Multichannel Image Detectors Volume 2 9780841208148, 9780841210646, 0-8412-0814-X

Table of contents :
Title Page......Page 1
Half Title Page......Page 3
Copyright......Page 4
ACS Symposium Series......Page 5
FOREWORD......Page 6
PdftkEmptyString......Page 0
PREFACE......Page 7
1 Guidelines for the Selection of Four Optoelectronic Image Detectors for Low-Light Level Applications......Page 9
Detectors for Low-Light Level Operation......Page 10
Silicon Intensified Target (SIT) Vidicon and Intensified SIT (ISIT)......Page 11
Self-Scanned Silicon Photodiode Array (SPD)......Page 12
Reliability and Repeatability......Page 13
Formats......Page 16
Light Shocks......Page 17
Spectral Response......Page 18
Resolution......Page 20
Random Access Capability......Page 23
Blemishes......Page 25
Response Variations......Page 26
Gain......Page 28
Dynamic Range......Page 31
Signal Storage Capability......Page 32
Blooming......Page 33
Distortion......Page 34
Variable Dimensions......Page 35
Conclusions......Page 36
Literature Cited......Page 37
2 The Silicon-Intensified Target Vidicon Detector Operation, Characterization, and Application in Atomic Spectroscopy Research......Page 38
Operation of the SIT Vidicon......Page 39
Vidicon Components Effect on Detection......Page 42
Examples of Experiments Employing SIT Vidicon Detection......Page 50
Literature Cited......Page 61
Introduction and Review......Page 64
Automatic Optimization of Integration Time......Page 68
Instrument Optics......Page 71
The Microcomputer and Data Acquisition System......Page 72
Performance......Page 74
Application to MuItiwavelength ORD Spectrometry......Page 77
Conclusions......Page 78
Literature Cited......Page 79
4 Inductively Coupled Plasma-Atomic Emission Spectroscopy with Multichannel Array Detection......Page 81
Apparatus And Procedure......Page 83
Results And Discussion......Page 86
Conclusion......Page 119
Literature Cited......Page 120
Multielement Emission Spectrometry Using a Charge-Injection Device Detector......Page 123
Literature Cited......Page 137
6 Charge-Injection and Charge-Coupled Devices in Practical Chemical Analysis Operation Characteristics and Considerations......Page 139
Optical Systems for Use with Array Detectors......Page 156
Literature Cited......Page 159
7 Luminescence Measurements with an Intensified Diode Array......Page 161
Instrumentation......Page 162
Performance Characteristics......Page 165
Applications of the IDA......Page 168
Conclusions......Page 172
Literature Cited......Page 175
8 Time-Resolved Resonance Raman Spectroscopy of Radiation-Chemical Processes......Page 177
Description of the Apparatus......Page 179
Application......Page 182
Literature Cited......Page 187
9 Picosecond Emission Spectroscopy with Intensified Photodiode Arrays......Page 189
Apparatus......Page 191
Experimental Examples......Page 193
Instrument Performance......Page 202
Literature Cited......Page 204
10 Picosecond Spectroscopy and Applications to Chemical and Biological Systems......Page 206
Methods of Optical Detection......Page 207
Applications of Picosecond Spectroscopy......Page 217
Summary......Page 223
Literature Cited......Page 224
Streak Camera......Page 226
Jitter-Free Streak Camera......Page 227
Data Acquisition......Page 228
Applications......Page 229
Averaging of Very Weak Signals......Page 230
Fluorescence Depolarization Measurements......Page 233
Acknowledgment......Page 234
Literature Cited......Page 236
12 Analytical Chemistry with Spatial Resolution: Obtaining Spectral Images with Multichannel Detectors......Page 237
Conceptual Framework......Page 238
The Multichannel Imaging Spectrometer......Page 242
Detection of Objects Embedded in Scattering Media: The Multichannel Disadvantage.......Page 248
Literature Cited......Page 254
13 Detection of Extreme UV and Soft X-Rays with Microchannel Plates: A Review......Page 256
Quantum Detection Efficiency......Page 257
MCP Gain and Pulse Height Distribution......Page 259
Time Response and Count Rate Performance......Page 262
Cesium Iodide......Page 263
Magnesium Fluoride and Lithium Fluoride......Page 264
Phosphor/photodetector Systems......Page 267
Multiwire Readout Systems......Page 268
CODACON Readout System......Page 269
Multi-Anode Microchannel Plate Array......Page 271
Wedge and Strip Readout Systems......Page 273
Literature Cited......Page 275
14 Multichannel Extreme UV Spectroscopy of High Temperature Plasmas......Page 279
General Considerations......Page 280
Intensified Photodiode Array......Page 283
Applications of the IPDA......Page 291
Discussion......Page 294
Literature Cited......Page 298
15 Spectrometers for Rocket, Balloon, and Spacecraft Experiments......Page 299
1. Rocket-borne Spectrometer for the Study of Dynamic Molecular Processes in the Atmosphere......Page 301
2. Stratospheric Aerosol Measurements Using the CCD-Based Spectrometer......Page 311
3. IR Spectrometer for Balloon and Satellite Experiments......Page 314
3a. Balloon-borne IR Spectrometer—Pyroelectric Vidicon Sensor......Page 316
4. Spectral Data Processing......Page 319
5. Conclusion......Page 321
Literature Cited......Page 322
Author Index......Page 324
Β......Page 325
D......Page 326
I......Page 327
L......Page 328
Ρ......Page 329
R......Page 330
S......Page 331
X......Page 332

Citation preview

Multichannel Image Detectors

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

ACS

S Y M P O S I U M S E R I E S 236

Multichannel Image Detectors Volume 2 Yair Talmi, EDITOR Princeton Instruments, Inc.

Based o n a symposium cosponsored by the D i v i s i o n o f A n a l y t i c a l Chemistry and the D i v i s i o n o f Physical Chemistry at the 184th Meeting of the A m e r i c a n C h e m i c a l Society, Kansas City, M i s s o u r i , September 12-17, 1982

American Chemical Society, Washington, D.C. 1983 In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Library of Congress Cataloging in Publication Data (Revised for vol. 2) Multichannel image detectors. ( A C S symposium series, I S S N 0097-6156; 236) "Based on symposia sponsored Analytical and Physical Chemistr Chemical Society; v. 1, 176th meeting, M i a m i Beach, Fla., Sept. 11-12, 1978; v. 2, 184th meeting, Kansas City, M o . , Sept. 12-17, 1982." Includes bibliographies and index. 1.Spectrum analysis—Congresses. 2. Spectrum analysis—Instruments—Congresses. 3. Image processing—Equipment and supplies—Congresses. I. Talmi, Yair, 1942. II. American Chemical Society. Division of Analytical Chemistry. III. American Chemical Society. Division of Physical Chemistry. IV. American Chemical Society. Meeting (176th: 1978: M i a m i Beach, Fla.) V. American Chemical Society. Meeting (184th: 1982: Kansas City, Mo.) VI. Series: A C S symposium series; 102, etc. QD95.M975 1983 I S B N 0-8412-0814-X

543'.0858

79-12441

Copyright © 1983 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. P R I N T E D IN THE U N I T E D S T A T E S O F

AMERICA

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

ACS Symposium Series M . Joan Comstock, Series Editor

Advisory

Board

D a v i d L.

Allara

Robert O r y

Robert Baker

Geoffrey D . Parfitt

Donald D . Dollