanced ionization creates a pencil of ions along the path of the laser in the flame. The ions are collected by the electrode at a distance from the point of laser impact and the time from their creation to the collection measured for a series of applied potentials. The observed OG signal results from the increased collisional ionization rate of the neutral atoms in an electronically excited state relative to that in the ground state. Sodium and Uranium were introduced into the flame as salt solutions aspirated into the burner. The tunable pulsed dye laser is tuned to an electronic transition of the neutral metal atom and a typical OG signal is schematically shown in Fig. 6.17. The first feature of the trace is the electron signal that peaks at about 0.8 µs. This is followed by the ion signals due to the sodium ions arriving 10 µs after the laser pulse and the uranium ions arriving 21 µs after the laser pulse. These experiments directly measure the velocity of metal ions in a flame that leads to the determination of the mobility of the ion. A comparison of the direct experimental determinations of ion mobilities, of Li, Na, K, Ca, Fe, Sr, Ba, In, Tl, and U, with the Langevin theory [38] shows substantial improvement over previous studies. The chemistry of soot formation in flames is a complex field where the key precursors leading to the formation of carbon particles are not very well understood. There are fundamental processes involving the roles of ions and polycyclic aromatic compounds that need to be studied. Smyth and Mallard [37] have carried out OG spectroscopy of flames to elucidate the formation of small particles in a C2H2/air flame. It has been found that laser-induced ionization signals observed, at the sooting limit, may be attributed to rapid heating of small particles with subsequent thermal ionization. A rough estimate of the particle mass (2300–6300 amu) and particle size (1.6–2.2 nm) for the ionized species has been obtained. These results indicate that the OG signals are due to very small particles which appear early in the soot formation process. LOGS has been used to explore the limits of detection for uranium and selected volatile, toxic trace metal particles in the off-gases of mixed hazardous waste [39]. In thermal treatment systems, metal species occur as airborne metal oxide or metallic particles rather than as free, single atoms. This study aims at establishing the conditions under which this technique can provide accurate, ultra-sensitive measurements of concentrations of uranium and other radionuclides.

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4.4  Electron-photodetachment studies by optogalvanic spectroscopy In the positive column of discharge through electronegative gases a sheath field, between the plasma and the wall of the discharge tube, prevents the negative ions from diffusing to the walls. This slow diffusion of negative ions compared to electrons gives rise to a negative ion density about two orders of magnitude larger than the electron density. Webster [40] has designed and constructed a dc discharge cell with adjustable electrode positions and an orthogonal geometry between the laser beam and the discharge axis. In this work visible dye laser and infrared diode laser have been used to investigate selected neutral, radical, and negative ion species. McDermid and Webster [41] have described an OG experiment in which electrons detached from the negative ions formed in the discharge are observed as a function of incident laser wavelength. Measurement of electron affinities from Cl− and I− negative ions are described. Suzuki and Kasuya [13] observed the O− and O2− OG signals with a cw laser in 13.56 MHz O2 discharge and found that the signal intensity near photodetachment threshold of O− is proportional to the photodetachment cross-section. The transition strengths for the fine structure components were found to compare well with the theoretical predictions of Rau and Fano [42]. LOGS has been successfully applied to investigate electron affinity using I− [32] and CN− [43] in hollow cathode dc discharge. Negative ions provide a fertile field for spectroscopic investigations since very little is known about their structure and the electronic states. Electron photodetachment is a powerful method for exploring these ubiquitous and unique quantum systems. When negative ions are irradiated, the following process can occur: A − + hν → A + e− (6.3) In photodetachment spectroscopy, the process is monitored as a function of photon energy. The outermost electron in a negative ion is very weakly bound. It is far from the nucleus and it is bound only due to polarization of other electrons in the system. In contrast, the outermost electron in an atom or in a positive ion, is bound in a long range Coulomb potential. The binding energies of negative ions are an order of magnitude smaller than in neutral atoms and the short range binding force can only support a few, if any, bound states in these systems. However, these weakly bound systems are abundant in any environment such as solar atmosphere and interstellar medium and in plasma. There is enough evidence that negative ions prevent the growth of bacteria and fungi on solid media and exert a lethal effect on vegetative forms of bacteria suspended in water [44]. The lack of long range Coulomb force enhances the importance of inter-electronic interactions. Thus the independent particle model which is adequate for describing atomic structure, breaks down for negative ions. Experimental investigations of negative ions may thus serve as a useful probe of electron correlation and can be used to test theoretical models that go beyond the independent particle approximation. In Li−, the electron correlation energy is found to be 3 times larger than electron affinity [45]. The two-electron system H− is a model case for many calculations and the electron correlation in this system also dominates the binding energy [46]. The lack of bound

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excited states in negative ions leads to the fact that, with very few exceptions, the only atomic parameter that can be determined for these species is the electron affinity Ea of the parent atom or molecule. Ea is the energy released when an additional electron is attached to a neutral atom forming a negative ion. As discussed earlier, the electron photodetachment from I− corresponds to the onset of production of I atoms in the 2P3/2 state for a laser wavelength of 405.18 nm which is equivalent to a photon energy of 3.059 eV. Thus an experimental value of Ea = 3.0591 ± 0.0001 was obtained from the OGS using a dye laser by Webster et al. [32].

4.4.1  Photodetachment threshold According to Wigner law [47], the cross-section for photodetachment of an s-wave electron near the threshold photon energy, E of electron detachment is given by the following relations: 1

σ ( E ) = C ( E − E0 ) 2 for E ≥ E0 (6.4) = 0 for E < E 0

where σ(E) is the cross section for photodetachment, E0 is the photodetachment threshold, and C is a constant. The spectral features get complicated under high resolution due to fine structure and hyperfine structure of atomic energy levels. The isotopes of chlorine 35 Cl (about 75%) and 37Cl (about 24%) both have a nuclear spin of 3/2. The ground state of chlorine atom 2P3/2 with electron configuration 1s22s22p63s23p5 splits into four hyperfine structure levels corresponding to F = 0, 1, 2, 3 where F = 0 level has the lowest energy. The ground state of negative chlorine ion is 1S0 with electron configuration 3p6 and it has neither fine structure nor hyperfine structure. Thus with one initial state and four possible final states, there are four independent channels that have to be taken into account in the analysis of LOG spectra. The relative cross section for different channels is proportional to the multiplicity of the final state of the neutral atom, that is, (2F + 1). Berzinsh et al. [48] have measured the electron affinity of chlorine at high resolution using an excimer pumped pulsed dye laser beam of 20 ns duration propagating parallel or antiparallel with respect to the beam of negative chlorine ions. The laser bandwidth was 4.3 GHz and the electron affinity of Cl was found to be 29138.59 ± 22 cm−1 corresponding to 3.61 eV. The electron affinity EEa(xCl) of the isotope xCl is obtained as the average value of the photodetachment threshold, Eth obtained with parallel and antiparallel laser and ion beams (designated p and a, respectively): EEa ( x Cl) = ( Etha + Ethp ) / 2 (6.5) The isotope shift of electron affinity in the negative chlorine ion has been experimentally determined by measuring the difference in electron affinity for the two isotopes. The frequency shift corresponding to the difference in electron affinity is given by: ∆EEa = EEa ( 37 Cl) − EEa ( 35 Cl) (6.6)

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Combining Eqs. (6.5) and (6.6), we get ∆EEa = [ Etha ( 37 Cl) + Ethp ( 37 Cl) − Etha ( 35 Cl) − Ethp ( 35 Cl)] / 2 (6.7) = [ Etha ( 37 Cl) − Etha ( 35 Cl)] / 2 − [ Ethp ( 35 Cl) − Ethp ( 37 Cl)] / 2 Thus four threshold measurements are to be carried out for one determination of the difference in the electron affinity of the two isotopes. The experimental value was found to be (∆EEa)/h = 340 MHz. In 35Cl the F = 0 ground level is 700 MHz below the unperturbed level and the corresponding shift in 37Cl is 586 MHz as shown in Fig. 6.18. Therefore subtracting the difference between these two values from experimental (∆EEa)/h gives the corrected isotope shift of the electron affinity of Cl as 226 MHz. Kitsopoulos et al. [49] have designed and constructed a high resolution photodetachment spectrometer with 3 cm−1 (0.37 meV) resolution. It has been used for threshold photodetachment measurements of I− and SH− negative ions. In the latter case, individual rotational transitions between the ion and neutral molecule appear as clearly resolved peaks. Klein et al. [43] were the first to use laser OGS to measure the photodetachment threshold of molecular negative ion CN− leading to an electron affinity of 3.821 ± 0.06 eV for the parent molecule CN. Greenberg et al. [50] have measured the negative ion density in NF3 discharge based on the photodetachment process: F− + hν → F + e−. The increase in free electron

Figure 6.18  The energy level diagram of the ground states of Cl− and Cl with zero energy set to the lowest hyperfine level in the ground state of the two isotopes. Adapted from Ref. [48].

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density as a function of laser fluence was monitored following the photodetachment of F−. These measurements indicate that the principal negative charge carriers in NF3 discharge are negative ions and not free electrons. Hotop et al. [51] investigated the photodetachment of Se− ions in a crossed beam experiment with a pulsed tunable laser of bandwidth 1–2 A. The threshold behavior of photodetachment cross-section was compatible with Wigner law over only about 5 meV above threshold. Some differences were observed for the threshold behavior of transitions terminating in different final states of Se (3P2,1,0). The observation of 4 out of 6 possible fine structure transition thresholds unambiguously determine the electron affinity Ea(Se) = 16297 cm−1 or 2.0206 eV.

4.5  Intracavity optogalvanic spectroscopy Intracavity OGS is an ultrasensitive laser-based technique for detection of 14C-labeled carbon dioxide [52]. The strongest lasing transitions in 14CO2 laser are at 11.8 µm {P(20)} and at 11.3 µm {R(20)}, significantly longer in wavelength than the lasing transitions of the other stable isotope CO2 lasers. The experimental system also includes a small 12CO2 laser for normalization, which operates at 10.6 µm and the emission is in resonance with only 12CO2 molecules in the external reference cell. The analyte cell is placed inside the 14CO2 laser cavity. The partial transmittance of the front mirror of the 14CO2 laser makes it possible to allow 1.5 W of the 10 W 12CO2 laser beam to be incident onto the intracavity sample cell. Both lasers are modulated at different frequencies and 12CO2 and 14CO2 OG signals are acquired simultaneously by phase sensitive lock-in technique. Each sample cell is capacitively coupled to a tunable, low power RF oscillator circuitry that initiates and sustains the glow discharge. Since glow discharge is generated via the RF voltage applied to the electrodes mounted outside the analyte cell, the electrodes do not come into direct contact with the sample being analyzed. Measuring the OG signals simultaneously for both cells and taking the double ratio of signals eliminates errors due to laser fluctuations. 12CO2 normalization is achieved by the OG signal from the external cell, while the 14CO2 is normalized to the OG signal from the laser tube itself. Using this ultra sensitive OG detection technique, limits of detection for 14C/12C ratios near 10−15 have been obtained. With a 15 W 14CO2 laser, a linear calibration with samples from 5 × 10−15 to greater than 1.5 × 10−12 in ratios has been demonstrated [53]. Possible applications of this technique include microdosing studies in drug development, individualized sub-therapeutic tests of metabolism, carbon dating and real time monitoring of atmospheric radiocarbon. The most recent work using this technique has, however, shown serious problems of reproducibility in the detection of radiocarbon and this method is not a viable method [54].

4.6  Wavelength calibration LOGS is a simple and accurate method to calibrate the wavelength of tunable lasers. A tiny fraction of the tunable laser output is taken out to irradiate a suitable discharge

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lamp whose OG spectrum is recorded simultaneously with the unknown spectrum under investigation. The main requirement for the standard lines in the calibration of wavelength is that these should be sharp, strong, and evenly spread over the UV, visible and near IR regions. OG spectra of neon, argon, thorium, and uranium adequately satisfy these conditions and have been widely used for the calibration of unknown spectra. Hollow cathode lamps with various species sputtered from the cathode and filled with neon or argon have been preferred sources in wavelength calibration [55]. Bandwidth of multimode lasers can also be very accurately determined from the lOG spectrum. Since the linewidths of thermalized discharge species are relatively small, the bandwidth of an exciting broadband laser is easily derived from the recorded OG profiles. As an example, the linewidth of neon spectral line at 588.2 nm in a hollow cathode lamp is approximately 0.004 nm at a thermalized temperature of 1800 K. King et al. [6] have shown that the bandwidth of laser can be determined by measuring the full width at half maximum of the recorded OG line profile.

4.7  Laser frequency and power stabilization The OG signal from a gas discharge can be used to frequency lock pulsed as well as cw tunable lasers. The technique is based on the impedance change in the discharge as a function of the output power of the excitation laser. The electrical signal for the feedback loop is derived from a change in the voltage across the discharge as the laser is tuned to an optical transition of a species present in the discharge. In this technique no optical detector is required and therefore the scattered excitation light no longer limits the usefulness of the locking scheme. Green et al. [56] used OG signal to lock a cw dye laser to several transitions in Na-Ne and Ba-Ne hollow cathode lamps. Dev et al. [57] have used an external cavity OG cell for the stabilization of a cw CO2 laser.

4.8  Rydberg states of atoms Rydberg formula was developed to describe energy levels of hydrogen atom but it can be used to describe highly excited states of a multi-electron atom or molecule. Such states, called Rydberg states, converge on an ionic state with an ionization energy threshold associated with a particular ionic core configuration. LOGS is a novel technique for the investigation of Rydberg states of atoms and molecules, with very large principal quantum numbers (n = 20 to n ≥ 100), in discharge cells. Low pressure glow discharge is characterized by high electron temperatures in the range of 104–106 K and has significant population density in the excited states particularly the long lived metastable states. Such metastable levels have been used as intermediate levels in twophoton excitation of Rydberg states [58]. Baig and coworkers have studied the highly excited odd-parity states of mercury using two-photon and two-step excitation from the metastable state using a rf discharge cell. The 6s6p3P0 metastable state was used in the observation of the 6snp3P0(13  ≤  n  ≤ 27), 6snp3P2(13  ≤  n  ≤ 30), and 6snf3F2(10  ≤  n  ≤ 42) Rydberg series [59], and 6s6p3P2 metastable state was used in a two-step excitation via 6s7s3S1 intermediate level to record the 6snp3P0(10  ≤  n  ≤ 18), 6snp3P1(10 ≤ n ≤ 41), 6snp3P2(10 ≤ n ≤ 70), and 6snp1P1(10 ≤ n ≤ 42) Rydberg

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series [60]. The corresponding even-parity Rydberg series 6sns3S1(13 ≤ n ≤ 50), 6snd1D2(6  ≤  n  ≤ 18), 6snd3D1(6  ≤  n  ≤ 14), 6snd3D2(6  ≤  n  ≤ 15), and 6snd3D3(6  ≤  n  ≤ 59) have been reported in a subsequent publication [61] where the detection of parity forbidden transitions 6snp3P1(44 ≤ n ≤ 50) in the presence of krypton as the buffer gas in the dc discharge has been attributed to the increase in transition probability with increase of atomic weight of the buffer gas.

4.9  Understanding the physics of OGS The very sensitive technique of OGS is known to suffer from stability and reproducibility problems. Attempts are being made to get a deeper knowledge of the underlying physics to improve the performance of OGS. Hollow cathode discharge is known as a reservoir of sputtered atoms and the analysis of OGS is based on the understanding that the sputtered atoms which are not returned back to the cathode are thermalized by the buffers in the cathode dark space. Zhechev and Steflekova [62] have studied the conductivity of an ensemble of sputtered atoms in a hollow cathode discharge as a function of their alignment and orientation. Linearly polarized resonant light orients the magnetic states of sputtered atoms at either m = 0 →  m = 1 or m = 0 →  m = −1. The alignment as well as orientation of atoms manifest themselves optically in linearly or circularly polarized spontaneous emission. This also implies that electron–atom interactions depend on the coherence induced by the incident polarized light since resonant light absorption is the necessary condition for OGE in discharge plasma. OGE is the galvanic manifestation of the redistributing action of light absorbed in a steady state population. Time resolved OGS would reveal the transient processes of relaxation related to the conductivity of the light absorption in the discharge plasma. Both time resolved and amplitude-OG signals have been detected for the same optical transition depending on the polarization of absorbed light. The differences, in OG efficiency, time evolution, and sensitivity to discharge current and laser power, are ascribed to the rate constants of the decay processes. Berglund [63] has designed and constructed a miniature high-precision, isotoperesolving molecular spectrometer based on the OGE. The most important component of this microsystem is a microplasma source in the form of a split ring resonator. The plasma sources developed in this work are the first ever miniature devices to be used in OGS. These sources have been manufactured in both printed circuit board and alumina for compatibility with other devices in the spectrometer. Preliminary work on the microsystem has confirmed that OGS scales well with miniaturization. The signal strength does not decrease as the volume is reduced and the stability and reproducibility are greatly increased. A major benefit of the miniature sample cell is the miniscule amount of sample it requires. The use of this instrument for exploration of planets and moons, is expected to help the detection of extraterrestrial life. Zhechev [64] has detected an anomalous OG signal in the hollow cathode discharge which is found to be less dependent on the absorbing optical transition and it is more informative on the parameters of the plasma. This response manifests itself both in the amplitude and time-resolved OG reactions and can be used as a sensitive tool in monitoring procedure.

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5 Conclusion The scientific observation that current in a stable dc discharge is increased or decreased, when external light is incident on the discharge tube, gave birth to a novel spectroscopic detection technique. The OGE is a change in the electrical properties of a discharge caused by illuminating the discharge with radiation having a wavelength corresponding to an atomic or molecular transition in the discharge. OGS does not require a photomultiplier tube or photodiode detector to obtain atomic transition in the spectra, because the gas discharge itself serves as a resonant photodetector. The advent of tunable laser sources in the 1970s led to very rapid developments of experimental systems to explore the structure and dynamics of short lived atomic and molecular species in electric discharge sources as well as in stable flames. By suitable selection of the exciting laser radiation it is possible to detect multiple elements in the same discharge or flame source. The puzzling dynamics of the OGE involves a variety of atomic and molecular processes and several models have been developed to understand the physics of this widely used experimental technique. Applications of OG detection in moderate resolution spectroscopy, in Doppler-free spectroscopy and in analytic studies are discussed in this article. Intracavity OGS and the use of miniature sample cells have greatly enhanced the application of this detection technique. Studies on negatively charged ions and ionic molecules have provided very useful data from LOGS on electron-photodetachment. The role of LOGS in stabilizing the frequency and power output of lasers has been briefly described. Electric discharge plasma involves complex atomic and molecular interactions with the optical radiation and there is a need for further experimental and theoretical research to fully utilize many applications of the technique of LOGS.

Acknowledgments I take this opportunity to express my gratitude to Dr. K.C. Smyth, who during a short visit to our laboratory in BHU in 1988, greatly enthused me to work in the field of optogalvanic spectroscopy. My daughters, Dr. Punam Rai and Dr. Vineeta Singh take care of my health while Sudheer, Sangeeta, and Michelle look after my other needs. This article is a token of my affection to the grand children Leo and Mia who keep me thoroughly entertained.

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[55] X. Zhu, A.H. Nur, P. Mishra, Laser optogalvanic wavelength calibration with a commercial hollow cathode iron-neon discharge lamp, J. Quant. Spectrosc. Radiat. Transfer 52 (1994) 167. [56] R.B. Green, R.A. Keller, G.G. Luther, P.K. Schenck, J.C. Travis, Use of an optogalvanic effect to frequency-lock a continuous wave dye laser, IEEE J. Quant. Electron 13 (1977). [57] V. Dev, D.J. Biswas, U.K. Chatterjee, Parametric studies of the optogalvanic effect in a low pressure CO2 cell, Appl. Phys. B 50 (1990) 67. [58] H. Rinneberg, J. Neukammer, High resolution laser spectroscopy of odd-parity Barium Rydberg states populated by forbidden atomic transitions, J. Physique Colloque 44 (1983) C7–177. [59] M.A. Zia, B. Suleman, M.A. Baig, Two-photon laser optogalvanic spectroscopy of the Rydberg states of mercury by RF discharge, J. Phys. B 36 (2003) 4631. [60] M.A. Zia, M.A. Baig, Two-step laser optogalvanic spectroscopy of the odd-parity Rydberg states of atomic mercury, Eur. Phys. J. D 28 (2004) 323. [61] M.A. Zia, M.A. Baig, Laser optogalvanic spectroscopy of the even-parity Rydberg states of atomic mercury, J. Opt. Soc. Am. B 22 (2005) 2702. [62] D. Zhechev, V. Steflekova, Optogalvanic effect on degenerate magnetic states of sputtered atoms in a glow discharge, J. Phys. 558 (2014) 012032. [63] M. Berglund, Miniature plasma sources for high-precision molecular spectroscopy in planetary exploration, Doctoral Thesis (Summary), Doctoral Thesis (Summary), Uppsala University (2015). [64] D. Zhechev and N. Parvanova, Anomalous optogalvanic signal spectriometric applications, Opto-Electronics Review, 11 (2003) 31.

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Ichiro Tanabe Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan Chapter outline 1 Introduction 245 2 Study of optical properties of TiO2 using radiation spectroscopy and theoretical simulation  246 3  ATR spectroscopy for semiconductor materials  248 3.1  ATR-FUV instrument  248 3.2  ATR-FUV measurements of semiconductor powders  254 3.3  Photon-induced spectral changes of TiO2  257

4  DUV Rayleigh scattering spectroscopy for individual TiO2 nanocrystals   260 5  Applications of UV Raman spectroscopy for semiconductor nanocrystals  263 5.1  Study of TiO2 phase transformation using UV Raman spectroscopy  263 5.2  UV Raman spectroscopy of zirconia nanocrystals  266

6  Summary and future outlook  268 References  269

1 Introduction Inorganic semiconductor materials are commonly used in various fields, such as photocatalysis and solar cells [1–10]. In particular, titanium dioxide (TiO2) is a typical material. The functions of semiconductor materials can be observed under irradiation using the light of higher energy than their bandgap energy [11]. For example, the bandgap energy of anatase TiO2 is approximately 3.2 eV, and thus, anatase TiO2 can be used as catalyst under ultraviolet (UV, ≤400 nm), deep-UV (DUV, ≤300 nm), and far-UV (FUV, ≤200 nm) light irradiation [12]. Therefore, their optical characteristics in these regions are essential and fundamental properties of semiconductor materials. The optical properties of TiO2 in the UV, DUV, and FUV regions are outlined in Section 2. These studies were mainly done by radiation spectroscopies and theoretical calculations. Section 3 focuses on FUV spectroscopy. Until recently, spectral measurements in the FUV region required a high vacuum atmosphere and expensive instruments, and thus, previously reported studies in the FUV region were limited to fundamental works. However, owing to the advent of a novel FUV spectroscopy technology using an attenuated total Molecular and Laser Spectroscopy. http://dx.doi.org/10.1016/B978-0-12-818870-5.00007-1 Copyright © 2020 Elsevier Inc. All rights reserved.

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reflectance (ATR) system, FUV spectral measurements have become more easily accessible. The ATR-FUV spectral measurements of water and ionic liquids are introduced in Section (3.1) as typical examples. Studies on the optical properties of semiconductor materials in the FUV region were performed using the ATR-FUV methods, and the results are summarized in Section (3.2). In addition, recently, a new system that enables spectral measurements under UV and visible light irradiation was developed. Section 4 describes the recently reported investigations that used resonant Rayleigh scattering spectroscopy. This technique could help analyze the optical properties and determine the intrinsic bandgaps of single TiO2 nanocrystals depending on their size. Section 5 reviews UV Raman scattering spectroscopy and its applications for semiconductor materials. The phase transition process of semiconductor nanocrystals such as TiO2 and ZrO2 (Sections 5.1 and 5.2, respectively) was analyzed combining UV and visible Raman scattering spectroscopy techniques, and the results were comprehensively discussed. Last, Section 6 summarizes the chapter and depicts the future outlook.

2  Study of optical properties of TiO2 using radiation spectroscopy and theoretical simulation The optical properties of TiO2, which is a very common semiconductor material, have been investigated using various techniques. Radiation spectroscopy is a typical and powerful method because its bright light enables reflection spectrum measurements with high signal-to-noise ratios. Optical constants (i.e., the refractive index, n, and extinction efficiency, k) can be determined based on the obtained reflection spectra and the Kramers–Kronig conversion. Because rutile TiO2 is the most stable crystal phase of TiO2, early studies on the optical properties of TiO2 mainly focused on rutile TiO2 single crystals [13–17]. In 1965, Cardona and Harbeke [13] measured the reflection spectra of rutile and determined

Figure 7.1  (A) Refractive index, n and (B) extinction efficiency, k of rutile TiO2 under perpendicular and parallel polarization conditions (E⊥c and E||c, respectively) [13].

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its optical constants, including their dependence on polarization (E⊥c and E||c), as illustrated in Fig. 7.1. In 1972, Fisher construed the reflection spectra measured by Cardona and Harbeke [13] using molecular-orbital (MO) energy-level diagrams deduced from X-ray spectra [18]. In 1992, Glassford and Chelikowsky performed ab initio pseudopotential calculations and compared the calculated and experimental reflection spectra, as presented in Fig. 7.2 [19]. Vos et al. also reported reflection spectral measurements [13] and assigned them to electronic energy levels in 1977 [15]. In 1993, the growth of anatase TiO2 single crystal was reported by Berger et al. [20]. Hosaka et al. [21,22] measured the reflection spectra of anatase TiO2 single crystals (Fig. 7.3), and assigned the bands using MO calculations. According to their reports, the valence bands of TiO2 mainly comprised O(2p) orbitals while the Ti(3d) and Ti(4s) orbitals were the main contributors to the conduction bands. In other words, the spectral structure of TiO2 was mostly due to the electron transition from the O(2p) states to the Ti(3d), Ti(4s), and Ti(4p) ones. In addition, it was revealed that the spectral structure of anatase was very similar to that of rutile, yet the peaks in the anatase spectrum shifted toward lower energies than that of rutile. The absorption edge of anatase (approximately 3.3 eV) was determined to be 0.3 eV larger than that of rutile. Moreover, the optical constants of anatase TiO2 were calculated based on reflection spectra and the Kramers–Kronig conversion [21,22].

Figure 7.2  Calculated and experimental (solid and dashed lines, respectively) reflection spectra of rutile TiO2 for (A) perpendicular (E⊥c) and (B) parallel (E||c) polarization conditions [19].

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Figure 7.3  Reflection spectra of anatase TiO2 single crystal [23].

As described earlier, the optical constants of TiO2 could be determined using the Kramers–Kronig conversion of the experimentally obtained reflection spectra. Moreover, while radiation spectroscopy measurements provide reflection spectra over a wide wavelength range, they involve complex experimental installation and ultrahigh vacuum environment. Therefore, most studies that focused on radiation spectroscopy were limited to basic investigations and used single crystal TiO2. In 2011, Sério et al. [23,24] measured and reported the transmission spectra of TiO2 thin films, as illustrated in Fig. 7.4. They prepared anatase TiO2 films on calcium fluoride substrates using the magnetic sputtering method under various oxygen partial pressures (10%, 20%, and 25% O2). The spectra in Fig. 7.4A presents very weak and broad absorption bands at approximately 200 and 260 nm, which were assigned to the eg(σ) → t2g(π*) and t2g(π) → t2g(π*) transitions, respectively, using MO energy level diagrams and calculated band structures reported by other scholars [22,25]. Although the spectra were measured in the 320–115 nm region using a synchrotron radiation spectrometer, the 145 nm feature in Fig. 7.4A was not due to TiO2 but was attributed to the adsorbed H2O.

3  ATR spectroscopy for semiconductor materials 3.1  ATR-FUV instrument As mentioned in the introduction, the optical properties in the FUV and DUV regions provide rich information on the electronic states of materials. However, great care is required when measuring absorption spectra in these regions because of the large absorbance of the materials. For example, the absorbance index, α, of TiO2 in the DUV region is 106–107 cm−1 [4]. Spectral measurements in the FUV region are more challenging because oxygen molecules and water vapor in the air also absorb FUV light strongly. Therefore, spectroscopic investigations in the FUV region should be performed using synchrotron radiation and ultrahigh vacuum atmosphere.

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Figure 7.4  Absorption spectra of (A) anatase TiO2 thin films and (B) H2O [23].

In 2007, a novel FUV spectroscopy technique, which utilized an ATR system was developed by Ozaki’s group [16–28]. The schematic outline of the ATR-FUV instrument is depicted in Fig. 7.5. The incident light from a deuterium lamp passes through a monochromator and is split into a reference beam and a sample beam using a magnesium fluoride half mirror. Subsequently, the two beams pass through a quartz plate coated with sodium salicylate, which fluoresces. Last, the fluorescence of samples is detected using a photomultiplier (PMT). During the ATR measurements, the evanescent wave generated at the interfacial area between the internal reflection element (IRE) and the sample is used as probe light. The penetration depth (dp) corresponds to the optical path length of the ATR system, and can be determined as follows: dp =

λ 2π n1 sin θ − ( n2 / n1 ) 2

2

where n1, n2, and θ are the refractive indices of the IRE and sample, and incident angle, respectively.

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Figure 7.5  Outline schematic of attenuated total reflectance-far ultraviolet spectrometer [27]. Here, IRE and PMT are the internal reflection element and photomultiplier, respectively; and dp, n1, n2, and θ are the penetration depth, refractive indices of the IRE and sample, and incident angle, respectively.

The value of dp in the FUV region is smaller than 50 nm. Therefore, the ATR method could significantly and easily shorten the light pass length, which is an important advantage of ATR-FUV spectroscopy. The optical system and sample compartment of ATR-FUV spectrometers are separated by the IRE, which is an important feature of this instrument. To remove oxygen molecules and water vapor, which absorb FUV light strongly, the optical system is purged with dry nitrogen gas, which does not absorb FUV light. This eliminates the need for the ultrahigh vacuum atmosphere during FUV spectroscopic measurements. Moreover, the sample compartment is exposed to air, and thus, the environment around the sample can be desirably controlled. The ATR method enables spectral measurements in the FUV region for liquid samples, because samples should not be kept in vacuum. Owing to these two characteristics of the ATR-FUV spectroscopes (i.e., the shortened light pass length and eliminating the need for vacuum atmosphere), spectral measurements in the FUV region have been performed for various samples [29–42]. The ATR-FUV spectra of water at different temperatures are presented in Fig. 7.6 as a typical example [29]. The incident angle was set at 60 degrees and the IRE was made of sapphire. In this case, the light pass length was less than 40 nm, and the FUV spectra were obtained without saturation. To measure the spectra of water using transmission

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Figure 7.6  Attenuated total reflectance-far ultraviolet spectra of H2O and D2O at different temperatures [29].

spectroscopy without saturation, thin water layers of sub-micrometer thickness should be prepared. However, for ATR-FUV measurements, water can simply be dropped onto the IRE. The n and k values of water (Figs. 7.7 and 7.8, respectively) could be determined by applying the Kramers–Kronig conversion to the ATR-FUV spectra. Recently, ATR-FUV studies on ionic liquids were performed by Tanabe et al. [43– 45]. Ionic liquids are liquid phase salts at ordinary temperature and pressure, which consist of anions and cations. Because of their unique properties, such as extremely low vapor pressure, high thermal stability, and wide potential window, ionic liquids have attracted significant attention in various fields, including electrochemistry and synthetic chemistry [46–50]. One of the attractive characteristics of ionic liquids is the tunability

Figure 7.7  Refractive index, n, of (A) H2O and (B) D2O at different temperatures [29].

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Figure 7.8  Extinction efficiency, k, of (A) H2O and (B) D2O at different temperatures [29].

of their properties which can be achieved by selecting pairs of cations and anions. Ionic liquids also exhibit very large absorbance; the absorption coefficient, ε, of 1-Butyl3-methylimidazolium bis(trifluoromethylsulfonyl) amide ([BMIM][TFSA]) at 211 nm was reported to be approximately 5 × 103 mol−1 dm3 cm−1 [51]. Therefore, it can be difficult to measure the transmission spectra of neat ionic liquids without saturation. The ATR-FUV technique can easily measure the spectra of condensed-phase samples without saturation. Systematic spectral measurements of various kinds of ionic liquids were performed in combination with quantum chemical calculations. The investigated cations and anions are listed in Table 7.1. In addition, Fig. 7.9A–C present the ATR spectra of [BMPY][TFSA], [TMPA][TFSA], and [TOMA][TFSA], Table 7.1  Names and abbreviations of (A) cations and (B) anions. Name (A) Cation 1-Butyl-3-methylpyrrolidinium N,N,N-Trioctyl-N-methylammonium N,N,N-Trimethy-N-propylammonium 1-Ethyl-3-methylimidazolium 1-Butyl-3-methylimidazolium 1-Hexyl-3-methylimidazolium 1-Octyl-3-methylimidazolium (B) Anion Bis(trifluoromethylsulfonyl)amide Tetrafluoroborate Chloride Bromide Iodide

Abbreviation [BMPY]+ [TOMA]+ [TMPA]+ [EMIM]+ [BMIM]+ [HMIM]+ [OMIM]+ [TFSA]− [BF4]− Cl− Br− I−

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Figure 7.9  Experimental attenuated total reflectance spectra of (A) [BMPY][TFSA], (B) [TMPA][TFSA], (C) [TOMA][TFSA], and (D) calculated vertical transitions of [TFSA]− [44]. Here, [BMPY]+, [TMPA]+, [TOMA]+, and [TFSA]− are the 1-butyl-3-methylpyrrolidinium, N,N,N-trimethy-N-propylammonium, N,N,N-trioctyl-N-methylammonium, and bis(trifluoromethylsulfonyl) amide ions, respectively.

respectively, in the 145–300 nm range (where[TMPA]+, and [TOMA]+ are N,N,Ntrimethy-N-propylammonium and N,N,N-trioctyl-N-methylammonium, respectively). These three spectra did not exhibit any absorbance at wavelengths longer than 180 nm, and exhibited one peak at approximately 150 nm. Although conventional UV–Vis spectrometers cannot measure electronic absorption spectra in the FUV region, ATR spectrometers could easily generate them. According to the time-dependent density functional theory calculations, the [TFSA]− ion exhibited oscillator strengths in the wavelength range below 180 nm (Fig. 7.9D), while the [BMPY]+ and [TMPA]+ ions did not absorb light in the wavelength range above 140 nm. Therefore, the spectra in Fig. 7.9A–C are mostly due to the internal electronic excitation of the [TFSA]− ion. The ATR spectra of imidazolium-based ionic liquids that contain [TFSA]− and tetrafluoroborate ([BF4]−) ions and alkyl chains of different lengths: 1-ethyl-3-methylimidazolium ([EMIM]+), 1-butyl-3-methylimidazolium ([BMIM]+), and 1-octyl3-methylimidazolium ([OMIM]+) are depicted in Fig. 7.10A–F. The spectral shapes of these ionic liquids were similar. They presented two main peaks at approximately 170 and 210 nm regardless of the anion species and alkyl chain length. The peak wavelength at approximately 210 nm matched that in the transmission spectrum of [BMIM] [TFSA] reported by Katoh [51]. The ATR spectra revealed the definite absorption peak in the FUV region. Vertical transitions were calculated using the imidazolium cation in vacuum as model, as presented in Fig. 7.10G–I. The calculated vertical transition wavelengths corresponded to the experimentally observed peak wavelengths. These simulations indicated that the experimentally observed absorptions were mainly due to the cations,

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Figure 7.10  Experimental attenuated total reflectance spectra of (A) [EMIM][TFSA], (B) [BMIM][TFSA], (C) [OMIM][TFSA], (D) [EMIM][BF4], (E) [BMIM][BF4], and (F) [OMIM][BF4]; and calculated vertical transitions of (G) [EMIM]+, (H) [BMIM]+ and (I) [OMIM]+ ions [44]. Here, [EMIM]+, [BMIM]+, [OMIM]+, [TFSA]−, and [BF4]− are 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-octyl-3-methylimidazolium, bis(trifluoromethylsulfonyl) amide, and tetrafluoroborate ions, respectively.

although the contribution of the [TFSA]−ion to the spectra in Fig. 7.10A–C was minimal. Nishi et al. investigated the electronic structure of 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) using UV photoemission spectroscopy, inverse photoemission spectroscopy, and near-edge X-ray absorption fine structure spectroscopy, and reported that both the top of the occupied states and bottom of the unoccupied states were attributed to the cation [52]. These results also indicated that the absorbance at the lowest energy was assigned to the intramolecular excitation of the cation. In addition, electronic interactions between halide anions (Cl−, Br−, and I−) and imidazolium cations were also detected. Therefore, the ATR system can investigate both intra- and inter-ion electron transitions of neat ionic liquids systematically and easily. Other liquid samples, such as aqueous solutions [29–33], alkanes [34–36], alcohols [37], ketones [38], amides [39], nylons [40], and nanocarbons [41,42] were also studied using ATR-FUV spectroscopy.

3.2  ATR-FUV measurements of semiconductor powders Recently, ATR-FUV spectrometry has been used for inorganic semiconductor materials, such as TiO2 and ZnS [53–55]. Fig. 7.11A presents the ATR spectrum of ZnS nanoparticles, and Fig. 7.11B illustrates the reflection spectra of ZnS single crystal using synchrotron radiation spectroscopy, as reported by Hengehold et al. [56]. The n

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Figure 7.11  (A) Attenuated total reflectance spectrum of ZnS. (inset) Scanning electron microscopy image of ZnS nanoparticles. (B) Reflectivity, (C) refractive index, n, and (D) extinction efficiency, k, spectra in the 150–300 nm region of ZnS single crystal obtained using synchrotron radiation spectroscopy [54].

and k values (Fig. 7.11C,D, respectively) were calculated using the Kramers–Kronig conversion. Although both the ATR and reflectivity spectra (Fig. 7.11A,B, respectively) presented two bands at ∼180 and ∼220 nm, their peak wavelengths and intensity ratios were different. The peak wavelength and intensity ratio of the spectrum in Fig. 7.11A was more similar to the n spectrum in Fig. 7.11C than the k spectrum in Fig. 7.11D. That is, the ATR spectrum was more similar to the n spectrum than to the reflective or k spectra. This may be due to scattering effects (i.e., diffuse reflection) from nanoparticles. According to the Fresnel equations, the reflectance increases as n increases. When measuring the reflective spectra of single crystals, light is reflected in a specular manner. By contrast, nanoparticles scatter light in random directions, which results in an increase in optical density. Therefore, the effect of scattering is stronger in ATR spectra than in previously reported spectra. Using finite-difference time-domain simulations, the effects of the sample morphology on the ATR spectra were revealed [54]. ATR spectrometers can provide proper spectra of semiconductor materials without using high vacuum or requiring large spaces, and ATR systems enable the rapid and systematic investigation of the electronic states of various materials. The photocatalytic activity of TiO2 can be improved by modifying it using metal nanoparticles [57–63]. Fig. 7.12A depicts the ATR spectra of TiO2 and TiO2 modified with Au, Pd, and Pt nanoparticles. The characteristic absorption band present in the FUV region (∼160 nm) of these spectra, was assigned to the t2g(π) → eg(σ*) transition [21,25,53]. Upon depositing metal nanoparticles on TiO2, the absorption intensity in the longer wavelength region decreased, while the intensity in the shorter wavelength region increased (Fig. 7.12A,B). The degree of spectral changes was calculated as the ratio of the integrated intensity of absorption in the 150–180 nm range to that in the 270–300 nm range, and it was revealed that strong positive correlations existed between the calculated ratios and work functions of the modifying metals (Fig. 7.12C). These spectral changes can be explained as follows. When TiO2 comes into contact with a metal that presents a larger work function, the electrons flow from TiO2 into the metal until their Fermi levels become equal [4]. The work functions of TiO2, Au, Pd, and Pt are ∼4.0, 4.7, 4.9, and 5.7 eV, respectively, and therefore, the electrons were transferred from TiO2 to the metal nanoparticles. As a result, the higher energy TiO2 levels became unoccupied, which reduced the availability of electrons that can be

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Figure 7.12  (A) Attenuated total reflectance-far ultraviolet spectra of TiO2 and Au-, Pd-, and Pt-modified TiO2. (Inset) Scanning electron microscopy image of Pt nanoparticles-modified TiO2. (B) Difference spectra of TiO2 between spectra measured before and after metal modification. (C) Ratio of integrated intensity of absorption in 150–180 nm region to that in 270–300 nm region plotted against work function differences between TiO2 and each metal [53].

excited by longer wavelengths and led to the suppression of absorption in the longer wavelength range. On the other hand, metal nanoparticles deposited on TiO2 can act as sinks for the photoexcited electrons [58–63], which increased the charge-separation efficiency and absorption intensity in the shorter wavelength area of the spectra. Although further studies are needed to elucidate the causes of these spectral changes, it was already established that spectral changes strongly depend on the work functions of the modified metals.

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Figure 7.13  Photocatalytic activity versus work function difference between TiO2 and modifying metals [53].

Spectral variations imply electronic state changes of TiO2, and thus, the photocatalytic activities of the metal (Au, Pd, and Pt)-modified TiO2 samples should also change. The photocatalytic activities of TiO2 and metal (Au, Pd, and Pt)-modified TiO2 powders were estimated using the photoinduced degradation reaction of methylene blue aqueous solution. Fig. 7.13 illustrates the relationships between the work functions of the modifying metals and photocatalytic activities of the metal-modified TiO2 samples. The strong positive relationships indicated that the larger work functions of the modifying metals resulted in the surge in charge separation, which led to the higher photocatalytic activities of the metal-modified TiO2 samples. These findings were in agreement with those of previous studies, which reported clear correlations between the work function of the modifying metals and photocatalytic activities of metal-modified semiconductors [58]. These results indicated that the photocatalytic activity of modified TiO2could be systematically estimated using simple spectral measurements. Not only the metal species dependence but also the effects of the TiO2 phase (anatase or rutile) and modifying-metal particle size (5–60 nm) and shape (sphere, rod, or cube) were investigated using the same strategy [64,65]. Comparing anatase and rutile TiO2, it was determined that the electronic state changes of rutile TiO2 increased more than those of anatase TiO2 owing to the deposition of Pt nanoparticles on it [64]. Furthermore, the smaller Au nanoparticles induced larger electronic state changes, while the electronic states’ dependence of the nanoparticle shapes was not significant [65]. These results demonstrated the potential of FUV-DUV spectroscopy as a novel investigation method for the electronic states of semiconductor materials, which strongly relate to their photofunctional applications. The insight into the relationship between their electronic states and activities depending on various factors may lead to the development of highly efficient optical materials.

3.3  Photon-induced spectral changes of TiO2 The functions of TiO2 can be observed under light irradiation, and thus, spectroscopic analysis under light irradiation is desirable. Recently, Tanabe et al. introduced an

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Figure 7.14  Schematic diagrams of (A) sample chamber of the attenuated total reflectance-far ultraviolet spectroscope and (B) newly introduced external light-irradiation system [66].

external light irradiation system for the ATR-FUV spectrometer, and measured the spectra of TiO2 and Au-modified TiO2 (Au-TiO2) under UV and visible light irradiation (300–350 and 500–800 nm, respectively) [66]. Fig. 7.14 presents the schematics of the developed light irradiation system. A hole (1 cm in diameter) covered with a quartz window was punctured in the front wall of the sample chamber (Fig. 7.14A), and a mirror placed below the sapphire IRE (Fig. 7.14B). Fig. 7.15A illustrates the absorption spectra of Au-TiO2 before and after UV light irradiation. After UV light irradiation, the spectral intensity at approximately 160 nm decreased, while the intensity in the 180–300 nm region remained almost unchanged. This spectral change could be related to the electron transfer from TiO2 to the Au nanoparticles. When TiO2 comes into contact with a metal of higher work function than its own, TiO2 electrons flow toward the metal until the Fermi levels of the metal and TiO2 become equal [4]. At the same time, holes are left in the valence band of TiO2, and the Schottky barrier forms between TiO2 and Au. The Schottky barrier suppresses the recombination of the electrons in Au and holes in the TiO2, which indicates

Figure 7.15  Attenuated total reflectance-far ultraviolet spectra of Au-TiO2 before and after (A) ultraviolet (UV) and (B) visible light irradiation. Insets are the magnified plots in the 150–180 nm range [66].

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that the Au nanoparticles deposited on TiO2 function as an electron pool [58–63]. Consequently, the number of electrons in TiO2 decreases (Fig. 7.16A), which suppresses light absorption, as presented in Fig. 7.15A. After UV light irradiation, Au-TiO2 was irradiated with visible light, and the absorption spectra were measured. As depicted in Fig. 7.15B, after visible light irradiation, the spectral intensity at approximately 160 nm increased up to the original intensity (i.e., the intensity before UV light irradiation). This indicated that electrons were transferred from the Au nanoparticle to TiO2, which was opposite of the electron transfer direction during UV light irradiation. Au nanoparticles absorb visible light owing to localized surface plasmon resonance (LSPR) [67–70]. When Au nanoparticles deposited on TiO2 were irradiated using LSPR-wavelength light, the electrons in the resonant Au nanoparticles were transferred to TiO2 and positive charges were generated in the Au nanoparticles. This light-induced charge separation was reported by Tatsuma’s group [69,70], and was named as plasmon-induced charge separation. In this described case, the visible light irradiation caused the transfer of the electrons in the Au nanoparticles to TiO2, and the absorption of TiO2 recovered, as presented in Fig. 7.16B. The light-induced electron transfer between TiO2 and Au was reported by several researchers, as follows. Jacob et al. [60] and Subramanian et al. [61] measured the absorption spectra of Au-TiO2 under UV light irradiation and observed that the shift of the apparent Fermi level depended on the size of the Au nanoparticles. They concluded that the Fermi level shift was due to the electron transfer from TiO2 to Au. Kazuma et al. visualized the photo-induced charge distributions of Au nanoparticles on TiO2 using Kelvin probe force microscopy, and observed the electron transfer from TiO2 to the Au nanoparticles under UV light irradiation and the electron injection from the Au nanoparticles to TiO2 under visible light irradiation [71]. Furube et al. observed

Figure 7.16  Schematic diagram of electron transfer (A) from TiO2 to an Au nanoparticle under ultraviolet (UV) light irradiation and (B) from an Au nanoparticle to TiO2 under visible light irradiation [66].

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the electron transfer from the Au nanoparticles to TiO2 using femtosecond absorption spectroscopy [72]. The electron transfer between TiO2 and the Au nanoparticles improved the photocatalytic activity of the Au-modified TiO2 under UV and visible light irradiation because of the efficient charge-separations.

4  DUV Rayleigh scattering spectroscopy for individual TiO2 nanocrystals The band structure of semiconductor materials is a fundamental property and strongly affects the photocatalytic activity of materials. When the size of TiO2 nanocrystals is smaller than several tens of nanometers, their band structures alter depending on their size because of their crystallinities and quantum size effects [73]. The size dependence of the photocatalytic activity of TiO2 has been reported by several groups of researchers who used X-ray crystallography [74], UV–Vis spectroscopy [75], and FUV spectroscopy [64]. The measurement of the scattering from the TiO2 particles is another effective method [76]. In these studies, the size dependence was estimated as an ensemble average using narrowly distributed TiO2 nanocrystals. In 2014, Honda et al. investigated the bandgap variations dependence on crystal size, crystallinity, shape, and surface defects using resonant Rayleigh scattering spectroscopy from the DUV to the visible spectral region (220–500 nm) for single TiO2 nanocrystals [77]. They developed an original microscopy system that was switchable from dark-field imaging to scattering spectroscopy with high positional reproducibility of the switching optics. Because TiO2 is sensitive to UV light, Rayleigh scattering spectroscopy in the UV range reflected its electronic properties. Figs. 7.17 and 7.18 Figures 7.17 and present the dark-field images and resonant Rayleigh scattering spectra, respectively, of individual TiO2 nanocrystals in two different samples. Two kinds of commercial TiO2 nanocrystals were used, and their diameters were 6 and 30 nm (Figs. 7.17A and 7.18A, and Figs. 7.17B and 7.18B, respectively). The inset values in Fig. 7.17 are circularities of the bright spots. The dark-field images were collected by filtering the incident light in the 260–340 nm range to detect the UV-active crystals. Generally, scattering intensity is stronger for larger particles, and thus, the dark-field image of the 6 nm diameter TiO2 nanocrystals (Fig. 7.18A) presented lower signal-to-noise ratio than that of the 30 nm diameter TiO2 nanocrystals (Fig. 7.18B). The resonant Rayleigh scattering spectra of the 6 nm diameter TiO2 nanocrystals (Fig. 7.18A) exhibited a distinct peak at approximately 310 nm. The peak was observed at almost the same wavelength for five individual nanocrystals; the average wavelength was 310 ± 3 nm (approximately 4 eV). The small peaks around 450 nm were attributed to photoluminescence, which indicated that the TiO2 nanocrystals presented some defects. On the other hand, in Fig. 7.18B, the peak wavelength diverged from 300 nm (approximately 4.1 eV) to 350 nm (approximately 3.5 eV), and the average wavelength was 329 ± 6 nm (approximately 3.8 eV). The larger distribution of the scattering peak

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Figure 7.17  Dark-field images of individual TiO2 nanocrystals for (A) 6 and (B) 30 nm diameter samples. The circularity values for the bright spots were calculated and are indicated in top right corners of the images, when available [77].

Figure 7.18  Resonant Rayleigh scattering spectra of individual TiO2 nanocrystals for (A) 6 and (B) 30 nm diameter samples. The numbers adjacent to the plotted lines correspond to the dark field image panels in Fig. 7.17 [77].

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wavelength could be ascribed to the size and shape inhomogeneity of the 30 nm diameter TiO2 nanocrystals compared with 6 nm ones. The bandgap energy for each TiO2 nanocrystal was obtained from the scattering peaks corresponding to the transition energy between the maximum density of states from the valence band to the conduction band of TiO2. The scattering peak wavelength (i.e., the bandgap energy) was different for each TiO2 nanocrystal. Fig. 7.19 illustrates the relationship between the scattering peak wavelength and the cube root of the scattering intensity (I1/3). Larger particles present stronger scattering light intensities, and hence, I1/3 is proportional to the crystal radius. It has been theoretically predicted that the bandgap energy is inversely proportional to the crystal radius because of the quantum effect. Fig. 7.19 roughly demonstrates the theoretically expected negative relationship between the scattering intensity, which is proportional to the crystal size, and bandgap energy. Honda et al. discussed the characteristic X, Y1, and Y2 points in Fig. 7.19, which were related to the resolution of the optical system and effects of crystallinity of the individual TiO2 nanocrystals [77]. More details are included in the original article. The variations in the bandgap energy determined using resonant Rayleigh scattering spectroscopy are essential for the photocatalysis applications of TiO2 [78], and could be useful for various materials, such as UV light emitting diodes [79] and UV-plasmonic nanomaterials [80].

Figure 7.19  Relationship between bandgap energy and cube root of scattering intensity (I1/3) of TiO2 nanocrystals. The gray (red online) and black dots indicate the 6 and 30 nm diameter TiO2 nanocrystals, respectively. See original article for more details on the X, Y1, and Y2 labels [77].

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5  Applications of UV Raman spectroscopy for semiconductor nanocrystals 5.1  Study of TiO2 phase transformation using UV Raman spectroscopy As noted in Section 2, TiO2 presents two common crystal phases; rutile and anatase. Because the rutile phase is more stable than the anatase one, anatase TiO2 transforms into rutile TiO2 over a wide range of temperatures. These two types of TiO2 present different properties: for example, anatase TiO2 is more suitable as catalysts [81–83], and thus, understanding the phase transformation of TiO2 is highly important. The crystal type of TiO2 can be identified using various methods, such as Raman spectroscopy [84,85], infrared spectroscopy [86,87], and X-ray diffraction (XRD) [88,89]. Zhang et al. [90], Zhang et al. [91], and Su et al. [92] used UV Raman scattering spectroscopy to investigate the TiO2 phase transitions. The thermally induced phase transformation of TiO2 from anatase into rutile was analyzed in 2006 using XRD and visible and UV Raman scattering spectroscopy (Visible- and UV-Raman, respectively) [90]. Anatase TiO2 nanocrystals (several dozen nanometers in size) were synthesized, and their phase transformation was observed over the temperature range of 200°C–800°C. As the temperature increased, the TiO2 nanocrystals partially transitioned from anatase into rutile phase. The transformation ratio was estimated using the three earlier-mentioned methods. Fig. 7.20A compares the temperature dependence of the rutile concentration determined using XRD and Visible-Raman. The XRD patterns and Raman spectra and their assignments are not presented here (for more details, see Ref. [90]). While the XRD and Visible-Raman results were in good agreement, there were significant differences between the XRD patterns and UV-Raman spectra, as illustrated in Fig. 7.20B. According to the XRD and Visible-Raman results, the phase transformation from anatase to rutile started at 550°C, and almost the entire amount of TiO2 was

Figure 7.20  Rutile content of TiO2 estimated using (A) X-ray diffraction (XRD) and visible Raman scattering spectroscopy (Visible-Raman) and (B) XRD and ultraviolet Raman scattering spectroscopy (UV-Raman) at different temperatures. The excitation wavelengths were 532 and 325 nm for Visible- and UV-Raman, respectively [90].

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transformed into rutile at 700°C. However, the UV-Raman study demonstrated that the predominant phase of the TiO2 powder until 680°C was anatase, and the rutile phase first appeared at 700°C. These differences were due to the detection depth of the measuring methods. Both XRD and Visible-Raman are bulk-sensitive methods because the probe radiation (X-rays and visible light) can pass through TiO2. By contrast, the incident excitation light for UV-Raman is strongly absorbed by TiO2, and thus, the Raman signals originate mostly from the surface of TiO2. In other words, UV-Raman is a surfacesensitive technique while Visible-Raman is a bulk-sensitive one. Using these results and transmission electron microscopy (TEM) observations, Zhang et al. proposed the phase transformation scheme illustrated in Fig. 7.21. As the temperature increased, the anatase nanocrystals agglomerated into larger particles (Fig. 7.22). The phase transformation first started in bulk at 550°C while the surface consisted of anatase phase until 680°C. Additionally, a lanthan (La2O3) layer coated on the TiO2 surface strongly suppressed the aggregation and phase transition.

Figure 7.21  Proposed scheme for TiO2 phase transformation [90].

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Figure 7.22  Transmission electron microscopy images of TiO2 calcined at (A) 500, (B) 600, and (C) 800°C [90].

Zhang et al. also investigated the size effects of TiO2 nanocrystals utilizing the same methods listed earlier. Six kinds of anatase TiO2 nanocrystals of different diameters (7, 10, 15, 25, 60, and 300 nm) were prepared and analyzed. The experimental results using XRD, Visible-Raman, UV-Raman, and TEM measurements revealed that three types of phase transition processes occurred depending on the nanocrystal size, as depicted in Fig. 7.23. The proposed schemes started with the agglomeration of TiO2 nanocrystals followed by the phase transition at a subset of anatase interfaces for the relatively small ( 60 nm diameter, Fig. 7.23C), the agglomeration did not progress much even at high temperature, and the phase transition started from the anatase surface. When the particle size was in the range of 10–60 nm, the phase transition started at the insets of anatase TiO2, and the predominant phase of the outer area was anatase until the temperature exceeded 900°C(Fig. 7.23B). Such systematic investigations were performed by combining bulk-sensitive Visible-Raman and surface-sensitive UV-Raman.

5.2  UV Raman spectroscopy of zirconia nanocrystals UV Raman spectroscopy can be used not only for TiO2 but other semiconductor materials as well. Li’s group [93–95] studied the phase transition process of zirconia (ZrO2) using UV- and Visible-Raman. ZnO2 exhibits three main phases; that is, monoclinic, tetragonal, and cubic. At first, Zr(OH4) powder was prepared, which was used as ZrO2 precursor, and the temperature dependence of the Visible- and UV-Raman spectra in the temperature range of 400°C–700°C (Fig. 7.24) was, subsequently, investigated [93].

Figure 7.24  (A) Visible- and (B) UV-Raman spectra of Zr(OH)4 powder calcined at 400, 500, and 700°C. The excitation wavelengths were 532 and 244 nm for Visible- and UV-Raman, respectively [93].

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The signal-to-noise ratio of the UV-Raman spectra was much higher than that of the Visible-Raman spectra. This was attributed to the excitation wavelength of UV-Raman (244 nm) overlapping with the absorption of ZrO2, which led to the occurrence of the resonant-Raman effect. The most distinctive spectral differences induced by the higher calcination temperature from 400°C to 700°C was observed at approximately 380 cm−1 (labeled as 379 and 377 cm−1 in the Visible- and UV-Raman spectra in Fig. 7.24A,B, respectively), which was the Raman signal from the monoclinic phase of ZrO2. For the Visible-Raman spectra (Fig. 4.24A), the Raman peak at approximately 380 cm−1was negligibly small at the calcination temperature of 400°C. By contrast, the UV-Raman spectra presented the peak at approximately 380 cm−1even at the calcination temperature of 400°C. These results indicated that the phase transition from tetragonal to monoclinic started near the surface of the particles, at a lower temperature, as presented in Fig. 7.25. Li et al. investigated the effects of doping on the ZrO2 phase transition [94]. Fig. 7.26 presents the effects of lanthanum oxides (Y2O3 and La2O3) doping. For pure ZrO2, as described earlier, the phase transition from tetragonal to monoclinic started at 400°C and proceeded as the temperature increased. On the other hand, upon doping

Figure 7.25  Proposed scheme for ZrO2 phase transformation. The light gray, dark gray, and gray areas (green, blue, and red online) represent amorphous Zr(OH)4, tetragonal ZrO2, and monoclinic ZrO2, respectively [93].

Figure 7.26  Proposed schemes for phase transformations of ZrO2, Y2O3-ZrO2, and La2O3-ZrO2 [94].

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Figure 7.27  Proposed diagram for tetragonal to monoclinic phase transformation of (ZrO2)0.92(Se2O3)0.02 [95].

with Y2O3, Y2O3-ZrO2 was still amorphous at 400°C, and then, the tetragonal phase coated with cubic phase formed at 400°C. By contrast, when La2O3 was used as the dopant, La2O3-ZrO2 remained amorphous at 500°C. At the higher calcination temperature, tetragonal phase formed and no more phase transitions were observed until 800°C. Li et al. [94], discussed the lanthanum oxide concentration dependence and its mechanism. In addition, for (ZrO2)0.92(Se2O3)0.02, a very unique calcination temperature dependence was determined for the tetragonal/monoclinic phase transition (Fig. 7.27) [95]. These results clearly demonstrated the high utility of UV-Raman spectroscopy for phase measurements near the surface. The phase transition processes of semiconductor materials can be comprehensively investigated combining UV-Raman with Visible-Raman and XRD spectroscopy, the last two being bulk-sensitive methods.

6  Summary and future outlook The spectroscopic investigations of semiconductor materials utilizing FUV, DUV, and UV light were reviewed in this chapter. The optical constants of TiO2 in a wide wavelength range were determined using common and traditional radiation spectroscopy methods, while theoretical calculations facilitated band assignments (Section 2). Recently developed ATR-FUV spectrometers could easily and systematically measure spectra of various materials in a wide spectral range, including the FUV region. The additionally introduced external light-irradiation system was used to detect the spectral changes of Au-modified TiO2under UV and visible light irradiation (Section 3). Resonant DUV Rayleigh scattering spectroscopy provided the bandgap energy of single TiO2 nanocrystals depending on the crystal sizes (Section 4). Moreover, UV and visible Raman scattering spectroscopy techniques were used to investigate the phase transitions of semiconductor nanocrystals.

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In summary, semiconductor materials present strong absorbance in the DUV and FUV regions, as described in Sections 2 and 3. Material properties, such as their bandgap energy and phase transitions could be determined utilizing absorbance, Rayleigh scattering (Section 4) and Raman scattering (Section 5) techniques. Inorganic semiconductors present practical applications in various fields, such as solar cells and photocatalysis, and FUV and DUV spectroscopy could contribute not only to the fundamental understanding of their properties but also to developing more sophisticated applications

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Marcelo G. Vivasa, Daniel L. da Silvab, Cleber R. Mendoncac, Leonardo De Bonic a Optical Spectroscopy and Photonics Laboratory, Federal University of Alfenas, Poços de Caldas, MG, Brazil; bDepartment of Natural Science, Mathematics and Education, Federal University of São Carlos, Araras, SP, Brazil; cPhysics Institute of São Carlos, University of São Paulo, São Carlos, SP, Brazil Chapter outline 1 Introduction 275 2 Microscopic description of the nonlinear optical response: Electronic first-order hyperpolarizability  279 3 Hyper-Rayleigh scattering technique  282 4 Theoretical calculation  284 4.1 Importance of symmetry on the second-order NLO responses  284 4.1.1 Intrinsic symmetry of permutation  285 4.1.2 Kleinman’s symmetry  285 4.1.3 Molecular symmetry  286 4.2 Molecular first-order hyperpolarizability by hyper-Rayleigh scattering experiment  286 4.3 Schemes for the determination of the molecular hyperpolarizabilities  288

5 First-order hyperpolarizability in push-pull octupolar molecules  291 5.1 Enhancing the electronic first-order hyperpolarizability  291 5.2 Comparison between experimental and theoretical data for dynamic first-order hyperpolarizability  295 5.3 Molecular branching effect on the dynamic first-order hyperpolarizability  296 5.4 Quantifying molecular interaction via HRS signal  298

6 Discussing HRS results based on quantum chemical results  300 7 Final Remarks  309 References  311

1 Introduction Nonlinear optics (NLO) is a field in physics that studies optical phenomena in which the induced polarization ( P) in a material is nonlinear with respect to the applied  electric field ( E ). This area of research arose in 1961 with the discovery of the second harmonic generation (SHG) by Franken et al., [1] after the advent of the laser by Maiman [2]. In SHG, a laser beam of high irradiance interacts with a material such Molecular and Laser Spectroscopy. http://dx.doi.org/10.1016/B978-0-12-818870-5.00008-3 Copyright © 2020 Elsevier Inc. All rights reserved.

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that its optical response generates a new photon with twice the frequency of the incident photons. Since then, the NLO has emerged as a fascinating research field with great opportunities for both fundamental and applied sciences. Later, in the 1990s, organic molecules have appeared as promising NLO materials, displaying effects such as multi-photon absorption, nonlinear refraction, electronic firstorder hyperpolarizability, among others [3–18]. After that, molecular nonlinear optics has been a subject of continuous interest in connection with several applications in different areas, such as materials science, optical telecommunications, 3D data storage, fast information and signal processing, microfabrication, in biology [19–28]. This has motivated numerous experimental and theoretical studies aiming at the design of organic materials with large NLO responses. In particular, the design of organic molecules presenting enhanced second-order nonlinear optical response has been very active, mainly driven by their use as electro-optical components such as more efficient frequency doubling, second harmonic microscopy, fast electro-optic (EO) modulation, ultrafast lasers, and even to identify the interaction and affinity between biological molecules [17,29–33]. Fig. 8.1 illustrates three different applications that use materials with high secondorder NLO response. Fig. 8.1A displays waveguides fabricated onto organic crystals

Figure 8.1  Applications of second-order nonlinear optical process. (A) Microfabrication of waveguides in organic crystal with SHG features. Inset shows the guided mode intensity at 800 nm. (B) Electro-optical modulator. (C) Image of fibrotic mouse kidney obtained combining SHG and two-photon fluorescence microscopy. Figure A, Reprinted with permission from Ref. [35]. Copyright (2018), Elsevier; Figure B, Reprinted with permission from Ref. [36]. Copyright (2018), Optical Society of America; Figure C, Reprinted with permission from Ref. [37]. Copyright (2016), Nature Group.

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(L-threonine crystal) via femtosecond laser micromachining. L-threonine crystal has considerable second-order optical susceptibility, in particular, ones related to SHG [34]. According to Ref. [35], the waveguides fabricated in L-threonine presented normalized power conversion efficiency for SHG at 377 nm of (10.3 ± 0.4)% (MWcm2)−1. In the inset of Fig. 8.1A, it is shown the SHG guided mode intensity at 377 nm. Fig. 8.1B shows a silicon-organic electro-optic modulator design, which is a Mach-Zehnder interferometer [36]. Such device uses two organic materials named JRD1 and YLD124 with high first-order hyperpolarizability. Depending on the organic material employed, it is possible to modulate the signal to achieve ultra-high EO activity due to distinct intermolecular interactions of the NLO chromophores. Through this hybrid architecture, the authors achieved the highest EO figure of merit in a high-speed modulator at any operating wavelength [36]. On the other hand, Fig. 8.1C presents a biological application of the second-order nonlinear optical response. By using a combination of SHG and twophoton fluorescence microscopy with Bessel beam excitation, the authors performed images with ultrahigh resolution of the histological section of fibrotic mouse kidney [37]. As noted in Fig. 8.1C, this technique allows obtaining images with tiny details and higher resolution than conventional optical microscopy. In this context, to study novel materials with large broadband nonlinear optical properties and ultrafast response time, it is crucial to decrease the irradiance threshold necessary to generate NLO effects and, consequently, develop more efficient photonics devices. Therefore, in order to characterize the NLO response of novel materials, it is needed to employ optical techniques with high accuracy. Among the techniques widely used to characterize the second-order nonlinear optical response of materials, we can highlight the hyper-Rayleigh scattering (HRS) due to its high sensibility and fast response time. HRS is an optical parametric effect in which two incident photons of frequency w are annihilated to create a scattered photon of 2w [38–40]. Unlike SHG, the HRS signal is isotropic, incoherent, and dephased with the excitation photon. Fig. 8.2 illustrates a representative design and energy diagram that emphasize the difference between (A) SHG and (B) HRS processes. It is essential to mention that both have a low probability of occurrence and, therefore, it is necessary to employ high irradiance (from MW/cm2 to GW/cm2) to observe its manifestation. However, as the HRS is an incoherent process it needs more irradiance to be observed. Moreover, NLO effects are ruled by the Heisenberg uncertainty principle, that is, the temporal overlap (∆t) between the photons involved in the process needs to satisfy  the relation ∆E ∆t ≥ , in which ∆E is the excitation photon energy and  is the reduced 2 Planck’s constant. Hence, both statements suggest the use of ultrashort laser pulses. From the classical analysis of induced dipole in a material, it is possible to show that the HRS is related to the second-order polarizability term, also known as the first-order hyperpolarizability, and, therefore, it is a nonlinear optical phenomenon. The first-order hyperpolarizability is absent  in centrosymmetric molecules because a perturbation caused by an electric field ( E ) leads to a polarization that does not alter its direction when the electric field is reversed (−E ) . It happens because in a centrosymmetric material the interaction potential is symmetrical with respect to its inversion center. On the other hand, an oscillating electric field such as a laser should cause a

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Figure 8.2  Representative design and energy diagram for the (A) SHG and (B) HRS. The main difference between the SHG and HRS are illustrated as: (1) the signal is anisotropic (SHG) and isotropic (HRS); (2) coherent (SHG) and incoherent (HRS); (3) in phase (SHG) and dephasing (HRS) with the excitation photon.

shift in the electron position to the  left  and/or  right depending on the direction of the oscillating electric field, that is, P −E = −P E . From the macroscopic point of view, the total induced polarization in the perturbative regime is given by: [41]

( )

( )

     1  (2)  1  (3)     E j (ω ) Ek (ω ) El (ω ) +… P (ω ) = ε 0  χ ij(1) ⋅ E j (ω ) + χ ijk : E j (ω ) Ek (ω ) + χ ijkl 2! 3!   (8.1)  (1) in which ε0 is the vacuum dielectric constant, χ ij is the linear (first order) optical susceptibility (rank-2 tensor) andit is related to the linear refractive index, Rayleigh (2) scattering and linear absorption; χ ijk is the second-order optical susceptibility (rank-3 tensor) and gives origin to the second harmonic generation, HRS, sum- and differ (3) ence-frequency generation and optical ratification; χ ijkl is the third-order optical susceptibility (rank-4 tensor) and describe the nonlinear refractive index (Kerr effect) and two-photon absorption, third-harmonic generation. Also, i, j, k, lare the components   of the electric field along the molecular axis. Thus, the relation P −E = −P E provides the following result:

( )

        1  (2) 1  ε 0  χ ij(1) ⋅ −E j + χ ijk : −E j −Ek + χ (3) −Ek −El +… = jkl  − E j   2! 3!  1  (2)   1  (3)      −ε 0  χ ij(1) ⋅ E j + χ ijk : E j Ek + χ jkl  E j Ek El +…   2! 3!

( )

( )(

)

( )(

( )

)( )

(8.2)

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Thus, for Eq. (8.2) to be satisfied,  (2)it is necessary that all even terms in the polarization expansion vanish, that is, χ ijk = 0. It is worth mentioning that  the (2)connection between the microscopic and macroscopic quantities is given by β ijk ∝ χ ijk N , in  which βijk is the first-order hyperpolarizability and N is the number of molecules per cubic centimeter. Therefore, the HRS is highly sensibly to molecular symmetry and, because of this, it has been explored to quantify the interaction between biological molecules with great success [42,43]. Thus, in this chapter, we describe the potential of the HRS technique to investigate the first-order hyperpolarizability of organic and biological molecules. For that, we initially present fundamentals of the first-order hyperpolarizability induced in materials. Three of the most commonly used approaches are presented, namely, the finite field (FF) method, the sum-over-states (SOS) method, and the response function (RF) formalism, as well as the related quantum chemical methods most used with each one of them. We describe some additional information to the experimental data that can be provided by theoretical calculations, such as the depolarization ratio and the multipolar components of the first-order molecular hyperpolarizability (β). These models are used to aid in fundamental understanding that can be employed, at the molecular level, to reach remarkable first-order hyperpolarizability (>10−27 cm5/esu). Moreover, we present in detail the HRS experimental setups used to quantify β. Finally, recent results about the first-order hyperpolarizability are described and discussed for organic molecules with distinct molecular structures, as well as the use of the HRS effect as a technique to quantify specific interactions in biological materials.

2  Microscopic description of the nonlinear optical response: Electronic first-order hyperpolarizability The interaction  between an atomic or molecular system and an oscillating electromagnetic field ( E ) can be described on the light of a semi-classical framework. In this context, the interaction Hamiltonian (Hint) at the dipole electric approximation is given by [44]:   H int = µ ⋅ E, (8.3)  in which µ is the atomic or molecular dipole moment. In the perturbative regime, that is, considering the magnitude of the incident electric field much smaller than the interatomic electric field, theinduced dipole moment can be express as a power series of the electric field strength E :   1       1   1  µi = µi0 + α ij ⋅ E j + βijk : E j Ek + γ ijkl  E j Ek El +… 1! 2! 3!

(8.4)

  α ij is the first-order polarizability tensor (or in which µi0 is the static dipole moment,  zero-order hyperpolarizability),βijk is the second-order polarizability tensor (or firstorder hyperpolarizability), and γ ijkl is the third-order polarizability tensor (or secondorder hyperpolarizability).

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Within the perturbative regime, one can  use the  time-dependent perturbation theory  to obtain the linear (α ij ) and nonlinear (βijk and γ ijkl ) optical responses of the material at the presence of the electric field. However, this theory can be employed only if the perturbation acts on the molecular system on a very short (t→0). Addition time-scale  ally, to observe the nonlinear optical effects related to βijk and γ ijkl it is necessary to use high irradiance because the probability of occurrence of NLO phenomena is very low. Ultrashort time-scale and high irradiance can be obtained employing laser pulses with a temporal duration from nanosecond to femtosecond. Here, we are interested in describing the first-order hyperpolarizability magnitude through the molecular parameters (transition dipole moment, permanent dipole moment difference, linewidth, etc.) that can be obtained, for instance, from conventional optical spectroscopy, such as steady-state absorption and fluorescence [45]. In this context, Orr and Ward [44] published a pioneering paper on the application of the perturbation theory to obtain an estimative of the microscopic nonlinear optical response of an isolated system. According to such works [44,46], the degenerate first-order hyperpolarizability can be expressed, within the SOS approach, as:  gµ m mµ n nµ g  1 i j k  β (−ωσ , ω1 , ω 2 ) = 2 P (i, j, k; −ωσ , ω1 , ω 2 ) ∑ ∑   − − ω ω ω ω   ( )( ) σ gm gn 2 n≠g m≠g 

(8.5)

in which, the indexes n and m represent the excited states and m µ n = m µ n − g µ g δnm. Here, µi, µj, and µk are, respectively, ith, jth, and kth cartesian component of the dipole moment operator. P (i, j, k , −ωσ , ω1 , ω 2 ) is the permutation operator with ωσ = ω1 + ω 2 . The summation is made over all possible combinations of n and m and for any permutation of (i,j,k) an equivalent permutation of (−ωσ , ω1 , ω2 ) should be performed. From the quantum mechanics point of view, second-order nonlinear optical phenomena are forbidden in centrosymmetric materials due to dipole-electric selection rules. To make it clearer, let us considerer a two-energy level approach for centrosymmetric molecules. In this case, the two real states involved in the optical transition (ground and first-excited states) have wavefunction with distinct parity symmetry (one-photon transition, Laport’s rule, gerade→ungerade or ungerade→gerade). Therefore, Eq. (8.5) can be rewritten as:

β ( −ω σ ; ω 1 , ω 2 ) ∝ ∑ ∑ n≠ g m≠ g

g µi n n µ j n n µ k g

(ω

ng

)(

− ω σ ω ng − ω 2

)

(8.6)

So, ∞

g µi n n µ k g n µ j n =

∫ ψ µ (ψ ψ ) µ ψ ψ * g

i

−∞

∞

ψ µ ψ  dV = 0 )ψ  ∫ ψ(µµ  * g

−∞



i

odd

k

g

* n

j

even

n

n

* n

k

g

* n

µ jψ n dV = (8.7)

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Figure 8.3  Representative diagrams for the HRS considering (A) two-energy level and (B) three-energy level approach.

Eq. (8.6) tends to zero because the integrand is an odd function in a symmetrical domain. Therefore, in centrosymmetric materials the selection rules for second-order nonlinear optical effect follow the same rules that the ones for two-photon transition, that is, same parity, ungerade→ungerade or gerade→gerade [47]. To ascribe correctly the first-order hyperpolarizability is necessary to consider the electronic structure of the material to be investigated. To small organic molecules, in general, the first-order hyperpolarizability can be determined with high accuracy considering a two-energy (n = m) or three-energy (n ≠ m) level model. Fig. 8.3 displays the representative diagrams for the two-energy (Fig. 8.3A) and three-energy (Fig. 8.3B) level approach. As can be noted, for the 2LM (two-level model) the first-order hyperpolarizability is ruled by only the transition dipole   moment (µ gn ) and permanent dipole moment difference (∆µ gn ) that connect the two states involved in the optical transition. This model assumes that only one excited state (generally lowest-energy one) effectively contribute to the first-order hyperpolarizability. The dynamic first-order hyperpolarizability within the 2LM can be expressed as ( n = m = 1 and g = 0 ):   2 2 ∆µ 01 µ 01 ω 01   2 3 3 β (−2ω; ω , ω ) = 2 2 ∆ µ µ = D ω , ω (8.8) ( ) 01 01 01 2 2 2 (ω 01 − 4ω 2 )(ω 01 −ω2) 2 ( ω01 ) 4 ω 01 is the frequency dispersion factor that 2 2 −ω2) (ω − 4ω )(ω01 takes into account the resonance enhancement effect (represented in the Fig. 8.3 by the detuning ∆ = ω gn − 2ω ), w is the angular frequency of the incident laser light, and w01 is the frequency of the electronic transition from the ground to the first excited  state. µ 01 is the transition dipole moment between the singlet ground (S0) and first ex   cited (S1) states, ∆µ 01 = µ11 − µ 00 is the permanent dipole moment difference between the ground and first excited states. In general, a 2LM is used to describe molecules that have predominantly dipolar character. In this context, more complex molecules which have higher order

in which D(ω , ω 01 ) =

2 01

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contributions for the NLO response, such as quadrupolar and octupolar molecular systems, the 2LM is not enough to correctly describe the first-order hyperpolarizability. Thus, the simplest framework capable of accounting for multipolar nonlinearities is the three-level model (3LM) approach proposed in Ref. [48,49]. In this model, the molecular first-order hyperpolarizability tensor (βijk) has nonzero components associated with dipole moments that make the coupling among the three lowest-energy states. Hence, assuming n ≠ m in Eq. (8.6) and performing all possible permutation, the following expression for the first-order hyperpolarizability in a 3LM [48–50] can be derivate:    g µi m m µ j n n µ k g + g µi m m µ k n n µ j g +    (ωmg − 2ω )(ωng − ω ) (ωmg − 2ω )(ωng − ω )    g µ j m m µi n n µ k g  g µ k m m µi n n µ j g + +  β (−2ω; ω , ω ) ∝ ∑  (ωmg + ω )(ωng − ω ) (ωmg + ω )(ωng − ω ) n ,m ≠ g      g µ k m m µ j n n µi g + g µ j m m µ k n n µi g    ω mg + ω )(ω ng + 2ω ) ω mg + ω )(ω ng + 2ω ) ( (   (8.9) On the other hand, Eq. (8.9) can be rewritten considering the maximum contribution of the matrix elements for the first-order molecular hyperpolarizability, as [50,51]:  | ∆µ 01 || µ 01 |2 | ∆µ 02 || µ 02 |2 | µ 01 || µ12 || ∆µ 02 |  3 β (−2ω; ω , ω ) = D(ω , ω 01 )  + +  2 2 (ω 02 )2 (ω 01 )(ω 02 )   (ω 01 )

(8.10)

in which the first two terms are dipolar terms for the states n = 1 and m = 2    and the third term is related to the higher order contribution. ∆µ 02 = µ 22 − µ 00 is the permanent dipole moment difference between the ground and second excited states,  and µ12 is the transition dipole moment between the first and second excited states. The molecular parameters involved in HRS, considering a three-level model, are il    lustrated in Fig. 8.3B. It is worth mentioning that µ 01 , µ 02 , ∆µ 01 , and ∆µ 02 can be obtained from steady-state absorption and fluorescence spectroscopy, as shown in details in Ref. [45]. In contrast, it is necessary to use the white-light transient absorp tion spectroscopy in a pump-probe configuration to find µ12 .

3  Hyper-Rayleigh scattering technique Fig. 8.4 illustrates the conventional HRS setup as described in Ref. [52]. A pulsed laser beam (from ns to fs) tuned at infrared (in general) goes to the dielectric mirrors set, which acts filtering higher harmonic contributions from the laser system to guarantee that irradiance used is related to only the excitation beam. Polarizers and a half wave-plate are employed to control, at the same time, the irradiance and the

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Figure 8.4  Conventional HRS setup.

polarization state of the excitation laser. After that, the beam is focused on the cuvette containing the liquid sample. The SH signal due to the HRS is collected at 90o from the excitation beam through the iris that to select the solid angle of detection and a telescope that collimates the SHG signal onto the detector. A band-pass filter is added to remove the excitation laser beam and a monochromator tuned at the SHG wavelength is used to collect only the scattering photon at 2w. Finally, the HRS signal is amplified through the photomultiplier (PMT) and plotted in the computer scream with the auxiliary of the dedicated software. Thus, the quadratic dependence between the incident (w) and scattering (2w) photons is available. Calculating the angular coefficient (α) of the straight line (I(2w)/I2(w))of such data for the molecule studied and one for a reference molecule, the dynamic first-order hyperpolarizability can be evaluated according to:

α sample β sample = βreference α reference

(8.11)

p-Nitroaniline dissolved in chloroform (βPNA = 17.5 × 10−30 cm5/esu at 1064 nm [53,54]) is widely used as reference material. Recently, Franzen et al. [53] published an extension of the conventional HRS technique. The HRS setup is shown in Fig. 8.5A. In fact, this technique has taken advantages on a mode-locked and Q-switched Nd:YAG laser that delivers a train of pulses separated by about 13 ns. Each pulse of the envelope has duration of 100 ps (FWHM). The pulse train can be visualized in Fig. 8.5B. Hence, it allows us an enhanced HRS signal-to-noise ratio and to get a faster acquisition process because a complete irradiance scan is obtained for each pulse train profile, providing a better statistical

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Figure 8.5  (A) Hyper-Rayleigh scattering experimental setup and (B) Q-switched and mode locked laser envelope. Reprinted with permission from Ref. [55]. Copyright (2018), American Chemical Society.

ensemble. At the same time, no modification along the experimental setup is required to control the irradiance, which optimizes the experiment time. During background determination, a computer controlled shutter is used to block the laser beam. It allows obtaining HRS curves without spurious effects from electronics or environment since the HRS signal is very sensitive. In Fig. 8.5A, two polarizers are employed to control the maximum level of excitation irradiance and, at the same time, also keeping the laser polarization always linear (vertical). HRS signal with a higher signal-to-noise ratio is obtained by performing a great number of measurements. A reference photodetector helps to improve the signal-to-noise ratio, just by introducing a beam splitter before the focusing lens and monitoring in real time, the laser intensity pulse train distribution. The laser portion that passes by the beam splitter is focused by a set of divergent and convergent lenses in order to expand the beam and achieve small Rayleigh parameter. It is used to avoid possible damages to the cuvette walls and focus at the middle point of a 1 cm fused silica cuvette. The scattered light (HRS signal) at the double frequency (532 nm) is collected perpendicular to the pump beam propagation, which helps to avoid possible interferences between laser and measured signal at the PMT. Also, to improve the signal-tonoise ratio, the setup presents a spherical mirror which reflects part of the signal that is scattered to the opposite direction of the photomultiplier. The spherical back mirror increases the HRS signal of about 100%. A telescope is used between the sample and the PMT to achieve a high solid angle. A narrow band-pass filter (10 nm) allows only the 532 nm scattered signal (nonlinear emission) to be detected by the PMT.

4  Theoretical calculation 4.1  Importance of symmetry on the second-order NLO responses The symmetry is an essential aspect in the description of NLO properties of molecules and materials. In fact, it is always helpful to consider the symmetry in the second-order nonlinear processes because it can elucidate the tensor properties and often makes the values of several independent components of β that are nonzero identical. Thus, considering symmetry aspects facilitates the description of the first-hyperpolarizability

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of a molecule, because it allows one considerably reduce the number of independent terms of β. In the most general case the first-hyperpolarizability tensor contains 27 independent components, collected in a 9 × 3 matrix. Hyperpolarizabilities and macroscopic susceptibilities exhibit three types of symmetry: permutation symmetry, time-reversal symmetry, and spatial symmetry [56]. The permutation and time-reversal symmetries are fundamental properties of the NLO properties themselves, while the spatial symmetry reflects the structural properties of the material medium. The consequences of these three types of symmetry on the form of the β tensor are discussed later in the case of an SHG process.

4.1.1  Intrinsic symmetry of permutation In the case of two indiscernible incident photons (i.e., with the same frequency and polarization state), the following permutation can be applied on the components of the β tensor:

β (8.12) ijk = β ikj and consequently, the number of independent components in the β can be reduced from 27 to 18.   β xxx β =  β yxx   β zxx 

β xyy

β xzz β xyz

β xxz

β yyy

β yzz β yyz

β yxz

β zyy

β zzz β zyz

β zxz

 β xxy  β yxy   β zxy  

(8.13)

4.1.2  Kleinman’s symmetry If the frequencies of all optical fields involved (excitations and response) in NLO process are far from any transition frequency (absorption) of the molecule under study, the overall permutation symmetry can be applied to the Cartesian components of the hyperpolarizability tensor without any changes of their values. This property is known as the time-reversal and was first formulated by Kleinman:

β ijk = β kji

(8.14)

Assuming the Kleinman’s conditions, the number of independent (nonzero) components in the β tensor is reduced to 10:   β xxx β =  β yxx   β  zxx

β xyy β yyy β zyy

 β xzz  β yzz  + β xyz   β zzz  

(8.15)

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Molecular and Laser Spectroscopy

4.1.3  Molecular symmetry Assuming the electric field of the incident light process is not small (compared with the intra-atomic field binding the electron to the nucleus), one can express the induced molecular dipole moment as a power series of the electric field strength E: 0 µ (8.16) i = µ i + α ij E j + β ijk E j E k + γ ijkl E j E k El + ...

The number of independent components in the β tensor also depends on the symmetry of the molecular system. In the particular case of centrosymmetric molecules (i.e., molecules presenting inversion symmetry), the coordinates can be transformed as: x → − x , y → − y, z → − z (8.17) Consequently, in centrosymmetry molecules, if one changes the sign of the applied electric field the dipole components will change sign as well, that is:

µ (8.18) x (− E x ) = −µ x ( E x ), µ y (− E y ) = −µ y ( E y ), µ z (− E z ) = −µ z ( E z ) In this way, considering a centrosymmetric molecule, for which the permanent dipole moment is null (µi0 = 0 ) and the electric-field induced change follows the relation µi (−Ei ) = −µi ( Ei ) , and examining the effect of the symmetry on the β tensor components, we have:

µ (8.19) i ( Ei ) = α ij ( E j ) + β ijk ( E j )( E k ) + γ ijkl ( E j )( E k )( El ) + ... µi (−Ei ) = α ij (−E j ) + βijk (−E j )(−Ek ) + γ ijkl (−E j )(−Ek )(−El ) + ... µ i (−Ei ) = α ij ( E j ) + βijk ( E j )( Ek ) − γ ijkl ( E j )( Ek )( El ) + ...

(8.20)

The principle of Neumann states that if a molecule or crystal is invariant concerning certain symmetry elements, any of their physical properties must also be invariant concerning the same symmetry elements. Therefore, a symmetry operation is required to leave the sign and magnitude of physical properties unchanged. Thus, from Eqs. (8.19) and (8.20) and based on the principle of Neumann [56], one can conclude that βijk = 0 whatever i, j, and k for centrosymmetric molecules. In addition, as general rule, all even-order nonlinear properties vanish in centrosymmetric systems.

4.2  Molecular first-order hyperpolarizability by hyper-Rayleigh scattering experiment In the HRS experiment, the scattered light is given as the simple incoherent contribution of the scatterers. The intensity of the light scattered by a single molecule at the harmonic wavelength (2w) is proportional to the orientational averaged first-order 2 hyperpolarizability squared β HRS (the brackets indicate orientational averaging) 2 [38]. The relation between β HRS and the components of the molecular first-order

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287

hyperpolarizability tensor (βijk) depends on the polarization state of the fundamental and harmonic beams, the spatial geometry of the experimental setup and the molecular symmetry [57]. In the classical HRS experiment, the 90° angle of spatial geometry is used. Assuming such geometry and adopting the laboratory coordinate system of reference (X, Y, Z) and the molecular coordinate system of reference (x, y, z), β HRS can be written as follows for a fundamental light beam propagating in the X-direction and vertically polarized in the Z-direction [58,59].

β HRS =

2 β HRS =

2 2 β ZZZ + β XZZ

(8.21)

2 2 In which β ZZZ and β XZZ are macroscopic hyperpolarizability averages in which the first subscript (X or Z) refers to the polarization state of the 2w harmonic light (in the laboratory coordinate system of reference), assuming that both polarization states are detected with equal sensitivity. The associated depolarization ratio (DR) of the molecular scatterers, defined as the ratio of the vertical (Z) and horizontal (X) components of the HRS light, is given by: 2 β ZZZ I 2ω DR = Z2ω = 2 IX β XZZ

(8.22)

The macroscopic averages in Eq. (8.21) can be written as a function of the molecular first-order hyperpolarizability tensor components (molecular system of reference). Assuming the Kleinman’s symmetry conditions (invariance of the tensor components with respect to permutation of the Cartesian indices), [60] valid for transparent and weakly dispersive optical materials, the macroscopic first-order hyperpolarizability averages can be written as:





2 β ZZZ =

1 6 9 3 12 βiii2 + ∑ βiii βijj + ∑ β jii2 + β jii β jkk + βijk2 ∑ ∑ 7 i 35 i ≠ j 35 i ≠ j 35 i ≠ j ≠ k 35

(8.23)

2 β XZZ =

1 2 11 1 8 βiii2 − βiii βijj + β jii2 − β jii β jkk + βijk2 ∑ ∑ ∑ ∑ 35 i 105 i ≠ j 105 i ≠ j 105 i ≠ j ≠ k 35

(8.24)

Whereas the use of the more classical Cartesian description in the earlier-mentioned equations is a usual procedure, the consideration of their spherical counterparts is less common. This allows the molecular β tensor (symmetric 3rd rank tensor) to be decomposed, under the Kleinman’s symmetry, as the sum of dipolar (J = 1) and octupolar (J = 3) tensorial components [61–63]. In addition to this conceptual advantage, it allows to track experimental results down to their basic molecular origin. Finally, a practical advantage of the spherical tensor formalism resides in the rotational invariance, avoiding possible sources of misfit with experimental data due to arbitrary orientation of the Cartesian axes. Using mixed Cartesian-spherical formalism, the

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2 2 and β XZZ macroscopic averages and β HRS can be rewritten as a function of β ZZZ the squared norms of the irreducible J spherical components as:







2 β ZZZ =

9 6 2 2 β J =1 + β J =3 45 105

(8.25)

2 β ZXX =

1 4 2 2 β J =1 + β J =3 45 105

(8.26)

10 10 2 2 β J =1 + β J =3 45 105

β HRS =

(8.27)

The relationship between the two spherical tensorial components and the Cartesian tensorial components of the molecular β tensor can be found somewhere else [59]. The nonlinear anisotropy parameter (ρ) is defined as: ρ = β J =3

2

β J =1

2

(8.28)

and provides a quantitative classification of the molecular system in terms of its more or less pronounced octupolar/dipolar character. Such a parameter can be used to compute the relative contribution φ ( β J ) of the octupolar φ ( β J =3 ) = ρ / (1 + ρ ) and dipolar φ ( β J =1 ) = 1 / (1 + ρ ) components to the first-order hyperpolarizability tensor (β). Therefore, in this representation pure dipolar and octupolar molecules refer to those having, respectively, no octupolar (φ ( β J =3 ) = 0 ) and no dipolar (φ ( β J =1 ) = 0 ) contributions in their HRS response. Due to the advantages described earlier, the spherical representation of the first-order hyperpolarizability is frequently adopted to theoretically describe the molecular property and discuss the experimental data. The nonlinear anisotropy ρ is related to the depolarization ratio (DR) of the molecular scatterers according to:  2 2   1+ 7 ρ  DR = 9  12  1+ ρ 2    7

(8.29)

4.3  Schemes for the determination of the molecular hyperpolarizabilities The frequency dispersion and the electronic correlation play an important role in the evaluation of the first hyperpolarizabilities and are necessary for comparison with experimental values. To estimate both effects, the multiplicative scheme can be applied. In this scheme, the Hartree Fock and time-dependent Hartree Fock (TDHF) [64] methods are respectively applied for obtaining the static and dynamic first hyperpolarizabilities of the chromophore, while a post-HF method, such as the MP2 method, for

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289

example, is employed to take into account the electron correlation effects. It consists of multiplying the static and electron correlated value of the static hyperpolarizability ( βCORR (0; 0, 0)) by a corrective dispersion factor:

βCORR (−2ω; ω, ω) = βCORR (0;0,0) ×

βTDHF (−2ω; ω, ω) β HF (0;0,0)

(8.30)

A similar scheme can be applied to the depolarization ratios (DR) of the first hyperpolarizability: DRCORR (−2ω; ω , ω ) = DRCORR (0; 0, 0) ×

DRTDHF (−2ω; ω , ω ) DRHF (0; 0, 0)

(8.31)

This multiplicative correction approximation is a powerful tool to estimate the correlated dynamic β values [65]. A second approach, called the coupled-perturbed (CP) schemes, [66,67] has also been widely used to investigate the molecular properties of molecules responding to an external electric field. In these schemes, an external electric field is added to the effective Fock/ Kohn—Sham Hamiltonian of the HF and DFT methods. Based on perturbation theory, the wave function and the Fock/KS operators are expanded in various orders of the electric field and the equation of each order is obtained. Solving these equations leads to the first (D(1)) and higher (D(n+1)) order response of the density matrix, which are necessary for calculating respectively the polarizability and nthorder hyperpolarizabilities. Within the coupled-perturbed scheme, the responses of the density matrix are computed by an iterative solution of the Coupled-perturbed self-consistent-field (CPSCF) equations. These SCF theories include the Hartree–Fock (HF), Kohn–Sham density functional theory (KS-DFT) and hybrid HF-DF methods. The equations used in CPHF are similar to those used in CPKS. The main difference is that the HF exchange potential in CPHF method is replaced by an (approximate) exchange-correlation (xc) potential in CPKS method. Another scheme to estimate the NLO properties taking into account the electron correlation effects is to use the SOS methods. SOS expressions are based on the manybody perturbation theory and were already presented (see Section 2). The sum-over-states method [44,68,69] provides accurate calculated response properties. The advantage of this approach over response-theory approaches is that it reveals which excited states contribute most to a given response property. Moreover, the frequency dependence of responses is easily included. However, the method is computationally demanding because it requires the explicit calculation of the contributions of all possible excited states to the property, and the numerical evaluation of a sum over many terms is subject to roundoff errors. In general, the high number of excited states needs to obtain converged results limits that its use is feasible only in combination with semiempirical methods.

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All the previous schemes of calculating NLO properties are analytical and they do not suffer from any intrinsic numerical ill-conditioning. However, depending on the quantum chemical method choice such schemes are computationally demanding, what limits the system size to which they can be applied. The finite field (FF) method is widely used for calculating the molecular hyperpolarizabilities of molecules and oligomers because of its low computational cost and ease of implementation. The method only requires calculating the energy of the molecule in external fields. Unlike the previous schemes, it does not require analytical derivatives concerning the field components or any information about excited states. This makes it applicable to all levels of theory and it is an ideal technique to test new electronic structure methods. The dependence of the energy E of a molecule on an external static homogenous electric field F can be written as a Taylor expansion of the form: 1 ∂E 1 ∂2 E 1 ∂3 E 2 E ( F ) = E (0) + F+ F + F 3 + ... 2 3 1! ∂ F 2! ∂ F 3! ∂ F 0 0 0

(8.32)

1 1 E ( F ) = E (0) + µ F + α F 2 + β F 3 + ... (8.33) 2 6 The FF expressions for calculating the response properties are obtained by arranging Eq. (8.33) as: ∂2 E α = − 2 = lim ∂F 0 F→0

2 E (0) − E ( F ) − E (− F ) F2

(8.34)

∂3 E β = − 3 = lim ∂F 0 F→0

E (+2 F ) − E (−2 F ) − 2 [ E (+ F ) − E (− F )] 2F 3

(8.35)

in which E is the energy of the molecular system and F is the amplitude of the external electric field. The FF method has some limits and drawbacks. It is limited to the calculation of static response properties because time-dependent fields are too complicated to be handled straightforwardly. But the most crucial downside is the well-known dependence of the calculated response properties on the choice of the initial field strength for doing the calculation. Evaluation of response properties at too small fields leads to noise due to the finite convergence thresholds for the energy and wave function optimization. Choosing a too strong field strength also leads to inaccuracies that stem from two factors. First, the higher order terms in the Taylor expansion in Eq. (8.33) are not negligible anymore and contribute to the energy of the molecule. The second effect, and more problematic, is the change in the electron configuration of the molecule at certain field strength. For sufficiently strong fields, an excited state at a zero field will become lower in energy than the former ground-state. Hence, all properties

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evaluated at such field strengths reflect the behavior of this excited state. Moreover, strong enough field strengths can lead to the ionization of the molecule. Therefore, obtaining meaningful molecular response properties by the FF method depends on doing the calculation in a window of feasible field strengths that have a lower bound of noise and an upper bound of results corresponding to the excited or ionized states of the system. Although this is a serious problem of the FF method, there is no systematic way of estimating the optimal field strength for evaluating these properties and avoiding non-meaningful results. Another problem of the FF method is that calculated FF properties need further refinements to eliminate contributions from the higher derivative terms in the Taylor expansion. To remove higher-order contributions in the FF differentiation, the Romberg procedure can be applied in combination with field amplitudes ranging from ±0.0004 to ±0.0064 a.u. Knowing the successive estimates of β for increasing field amplitudes a general recursive procedure can be used. The iterative Romberg expression is given by:

β kn =

4 n β kn−1 − β kn+−11 4 n−1

(8.36)

in which n is the order of the iteration and k defines the field amplitude: Fk = ( 2 k −1 ) F0 .

5  First-order hyperpolarizability in push-pull octupolar molecules 5.1  Enhancing the electronic first-order hyperpolarizability An interesting molecular strategy to enhance and tune the nonlinear optical response of organic molecules is to syntheses molecules with octupolar structure (Y-shaped) like those shown in Fig. 8.6 [70,71]. These octupolar structures reported in Ref. [70,71] are composed by a triarylamine core bearing two 4-di(4’tert-butyl-biphenyl) (tBu, Group I) or 3,3’-bis(trifluoromethyl)phenyl arms (CF3, Group II) and a third group with varying electron-withdrawing strength (EWG  → (H  CHCl3 > CCl4. This series is in better agreement with the experimental βHRS measurements (CH3– CN > CCl4 > CCl3–CN > CH2Cl2 > CHCl3) than the hierarchy in the βHRS values obtained at the HF level. The only remaining discrepancy between the theoretical and experimental results concerns the CCl4 molecule, whose relative βHRS value is strongly underestimated by the calculations. In this last case, using more elaborate computational procedures including higher-order (quadruple) electron excitations or zero-point vibrational contributions might improve the consistency between theory and experiments. Moreover, as already mentioned, it is known that the strong hyperRayleigh response of CCl4 has a coherent contribution related with its interaction with the surrounding molecules.

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Table 8.8  βLiq/βGas ratios, as calculated at the TDHF and CCSD(T) levels of approximation using the d-aug-cc-pVTZ basis set in comparison to HRS experiments. Dynamic (λ = 1064 nm) CCSD(T) results were obtained using the multiplicative scheme. Molecule Dynamic HF Static CCSD(T) Dynamic CCSD(T) Experimental CCl4 CHCl3 CH2Cl2 CCl3–CN CH3–CN

1.39 1.51 1.78 0.81 3.43

1.47 2.09 2.30 2.36 3.66

1.43 1.55 1.72 1.04 2.77

1.53 ± 0.10 (Ref. [67]) 1.70 ± 0.10 (Ref. [67]) … … 1.90 ± 0.10 (Ref [67])

Because these intermolecular interaction effects can also be significant for the other molecules investigated here, making questionable any comparison between experimental and theoretical data, for a deeper discussion the authors turned to the calculated and measured βLiq/βGas ratios (see Table 8.8) [67]. The dynamic CCSD(T) values, calculated with and without using the IEF-PCM treatment, were obtained using the multiplicative scheme approximation in which the static CCSD(T) values are corrected by the ratio between the TDHF and CPHF values. The results reported in Table 8.8 show that CCSD(T)(IEF-PCM)/d-aug-cc-pVTZ calculations corrected using the multiplicative scheme provide βLiq/βGas ratios in good agreement with the experimental data, though the calculated value is overestimated for CH3–CN. The agreement with experiments is poor when frequency dispersion effects are not considered. The agreement is also better at the CCSD(T) level than at the HF level, which demonstrates again the importance of electron correlation. The βJ-components of the hyperpolarizability tensors were used to provide further insights into the symmetry-NLO property relationships in the series of molecules, as well as into the performances of the different computational schemes with respect to experimental measurements. For λ = 1064 nm, the total βHRS responses, the dipolar (βJ=1) and octupolar (βJ=3) tensorial components, as well as the depolarization and ρ ratios were calculated at the HF(IEF-PCM)/d-aug-cc-pVTZ and CCSD(T)(IEF-PCM)/daug-cc-pVTZ levels. The theoretical and experimental data are compared in Fig. 8.15, which portray the evolution of DR as well as of the octupolar Φ ( β J =3 ) and dipolar Φ ( β J =1 ) contributions to the second-order NLO responses as a function of ρ. These results showed that both the HF and CCSD(T) levels of theory (except the HF method for CCl3–CN) overestimate the octupolar character of the molecules, leading therefore to ρ values larger than the measured ones. The calculated values are nevertheless in qualitative agreement with experiments because all compounds, except CH3–CN, are predicted to display a dominant octupolar character, and because the hierarchy of the anisotropy factors matches with the experimental one. Finally, the study demonstrated that HRS is a technique of choice to probe the symmetry of the molecules since it allows a fine decomposition of the multipolar contributions of the NLO responses. The work also illustrates the difficulties of the accurate computation of the NLO properties, in particular for molecules in which the octupolar contribution is dominant, while the NLO responses of dipolar systems are better reproduced.

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Figure 8.15  Evolution of the depolarization ratio DR (left) and the octupolar and dipolar contributions (right) concerning the nonlinear anisotropy parameter. Black squares correspond to measured values, and (red circles) correspond to dynamic CCSD(T) results obtained using the multiplicative scheme. Reprinted with permission from Ref. [59]. Copyright (2012), American Institute of Physics.

7  Final Remarks Herein, we have concentrated our effort on the study of the first-order hyperpolarizability of organic molecules through the Hyper-Rayleigh technique and the application of quantum chemical calculations. We show that a very effective molecular strategy to obtain high DFH values is synthesizing organic molecules with push-pull octupolar structures. In such structures composed by a triarylamine core bearing two 4-di(4’tert-butyl-biphenyl) and a third group with varying electron-withdrawing strength was observed an increase of 3.5 times in the effective DFH as EWG is changed from CN to NO2. This outcome is still more interesting because the number of effective πelectrons from one molecule to another is unchanged practically. This indicates that the NO2 EWG modified completely the electronic distribution along the branches in the octupolar structure contributing to achieve considerable DFH value on the order of 10−28 cm5/esu. These results were corroborated through the CPKS calculations. Another important outcome ascribed in this chapter was the branching effect, that is, making a linear combination of dipolar molecules, on the DFH. In Ref. [55], we reported a great enhancement on the effective DFH (take into account the number of effective π-electron) for the V-shaped (two-branch) molecule as compared to the dipolar structure. DFH values on the order of 10−27cm5/esu were reported for these structures. This important outcome was ascribed to the intramolecular cooperative effect between the branches at the excited state for the V-shaped molecule. In the same work, the anticooperative effect was reported for the octupolar (three-branch) molecule because it was observed to have an effective DFH smaller than one in the dipolar molecule. These findings can contribute to a better understanding of the underlying mechanisms that rule the DFH of organic molecules in solution.

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Also, the use of the HRS technique as a molecular probe to investigate the affinity between biological molecules is reported based on the results of Ref. [42]. In that study, the HRS technique was used to obtain information about the binding of hemoglobin (Hb) variants on erythrocyte skeletal protein (spectrin) because it is a nondestructive optical method and susceptible to the molecular symmetry. The finding showed that the HRS technique is very faster and sensible to detect such interactions than other techniques employed for the same propose. Throughout the chapter, we also illustrated that the HRS technique could be combined with the use of a theoretical approach to get a deeper understanding of the firstorder hyperpolarizability of a molecular system. Such theoretical approaches combine the application of quantum chemical methods with a numerical or analytical scheme, such as the FF scheme, for example, and can provide relevant information about the molecular system being studied. As presented, such theoretical approaches can elucidate the tensor properties of the first-order hyperpolarizability, the role of the frequency dispersion on the first-order hyperpolarizability and to provide a quantitative classification of the molecular system in terms of its more or less pronounced octupolar/dipolar character (based on the computation of the nonlinear anisotropy parameter). Besides, it was illustrated that if a polarizable continuum solvation model is combined with a quantum mechanical description of the molecular system, then the theoretical approaches can also provide information about the role of solvent effects (the electrostatic solute-solvent interaction to be more precise) on the first-order hyperpolarizability of the solvated molecule. It was also discussed that for a quantitative comparison with experimental values, the choice of the quantum chemical method (and basis set) to be used is a crucial issue, mainly because the frequency dispersion and the electronic correlation play an important role in the evaluation of the first-order hyperpolarizability. The ab initio post-Hartree-Fock methods and DFT based methods are the most used ones in the quantum mechanical estimate of the molecular first-order hyperpolarizability. While the former is in most cases too computationally expensive to be efficiently applied, the latter has proved to be a very suitable tool in the field of nonlinear optical properties. The main drawback of DFT based methods is that unfortunately the exact exchange-correlation energy functional is not known and the accuracy of the semiempirical exchange-correlation functionals proposed is so far unable to be systematically improved with as much rigor and generality as observed in the wave function based methods.

Acknowledgments Financial support from FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo, grants 2011/12399-0, 2015/20032-0, 2016/20886-1 and 2018/11283-7), FAPEMIG (Fundação de Amparo à Pesquisa do Estado de Minas Gerais, grant APQ-01469-18), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico, grant 425180/2018-2), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior(CAPES) and the Air Force Office of Scientific Research (FA9550-15-1-0521) are acknowledged.

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Heterodyne-detected chiral vibrational sum frequency generation spectroscopy of bulk and interfacial samples

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Taka-aki Ishibashia, Masanari Okunob a University of Tsukuba, Tsukuba, Japan; bThe University of Tokyo, Tokyo, Japan

Chapter outline 1 Introduction 315 2 Principles of chiral VSFG spectroscopy  317 2.1  2.2  2.3  2.4  2.5  2.6 

What is VSFG spectroscopy?  317 VSFG susceptibility and its symmetric properties  318 VSFG susceptibility and molecular hyperpolarizability  319 Relation between chiral VSFG susceptibility and the symmetry of Raman tensor  321 Polarization combinations for chiral and achiral SFG measurements  322 Modes of SFG signal measurement  324 2.6.1  Narrowband IR scheme and multiplex scheme of SFG spectral measurement  324 2.6.2  Intensity measurement and phase-sensitive measurement of SFG signals  324

3 Experimental setup and the analysis of observed data in HD chiral VSFG  326 3.1 Multiplex HD VSFG spectrometer  326 3.2  Method for analyzing raw data to calculate the susceptibility of a sample  328

4 Applications of HD chiral VSFG spectroscopy  330 4.1  Neat liquid limonene  330 4.2  Vibrationally-electronically doubly-resonant chiral SFG of chiral solutions  332 4.3 Vibrationally-electronically doubly-resonant chiral SFG of chiral monolayers – electronic excitation profiles of complex chiral susceptibilities  335 4.4  Polymer thin films – bulk-or-interface assignment by polarization dependence  338 4.5  Air/protein solution interfaces  341

5 Concluding remarks  343 References  344

1 Introduction Sum frequency generation (SFG) is a second-order nonlinear optical process that converts two intense input lights into a new light the frequency of which is the sum of the input frequencies. The SFG process is electric dipole forbidden in a centrosymmetric medium. By virtue of this selection rule, SFG is often used to selectively observe an interface between two centrosymmetric bulk media because the inversion symmetry is Molecular and Laser Spectroscopy. http://dx.doi.org/10.1016/B978-0-12-818870-5.00009-5 Copyright © 2020 Elsevier Inc. All rights reserved.

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broken at the interface [1]. The chirality of a sample also breaks the inversion symmetry. Therefore, a randomly oriented ensemble of chiral molecules is SFG active when one enantiomer outnumbers the other in the sample; one can detect the chirality of the ensemble by SFG [2]. A similar but more elaborate symmetry argument shows that the chirality of azimuthally isotropic interfaces is also detectable by SFG. When one of the input lights is in the infrared (IR) region, and its frequency approaches a vibrational resonance of the sample, the SFG process is enhanced. Therefore, vibrational spectroscopy via the SFG process, that is, vibrational SFG (VSFG) spectroscopy is possible [1,3,4]. We call chirality detection by vibrational SFG spectroscopy chiral VSFG. Chiral VSFG is applicable to both isotropic bulk and azimuthally isotropic interfacial samples [5,6]. The first reliable chirality detection by SFG spectroscopy was carried out on chiral organic liquids of limonene by Shen and coworkers [7]. Subsequently, electronically-resonant chiral SFG spectra of organic solutions [8], vibrationallyelectronically doubly-resonant chiral SFG of organic solutions [9], and films of polymers and J-aggregates were reported [10,11]. Recently, chiral VSFG has been actively applied to polypeptides and proteins at interfaces. Yan and coworkers proposed close correlations between secondary structures and chiral VSFG spectra of them [12–14]. The advantage of chiral VSFG over conventional chiral vibrational spectroscopies, such as vibrational circular dichroism (VCD) and Raman optical activity (ROA) [15,16], is its high sensitivity. One reason is that chiral VSFG is an electric dipole allowed process unlike the conventional chiral spectroscopies, which rely on higher order interactions between molecules and light, such as magnetic dipole and electric quadrupole couplings. The other reason is that chiral VSFG is background free. Chiral VSFG can be obtained directly in some chiral-specific polarization combinations. In contrast, VCD and ROA measure the difference in their signals (absorption in VCD and scattering intensity in ROA) for left and right circularly polarized light. In this case, a tiny chiral signal can be overwhelmed by large achiral signals. Therefore, the background free feature is highly effective to detect chiral signals We have developed a heterodyne-detected (HD) chiral VSFG spectrometer combined with a multichannel detection for the first time [17]. Heterodyne detection is a technique to measure the electric field of light with phase information instead of the light intensity [18–20]. Employing the heterodyne detection in chiral VSFG spectroscopy has advantages in the amount of information and sensitivity over the conventional homodyne-detected chiral VSFG that measures only the intensity of the SFG signal. In this article, first, we introduce the basic theory of chiral VSFG spectroscopy to the minimum extent in order to help readers unfamiliar with SFG spectroscopy follow this article. The readers are encouraged to refer to excellent textbooks and reviews [3–6,21–24] for more detailed aspects of SFG, VSFG, chiral VSFG, and nonlinear spectroscopy in general. Then, we describe the HD VSFG chiral spectrometer we have developed and the method of data analysis to obtain VSFG spectra from raw data, and introduce the applications of HD chiral VSFG spectroscopy to chiral liquids [17], chiral solutions [25], chiral monolayers on water [26], chiral polymer films [27], and proteins at air-water interfaces [28].

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2  Principles of chiral VSFG spectroscopy 2.1  What is VSFG spectroscopy? In SFG spectroscopy, two laser beams, one in the UV or visible region with a frequency of w1 and the other in IR with a frequency of w2, are superimposed in a sample, and the light generated at the sum of the two frequencies (ws = w1 + w2), the SFG signal, is detected. (Fig. 9.1) The electric field of the SFG signal (Es) is related to those of w1- and w2-beams (E1, E2) as follows: (2) Es ∝ χ E1 E2 ,

(9.1)

where χ(2) is the second-order nonlinear susceptibility, a third-rank tensor, which has 27 elements. Note that tensor elements are generally complex numbers because they describe the frequency response of sample materials. The SFG signal is generated in well-defined reflected directions determined by the condition that the input and signal wavevector components along the interface must be matched as follows: k1|| + k2|| = ks|| ,

(9.2)

where k1ǁ, k2ǁ, and ksǁ are the wavevector components of w1, w2, and the signal lights, respectively, and ǁ symbol denotes that the components are parallel to the interface. Usually, w1- and w2-beams are introduced in the same incident plane so that the wavevector of the signal beam is also in the same plane. When w2 light is in resonance with a vibrational frequency of the sample, χ(2) can be greatly enhanced. (Fig. 9.2) Therefore, by measuring the SFG light as a function of the IR frequency w2, we can conduct vibrational spectroscopy. SFG spectroscopy conducted under vibrational resonant conditions is called VSFG spectroscopy. In this review, chirality detection of isotropic and azimuthally isotropic interfacial samples by VSFG spectroscopy (chiral VSFG spectroscopy) is discussed.

Figure 9.1  Schematic diagram describing the SFG experiment in reflection with the definition of the incidence angles and laboratory axes. The directions of P and S polarized electric fields are also shown.

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Figure 9.2  Energy-level diagram schematics of SFG processes. (A) Non-resonant SFG, (B) vibrational resonant SFG (VSFG), (C) vibrationally-electronically doubly-resonant (DR) SFG.

2.2  VSFG susceptibility and its symmetric properties Isotropic bulk samples have the full rotation symmetry, and azimuthally isotropic interfacial samples have C∞ symmetry. As a result of these macroscopic symmetries, several properties of a VSFG susceptibility tensor are derived under the electric dipole approximation [2]. In the following description, three indices I, J, and K of a Cartesian (2) element, χ IJK , refer to the polarization directions of SFG, w1, and w2 electric fields, respectively. The laboratory-frame (XYZ) is assumed so that Z-axis is parallel to the interface normal, and the incident plane of w1- and w2-beams is XZ-plane (see Fig. 9.1). (2) (2) (2) (2) (2) (2) 1. For both bulk and interface samples, six elements of χ XYZ , χ YZX , χ ZXY , χ ZYX , χ YXZ , and χ XZY can be non-zero only if the sample is chiral. These elements will be called chiral elements in this article. A chiral element of one enantiomer is equal to the negative of the corresponding (2) (2) element of the other enantiomer, that is, χ IJK (enantiomer1) = − χ IJK (enantiomer 2). 2. For interfacial samples, only three among the six chiral elements are independent:

(2) (2) (2) (2) (2) (2) χ XYZ = − χYXZ , χYZX = − χ XZY , χ ZXY = − χ ZYX .

(9.3)

They are antisymmetric for exchanging X and Y indices. 3. For bulk samples, all the six chiral elements are interrelated with one another: (2) (2) (2) (2) (2) (2) χ XYZ = χYZX = χ ZXY = − χ ZYX = − χ XZY = − χYXZ .

(9.4)

4. For achiral bulk samples, there is no non-zero element. 5. For interfacial samples, other than the chiral elements, seven elements can be non-zero regardless of whether the sample is chiral or achiral, and only four of them are independent: (2) (2) (2) (2) (2) (2) (2) χ XXZ = χYYZ , χ XZX = χYZY , χ ZXX = χ ZYY , χ ZZZ .



(9.5)

These elements will be referred to as achiral elements in this article. An achiral element of one enantiomer is equal to the corresponding element of the other enantiomer.

Note that these properties are valid not only for VSFG processes but general SFG processes as long as they are non-degenerate. The non-zero elements and their

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Table 9.1  No-zero Cartesian elements of χ(2) tensors for isotropic interfaces and isotropic bulk samples. Non-zero component Sample (symmetry)

Achiral

Chiral interface (C∞)

(2) (2) (2) (2) (2) χ XXZ = χ YYZ , χ XZX = χ YZY , χ ZXX (2) (2) = χ ZYY , χ ZZZ

Chiral bulk (full rotation)

None

Achiral interface (C∞v)

(2) (2) (2) (2) (2) χ XXZ = χ YYZ , χ XZX = χ YZY , χ ZXX (2) (2) = χ ZYY , χ ZZZ

Achiral bulk (full rotation, inversion)

None

Chiral (2) (2) (2) χ XYZ = − χ YXZ , χ YZX (2) (2) (2) = − χ XZY , χ ZXY = − χ ZYX (2) (2) (2) (2) χ XYZ = χ YZX = χ ZXY = − χ ZYX (2) (2) = − χ XZY = − χ YXZ

None None

interrelationship are summarized in Table 9.1. Hereafter, we consider only isotropic bulk and azimuthally isotropic interfacial samples.

2.3  VSFG susceptibility and molecular hyperpolarizability In the case of a sample composed of molecules, its SFG susceptibility χ(2) may be written using the molecular hyperpolarizability β as: N (2) χ IJK = β IJK ε 0

(I , J , K = X ,Y , Z )

(9.6)

where the brackets denote an average. N is the number of molecules per volume for a bulk sample, while it is that per unit area for an interfacial sample. In the international system (SI), the unit of ε0 is Farad per meter (F·m–1 = C·V–1·m–1), and that of β is C·m3·V–2. Therefore, the unit of the susceptibility of a bulk sample and an interfacial sample is m·V–1 (meter per volt) and m2·V–1 (square meter per volt), respectively. When the intermolecular interaction is weak and β in the molecular frame depends only slightly on the orientation, the sample is regarded as a collection of noninteracting molecules, which are identical but oriented variously. In such a case, the averaging procedure is simplified as: N (2) χ IJK = ε0

∑

ξ ,η ,ζ = x , y , z

( Iˆ i ξˆ )( Jˆ i ηˆ )( Kˆ i ζˆ ) βξηζ

(I , J , K = X, Y , Z )

(9.7)

where x, y, and z are the molecular-fixed axis, the symbols such as Iˆ and ξˆ are the unit vector along the direction of I-axis and ξ-axis, respectively, and ‘•’ denotes the

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inner product. Molecular orientations at interfaces are often inferred by analyzing the (2) experimentally obtained values of χ IJK based on Eq. (9.7) [29–34]. The quantum mechanical formula of the molecular hyperpolarizability β corresponding to the SFG process can be obtained by perturbative calculation on the density matrix formalism. The hyperpolarizability may be expressed as the sum of vibrational non-resonant and resonant terms (βNR, βR) as follows [4,21,23,35]: βξηζ = β NR,ξηζ + β R,ξηζ

(ξ ,η , ζ = x , y, z ),

αξη dζ 1 β R,ξηζ = − ∑ , 2 v ω 2 − Ω vg + iΓ vg

(9.8)

(9.9)

d = v µζ g , ζ

(9.10)

 g µη n n µξ v  1  g µξ n n µη v αξη = ∑  + ,  n  Ωng − (ω1 + ω 2 ) − iΓ ng Ωnv + (ω1 + ω 2 ) + iΓ nv 

(9.11)

where dζ is ζ-element of the IR transition moment from the ground state (g) to a vibrationally excited state (v), and αξη is ξη-element of the Raman tensor for the transition from the state v to g. (See Fig. 9.2.) µξ is the electric dipole moment operator projected on ξ-axis, and ħΩab (≡ Ea – Eb) is the energy difference between state a and state b. Γab is the damping constant of the off-diagonal ab element of the density matrix. Therefore, Γvg corresponds to the bandwidth (half width at half maximum) of a vibrational transition, while Γng and Γnv correspond to that of the electric transition from the electronic ground state to the electronically excited state of the intermediate vibronic state (n). Eq. (9.9) shows that an SFG active vibrational mode must be active for both IR and Raman transitions. Note that the Raman tensor can approximately be considered as a real constant when both w1 and ws = w1 + w2 are far from electronic resonance. This is because Γng and Γnv are usually much smaller than Ωng and Ωnv. The concrete formula of βNR, which can be found in the literature [21,35], indicates that it slightly depends on w2 and can be approximately regarded as constant for a narrow w2 range. βNR is in general complex, but as in the case of the Raman tensor, βNR in molecular origins is practically real in the electronic non-resonant condition. The susceptibility of the sample can also be expressed as a sum of the vibrational non-resonant and vibrational resonant terms as follows: Av (2) (2) χ (2) = χ NR + χ R(2) = χ NR +∑ . ω − Ω 2 vg + iΓ vg v

(9.12)

Here, Ωvg and Γvg in Eq. (9.9) are often written as Ωv and Γv for simplicity. Av is real when both w1 and ws are far from electronic resonance as it corresponds to the product

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321

of the IR transition moment and Raman tensor. Note that when a sample is comprised (2) of a molecular layer on a metal substrate, χ NR due to the substrate can be complex even in electronic non-resonance [3].

2.4  Relation between chiral VSFG susceptibility and the symmetry of Raman tensor As explained in Section 2.3, the VSFG susceptibility of a molecular system is approximately the orientationally averaged molecular hyperpolarizability β, which is proportional to the product of the Raman tensor and IR transition moment. The symmetry of the Raman tensor is closely related to the selection rules of chiral VSFG signals. Any Raman tensor (α), a second rank tensor, can be decomposed into the sum of the symmetric part (S) and antisymmetric part (A),

α ij = Sij + Aij ,

Sij ≡ (α ij + α ji ) / 2 = S ji ,

Aij ≡ (α ij − α ji ) / 2 = − Aji .

(9.13)

It is well known that the Raman tensor is symmetric in the electronic non-resonant condition [36,37]. For an isotoropic bulk sample, its chiral elements of VSFG susceptibility, which are obtained by averaging β isotropically, are [7]

χ chiral

(2) (2) (2) (2) (2) (2) = χ XYZ = χYZX = χ ZXY = − χ XZY = − χ YXZ = − χ ZYX

∝ β yzx − β zyx + β zxy − β xzy + β xyz − β yxz

∝ {(α yz − α zy ) d x + (α zx − α xz ) d y + (α xy − α yx ) d z } = − ( Ayz d x + Azx d y + Axy d z ) (9.14)

The six chiral elements are interrelated and they are non-zero only if the Raman tensor is asymmetric, that is, the antisymmetric part of the Raman tensor is non-zero. This restriction limits the sensitivity of chiral VSFG of bulk samples because the antisymmetric part of a Raman tensor is much smaller than the symmetric part when both w1 and ws are far from electronic resonance [36,37]. However, in electronic resonant conditions, a Raman tensor can have a large antisymmetric part, leading to great enhancement of the sensitivity of chiral VSFG. This subject will be discussed in Section 4.2 and 4.3. The restriction is partly lifted for interfacial samples that are azimuthally isotropic, or uniaxial. Moad and Simpson gave explicit expressions that relate an SFG susceptibility with a molecular hyperpolarizability for such a sample (Eqs. A17–23 in Ref. [35]). By substituting the product of the Raman tensor and IR dipole for β in the expressions, one gets relationships between the non-zero χ(2) Cartesian elements and the symmetry of the Raman tensor for a uniaxial system as follows: (2) (2) (2) 1. The achiral Cartesian elements that depend solely on the symmetric part: χ XXZ = χ YYZ , χ ZZZ 2. The achiral Cartesian elements that depend on both the symmetric and antisymmetric parts: (2) (2) (2) (2) χ XZX = χ YZY , χ ZXX = χ ZYY

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(2) (2) 3. The chiral Cartesian elements that depend on solely on the antisymmetric part: χ XYZ = − χ YXZ 4. The chiral Cartesian elements that depend on both the symmetric and antisymmetric parts: (2) (2) (2) (2) χYZX = − χ XZY , χ ZXY = − χ ZYX . The relationship 4 shows that some chiral susceptibility elements can be non-zero even if the Raman tensor is purely symmetric. As mentioned earlier, a Raman tensor is symmetric in the electronic non-resonant condition. Therefore, chiral VSFG can be considered interface specific if the electronic non-resonant condition is rigorously fulfilled.

2.5  Polarization combinations for chiral and achiral SFG measurements In SFG measurement, the contributions of χ(2) tensor elements to the observed signal are controlled by selecting the polarization states of the SFG signal, w1, and w2 lights. Linearly polarized states called P and S polarization are usually used; P polarized light has the electric field in the incidence plane and S polarized light has the electric field perpendicular to the plane. A polarization condition of SFG measurement is specified by three characters corresponding to the polarization states of the SFG signal, w1, and w2 lights, respectively. For example, PSP signifies that the SFG signal is P polarized, w1 light is S polarized, and w2 light is P polarized. There are 8 (= 23) polarization combinations with P or S polarized SFG signal, w1, and w2 lights. However, not all combinations are effective in the electric dipole approximation because of the symmetric properties of χ(2) tensor elements. SSP, SPS, PSS, and PPP combinations are effective for achiral and chiral interfacial samples, while SPP, PSP, and PPS combinations are so for chiral interfacial and bulk samples. Therefore, it is possible to probe the chiral and achiral elements separately by selecting a proper polarization combination. The chiral elements exclusively contrib(2) (2) (2) (2) ute to the SFG signals detected with SPP ( χYZX , χYXZ ), PSP ( χ XYZ , χ ZYX ) , and PPS (2) (2) ( χ ZXY , χ XZY ) combinations, while the achiral elements to those detected with SSP (2) (2) (2) (2) (2) (2) (2) ( χYYZ ) , SPS ( χYZY ), PSS ( χ ZYY ) , and PPP ( χ XXZ , χ XZX , χ ZXX , χ ZZZ ) combinations. In the parentheses, specific contributing elements are given. As an example, we consider PSP combination. P polarized field consists of X and Z components, while S polarized field solely of Y component. Therefore, only the susceptibility elements, such as (2) ( χ IYK ) ( I , K = X , Z ) , can contribute to the signal in the PSP combination. Inspecting (2) (2) Table 9.1, we find that only χ XYZ and χ ZYX , both of which are the chiral elements, meet the requirement, and conclude that only chiral samples give the SFG signal in the PSP combination. In the same way, the Cartesian elements for other polarization combinations can be derived. We note that the chiral polarization combinations (PSP, SPP, PPS) contain “P” an even number (2) of times, while the achiral polarization combinations do “P” an odd number (1 or 3) of times. The most important consequence of the exclusive nature in the chiral and achiral polarization combinations is that it permits the background-free observation of the chiral SFG signal. We can measure chiral signals due to chiral elements without the interference from achiral signals due to achiral elements, which are usually much larger than chiral signals. It is also practically important that the chiral polarization combinations utilize only linear polarization states of light. The high purity of a linear

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323

polarization state can easily be achieved because dispersion-free polarizers for UV, visible, and IR lights with excellent extinction ratios are readily available. These advantages greatly contribute to the high sensitivity of chiral SFG spectroscopy. The SFG signal field with a polarization combination can be expressed with a so(2) called effective susceptibility, χ eff ,LMN as follows: (2) EL (ω s ) ∝ χ eff, LMN EM (ω 1 )EN (ω 2 ) ( L, M , N = P or S),

(9.15)

where EL(wS), EM(w1), and EN(w2) are the Fourier components of SFG, w1, and w2 light fields. Effective susceptibilities are linear combinations of Cartesian elements of χ(2) tensor. Their concrete forms for SFG signals generated in the reflection direction are given as: (achiral polarization combinations) (2) χ eff,SSP = LYY (ω s ) LYY (ω 1 ) LZZ (ω 2 ) sin θ 2 χ YYZ ,

(9.16)

(2) χ eff,SPS = LYY (ω s ) LZZ (ω 1 ) LYY (ω 2 ) sin θ1 χ YZY ,

(9.17)

(2) χ eff,PSS = LZZ (ω s ) LYY (ω 1 ) LYY (ω 2 ) sin θ s χ ZYY ,

(9.18)

(2) χ eff,PPP = − L XX (ω s ) L XX (ω 1 ) LZZ (ω 2 ) cos θ s cos θ1 sin θ 2 χ XXZ

− L XX (ω s ) LZZ (ω 1 ) L XX (ω 2 ) cos θ s sin θ1 cos θ 2 χ XZX + LZZ (ω s ) L XX (ω 1 ) L XX (ω 2 ) sin θ s cos θ1 cos θ 2 χ ZXX

+ L XX (ω s ) LZZ (ω 1 ) LZZ (ω 2 ) sin θ s sin θ1 sin θ 2 χ ZZZ .

(9.19)

(chiral polarization combinations) (2) (2) χ eff,SPP = LYY (ω s ) L XX (ω 1 ) LZZ (ω 2 ) cos θ1 sin θ 2 χ YXZ



(2) + LYY (ω s ) LZZ (ω 1 ) L XX (ω 2 ) sin θ1 cos θ 2 χ YZX ,

(9.20)

(2) (2) χ eff,PSP = − L XX (ω s ) LYY (ω 1 ) LZZ (ω 2 ) cos θ s sin θ 2 χ XYZ



(2) + LZZ (ω s ) LYY (ω 1 ) L XX (ω 2 ) sin θ s cos θ 2 χ ZYX ,

(9.21)

(2) (2) χ eff, − L XX (ω s ) LZZ (ω 1 ) LYY (ω 2 ) cos θ s sin θ1 χ XZY PPS =



(2) + LZZ (ω s ) L XX (ω 1 ) LYY (ω 2 ) sin θ s cos θ1 χ ZXY .

(9.22)

The experimental scheme and the coordinate systems assumed are shown in Fig. 9.1. θ1 and θ2 are the incidence angles of w1 and w2 beams, and θs is the emitting angle of

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the SFG signal. LIIs (I = X, Y, Z) are the Fresnel factors, which relate the input fields to the fields in a sample, and also relate the induced nonlinear polarization to the signal field. They are functions of refractive indices of samples and the incidence and emitting angles. The concrete forms of the Fresnel factors are given in Appendix A. The chiral Cartesian elements of one enantiomer are the negatives of the corresponding elements of the other enantiomer, and the effective susceptibility for a chiral polarization combination is a linear combination of the chiral Cartesian elements. As a result, in a given chiral polarization combination, the effective susceptibility of one enantiomer and that of the other have the same magnitude but opposite signs, giving the SFG signal fields with opposite signs. Therefore, we can distinguish one enantiomer from the other by measuring the sign, or phase of the SFG field.

2.6  Modes of SFG signal measurement 2.6.1  Narrowband IR scheme and multiplex scheme of SFG spectral measurement Usually, either of the following two schemes of measurement is employed to record VSFG spectra; one is the narrowband IR scheme, and the other is the multiplex scheme. In the narrowband IR scheme, the narrow band IR (w2) light is used and the SFG light intensity detected with a single channel detector is recorded as a function of IR frequency. This scheme has been used since the first observation of VSFG spectra from an adsorbate monolayer in 1987 [38]. In the multiplex scheme, broadband IR light (w2) and narrowband visible/UV light (w1) are employed to generate a broadband SFG signal, which is spectrally dispersed and then is detected with an optical multichannel detector [39–42]. A vibrational spectrum of a finite frequency range (typically 100–300 cm–1) can be obtained without scanning IR frequency; therefore this scheme is particularly suitable to time-resolved measurements [43,44]. There is another measuring scheme with broadband IR light, where the IR light is modulated with a Michelson type interferometer; however, this scheme has been rarely employed to measure ordinary one-dimensional VSFG spectra [45,46].

2.6.2  Intensity measurement and phase-sensitive measurement of SFG signals In conventional SFG spectroscopy, the intensity (Is) of the signal light from a sample is detected. We refer to this method as the homodyne detection in this article. Is(ws) is 2 proportional to the square modulus of the SFG signal field, Es, L (ω s ) , which is, in turn, 2 (2) proportional to the square modulus of the susceptibility, χ eff ,LMN . As a result, the phase (2) information of the effective susceptibility χ eff ,LMN is entirely lost. The SFG susceptibility is a complex quantity, and the phase (argument), as well as the magnitude, is significant. The phase of the SFG susceptibility has important relationships with the polar orientation and chirality of molecules. The phase of the susceptibility is changed by π, when the polar orientations of molecules at the interface are reversed [20], or chiral molecules in the sample themselves are converted into their mirror image [7].

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325

When we measure chiral VSFG signals in a chiral polarization combination with the homodyne detection, the sign (or phase) information critical to distinguish enantiomers is lost; we can judge whether the sample is chiral or not, but we cannot tell which enantiomer is dominant. This disadvantage of the homodyne detection can be partially circumvented by using so-called PMP polarization combinations, where M (“mixed”) refers to a linear polarization of w1 light, the electric field of which is at ± 45° from the incidence plane [7]. The effective susceptibilities for these combinations are: (2) χ (2) = χ (2) ± χ eff 2, ,PPP ) / eff ,± ( eff ,PSP

(9.23)

and the signal intensity difference between + and – combinations can be expressed as: 2

2

(2) (2) (2) (2) * (2) * (2) I − I ∝ χ eff = χ eff ,+ − χ eff ,− ,PSP ( χ eff ,PPP ) + ( χ eff ,PSP ) χ eff ,PPP . + −

(9.24)

The difference signal is inverted when the sample is switched from one enantiomer to (2) (2) the other because the phase of χ eff ,PSP is changed by π, while that of χ eff ,PPP remains the same. Therefore, we can distinguish enantiomers by the sign of the difference signal. However, this method is technically cumbersome. Moreover, it is impossible to (2) (2) extract χ eff ,PSP spectrum from observed data in the absence of χ eff ,PPP spectrum with phase information. To determine the phase of the SFG susceptibility, the phase of the SFG signal field relative to w1 and w2 fields is necessary. Mixing the SFG field from the sample with a reference field of a known phase enables us to determine the relative phase of the SFG field. Shen and coworkers have first demonstrated such a phase measurement of VSFG signals on a pentadecane acid monolayer at the air/water interface in the narrowband IR scheme [47]. Later, the technique has been improved and applied to a crystalline quartz/water interface [48] and air/water interfaces [49] to study polar orientations of water molecules. This method was named phase sensitive VSFG (PSVSFG) spectroscopy [50]. Phase measurement of the SFG susceptibility has been extended to the multiplex scheme by Tahara and coworkers, first for electronic SFG of p-nitroaniline at an air/silica interface [19] and then for VSFG of surfactants at air/ water interfaces [20]. They called the technique heterodyne-detected (HD) SFG or VSFG (HD SFG or HD VSFG) spectroscopy. The SFG light field generated in a medium other than the target sample with w1 and w2 lights from the identical light source is usually employed as the reference field, which is called the local oscillator (LO). The superposition of the two SFG fields results in the light intensity expressed as follows: I HD (ωs ) ∝

=

(2) Esample (ωs ) + ELO (ωs ) 2

2

2

(2) (2) (ωs ) Esample (ωs ) + ELO (ωs ) + 2 Re ELO (ωs )* Esample

(9.25)

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(2) where Esample (ωs ) is the SFG signal field of the target sample, and ELO (ωs ) the field of the local oscillator. The magnitude of the interference term (the last term) depends on (2) (2) the relative phase between Esample and ELO, and the phase of Esample can be obtained by modulating the phase of ELO. The specific procedure to extract the phase of the susceptibility of the sample will be presented in Section 3. Employing the heterodyne-detection for chiral SFG spectroscopy has three advantages: (1) the heterodyne detection enables us to measure the SFG susceptibility χ(2) with the phase that carries direct information concerning the chirality of the sample; (2) the LO also serves as an “amplifier” of the SFG field of the sample, resulting in enhancing the sensitivity of chiral SFG; (3) the location of the signal source, bulk or interface, can be inferred from the observed phase as will be explained in Section 4.1.

3  Experimental setup and the analysis of observed data in HD chiral VSFG We have measured the phase of the chiral VSFG susceptibility for the first time with a multiplex heterodyne-detected (HD) VSFG spectrometer [17]. In this section, we will describe our spectrometer and the method for analyzing raw data to obtain the chiral and achiral susceptibility of a sample.

3.1 Multiplex HD VSFG spectrometer Our spectrometer was constructed after the work by Tahara and coworkers [20]. It adopts a frequency-domain interferometric technique [51,52] in which a frequencyindependent time delay, T, is introduced into the LO field by passing the LO light beam through a glass plate whose refractive index can be regarded as constant for a certain frequency range of the LO field (ws). The introduced time-delay corresponds to phase modulation proportional to wsT, resulting in a fringe pattern on the detected spectrum IHD(ws). We separate the fringe component obtained from the target sample by the Fourier analysis and compare with that obtained from the standard sample with a known susceptibility to deduce the complex susceptibility of the target sample. A schematic diagram of our multiplex HD-SFG spectrometer is shown in Fig. 9.3 [17]. A femtosecond regenerative amplifier (Legend Elite, Coherent) is used to generate fundamental pulses (repetition rate: 1 kHz, center wavelength: 800 nm, pulse width: ∼100 fs, pulse energy: ∼3.5 mJ). One-third of the output is used to pump a white-light-seeded optical parametric amplifier (TOPAS-C, Coherent) to generate a broadband IR beam. The remaining two-thirds are introduced into a narrow band second harmonic generator (SHBC, Coherent), which generate a second harmonic (400 nm) with a picosecond time duration (∼5 ps) and a narrow bandwidth (8∼10 cm–1) [41,42]. The output of SHBC then pumps the other white-light-seeded optical parametric amplifier (TOPAS 400, Coherent). The signal output or the

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327

Figure 9.3  Schematic diagram of the experimental setup for multiplex HD chiral VSFG spectroscopy.

second harmonic of the signal or idler output of the 400-nm-pumped optical parametric amplifier is used as w1 light, which is wavelength-tunable in the UV/visible region from 235 to 795 nm. The w1 (UV/visible) and w2 (IR) beams are overlapped in a y-cut quartz thin plate, whose thickness is 10 µm, to generate a broadband SFG signal as the LO. The LO is delayed relative to the IR and UV/visible pulses in time by 2.5 ps with a fused silica plate (1.5 mm thickness) between the quartz plate and the concave mirror. The LO, and w1 and w2 beams transmitted through the quartz plate are refocused onto the sample by a concave mirror. The incident angles of the w1 and w2 beams are ∼73° and ∼62°, respectively. The reflected LO and the SFG signal generated at the sample pass through a polarization analyzer for selecting the polarization of the detected light. They were subsequently introduced into a polychromator (TRIAX550, Horiba Jobin Yvon, focal length: 550 mm, grating: 2400 grooves/mm, 330 nm blaze) after a low-dispersive pre-monochromator (CT25-UV, JASCO, focal length: 250 mm, dispersive elements: a prism (silica or S-BSM2), or a grating (100 grooves/mm, 780 nm blaze)) and interfered with each other in the frequency domain. The intensity spectrum with an interference fringe is finally detected by a UV-sensitive back-illuminated LN-cooled CCD camera (LN/CCD-1340/400-EB, Roper Scientific). To calibrate the data, we need to collect the signals from the sample and a reference sample at the same experimental conditions. A z-cut crystalline quartz plate is usually used as the reference sample.

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3.2  Method for analyzing raw data to calculate the susceptibility of a sample Raw data acquired by the experimental procedure have to be analyzed to obtain the (2) SFG field from a sample, Esample (ωs ) . The analyzing procedure for calculating the susceptibility of a sample from observed raw data is described in this section [17,19,20]. The total electric field analyzed with the polychromator equipped with the multichannel detector can be expressed using the Fourier transform as: − iω s t    + ELO (ω s )e− iω s ( t −T )  dω s , (9.26) E (t ) = Esample (t ) + ELO (t − T ) = ∫  Esample (ω s )e

where E sample (t ) is the SFG field generated at the sample, E LO (t − T ) is the LO field with the time delay T by the delay plate. E LO (t ) should be regarded as the LO field if the delay plate were not inserted. Here, the tilde (∼) is used to denote a quantity that varies rapidly in time. Then, the observed raw spectrum (Fig. 9.4A) is: I HD (ω s ) ∝

=

Esample (ω s ) + ELO (ω s )eiω sT 2

Esample (ω s ) + ELO (ω s )

2

2

(9.27)

* * + E sample (ω s )ELO (ω s )e+ iω T + Esample (ω s )ELO (ω s )e− iω sT .

The fourth term can be separated because the inverse Fourier transform of the fourth term is localized at around t = –T, while the first and second terms appear at around t = 0 and the third term at around t = T as shown in Fig. 9.4C. We convert IHD(ws) into its inverse Fourier transform, filter out except the fourth term with an appropriate

Figure 9.4  Raw HD spectra (IHD) of a sample (neat liquid R-limonene) (A) and the reference (a z-cut crystalline quartz plate) (B). The inverse Fourier transforms of (A) and (B) are shown in (C) and (D), respectively. The filter function is also plotted to the right ordinate in (C) and (D). Imaginary (solid line) and real (dashed line) parts of the filtered heterodyne spectra of the sample (E) and the reference (F).

Heterodyne-detected chiral vibrational sum frequency generation spectroscopy

329

numerical filter (Fig. 9.4C), and then transform the filtered time-domain data back into a frequency-domain spectrum (Fig. 9.4E). The complex interferogram of the sample thus obtained is: * − iω sT . Asample (ω s ) ∝ Esample (ω s )ELO (ω s )e

(9.28)

The SFG signal field can be expressed as the product of the effective susceptibility of the sample, w1 field, and w2 field; and the proportional constant depends on whether the sample is interfacial and bulk as follows: (2) Esample (ωs ) = C i χ eff ,sample E (ω1 ) E (ω 2 ),

(for an interfacial sample),

−1 (2) Esample (ω s ) = −C ∆kZ χ eff,sample E (ω 1 )E (ω 2 ),

(9.29)

(for a bulk sample),

(9.30)

where C is a constant determined by the experimental setup. ∆kZ is defined as: 1 1 nb (ω 1 )2 − na (ω 1 )2 sin 2 θ1 + λ λ2 ∆kZ = 2 π 1 1 2 2 2 + nb (ω s ) − na (ω s ) sin θ s λs

nb (ω 2 )2 − na (ω 2 )2 sin 2 θ 2 , (9.31)

where λ1, λ2, and λs are the wavelengths in vacuum of w1, w2, and SFG lights, respectively. na and nb are the refractive indices of media a and b, respectively. (See Fig. 9.1.) ∆kZ is known as the phase mismatch in the bulk sample media, and its inverse ∆kZ–1 is sometimes called the coherence length [4,53,54]. In the following section, equations for an interfacial sample will be given. Corresponding equations for a bulk sample can (2) −1 (2) be obtained by replacing χ eff ,sample with i ∆kz χ eff ,sample . We also rewrite ELO(ws) to factor in the reflectivity of the LO field, rsample(ws), at the sample interface as: ELO (ωs ) = rsample (ωs ) ELO,0 (ωs ).

(9.32)

Taking Eqs. (9.28), (9.29), and (9.32) together, the complex interferogram of an interracial sample is: (2) * * − iω sT . Asample (ω s ) ∝ C i χ eff,sample (ω s )E (ω 1 )E (ω 2 )rsample (ω s )ELO,0 (ω s )e

(9.33)

The raw data measured on the reference sample, the z-cut quartz, is also converted into the complex interferogram in the same procedure as: −1 (2) * * − iω sT . Aquartz (ω s ) ∝ −C ∆kZ,quartz χ eff,quartz E (ω 1 )E (ω 2 )rquartz (ω s )ELO,0 (ω s )e

(9.34)

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(See Fig. 9.4B, D, F.) Here we use Eq. (9.30) for a bulk sample because the SFG signal of the quartz is from bulk. Combining Eq. (9.33) with Eq. (9.34), we obtain: (2) χ eff,interfacesample (ω s ) = i

* −1 ∆kZ,quartz rquartz (ω s ) Ainterfacesample (ω s ) * rsample (ω s )

Aquartz (ω s )

(2) χ eff,quartz

(9.35)

for an interfacial sample. Note that the position and orientation of the quartz plate should be as similar as possible to those of the sample. Otherwise, Cs in Eqs. (9.33) and (9.34) become different, leading to an error in the phase of the effective susceptibility of the sample. We usually control the position in Z direction (normal to the surface of a sample) with an accuracy of less than 1 µm by using a displacement sensor (LT-8110, Keyence).

4  Applications of HD chiral VSFG spectroscopy 4.1  Neat liquid limonene Fig. 9.5A shows the imaginary and real parts of HD chiral VSFG spectra of R-, S-, and racemic mixture of limonene [17]. Limonene is a typical chiral organic liquid, whose structures are shown in Fig. 9.5D. The electronic resonance of limonene in the longest wavelength region is around 190 nm, which is far from the wavelength of the visible laser (532 nm) in the experiment. In the imaginary parts, it is clearly seen that the enantiomers showed several stretching bands with opposite signs, while the racemic mixture showed no discernible signal. These results show that enantiomers can be distinguished by the phase of the electric field of a chiral VSFG signal, which is obtained from the heterodyne detection. Chiral VSFG signals can be generated both from chiral bulk and chiral interfaces with uniaxially oriented structures. It is required to determine whether the bulk or the interface contributes to a chiral signal when chiral molecules locate both in bulk and at the interface. In the electric dipole approximation, the phase of the sum frequency electric field differs by π/2 depending on whether the SFG signal is from the bulk or the interface as mentioned in Eqs. (9.29) and (9.30) [4,55]. Fig. 9.5A shows the imaginary and real parts of HD chiral VSFG spectra obtained by using Eq. (9.30) on the assumption that the chiral VSFG signals come from bulk, while Fig. 9.5B those obtained by using Eq. (9.29) on the assumption that the chiral VSFG signals come from the interface. As explained in Section 2.3, when both SFG signal (ws) and visible light (w1) are far from electronic resonances, the vibrational resonant term is the sum of the complex Lorentz functions with real numerators, whose imaginary and real parts are peak and dispersive line shape (Fig. 9.5C). Therefore, by comparing Fig. 9.5A–C, we conclude that the chiral VSFG signal in reflection from neat liquid limonene originated from the bulk not from the interface.

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Figure 9.5  HD-chiral VSFG spectra from R-, S-, and a racemic mixture of limonene in the PSP polarization combination. (A) Imaginary parts and real parts of the spectra on the correct assumption that the signals come from bulk. (B) Imaginary parts and real parts of R-, S-, and a racemic mixture of limonene on an incorrect assumption that the signals come from the interface. The units of vertical axes in Fig. 9.5A,B are different because their assumed spatial origins are different (See Section 2.3). (C) The imaginary and real parts of a Lorentz function with a real numerator, L(ω ) = −1 / (ω − Ω + iΓ). (D) Molecular structures of limonene. Reproduced from M. Okuno, T. Ishibashi, The Journal of Physical Chemistry Letters 5 (2014) 2874 with the permission. Copyright (2014) American Chemical Society.

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In this study, we employed the reflection geometry, whose coherence length is much shorter than that of the transmission geometry. The coherence length, in which the VSFG signal constructively generates, corresponds to the effective sample thickness for a bulk sample. In our measurement, the coherence length is estimated to be ∼30 nm, meaning that the chiral VSFG signal was generated in a very thin volume. Before our work, Shen et al. had reported that they could detect chiral VSFG signals only in the transmission geometry, whose coherence length was several µm, while not in the reflection geometry [7], whose coherence length was shorter than that of transmission by two orders of magnitude [53]. In our case, the signal was amplified by the LO field employed in the heterodyne detection, which resulted in the improvement of the detection limit. Thus, the heterodyne detection enables us to detect signals which were not measurable in conventional (homodyne-detected) chiral VSFG spectroscopy.

4.2  Vibrationally-electronically doubly-resonant chiral SFG of chiral solutions When the SFG signal wavelength approaches an electronic transition of the molecule, the signal greatly increases, just as resonance Raman scattering. VSFG under electronic resonance is often called as vibrationally-electronically doubly-resonant (DR) SFG. The intensity enhancement due to the electronic resonance effect in the homodyne detected chiral VSFG bands was reported to be 105∼106 [9]. It has been demonstrated that DR chiral SFG spectroscopy detects optical activity of molecules with chromophores with high sensitivity [9–11,56]. We have developed heterodyne-detected doubly-resonant (HD-DR) chiral SFG spectroscopy, which allows us to detect and distinguish chirality with high sensitivity [25]. We measured acetone solutions of R- and S-1,1’-bi-2-naphthol (BINOL). Fig. 9.6 shows the electronic absorption spectrum of BINOL in acetone. In order to obtain the electronic resonance effect around 335 nm, we set the wavelength of the visible beam to ∼350 nm in the near-UV region. The spectra were measured with an exposure time of 1 min. In the previous study of the same sample, it was suggested that chiral SFG signals are generated from bulk [9]. Fig. 9.7 shows the imaginary and real parts of HD-DR chiral SFG spectra from 300 mM R- and S-BINOL solutions in acetone. The two HD-DR chiral SFG spectra showed mirror-image features with opposite phases, demonstrating that the heterodyne detection enables us to distinguish the enantiomers in the doubly resonance condition. We performed the experiment of the concentration dependence of HD-DR chiral signals to evaluate the detection limits and linear scaling character of HD-DR chiral spectroscopy. In Fig. 9.8, the amplitude of the band at 1379 cm−1 extracted from the real part was plotted against the (enantiomer) concentration. The result suggests that the 20 mM solution was successfully detected with 1 min exposure time. The concentration of neat limonene liquid is 6.2 M, which is more than 102 times higher than that of the 20 mM BINOL solution. This means that the electronic resonance effect enhanced an HD chiral VSFG signal amplitude by a factor of 102. Fig. 9.8 also demonstrates that the signal amplitudes are almost linear to the molecular (enantiomer)

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Figure 9.6  Molecular structures of R- and S-BINOL. The electronic absorption spectrum of 1.2 × 102 µM BINOL solution in acetone. Reproduced from M. Okuno, T. Ishibashi, Analytical Chemistry 87 (2015) 10103 with the permission. Copyright (2015) American Chemical Society.

Figure 9.7  HD-DR chiral SFG spectra from 300 mM S- and R-BINOL acetone solutions. (A) Imaginary parts and (B) real parts. The spectra were measured with an exposure time of 1 min. Reproduced from M. Okuno, T. Ishibashi, Analytical Chemistry 87 (2015) 10103 with the permission. Copyright (2015) American Chemical Society.

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Figure 9.8  Scaling of the HD-chiral VSFG amplitude with the concentrations of S- and R-BINOL solutions. Reproduced from M. Okuno, T. Ishibashi, Analytical Chemistry 87 (2015) 10103 with the permission. Copyright (2015) American Chemical Society.

Figure 9.9  Imaginary spectra from a pure S-BINOL solution of 300 mM (red line) and the mixture of S- and R-BINOL (6:4) (blue line). The spectrum of the mixed solution is enlarged 5 times (dotted line). Reproduced from M. Okuno, T. Ishibashi, Analytical Chemistry 87 (2015) 10103 with the permission. Copyright (2015) American Chemical Society.

concentration, suggesting that HD chiral VSFG spectroscopy can directly provide the quantitative information on enantiomers. In addition, we evaluated an enantiomeric excess by HD-DR chiral SFG spectroscopy. Fig. 9.9 shows the imaginary spectra of a pure S-BINOL solution and a mixed solution of S- and R-BIONL solutions of 300 mM with the volume ratio of 6:4, equivalent to a pure S-BINOL solution of 60 mM. The spectra are similar to each other, indicating that the enantiomeric excess of the mixed solution was properly estimated. In this study, we showed that HD-DR chiral SFG spectroscopy is useful as an analytical tool for quantitative determination of chirality with high sensitivity.

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4.3  Vibrationally-electronically doubly-resonant chiral SFG of chiral monolayers – electronic excitation profiles of complex chiral susceptibilities As demonstrated in the previous section, the sensitivity of chiral VSFG spectroscopy is significantly enhanced under electronic resonant conditions. With the enhanced sensitivity, even the chirality of monolayers can be detected [9]. Moreover we successfully distinguished the chirality senses of monolayers on water in situ by HD-DR chiral SFG spectroscopy [26]. The monolayers of chiral amphiphiles with the 1,1’-binaphthyl skelton [57] (R-1 and S-1 in Fig. 9.10) at the air/water interface were measured. Fig. 9.11 shows the imaginary and real parts of complex chiral VSFG spectra from Langmuir monolayers of R-1 and S-1. The enantiomers displayed opposite signs in the complex spectra, demonstrating that the chirality senses of the monolayers were properly distinguished. To our best knowledge, this result is the first report on the distinction of the chirality senses of monolayers on water. It is virtually impossible to measure the chirality of Langmuir monolayers directly by conventional chiroptical techniques such as CD, VCD, and ROA. In contrast, our method, HD chiral VSFG spectroscopy, readily distinguished the chirality senses of the Langmuir monolayers in situ, showing its great potential for various unique applications. We also investigated the mechanism of the electronic resonance effect in chiral VSFG signals. As described in Section 2.3, the susceptibility of a sample is proportional to the average of the hyperpolarizabilities of molecules, and vibrational resonant terms of the hyperpolarizability tensor are the product of the Raman tensor and the IR transition moment. As a result of orientational averaging of the molecular hyperpolarizability, only the antisymmetric part of the Raman tensor contribute to chiral elements of the susceptibility of bulk liquid samples [7]. In contrast, both the

Figure 9.10  Molecular structures of R- and S-form binaphthyl derivatives (R-1 and S-1). Reproduced from M. Okuno, D. Ishikawa, W. Nakanishi, K. Ariga, T. Ishibashi, The Journal of Physical Chemistry C 121 (2017) 11241 with the permission. Copyright (2017) American Chemical Society.

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Figure 9.11  HD-DR chiral SFG spectra from S- and R-binaphthyl derivative monolayers on water. (A) Imaginary parts and (B) real parts. Reproduced from M. Okuno, D. Ishikawa, W. Nakanishi, K. Ariga, T. Ishibashi, The Journal of Physical Chemistry C 121 (2017) 11241 with the permission. Copyright (2017) American Chemical Society.

symmetric and antisymmetric Raman tensors can contribute to chiral elements of interfaces. However, no direct experimental evidence that the symmetric Raman tensor actually contributes to chiral VSFG signals had been presented before our investigation. We proposed that the symmetric Raman tensor contributes to the chiral VSFG signals from the oriented monolayers on the basis of experimentally obtained electronic excitation profiles of vibrationally resonance susceptibilities. By varying the visible wavelength, we obtained electronic excitation profiles of the amplitude and phase of VSFG bands. Only the excitation profile of the signal intensity, the square of the amplitude, had been used to evaluate the electronic resonance effect before [9,11]. In this study, taking advantage of the heterodyne detection, we obtained the excitation profile of the phase of DR-VSFG bands, which may provide us helpful information on the electronic structures of the molecules. Fig. 9.12A–C show the electronic excitation profiles of the amplitude and phase of the chiral VSFG signal (PSP polarization combination) from a BINOL solution in tetrahydrofuran (THF), the achiral signal (SSP polarization combination), and the chiral signal (PSP polarization combination) from the monolayers.

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Figure 9.12  Electronic excitation profiles of the amplitudes (solid lines, left axes) and phases (dotted lines, right axes) of the 1350 cm−1 band for chiral SFG (PSP) signals from (A) the S-BINOL solution, (B) the achiral SFG (SSP) signals, and (C) chiral SFG (PSP) signals from the S-1 monolayer. The amplitudes and phases are plotted against the wavelengths of the SFG signals. Reproduced from M. Okuno, D. Ishikawa, W. Nakanishi, K. Ariga, T. Ishibashi, The Journal of Physical Chemistry C 121 (2017) 11241 with the permission. Copyright (2017) American Chemical Society.

The previous study showed that chiral VSFG signals from a BINOL solution in THF originate from bulk, meaning that only the antisymmetric tensor contributes to the signal [8]. In contrast, the achiral signal from the monolayers in the SSP polarization combination originates solely from the symmetric Raman tensor [35]. The observed amplitude and phase of the chiral signal from the bulk solution in PSP combination (Fig. 9.12A) depended on the excitation wavelength more steeply than those of the achiral signal from the monolayer in SSP combination (Fig. 9.12B). The amplitude of the bulk solution dramatically decreases as the excitation wavelength moves from the electronic resonance to long wavelengths. Thus, the chiral VSFG signal from the bulk solution showed steep dependence on the excitation wavelength. Shen and coworkers theoretically showed that the antisymmetric Raman tensor more sharply depends on the excitation wavelength around the electronic resonance than the symmetric one under the Born-Oppenheimer adiabatic approximation with nonadiabatic corrections [58]. The observed difference in the excitation profiles of the chiral signal from the bulk solution and the achiral signal from the monolayer reflects the difference in the contributions from the symmetric and antisymmetric Raman tensors. The excitation profiles of the PSP chiral signals from the monolayer (Fig. 9.12C) were similar to those of the achiral signals from the monolayer rather than those of the chiral signals from the bulk solution. The similarity indicates that the chiral signal from the monolayer originated not from the antisymmetric Raman tensor but the symmetric one as the SSP achiral signal from the monolayer. The present result is the first experimental evidence that the symmetric Raman tensor plays an essential role in the chiral VSFG signal in the interfacial systems of uniaxially oriented molecules.

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4.4  Polymer thin films – bulk-or-interface assignment by polarization dependence Chiral VSFG signals from bulk media are zero under the rigorously electronic nonresonant condition, while significant chiral VSFG signals can be generated from interfacial systems with uniaxially oriented structures even under the same condition. PSP, SPP, and PPS polarization combinations are chiral-specific both for bulk and interfacial samples. In Section 4.1, we mentioned that the spatial origin of a chiral VSFG signal can be deduced from the phase of a chiral VSFG band. This method is simple and effective in the case that spectral features are uncomplicated. However, it is difficult to apply to spectra that have several vibrational bands overlapping with one another. We proposed a new approach based on the polarization dependence of a chiral VSFG signal to ascribe its spatial origin [27]. We showed that chiral VSFG signals in reflection from bulk, in which molecules randomly oriented, heavily depend on polarization combinations, while those from interfaces, at which molecules azimuthally isotropically orient, much less. By comparing the ratio of the effective susceptibilities in different polarization combinations with simulated values for the bulk and interfacial samples, the spacial origin of chiral VSFG signals could be estimated. Actually we successfully took this approach to infer the origin for thin films of poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA), which form left- and right-handed helical structures, respectively. By using Eqs. (9.20) and (9.21), we derived the effective second-order nonlinear susceptibilities in reflection for both bulk and interfacial chiral samples in the SPP and PSP polarization combinations. In the case of bulk chiral samples, the substitution of Eq. (9.4) allows us to express the effective second-order nonlinear susceptibilities for semi-infinite bulk chiral samples in the SPP and PSP polarization combinations as: (2) χ eff ,SPP = {− LYY (ω s ) L XX (ω1 ) L ZZ (ω 2 ) cos θ1 sin θ 2



+ LYY (ωs ) LZZ (ω1 ) LXX (ω 2 )sin θ1 cos θ 2}χYZX ,

(9.36)

(2) χ eff ,PSP = {− L XX (ω s ) LYY (ω1 ) L ZZ (ω 2 ) cos θ s sin θ 2



− LZZ (ωs ) LYY (ω1 ) LXX (ω 2 )sin θ s cos θ 2}χYZX .

(9.37)

In the case of interfacial samples, Eqs. (9.20) and (9.21) can be simplified under the electronic non-resonant condition in a strict sense. In this case, a Raman tensor is purely symmetric, resulting in that the corresponding vibrationally resonant SFG susceptibility is symmetric with respect to the first and second indices. Hence, χXYZ = χYXZ and χYZX = χZYX. In addition, azimuthally isotropic symmetry yields χXYZ = –χYXZ as shown in Eq. (9.3), resulting in χXYZ = χYXZ = 0. Thus, for isotropic interfaces under the electronic non-resonant condition, we obtain, (2) ′ (ω1 ) LXX (ω 2 )sin θ1 cos θ 2 χYZX , χ eff ,SPP ∝ LYY (ω s ) L ZZ (9.38)

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Figure 9.13  The ratios of the effective second-order nonlinear susceptibilities in the SPP (2) (2) and PSP polarization combinations, χ eff ,SPP / χ eff ,PSP , for an interfacial system (A) and a bulk system (B). The ratios are plotted against the incidence angles of the visible beams with those of the infrared beams shown in the legend. The positive and negative IR incidence angle correspond to the co- and counter-propagating geometries. (See Fig. 9.1 for a co-propagating geometry.) Reproduced from M. Okuno, T. Ishibashi, The Journal of Chemical Physics 149 (2018) 244703 with the permission. Copyright (2018) American Institute of Physics.

(2) ′ (ωs ) LYY (ω1 ) LXX (ω 2 )sin θ s cos θ 2 χYZX . χ eff ,PSP ∝ LZZ

(9.39)

ZZ-component of the Fresnel factor for the interface is different from that for the bulk, ′ while XX- and YY-components are common to the interface and the bulk. Here, LZZ and LZZ denote the ZZ-components for the interface and the bulk, respectively. (See (2) (2) Appendix A) Eqs. (9.36)–(9.39) show that both χ eff ,SPP and χ eff ,PSP are proportional to a single Cartesian element, χYZX, regardless of interfacial samples or bulk samples. (2) (2) (2) (2) In Fig. 9.13, the ratios of χ eff ,SPP to χ eff ,PSP , χ eff ,SPP / χ eff ,PSP , calculated using Eqs. (9.36)–(9.39) are plotted as a function of the incident angles of visible and IR beams for interface and bulk systems. The ratios for the interface and the bulk are significantly different. In particular, when both θ1 and θ2 are between 45° and 80°, corresponding to typical VSFG setups employing co-propagating IR and visible beams, the ratio is 0.9∼1.2 for the interface, while it is −0.3∼0.3 for the bulk. In the measurement of chiral VSFG signals from the polymer thin films, we employed a co-propagating geometry, and the incidence angles of the visible and IR lasers were 70° and 60°, respectively. In this case, χSPP/χPSP from the interface is estimated to be approximately 1.0, while that from the bulk was −0.03. The values for the interface and the bulk are so different that we may easily judge which of the interface and the bulk fits an experimental result. Thus, one could deduce the spatial origin of a chiral VSFG signal from the ratio χSPP/χPSP. When the ratio is much smaller than 1, we deduce that the origin would be bulk. Otherwise, we conclude that contribution from the interface cannot be negligible. Fig. 9.14A–D show the imaginary and real parts of HD chiral VSFG spectra in PSP and SPP polarization combinations from thin PLLA and PDLA films, whose

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Figure 9.14  (A) Imaginary and (B) real parts of the HD-chiral VSFG spectra from the thin films of PLLA (red) and PDLA (blue) in the PSP polarization combination. (C) Imaginary and (D) real parts of the HD-chiral VSFG spectra from the thin films of PLLA (red) and PDLA (blue) in the SPP polarization combination. Reproduced from M. Okuno, T. Ishibashi, The Journal of Chemical Physics 149 (2018) 244703 with the permission. Copyright (2018) American Institute of Physics.

thicknesses were estimated to be 200–400 nm. All the spectra were calculated from raw data on the assumption that chiral VSFG signals came from the interfaces. The spectra of the PLLA and PDLA films showed similar features except that their signs were opposite. This observation demonstrates that we properly distinguished the chirality senses of the polymer films by their chiral VSFG spectra. Importantly, the PSP and SPP spectra of each enantiomer were similar in spectral features: band shape, amplitude, and sign. If the chiral VSFG signals came solely from bulk, the SPP signal should be much smaller than the PSP signal. In contrast, the observed ratio was roughly consistent with the calculated ratio for interface samples under the electronic non-resonant condition, under which only the symmetric Raman tensor contributes to chiral VSFG signals. The agreement between the observed and calculated values suggests that the signals from the films were originated from the interfaces and were contributed from the symmetric Raman tensor.

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It should be noticed that the spectra from the PLLA and PDLA films were so complicated that we cannot deduce their spatial origin from the phases of chiral VSFG bands as we did for liquid limonene in Section 4.1. In this study, we proposed a new method to deduce the spatial origin of chiral VSFG signals from their polarization dependences. The ratio of effective susceptibilities in the SPP and PSP polarization combinations is a useful measure that could enables us to readily judge whether the bulk or the interface contributes to the chiral VSFG signals.

4.5  Air/protein solution interfaces Recently, chiral VSFG spectroscopy has been attracting much attention as a powerful tool for analyzing secondary structures of peptides and proteins at interfaces. Yan and coworkers suggested that chiral VSFG signals from peptides and proteins in the amide I and NH stretching regions were sensitive to their secondary structures based on their homodyne detected experiments [59–61]. We measured both achiral and chiral HD VSFG spectra from three proteins adsorbed at air/aqueous solution interfaces [28]. The concentrations of the proteins were set to 2 mg/mL. The secondary structure contents of the proteins in solution phase are shown in Table 9.2 [62–64]. Bovine serum albumin (BSA), pepsin, and concanavalin A were selected as samples because their secondary structures in aqueous solutions had been well characterized by other methods, including X-ray diffraction and vibrational spectroscopy. BSA is rich in αhelices [62], while pepsin and concanavalin A contain abundant antiparallel β-sheets [63,64]. Fig. 9.15A–F show the imaginary and real parts of HD achiral VSFG spectra from the adsorbed proteins at the air/solution interfaces in the CH/OH stretching and amide I regions in the SSP polarization combination. The achiral spectra of the three proteins at the air/solution interfaces displayed spectral features similar to one another, implying that the achiral spectra were not sensitive to the secondary structures at the interfaces. It is well established that amide I bands in IR and Raman spectra from bulk solutions of proteins sensitively reflect their secondary structures. In contrast, all the achiral VSFG spectra of the three proteins at the interfaces exhibited a positive amide I band at approximately the same frequency around 1650 cm−1, indicating that the achiral amide I band in HD-VSFG spectra at air/water interfaces was not sensitive to the secondary structures, being different from IR and Raman spectra. Another likely interpretation is that disordered part of proteins, whose orientation may be controlled by the hydrophobic interaction at the air/water interface, gave rise to the achiral amide I band. Table 9.2  Secondary structural content (%) reported in the literature. Protein

α-helix

β-sheet

β-turn

BSA [62] Pepsin [63] Concanavalin A [64]

67 12 2

0 46 64

10 20 22

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Figure 9.15  HD-achiral (SSP)VSFG spectra from the air/BSA solution (A,D), the air/pepsin solution (B,E), and the air/concanavalin A solution (C,F) interfaces in the SSP polarization combination. (A–C) are in the CH-OH stretching region and (D–F) in the amide I region. Red and blue lines are imaginary and real parts, respectively. The intensity scales are normalized with the intensity of z-cut quartz. Reproduced from M. Okuno, T. Ishibashi, The Journal of Physical Chemistry 119 (2015) 9947 with the permission. Copyright (2015) American Chemical Society.

Figure 9.16  HD-chiral (PSP) VSFG spectra from the air/BSA solution (A,D), the air/pepsin solution (B,E), and the air/concanavalin A solution (C,F) interfaces in the PSP polarization combination. (A–C) are in the CH and OH stretching region and (D–F) in the amide I region. Red and blue lines are imaginary and real parts, respectively. The intensity scales are normalized with the intensity of z-cut quartz. Reproduced from M. Okuno, T. Ishibashi, The Journal of Physical Chemistry 119 (2015) 9947 with the permission. Copyright (2015) American Chemical Society.

Next, Fig. 9.16A–F show the imaginary and real parts of HD chiral VSFG spectra from the three proteins at the air/solution interfaces in the NH stretching and amide I regions. They were observed in the PSP polarization combination. We assumed that

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chiral signals came not from the bulk but from the interfaces in the spectral analysis. The assumption was justified by the imaginary and real parts of the obtained spectra, which have reasonably peak and dispersive shapes, respectively. In the spectra from pepsin and concanavalin A, which are rich in antiparallel βsheets in their aqueous solutions, one can see HD chiral VSFG bands with the positive sign both in the NH stretching and amide I regions. The amide I band located around 1630 cm−1, corresponding to previous studies by Yan's group. This observation indicates that the amide I observed from pepsin and concanavalin A originated from antiparallel β-sheets. Moreover, interestingly, a weak but significant amide I band was also observed from BSA, which is rich in α-helices in solution. Provided that the α-helix structure does not give the amide I band while the antiparallel β-sheet gives both the amide I and NH stretching bands as Yan and coworkers suggested [60], the observed amide I band from BSA suggests the conformational change in the protein to antiparallel β-sheets at the interface. The results of this study demonstrate that we can infer secondary structures at interfaces by referring to chiral VSFG spectra. In this study, all vibrational bands in both the NH stretching and amide I regions have positive signs in the imaginary parts of chiral VSFG spectra. The sign of the bands should contain the information on molecular orientations at interfaces. However, the model that explains the sign of the chiral VSFG signals has not been developed yet.

5  Concluding remarks Heterodyne-detected (HD) chiral VSFG spectroscopy is a powerful method for detecting and distinguishing the chirality of an isotropic bulk or azimuthally isotropic interfacial sample. The sensitivity of the method was so highly enhanced under electronic resonant conditions that we successfully distinguished the chirality senses of the binaphthyl derivative monolayers by the method without difficulty; discrimination between enantiomeric monolayers has never been achieved by conventional chiral vibrational spectroscopies. We also demonstrated that HD chiral VSFG spectra of the helical polymer films were practically interface-specific when measured in the reflection mode under the electronic non-resonant condition. One of the most important issues to be addressed concerning chiral VSFG is how to predict theoretically chiral VSFG band amplitudes and phases for a bulk or interfacial, chiral molecular system with a given absolute configuration. Little research has been reported especially on the method for calculating chiral VSFG spectra of bulk samples. This situation is in contrast with the case of CD, VCD, and ROA, for which quantum chemical calculations reproduce spectra relatively well in many instances. A challenge in developing such a method for chiral VSFG spectra seems to be the establishment of an efficient, reliable way to precisely evaluate the antisymmetric part of Raman tensors because such evaluation requires calculations beyond adiabatic approximations [36,37,58]. The interpretation of an observed chiral VSFG spectrum from an interface depends on both orientational distribution and hyperpolarizabilities of molecules involved. The phase of the susceptibility given by HD chiral VSFG should be valuable in the

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orientational analysis of chiral molecules at interfaces. HD chiral VSFG is suitable for time-resolved measurements because of the multiplex scheme. The time-resolved measurements would be extremely cumbersome, if not impossible, to perform with the scanning scheme. Time-resolved HD chiral VSFG combined with the pump-probe technique would open a way to study unexplored ultrafast dynamics of chirality-related photochemistry. We hope HD chiral VSFG will find many novel and practical applications, and develop into a tool of choice for studying chirality in the near future.

References [1] Y.R. Shen, Surface properties probed by second-harmonic and sum-frequency generation, Nature 337 (1989) 519–525. [2] J.A. Giordmaine, Nonlinear optical properties of liquids, Phys. Rev. 138 (1965) 1599– 1606. [3] C.D. Bain, Sum-frequency vibrational spectroscopy of the solid-liquid interface, J. Chem. Soc. Faraday T 91 (1995) 1281–1296. [4] Y.R. Shen, Fundamentals of Sum-Frequency Spectroscopy, Cambridge University Press, Cambridge, (2016). [5] M.A. Belkin, Y.R. Shen, Non-linear optical spectroscopy as a novel probe for molecular chirality, Int. Rev. Phys. Chem. 24 (2005) 257–299. [6] N. Ji, Y.R. Shen, A novel spectroscopic probe for molecular chirality, Chirality 18 (2006) 146–158. [7] M.A. Belkin, T.A. Kulakov, K.H. Ernst, L. Yan, Y.R. Shen, Sum-frequency vibrational spectroscopy on chiral liquids: a novel technique to probe molecular chirality, Phys. Rev. Lett. 85 (2000) 4474–4477. [8] M.A. Belkin, S.H. Han, X. Wei, Y.R. Shen, Sum-frequency generation in chiral liquids near electronic resonance, Phys. Rev. Lett. 87 (2001) 113001/113001-113001/113004. [9] M.A. Belkin, Y.R. Shen, Doubly resonant IR-UV sum-frequency vibrational spectroscopy on molecular chirality, Phys. Rev. Lett. 91 (2003) 213907/213901-213907/213904. [10] M. Oh-e, H. Yokoyama, S. Yorozuya, K. Akagi, M.A. Belkin, Y.R. Shen, Sum-frequency vibrational spectroscopy of a helically structured conjugated polymer, Phys. Rev. Lett. 93 (2004) 267402. [11] T. Nagahara, K. Kisoda, H. Harima, M. Aida, T. Ishibashi, Chiral sum frequency spectroscopy of thin films of porphyrin J-aggregates, J Phys Chem B 113 (2009) 5098–5103. [12] L. Fu, G. Ma, E.C.Y. Yan, In situ misfolding of human islet amyloid polypeptide at interfaces probed by vibrational sum frequency generation, J. Am. Chem. Soc. 132 (2010) 5405–5412. [13] L. Fu, J. Liu, E.C.Y. Yan, Chiral sum frequency generation spectroscopy for characterizing protein secondary structures at interfaces, J. Am. Chem. Soc. 133 (2011) 8094–8097. [14] E.C.Y. Yan, L. Fu, Z. Wang, W. Liu, Biological macromolecules at interfaces probed by chiral vibrational sum frequency generation spectroscopy, Chem. Rev. 114 (2014) 8471– 8498. [15] L.D. Barron, Molecular Light Scattering and Optical Activity, Cambridge University Press, Cambridge, (2004). [16] L.A. Nafie, Vibrational Optical Activity: Principles and Applications, John Wiley & Sons Ltd, West Sussex, (2011).

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[37] D.A. Long, The Raman Effect, John Wiley & Sons, Ltd, West Sussex, (2002). [38] X.D. Zhu, H. Suhr, Y.R. Shen, Surface vibrational spectroscopy by infrared-visible sum frequency generation, Phys. Rev. B 35 (1987) 3047–3050. [39] E.W.M. Van Der Ham, Q.H.F. Vrehen, E.R. Eliel, Self-dispersive sum-frequency generation at interfaces, Opt. Lett. 21 (1996) 1448–1450. [40] L.J. Richter, T.P. Petralli-Mallow, J.C. Stephenson, Vibrationally resolved sum-frequency generation with broad-bandwidth infrared pulses, Opt. Lett. 23 (1998) 1594–1596. [41] T. Ishibashi, H. Onishi, Multiplex infrared-visible sum-frequency spectrometer with a phase-conjugated pulse mixing device for narrow-bandwidth visible probe generation, Appl. Spectrosc. 56 (2002) 1298–1302. [42] T. Ishibashi, H. Onishi, A multiplex infrared-visible sum-frequency spectrometer with wavelength tunability of the visible probe, Appl. Phys. Lett. 81 (2002) 1338–1340. [43] M. Bonn, C. Hess, S. Funk, J.H. Miners, B.N.J. Persson, M. Wolf, et al. Femtosecond surface vibrational spectroscopy of CO adsorbed on Ru(001) during desorption, Phys. Rev. Lett. 84 (2000) 4653–4656. [44] P.C. Singh, S. Nihonyanagi, S. Yamaguchi, T. Tahara, Ultrafast vibrational dynamics of water at a charged interface revealed by two-dimensional heterodyne-detected vibrational sum frequency generation, J. Chem. Phys. 137 (2012) 094706-1–094706-6. [45] J.A. McGuire, W. Beck, X. Wei, Y.R. Shen, Fourier-transform sum-frequency surface vibrational spectroscopy with femtosecond pulses, Opt. Lett. 24 (1999) 1877–1879. [46] J.A. McGuire, Y.R. Shen, Signal and noise in Fourier-transform sum-frequency surface vibrational spectroscopy with femtosecond lasers, J. Opt. Soc. Am. B 23 (2006) 363–369. [47] R. Superfine, J.Y. Huang, Y.R. Shen, Phase measurement for surface infrared–visible sumfrequency generation, Opt. Lett. 15 (1990) 1276–1278. [48] V. Ostroverkhov, G.A. Waychunas, Y.R. Shen, Vibrational spectra of water at water/αquartz (0 0 0 1) interface, Chem. Phys. Lett. 386 (2004) 144–148. [49] N. Ji, V. Ostroverkhov, C.S. Tian, Y.R. Shen, Characterization of vibrational resonances of water-vapor interfaces by phase-sensitive sum-frequency spectroscopy, Phys. Rev. Lett. 100 (2008) 096102-1–096102-4. [50] Y.R. Shen, Phase-sensitive sum-frequency spectroscopy, Annu. Rev. Phys. Chem. 64 (2013) 129–150. [51] L. Lepetit, G. Cheriaux, M. Joffre, Linear techniques of phase measurement by femtosecond spectral interferometry for applications in spectroscopy, J. Opt. Soc. Am. B 12 (1995) 2467–2474. [52] P.T. Wilson, Y. Jiang, O.A. Aktsipetrov, E.D. Mishina, M.C. Downer, Frequency-domain interferometric second-harmonic spectroscopy, Opt. Lett. 24 (1999) 496–498. [53] X. Wei, S.-C. Hong, A.I. Lvovsky, H. Held, Y.R. Shen, Evaluation of surface vs. bulk contributions in sum-frequency vibrational spectroscopy using reflection and transmission geometries, J. Phys. Chem. B 104 (2000) 3349–3354. [54] X. Wei, S.C. Hong, X.W. Zhuang, T. Goto, Y.R. Shen, Nonlinear optical studies of liquid crystal alignment on a rubbed polyvinyl alcohol surface, Phys. Rev. E 62 (2000) 5160–5172. [55] K. Kemnitz, K. Bhattacharyya, J.M. Hicks, G.R. Pinto, K.B. Eisenthal, T.F. Heinz, The phase of second-harmonic light generated at an interface and its relation to absolute molecular orientation, Chem. Phys. Lett. 131 (1986) 285–290. [56] B. Busson, A. Tadjeddine, Chiral specificity of doubly resonant sum-frequency generation in an anisotropic thin film, J. Phys. Chem. C 112 (2008) 11813–11821. [57] D. Ishikawa, T. Mori, Y. Yonamine, W. Nakanishi, J.P. Hill, K. Ariga, et al. Mechanochemical tuning of the binaphthyl conformation at the air-water interface, Angew. Chem. Int. Ed. Engl. 54 (2015) 8988–8991.

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Appendix A Fresnel factors ZZ-component of the Fresnel factors has different forms for the bulk and the interface, while XX- and YY-components are common to them. In this review, ZZ-component for ′ , while that for the bulk as LZZ . LXX , LYY , LZZ , and LZZ ′ the interface is designated as LZZ for the geometry shown in Fig. 9.1 are as follows: 2na (ω ) cos γ LXX (ω ) = , n ( ω ) cos γ + nb (ω ) cos θ a

(A1a)

2na (ω ) cos θ LYY (ω ) = , na (ω ) cos θ + nb (ω ) cos γ

(A1b)

2

 na (ω )  2nb (ω ) cos θ LZZ (ω ) =   , na (ω ) cos γ + nb (ω ) cos θ  nb (ω ) 

(A1c)

2

 n (ω )  ′ (ω ) = LZZ (ω )  b LZZ  ,  n′(ω ) 

(A1d)

where θ is the incidence angle of an input beam, or the outgoing angle of the SFG signal beam. γ is the refractive angle into the medium b defined by na sin θ = nb sin γ ,

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where na and nb are the refractive indices of medium a and b, respectively. n′ is the refractive index of the interfacial layer [33,54]. n′ in the works of this article were calculated using the following expression [33,65]: n′ =

na2 + nb2 + 4 . 2(na−2 + nb−2 + 1)

(A2)

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Xiaoxia Han, Bing Zhao State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, P. R. China Chapter outline 1 SERS and its mechanisms: a brief introduction  350 2 SERS-active substrates  351 2.1 Noble metals  352 2.2 Transition metals  353 2.3 Semiconductors 353 2.3.1 Metal oxides  353 2.4 Semiconductor-metal heterostructures  357

3 Mechanism of SERS on semiconductor nanomaterials  357 3.1 3.2 3.3 3.4 3.5

Plasmon resonance  358 Mie resonance  358 CT resonance  359 Exciton resonance  360 Key points of SERS on pure semiconductor nanomaterials  361

4 Applications 361 4.1 Probing CT in dye-sensitized solar cells  361 4.1.1 ZnO-TiO2/N3/Ag  362 4.1.2 Au@Ag/N3/TiO2  362 4.2 Chemical and biological sensing  363 4.2.1 Small ions and toxic molecules  363 4.2.2 Protein biomarkers  367 4.2.3 Cell viability and apoptosis assays  368 4.3 Probing intermolecular interactions  371 4.3.1 The effect of hydrogen bonds on CT  371 4.3.2 Enantioselective discrimination by hydrogen binding  373 4.3.3 ET between redox proteins  375

5 Conclusions and outlook  376 Reference  376

Surface-enhanced Raman scattering (SERS) is first discovered in the mid-1970s. With over 40-years developments, SERS active nanomaterials have extended from noble metals and transition metals to semiconductor materials. Meanwhile, the emergence of novel theories enables better understanding of SERS mechanisms. Owing to the ultrahigh sensitivity and selectivity, SERS allows fast, non-invasive and in situ detection of target molecules. Beyond basic studies in physical chemistry, recent years SERS Molecular and Laser Spectroscopy. http://dx.doi.org/10.1016/B978-0-12-818870-5.00010-1 Copyright © 2020 Elsevier Inc. All rights reserved.

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has exhibited great potential in numerous promising applications in nanomaterials and nanotechnology, chemical analysis, biophysical chemistry, and clinical medicine. In the following sections, the main mechanisms of SERS and a variety of SERSactive nanomaterials are outlined; SERS on semiconductor nanomaterials is discussed in detail; applications of SERS in dye-sensitized solar cells, chemical and biological sensing, and intermolecular interactions are respectively introduced.

1  SERS and its mechanisms: a brief introduction SERS is a surface-sensitive phenomenon (Fig. 10.1A), in which Raman signals of the adsorbed molecules can be significantly enhanced by 106–1014. Owing to the ultrahigh sensitivity and selectivity, SERS allows fast, non-invasive and in situ detection of target molecules, and has attracted increasing interest in chemical analysis, material science, nanotechnology, and biomedicine [1,2]. Raman intensities of molecular vibrations will be greatly enhanced when the Raman excitation wavelength approaches or falls in the molecular electronic absorption region, called resonance Raman scattering. Surfaceenhanced resonance Raman scattering (SERRS) is a form of SERS with a combination of surface enhancement provided by immobilizing an analyte onto a SERS-active substrate and a molecular resonance excited by an appropriate laser wavelength [4]. SERRS has been widely used as powerful tools for ultrasensitive chemical analysis down to a single-molecule level under favorable circumstances. Generally, the mechanisms of electromagnetic and chemical enhancement are commonly accepted to interpret the observed SERS effect. The electromagnetic enhancement (EM) effect originated from the local electric field generated by the collective oscillations of surface plasmons in metallic particles. Enhancement factors of 1010–1014 have been reported to occur at the regions of high electromagnetic fields associated with localized plasmonic resonances, which enable the detection of single molecules [5,6]. Numerous studies focused on the plasmon-enhanced Raman scattering (PERS), which has been widely applied even in the research fields of in vivo biomedical applications [7]. On the other hand, it is known that high electric fields can occur near the surfaces with high curvatures also in non-plasmonic materials [3,8]. Our study regarding SERS of cytochrome b5 on titanium dioxide (TiO2) electrodes revealed that the increased SERS hot spots at the TiO2/water interface with increasing anisotropy of the halfellipsoids used to mimic the surface roughness (Fig. 10.1B) [3]. TiO2 can be used directly for biocompatible immobilization and its EM enhancement ability allows SERS spectroscopic monitoring of electron-transfer processes of redox proteins. The well-accepted plasmon theory alone could not explain all the SERS regarding varieties of molecules and substrates. For example, with regard to the molecules with similar Raman cross-sections, those with the ability to be chemically adsorbed on the metal surface displayed the largest enhancements. The chemical mechanism (CM) is expected to explain these phenomena. The intensity of a Raman band is proportional to α [2]. The polarizability (α) is associated with A, B, and C enhancement mechanism according to the vibronic theory proposed for resonance Raman scattering

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Figure 10.1  Schematic diagram of SERS on roughened nanomaterials (A) and (B) calculated electric field enhancement of TiO2 nanostructures using half ellipsoids with an aspect ratio of 3 (left) and 6 (right) [3].

Figure 10.2  The major effect factors of molecular polarizability, the equation of degree of CT and schematic diagram of CT.

by Albrecht et al. [9]. Expanding the transition dipole moment in terms of normal coordinates of the ground state, the A, B, and C scattering terms can be identified. The A-term describes the Franck–Condon contribution and the B and C-term describes Herzberg–Teller type activities [10]. Lombardi et al. proposed a CT theory based on the Herzberg–Teller theory [11]. In a metal-molecule system, the conduction band orbitals of the metal lies between the highest occupied molecular orbital (HOMO) and its first excited unfilled level lowest unoccupied molecular orbital (LUMO), and CT can occur either from a metal cluster to molecules, or from molecules to a metal cluster depending on the relative energies of the metal Fermi level and the HOMO and LUMO levels of the adsorbed molecule (Fig. 10.2) [12]. Furthermore, they introduced a quantitative measure of the degree of CT (ρCT), which was used to examine the CT contributions to the SERS spectra under various conditions.

2  SERS-active substrates The fast growing research field of SERS relies on the development of SERS-active substrates. The intrinsic properties of the noble metals, transition metals, and semiconductors enable these materials to be utilized in various study fields of fundamental and practical applications. Owing to the higher enhancement abilities than other

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Figure 10.3  Timeline of the development of SERS-active materials.

SERS-active materials, noble metals (silver and gold) have more widespread applications in analytical chemistry including ultrasensitive sensors and biomedicine. SERS on transition metals is mainly applied in electrochemistry and surface catalysis. On the other hand, the CT enhancement mechanism is better and deeply understood on the basis of SERS on semiconductor nanomaterials. The timeline of the development of SERS-active materials is plotted in Fig. 10.3 [13].

2.1  Noble metals SERS activities of noble metals (Au, Ag, and Cu) were first found and extensively applied in surface physical chemistry, biomedicine, food chemistry, and forensic biology. Although SERS on other materials like transition metals and semiconductors are well developed, the highest enhancement ability was observed afterward, on Ag nanomaterials, which enables ultrasensitive sensors to be useful in practical applications. Owing to the better biocompatibility, Au nanoparticles are usually prepared with Raman reports and antibodies/aptamers, which are applied in immunoassays and biosensing [14,15]. The biocompatibility of the Ag and Au can be further improved by surface coating (by polymers, silica et al.), allowing in situ probing specific biomarkers or microenvironment in disease-related cells [16,17]. The enhancement abilities of the noble metals can be modulated by adjudging the morphology, size, and dimensions. However, random aggregation of Ag/Au nanoparticles usually causes fluctuant SERS intensities, bringing difficulties for quantification. Accordingly, patterned and highly uniform SERS-substrates were developed and significantly improved the SERS spectral reproducibility for in vitro molecular sensing [18]. Alternatively, Raman reporter-embedded core-shell structures are prepared for stabling the Raman reporters and avoiding desorption and enzymatic degradation in physiological conditions, which have been proved to have promising applications in vivo biomedicine [19,20]. Moreover, as a novel readout method, SERS frequency shifts with excellent spectral reproducibility have been applied in the detection of nucleic acids, proteins, and protein assembly [21–24]. The shell-isolated nanoparticles were reported in 2010 [25]. The ultrathin coating with a silica or alumina shell keeps the nanoparticles from agglomerating, separates them from direct contact with the adsorbed targets. The shell-isolated nanoparticleenhanced Raman spectroscopy (SHINERS) significantly expands the flexibility of

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SERS for useful applications in the materials and life sciences, as well as for food safety, drugs, and environment pollutants [25,26].

2.2  Transition metals Since 1996, Tian and the coauthors have found the SERS activities of transition metals (Pt, Ru, Rh, Pd, Fe, Co, and Ni) [27]. They developed various roughening procedures and optimized the performance of the confocal Raman microscope, and the enhancement factors for transition metals were calculated to be 10–104. The SERS behavior of transition metal electrodes and nanorod arrays were explained by the theories considering the electromagnetic and chemical contributions. The SERS on the transition metals were useful in surface adsorption, electro-catalysis and corrosion of transitionmetal-based systems. Furthermore, noble-transition metal core-shell structures were synthesized with different shell thicknesses, and their ultraviolet laser excited SERS enhancements were investigated. The enhancement ability was found to be dependent on the shell thickness and the properties of the transition metals [28]. A potential-dependent chemisorption of carbon monoxide at a Au core-Pt shell demonstrated that the chemical bonding of COB (bridge binding) is much more sensitive to the changes in the interfacial electric field than that of COL (linear binding) [29].

2.3 Semiconductors SERS on semiconductors was first observed in NiO by Yamada et al. in 1982 [30] and has been extended from metal oxides and silver halides to single elemental semiconductors, semiconductor sulfide, and organic semiconductors. The CT enhancement mechanism is believed to have a major contribution to the observed high SERS signals in these cases. The morphology, enhancement factors, and probe molecules of SERS-active semiconductors of metal oxides and other nanomaterials are summarized as listed in Table 10.1.

2.3.1  Metal oxides Metal oxides are the most extensively investigated semiconductors in the research field of SERS. Earlier related studies investigated SERS from the crystal TiO2 (001) and NiO (110) surface [30 31]. The chemical binding between the nitrogen of pyridine and the atomic sites of NO or TiO2 resulted in the enhancement of Raman scattering due to CT excitation of the adsorbent-adsorbate interaction, which was similar with the mechanism of resonance Raman scattering effect. SERS was observed on mesoporous anatase [32] and the results revealed the formation of the bidentate or bridging linkage between TiO2 and the Ru-bpy dyes. However, the enhancement factors (EF) were found to be rather low (below 10). Noted that TiO2 nanomaterials with smaller sizes (≤ 10 nm) were proved to be capable of displaying much higher Raman enhancements (103 or higher) [33–36]. In a case of TiO2 photonic microarray, SERS sensitivity can be improved by the enhanced matter-light interaction through repeated and multiple light scattering (Fig. 10.4A) [37]. Recently, novel two-dimensional amorphous TiO2 nanosheets were successfully developed and an ultrahigh enhancement factor of 1.86 × 106 was obtained for 4-mercaptobenzoic acid (4-MBA). Density

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Table 10.1  SERS-active semiconductors [44]. Morphology/ Semiconductors Size NiO

Probes

EF

Crystal surface Pyridine (100) Crystal Pyridine surface(001) Mesoporous film Ru-bpy

Low

103 1–102

TiO2 TiO2

10 nm particle 6.8–14.2 nm particle 2/5 nm colloid 3 nm colloid

TiO2

5 nm colloid

TiO2

3D nanostructure

TiO2

Microarray

TiO2

Roughened electrode 2/5 nm colloid

TiO2 TiO2 TiO2 TiO2

TiO2 TiO2 TiO2 ZnO ZnO ZnO ZnO ZnO Fe2O3 Fe2O3 Fe3O4 Fe3O4 Cu2O Cu2O CuO

Particle Nanosheet 50 nm colloid 20 nm nanocrystal 18–31 nm nanocrystal porous ZnO nanosheet Amorphous nanocages Sphere/ spindle/cube nanocrystals 70–80 nm colloid 120 nm colloid 9 nm nanoparticle Roughened electrode 300 nm nanospheres Roughened electrode

4-MBA 4-MBA

Low Low

Enhancement Refermechanism ences Resonance Raman Resonance Raman Resonance Raman CT CT

[30] [31] [32] [33] [45]

Enediol molecules 103 CT Alizarin red S/ CT phosphate Nitrothiophenol 102–103 CT isomers Crystal violet 106 Multiple SERS mechanisms MB 104 Matter-light interaction Cytochrome b5 10 EM

[34 46] [31 47]

Salicylate / salicylic acid Dye 4-MBA D266 4-MPY/BVPP

CT

[49]

CT CT CT CT

[50] [51] [52] [39]

CT

[40]

106 50 103

4-MBA /4-MPY 4-MBA

[36] [48] [37] [3]

[53]

4-MBA/4-MPY/4- 105 ATP 4-MPY 104

CT

[41]

CT

[42 54]

Pyridine 104 TSPP 30 organic molecules Pyridine Low

EM CT EM CT

[55] [56] [43] [57]

CT/EM

[38]

4-MBA BHA

105

[58]

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Table 10.1  SERS-active semiconductors [44] (Cont.) Morphology/ Semiconductors Size CuO 80 nm nanocrystals Al2O3 80 nm AAO film Ag2O 28 nm colloid SnO2 40 nm particle Pb3O4 100 nm particle MoO3 Nanobelt W18O49 Nanowire AgX Colloid Ag2S Graphene Si Si Ge CdS CdTe CdSe PbS ZnS ZnSe GaP ITO DFH-4T

11 nm particle Layer 200–370 nm particle Nanowire Nanowire 8 nm particle 3 nm particle QDs QDs 8.2 nm QDs 3 nm particle Thin film 40–100 nm particle 20–70 nm particle ivy-like film

Probes 4-MPY

EF 102

Cytochrome c Pyridine 4-MBA Mercaptopyridine 4-MBA Rodamine 6G Dyes

30 105 High 102 103 105 50–100

4-Mpy Dye molecules 4-MBA

Enhancement Refermechanism ences CT [59] [60] [61] [62] [63] [64] [65] [66–69]

2–17 High

EM EM CT CT CT CT Resonance Raman CT CT Mie resonance

Rodamine 6G N719 4-Mpy 4-Mpy

Low Low 102 104

CT CT CT CT

[73] [73] [74] [75]

Pyridine 4-Mpy 4-Mpy 4-Mpy CuPc

105 103 103 106 700

CT CT CT CT/EM EM

[76] [77] [78] [79 80] [81]

Cyt c

500

CT

[82]

MB

103

CT

[83]

[70] [71] [72]

functional theory (DFT) results revealed that the low coordination number of surface Ti atoms and a large number of surface oxygen defects facilitate CT from the 4-MBA to the TiO2 substrates. Copper oxides (Cu2O and CuO) were also found to be SERS-active with a high Raman enhancement of 105 on Cu2O nanospheres owing to the static chemical enhancement, resonant chemical enhancement, and electromagnetic enhancement (Fig. 10.4B) [38]. Our study on SERS of zinc oxide (ZnO) demonstrated that ZnO nanocrystals (20 nm) can display an EF of 103 for the Raman probe of para-mercaptopyridine (4-Mpy) [39]. Moreover, we found a size-dependent CT resonance on ZnO with diameters ranging from 18 to 31 nm. In both cases, the chemical enhancement is most likely responsible for the observed Raman enhancement [40]. Recent study indicated that the amorphous ZnO nanocages displayed an enhancement factor of up to 6.62 × 105 for 4-MBA, 4-MPY, and 4-aminothiophenol (4-ATP), the authors found

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Figure 10.4  The morphology of semiconductor nanomaterials. (A) SEM images of TiO2 inverse opal; (B) Cu2O nanospheres [38]; (C) the side view of the GeNT arrays fabricated on the template of SiNWs with oxide layer (D) SEM image of Si NPs with a size of 370 nm [72]; (E) TEM image of PbS nanoparticles [77]; and (F) 4-Mpy capped CdS microclusters [74]; top-view (G) and cross-sectional (H) SEM images of the nanostructured DFH-4T films [83]. Part A: Adapted with permission from [37], copyright (2014) American Chemical Society; Part C: adapted with permission from [73], copyright (2011) American Chemical Society.

that the enhancement of the ZnO–molecule interactions promoted the interfacial CT process, improving the sensitivity of semiconductor-based SERS [41]. Apha-Fe2O3 nanocrystals were found able to enhance the Raman signal of 4-Mpy with an EF of 104 due to the CT mechanism [42]. Recent study by Gilbert et al. showed that magnetite Fe3O4 can electromagnetically enhance Raman signals of diverse surface-adsorbed organic molecules, which provides a new approach for establishing the surface interactions of environmentally relevant organic ligands and mineral surfaces [43]. Noted that WO2.72 (W18O49) with oxygen vacancies displayed a large EF of 3.4 × 105, which was believed to be attributed to molecule resonance of R6G, exciton resonance of W18O49 defect states, and the photon induced CT resonance together with the ground-state CT resonance from matched energy level between W18O49 and the R6G molecules [65]. Single elemental semiconductors (Si, Ge, and graphene) were proved in recent years to be capable of displaying Raman enhancement based on the CT mechanism. By exploiting terminal hydrogen atoms on the surface of SiNW and GeNT (Fig. 10.4C) arrays, an efficient CT process between the molecules and the nanostructured Si or Ge substrates was successfully realized. Rodriguez et al. found light induced Mie resonances of high refractive index of silicon nanoparticles and generate strong evanescent electromagnetic field. Moreover, they found the silicon (Fig. 10.4D) presented stronger field enhancement due to Mie resonances at larger sizes than the gold [72]. A clear Raman enhancement effect was observed on the surface of monolayer graphene in 2010 by Jing Zhang et al., which is the first report about SERS on the surface of graphene [71]. Here, Raman enhancement factors are found to be quite different

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for different peaks of dye molecules, changing from 2 to 17, which were attributed to the CT enhancement between graphene and the molecules. Soon after, the firstlayer effect in graphene-enhanced Raman scattering (GERS) was found and GERS is strongly dependent on the distance between graphene and the adsorbed molecule, convincing the chemical-enhanced mechanism from graphene [84]. Moreover, the nanoparticle size and excitation wavelength-dependent SERS of 4-Mpy on the PbS quantum dots (QDs) (Fig. 10.4E) were studies by Lombardi et al. The results showed a maximum of the Raman band intensity for the b1 and b2 lines occurs at ∼8.2 nm PbS QDs, and a maximum in the intensity at approximately 525 nm laser wavelength. We observed SERS spectra of 4-Mpy on 8 nm CdS cluster (Fig. 10.4F) with an EF of 102 in 2007. The SERS spectra on the CdS exhibited some differences (e.g., shifted and sharpened bands) when compared with that on Ag substrates, indicating the possibility of a different enhancement mechanism on the CdS surface [74]. SERS on pure organic semiconductors was observed in 2017 by Demirel et al. [83] The α, w-diperfluoro-hexylquaterthiophene (DFH-4T) nanostructured films were found SERS-active with an enhancement factor up to 3.4 × 103 for the probe molecule methylene blue (MB). The metal-free DFH-4T films displayed SERS via a resonance CT mechanism between the DFH-4T substrate and the MB, which was supported by the comparative morphological experiments and electronic structure calculations. Both of the CT energetics and the substrate morphology were found important for the Raman signal enhancement. This study brings a simple way of the fabrication of SERS-active nanostructured platforms enabling high chemical enhancements and few-to-single molecule level sensing.

2.4  Semiconductor-metal heterostructures Semiconductor–noble metal heterostructures are most widely investigated owing to the higher Raman enhancement ability attributed to Ag or Au in visible laser wavelength regions. In these studies, semiconductors and metals are assembled either in the direct contact form or in a molecule-bridged sandwich form. The morphology of the semiconductors is variable for nanoparticles, nanorods, nanosheets, nanochains, and flower-like structures. The Raman enhancement of molecules in semiconductor– metal heterostructures strongly depends on the nature of metals, probe molecules, and assembly ways, which will affect the CT direction and the additional EM field.

3  Mechanism of SERS on semiconductor nanomaterials A large number of researches indicate that SERS on semiconductors is dependent on the material size, surface defects, shape, and morphology of nanostructures. The enhancement mechanisms of both EM (plasmon and Mie resonance) and CM (CT and exciton resonance) were explored and discussed for the SERS observed on semiconductors. In some cases, the combination of surface plasmon, CT, and band gap resonances is considered to be most likely the contributing factor in the observed Raman signal enhancement on semiconductor materials [48 79].

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3.1  Plasmon resonance In the case of semiconductor materials, the surface plasmon frequency can be determined by the following equation: 1/2

 4π ne 2  ω sp =  (10.1)   meε ∞  Here, n is the electron carrier density and me is the effective electron mass. ε∞ is the high frequency dielectric constant. The surface plasmon frequency (wsp) rises in proportion to electron density (n) [85 86]. Thus, the conduction band plasmon of semiconductor materials is assumed to fall in the infrared region attributed to the small electron density. In contrast, the electron density of valence electrons in solids is as large as 1022–1024 cm–3, and therefore the valence band plasmon of semiconductors is expected to be in the ultraviolet region with the plasmon excitation energy ranging from 4 to 30 eV. Therefore, the bulk conduction and valence band plasmons are far from the visible region, in which most commonly used laser wavelengths are involved. However, it is well known that owing to the variety in geometry the plasmon resonances of semiconductors can shift as observed in metals. By fabrication of a TiO2 3D nanofiber network, Maznichenko et al. found there was a red shift of rutile TiO2 plasmon from the vacuum-ultraviolet (VUV) closer to the 532 nm laser excitation, whose contribution to the high SERS enhancement was not negligible [48]. Another example is CuTe nanocrystals with optimized shapes were found to be capable of displaying a strong near-infrared optical absorption, which could be used as plasmonic sensors for a wide family of molecules with SERS [87].

3.2  Mie resonance High electric fields can generate around semiconductor micro- or nanostructures with larger diameters, which originate from the optical resonances of the whispering gallery modes (WGMs), also known as Mie resonances. Strong radial dependence of metallic plasmon resonances was originally exploited by Mie in 1908 [88]. Near-field scattering efficiency (QNF) is the ratio of the power of the scattered light to that of the incident light [89]. ∞

{

2 2 2 QNF = 2∑ an ( n + 1) hn( 2−)1 ( ka ) + n hn( 2+)1 ( ka )    n =1 2 2 2 + ( 2 n + 1) bn hn( ) ( ka ) 

}

( 2)

(10.2)

Where an and bn are the scattering coefficients, hn is the Hankel function of the second kind. The parameter a is the radius of the sphere and k (=2π/λ) is wave number

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Figure 10.5  The CT pathways in semiconductor-molecule systems (A) HOMO-to-CB, (B) CT complex-to-CB, (C) VB-to-LOMO, (D) surface state-to-molecule LOMO, and (E) CB-toHOMO [44].

of the incident light. Accordingly, the Mie scattering near-field intensity display resonances as a function of the size parameter x = 2πa/λ. The first application of the Mie scattering theory to semiconductors was done by Hayashi et al. [81]. They observed SERS from copper phthalocyanine (CuPc) nanoparticles, and found the importance of the EM mechanism even for a nonmetallic particle. According to the Mie theory, Lombardi et al. optimized the size of ZnSe nanoparticles and observed an enhancement factor of 2 × 106 [79]. Moreover, they found that there was an excellent agreement between the observed spectra from the monolayer arrays of silica and a single particle calculated spectrum [90]. Alessandri et al. found that a remarkable enhancement of Raman sensitivity can be obtained by SiO2-TiO2 coreshell microsphere based resonators, which is attributed to the synergistic contribution of the high-refractive index of TiO2 shell and colloidal crystal-based cavities [91].

3.3  CT resonance Semiconductors have an energy gap between a full valence band and an empty conduction band, and thus the CT between semiconductor nanomaterials and the adsorbed molecules depends on the vibronic coupling of the conduction band and valence with the molecular excited state and ground state. Briefly, the CT in a semiconductor-molecule system can occur in the following five pathways: [33,34,62,92–95] 1. HOMO-to-CB. The electron occupying the ground state of the molecule is directly excited by the incident light from the HOMO of the adsorbed molecule to an energy level of the conduction band in semiconductor. Then the excited electron immediately transits back to a certain ground vibrational energy level of the molecule and a Raman photon is subsequently released (Fig. 10.5A).

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2. CT complex-to-CB. Chemical bonding between the molecule and semiconductor reduces the formation of a CT complex, resulting in the enhancement of the polarizability and the Raman signals of the original adsorbed molecule (Fig. 10.5B). 3. VB-to-LUMO. The electron in the valence band of the semiconductors is excited to the high energy level LUMO of the adsorbed molecule, and quickly transits back to the valence band, releasing a Raman photon (Fig. 10.5C). 4. Surface state-to-LUMO. The electron is firstly excited from the valence band of semiconductor to the surface defect, resulting in the formation of a surface state. Subsequently, the electron is further excited from the surface state to the LUMO of the molecule, releasing a Raman photon by transiting back to the surface state (Fig. 10.5D). 5. CB-to-HOMO. Some dye molecules can easily be excited by the visible light to a higher energy level of LUMO, and the electron is then injected into a matching energy level in the conduction band of semiconductors through resonant tunneling. The electron finally transits back to a ground vibrational energy level of the molecule with a Raman photon being released (Fig. 10.5E).

In our group, we extensively studied SERS on semiconductor (TiO2, ZnO, CdTe)– Ag (Au) hybrid structures, and three CT pathways are summarized based on our studies. 1. Semiconductor-to-molecule-to-metal. In a sandwich structure of the TiO2–MBA–Ag colloid, the TiO2–Ag provides an additional CT and EM besides the intrinsic TiO2-to-molecule CT and the EM from the Ag SPR effect, which is responsible for the observed much higher enhancement of the probe. Here, Ag NPs with a high electronegativity act as electron acceptors, allowing ET from TiO2 to Ag bridged by 4-MBA [96]. 2. Metal-to-semiconductor-to-molecule. A novel laser-driven photo-induced interfacial CT was observed in a Cu–ZnO–PATP film. Here, SERS spectra combined with a well-characterized surface morphology and optical spectroscopy indicate that with ultraviolet excitation, multiphonon resonant Raman scattering results in addition to strong enhancements [97]. Additionally, the direct connection of semiconductors (TiO2 or ZnO) and metals would lead to the CT from metals to molecules (4-MBA or PATP) bridged by semiconductors. In the case of an Ag–TiO2–MBA assembly, for example, the deposited Ag on TiO2 can inject additional electrons into molecules adsorbed on the TiO2 surface through the CB of TiO2 NPs [98 99]. 3. Metal-to-molecule-to-semiconductor. The CT from metals to semiconductors bridged by molecules was proposed in the self-assembled metal molecule-semiconductor films. In the case of the Ag-MPH-TiO2 assembled films, the enhancement of b2 modes of the 4-mercaptophenol (4-MPH) molecule is associated with the CT between the Ag nanoparticles and the 4-MPH-TiO2 complex, which dependents on whether the incident laser has sufficient energy to excite the CT electronic transitions from 4-MPH molecules to TiO2 [100–103].

3.4  Exciton resonance In semiconductor materials, the electron can be excited from the valence band into the conduction band, generating electron–hole pairs in solids known as excitons. The exciton Bohr radius is the distance in an electron-hole pair. When the particle size of a spherical semiconductor nanoparticle is smaller than the exciton Bohr radius, the exciton levels tend to diverge due to the effects of quantum confinement, which causes strong size dependence of the SERS spectra. Moreover, the quantum confinement will affect the location of the CT resonances in molecule-semiconductor systems. In

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an exciton resonance process, a vibrational quantum of energy would transfer to the vibrational level of the molecule, while a Raman photon would be radiated from the molecule at a certain vibrational state. An exciton resonance is believed to be capable of not only increasing the enhancement factor, but also influencing the selection rules and the spectral appearance [86].

3.5  Key points of SERS on pure semiconductor nanomaterials On the basis of the related studies, several important points including the material size, surface defects, and morphology are found to have significant impact on the semiconductor-enhanced Raman scattering. These key factors are summarized as follows: 1. Material size effect. Semiconductor nanoparticles with the size near Bohr radius of bulk exciton, the electron energy levels are discrete rather than continuous due to the quantization effects. In this case, the allowed transition levels in semiconductors would be plummet, resulting in the decrease in the CT contribution to SERS. 2. EF. The contribution from chemical mechanism is theoretically believed to be less than 102 in metal SERS-active substrates. However, the EF observed and calculated on semiconductor surfaces ranges from 10 to 106, which suggest that the explanation based on chemical mechanism for metal-based SERS does not fulfill that for SERS on semiconductors. 3. Spectral profiles. The profiles of Raman spectra observed on semiconductor materials are quite different with those observed on metal substrates, in which the Raman enhancement of selective bands varies with different excitation wavelengths. Moreover, compared with that observed on metal substrates, the half-peak width of Raman bands observed on semiconductors is narrower due to much smaller particle size and narrower size distribution. These evidences all indicate that SERS on semiconductors is a quasi-resonance phenomenon. 4. Surface defects. The surface defects can bind electrons and form the surface states levels, providing more energy levels for CT transitions and increase the CT contribution to SERS. It should be pointed out that the surface defects are close to or at a semiconductor surface, and thus the CT contribution originated from the surface defects might be offset by the interaction between adsorbed molecule and the surface defects. 5. Chemical binding. Chemical bonding is not essential for the CT transition, especially for those dye molecule-semiconductor systems, but it plays an important role in molecular adsorption and stability on semiconductor surfaces.

4 Applications 4.1  Probing CT in dye-sensitized solar cells Recent years, dye-sensitized solar cells (DSSCs) have gained increasing attention owing to the properties of high efficiency, low cost, flexibility, and ease of manufacture. Noble metal nanoparticles such as Ag and Cu, have been utilized to improve the efficiency of DSSCs, where the localized surface plasmon generated around the surface of metal nanoparticles, increasing the optical absorption and enlarge the spectral response region of a material [104]. In our research group, we applied SERS to probe CT in dye-sensitized solar cells with semiconductor-metal assemblies.

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Figure 10.6  Proposed models for the CT mechanism in (A) ZnO-TiO2/N3/Ag [105] and (B) Au@Ag/N3/TiO2 assemblies. Adapted with permission from [106], copyright (2018) American Chemical Society.

4.1.1 ZnO-TiO2/N3/Ag A number of studies have focused on improving the performance of DSSCs by a composite semiconductor with an appropriate CB and VB, which can hinder the recombination of electron-hole pairs. Accordingly, we fabricated ZnO-TiO2/N3/Ag assemblies and the interfacial CT processes have been monitored by SERS. The dye loading capacity of the dendritic crystal ZnO-TiO2 was found to be much larger than that of the TiO2. In another word, the use of a ZnO-TiO2 composite semiconductor in the semiconductor/N3/Ag system can improve the adsorption capacity of N3, promoting the CT process. Here, the ZnO-TiO2 composite semiconductor exhibited a synergistic effect due to CT between its components. The electrons at the CB level of ZnO may inject into the CB level of TiO2 and then transfer to the Fermi level of Ag (Fig. 10.6A). Thus, a builtin electric field is formed from ZnO to Ag via TiO2. Introducing the Ag nanoparticles into the semiconductor/N3 assemblies can enhance its optical absorption, enlarge its spectral response region, induce localized surface plasmon resonance (LSPR), resulting in a new CT pathway (N3 to Ag), and reduce the energy threshold of the synergistic effect due to CT. Our results suggested that semiconductor/N3/Ag systems based on the ZnO-TiO2 composite semiconductor are an excellent potential structure for DSSCs.

4.1.2 Au@Ag/N3/TiO2 The improvement of the performance induced by a noble metal is of great importance in plasmon-enhanced DSSCs. Apart from the light harvest efficiency, the improvement of CT process is another crucial factor controlling the efficiency of DSSCs. Thus, it is essential to understand how metals impact the whole CT processes in DSSCs besides the SPR effect. In our recent study, the CT threshold in DSSCs can be reduced by introducing core–shell dual metals. We fabricated Au@Ag core–shell nanoparticles, which afforded a combination of the high surface plasmonic band extinction coefficient of

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Ag as well as unique synergistic effects due to CT within the Au@Ag composite structure. N3 molecules and various layers of TiO2 were respectively assembled, forming Au@Ag/N3/n-TiO2 assemblies. In this system, the CT processes could be tuned via layer-by-layer assembly of TiO2, and be estimated quantitatively via the ρCT. It was found that introducing Au as the core in Ag nanoparticles appropriately reduces the CT threshold, which arises from energy level equilibration in the Au@ Ag1-2/N3/n-TiO2 systems. The probable CT process in Au@Ag/N3/n-TiO2 is shown in Fig. 10.6B. The electrons are stimulated by photons from the HOMO level of N3, and then undergo four CT pathways: (1) the LUMO level of N3, (2) the Wf of Ag, (3) the Wf of Au, or (4) the CB of TiO2, to be then injected into the Au Wf via the Ag Wf. The last CT process forms a new CB3 for the Au@Ag/TiO2 complex, which lies between CB1 and CB2. The Au@Ag/N3/n-TiO2 system exhibits CT enhancement even under low-energy laser excitation.

4.2  Chemical and biological sensing As an ultrasensitive analytical tool, SERS has widespread applications in chemical and biological sensing. Recent years, our research group has developed a series of analytical methods for small ions, toxic molecules, biomarkers, cell viability, and apoptosis based on novel metals and semiconductors.

4.2.1  Small ions and toxic molecules Small toxic molecules in water and food have a strong impact on human health. Nitrite may cause irreversible oxidation of hemoglobin to methemoglobin and thus abolishes the oxygen transport function [107]. Thus excessive nitrite uptake contributes to a variety of negative health effects, such as DNA mutation, cancer, and chronic obstructive pulmonary disease [108 109]. Cyanide can tightly bind to ferric hemes and thus blocks intracellular oxygen reduction. H2O2, known as a cytotoxic and signaling molecule, is widely utilized as a target in clinical diagnosis, food and pharmaceutical analysis [110]. We developed magneto-plasmonic biosensor for detecting and decontaminating small toxic molecules, consisting of Fe3O4 cores in a silica matrix, and a shell of Ag nanoparticles coated by chitosan. Myoglobin (Mb) was covalently attached to the chitosan shell by using glutaraldehyde as a linker. SERRS spectroscopy was then employed to monitor binding of small toxic molecular species (NO2−, CN−, and H2O2) (Fig. 10.7A) [111]. The Fe3O4-Ag hybrid nanoparticles exhibit tailored plasmonic properties for SERRS spectroscopic detection of the binding of target molecules by the heme group of the Mb. This study provides a possibility to quantify the concentration of nitrite, cyanide, and hydrogen peroxide in solution with detection limits that are lower than the maximum contaminant level (MCL) values. Moreover, the dispersed Fe3O4-Agchitosan nanoparticles with bound toxic targets can be easily collected by an external magnet, representing a convenient way for decontamination. Furthermore, the strategy of Mb immobilization can be employed for immobilizing other protein (e.g., enzymes and antibodies) for a variety of applications in bioanalysis and biosensing.

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Figure 10.7  Magnetic silver hybrid nanoparticles for SERRS spectroscopic detection and decontamination of small toxic molecules (A) [111]; (B) application of SERS in selective determination of chromium(VI) in water[35]. Published by The Royal Society of Chemistry.

The important merit for semiconductor-based SERS sensors is that the stability and reproducibility of the SERS sensor can be significantly improved by employing the semiconductor nanoparticles because these homogeneous colloidal systems display highly reproducible spectra unlike those rely on “hot spots” for enhancement. SERS on semiconductors based on chemical enhancement is applicable of determining phosphate and Cr(VI) ions. In the case of phosphate, the pyrocatechol adsorbs on a TiO2 surface via a bidentate chelation mode, resulting in the formation of a new ligand-to-metal CT transition located (SERS on).The chelation between the pyrocatechol and colloidal TiO2 was inhibited when phosphate anions pre-adsorbed on the colloidal TiO2 surface under acidic pH conditions, resulting in a gradual decrease in the SERS intensity of the pyrocatechol molecule [47]. Thus in this way, quantitative analysis of phosphate anions was achieved by measurement of “turn-off” SERS based on the first-layer effect of a chemical mechanism.

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Cr(VI) exists as an oxyanion (CrO42−), which is highly toxic and has been extensively used in industrial processes and has become one of the major environmental hazards [112]. Cr(III) is relatively non-toxic and is regarded as an essential trace element associated with the metabolism of carbohydrates and lipids. However, because of the lack of efficient testing method, the estimated MCL by the World Health Organization (WHO) includes the total amount of Cr. A method combined sensitivity and easy manipulation is highly required for selective detection of Cr(VI) in aqueous solution. A CT complex, alizarin red S (ARS)-sensitized colloidal TiO2 nanoparticles, with a facile synthetic route are used to determination of Cr(VI) in water (Fig. 10.7B). Raman signal mechanism of the ARS–TiO2 complexes was explored and the results suggested that the molecular polarizability tensor can be enhanced due to the vibronic coupling of the conduction band states of the semiconductor with the excited states of the probe molecule through a Herzberg–Teller coupling term. The SERS intensities of the ARS–TiO2 complexes have been found to be sensitive to the Cr(VI) concentration due to co-catalysis, providing a novel way of the determination of Cr(VI). Colloidal TiO2 nanoparticles can be employed both as an effective substrate to elicit the SERS signals, and as a catalytic center to induce the self-degradation of the ARS response to Cr(VI). Based on this “turn-off” SERS strategy, other metal ions may also be detected by utilizing different semiconductor enhancement systems in which the energy level of the semiconductor is matched with the redox potential of the target metal ions. Estrogens are a broad class of compounds functioning as primary female sex hormones that exert many physiological effects. Increasing evidence has indicated that many adverse health consequences are associated with relatively high exposures to environmental estrogens and estrogen-like compounds, and the link between some diseases (e.g., breast cancer) and hormones, estrogens in particular, has been a source of great concern among scientists [113 114]. The development of rapid and highly sensitive detection methods for estrogens is very important and necessary to control hormonal concentration below safety standards. Traditional and commonly-used immunochemical methods require complicated antibody preparation and cannot always discriminate between specific and non-specific binding, especially when the estrogen concentration is very low, which may lead to false results and thus restrain their practical applications [115]. We have developed a SERRS-based approach for recognition of phenolic estrogens [116]. A key feature of this study is an azo coupling reaction between phenolic estrogens and Pauly’s reagent, which is used not only to enhance the affinity of the analytes to metal nanoparticle but also to amplify SERS signals by the SERRS effect of the products. The proposed method is simple (mixing the estrogens, Pauly’s reagent, and silver colloid without separation or purification), rapid (completion of the coupling reaction and SERS measurement within 2 min), ultrasensitive (down to the subnanomolar level in solution), and applicable to most molecules with phenol groups. Moreover, unlike immunochemistry-based methods, nonspecific binding and false positive results can be eliminated, because each SERRS spectrum of the azo dyes is specific for its corresponding estrogen, which enables to detect multiple phenolic estrogens simultaneously in a mixed solution (Fig. 10.8).

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Figure 10.8  (A) Coupling reaction-based ultrasensitive detection of phenolic estrogens using SERRS [116] and (B) magnetic-TiO2 nanocomposites for SERRS spectroscopic determination and degradation of toxic anilines and phenols [117].

As a further study, we have developed Fe3O4-TiO2 nanocomposites for a more widespread application in determination and degradation of toxic anilines and phenols [117]. Benzidine and its derivatives are important precursors in the synthesis of dyes, and previous studies demonstrated that benzidines have the potential to metabolize to carcinogenic amines in human bodies, and thus, increasing attention is paid on the potential risk (e.g., bladder cancer) of human exposure to products containing benzidine and its congener-based dyes [118 119]. The property of directed translocation of substrates constitutes a wide range of applications of magnetic nanoparticles in catalysis, biological separation, and biomedicine [120]. TiO2 is a non-toxic and biocompatible material with unique optical and photocatalytic properties, which combined with magnetic materials, will enable simultaneously analyte binding, optical, catalytic, and magnetic properties. Mesoporous Fe3O4-TiO2 nanocomposites functionalized with PATP were accordingly prepared to capture toxic anilines and phenols by azo coupling. Loading the nanocomposites with Ag nanopartices offers the possibility for a sensitive quantitative determination of target compounds by SERRS, which allows multiplex detection because of the specific vibrational fingerprints. Sensitivity and selectivity can be further enhanced by concentrating the hybrid particles by an external magnet. Apart from the analytical power, the bound toxic compounds can be degraded via TiO2-assisted photocatalysis, and our results indicated that the degradation process can be enhanced in the presence of plasmonic Ag nanostructures. Therefore the

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Fe3O4-TiO2 nanocomposites represent promising devices for toxicity assessment and elimination in practical applications for safety assessment of food, environment, and commercial products.

4.2.2  Protein biomarkers Our recent study is focusing on developing SERS-based detection methods for α-Fetoprotein (AFP), an important liver tumor biomarker. Liver cancer is the second most common cause of death from cancer worldwide. The glycoprotein AFP has been developed as a biomarker for the early diagnosis of liver cancer [121]. On the other hand, the Lens culinaris agglutinin (LCA)-reactive fraction of AFP (AFP-L3) has recently been proposed as a more specific and important biomarker for hepatocellular carcinoma (HCC) diagnosis [122]. In general, the concentration of AFP in healthy people is below 10 ng/mL, while AFP-L3 is detected only in the patients with HCC. In oncology, when the AFP-L3% (the percentage of AFP-L3 in total amount of AFP) is ≥10%, a diagnosis of HCC should be highly considered. Measurement of AFPL3 exhibits dramatically improved specificity and sensitivity compared with the total AFP concentration [123]. Raman shifts of the probes are sensitive to the protein ligands, and recent years Raman frequency-shifts have been proved to be capable of improving the accuracy of the SERS-based quantification of targets, displaying great potential in biosensing [22 124]. We designed a SERS-based sandwich immunoassay based on two independent SERS probes, MBA, and 5,5-dithiobis(succinimidyl-2-nitrobenzoate) (DSNB), for evaluation of their frequency-shift and intensity changes (Fig. 10.9A). The frequency shifts of the MBA was used for the quantitative determination of AFP and the SERS intensities of a typical DSNB band was applied for quantification of the AFPL3, and thus the AFP-L3% can be finally determined. This approach with excellent reproducibility and high accuracy significantly simplifies the conventional detection procedure for AFP-L3%, and it shows acceptable accuracy with potential for reduced economic cost. The SERS-based immunochips for AFP-L3% is expected to be of great convenience for not only the early diagnosis of HCC but also for other hepatopathies. Furthermore, to eliminate the limitation in the dependence of antibodies, we explored the possibility of determine AFP by SERS without AFP antibodies. Currently, the majority of detection methods for protein biomarkers are based on specific antibodies with chemiluminescent and fluorescent labels [125 126]. However, the high cost in the producing process limits the applications of antibodies especially for monoclonal antibodies. Aptamers with higher stability and relatively lower cost than antibodies have attracted increasing interest in recent years [127]. However, one specific aptamer is required for each biomarker, leading to a longer lead time for aptamer screening and only a few effective aptamers are now available, which limits its wide applications in clinical diagnosis. To tackle the challenge of antibody-free discrimination of protein biomarkers, a linker molecule, perylenetetra carboxylic acid (PTCA) is used for the first time to probe the bound target proteins by SERS spectroscopy (Fig. 10.9B). In combination with the symmetry breaking and frequency shifts of the linker, 10 proteins with diverse

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Figure 10.9  (A) Multiplex immunochips for high-accuracy detection of AFP-l3% based on SERS; (B) antibody-free discrimination of protein biomarkers in human serum based on SERS. Part A: Adapted with permission from [128], copyright (2017) American Chemical Society; Pat B: Adapted with permission from [129], copyright (2018) American Chemical Society.

molecular weight and structure are discriminated with the aid of hierarchical cluster analysis (HCA). The feasibility of the proposed approach was further utilized for the identification of AFP in human serums. We found that the SERS spectral variation is both AFP and human serum dependent, and more importantly, there is almost no overlap between the spectral changes of AFP and human serum. The result indicated that the AFP had a relatively higher affinity to the substrates among other proteins in the serum, and it also demonstrated that the sensitivity of the proposed method is high enough to identify 5 ng/mL of AFP in human serum. The feasibility of the method to discriminate the patients with liver cancer was confirmed by testing 21 clinical samples with different AFP concentrations.

4.2.3  Cell viability and apoptosis assays Cell viability assays are useful in screening chemicals for their cytotoxicity to various cells, and has also been used in cancer chemotherapy to select an anticancer drug as well as its dose. MTT [3-(4,5-Dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide] has been among the most commonly used dye that produce an intense color

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due to the formation of formazan. MTT assays have been used for studying the chemosensitivity, radiosensitivity, and toxicity of drugs in human tumor cell lines [130 131]. The reliability of MTT assays based on UV–visible (UV–vis) absorption spectroscopy has been readily proven; however, it is still challenging to improve the detection sensitivity and simplify the complicated experimental procedure. We combined MTT assays and SERRS, and developed a SERRS-based MTT assay, which has three crucial features: (1) resonance Raman effect of the MTT formazan with excitation wavelength of 633 nm; (2) Au nanopartices were used as both a primary amplifier for the Raman signals and a probe that could directly concentrate formazan molecules. Moreover, the fluorescence interference is quenched by the AuNPs; (3) this method is easier to perform than the traditional MTT assays, requires fewer samples (0.1 mg/mL MTT), a shorter time (30 min), and without the interference from the involved media (serum, residual MTT, and drugs). The results show that the method is reproducible and accurate from the dose–response curve, enabling the practical application of this SERRS-based MTT assay for the immediate detection of formazan at very low levels in assessment of cell viability and proliferation. Furthermore, we explored the feasibility of the strategy in assessment of single-cell viability. Single-cell viability assays are helpful for better understanding of intracellular metabolic processes. It has been acknowledged for decades that impaired mitochondrial metabolism can lead to cancer development, which is strongly supported by the biochemical evidence that mitochondrial activity is a tumor suppressor [134 135]. Popular approaches for the detection of single-cell viability focused on differentiate live and dead cells. However, few studies are capable of quantifying viable single cells. The ability of resonance Raman spectroscopy to characterize cells at the molecular level renders it a novel method for single-cell-level detection of viability [136]. We have developed a novel method for quantifying single-cell viability by a resonance Raman scattering-MTT assay. The Raman intensity of MTT formazan with 633 nm laser excitation was found at least fivefold higher than that of MTT. This feature offers the possibility to detect either the mixture of the two molecules by changing excitations) or just the formazan portion by using 633 nm laser excitation. Moreover, if the Raman scattering is collected where the laser spot is concentrated near the center of a single cell, semi-quantitative detection of single cell activity can be achieved. These results show that the RRS-MTT method can be used for measurement if the single cell is alive, as well as for determining the different activity levels of each single cell, thereby facilitating in situ monitoring of individual cell viability. The in situ imaging of the human lung cancer cell line A549 (Fig. 10.10B) demonstrated that this resonance Raman-based MTT method is feasible and reliable in monitoring singlecell activity when MTT and other interference molecules are absent inside the cells. SERS is recently applied to detect apoptosis combined with nickel nanowires (Ni NWs) (Fig. 10.11) in our group. Besides electron transfer function in the respiratory chain, cytochrome c (Cyt c) is also involved in initiation of apoptosis, including binding into cardiolipin and release from mitochondria to the cytoplasm [137 138]. Cyt c is free within the intermembrane of mitochondria during the release process and in the cytoplasm after release. It will be of great significance for early apoptotic cell detection if a label-free approach is developed to directly probe Cyt c release in real time.

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Figure 10.10  (A) A rapid and ultrasensitive method for cell viability based on SERRS of MTT formazan; (B) A cartoon demonstrating Raman-MTT based assays for cell viability and the in situ imaging of the human lung cancer cell line A549, which shows the peak intensity of 722 cm−1 distribution in the A549 cells [133]. Part A: Adapted with permission from [132], copyright (2013) American Chemical Society; Part B: Reproduced by permission of The Royal Society of Chemistry.

Traditional detection method for determination of the released Cyt c from mitochondria is Western blotting [139 140]. In recent years, several novel approaches have been developed such as fluorescence-based strategies [141 142] with highly selectivity and sensitivity. However, almost all these methods require time-consuming procedures for the preparation of specific aptamers or antibodies in order to capture Cyt c based on bio-recognition. Moreover, none of these methods can distinguish the redox states of the released Cyt c. The as-prepared Ni NWs with magnetic property were capable of selectively reducing Cyt c among diverse heme proteins. More importantly, the Ni NWs were successfully used for rapid Cyt c reduction and determination of the released Cyt c from mitochondria in apoptotic Hela cells. It is noted that the redox states of the released Cyt c can easily be determined with the aid of the Ni NWs, and label-free highly sensitive Cyt c quantification can be achieved with the Ag nanopartices by SERS

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Figure 10.11  Schematic diagram of in situ detection of the Cyt c release from mitochondria in actinomycin D-induced apoptotic Hela cells by RR/SERS spectroscopy. (A) RR spectra of the Cyt c in isolated mitochondria solution before (black) and after (red) reduction by the Ni NWs, and RR spectrum of the reduced Cyt c; (B) SERS spectra of reduced Cyt c at concentrations of 1 µM (dark cyan) and 1 nM (orange), respectively; (C) Cyt c concentration-dependent relative SERS intensities of the band at 1374 cm-1. Adapted with permission from [143], copyright (2019) American Chemical Society.

spectroscopy. Without the Ni, it will be impossible for SERS to determine the Cyt c redox states outside mitochondria. Moreover, the high sensitivity is also attributed to the combination of Ni and Ag. The reduction reaction can complete within 1 min and the Ni NWs can be conveniently collected and separated by an external magnet, which display great potential in the applications of Cyt c function investigation in apoptosis.

4.3  Probing intermolecular interactions SERS is capable of provide more detailed structural information of target molecules owing to the excellent sensitivity and selectivity, which offers a new way of probing intermolecular interactions. We attempted to explore the effect of intermolecular H-bonding on CT, and a detection method for enantioselective discrimination was accordingly developed. On the other hand, Ni nanostructured supports were found to allow the investigation of ET between redox proteins.

4.3.1  The effect of hydrogen bonds on CT The effect of intermolecular interaction on SERS profiles has been simply neglected in many cases, which may restrict some of the practical applications and also lead to controversial conclusions. Hydrogen-bonding (H-bonding) is known to strongly affect

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the intrinsic properties and performance of molecules [144], and the possible modifications to the polarizability of a molecule caused by H-bonding are still ambiguous. A better understanding of H-bonding via SERS spectroscopy is thus necessary for a deeper comprehension of the modification to the electronic structure of a molecule resulting from H-bonding interaction. We have systematically studied the CT-induced SERS spectra of MBA under the influence of different intermolecular H-bonds. A self-assembled monolayer of 4-MBA at the surface Ag nanoparticles was used as a model system. By comparing SERS spectra obtained under normal and basic conditions, the effect caused by H-bonding can be further understood based on the relative intensities and frequency shifts of the SERS signals. The SERS results revealed an important influence of the intermolecular H-bonding on the electronic structure of the molecules. The enhanced b2 mode of 4-MBA can be considered as a manifestation of the CT transition from the Fermi level of the Ag nanoparticles to the LUMO of the adsorbed MBA molecules promoted by H-bonding. This study paves the way to probe the significant influence of intermolecular H-bonding on SERS spectroscopy. Moreover, the intermolecular interactions in a composed system of p-aminothiophenol (PATP) and benzoic acid (BA) were studied by SERS [145]. The results suggested that H-bonds formed through intermolecular interactions between the –NH2 and –COOH groups and promoted the CT transition from the Ag substrate to the adsorbed PATP molecules. Accordingly, the intensities of the non-totally symmetric vibrations (the b2-type bands) of PATP are influenced through the Herzberg–Teller contribution (Fig. 10.12).

Figure 10.12  (A) Ag NPs/MBA/Aniline assembly, (B and C) normalized SERS spectra of MBA for the band at 1075 cm–1 upon exposure to varying of aniline and the concentrations of aniline are 0, 10–8, 10–7, 10–6, 10–5, 10–4, 10–3, and 10–2 M (A–H, respectively). Adapted with permission from [146], copyright (2014) American Chemical Society.

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4.3.2  Enantioselective discrimination by hydrogen binding Molecular chirality is a prominent characteristic of biological processes, and enantiomers of a chiral molecule may exhibit striking differences in physiological responses. The exploitation of approaches to discriminate between enantiomers is critical in the fields of science and technology. Considerable efforts have been devoted to the development of specific chiral selectors that enable enantioselective discrimination by vibrational circular dichroism, UV/Vis, fluorescence, Raman optical activity, and NMR spectroscopy. However, most of these techniques are time-consuming and require complicated syntheses and pretreatment steps. Therefore, a simple, generic, and efficient technique to realize enantioselective discrimination is urgent needed. There have been few precedents for chiral discrimination by SERS, and they have used specific chiral molecules as selectors to obtain discernible spectra between the enantiomers of a chiral molecule, which suffered from a lack of generality and poor distinction between enantiomer [147 148]. It was still difficult for SERS-based enantioselective detection because the difficulties in the fabrication of a SERS receptor with the specific stereochemical properties. Changes in the relative Raman intensities of the adsorbates in a SERS spectrum can be useful for estimating the occurrence of a photoinduced CT process [151], which should be observed when the adsorbed probe molecule interacts with the two enantiomers of a chiral molecule. Herein, we developed, for the first time, a simple and SERSbased method for enantioselective discrimination based on intermolecular hydrogen bonding and CT contributions in a complex consisting of Ag nanoparticles and 4-MPY (Ag–MPY) covalently bonded by S-Ag bonds. After fabrication, the Ag–MPY complex was immersed in various chiral molecules. The results showed that the pair of bands at 1202 and 1220 cm−1 can be used as an indicator and as a direct response to the enantioselectivity through the formation of hydrogen bonds between MPY and the chiral alcohol. The calibration curve (Fig. 10.13A) revealed a linear correlation with a coefficient of R2 = 0.9948, indicating the feasibility of the proposed method for enantioselective discrimination. These results provide important improvements in the field of label-free enantioselective discrimination without the employment of any chiral agents. Moreover, to further confirm the universality of the SERS-based approach and to explore the possible mechanism of enantioselective discrimination, we utilized an achiral SERS probe molecule PATP, as the chiral selector. Excitation wavelengthdependent SERS experiments were conducted for the Ag-PATP complex immersed in racemic TFIP and R-TFIP with excitations at 514, 633, and 785 nm. As shown in Fig. 10.13B, when racemic TFIP was involved, both a dramatic decrease in the intensity of the b2 modes of PATP and a weakened SERS signal intensity occurred with the laser excitation of 514 and 633 wavelengths. The degree of CT of PATP in this chiral discrimination system demonstrated the key effect of CT contribution on the SERS signal of PATP with interaction with R-TFIP. The mechanism of this CT-induced chiral discrimination with regards to the corresponding energy levels of Ag and PATP was explored and discussed (Fig. 10.13B, right). Accordingly to the energy levels, the laser of a 633 nm laser (ca. 1.96 eV) can promote an electron from the Fermi level of Ag to an energy level (2.44 eV) just below

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Figure 10.13  (A) Normalized SERS spectra of the Ag–MPY complex in the presence of TFIP with various ee values and the correlation of the relative intensity ratio of I1202/I1220 with the ee values (in %); [149] (B) SERS spectra and the degree of CT (ρCT) of the Ag-PATP complex after immersion in racemic TFIP and R-TFIP with excitation wavelengths of 514 (A), 633 (B), and 785 nm (C); (right) the energy level diagram of the Ag-PATP complex assembly at the energy of the 633 nm laser excitation. Adapted with permission from [150], copyright (2016) American Chemical Society.

the LUMO level of the adsorbed PATP and close to the CT resonance region, allowing a CT resonance. When PATP interacts with TFIP by intermolecular hydrogen bonding, the intrinsic differences between the two S and R enantiomers resulted in different bonding geometries in the structure of the Ag-PATP complexes. This is evidenced by an obvious red shift occurred in the frequency of the a1 mode at 1075 cm−1 in the SERS spectra upon Ag-PATP changing from the immersion in racemic TFIP to that in R-TFIP with all the three laser excitations. In the case of the 514 and 633 nm excitations, the laser energy was sufficient to induce a CT transition for Ag-PATP. However, in the case of 785 nm excitation, the laser excitation is insufficient to cause electron transfer, and therefore, there was no noticeable CT contribution for the Ag-PATP complex with the introduction of R-TFIP at this laser excitation.

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4.3.3  ET between redox proteins Electron transfer (ET) is a fundamental reaction in a variety of biological processes, for example, photosynthesis, respiratory chain, and metabolism. To utilize dedicated redox proteins for biotechnological applications, it requires adsorption of the biomolecules on conductive support materials, in which ET is largely controlled by the affinity of a certain protein to a given surface and its orientation in the immobilized state. The surface properties of the support material control important parameters like biocompatibility, protein coverage, and electronic communication between metal and protein. Structural information on surface bound molecules can be obtained via SERS. Efficient plasmonic field enhancement in the violet spectral region requires the use of Ag supports [152]. However, direct adsorption of proteins to a bare Ag surface frequently causes degradation due to the inevitable presence of Ag+ cations and the high electrostatic fields in the metal/solution interface. We used a Ni support for immobilizing cytochrome b5 (Cyt b5) and myoglobin (Mb) in a functional intact manner and to explore the potential of SERRS to monitor heterogeneous and inter-protein ET processes of adsorbed proteins (Fig. 10.14). The results demonstrated that the Ni surface exhibits a high affinity for protein binding and furthermore, due to the low electric fields present at the Ni interface, protein denaturation is prevented even for direct adsorption of the protein to the metal surface. The orientation of the redox protein on the Ni surface was favorable for an efficient ET between Ni and Cyt b5. At the same time the flexibility and biological function of the immobilized Cyt b5 was preserved such that it was capable to act as an electron relay for the reduction of met-Mb. Our results open up a new possibility for Ni electrodes as a promising support for bioelectronic devices and biosensors on the one hand and for SERS spectroscopic investigations on the other hand.

Figure 10.14  Schematic diagram of study on ET between Cyt b5 to Mb on Ni support (A); (B) time-dependent changes of the SERRS spectra of Cyt b5 immobilized on Ni surfaces after addition of metMb; (C) ET between Ni support and Cyt b5, Cyt b5 and Mb [153].

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5  Conclusions and outlook In summary, we introduced SERS-active semiconductor nanomaterials and the related enhancement mechanism. All the results cited here demonstrated that semiconductors have been promising SERS-active substrates with a considerably large Raman enhancement factor under optimized conditions, excellent chemical and thermal stability, spectral reproducibility, and superior selectivity. SERS on semiconductors may be applicable in electrochromic devices for interfacial characterization such as in probing the adsorption configuration of dye molecules and the efficiency of the charge CT process, which indicated a great potential application in dye-sensitized solar cells. On the other hand, the SERS-based applications in chemical and biological sensing, and probing intermolecular interactions were outlined according to the related progress achieved in our group. The enhancement mechanism of SERS on semiconductors is however still something unclear. As for the SERS induced by plasmonic nanoparticles, one can experimentally identify the plasmon “resonance” which enhances Raman enhancement. However, in the case of SERS based on the CT resonance, the direct identification of resonance is still not achieved even though there are some possible discussions based on various calculation methods [154,155]. Moreover, so far only a few organic molecules are found to be selectively enhanced by semiconductors, which limit the application fields of semiconductor-induced SERS. Thus, more deep studies are necessary to be conducted for a better understanding of the SERS on semiconductors, and further improvements in spectral reproducibility of SERS-based sensors will make them more versatile and applicable.

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Jan Krajczewski, Andrzej Kudelski Faculty of Chemistry, University of Warsaw, Poland

Chapter outline 1 Introduction 387 2  Interaction of light with the plasmonic nanoparticles   388 3  Synthesis of plasmonic cores for SHINERS nanoresonators  391 4  Formation of the protecting layer  394 5  Example applications of SHINERS spectroscopy  395 6 Summary 409 References  409

1 Introduction Plasmonic nanoparticles are used for many different purposes. One of the important fields of application of plasmonic nanostructures is carrying out analysis of various surfaces using different optical spectroscopies. These analytical methods utilizing plasmonic nanoparticles are based on a significant increase in the intensity of the electromagnetic field in the proximity of illuminated plasmonic nanoparticles. The local field enhancement induces increase in the efficiency of various optical processes such as: fluorescence [1], second harmonic generation [2], Raman scattering [2], Raman optical activity [3], hyper-Raman scattering [4], coherent anti-Stokes Raman scattering [2], or infrared absorption [5]. Since light can easily penetrate through some dense media (e.g., through air or water) optical measurements utilizing plasmonic nanoresonators can be used to carry out optical chemical analysis of so-called buried interfaces (surfaces between two “dense” media, as the interfaces between the solid samples and the liquids), which include interfaces of various biological samples in the “natural” environment. Surfaces of biological objects have to be often analyzed under “normal” conditions because such objects are usually damaged after introducing to the vacuum. The most important, from the practical point of view, method of the surface analysis using plasmonic nanostructures is analysis utilizing Raman scattering. Direct interaction between the plasmonic metal structures and various biological molecules (e.g., peptides) may lead to a change in the structure of the analyzed biomolecules. Moreover, when the goal of the investigation is to determine the mechanism of the interaction between the adsorbate and the adsorber, Molecular and Laser Spectroscopy. http://dx.doi.org/10.1016/B978-0-12-818870-5.00011-3 Copyright © 2020 Elsevier Inc. All rights reserved.

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it is important to prevent the direct interaction between the analyzed adsorbed molecules and the surface of the plasmonic metal nanoresonator (if such interactions would occur the measured spectra would be dominated by the contribution from the molecules of the adsorbate interacting with the surface of the plasmonic nanoresonators, not by the contribution from the molecules of the adsorbate interacting with the analyzed surface). To prevent the direct contact between the metallic plasmonic resonators and the investigated surfaces (or the analyzed adsorbate), the plasmonic nanoresonators are covered with very thin and relatively inert protecting layer. The technique utilizing surface-protected plasmonic nanoresonators used to carry out Raman measurements is called shellisolated nanoparticle-enhanced Raman spectroscopy—SHINERS. SHINERS spectroscopy was developed by Tian et al. in 2010 [6]. This technique seems to be very useful tool to carry surface analysis of various structures, for example, by the deposition of the SHINERS nanoresonators on living cells one can record in in situ conditions the Raman spectrum dominated by the contribution from the species present in the outermost parts of the cells of various organisms [6,7]. In this chapter, we present the mechanism of the enhancement of the efficiency of generation of Raman signal by SHINERS nanoresonators, the methods of the synthesis of the plasmonic cores of SHINERS resonators, the methods of the deposition of the protecting layers, and examples of the applications of SHINERS spectroscopy (in chemical and biochemical analysis and in catalysis).

2  Interaction of light with the plasmonic nanoparticles When nanoparticles formed from metals with a negative real and small positive imaginary dielectric constant (e.g., nanoparticles of gold or silver) are illuminated by the electromagnetic radiation, the electric field of the electromagnetic wave induces collective oscillations of surface conduction electrons called surface plasmons [8]. The oscillation of electrons in the metal nanoparticles leads to the creation of the electric dipole (see Fig. 11.1) and, in consequence, to the amplification of the electric field in close proximity of the nanoparticle. For a spherical metal nanoparticle the magnitude of the induced dipole (p) is proportional to [10]: p~

ε M ( λ ) − ε out ( λ ) ε M ( λ ) + 2ε out ( λ )

Figure 11.1  Schematic illustration of plasmon oscillation for a metallic sphere, showing the displacement of the conduction electron charge cloud relative to the nuclei. Reproduced with permission from [9]. Copyright 2002 American Chemical Society.

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where: λ is the wavelength of the excitation radiation, εM(λ) and εout(λ) are the dielectric functions of the metal and the surrounding medium, respectively. As can be seen from the earlier equation, when the value of εM(λ) is close to the value of −2εout(λ), the denominator of this equation is close to zero, and hence strong electric dipole is induced which leads to very large local intensity of the electric field. Since εM(λ) is a complex number, it is obviously not possible to satisfy in full the condition εM(λ) = −2εout(λ) exactly (which would imply p → ∞). In some cases, the imaginary part of Im[εM(λ)] at a given λ is small (e.g., for Ag when visible radiation is used), and, in these cases, the resonance effect could be observed (so-called surface plasmon resonance) and the local field enhancement will be limited only by how small Im[εM(λ)] is at that particular λ. This enhancement effect is illustrated by Fig. 11.2, which shows the fourth power of the electric field enhancement factor (Gloc)—for explanation why the fourth power of the field enhancement is presented vide infra—in the proximity of illuminated spherical silver nanoparticle surrounded by the SiO2 layer. Further simulations showed that for anisotropic plasmonic nanoparticles, the highest field enhancements are observed on sharp structures on their surface (such as sharp apexes and edges)—see Fig. 11.3A. For agglomerates and aggregates of plasmonic objects, very high field enhancement is also induced in the slits between plasmonic nanostructures—see Fig. 11.3B. As mentioned in the Introduction, the local enhancement of the intensity of the electric field induces local increase in the efficiency of various optical processes. Undoubtedly, from the practical point of view, the most important spectroscopic application of the plasmonic nanoresonators is inducing an increase in the efficiency of the generation of the Raman signal. This effect is called

Figure 11.2  (A) Calculated distribution of the fourth power of the electric field enhancement factor (Gloc) in the proximity of illuminated spherical silver nanoparticle surrounded by the SiO2 layer. The wave vector k and the polarization of incident light are denoted by arrows. The excitation wavelength is 785 nm. (B) Theoretically calculated Gloc values versus the incident wavelength at a given point A (denoted by a red dot), which is about 1 nm away from the surface of the Ag core. Optical constants of the silica shell and the medium are 1.44 and 1.33, respectively. Reproduced with permission from Ref. [11]. Copyright 2009 American Chemical Society.

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Figure 11.3  (A) Electric field enhancement contours around the silver prism for a plane that is perpendicular to the trigonal axis and that passes midway through the prism. The prism is illuminated by the light that has k along the trigonal axis and E along the abscissa. The size of the prism: side length 100 nm, thickness 16 nm. The wavelength of the excitation radiation: for the left Image 770 nm, and for the right Image 460 nm. (B) Calculated distribution of the fourth power of the electric field enhancement in the proximity of the illuminated dimer of Ag@SiO2 nanoparticles (upper panel) and SiO2@Ag nanoparticles (bottom panel). Part A: Reprinted with permission from Ref. [9]. Copyright 2003 American Chemical Society; Part B: Reprinted with permission from ref. [15]. Copyright 2015 the Owner Societies of PCCP.

surface-enhanced Raman scattering (SERS). In SERS spectroscopy, the increase in the intensity of the measured Raman signal is roughly proportional to the fourth power of the field enhancement [8,12]. This fourth power dependence of the SERS enhancement factor on the field enhancement often leads to a very large increase in the efficiency of the generation of the measured SERS signal, and, for example, in some

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cases, it is possible to record SERS signal even from a single molecule [13,14]. Due to these very large achievable enhancements factors, SERS spectroscopy is one of the most sensitive analytical tools. In other spectroscopies utilizing plasmonic nanoresonators, the correlation between the field enhancement and the increase in the intensity of the optical signal is usually different, for example, in surface-enhanced infrared adsorption the increase in the efficiency of the adsorption of infrared radiation is proportional to the second power of the electromagnetic field enhancement [8]. An important issue when the plasmonic nanoresonators are used to enhance the efficiency of various optical processes is keeping molecules understudy in a very close distance to the surface of the plasmonic nanoresonator. Since for different optical process, the dependences of the increase in the efficiency of the process on the field enhancement are different, the correlations between the enhancement factor and the distance to the surface of the plasmonic nanoresonator are also different. For example, in the case of SERS measurements, the achievable SERS enhancement decreases as a function of r−10 with increasing distance from the plasmonic nanoparticle [16].

3  Synthesis of plasmonic cores for SHINERS nanoresonators The first SHINERS experiments have been carried out using, as plasmonic cores, spherical gold nanoparticles synthesized by the standard citrate method [6,17,18]. In 2012 Tian et al. [19] and Kudelski and Wojtysiak [20] carried out for the first time the SHINERS measurements using silver nanoresonators instead of the gold one, and they showed, that, in some cases, application of the silver plasmonic nanoresonators allows for a significant increase in the sensitivity of the SHINERS measurements. The silver plasmonic cores used for the first SHINERS measurements have been also spherical and have been synthesized using a standard citrate method [19,20]. As it has been shown in many theoretical simulations, the highest field enhancement is induced on sharp structures on the surface of the illuminated plasmonic nanoparticles [9,12,21]. Therefore, one may expect that an effective method of increasing the efficiency of the intensity enhancement of the electric field by the plasmonic nanoparticles should be the formation of various sharp structures on their surfaces, such as apexes and edges. Moreover, for anisotropic nanoparticles, the wave number of the radiation which induces the surface plasmon resonance can be changed in a significantly broader range than for spherical nanostructures. Due to expected higher activity in the SERS measurements and because of an easier possibility of changing of the condition of the surface plasmon resonance, some anisotropic plasmonic nanostructures having the shape of a cube, rod, dipyramid, decahedral, and a star have been tested as plasmonic cores of SHINERS nanoresonators [7,22,23]. As it has been expected, when anisotropic plasmonic nanoparticles containing many sharp apexes and edges were used as SHINERS nanoresonators, the intensity of the measured Raman signal was significantly larger than in the case of experiments carried out using spherical plasmonic nanoparticles having roughly the same size and produced from the same

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amount of plasmonic material. For example, in the case of nanoparticles having the shape of decahedrons or dipyramids, this increase in the SERS activity is equal to about one order of magnitude [7,23]. Anisotropic plasmonic nanostructures can be often synthesized using relatively simple chemical methods. For example, gold nanorods and dipyramids could be obtained using so called seed-mediated growth synthesis [24,25]. In this process, anisotropic nanoparticles are formed as a result of the growth (growth of nanoparticles is achieved by a reduction of the metal precursor, for example, HAuCl4, by a weak reducing agent like ascorbic acid) of initially spherical seeds in solutions containing various surfactants. Silver decahedrons are usually obtained by the photochemical synthesis [7]. In this synthesis aqueous sol of spherical silver nanoparticles is transformed under the light exposure into the sol containing anisotropic nanostructures. Anisotropic plasmonic nanoparticles could also be synthesized using various compounds that are selectively adsorbed on various crystalline facets and hence they strongly influence the growth process. For example, reduction of silver ions in hot dimethyl formamide in the presence of polyvinyl pyrrolidone leads to formation of silver nano-decahedrons [26], while reduction of silver ions in hot ethylene glycol in the presence of sulfide ions leads to formation of silver nanocubes [27]–all these kinds of nanoparticles have been used as plasmonic cores in SHINERS nanoresonators [7,28]. The other interesting kind of plasmonic cores for SHINERS nanoresonators are hollow nanostructures. Hollow nanostructures exhibit plasmonic properties different and often superior to the respective solid nanostructures [29,30]. For example, the position of the plasmon resonance band for the spherical hollow gold nanoparticles can be changed (by varying the diameter of the nanoparticle and the thickness of the shell) in a significantly broader wavelength range than in a case of solid nanostructures [29,30]. The achievable significant red-shift of the plasmon band for hollow gold nanostructures facilitates the various biomedical application of such nanoparticles because it allows tuning the position of the plasmonic band to the transparent window of many biological tissues (800–1200 nm) [31,32]. Therefore, SHINERS nanoresonators containing hollow plasmonic cores could be especially useful for experiments in which irradiation with the red light or infrared radiation is required. In 2015, Abdulrahman et al. [33] reported the first example of SHINERS measurements using hollow silver and hollow gold nanostructures as plasmonic cores of SHINERS nanoresonators. Hollow silver nanoparticles were synthesized according to the modified method proposed by Moshe and Markovich [34]. This method involves two steps. First, Ag2O nanoparticles are formed in an alkaline solution. Then, obtained Ag2O nanoparticles are reduced by the addition of freshly prepared solution of sodium borohydride—since the reaction starts from the outermost parts of the Ag2O nanoparticles and the volume of silver that is formed by the reduction of silver oxide is significantly smaller than the volume of the reduced oxide, hollow silver nanostructures are formed. Hollow gold nanospheres were synthesized using cobalt nanoparticles as sacrificial templates [35]. In the first step of the synthesis, cobalt nanoparticles are formed by the reduction of cobalt chloride by sodium borohydride. Then, the obtained cobalt nanoparticles are added to the solution containing chloroauric acid, and due to the galvanic replacement reaction between metallic cobalt and HAuCl4, gold replaces

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metallic cobalt. After the replacement reaction is finished, the unreacted cobalt is dissolved by aerating of the sol of Co@Au nanoparticles, and, as a result, hollow gold nanostructures are formed. By changing the ratio of the numbers of moles of HAuCl4 and Co it is possible to control the size of the cavity in gold nanoparticle and hence to control their optical properties (see Fig. 11.4). Analogously to SHINERS nanoresonators containing anisotropic cores also SHINERS nanoresonators containing hollow cores are in some cases significantly more efficient in the increasing efficiency of the generation of Raman signal than the solid nanostructures having similar size—especially when it is possible to significantly change the frequency of the surface plasmon resonance for the obtained systems [33].

Figure 11.4  (A) The visual appearance and the respective UV–vis extinction spectra of colloids obtained by the galvanic replacement reaction between cobalt nanoparticles and the solution of HAuCl4. The ratio of numbers of moles of HAuCl4 and Co used in the reaction was equal to (a) 0.10:1, (b) 0.15:1, (c) 0.25:1, (d) 0.33:1, and (e) 0.50:1. (B) TEM images of the nanostructures obtained when the ratio of the numbers of moles of HAuCl4 and Co was equal to (B1) 0.10:1, (B2) 0.25:1, and (B3) 0.50:1. Reproduced with permission from Ref. [33]. Copyright 2015 American Chemical Society.

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4  Formation of the protecting layer As mentioned in the Introduction, to obtain SHINERS nanoresonators, the plasmonic cores have to be covered with the protecting layer. Protecting layers are typically formed from various relatively chemically inert oxides, such as SiO2, MnO2, TiO2, or ZrO2. However, in some cases, organic polymers or carbon shells are also used to protect plasmonic cores. Nowadays, the cores of SHINERS nanoresonators are usually covered by the SiO2 layer, which is formed by the decomposition of Na2SiO3 or decomposition of tetraethyl orthosilicate. The deposition of the nanometric silica layer by the decomposition of Na2SiO3 is usually carried out using the procedure proposed by Mulvaney et al. [36]. Briefly speaking, to the sol of metal nanoparticles (Ag or Au) a solution of Na2SiO3 acidified with HCl is added, and the reaction mixture is kept under stirring for a relatively long time (even up to 6 days). In the case of deposition of silica on highly anisotropic nanoparticles their surfaces are often protected by the layer formed from alkanethiols, for example, from 16-mercaptohexadecanoic acid [36]. As mentioned earlier, silica layers can be also formed by the decomposition of tetraethyl orthosilicate that is catalyzed by ammonia or amines (like dimethylamine). This reaction is usually carried out in organic solvents like isopropanol [22] or ethanol [37]. The thickness of the formed SiO2 shells depends on the initial concentration of the added tetraethyl orthosilicate. An important advantage of this method is a relatively short time required to deposit even thick SiO2 layer—usually only ca. 15 minutes. An important drawback of protecting layers formed from silica is their low durability in some solutions, for example, solutions having high or low pH. Therefore, for SHINERS investigations in the alkali media Tian et al. suggested using Au@MnO2 nanoresonators [38]. Layers of manganese oxide on gold cores have been deposited by the reduction of KMnO4 by K2C2O4 carried out in the presence of gold nanoparticles in a solution with pH equal to 9.5 [38]. The thickness of the formed MnO2 layer can be controlled by the change of the concentration of KMnO4. To deposit MnO2 layer on silver cores the described above procedure has to be slightly modified—silver cores dissolve themselves when added to such a reaction mixture [39]. In order to deposit MnO2layer on Ag cores the reduction of KMnO4 by K2C2O4 has to be carried out at significantly higher pH (optimally at pH equal to ca. 12) [39]. Also, SHINERS nanoresonators protected by the ZrO2 layers are significantly more stable in acidic and alkaline solutions than the nanoresonators protected by the silica shells. Moreover, since zirconia has exceptionally large refractive index, deposition of the ZrO2 layer significantly shifts the frequency of the plasmon resonance [40]. Zirconia layer is usually deposited by the decomposition of zirconium(IV) isopropoxide [40]. SHINERS nanoresonators can be also protected by the carbon layer or by the layer formed from some polymers. To form carbon shells, Yang et al. deposited p-mercaptobenzoic acid on the surface of silver nanoparticles and then carbonized formed organic layer in the concentrated solution of sulfuric acid [41]. TEM analysis of the obtained nanostructures showed that after this process the silver cores are covered by the carbon layer with a thickness of about 2 nm [41]. SHINERS nanoresonators protected by the layer of polydopamine have been synthesized by Ye et al. [42]. Polydopamine layers on

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Figure 11.5  (A) SERS spectra of pyridine adsorbed on a smooth gold surface which has been covered with Au@SiO2 nanoparticles having the diameter of the gold core of 55 nm. Measurements have been carried out for Au nanoparticles covered with SiO2 layers with different thicknesses. (B) Determined experimentally and calculated theoretically the dependences between the thickness of the SiO2 shell and the intensity of the measured SHINERS spectrum of pyridine. Reproduced with permission from Ref. [6]. Copyright 2010 Springer Nature.

gold cores have been formed by the self-polymerization of dopamine, and the thickness of the layer was controlled by the change of the concentration of dopamine. An important problem in the synthesis of SHINERS nanoresonators is controlling the thickness of the protecting layer. If the layer is too thin, the probability of the appearance of pine-holes increases. However, increasing of the thickness of the protecting layer deposited on the plasmonic cores leads to a decreasing in their SERS activity (see the section “Interaction of light with the plasmonic nanoparticles”). An example dependence between the shell thickness and the intensity of the SHINERS spectra are presented in Fig. 11.5 [6].

5  Example applications of SHINERS spectroscopy Plasmonic nanoparticles covered with the protecting layers (e.g., from SiO2) have been used to detect and determine many compounds both in the bulk samples and when they are adsorbed on various surfaces. It allows, for example, to use SHINERS spectroscopy to determine the mechanism of various surface processes, including the mechanism of some electrochemical and catalytic reactions. SHINERS spectroscopy has been also used to analyze surfaces of various biological samples. In many cases the deposited protecting layer plays only one role from the three mains roles of the protecting layer in the standard SHINERS measurements, for example, deposited layer is only used to increase the stability of the plasmonic core (the is no need to prevent the direct interaction between the adsorbate and the metallic core or to prevent agglomeration of plasmonic nanoparticles). Therefore, there are many non-typical SHINERS

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experiments and in some cases experiments carried out using surface-protected plasmonic nanoparticles are even not called SHINERS measurements. However, to present broader image of the practical applications of plasmonic nanoparticles covered with the protecting layer in the Raman measurements we have briefly described these types of experiments also. SHINERS spectroscopy has been used to detect and determine the concentration of many various chemical compounds or biological objects. One of the first systems analyzed using SHINERS spectroscopy is the skin of an orange fruit contaminated with methyl parathion (which is an efficient insecticide) [6]. Raman spectra of pure orange skin is dominated by two bands at 1156 and 1528 cm−1. These Raman bands are assigned to the molecules of carotenoid which is present at the skin [6]. The Raman spectra of the pure orange skin and the orange skin contaminated with methyl parathion are practically identical (it is not possible to observe any significant differences between these Raman spectra), and therefore, standard Raman spectroscopy could not be used to detect this insecticide. However, deposition of SHINERS nanoresonators on the surface of the contaminated orange skin allows on identification in the measured Raman spectrum of a new Raman band at 1350 cm−1, which could be assigned to the vibration of the methyl parathion molecule (see Fig. 11.6). The first SHINERS detection

Figure 11.6  Raman spectra of (a) solid methyl parathion, (b) skin of the orange fruit, (c) skin of the orange fruit contaminated by methyl parathion, (d) skin of the orange fruit covered with hollow-Ag@SiO2 nanoparticles, and (e) skin of the orange fruit contaminated by methyl parathion and covered with hollow-Ag@SiO2 nanoparticles. To contaminate the orange skin with methyl parathion 0.5 µg of methyl parathion was deposited on the area of ca. 10 cm2 of the surface of the orange fruit. Reproduced with permission from Ref. [33]. Copyright 2015 American Chemical Society.

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of methyl parathion has been carried out using Au@SiO2 nanoparticles [6]. Later, other nanoresonators also have been used, for example Ag@SiO2 [20], hollow–Ag@ SiO2 [33], or Ag@MnO2 nanoparticles [39]. The detection limit of methyl parathion in SHINERS measurements using, for example, hollow–Ag@SiO2 nanoparticles was estimated to be ca. 3 ± 2 ng [33]. As mentioned in the section “Interaction of light with the plasmonic nanoparticles,” especially large enhancement factors of the electromagnetic field are observed on sharp apexes and edges of anisotropic plasmonic nanoparticles. Therefore, application of anisotropic nanoparticles as SHINERS nanoresonators should allow detecting pesticides in a lower concentration range than when spherical nanoparticles are used. Application of anisotropic SHINERS nanoparticles in detection of a pesticide was tested by Kolataj et al. [23], who decided to detect the pesticide thiram using silicacovered dipyramidal Au SHINERS nanoresonators. Kolataj et al. found that using these nanoresonators allows detecting thiram on tomato skin with a limit of detection estimated as 0.9 ng/cm2 [23]. In order to detect thiram, a contaminated tomato skin was covered by the sol of dipyramidal-Au@SiO2 nanoparticles (100 µL of such sol was deposited on 1 cm2 of the tomato skin) and the solvent was evaporated. In the recorded SHINERS spectra (see Fig. 11.7) one can identify bands characteristics for thiram: at 548 cm−1 due to the ν(S–S) vibration, at 924 cm−1 due to ν(P–N), at 1138 cm−1 due to ρ(CH3) + ν(C–N), at 1369 cm−1 due to ρ(CH3) and at 1496 cm−1 due to ν(C–N) vibration. Band located at 1163 cm−1 could be assigned to the ν(C–C) stretching vibration of the carotenoids molecules, which are present in the tomato skin

Figure 11.7  Raman spectrum of “clean” tomato skin (before and after covering with dipyramidal-Au@SiO2 nanoparticles) and Raman spectra of tomato skin contaminated with thiram on which dipyramidal-Au@SiO2 nanoparticles have been deposited. Surface concentration of thiram: 120 pg cm−2, 1.2 ng cm−2, 12 ng cm−2, and 120 ng cm−2. Arrows point to the band due to the ν(C–C) stretching vibration of carotenoid molecules present in the tomato skin. Reproduced with the permission from Ref. [23].

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[23]. The surface concentration of thiram was calculated from the intensity of its Raman band in the measured SHINERS spectra. An interesting example of SERS analysis using surface-protected nanoresonators is detection of highly toxic herbicide paraquat. Some reports connected poisoning by paraquat or its derivatives with the development of Parkinson’s disease [43]. Unfortunately, standard SERS determination of paraquat using bare gold nanoparticles is not possible in human urine or plasma because is such conditions molecules of paraquat could not reach the surface of the gold nanoparticles—in these cases, the surface of the unprotected gold is covered by a thick layer of another present in these samples species (mainly proteins). Therefore, to detect paraquat using SERS spectroscopy, ultrathin layer of silica has to be deposited on the surfaces of gold nanoparticles. Application of Au@SiO2 nanoresonators allowed to detect paraquat using SERS spectroscopy in a wide concentration range (in urine from 1 to 20 mg/L). Recorded spectra of paraquat on Au@SiO2 nanoresonators exhibit four strong bands: at 839 cm−1 assigned to the C–N stretching mode, at around 1183 cm−1due to the C = C bending vibration, at 1293 cm−1which belongs to the C–C distortion mode, and the band at 1643 cm−1, which is due to the CN stretching vibration of paraquat- the detection of this herbicide is based on the measurement of the intensity of the Raman band due to the CN stretching vibration [44]. In addition to bands of paraquat in the recorded SHINERS spectra, matrix bands at 620, 730, and 1350 cm−1 are also visible. However, the presence of these bands does not affect the detection of paraquat [45]. SHINERS measurements using Au@SiO2 nanoparticles allows to detect paraquatin urine or plasma at such low concentration as 0.5 µg/dm3 [45]. One of the simplest examples of the analytical applications of surface-protected nanoresonators is the use of Au@SiO2 nanoparticles for the detection of cyanide ions in water samples [46]. To carry out such measurements, Gao et al. [46] created gold plasmonic cores covered with the silica shells that, on purpose, contained some pinholes. Gao et al. [46] found that the pH of the reaction solution is the key factor for the formation of the pinhole silica shell around nanoparticles, therefore, the deposition of silica has been carried out by the decomposition of sodium silicate in pH higher than 10.2 [46]. The silica shell outside the Au core significantly improves the stability of the plasmonic nanoresonator by preventing the dissolution of gold in a cyanide solution, therefore, although it was not a typical SHINERS experiment (adsorbed species interact directly with the plasmonic core), the carried-out measurements have been qualified by Gao et al. as the SHINERS ones [46]. The SERS spectrum of cyanide anions adsorbed on gold is dominated by a strong sharp band at 2135 cm−1 and two other minor bands at around 297 and 390 cm−1. The band at 297 cm−1 could be assigned to the Au–CN bending mode, while the band at 390 cm−1 is assigned to the Au–C stretching vibration. The sharp strong band around 2135 cm−1 can be assigned to the CN stretching vibration [47]. Application of Au@SiO2 nanoparticles allows detecting cyanide ions in the concentration range between 1 and 500 µg/L [46]. Estimated SERS enhancement factor was equal to 7.5 × 107. What is important, described system exhibits high selectivity toward cyanide ions. The intensity of the band at 2135 cm−1 remains stable even after addition of tenfold excess of another ion. Therefore, application of such SHINERS nanoresonators allows detecting cyanide ions even in real samples like tap or lake water. Other important compounds that can be determined using SHINERS spectroscopy are nitrites. Nitrite ions (NO2−) are serious hazards for human health [48] and for

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environment [49], and therefore, the US Environmental Protection Agency established the maximum contaminant level allowed for nitrite ions in drinking water (equal to 1 mg/L) [50]. Nitrite ions do not produce strong Raman spectrum. Also addition of gold nanoparticles does not lead to the strong SERS spectra of nitrite ions. However, it has been shown that nitrite ions could be detected by using them to the diazotization of p-nitroaniline in acid media. Diazotization leads to the consequent coupling reaction and to the formation of azo dye, which has very large cross-section for Raman scattering. Recorded SHINERS spectra of the azo dye were different than its normal Raman spectrum, indicating the interaction of the molecules of the dye with Au@ SiO2nanoresonators. In SHINERS spectra of the formed azo dye one can observe many various bands, however, the detection and the determination of this compound was based on the intensity of the following three Raman bands: at 1137 cm−1 assigned to the C–N = N vibration of the phenyl rings, at 1395 cm−1 connected with the C–C vibration coupled with the N = N stretch and at 1432 cm−1 which could be assigned to the N = N stretching vibration of the trans isomer. In all three cases linear response could be observed in the concentration range from 0.5 to 6.0 mg/L with high correlation coefficient (see Fig. 11.8) [50]. The limit of detection (defined as the concentration of the analyte that is required to produce response of three times higher than the standard deviation of the noise level) was estimated at 0.07 mg/L [50]. The described method was successfully applied for the analysis of the real samples: drinking water, tap water, and mineral water. What is important, above mentioned method exhibits high selectivity against nitrite ions. Addition of even 50 times higher concentrations of

Figure 11.8  (A) Scheme showing the formation of azo dye due to the coupling reaction between p-nitroaniline and nitrite ion, (B) SHINERS spectra of formed azo dye from the nitroanaline with different concentration of nitrite ions, (C) linear response of peak located at 1137, 1395, and 1432 cm−1 against nitrite ions concentrations. Reproduced with permission from Ref. [50].

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other cations or ions did not interfere the determination of nitrite ions, because variation of the area of the band at 1395 cm−1 is less than 15% [50]. In a similar way, one can determine the concentration of iodates in iodized NaCl salt and water samples [51]. This method was based on oxidation of hydroxyloamine by iodates that lead to the formation of nitrite ions. Subsequently, formed nitrite ions react with p-nitroaniline, which leads to the formation of diazonium ions. Diazonium ions were coupled with N-(1-naphthyl) ethylenediamine dihydrochloride and, as a result, color azo dye was created. Linear response of the intensity of the Raman spectra of the azo dye was observed for the concentration range of iodates between 7.5 and 130 µg/L with high correlation coefficient. The limit of detection of iodates was estimated as equal to 2.0 µg/L [51]. Indirect detection using surface-protected plasmonic nanoparticles is also possible for mercury ions [52]. In this case, at first, Au@Ag nanorods were synthesized and coated with the nanometric layer of silica. The rhodamine 6G-derived Schiff base was also synthesized and then bonded to the SiO2-coated Au–Ag nanorods functionalized with the amino group. In the presence of Hg2+ ions the linker via which rhodamine 6G is bonded to the SiO2-coated Au–Ag nanorods hydrolyze, molecules of rhodamine 6G are detached from the SERS nanoresonator, and it leads to a remarkable decrease in the intensity of the measured SERS signal. The linear relationship between the concentration of mercury ions and the intensity of the measured SERS signal is observed for the concentration range of Hg2+ ions between 1 pM and 0.1 µM [52]. Also copper cations can be detected at picomolar concentrations by SHINERS measurements using Ag@TiO2 nanoparticles with attached 2,2’-bipyridine ligand [53]. Au@SiO2 nanoparticles can be also used for Raman determination of the concentration of perchlorate ions in a wide concentration range with the detection limit estimated on 10−6 M (0.1 mg/L) [54]. Determination of perchlorate ions is possible due to high activity in Raman scattering of the symmetric stretching vibrations of ClO4− ions which induce the appearance of the strong characteristic Raman band located at 950 cm−1 in case of solid salts or at 934 cm−1 in case of aqueous solutions [54]. Another group of chemical compounds that can be detected using Raman spectroscopy and surface-protected plasmonic nanoresonators is highly toxic organophosphorous nerve agents. Nerve agents are divided into two classes: G- and V- agents. Tabun, sarin, soman, and cyclosarin are examples of G-agents, while very toxic VX, short for “venomous agent X,” is one of the best-known examples of the V nerve agents. Wu et al. showed that VX can be detected directly at a 20 ng mL−1 level using Raman spectroscopy and the Au@SiO2 nanoparticles with pinholes in the silica layer as the SERS substrate [55]. Moreover, combined with a specific alkaline keto-oxime transformation approach, Gagents can be measured as low as 10 ng mL−1 within several minutes with excellent discrimination from V-agents and other common organophosphorous pesticides [55]. SERS measurements using Au@SiO2 nanoresonators can be also used to detect formaldehyde [56]. Formaldehyde does not give strong characteristic Raman spectra, therefore, before its detection, formaldehyde was transformed into formaldehyde azine in the reaction with 3-methyl-2-benzothiazolinone hydrazine—see Fig. 11.9A). Formed formaldehyde azine, when adsorbed on Au@SiO2 nanoparticles, generates strong SERS spectrum with four characteristic Raman bands at 873, 1275, 1401, and 1511 cm−1. The band located at 873 cm−1 is assigned to the bending vibration of the C–H groups, strong

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Figure 11.9  (A) Scheme of the chemical reaction between 3-methyl-2-benzothiazolinone hydrazine (MBTH) and formaldehyde. (B) Scheme of the rapid on-site formaldehyde detection from aquatic products. Reproduced with permission from Ref. [56]. Copyright Royal Society of Chemistry, 2013.

bands located at 1275 and 1401 cm−1 are attributed to the stretching modes in the = C–N and = N–N moieties, respectively, while the band at 1511 cm−1 is assigned to vibration of the benzene ring. The quantitative determination of formaldehyde was based on the measurements of the intensity of the band located at 1275 cm−1[56]. The developed method exhibits good linear response in the concentration range from 0.4 to 4.8 µg/dm3 and the limit of detection was estimated on 0.17 µg/L [56]. Another important compound that can be detected using SERS spectroscopy is melamine (2,4,6-triamino-1,3,5-triazine). Melamine is sometimes illegally added to various food products in order to increase the apparent protein content. Unfortunately, melamine may cause kidney stones, kidney failures or even death [57]. SERS detection of melamine in milk is possible using Au@SiO2 nanoresonators [58]. In this case SiO2 shell-isolated gold nanoparticles are used instead of the standard gold nanoparticles in order to reduce the distortion of the Raman signals originating from the charge transfer and electronic tunneling effects. Typical Raman spectrum of crystalline melamine exhibits few strong bands, with the strongest one located at 676 cm−1 and assigned to in-plane ring breathing vibration of the triazine ring. In the case of SERS detection of melamine, the linear relationship between the intensity of Raman band located at 676 cm−1 and melamine concentration is observed in the concentration range between 0.5 mg/L and 5 mg/L with high correlation coefficient 0.9895—see Fig. 11.10 [58]. SERS detection of melamine could be also carried out on silver nanoparticles coated by the carbon layer [59]. Such nanoresonators allow to detect melamine even in concentration of 10−4 M [59]. Au@SiO2 core/shell nanoparticles could be also applied for SERS detection of many important from the clinical point of view molecules, like glucose or uric acid [60]. Normal Raman spectrum of crystalline glucose exhibits bands located at 1332 and 1362 cm−1, which are attributed to the C–H bending modes, bands at 1267, 1153, 1122 cm−1 which are due to the various vibrations of the C–C–C–O chain, at 1078 cm−1 due to the C1–OH stretching mode, at 927 cm−1 due to the O–C1–H1 bending and at 858 cm−1 due to the C–C stretching. SERS spectra of glucose are slightly different from those of the crystalline samples due to the influence of the

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Figure 11.10  (A) Raman spectrum of crystalline melamine, (B) SERS spectra of melamine with different concentrations measured on Au@SiO2 nanoparticles, (C) linear relationship between the normalized intensity of the Raman band located at 676 cm−1 and the concentration of melamine. Reproduced with permission from Ref. [58].

interaction of molecules of glucose with the plasmonic surface. It was observed that the SERS bands of glucose at 935 and 1345 cm−1 could be still detected in the recorded spectra even at the glucose concentration of 2 ng/L (10−12 M), and that their intensity increases with an increase in glucose concentration—the linear range of the response is observed in the wide range of the glucose concentration between 10−3 M and 10−12 M [60]. In a similar way as glucose, uric acid could be also detected using SERS spectroscopy and Au@SiO2 nanoparticles. Similarly to glucose, SERS spectra of uric acid are also slightly different from those of the crystalline compound due to the influence of the interaction with the plasmonic surface. Quyen et al. showed that the intensity of the SERS band of uric acid at 640 cm−1 increases significantly with increasing concentration of this compound and that the detectable concentration range of uric acid in solution using this method is between 1.7 ng/L and 0.3 mg/L (10−11 M to 1.7 × 10−4 M) [60]. It is worth mentioning that the lowest detectable concentration of uric acid was estimated at 10−11 M, which is significantly lower than the detection limit of uric acid measured by cyclic voltammetry [61]. Silver nanoparticles coated by the nanometric layer of silica have been also used for the detection of proteins, like myoglobine [62]. In the presence of Cu2+ cations myoglobin conjugated Raman tags Ag@SiO2 nanoparticles specifically bound to

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iminodiacetic acid functionalized gold wafers through the coordination interactions of iminodiacetic acid—Cu2+—histidine residues available on the myoglobin surfaces. The formation of these nanoconjugates leads to increase in the measured intensity of the Raman band at 1635 cm−1 characteristic for the bending vibration of the surface hydroxyl groups. The intensity of this band linearly increases with increasing concentration of myoglobin in the analyzed sample. A linear response was observed in the concentration range from 0.01 to 10 µg/mL, and the detection limit of this technique was estimated on 1.5 ng/mL [62]. An interesting class of SHINERS nanoresonators is nanoresonators containing organic shells. For example, gold cores coated by poly(2-aminothiophenol) (PAT) have been used to detect trinitrotoluen (TNT) [63]. Due to the strong electron-withdrawing effect of the nitro group, TNT forms Meisenheimer complexes with the amino groups which come from PAT deposited on gold nanoparticles (see Fig. 11.11). Formation of Meisenheimer complexes is observed even when traces of TNT are present in the gas phase. The detection of TNT is based on the measurement of the intensity of the characteristic Raman bands of formed complex [63]. The described TNT sensor exhibits high selectivity because the presence in the analyzed sample of dinitrotoluene, nitrophenol, nitrobenzene, or toluene does not generate appearance of any significant Raman signal. SHINERS nanoresonators have been also formed by coating of gold plasmonic cores by another polymer shell, for example, polydopamine (PDA) [42]. Gold cores were prepared by the citrate method, while polydopamine layer was synthesized by

Figure 11.11  General scheme of the formation of Meisenheimer complexes between molecules of TNT and Au@PAT nanoparticles. Formation of Meiseinheimer complex allows detecting of TNT molecules using SHINERS spectroscopy. Reproduced with permission from Ref. [63]. Copyright Royal Society of Chemistry, 2012.

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the self-polymerization of dopamine and its thickness was controlled by changing the dopamine concentration. Formed PDA shells were very stable, for example, 1.3 nm PDA shell deposited on gold cores was chemically inert and stable in both strongly acidic and alkaline solutions, whereas silica layer deposited on standard Ag@SiO2 nanoparticles dissolves in alkaline solutions. Ye et al. showed that Ag@PDA SHINERS nanoresonators can be used for label-free and quantitative detection of benzotriazole, an important corrosion inhibitor, through utilizing a presumed π–π stacking interaction [42]. A broad linear range from 10−4 to 10−8 M was achieved with a limit of detection of 1 nM [42]. The limit of detection of benzotriazole using SHINERS spectroscopy is about 200 times lower than the maximum allowable level of benzotriazole in water, and is also significantly lower than that of some modern methods of benzotriazole detection such as fluorescence, liquid chromatography, and gas chromatography coupled with mass spectrometry [42]. Interesting example of surface protected SERS nanoresonators are gold cores coated by pNIPAM (poly-(N-isopropylacrylamide)) [64]. In the first step of the preparation of such nanoresonators, thin polystyrene (PS) shell was deposited on gold nanoparticles. Subsequently, N-isopropylacrylamide (NIPAM) monomer was polymerized on surfaces of Au@PS nanoparticles using 2,2’-azobis(2-methylpropionamidine) dihydrochloride as an initiator. Formed Au@pNIPAM nanoparticles allowed, for example, to record for the first time the SERS spectrum of 1-naphtol (it was not realized in standard SERS measurements because of low affinity of 1-naphtol to metallic surfaces). Au@pNIPAM nanoparticles have been also tested as nanoresonators for Raman detection of 1-naphthalenethiol. A very interesting property of pNIPAM layer is transformation from a hydrophilic, water-swollen to hydrophobic globular form [64]. This transformation is induced by the change of temperature. Therefore, when 1-naphthalenethiol is added to swollen Au@pNIPAM nanoparticles, analyte molecules could easily diffuse through the polymer network and could be chemisorbed on the Au surface what is reflected in high intensity of the measured SERS spectra [64]. Increasing the temperature does not change the intensity of the measured SERS spectra. However, when 1-naphthalenethiol is added to Au@pNIPAM nanoparticles at 60oC, the measured SERS spectra has low intensity because analyte molecule could not penetrate transformed polymer shell. It is worth mentioning that if the molecules of analyte are close (2–3 nm) to the metal surface, fluorescence is quenched, but significant increase of the Raman spectrum is observed. On the other hand, when molecules of analyte are away from the plasmonic surface (e.g., ca. 10 nm), Raman spectra weakens, but fluorescence intensity increases. Therefore, depending on the temperature, Au@pNIPAM nanoresonators could be used to effectively enhance fluorescence or Raman scattering. Such nanostructures could be also used as SHINERS platform suitable for gas sensing [65]. Molecules of gas pollutant could be trapped near metallic surface, what allows to detect, for example, a well-known polyaromatic hydrocarbonpyrene. The other promising field of application of SHINERS nanomaterials is analysis of some biological samples. It was reported that when SHINERS nanoresonators have been deposited on various cells, it was possible to distinguish on the basis of the measured SHINERS spectra between normal and pathologically changed cancer cells [66,67], for example, SHINERS nanoresonators can be applied to distinguish between

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normal breast tissue (NB), fibroadenoma (FD), atypical ductal hyperplasia (ADH), ductal carcinoma in situ (DCIS) and invasive ductal carcinoma (IDC) [68]. Kast et al. showed that in the Raman spectra of healthy, unaltered breast tissue one can identify bands at 1300, 1442, and 1745 cm−1, which could be assigned to the vibrations of lipid molecules [69]. Pathologically changed tissues exhibit similar spectra, however, in each case some spectral differences occur, which allow identifying various types of cancer (see Fig. 11.12) [68]. Spectra recorded for fibroadenoma (FD) exhibit strong band at 665 cm−1, which is characteristic for the C–S stretching mode of cysteine

Figure 11.12  SHINERS spectra (solid line) and normal Raman spectra (dotted line) acquired from fresh frozen sections of normal breast tissue (NB) as well as those displaying the following breast lesions: fibroadenoma (FD), atypical ductal hyperplasia (ADH), ductal carcinoma in situ (DCIS), and invasive ductal carcinoma (IDC). Reproduced with permission from Ref. [68]. Copyright Royal Society of Chemistry, 2015.

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[70]. Spectra recorded for ADH, DCIS and IDC in comparison to the SHINERS spectra of a normal breast tissue show stronger bands located at 1004, 1033, 1610, and 1658 cm−1 from the vibrational modes of proteins and at 970, 1090, and 1157 cm−1 from the vibrational modes of DNA. A higher concentration of proteins and DNA in the pathologically changed tissues could be used as an indicator of such changes. It means that pathologically changed tissues exhibit slightly different SHINERS spectra, which allow identifying various types of cancer [69]. Quyen et al. proposed an interesting method for detection of a tumor marker carcinoembryionic antigen (CEA) using Raman spectroscopy and gold nanorods covered with a silica layer [28]. CEA is a tumor marker that is found in over 95% of all colon tumors and in more than 50% of lung tumors. Quyen et al. showed that using gold nanorods coated by nanometric layer of silica upon which rhodamine 6G was applied as a Raman reporter molecule allows on detection of the CEA marker even in a very low concentration range [28]. The detection procedure starts from the addition of poly(ethylene glycol) bis(carboxymethyl) ether (PEGC) to the suspension of Au@SiO2 nanorods with attached rhodamine 6G dye; this step generates carboxylamine binding. Following addition of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) leads to carboxylate termination, what allows for esterification with N-hydroxysuccinimide (NHS). In the last step, monoclonal antibody was added to form NHS activated R6G/ Au@SiO2 nanoparticles. The general scheme presenting all the above steps is shown in Fig. 11.13. It was found that the recorded signal from Raman reporter molecule (R6G) was about 360 times larger than in the case when R6G molecules are adsorbed on a silicon wafer. Quyen et al. found that the intensity of the R6G Raman band at 1619 cm−1 increases linearly with increasing concentration of CEA antigen in the range from 10 ng/mL to 100 pg/mL [28]. The detection limit, which was defined as the concentration of CEA for which the intensity of the band at 1619 cm−1 is 3 times higher than the background signal, was estimated on 0.86 fg/mL. The described above system could be applied for clinical diagnosis in case of human with colon WiDr cells. Detection of tumor markers could allow for early detection and treatment of cancers. Detectable CEA antigen concentration in real clinical samples was estimated at 0.2 pg/mL. Therefore, this method could be used for early detection of cancer. SHINERS measurements could be applied to investigate adsorption and desorption processes. For example, benzotriazole (BTAH) is well known as an effective corrosion inhibitor of copper because of its ability to make a coordination polymer film on the copper surface that provides a barrier to Cu oxidation. Measured SHINERS potential dependent spectra display reversible film formation on polycrystalline Cu and irreversible film formation on single-crystal Cu [71]. SERS spectroscopy carried out on small gold satellites self-assembled onto larger silica-isolated gold cores have been used for monitoring gold-catalyzed reduction of p-nitrothiophenol (pNTP) to p-amino thiophenol (pATP) [72]. Due to the presence of thiol groups both molecules (pNTP and pATP) are chemisorbed on gold surfaces. This catalytic reduction is initiated by the addition of sodium borohydride. The kinetic of the process was monitored in situ by collecting SERS spectra at different reaction times. It was proposed that this reaction is catalyzed via electron transfer between catalytically active gold surfaces and nitro groups. It was observed that large

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Figure 11.13  General scheme presenting preparation of sensor for detection CEA antigen which utilized modified Au@SiO2 nanorods. Reproduced with permission from Ref. [28]. Copyright Royal Society of Chemistry, 2014.

silica-covered gold nanospheres without small gold satellites did not catalyze this process. Quantitative information about relative concentration of p-NTP and p-ATP could be determined by the comparison of the peak intensities of characteristics bands at 1569 and 1591 cm−1 (see Fig. 11.14). The same reaction could be also carried out and monitored using platinum nanoparticles attached to the Au@SiO2 nanostructures [73]. Also catalytic oxidation of CO could be monitored with SHINERS spectroscopy when using as nanoresonators, for example, Pt-Fe or Pd nanoparticles deposited on the Au@SiO2 nanostructures [74]. Such measurements gave the direct evidence that in case of oxidation of CO on Pt-Fe nanoalloys, ferrous center could weaken the Pt–C bond and activate O2 at room temperature, which led to CO oxidation by the Langmuir-Hinshelwood mechanism [74]. In a case of Pd catalyst SHINERS experiments combined with DFT calculations showed that active O2 species are not formed until CO begins to desorb, causing the reaction to follow Eley-Rideal mechanism [74].

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Figure 11.14  SERS spectra recorded during the reduction of p-NTP to p-ATP catalyzed by the gold nanoparticles. Spectra have been recorded at different reaction times. Reproduced with permission from Ref. [72]. Copyright 2013 American Chemical Society.

In another study hydrogenation of aqueous solution of ethyl puryvate on singlecrystal Pt electrodes was examined by cyclic voltamperometry and by SHINERS spectroscopy using Au@SiO2 nanoresonators [75]. Obtained results allow to detect two intermediates half hydrogenation state, formed by addition of a hydrogen atom to the keto carbonyl group, and another species identified as intact chemisorbed ethyl puryvate bound in a µ2(C,O) configuration [75]. It was found that populations of these two species were sensitive to the Pt surface structure µ2(C,O), ethyl puryvate adsorbate was dominant at pristine Pt(111) and Pt(100), the half hydrogenation state was only observed at these electrodes after the introduction of defects by electrochemical roughening. Application of Au@SiO2 nanoresonators allowed also to investigate using SHINERS spectroscopy the adsorption and reaction behaviors of poly-ethynylaniline on single-crystal Au(111) electrodes [76]. Gold nanoparticles coated by SiO2 nanolayer were also used to determine the intermediates of nitrate reduction on various copper crystalline faces: Cu(100), Cu(111), and Cu(110) [77]. SHINERS measurements showed that the intermediates created on all these three faces are similar, suggesting that the same mechanism is operative on all of them. SHINERS measurements can be carried out even at relatively high temperatures because gold nanoparticles covered with SiO2 or TiO2 nanolayers are stable in such

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conditions. For example, Hartman and Weckhuysen showed that SHINERS spectroscopy, when using nanosized Ru and Rh hydrogenation catalysts deposited on Au@ SiO2 or Au@TiO2 nanostructures as nanoresonators, could be used to monitor CO hydrogenation process even at 400oC [78]. Ruthenium and rhodium layers were deposited on SHINERS nanoresonators by wet impregnation method. For example, the position of the recorded Raman bands at 485 and 2020 cm−1 in experiments carried out with Ru clusters (associated with the stretching vibration of Ru–CO and RuC–O, respectively) suggest that the molecules of carbon monoxide were mainly adsorbed in a linear position on the surface of ruthenium [78].

6 Summary In this chapter, we have provided an overview of the state of the art of SHINERS spectroscopy. We have presented the mechanism of the large enhancement of the efficiency of generation of Raman signal by SHINERS nanoresonators, the methods of the synthesis of various plasmonic metal nanoparticles which are used as cores in SHINERS nanoresonators, the methods of deposition of the protecting layers on plasmonic cores and the examples of various applications of SHINERS spectroscopy. We hope that the presented examples shall convince the Readers that the Raman spectroscopy using surface protected plasmonic nanoresonators is a useful and sensitive tool to carry out chemical analysis of various surfaces, including surfaces of biological objects in in situ conditions.

Acknowledgments This work was financed out of funds of the National Science Centre, Poland, No. 2017/25/B/ ST5/01997.

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[71] N.R. Honesty, A.A. Gewirth, Shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) investigation of benzotriazole film formation on Cu(100), Cu(111), and Cu(poly), J. Raman Spectrosc. 43 (2012) 46–50, doi: 10.1002/jrs.2989. [72] W. Xie, B. Walkenfort, S. Schlücker, Label-free SERS monitoring of chemical reactions catalyzed by small gold nanoparticles using 3D plasmonic superstructures, J. Am. Chem. Soc 135 (2013) 1657–1660, doi: 10.1021/ja309074a. [73] H. Zhang, X.G. Zhang, J. Wei, C. Wang, S. Chen, H.L. Sun, et al. Revealing the role of interfacial properties on catalytic behaviors by in situ surface-enhanced Raman spectroscopy, J. Am. Chem. Soc. 139 (2017) 10339–10346, doi: 10.1021/jacs.7b04011. [74] H. Zhang, C. Wang, H.L. Sun, G. Fu, S. Chen, Y.J. Zhang, et al. In situ dynamic tracking of heterogeneous nanocatalytic processes by shell-isolated nanoparticle-enhanced Raman spectroscopy, Nat. Commun. 8 (2017) 15447, doi: 10.1038/ncomms15447. [75] S. Guan, O. Donovan-Sheppard, C. Reece, D.J. Willock, A.J. Wain, G.A. Attard, Structure sensitivity in catalytic hydrogenation at platinum surfaces measured by shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS), ACS Catal. 6 (2016) 1822–1832, doi: 10.1021/acscatal.5b02872. [76] J. Wang, J.C. Dong, J. Yang, Y. Wang, C.J. Zhang, M.M. Xu, et al. In situ SERS and SHINERS study of electrochemical hydrogenation of p-ethynylaniline in nonaqueous solvents, Electrochem. Commun. 78 (2017) 16–20, doi: 10.1016/j.elecom.2017.03.015. [77] D.P. Butcher, A.A. Gewirth, Nitrate reduction pathways on Cu single crystal surfaces: Effect of oxide and Cl−, Nano Energy. 29 (2016) 457–465, doi: 10.1016/j.nanoen.2016.06.024. [78] T. Hartman, B.M. Weckhuysen, Thermally stable TiO2- and SiO2-shell-isolated Au nanoparticles for in situ plasmon-enhanced Raman spectroscopy of hydrogenation catalysts, Chem.: Eur. J. 24 (2018) 3733–3741, doi: 10.1002/chem.201704370.

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Motohiro Bannoa, Hiroharu Yuia,b a Department of Chemistry, Faculty of Science, Tokyo University of Science, Tokyo, Japan; b Water Frontier Science & Technology (W-FST) Research Center, Tokyo University of Science, Tokyo, Japan Chapter outline 1 The brief history and the principle of stimulated Raman scattering microscopy  416 1.1 Principles of spontaneous and coherent Raman scattering  416 1.2 Application of coherent Raman scattering to microscopic imaging  419 1.3 Difficulties in conventional stimulated Raman scattering microscopy and possible solutions  421

2 Interferometric approach for obtaining the phase information from the stimulated Raman scattering signal  423 2.1 Principle of stimulated Raman scattering interferometry  423 2.2 Instrumental setup  426 2.3 Results and discussion  427

3 Differential interference contrast stimulated Raman scattering microscopy  430 3.1 Principle of differential interference contrast–stimulated Raman scattering microscopy  430 3.2 Instrumental setup  432 3.3 Results and discussion  433

4 Near–infrared stimulated Raman scattering photoacoustic spectroscopy  434 4.1 Principle of near–infrared stimulated Raman scattering photoacoustic spectroscopy  434 4.2 Instrumental setup  436 4.3 Results and discussion  437

5 Future plans: introduction of wave-front modulation technique  439 5.1 Improvement of the lateral resolution by spot shaping based on Fourier optics  439 5.2 Acceleration of imaging by multi-focus stimulated Raman scattering microscopy  440 5.3 Image correction by the technique based on adaptive optics  442

6 Conclusion 442 References  443

Molecular and Laser Spectroscopy. http://dx.doi.org/10.1016/B978-0-12-818870-5.00012-5 Copyright © 2020 Elsevier Inc. All rights reserved.

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1  The brief history and the principle of stimulated Raman scattering microscopy 1.1  Principles of spontaneous and coherent Raman scattering Spectroscopic methods for obtaining chemical-contrast information have been intensively developed and applied in many fields, for example, material development, biological science, and medical analyses. When spectroscopic methods based on infrared (IR) absorption and Raman scattering are applied to samples, their vibrational spectra can be obtained. These vibrational spectra are changed sensitively depending on the molecular species contained in the sample and their microscopic environments, such as local molecular structures, the types of crystal structures, and inner or applied stresses in molecular assemblies and crystals. Because of these useful features, vibrational spectroscopy has attracted much attention as an effective method to investigate local molecular environments with a noninvasive and nondestructive manner for material, biological, and medical samples. Raman scattering was first discovered at the beginning of the 1920s by C.V. Raman and K.S. Krishnan [1,2]. They found that light scattered by a medium contains components with wavelengths different from that of the incident light. After this discovery, it was revealed that the frequency difference between the incident and scattered lights corresponds to the eigenfrequencies of kinetic degrees of freedom of the sample. Compared to the IR absorption spectroscopy, one of the most important properties of Raman spectroscopy is that it is based on the lightscattering phenomena. Thus it is applicable to samples containing large amounts of water, such as living cells and biological tissues because water strongly absorbs light in the IR region. Nowadays, Raman scattering is usually applied for the measurements of the vibrational spectra of various molecules, materials, and biological samples. It is interpreted that the Raman-scattered light is generated by the frequency beat of the incident electromagnetic wave and the vibrational modes of the sample molecule. When an electromagnetic wave with a frequency of ν0 is introduced to the sample, Raman-scattered light with frequencies of νs and νas, where (12.1) νs = ν0 −ν (12.2) ν as = ν 0 + ν In Eqs. (12.1) and (12.2), ν0 represents the frequency of a vibrational mode in the sample molecule. The generated lights with the frequencies of νs and νas are called the Stokes- and anti-Stokes-scattering lights, respectively. The energy diagrams for the Stokes and anti-Stokes Raman scatterings are shown in Fig. 12.1. In Raman scattering spectroscopy, the spectral components of the Stokes- and/or the anti-Stokes-scattering lights, generated spontaneously by the introduction of the incident light, are measured. Therefore the generations of the Stokes- and anti-Stokes-scattering lights are often

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Figure 12.1  Energy diagrams of Raman scattering. (A) Stokes Raman scattering and (B) anti-Stokes Raman scattering.

called “spontaneous Raman scattering.” This is a contrast word to “coherent Raman scattering,” which will be explained later. Alternatively, in order to cause the Raman scattering process in the sample, two beams can also be introduced to the sample simultaneously. When the frequency difference between the two beams corresponds to the vibrational frequencies of a molecule contained in the sample, the beat of the two electromagnetic fields is resonant with the corresponding vibrational mode. This resonance causes the excitation of the corresponding vibrational mode. Due to the vibrating polarization of the excited molecules, an electromagnetic field is generated. The frequencies and the propagation directions of the incident beams are modulated by the generated electromagnetic field. The modulations of the incident beams after the sample, or the newly generated electromagnetic waves with frequencies different from those for the incident beams, can be detected as the signals. This process is called “coherent Raman scattering,” in contrast to the term “spontaneous Raman scattering,” described above. The signal due to coherent Raman scattering is generated only when the beat of the incident electromagnetic waves is resonant with the molecular vibrational modes in the sample. Therefore by changing the frequencies of the two beams, chemicalcontrast information can be obtained, as well as spontaneous Raman scattering. The coherent Raman scattering was first reported in the 1960s [3,4], and after that, the spectroscopic methods based on the coherent Raman scattering have been developed and extended. Let us think that two beams with the frequencies of ν0 (pump light) and (ν0–ν) (Stokes light) are introduced to a molecule. The molecule has a vibrational mode with the frequency of ν. The frequency difference between the two incident lights corresponds to the frequency of the vibrational mode. In this case, by the interaction between the two incident lights and the electromagnetic field generated by the polarization of the vibrational mode, four signals by the coherent Raman scattering process are generated. The four signals are called as follows: 1 2 3 4

Coherent anti-Stokes Raman scattering (CARS), frequency: ν0 + ν Stimulated Raman loss (SRL) or inverse Raman scattering, frequency: ν0 Stimulated Raman gain (SRG), frequency: ν0–ν Coherent Stokes Raman scattering (CSRS), frequency: ν0–2ν

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Figure 12.2  Schematic picture of the generations of the four coherent Raman scattering signals. When the pump and Stokes lights are introduced noncoaxially, the four signals propagate to directions different from each other. When introduced coaxially, the four signals are emitted to the same direction as the incident lights.

The image of the signals and the propagating directions for the four coherent Raman scattering signals are shown in Fig. 12.2. The process generating the SRL and SRG signals is called stimulated Raman scattering (SRS). There are several common features for the four coherent Raman scattering signals. First, these methods can be applied to samples containing large amounts of water as mentioned. Second, since the coherent Raman scattering is caused by the irradiations of light, it is totally a noninvasive method. These two features are also true for spontaneous Raman scattering. Third, because the coherent Raman scattering signals are generated by third-order optical process, the signal intensities depend on the amplitudes of the incident electric fields in a nonlinear way. Due to this dependence of the signal intensity on the incident electric fields, the signal is generated from only where the amplitudes of the incident electric fields exceed certain thresholds, and the area generating the signal is spatially restricted. This restriction results in the improvement of the spatial resolution, compared to spontaneous Raman scattering. Finally, the coherent Raman scattering signals are generated as beams with high directivity and coherence. Due to its high directivity, coherent Raman scattering signals can be collected more easily and effectively than that for spontaneous Raman scattering, which is generated and propagated to almost whole directions from the sample. In addition, when the sample emits fluorescence by the light irradiation, strong fluorescence signal generally drowns out weak Raman scattering signals. However, even for this adverse case, by applying the coherent Raman scattering, the interference from the fluorescence can be highly suppressed by detecting the signals far enough from the sample due to its signal directivity. As shown above, the four coherent Raman scattering signals have frequencies different from each other. The signals based on the coherent Raman scattering are emitted to the directions satisfying the phase matching condition [5,6]. When the two incident lights, namely, the pump and Stokes lights, are introduced to the sample noncoaxially, the four signals propagate to directions different from each other. The SRL and SRG signals are generated to the same directions as the pump and the Stokes lights, respectively, while the CARS and the CSRS signals go different paths from the incident lights. Therefore by applying the CARS or CSRS signals, background-free measurement is expected to be achieved. Furthermore, the frequency of the CARS signal is

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higher than those of incidents lights. This leads to the strong advantage toward the interference from the fluorescence by the incidents lights by utilizing optical bandpass filters. Due to these features, CARS has been widely applied in many methods, such as microscopic methods [6–12]. However, the CARS and CSRS signals often contain contributions from other transitions of the overtones and combinational tones of other vibrational modes. By the overlaps of the signals from the desired vibrational mode and the other modes, the spectral pattern is possibly distorted [6,7]. This effect is called “nonresonant background,” and due to this effect, the molecular selectivity and the dynamic range are lowered. Instead, the application of the SRG and/or SRL signals enables one to overcome the difficulty.

1.2  Application of coherent Raman scattering to microscopic imaging The spatial resolution is one of the most important parameters determining the performance of the vibrational microspectroscopic imaging. For vibrational microspectroscopic measurement with high spatial resolution, Raman scattering caused by light from UV to near-IR (NIR) region is more suitable than IR absorption. This is because wavelength of the IR light is longer by a factor of 5 to more than 20 than that of the UV to NIR light. Actually, for many confocal and/or multi-focus Raman microscopes, the light in the VIS to NIR region is utilized for the sample excitation, and the spatial resolutions reach micrometer order due to the diffraction limit of the light in the VIS to NIR region [13–15]. More recently, for further improvement of the spatial resolution, apparatuses where IR absorption or Raman scattering is combined with scanning probe microscopy have been developed. They are called nano-IR [16] and tip-enhanced Raman scattering [17], respectively. In these methods, the spatial resolution of nanometer order has been achieved and been gradually applied in many fields. These methods are most powerful from the viewpoint of lateral spatial resolutions, but their applications are limited to utmost surfaces where the scanning probe can approach. Raman scattering microscopy has also been expected to be applied for nondestructive analyses for spatial distribution of molecules and buried chemical structures in multilayered materials, such as those for organic electroluminescence and organic thin-film solar cells. However, these materials consist of thin layers with the thicknesses of less than sub-micrometers [18,19], which are less than the wavelength of light in the VIS to NIR region. Therefore it is difficult to measure the spatial distribution of a certain chemical species with enough high-depth resolution. Further, measurements for parts deeply buried in media that strongly scatters light due to the inhomogeneity of the refractive index inside have been strongly desired. Biological tissues are representative samples. In addition, biological samples are easily damaged by light irradiation with UV and VIS. For such measurements, NIR light is suitable for the excitation of the Raman scattering process. This is because NIR light with long wavelength is comparatively difficultly scattered even in the strong lightscattering medium. In addition, the damage to the sample by the irradiation of the excitation light can be suppressed to a certain extent when the wavelength of the incident light is long. Therefore by applying the NIR light, it is expected that chemical-contrast

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imaging is enabled for deeper parts in the biological samples with little damages to the samples. The application of the NIR light has another advantage. Many biological samples emit fluorescence, which interferes with the observation of the Ramanscattered light, by the light irradiation. When the NIR light is applied, the fluorescence emission is highly suppressed. However, the Raman scattering efficiency is lowered due to the long wavelength of the NIR light, resulting in the lowering of the contrast in the obtained image. By applying coherent Raman scattering, these disadvantages can be overcome, and chemical-contrast imaging with the following features can be enabled: 1 2 3 4

High spatial resolution Applicable to buried samples Contactless way NIR excitation

Among the four coherent Raman scattering, CARS spectroscopy is one of the most established ways to be applied for microscopy [6–12]. CARS microscopy is a powerful tool for chemical-contrast imaging of inhomogeneous samples. However, in CARS spectra, large contributions from the nonresonant background are observed, as mentioned above [6,7]. Due to the nonresonant background, the dynamic range of the detector is highly compressed. In addition, the shapes of the vibrational bands are distorted due to the spectral overlap with the nonresonant background [6,7]. Because of the nonresonant background, sensitive discussion about the molecular environment often becomes difficult in the CARS spectroscopy. SRL and SRG are other possible candidates among the four coherent Raman signals to be applied for microscopy. The SRL and SRG signals go with the incident pump and Stokes beams, respectively. Therefore to measure the signal intensities, only the SRL and SRG signals must be extracted from the incident beams. However, it is usually difficult to extract these signal intensities because the amplitudes of the SRL and SRG signals are smaller by a factor of less than 10−4 than that for the incident lights [6,20]. However, by applying the following method, the SRL and SRG signals can be effectively extracted from the incident beams. The scheme is shown in Fig. 12.3. Here, the amplitude of the incident Stokes beam is assumed to be modulated with the frequency of f, and that of the pump beam is not. When the SRS process arises in the sample, the SRL and the SRG signals are generated. Due to the amplitude modulation of the Stokes beam, the amplitude of the SRL signal is modulated with the same frequency f. By a photodetector, the total intensity of the pump beam after passing through the sample and the SRL signal is measured. By processing the measured total intensity with a lock-in amplifier and extracting the component modulated with the frequency f, only the intensity of the SRL signal can be obtained. Actually, by applying the present method, the SRS signals have been effectively obtained, and the SRS process has been applied for microscopy [6,20–27]. In these methods the spectral distortion observed in CARS microscopy does not appear. Therefore sensitive discussion on the molecular environment is enabled by applying SRS spectroscopy.

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Figure 12.3  Schematic picture of the measurement of the SRS (SRL) signal. A periodic function with the frequency of the megahertz order is input to the EOM, resulting in the amplitude modulation of the Stokes beam. The intensity of the pump (and SRL signal) after the sample is measured with the photodetector, and it is processed by a lock-in amplifier. (EOM, Electro-optic modulator; SRL, stimulated Raman loss; SRS, stimulated Raman scattering.)

1.3  Difficulties in conventional stimulated Raman scattering microscopy and possible solutions As described in the previous section, SRS has been applied as a base of a chemicalcontrast microscopic method. However, there are still several “limits” for this method. In the present chapter, several attempts to overcome these limits are briefly introduced. One of the “limits” in SRS microscopy is about the spatial resolution along the incident light propagations, namely, the depth resolution. For SRS microscopy, it is limited to several hundreds of nanometers [21]. Therefore SRS microscopy is difficultly when we apply it to the imaging of samples with the thickness of less than micrometers, such as multilayered polymer materials and semiconductors with micro- to nanostructures described above as examples. To overcome this difficulty, we thought it would be effective to measure the phase of the SRS signal by combing with optical interferometry, which had not yet been realized. By measuring the phase, a chemicalcontrast imaging with the spatial resolution of the order of nanometers in the depth direction shall be achieved. In an optical interferometry, a beam is first separated into two. These two beams go different pathways and again united to one beam. The intensity of the united beam is measured while changing the relative optical path length between the two beams separated first. When the phases of the two beams coincide with each other, that is, the maxima for the amplitudes of the two beams are overlapped, the amplitude of the united beam becomes the largest. In contrast, when he phases of the two beams have π/2 retardation, that is, the maximum of one beam and the minimum of the other are overlapped, the intensity of the united beam shows the smallest value. Therefore by measuring the intensity of the united beam generated by the spatial overlap of two

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coherent beams, the phase difference between the two beams, or the difference in the optical path lengths of the two beams, can be quantitatively estimated. As described above, the signals of coherent Raman scattering are beams with high coherence. Therefore they can interfere with other coherent beams with the same wavelengths. By applying a beam through the coherent Raman scattering process from the sample is used as one of the two beams in interferometry, the phase of the signal beam is expected to be measured quantitatively. As the spatial resolution of optical interferometry is less than nanometers [28], the depth resolution of microscopy based on coherent Raman scattering would be improved to the corresponding level. Another advantage of the combination of coherent Raman scattering and interferometry is that the chemical-contrast signal from the sample is detected in a heterodyne way. By applying the heterodyne detection, the signal intensity can be enhanced, and high-contrast imaging is enabled. As described above, the signal intensity for NIR Raman spectroscopy is lowered, although there are many advantageous features. However, by applying the heterodyne detection, this disadvantage about the low signal intensity can be overcome to some extent. So far, CARS spectroscopy has been combined with interferometry [29,30]. In these measurements, the CARS interference signal is applied for chemical-contrast imaging. However, to measure the CARS interference spectroscopy, a reference beam with the same frequency as the CARS signal must be prepared. Alternatively, when the SRS signal is combined with interferometry, the wavelengths of the SRL and SRG signals correspond to those of the pump and Stokes lights, respectively. Therefore as the reference beam, a portion of the pump or the Stokes beam separated before the sample can be easily applied. In addition, the phase for the CARS signal is neither quantified, nor is utilized for the improvement of the depth resolution in Refs. [29,30]. The development of the apparatus combining SRS spectroscopy and optical interferometry, or SRS interferometer, and its application to the improvement of the depth resolution by utilizing the phase information quantified from the SRS interference signal will be introduced in Section 2 [31]. For the developed SRS interferometer, the pump and the SRL signal, and the reference beams propagate spatially difference pathways. Due to the difference in the pathways, the relative phase difference between the SRL signal and the reference beam is readily distorted by the density fluctuation of air near these pathways. In order to overcome this difficulty, a differential interference contrast (DIC)–SRS microscope has been developed [32]. The detail will be mentioned in Section 3. Third, the development of an SRS photoacoustic spectrometer will be also introduced. In the conventional microscopic methods based on both spontaneous and coherent Raman scattering, it is difficult to obtain the signal from a part deeply buried in highly light scattering media. So far, Raman spectroscopy has been applied to early diagnostics of disease for subcutaneous tissue [33]. However, for these applications, the incident lights for the Raman scattering measurements are easily scattered by the tissue in front of the target point. Even if the target point is irradiated by the incident beams and the Raman scattering signal is generated, the signal would be readily scattered by the tissue. Because of this reason, the measurement based on Raman scattering has been limited to parts with the depth of less than millimeters [34]. In order to

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enable the measurement of the Raman scattering signal from the deeply buried parts in biological tissue, SRS spectroscopy and photoacoustic spectroscopy are combined [35]. In Section 4, the development and application of an SRS photoacoustic spectrometer will be introduced. Finally, in Section 5, the introduction of the wave-front modulation technique to the SRS microscope is described. By applying the technique, the shape of the focal spot for the incident beam can be arranged in an arbitrary way. The improvement of the spatial resolution along the lateral direction is explained.

2  Interferometric approach for obtaining the phase information from the stimulated Raman scattering signal 2.1  Principle of stimulated Raman scattering interferometry The principle of a simple optical interferometer is described as follows. Let us consider a case that two coherent beams, which are electromagnetic waves, are spatially overlapped. When the local maxima of the electric fields are overlapped, the electromagnetic fields constructively interfere with each other. On the other hand, when the local maximum of one field and the local minimum of the other field are overlapped, the fields destructively interfere. By measuring the total intensity of the electric field generated by the overlap of two electromagnetic waves, the phase difference between the two beams can be quantified. From the phase difference, the difference in the optical path lengths of the two beams can be estimated using the wavelength of the beams. Here, one beam passes through the sample (signal beam) and the other beam, separated first from the sample beam, passes another path (reference beam). After overlapping the signal and reference beams, by measuring the total intensity while scanning the optical path length of the reference beam, the interference pattern between the two beams is heterodyne-detected. From the interference pattern, the phase of the signal beam can be measured, and imaging with high spatial resolution is enabled. In addition, due to the heterodyne detection, the intensity detected by a photodetector is enhanced, and detection with higher contrast is enabled. If Raman spectroscopy and optical interferometry are combined, a super-high resolution chemical-contrast imaging method is expected to be developed. In addition, when NIR laser beams are applied for extending the applicable samples, as described in the previous section, the low signal intensity due to the long wavelength of the incident beams is expected to be reinforced by the signal enhancement of the heterodyne detection. To realize the combination of Raman spectroscopy and optical interferometry, it is essential to apply a coherent optical signal from the sample that has information on the vibrational modes of molecules contained in the sample. As the coherent chemical-contrast signal from the sample, the SRS (both SRL and SRG) signals mentioned in the previous section are considered. Actually, the combination of SRS spectroscopy and optical interferometry has been realized by the following method. The scheme of the signal detection is shown in Fig. 12.4. Before describing the SRS interference signal, let us consider the simple optical interference signal. The

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Figure 12.4  Schematic illustration of the detection for the SRS interference signal. Periodic functions with the frequencies of F1 and F2 are input to the EOMs 1 and 2, respectively. The target SRL interference signal is measured with a photodetector as the periodically changing components with frequencies of (F1 + F2) and |F1 − F2|. These components can be extracted by processing with a lock-in amplifier. (EOMs, Electro-optic modulators; SRL, stimulated Raman loss; SRS, stimulated Raman scattering.)

optical interference is formulated as follows. The electric fields of the pump light, Ep, and the reference light, ER, are written as E p = E p 0 cos ( 2π f0 t + φp ) (12.3) ER = ER 0 cos ( 2π f0 t + φR ) cos 2π F1t , (12.4) where Ep0 and ER0 are the amplitudes of the electric field for the pump and reference beams, respectively, f0 is the frequency of the both lights, φp is the phase of the pump light, and φR is the phase of the reference beam. Here, for sensitive detection, the amplitude of the reference beam is assumed to be modulated with the frequency of F1. Because the intensity detected with a photodetector is the square of the total electric field, the total intensity detected, Itot, is formulated as follows: 2

I tot = E p 0 cos ( 2π f0 t + ∆φ ) + ER 0 cos ( 2π f0 t ) cos 2π F1t  , (12.5) 1 2 1 2  ≅ E p 0 + ER 0 1 + cos ( 4 π F1t ) + E p 0 ER 0 cos ( 2π F1t ) cos ∆φ 2 2 where ∆φ represents the phase difference between the pump and reference beams (φp − φR). Here, the components with the frequency of around 2f0 are omitted because the periods of the vibrations of the components are much shorter than the response time of the detector used normally. The phase difference between the signal and reference

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beams ∆φ, results from the difference in the optical path lengths of the signal and reference beams. Thus ∆φ contains the information on the structures along the direction of the signal beam propagation in the sample. As shown in Eq. (12.5), the term containing ∆φ, which is the parameter to be measured, vibrates with the amplitude of Ep0ER0 and the frequency F1 [the third term of the second column in Eq. (12.5)]. When the amplitude of the component vibrating with the frequency F1 is measured, the phase difference ∆φ can be quantified. Application of a lock-in-amplifier is an effective way to extract the amplitude of a component vibrating with a certain frequency. Next, we consider the detection of the SRS interference pattern. Here, the interference between the SRL signal and the reference beam is considered. The SRG signal can also be applied as the signal rather than the SRL signal. Here, the amplitude of the Stokes light is modulated with the frequency of F2. In the present case the electric field due to the SRL signal, EL, on the signal light is formulated as EL = −EL 0 cos ( 2π f0 t + φp ) cos ( 2π F2 t ) , (12.6) where EL0 represents the amplitude of the electric field EL. It should be noted that the phase φp for EL is identical to that for Ep, as shown in Eq. (12.3). The minus signal on the right side of Eq. (12.6) describes that the SRL signal appears as the intensity decrease of the signal beam. When the SRG signal is applied, the sign should be changed to plus. The total intensity detected by the photodetector, IL,tot, between the SRL and the reference beam is written as I L ,tot = ( EL + ER )

2

= −EL 0 cos ( 2π f0 t + φp ) cos 2π F2 t + ER 0 cos ( 2π f0 t + φR ) cos 2π F1t  (12.7) 1 1 1 ≅ EL2 0 (1 + cos 4 π F2 t ) + ER2 0 (1 + cos 4 π F1t ) − EL 0 ER 0 cos ∆φ 2 2 2 [cos 2π ( F1 + F2 )t + cos 2π ( F1 − F2 )t ] 2

where the components with the frequencies around 2f0 are again omitted due to the same reason as in Eq. (12.6). The interference pattern between the SRL signal and the reference beam is observed at the frequencies of F1 ± F2, as shown in Eq. (12.7). The phase difference ∆φ contains the information on the structures along the direction of the pump beam propagation in the sample, as well as the simple optical interferometry shown in Eq. (12.5). The amplitude of the third term in the third column in Eq. (12.7) is proportional to EL0, ER0, and cos∆φ. When the detecting frequency is set to F1 + F2 or F1 − F2 , the interference signal can be measured. For example, when modulation frequencies F1 and F2 are set to 8 and 3 MHz, respectively, the SRS interference signals can be measured when the detection frequency is set to 11 or 5 MHz. It should be noted that EL0 and ER0 should be quantified for the quantification of the phase difference ∆φ. The amplitude EL0, which is the electric field amplitude of the SRL signal, is proportional to the concentration of the target molecule. In the following part the SRS interference signal from a bulk sample is measured and is processed. However, at the present stage, it is difficult to accurately quantify the concentration of the molecule

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in a certain part because the wave front of the SRL signal is readily distorted when this method is applied to a “real sample” containing tiny structures. The word “structures” contains not only the morphology of the sample but also the spatial inhomogeneity of components and the roughness of interfaces. To measure the concentration of the sample molecule quantitatively for such real samples, it is important to correct the effects by the wave-front distortion and sample roughness. When the amplitude of the interference pattern between the light elastically scattered by the sample (Rayleigh scattering) and the reference light, or Rayleigh interference signal, is measured, the effects can be estimated from the amplitude. If the SRS interference signal is calibrated by the Rayleigh interference intensity, more quantitative measurement should be enabled.

2.2  Instrumental setup The block diagram of the developed SRS interferometer is shown in Fig. 12.5 [31]. An output from a mode-locked Ti:sapphire oscillator (wavelength: 800 nm, time duration: 2 ps, repetition rate of pulses: 76 MHz, output power: 3.7 W) was separated into two, and they were introduced to two optical parametric oscillators (OPOs). Using the OPOs, two independently wavelength-variable NIR laser outputs (time duration: 2 ps, spectral width: 3 nm, repetition rate of pulses: 76 MHz, output power: 300–400 mW) were obtained. For causing the SRS process in the sample, one of the NIR beams was used as the pump beam, and the other was used as the Stokes one. The reference beam, used for the detection of the signal phase later, was separated from the pump beam. The pump and Stokes beams were spatially overlapped by a dichroic mirror, and they were focused on the sample coaxially by an objective lens [×50, numerical aperture (NA) = 0.65]. The optical delay between the pump and the Stokes beams was optimized by a mechanical stage located on the path of the pump beam. The SRS signal generated from the sample was spatially overlapped with the reference beam, as described above, by a beam splitter. The total intensity after the overlap was measured

Figure 12.5  Schematic diagram of the developed SRS interferometer. (BS, Beamsplitter; EOM, electro-optic modulator; OPO, optical parametric oscillator; SRS, stimulated Raman scattering.)

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with an InGaAs photodetector and was processed by a radio-frequency lock-in amplifier. The SRS interference pattern was measured by scanning the optical path length of the reference beam using a piezo stage. In the measurement the amplitude of the Stokes beam was modified with an electro-optic modulator (EOM), which is driven by a function generator. The input function was sine function with a frequency of 3 MHz. The amplitude of the reference beam was also modulated with a different EOM. The function for the modulation of the reference beam was sine function with the frequency of 8 MHz. For the measurement of the SRS interference signal, the detection frequency for the lock-in amplifier was set to 11 MHz, which is the sum frequency of the modulation frequencies for the Stokes and the reference beams. By applying these MHz-order modulations, the instrumental noise was suppressed to the shot-noise level [20]. As a test sample, a polystyrene (PS) film with a thickness of 100 µm was used. A triple-layered polymer film was also used as another test sample. The triple-layered film was prepared by the compression of a PS film sandwiched with two polyethylene terephthalate (PET) films. The thicknesses were 8 µm for the PS film and 13 µm for the PET films before the compression, respectively. The wavelengths of the pump and the Stokes beams were set to 1100 and 1236 nm, respectively. The wave-number difference between the pump and the Stokes lights was 1000 cm−1, corresponding to the wave number of the symmetric stretching mode of the phenyl rings in PS, 1003 cm−1. Under these conditions, the spatial distribution of the buried PS film was measured.

2.3  Results and discussion First, the SRS interference signal from the single PS film is shown in Fig. 12.6. The amplitude with a frequency of 11 MHz by the photodetector against the optical delay

Figure 12.6  SRS interference signal from a PS film. The intensity is plotted against the relative optical delay length between the pump and the reference beams. The wavelength of the Stokes beam was (A) 1236 nm (resonant condition) and (B) 1230 nm (off-resonant condition). The amplitude increase for the resonant condition is due to the generation of the SRS interference signal. The dotted curves represent the best fitted sine functions for the signals with a spatial period of 1100 nm. (PS, Polystyrene; SRS, stimulated Raman scattering.)

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between the pump and the reference beams is plotted. As compared to Fig. 12.6, either when the difference wave number between the pump and the Stokes beams corresponds to the wave number of the vibrational mode of PS (resonant condition) or not (off-resonant condition), a fringe pattern is observed. Because the spatial periodicity of the fringe pattern is 1100 nm, corresponding to the wavelength of the pump and the reference beams, it is concluded that the fringe pattern results from the optical interference between the components contained in the pump beam and the reference beam. In addition, the amplitude of the fringe pattern under the resonant condition is larger than that under the off-resonant condition, as shown in Fig. 12.6. The amplitude difference between the resonant and off-resonant conditions is thought to be the interference signal between the SRL signal and the reference beam. Therefore using the apparatus developed here, the SRS interference signal is experimentally observed for the first time. On the other hand, as described above, the fringe pattern due to optical interference is also observed under the off-resonant condition. This signal should result from the cross-phase modulation (XPM) [36]. When the Stokes beam is irradiated to the sample, the refractive index of the sample is changed. Due to the refractive index change, the divergence of the pump beam is changed, and the intensity measured by the photodetector is lowered by the divergence change. The background due to this XPM process is difficultly suppressed experimentally. However, the effect can be reduced by subtracting the signal under the off-resonant condition from that under the resonant condition. Second, in order to quantify the depth resolution of the developed SRS interferometer, the difference in the SRS interference pattern from the PS film is examined while changing the sample position along the direction of propagations of the incident beams or the depth direction. The depth position of the sample was changed with a piezo stage. The SRS interference signals from the PS film before and after changing the sample depth position by 100 nm are shown in Fig. 12.7. Here, since the SRL signal reflected from the sample is applied, the difference in the optical path length of the SRL signal corresponds to 200 nm. As shown in Fig. 12.7, the phase of the fringe pattern of the SRS interference signal is shifted by the sample position change. To quantify the phase change, the interference signals were fitted with a sine function with a spatial period of 1100 nm, corresponding to the wavelength of the SRL signal (and the reference beam). From the phase difference between the two patterns, the difference in the optical path length is calculated as 230 nm. From this result, it is revealed that difference of the sample depth in the order of sub-micrometers can be detected by the present SRS interference spectroscopy, which is difficultly detected by conventional microspectrocopic methods as confocal Raman scattering microscopy with the depth resolution of more than micrometers. Finally, tomographic imaging of the triple-layered polymer film prepared by thermal compression is introduced. The interference signals from the PS layer were measured while scanning the sample position along the lateral (x-axis) and the depth (z-axis) directions. The amplitude is plotted against the sample position as shown in Fig. 12.8. In Fig. 12.8A the plot of the amplitude of the interference signal between the Rayleigh-scattered light and the reference beam against the sample position is also

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Figure 12.7  SRS interference signal from the PS film at different depths. The sample position was moved by 100 nm with a piezo stage along the direction of the incident beam propagation. The dotted curves represent the best fitted sine functions for the fringe patterns. The phase difference between the two signals corresponds to an optical path length of 230 nm. (PS, Polystyrene; SRS, stimulated Raman scattering.)

Figure 12.8  Tomographic images for a triple-layered polymer film. The images are obtained with (A) Rayleigh scattering interference from the interfaces and (B) SRS interference signal from the PS layer. The picture (C) is the rough tomographic image constructed from the obtained images (A) and (B). The scale bar is shown in the downward of the figures. (PS, Polystyrene; SRS, stimulated Raman scattering.)

shown. When the incident beam is focused on the interface of two phases with different refractive indices, the amplitude of the Rayleigh-scattered light becomes large [37,38]. Therefore, in Fig. 12.8A, the areas where the amplitude is large indicate the positions of the interfaces between the two phases with different refractive indices, namely, the interfaces between PS and PET. From Fig. 12.8A, it is concluded that the

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interfaces between PS and PET locate at the depths of about 2 and 8 µm. On the other hand, for the plot of the amplitude of the SRS interference signal from PS against the sample position, shown in Fig. 12.8B, the amplitude shows its maximum at the area between the two interfaces estimated from the Rayleigh-scattering interference image, shown in Fig. 12.8A. Therefore it is experimentally observed that the component of the phase between the two interfaces is certainly PS. Therefore by applying the present method, the PS layer sandwiched by the two PET layers is successfully detected with the chemical selectivity. There has been a report that chemical-contrast imaging is accelerated by obtaining the spectral interference pattern between the SRS signal and a reference beam [39]. In contrast, the present study is the first application of the phase information of the SRS signal to imaging with nanometer-order accuracy obtained by combining interferometry. This is an example of the application of the SRS interferometer to a multilayered polymer film with a thicknesses of micrometer orders. However, as shown in the previous part, the sample position can be determined with an accuracy of nanometer order. Therefore it is expected that this method can be applied to chemical-contrast tomographic imaging of samples containing spatial inhomogeneity with the size of the sub-micrometer order. As described in the present section, it is concluded that obtaining the information on the phase of the SRS signal by the heterodyne detection is potentially applied to chemical-contrast tomographic imaging with high depth resolution.

3  Differential interference contrast stimulated Raman scattering microscopy 3.1  Principle of differential interference contrast–stimulated Raman scattering microscopy As mentioned in the previous section, by the development of the SRS interferometer, Raman spectroscopic measurement with the depth resolution of less than micrometers has been enabled. However, for the developed SRS interferometer, the pump and Stokes beams experience different optical paths from each other. This path difference possibly distorts the phase of the beams because of the density fluctuation of the air in the optical paths and reduces the spatial resolution at nanometer scale. To decrease the unfavorable affect from the density fluctuation of the air and enable applications to surface structures with further smaller structures with nanometer scale, a DIC–SRS microscope was developed. The schematic illustration of DIC microscopy is shown in Fig. 12.9. In the optical DIC microscopy the reference beam is separated from the signal beam by a Wollaston prism just before the focusing of the signal beam on the sample. The reflected signal and reference beams are again overlapped by the same Wollaston prism. The total intensity is measured after the overlapping of the signal and the reference beams [40]. Because the reference beam goes the path that is almost the same with that of the signal beam, the effect from the density fluctuation of the

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Figure 12.9  Schematic picture of DIC microscopy. A coherent beam is separated into two by the Wollaston prism. The separated two beams are focused on different points on the sample surface (left). The beams separated by the sample are overlapped by the Wollaston prism, and the intensity of the combined beam is measured. The intensity sensitively reflects the height difference between the points where the two beams are reflected, namely, ∆d (right). (DIC, Differential interference contrast.)

air can be highly suppressed. In addition, in the DIC microscopy, the phase difference between the signal and the reference beams can be measured as the signal intensity quantified by the photodetector. Therefore the scanning of the relative optical path length between the signal and the reference beam is not required. The measurement time for a two-dimensional imaging is actually shortened to less than several tens of minutes by applying the DIC method [32]. The DIC–SRS microscopy is a derivative of the optical DIC microscopy. The scheme is depicted in Fig. 12.10. First, the amplitude of the Stokes beam is modulated with, for

Figure 12.10  Schematic illustration of DIC–SRS microscopy. The set of the pump and the Stokes beams is separated into two by the Wollaston prism. The SRS signal is generated from the one point where one of the set of the pump and the Stokes beams are focused. The generated SRS (here, SRL of the reflected pump beam) signal is overlapped with the reference beam, and its intensity is heterodyne-detected. (DIC, Differential interference contrast; SRL, stimulated Raman loss; SRS, stimulated Raman scattering.)

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example, EOM. The set of the pump and the Stokes beams is separated into two sets by a Wollaston prism, and they are focused on different points in the sample, as well as a conventional optical DIC microscopy. For DIC–SRS microscopy, however, the intensities of the beams for the two sets are different by arrangement of the Wollaston prism. By this setup, the SRS process occurs at only the point where the stronger set of the pump and the Stokes beams is focused. After the reflections on the sample surface, the incident beams and the SRS signals are made coaxially again. After passing through a short-wave-pass filter, the intensity of the beams with the same wavelength as the pump beam is measured. By processing with a lock-in amplifier, the DIC–SRS signal is extracted. The brief formulation of the signal for DIC–SRS microscopy is shown as follows. The SRS signal is generated from around the focal points of the pump and Stokes beams when these points are overlapped. Because the amplitude of the Stokes beam was modulated by the EOM in the present experiment, as well as the SRS interferometer described above, the SRL signal appears as the component contained in the pump beam after the sample, modulated with the same frequency as the amplitude modulation for the Stokes beam. The total intensity of the electric field measured by the photodetector, indicated as Itot, can be written as I tot = ( E p + Er + ESRL ) (12.8) 2

where Ep, Er, and ESRL are the amplitudes of the electric fields for the pump beam, that of the reference beam, and that of the SRL signal, respectively. Among these amplitudes, only ESRL is modulated by the same frequency as the modulation frequency for that of the Stokes beam modulation. Using the lock-in amplifier, the amplitude for the component with the modulation frequency can be extracted. Therefore the DIC–SRS intensity, or IDS, can be formulated as I DS ∝ ( E p + Er ) ESRL . (12.9) In the above equation the square of ESRL is neglected because this value is small comparative to Ep and Er. The term (Ep + Er) changes depending on the relative phase difference between Ep and Er. The phase difference is due to the height difference between the points on the sample where the pump and the reference beams are reflected. Therefore by measuring the DIC–SRS intensity, the phase difference, or the height difference, can be obtained. Moreover, by measuring the DIC–SRS intensity while scanning the position of the focal points, the information on the surface morphology can be obtained with molecular selectivity.

3.2  Instrumental setup A DIC–SRS microscope was developed based on the SRS interferometer mentioned in Section 2. For DIC–SRS measurement, a Wollaston prism was located on the paths of the pump and Stokes beams. By the Wollaston prism, the pump and the Stokes beams were separated into two, respectively. The two sets of the separated pump and Stokes beams were focused on different points on the sample by the same objective

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lens. After the reflections on the sample, these beams were again collimated by the objective lens, and they were spatially united by the same Wollaston prism. The reflected beams were separated from the incident beams by a beam splitter and were detected by an InGaAs photodetector after passing through an optical short-wave-pass filter. By the filter, the Stokes beam reflected by the sample was cut. By the photodetector, therefore, only the SRL signal was detected. The developed apparatus was applied to the surface imaging of a silicon surface with the structures with depths of 50–300 nm. The wavelengths of the pump and Stokes beams were 1116 and 1185 nm, respectively. The wave-number difference between the two beams was 520 cm−1, corresponding to the Si–Si stretching mode in the silicon substrate.

3.3  Results and discussion The image obtained with the DIC–SRS measurement for the silicon surface is shown in Fig. 12.11. As shown in Fig. 12.11, the nanometer-order trenches are well observed with enough resolution. Therefore it is proved that DIC–SRS microscopy is actually an effective topographic way to observe such nanometer-order deep surface morphology with chemical contrast, which is difficultly measured by conventional Raman microscopic methods with the depth resolution of micrometer order, such as confocal Raman microscopy [32]. Next, the DIC–SRS microscopy is applied for the imaging of the surface morphology of a silicon substrate buried by water. It is well known that the surfaces of many an-

Figure 12.11  DIC–SRS image for the structure fabricated on the silicon substrate. The sizes of the structure are indicated in the top figure. The steps with the height of 50 nm are well resolved in the obtained image. (DIC, Differential interference contrast; SRS, stimulated Raman scattering.)

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imals and plants in nature are covered with nanometer-to-millimeter-order structures, and owing to these structures, highly water-repellent surfaces with the contact angle of more than 160 degrees are realized [41]. Recently, super-water-repellent surfaces have been fabricated based on this knowledge, especially in the material development field. As the origins of the super-water repellency, two models have been suggested. One is that the small structures on the surfaces are fully immersed by water, resulting in the expansion of the contacting area between the substrate and water, and the appearing contact area between the substrate and water becomes small. This mechanism was suggested by Wenzel [42]. In contrast, because it is suggested that air is trapped inside the small trenches, water on the surface contacts with both the surface material and the air. The surface tension of air can be regarded as 0, namely, it is highly water repellent. Due to the trapping of air, the appearing contact area of the surface and water becomes small. This model was suggested by Cassie and Baxter [43]. It is also suggested that the sliding angle of water on the surface is affected by these two states [44]. From these models and experimental results, it has been suggested that the actual spatial distribution of water around such small structures on the interfaces between the substrate and water should be experimentally observed, in addition to the measurements of the contact angles. Here, DIC–SRS microscopy is applied to the imaging of the small structures on a silicon substrate buried by water. As the sample, nanometer-to-micrometer-order structures on a silicon substrate was used. The obtained DIC–SRS image of the structures buried by water is shown in Fig. 12.12. The structures on the silicon surface were pillars with a height of 160 nm and a lateral size of 2 × 2 µm, and the gap between the pillars is 5.6 µm. As shown in Fig. 12.12, the pillars with a height of 160 nm are well observed in the obtained image. Therefore the depth resolution of the present method is enough to measure structures with the height of sub-micrometers, which is difficultly observed with conventional Raman microscopic methods. By calibrating the obtained image with the result for structures on a silicon substrate with a height of 120 nm, the difference in the optical path length between the beams focused on the top and bottom parts of the pillar structures buried by water is estimated as 430 nm. This difference in the optical path length is well explained when the 160-nm pillar structures are completely immersed by water, namely, the trenches on the silicon surface is fulfilled with water. As shown in the experimental result, DIC–SRS is an effective method for separation of the Wenzel and the Cassie–Baxter states. It is concluded that this method is useful for the analysis of surface conditions in the material field.

4  Near–infrared stimulated Raman scattering photoacoustic spectroscopy 4.1  Principle of near–infrared stimulated Raman scattering photoacoustic spectroscopy Chemical-contrast imaging of target samples located at deeper position through strongly scattering media has been highly required especially in medical fields. To

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Figure 12.12  SRS–DIC image for the pillar structures fabricated on the silicon substrate buried by water. The topographic image of the pillars obtained by laser microscopy is shown in the top. The structures observed in the SRS–DIC image correspond well to those for the image obtained by laser microscopy. (DIC, Differential interference contrast; SRS, stimulated Raman scattering.)

meet the requirements, we also developed the new apparatus by combining NIR SRS spectroscopy and photoacoustic one. When we induce the SRS processes by two NIR beams, less damage of the sample and less light–scattering are expected, which is quite important for the application to biological samples, compared to spontaneous Raman spectroscopy with UV or VIS excitation. The schematic illustration of the SRS photoacoustic process is shown in Fig. 12.13. In this method, the pump and Stokes beams are irradiated to the sample, as well as the other methods based on SRS. When a molecular vibrational mode contained in the sample is resonant with the frequency difference between the pump and the Stokes beams, the molecule is vibrationally excited. After the vibrational excitation, the

Figure 12.13  Schematic illustration of the generation of the photoacoustic signal.

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excess energy deposited to the vibrational mode is dispersed to kinetic modes of molecules around the vibrational mode. After the completions of the energy disperse processes, the energy is converted to local heat. Due to the local heating, a local volume of the molecule is expanded. Because the energy dispersion processes complete within the pico-to-nanosecond order, it is assumed that the expansion occurs instantaneously. After the local heating, the excess heat is dispersed in the sample. Due to the cooling process, the volume expanded returns to that before the expansion. A compressional wave, namely, ultrasonic wave, is emitted by these expansion and shrinking processes of the local volume [45–48]. A typical frequency of the ultrasonic wave generated by the photoacoustic process is in the megahertz order [45]. In this case the wavelength of the photoacoustic signal is longer by a factor of 106 than that of the incident light beams. The wavelength of a propagating acoustic wave is extremely long, and the wave becomes difficultly scattered by a strongly light-scattering medium, such as a biological tissue. Therefore by applying photoacoustic spectroscopy, it is expected that the chemical-contrast SRS signal from a part deeply buried in a strongly light-scattering medium is effectively detected.

4.2  Instrumental setup The block diagram of a developed SRS photoacoustic spectrometer is shown in Fig. 12.14. Briefly, the output from a Ti:sapphire regenerative amplifier (wavelength: 800 nm, pulse duration: 120 fs, spectral width: 100 cm−1, pulse repetition rate: 1 kHz, output power: 700 mW) was divided into two, and one was introduced to an optical parametric amplifier (OPA). The output from the OPA in the NIR region was focused on a BBO crystal, and second harmonic wave in the visible region was delivered. The delivered visible beam and the 800-nm output separated first were spatially

Figure 12.14  Schematic diagram of the near-IR SRS photoacoustic interferometer. By temporal synchronization between the optical chopper and the time-gate integrator, signal detection with a high signal-to-noise ratio is enabled. (IR, Infrared; SRS, stimulated Raman scattering.)

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Figure 12.15  Schematic illustration of the test sample for the SRS photoacoustic measurement. The SRS photoacoustic signals from the intralipid solution and the PS film are observed. The photographic image of the sample from the upward is shown in the right at the top. We can see the strong scattering of the incident light beams from the intralipid solution. (PS, Polystyrene; SRS, stimulated Raman scattering.)

overlapped by a dichroic mirror, and let them go the same path. These two beams were focused into the sample by a lens. The SRS photoacoustic signal from the sample was detected by a transducer (center frequency: 2.25 MHz). As shown in Fig. 12.14, the incident second harmonic beam of the OPA output was chopped by an optical chopper with a frequency of 500 Hz, corresponding to the half of the pulse repetition rate of the Ti:sapphire laser. The component modulated with 500 Hz was extracted from the intensity measured with the transducer by a boxcar integrator. The schematic illustration of the test sample used is shown in Fig. 12.15. In the sample a PS film with a thickness of 25 µm was buried in aqueous suspension solution of intralipid, which is often used as a model of biological tissues, especially for skins. The concentration of intralipid in the solution was 5 wt.%. The wavelength of the second harmonic wave of the OPA output was set to 650 and 644 nm. The wavenumber difference between the 650-nm second harmonic wave and the 800-nm wave corresponded to the wave number of the CH stretching mode of intralipid, while that between 644- and 800-nm waves corresponded to the wave number of the CH-stretching mode of PS. The depth of the PS film in the intralipid solution was set to 1.0 and 1.8 mm, as shown in Fig. 12.15.

4.3  Results and discussion The SRS photoacoustic signals from intralipid and PS are measured while scanning the sample position along the direction of the incident beam propagation. The plots of the SRS photoacoustic signals against the sample position are shown in Fig. 12.16. As shown in Fig. 12.16, when the incident beams are focused in the shallow part of the sample, only the SRS signal from intralipid is observed. As the focal points move to the deeper part of the sample, the signal from intralipid gradually decays at the depth of the PS film of 1.3 and 1.8 mm, respectively. At around the corresponding

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Figure 12.16  Dependence of the SRS photoacoustic signals from intralipid and the PS film on the depth of the focal point. The depth in the figure is indicated a certain starting depth for the measurements. The depth of the PS film in the sample is 1.3 and 1.8 mm for the top and bottom plots, respectively. The depth where the signal from the PS film is observed differs by 0.5 mm between the two measurements. (PS, Polystyrene; SRS, stimulated Raman scattering.)

depths, the SRS photoacoustic signal from the PS film corresponding to its thickness of about 0.1–0.2 mm is observed. These depth dependences of the SRS photoacoustic signal intensities correspond well to the structure of the sample shown in Fig. 12.15. Therefore it is concluded that the depth distribution of the SRS photoacoustic signal is successfully obtained with the present method. It should be noted that the light-scattering cross section for the present 5% intralipid suspension is about 10 times larger than that for typical human skins [49,50]. Therefore it is revealed that the SRS measurement for parts with a depth of more than 1 mm in highly light-scattering media, such as human skins, is enabled by applying the present method. There have been several attempts of the combination of SRS spectroscopy and photoacoustic spectroscopy [46–48]. However, in the previous studies, the object for the measurement was gas [46] or chloroform encapsulated in a glass capillary and buried in transparent water [47]. In the other case the powers of the incident laser beams were extremely high, and laser-induced breakdown of the sample occurred [48]. Therefore the present application of the NIR SRS photoacoustic spectroscopy successfully demonstrates deeper chemical-contrasted imaging in highly light-scattering media, such as biological tissues and materials containing optical inhomogeneity.

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5  Future plans: introduction of wave-front modulation technique As described above, the measurement of the SRS signal with super high resolution in the depth direction has been enabled by the newly developed apparatus. In order to extend its application toward the samples with surface roughness, inner inhomogeneity, and further high-speed imaging, etc., control units of the spatial phase patterns of the incident beams are also installed to the apparatus. As the control units of the spatial phase patterns, liquid-crystal-on-silicon spatial light modulators (SLM) are applied [51]. In the present section, the improvement of SRS microscopy by the introduction of the SLM is introduced as follows: 1 Improvement of the lateral resolution by spot shaping based on Fourier optics 2 Acceleration of imaging by multi-focus SRS microscopy 3 Image correction by the technique based on adaptive optics

5.1  Improvement of the lateral resolution by spot shaping based on Fourier optics The application of SLM is useful to the improvement of the spatial resolution of the SRS spectrometers along the lateral direction [51]. As mentioned in Sections 2 and 3, the spatial resolution along the depth direction for the SRS microscopy has been highly improved by combining with interferometry. However, the spatial resolution along the lateral direction is limited to several micrometers, as well as conventional Raman microscopic methods. To improve the spatial resolution along the lateral direction, we added SLM to the SRS microscope. The scheme is shown in Fig. 12.17. In the normal SRS microscope the pump and the Stokes beams are focused with an objective lens as

Figure 12.17  Scheme for the improvement of spatial resolution by spot shaping with SLM combined with SRS microscopy. From the red area where the spots of the pump and the Stokes beams are overlapped, the SRS signal is generated. By the spot shaping for the Stokes beam, the area is restricted to the center smaller circle with the diameter of less than micrometer. (SLM, Spatial light modulator; SRS, stimulated Raman scattering.)

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Figure 12.18  Spot shaping for the Stokes beam by the wave-front modulation technique. When the central phase pattern is projected to the SLM, the original shape of the spot (left) is changed to the concentric pattern (right). (SLM, Spatial light modulator.)

they are, and the shape of the focal spots is round. The spatial resolution for the SRS microscope is determined by the size of the round spot patterns. When the pattern of the focal spot for one of the incident beams is changed to a concentric circle, as shown in Fig. 12.17, the total size of the pattern is more than that determined by the diffraction limit. However, the diameter of the central circle can become less than the wavelength of the incident beam. When the focal spot for one of the incident beams for the SRS microscopy is arranged to the concentric pattern and that for the other incident beams is not, the SRS signal is generated from only the area of the centric small circle. By this restriction of the area for the SRS signal generation, the spatial resolution along the lateral direction is expected to be improved. To arrange the pattern of the focal spot, a computer-generated holograph (CGH) is projected on the SLM. The CGH is calculated by the iterative Fourier transform method [52]. When the CGH, shown in Fig. 12.18, is projected on the SLM, the focal pattern of the incident Stokes beam is arranged to the concentric pattern as shown in Fig. 12.18. To estimate the actual spatial resolution along the lateral direction, the SRS signal from a PS film is measured while scanning the sample position around the edge of the film. From the decay of the SRS signal intensity measured when the focal points cross the edge, the instrumental response function, which is an index of the spatial resolution, is obtained. The dependence of the SRS signal intensity on the sample position is shown in Fig. 12.19. As shown in Fig. 12.19, the slope of the signal intensity decay becomes steeper when the SLM is used. From the fitting of the slope with a Gaussian function, it is revealed that the instrumental response function is narrowed by about 15%. Therefore by applying the SLM, the spatial resolution of the SRS microscope can be improved without changing any conditions, including the wavelengths of the incident beams, the numerical aperture of the objective lens, and the optical elements used in the spectrometer.

5.2  Acceleration of imaging by multi-focus stimulated Raman scattering microscopy By applying the SLM, it is also enabled that the patterns of the focal points of the incident beams are changed to those with multiple points, as shown in Fig. 12.20. By

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Figure 12.19  Plot of the SRS signal intensity from the PS film on the sample position across the edge. The wave-front modulation by the SLM is on (red) and off (blue) (A). The plot around 5.5 µm is enlarged in (B). At the tops of the figures, the area where the PS film exists is roughly indicated. (PS, Polystyrene; SLM, spatial light modulator; SRS, stimulated Raman scattering.)

Figure 12.20  Photographic image of patterns with multi (4 × 4)-focal points generated by the wave-front modulation technique with the SLM. The scale bar corresponds to the length of 10 µm. (SLM, Spatial light modulator.)

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applying such multiple-point focusing and simultaneous measurements, the time for imaging should be shortened. As mentioned in the present section, the introduction of the SLM and the wave-front modulation technique, the performances of SRS microscopy are expected to be improved to a large extent.

5.3  Image correction by the technique based on adaptive optics In the SRS photoacoustic spectrometer mentioned in Section 4, the actual thickness of the embedded PS film is 25 µm. However, as shown in Fig. 12.16, the signal from the PS film ranges with a thickness of about 200 µm at the 1 mm or deeper position in the intralipid solution. From this result, when we applied the SRS photoacoustic spectrometer toward the depth profiling in highly scattering media, the resolution of the apparatus becomes remarkably lower than that in a transparent one. From the wavelengths of the incident beams and the numerical aperture of the objective lens used here, the depth resolution is calculated as several micrometers. Thus the lowering of the depth resolution should be due to the distortion of the wave front of the incident beams by the spatial inhomogeneity of the refractive index in the intralipid aqueous solution. When we apply the apparatus to the materials with surface roughness and/or tiny structures at the surface, the lowering of the depth resolution should also occur. When the spectroscopic methods are applied for the measurements for a sample deeply buried by a light-scattering media and/or with surface roughness, the distortion of the wave front is problematic for obtaining the tomographic image. Due to the wave-front distortion, the incident beams cannot be focused in an ideal way, and the areas of the focal spots are blurred. In addition, due to the wave-front distortion, the spectroscopic image is also distorted. To overcome the difficulty due to the wavefront distortion in inhomogeneous media, a technique of the adaptive optics is thought to be useful. By applying the technique, the wave-front distortion can be corrected, and an ideal focal spot is formed even in inhomogeneous media with a rough surface. Actually, the adaptive optics technique has been already introduced to Raman microscopy [53]. When the technique of the adaptive optics is introduced to our SRS microscope, high-spatial-resolution SRS measurement in the depth part of the sample is also expected for strongly scattering media with surface roughness, such as biological tissues.

6 Conclusion As described above, in the present chapter, the developments of novel SRS spectrometers to overcome several “limits” of conventional methods are reviewed. In the apparatuses the phase information of the SRS with chemical-contrast signal is extracted by combining with interferometry. By utilizing the phase information, the depth resolution has been highly improved. By combining SRS with photoacoustic spectroscopy, the detection of the chemical-contrast signal from parts deeply buried in highly lightscattering media has been achieved. In addition, the attempt of the control of the wave

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front for the incident beams by SLM is briefly reviewed for the improvement of SRS microscopy in the near future.

Acknowledgments The studies reviewed in this chapter were performed in the Department of Chemistry, Faculty of Science, Tokyo University of Science, Japan. The authors acknowledge the alumni of the group, Mr. Takayuki Kondo, Ms. Ami Nagashima, and Mr. Konosuke Onda for their hostile efforts. We also acknowledge Prof. Dr. Jun Taniguchi in Department of Applied Electronics, Faculty of Industrial Science and Technology, Tokyo University of Science, Japan for preparing silicon substrates with the patterned surface structures.

References [1] C.V. Raman, K.S. Krishnan, A New Type of Secondary Radiation, Nature 121 (1928) 501. [2] C.V. Raman, K.S. Krishnan, The Optical Analogue of the Compton Effect, Nature 121 (1928) 711. [3] E.J. Woodbury, W.K. Ng, Ruby Laser Operation in the Near IR, Proc. Inst. Radio Eng. 50 (1962) 2367. [4] P.D. Maker, R.W. Terhune, Study of Optical Effects Due to an Induced Polarization Third Order in the Electric Field Strength, Phys. Rev. 137 (1965) A801. [5] S. Mukamel, Nonlinear Optical Spectroscopy, Oxford University Press, Cambridge, (1995). [6] J.X. Cheng, X.S. Xie, Coherent Raman Scattering Microscopy, CRC Press, Boca Raton, (2013). [7] C.L. Evans, X.S. Xie, Coherent Anti-Stokes Raman Scattering Microscopy: Chemical Imaging for Biology and Medicine, Annu. Rev. Anal. Chem. 1 (2008) 883. [8] C. Krafft, M. Schmitt, I.W. Schie, D. Cialla-May, C. Matthäus, T. Bocklitz, et al. LabelFree Molecular Imaging of Biological Cells and Tissues by Linear and Nonlinear Raman Spectroscopic Approaches, Angew. Chem. Int. Ed. 56 (2017) 4392. [9] H. Kano, H. Segawa, M. Okuno, P. Leproux, V. Couderc, Hyperspectral coherent Raman imaging – principle, theory, instrumentation, and applications to life sciences, J. Raman Spectrosc. 47 (2015) 116. [10] C.H. Camp Jr., M.T. Cicerone, Chemically sensitive bioimaging with coherent Raman scattering, Nat. Photonics 9 (2015) 295. [11] M. Müller, A. Zumbusch, Coherent anti-Stokes Raman Scattering Microscopy, Chemphyschem 8 (2007) 2156. [12] A. Volkmer, Vibrational imaging and microspectroscopies based on coherent anti-Stokes Raman scattering microscopy, J. Phys. D: Appl. Phys. 38 (2005) R59. [13] N.J. Everall, Confocal Raman Microscopy: Why the Depth Resolution and Spatial Accuracy Can Be Much Worse than You Think, Appl. Spectrosc. 54 (2000) 1515. [14] A.Z. Samuel, S. Yabumoto, K. Kawamura, K. Iwata, Rapid microstructure characterization of polymer thin films with 2D-array multifocus Raman microspectroscopy, Analyst 140 (2015) 1847. [15] E. Hecht, Optics, fourth ed., Pearson Education Inc, New York, (2003).

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[16] A. Dazzi, R. Prazeres, F. Glotin, J.M. Ortega, Local infrared microspectroscopy with subwavelength spatial resolution with an atomic force microscope tip used as a photothermal sensor, Opt. Lett. 30 (2005) 2388. [17] R.M. Stöckle, Y.D. Suh, V. Deckert, R. Zenobi, Nanoscale chemical analysis by tip-enhanced Raman spectroscopy, Chem. Phys. Lett. 318 (2000) 131. [18] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, et al. Electroluminescence in conjugated polymers, Nature 397 (1999) 121. [19] G. Li, R. Zhu, Y. Yang, Polymer solar cells, Nat. Photonics 6 (2012) 153. [20] W. Min, C.W. Freudiger, S. Lu, X.S. Xie, Coherent Nonlinear Optical Imaging: Beyond Fluorescence Microscopy, Annu. Rev. Phys. Chem. 62 (2011) 507. [21] C.W. Freudinger, W. Min, B.G. Saar, S. Lu, G.R. Holtom, C. He, et al. Label-Free Biomedical Imaging with High Sensitivity by Stimulated Raman Scattering Microscopy, Science 322 (2008) 1857. [22] Y. Ozeki, W. Umemura, Y. Otsuka, S. Satoh, H. Hashimoto, K. Sumimura, et al. Highspeed molecular spectral imaging of tissue with stimulated Raman scattering, Nat. Photonics 6 (2012) 845. [23] E.R. Andresen, P. Berto, H. Rigneault, Stimulated Raman scattering microscopy by spectral focusing and fiber-generated soliton as Stokes pulse, Opt. Lett. 36 (2011) 2387. [24] M. Ji, M. Arbel, L. Zhang, C.W. Freudiger, S.S. Hou, D. Lin, et al. Label-free imaging of amyloid plaques in Alzheimer’s disease with stimulated Raman scattering microscopy, Sci. Adv. 4 (2018) eaat7715. [25] L. Wei, F. Hu, Z. Chen, Y. Shen, L. Zhang, W. Min, Live-Cell Bioorthogonal Chemical Imaging: Stimulated Raman Scattering Microscopy of Vibrational Probes, Acc. Chem. Res. 49 (2016) 1494. [26] W.J. Tipping, M. Lee, A. Serrels, V.G. Brunton, A.N. Hulme, Stimulated Raman scattering microscopy: and emerging tool for drug discovery, Chem. Soc. Rev. 45 (2016) 2075. [27] R.C. Prince, R.R. Frontiera, E.O. Potma, Stimulated Raman Scattering: From Bulk to Nano, Chem. Rev. 117 (2017) 5070. [28] M. Hytch, F. Houdellier, F. Hüe, E. Snoeck, Nanoscale holographic interferometry for strain measurements in electronic devices, Nature 453 (2008) 1086. [29] J.S. Bredfeldt, C. Vinegoni, D.L. Marks, S.A. Boppart, Molecularly sensitive optical coherence tomography, Opt. Lett. 30 (2005) 495. [30] E.O. Potma, C.L. Evans, X.S. Xie, Heterodyne coherent anti-Stokes Raman scattering (CARS) imaging, Opt. Lett. 31 (2006) 241. [31] M. Banno, H. Yui, Stimulated Raman Scattering Interferometer for Molecular-Selective Tomographic Imaging, Appl. Spectrosc. 71 (2017) 1677. [32] M. Banno, T. Kondo, H. Yui, Development of molecular-selective differential interference contrast microscopy utilizing stimulated Raman scattering, Opt. Lett. 43 (2018) 1175. [33] X. Feng, A.J. Moy, H.T.M. Nguyen, J. Zhang, M.C. Fox, K.R. Sebastian, et al. Raman active components of skin cancer, Biomed. Opt. Exp. 8 (2017) 2836. [34] P.T.C. So, C.Y. Dong, B.R. Masters, K.M. Berland, Two-Photon Excitation Fluorescence Microscopy, Annu. Rev. Biomed. Eng. 2 (2000) 399. [35] M. Banno, A. Nagashima, H. Yui, Stimulated Raman photoacoustic spectroscopy for chemical-contrast imaging of a sample deeply buried in scattering media, Analyst 141 (2016) 5747. [36] P. Samineni, B. Li, J.W. Wilson, W.S. Warren, M.C. Fischer, Cross-phase modulation imaging, Opt. Lett. 37 (2012) 800. [37] D. Huang, E.A. Swanson, C.P. Lin, J.S. Schuman, W.G. Stinson, W. Chang, et al. Optical Coherence Tomography, Science 254 (1991) 1178.

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[38] J.M. Schmitt, Optical Coherence Tomography (OCT): A Review, IEEE J. Sel. Top. Quantum Electron. 5 (1999) 1205. [39] F.E. Robles, M.C. Fischer, W.S. Warren, Dispersion-based stimulated Raman scattering spectroscopy, holography, and optical coherence tomography, Opt. Exp. 24 (2016) 485. [40] R. Hoffman, L. Gross, Reflected-light differential-interference microscopy: principles, use and image interpretation, J. Microsc. 91 (1970) 149. [41] W. Barthlott, C. Neinhuis, Purity of the sacred lotus, or escape from contamination in biological surfaces, Planta 202 (1997) 1. [42] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. 28 (1936) 988. [43] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546. [44] L. Gao, T.J. McCarthy, Wetting 101°, Langmuir 25 (2009) 14105. [45] L.V. Wang (Ed.), Photoacoustic Imaging and Spectroscopy, CRC Press, Boca Raton, 2009. [46] J. Barrett, M.J. Berry, Photoacoustic Raman spectroscopy (PARS) using cw laser sources, Appl. Phys. Lett. 34 (1979) 144. [47] V.V. Yakolev, H.F. Zhang, G.D. Noojin, M.L. Denton, R.J. Thomas, M.O. Scully, Stimulated Raman photoacoustic imaging, Proc. Natl. Acad. Soc. USA 107 (2010) 20335. [48] V.V. Yakolev, G.D. Noojin, M.L. Denton, B.A. Rockwell, R.J. Thomas, Monitoring stimulated Raman scattering with photoacoustic detection, Opt. Lett. 36 (2011) 1233. [49] H.J. van Staveren, C.J.M. Moes, J. van Marle, S.A. Prahl, M.J.C. van Gemert, Light scattering in Intralipid-10% in the wavelength range of 400 – 1100 nm, Appl. Opt. 30 (1991) 4507. [50] S.J. Matcher, M. Cope, D.T. Delpy, In vivo measurements of the wavelength dependence of tissue-scattering coefficients between 760 and 900 nm measured with time-resolved spectroscopy, Appl. Opt. 36 (1997) 386. [51] M. Banno, K. Onda, H. Yui, Improvement of Spatial Resolution for Nonlinear Raman Microscopy by Spatial Light Modulation, Anal. Sci. 33 (2017) 69. [52] J.W. Goodman, Introduction to Fourier Optics, third ed., Roberts & Compony Publishers, Greenwood Village, (2005). [53] A.J. Wright, S.P. Poland, J.M. Girkiu, C.W. Freudiger, C.L. Evans, X.S. Xie, Adaptive optics for enhanced signal in CARS microscopy, Opt. Exp. 15 (2007) 18209.

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Barbara Rossia, Cettina Bottaria,b, Sara Catalinic, Francesco D’Amicoa, Alessandro Gessinia, Claudio Masciovecchioa a Elettra Sincrotrone Trieste, Trieste, Italy; bDepartment of Physics, University of Trieste, Trieste, Italy; cEuropean Laboratory for Non-Linear Spectroscopy (LENS), Firenze, Italy Chapter outline 1 Introduction to resonance Raman spectroscopy  447 1.1 Light scattering and Raman effect  447 1.2 Resonance Raman scattering  451 1.3 Advantages and limitations of resonance Raman spectroscopy  452

2 Synchrotron-based ultraviolet resonance Raman setup at Elettra  453 3 Ultraviolet resonance Raman for investigation of structure and dynamics of peptides and proteins  456 3.1 Aqueous solvation of peptides  456 3.2 Isotope-labeling for monitoring structural conformations in peptides  459 3.3 Selectivity of synchrotron radiation-based ultraviolet resonance Raman for proteins  462

4 Ultraviolet resonance Raman study of deoxyribonucleic acid and their assemblies  466 4.1 Selectivity of ultraviolet resonance Raman on nucleobases  468 4.2 Conformational stability of deoxyribonucleic acid in aqueous solution  471 4.3 Thermal stability of deoxyribonucleic acid G-quadruplexes complexed with anticancer drug  473 4.4 Complementarity of ultraviolet resonance Raman and infrared spectroscopies for investigation of deoxyribonucleic acid  476

5 Final remarks and perspectives  477 References  478

1  Introduction to resonance Raman spectroscopy 1.1  Light scattering and Raman effect Light-scattering phenomenon refers to the process in which the incident photons with characteristic energy and momentum are diffused by matter. We can define the “elastic scattering” when the incident photon energy is the same of the diffused one while we refer to “inelastic scattering” when there is an energy variation. For simplicity, we will consider the case of a diatomic molecule [1,2]. In the scattering process the oscillating electric vector of the incident electromagnetic field induces Molecular and Laser Spectroscopy. http://dx.doi.org/10.1016/B978-0-12-818870-5.00013-7 Copyright © 2020 Elsevier Inc. All rights reserved.

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on the molecule an oscillating electric dipole moment µ . The induced electric dipole moment is proportional to the incident electromagnetic vector E through the so-called polarizability tensor α that is related to the tendency of the molecule to deform their electronic cloud: µ = αE Each component of the induced dipole µ can be expressed as z

µ ρ = ∑α ρσ Eσ σ =x

in which α ρσ are the components of the tensor α and Eσ = E ( 0 )σ cos 2πν 0 t , being ν 0 the frequency of the incident electromagnetic wave. Therefore the components of the induced dipole can be written as z

µ ρ = ∑α ρσ E ( 0 )σ cos 2πν 0 t σ =x

Assuming that the vibrations of the diatomic molecule are harmonic, the time dependence of the nuclear normal coordinates Qk can be expressed as Qk = Q ( 0 ) k cos 2πν t The variation of the polarizability α due to the vibrations of the molecule can be described by expanding each component in a Taylor series of Q, as

α ρσ = α ( 0 ) ρσ +

2  ∂α ρσ  1  ∂ α ρσ   Qk Ql +   Qk + ∑  2 k ,l  ∂Qk ∂Ql 0  k 0

∑  ∂Q k

By neglecting the higher order terms, the induced dipole moment becomes z

µ ρ (Qk ) = ∑α ( 0 ) ρσ E ( 0 )σ cos 2πν 0 t σ =x

+

 ∂α  1 z E ( 0 )σ Q ( 0 ) k  ρσ  cos 2π t (ν 0 + ν ) + cos 2π t (ν 0 − ν ) ∑ 2 σ =x  ∂Qk 0

By looking into the expression above, the radiation emitted by the oscillating dipole induced by the incident electromagnetic field on the molecule has three different components: z

•

∑α (0)

σ =x

ρσ

E ( 0 )σ cos 2πν 0 t that accounts for Rayleigh or elastic scattering;

Synchrotron-based ultraviolet resonance Raman scattering for material science z

•

449



(1 2) ∑ E ( 0 )σ Q ( 0 ) k (∂α ρσ ∂Qk )0 cos 2π t (ν 0 + ν ) + cos 2π t (ν 0 − ν ) that accounts σ =x

for anti-Stokes and Stokes Raman scattering, respectively.

For a given vibrational mode, the Raman scattering total intensity is 2

 ∂α  4 I (θ ) = B (ν 0 ± ν ) I 0  ρσ  sin 2 θ  ∂Qk 0 where I0 is the intensity of the incident light and θ is the angle between the induced dipole and the direction of propagation of the electromagnetic field. The expression reported above suggests that the Raman intensity mainly depends on two terms:

(

)

2

1. ∂α ρσ ∂Qk 0 that is related to the change induced in the polarizability of the molecule by the vibration; 4 2. (ν 0 ± ν ) that refers to the wavelength of the incident radiation.

According to the quantum theory, a radiation is emitted or absorbed as the result of the energy transfer between the electromagnetic field and the molecule. The quantummechanical description of the scattering process of the electromagnetic wave with energy E = hν 0 can be schematized as in Fig. 13.1. •

Rayleigh scattering [ E = hν 0 ]: this term describes the elastic scattering that is the dominant process (intensity ≈ 10 −6 lower than the intensity of excitation light). The interaction

Figure 13.1  Quantum-mechanical diagram describing the elastic and inelastic scattering processes.

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between light and matter does not change the energy state of the molecule and therefore the scattered photons have the same energy as the incident ones. • Stokes Raman scattering   E = h (ν 0 − ν )  : refers to the inelastic scattering process where there is an energy transfer from the electromagnetic wave to a vibrational level of the molecule. Therefore the scattered photons have a lower energy (red shifted) compared to the incident ones. The amount of transferred energy corresponds to the amount necessary to promote a vibrational transition. The Stokes component is the most commonly acquired in a Raman experiment because nevertheless its intensity is ≈ 10 −10 times lower than the excitation light, it remains the most intense component containing molecular vibration information. • Anti-Stokes Raman scattering  E = h (ν 0 + ν ) : it refers to the inelastic scattering process where there is an energy transfer from a molecular vibration level to the photons. Hence, the scattered photons have higher energy (blue shifted) compared to the incident ones. Antistokes profile contains the same molecular information of the Stokes component but it is less intense.

The transition from an initial state described by the wave function ψi to a final state ψ f induces a variation of the molecular dipole moment µ fi that can be written as µ fi = ψ f µˆ ψi

≠0

where µˆ corresponds to the permanent electric dipole operator for absorption or emission (infrared) process of an electromagnetic field E. For scattering phenomena, the induced dipole moment of the transition becomes µ fi = ψ f α ψi E where ψf and ψi are the vibrational wave functions of the final and initial vibrational states, respectively. In the quantum-mechanical description of the Raman effect, the total Raman intensity I (θ ) can be more rigorously defined by substituting the polarizability term (∂α ρσ ∂Qk )0 with the so-called Raman scattering tensor α ρσ  which components are obtained accordingly to the Kramers–Heisenberg fi expression: α ρσ  = 1 10 hc

 g M v vM g g1 Mσ v v M ρ g0  1 ρ σ ∑  ν − ν − ν + iΓ + ν − ν − ν + iΓ  v 0 exc 1 v v exc v v 

   

In the equation above, g0 and g1 are the vibronic wave functions of the ground electronic level and of the starting and final vibrational levels 0 and 1, v are the vibronic wave function describing the virtual state, Γ v is the width of the band associated with the v vibronic state and M ρ , Mσ are the electric dipole moments. In the case of spontaneous Raman scattering, the energy of the incident electromagnetic field ν exc does not coincide with any transition frequencies and under this conditions the Raman signal associated with a given normal mode is relative weak.

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1.2  Resonance Raman scattering Concerning the Raman intensity, it is necessary to distinguish between the two cases of conventional and resonance Raman (RR) scattering. In both cases, the transition involved in the scattering process occurs between vibrational states of the electronic ground state. The difference consists in the fact that in the conventional Raman, the electronic excited states take part to the process as virtual intermediate states, while in RR as real intermediate states. If the frequency of the incident electromagnetic field ν exc matches or is very close (pre-resonance condition) to the frequency of an electronic transition of the molecule (RR conditions), the Kramers–Heisenberg expression for the Raman scattering tensor can be simplified as α ρσ  = 1 10 hc

 g M v vM g  1 ρ σ 0

∑ v

 ν v − ν 0 − ν exc + iΓ v

   

(13.1)

Since only the terms having as denominator ν v − ν 0 − ν exc become dominant, the summation in the equation above is dependent on the vibrational quantum number of the electronic excited state v. If the RR conditions are satisfied, the scattering tensor α ρσ  becomes greater than in the conventional Raman case and the Raman inten10 sity I (θ ) associated with the corresponding normal mode is strongly enhanced. The imaginary part that appears in the denominator of expression (13.1) takes account that in resonance conditions, the effect of the electromagnetic perturbation is also related to the lifetime of the excited state, being Γ is a quantity inversely proportional to the lifetime of the vibronic excited states v. When the excitation energy falls into an electronic transition of a specific molecular portion (chromophore), we observe that some Raman vibrational modes result intensified more than other. The intensity of these Raman signals can be orders of magnitude higher (from 3 to 8) than those in out of resonance conditions. As a consequence, the sensitivity of RR technique is strongly increased. Moreover, by exploiting the RR effect, it is possible to analyze in a selective manner the signals coming from molecular portions of a heterogeneous sample or specific components in a complex mixture. Expression (13.1) can be further simplified, accordingly to the Born–Oppenheimer and the Condon approximations [1,2] as

α ρσ  = g M ρ e 10

0

e Mσ g



0

 1|v v|0   v − ν 0 − ν exc + iΓ v 

∑ ν v

The expression above is the so-called Albrecht’s term A for RR. The Albrecht term is A ≠ 0 if two conditions are fulfilled (RR selection rules): 1. The transition dipole moments g M ρ e

0

and e Mσ g

0

are both ≠ 0.

2. The Franck–Condon factors 1|v and v|0 are ≠ 0 for at least some values of v.

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Figure 13.2  Diagrams of the potential energy (V) as a function of the normal coordinate (Q) of the ground g and excited e electronic states.

While the RR condition (1) is satisfied if the corresponding electronic transition is allowed, the validity of condition (2) requires to consider the harmonic oscillator and the symmetry of the normal modes. Fig. 13.2 describes the RR process in terms of ground and excited electronic states as a harmonic potential. In correspondence of a fundamental band of the Raman spectra, the transition from the ground state i = 0 at the intermediate states and from the intermediate states at the final state f = 1 is non-zero if: 1. the potential curves of the involved electronic states are shifted with each other ( ∆Q ≠ 0 ); 2. there is a difference in the vibrational wave number between the ground and excited states, ν e ≠ ν g; 3. both the above conditions are satisfied.

It is to be noted that condition (2) is verified only for totally symmetric vibrational modes, unless molecular symmetry changes in the excited states.

1.3  Advantages and limitations of resonance Raman spectroscopy The vibrational spectroscopic techniques are powerful tool to study the molecular structural changes induced by chemical reactions and/or physical interactions of the system with the environment. Since Raman analysis can be useful to take out both qualitative and quantitative information of the investigated samples, it is usually applied in many research fields spanning from analytical to biological one. Compared to infrared absorption spectroscopy, both conventional and RR can be very valuable techniques for investigating samples in aqueous solution due to the relative low

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polarizability of water. The study of biologic molecules in physiologic conditions (i.e., relative low concentration in aqueous solution) is strongly recommended in order to mimic the real environment in which these systems exhibit their functionalities. Water shows a very strong infrared absorption and usually the OH bending signal covers the wave-number region between 1000 and 1800 cm−1 where the main vibrational features of organic and biological molecules fall. This seriously hampers the analysis of the vibrational fingerprints in the infrared spectra of samples dissolved in water solution. Conversely, RR allows one to analyze molecules dissolved in aqueous solutions without the necessity of isotopic substitution and in very high diluted conditions. This is mainly due to the greater sensitivity of RR with respect to its non-resonance counterpart that ensures a significant increment of the detection limit of the technique. As a matter of fact, while conventional Raman can detect samples with concentrations usually no lower than 0.1 M, RR technique is capable of analyzing samples with concentrations lower than 10−8 M. A second advantage of RR technique is related to the simplification of RR spectra compared to non-resonance ones, due to the selective enhancement of specific signals associated with the chromophores excited in the sample. This allows one to disentangle in the spectra of complex systems vibrational signals that usually are strongly superimposed in the conventional Raman profiles. By means of RR spectroscopy, it is also possible to collect the excitation profile of a particular Raman active mode that consists in the signal intensity as a function of the excitation frequency into the absorption band. These excitation profiles contain information on the vibronic transition associated with the particular normal mode before that the system relaxes from the electronic excited state levels. One limitation in the implementation of RR experiments is related to the self-absorption phenomenon that occurs when a portion of the scattered light is reabsorbed by the sample. This process reduces the total scattered light, also affecting the relative intensity of Raman peaks in the spectra. Another possible limitation of RR technique is the potential occurrence of the photodegradation of the sample, due to local heating and/or photoreactions in resonance conditions. In order to avoid sample damage, several strategies can be implemented as will be discussed in detail in the following section.

2  Synchrotron-based ultraviolet resonance Raman setup at Elettra Ultraviolet RR (UVRR) spectroscopy is a very valuable tool for collecting chemically specific information about a large variety of systems, due to the fact that organic molecules exhibit many and strong absorption transitions in the UV range. However, the full exploitation of UVRR spectroscopy would require to overcome some critical issues, manly related to the need of UV sources with appropriate characteristics: 1. Until now, little is known about the electronic transitions that occur in the UV region below 7 eV (≈180 nm) also due to the difficulty of vacuum-UV spectral measurements required for exploring this range. The importance of extending the UV domain lies in the possibility

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to cover the whole range of outer electronic transitions in matter (i.e., up to ≈10–15 eV) by selectively exploring specific orbitals and bands. 2. By using a continuously tunable excitation source, it is possible to map the whole resonances range of the sample in order to achieve a fine matching between the exciting radiation energy and the resonance conditions of specific chromophores. This allows one to perform, for examples, accurate UVRR measurements not biased by self-absorption effects.

The optical setup developed at the BL10.2-IUVS beamline (Elettra Synchrotron Facility, Trieste, Italy) exploits a tunable UV synchrotron radiation (SR) source for exciting and collecting UVRR spectra from different kinds of samples, that can be solid, liquid, and gels [3]. Fig. 13.3 shows the technical layout of the SR-based UVRR instrument. The Figure-8 32-mm undulator inserted in the radiation source of the beamline generates linear polarized SR with energy ranging from 4.4 to 11 eV (corresponding to wavelengths between about 113 and 280 nm) [4,5]. The main advantage of this undulator design is the strong reduction of the total on-axis power density that is achieved with no penalty on the useful photon flux in the first harmonic of the emission spectrum. The beam coming from the SR source is cleaned from the higher order harmonics of the undulator through two mirrors. A first gold-coated GLIDCOP, internally water-cooled, deviates the photons in the vertical plane with an angle of 60 degrees and a second externally water-cooled silicon mirror is used to bring back the beam parallel to the floor. Finally, a silicon switching mirror is used to guide the SR to the UVRR stage instead of a conventional high-resolution inelastic UV scattering (Brillouin) spectrometer [6]. Suitable UV-enhanced coating mirrors (with reflectivity of 90%–92% in the whole range 250–185 nm) are used to route and focus the SR radiation into the entrance slits of the monochromator (see Fig. 13.3). Overall the transport system delivers to the monochromator UV radiation with a power of ≈10 mW

Figure 13.3  Technical layout of the SR-based setup for UVRR measurements at BL10.2IUVS beamline (Elettra Synchrotron Facility, Trieste, Italy). (SR, Synchrotron radiation; UVRR, ultraviolet resonance Raman.)

Synchrotron-based ultraviolet resonance Raman scattering for material science

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Figure 13.4  Incident radiation beam power after the monochromator (see Fig. 13.3) as a function of the selected wavelength.

(at λ = 270 nm of wavelength) with a typical bandwidth of ∆λ / λ ≅ 0.01, corresponding to ≈350 cm−1. The SR radiation is monochromatized trough a Czerny–Turner monochromator (Acton SP2750 produced by Princeton Instruments) operating with three exchangeable flat holographic gratings with 1800, 2400, and 3600 grooves/mm. The maximum wavelength resolution at 270 nm provided by the monochromator is ≈0.012 nm, which corresponds to a half-width of ≈1.6 cm−1. Fig. 13.4 displays the beam-power of the radiation after the monochromator as a function of the selected wavelength. The possibility to obtain UVRR spectra even at these relatively low powers is an important advantage since it allows one to measure even organic samples, dye or chromophores, which are often subject of photodegradation when irradiated by UV light. After the monochromatization, the UV beam is collimated by a lens and transported to the Raman analyzer system (Fig. 13.3). There are two options for the sampling method: 1. the incident beam is focused on the sample and collected in a backscattering configuration trough plano-convex lens and mirrors; in this configuration, the typical size of the beam spot on the sample is of few mm2; 2. the incident light is focused on the sample trough the UV lens of microscope objectives (with magnification typically of 2, 10, or 40×) and the scattered radiation is collected in backscattering geometry.

The option (1) is mainly used for macro-Raman measurements carried out on bulk samples, such as solid, powders, liquids, and gels that are placed in suitable optical quartz cuvettes. The backscattering configuration and the geometrical characteristics of the Raman setup allow the use of a large variety of sample environments. Liquids, aqueous solution, and gels can be measured in specific sample holders able to thermalize the sample in a temperature range of 5°C–120°C. In order to prevent any possible photodecomposition of the samples due to the prolonged exposure to UV radiation, the cuvettes are subjected to continuous spinning during the running of the

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measurements in order to vary the sample volume illuminated by the radiation beam. Other methods can be implemented for preventing the radiation damage, such as flow systems that are very useful for liquid samples [7] or rotating sample holders to be used for measuring highly absorbing materials [8]. Sampling method (2) is used for micro-Raman experiments that are carried out on inhomogeneous samples, surfaces or films. In the latter case, a further charge-coupled device (CCD) microscope camera is employed for visualizing the position of the beam on the sample during the measurements. The Raman signal is analyzed by a three-stage spectrometer (TriVista 557, Princeton Instruments) where each stage is equipped with a selection of flat holographic gratings (1800 and 3600 groves/mm) optimized for both UV and visible radiation. The highest wavelength that can be obtained, which depends from the CT mechanical rotation capability, results to be 417 nm (2.98 eV) for the 3600 lines/mm gratings and 833 nm (1.49 eV) for the 1800 lines/mm grating. Finally, the scattered photons are detected by a peltier-cooled UV-enhanced CCD camera. The calibration of the spectrometer is standardized using cyclohexane. Polarized parallel (VV) and depolarized orthogonal (HV) UV Raman spectra can be collected by inserting in the optical path a Fresnel Rhomb Retarders (Half-Wave Retardance with broader wavelength range) and polarizers (see Fig. 13.3).

3  Ultraviolet resonance Raman for investigation of structure and dynamics of peptides and proteins UVRR technique is an ideal tool to probe the conformational rearrangements occurring in peptides and proteins and the role played by the water solvent in driving these structural changes. Raman spectra of these molecules typically result in a complex superposition of many signals that makes difficult the analysis. An appropriate tuning of the excitation wavelength [9–11] allows one to simplify the UVRR spectra, disentangling the signals arising from selected chromophores or molecular portions of the molecule. In addition, the high sensitivity of UVRR technique is crucial for ensuring to investigate peptides and proteins also in very diluted conditions while maintaining a satisfactory quality of the spectra.

3.1  Aqueous solvation of peptides Peptides are short chains of amino acids whose biological activity is strictly connected to the solvent properties. Besides the biomedical relevance of these small biomolecules, peptides can still be considered simple model systems to tackle the investigation of water (or solvent)–protein interactions at the atomic scale [12]. The backbones of peptides are characterized by amide linkage that connects two consecutive amino acids between the C atom of the carboxylic group of one and the N site of the nonside chain amino moiety of the other. The UVRR investigations of peptides in aqueous solutions gives the advantage to simultaneously analyze both the signals arising from

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Figure 13.5  (A) UVRR spectra of GSH aqueous solution obtained using excitation wavelength at 633, 266, 250, 226, and 210 nm; (B) UV–VIS absorption spectra of GSH aqueous solution and intensity ratio, observed in UVRR spectra, between the AII and AI bands. The sketch displays the structure of tripeptide GSH. (AI, Amide I; AII, amide II; GSH, glutathione; UVRR, ultraviolet resonance Raman; UV–VIS, ultraviolet–visible.) Adapted with permission from Ref. [15].

the backbone [13,14] and from the water molecules present in the hydration shell. As example, we report here the case of tripeptide glutathione (GSH) [15,16]. Fig. 13.5 displays the selectivity of the UVRR technique for monitoring specific vibrational signals in peptides. The Raman spectra of an aqueous solution of GSH have been collected by using different exciting radiations, ranging from 633 to 210 nm (Fig. 13.5A). Variations in the relative intensity of the Raman peaks of GSH are clearly evident depending on the exciting wavelength, especially by looking to the spectral region between 1200 and 1800 cm−1. In this wave-number region, we can recognize the characteristic vibrational signals of peptides associated with the primary amide vibrations that are mainly localized on the amidic linkage [17,18]. Amide I (AI), localized around 1650 cm−1, is mostly due to C=O stretching vibration. Amide II (AII), falling at about 1560 cm−1, derives by a combination of the out-of-phase NH bending and CN stretching. • Amide III (AIII) that is a very complex band taking account of different components between 1200 and 1400 cm−1 arising from the combination of in-phase NH bending and CN stretching. • •

By comparing the relative intensity of the amide bands with the signal associated with the SH stretching mode of GSH at ∼2600 cm−1, it is immediately visible that the use of shortest excitation wavelengths allows to selectively enhance in UVRR

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spectra the Raman peaks assigned to amide vibrations. This behavior is due to the approaching to resonance and preresonance condition of the amide π → π * transition at ∼190 nm going from visible to deep-UV excited radiation [19–21]. Interestingly, the spectra of Fig. 13.5A point out a more marked enhancement for the AII signal that is completely absent in the visible Raman spectrum of GSH and becomes dominant in the UVRR spectra collected using 226 and 210 nm as excitation wavelengths. This specific selectivity of AII band can be rationalized by considering the solvation effect on peptides in aqueous solution. The formation of hydrogen bond (HB) between the peptide sites and water molecules stabilize the ground-state dipolar reso− + nance structure O − C = NH 2 of amide bond over the neutral resonance structure ( O = C − NH 2 ) [14]. The contraction of the C–N bond length induces an increment of the displacement between the electronic ground and excited states along the C–N coordinate that results in an enhancement of the Raman cross section of the AII vibrational mode. The UVRR enhancement pattern of GSH is consistent with the nature of the electronic transition of amide groups in the tripeptide, as described in Fig. 13.5B. Here, the ratio between the area of AII and AI bands overlaps the UV absorption spectrum of GSH aqueous solution. As expected, the stronger upturn of the absorbance observed at low wavelengths is followed by a rapid increment of the AII/AI ratio, in agreement with the intensity variations observed in the corresponding UVRR profiles. The significant increase in the intensity of the amide signals in the Raman spectra of GSH, due to the resonance effect, can be exploited for investigating the peptide– solvent interactions occurring at the backbone sites of the tripeptide [15]. Fig. 13.6

(

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Figure 13.6  UVRR spectra collected at room temperature for diluted (20 mg/mL) and concentrated (160 mg/mL) solutions of GSH at pH = 3 (A) and pH = 8 (B). (GSH, Glutathione; UVRR, ultraviolet resonance Raman.) Adapted with permission from Ref. [15].

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points out the differences in the UVRR spectra of GSH collected on solutions at lowand high concentration of peptide in water and at different pHs. Since GSH exists in solution in different protonated forms, we must consider the protonated glycine at pH = 3 (Fig. 13.6A) and deprotonated at pH = 8 (Fig. 13.6B). The spectra of Fig. 13.6 give evidence of the sensitivity to the solvation effects of amide signals of GSH that undergo significant spectral modifications as a function of concentration of peptide in water. By analyzing the AI band, it is possible to take specifically information about the hydration effect on the C=O groups of GSH that can interact with water molecules as HB acceptors by forming up to two H-bonds on oxygen atom. The spectra in Fig. 13.6 show that the AI band of GSH appears broader in the diluted conditions with respect to high concentrated situation. This experimental finding suggests that in the highly solvated GSH, the C=O groups of tripeptide can be engaged in more than one H-bond with surrounding water molecules. This is consistent with the presence of C=O oscillators with a slight different strength that coexist in the same system giving rise to a broadening of the AI mode. Conversely, the decreasing of the number of water molecules available in the hydration shell for each peptide at high concentration of GSH results in a situation where the C=O groups of tripeptide are mainly involved in only one H-bond. Interestingly, this behavior is observed to be further promoted at basic pH (Fig. 13.6B), in accordance with the more marked capacity of the deprotonated form of GSH in decreasing the intermolecular order of water [15,16]. These solvation effects on the peptide sites of GSH are reflected also in the spectral changes observed for AIII and AS bands. They can be correlated with both the changes of the peptide’s dihedral angles and the establishment of H-bond links between the water molecules and the amide site [13,20–23].

3.2  Isotope-labeling for monitoring structural conformations in peptides If dissolved in deuterium oxide D2O, the hydrogens linked to S, N, and O atoms in the molecule of GSH can be exchanged with deuterium, leading to significant spectral modifications in the UVRR spectra of peptide in D2O compared to those obtained in H2O. This effect can be exploited in order to better resolve the normal mode composition of the Raman bands of peptides, especially for complex signals that take account for different types of vibrations. Fig. 13.7 shows the H/D isotopic substitution effect on the UVRR spectra of GSH collected at 5°C and 50°C. As expected, the frequency position of AI band is not affected by the H/D isotopic substitution due to the fact that this band accounts mainly for C=O stretching vibration. Conversely, the downshift of ∼106 cm−1 observed for AIII band, due to deuterium mass substitution, confirms the main character of N–H bending of the AII vibrational mode of GSH. Similarly, the deuterium substitution on the –N sites of GSH leads to a band shifts of ∼281 cm−1 for AIII3. By inspection of spectra in Fig. 13.7B, it can be noted that the bands AIII2 and AIII1 do not fully disappear in the spectra of GSH in D2O. This suggests that both these signals have an important N–H bending contribution although even CN, CO, and CαC molecular movements participate to the AIII2 and AIII1 vibrations.

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Figure 13.7  UVRR spectra collected at 5°C and 50°C for GSH dissolved in H2O (A) and D2O (B). (GSH, Glutathione; UVRR, ultraviolet resonance Raman.) Adapted with permission from Ref. [15].

The UVRR spectra in Fig. 13.7 evidence also the sensitivity of amide bands to the effect of temperature. This is mainly reflected by the observed shifts in the wavenumber positions of the primary amide signals, as reported in Fig. 13.8 for GSH in H2O at two different pHs. The temperature dependence observed for the amide bands position is strictly correlated with the progressive reduction of the HB interactions on amide sites of GSH due to thermal motion. The opposite behavior found for the bands upon the increasing of temperature, that is, blue shift for AI and red shit of AII and AIII (see Fig. 13.8), can be explained on the basis of the different normal modes composition of these signals [20,21]. Interestingly, the trends in Fig. 13.8 provide evidence that the pH does not seem to significantly affect the temperature dependence of amide bands of GSH. Compared with visible Raman spectra, UVRR profiles of GSH solutions show to be sensitive also to temperature-dependent conformational changes involving the trans–cis isomerization of GSH. This is marked by the raising upon the increasing of temperature of a specific signal at ∼1500 cm−1 that has been assigned to the cis-amide band, cis-AII [15]. Fig. 13.9 displays the area of cis-AII as a function of temperature for an aqueous solution of GSH at pH = 3. The trend evidences the presence of a double transition, most probably to be ascribed to the possible rotations around the two peptide bonds of GSH, as described in the sketches reported in the same panel. These conformational changes are promoted by the increasing of temperature. Interestingly, the intensity of the cis-AII band

Figure 13.8  Temperature dependence of wave-number position observed for amide I (A), II (B), and III3 (C) bands for GSH dissolved in H2O at pH = 3 and 8. (GSH, Glutathione.) Adapted with permission from Ref. [15].

Figure 13.9  Area of cis-amide signals cis-AII as a function of temperature for aqueous solution of GSH at pH = 3. The chemical structures reported on the graphs describe the possible cis and trans configurations assumed by the tripeptide GSH. (AII, Amide II; GSH, glutathione.) Adapted with permission from Ref. [15].

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increases with the enhancement of thermal motion without any frequency shift, while the trans-AII and -AIII bands downshift. This suggests that the cis-AII signal is mainly due to C–N stretching mode [24].

3.3  Selectivity of synchrotron radiation-based ultraviolet resonance Raman for proteins Proteins are large macromolecules present in living organisms, which play crucial and essential functions in all biological processes. They consist of one or more long chains of amino acid residues twisted into a three-dimensional (3D) characteristic shape. Since the 3D structures of proteins are strongly related to their function, it is very important to understand how these structural motives emerge from the folding process and can be affected by external conditions. Raman spectra of polypeptides and proteins usually contain many vibrational fingerprints that can be related to the 3D structure, the intermolecular interactions and the dynamics of these macromolecules. A variable wavelength excitation allows for selectively enhancing in the UVRR Raman spectra of protein the signals arising from specific chromophores, disentangling different spectral contributions [11,25]. Fig. 13.10 shows the selectivity of SR-based UVRR for investigating the protein hen egg white lysozyme (Lys), as an example. The UV absorption spectrum of Lys in the wavelength range 200–260 nm displays some features that arise from electronic transitions of different chromophoric segments of the macromolecule. UVRR excitation at 210 nm, close to the π–π* transition of the amide peptide bonds, results in a strong enhancement in the Raman spectra of the amide vibrations of the peptide backbone of proteins (Fig. 13.10A). These

Figure 13.10  Selectivity of SR-based UVRR experiments for a solution of lysozyme in phosphate buffer 10 mM pH 7: UV absorption spectrum of protein and UVRR spectra obtained using different excitation wavelengths, that is, (A) 210, (B) 228, and (C) 242 nm. (SR, Synchrotron radiation; UVRR, ultraviolet resonance Raman.)

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signals are markers of structural and functional significance for these macromolecules since they can be involved in intra- and intermolecular hydrogen bonding that hold in shape the secondary and tertiary structures of polypeptides [19–21,25–38]. Excitation at 225–245 nm within the absorption transitions of the aromatic rings of tyrosine (Tyr), tryptophan (Trp), and phenylalanine (Phe) shows UVRR spectra of Lys that are completely dominated by the vibrational signals of these aromatic amino acids (Fig. 13.10B and C) [25,29]. Due to the fact that the aromatic rings are more electron-rich compared to the side chains of most other amino acids, Tyr, Trp, and Phe can be involved in hydrophobic, cation-π, and HB interactions with nearby residues or surrounding molecules of solvent [30]. This explains since wave-number position and intensity of the Raman bands of aromatic residues are very sensitive to the local environment. Fig. 13.10 demonstrates that an accurate choosing of the UVRR excitation wavelengths allows one to separately explore different structural and dynamical aspects in protein biology. The Raman amide bands, very sensitive to polypeptide conformation, have been widely used to explore the changes occurring in protein structure, such as in the processes of protein folding, aggregation, and fibrillation [11,31,32]. These secondary structural changes can be correlated with the solvent exposure, ionization of Tyr side-groups, and hydrogen bonding that are reflected by spectral modifications in the bands of the aromatic amino acids, strongly enhanced in the UVRR spectra of proteins obtained with excitation wavelength at 230–270 nm [29,30,33–35]. An additional advantage of SR-based UVRR technique is that the continuous tunability of the SR source in the UV range permits to finely approach such preresonance condition that ensures a satisfactory enhancement of the desired Raman modes of proteins. However, at the same time it hampers the self-absorption that can decrease the observed Raman intensities and/or favor possible photo-damaging in the macromolecules [36]. Finally, it should be noted that the significant increment of the detection limit of UVRR with respect to conventional Raman technique allows one to investigate polypeptides and proteins in very high diluted conditions (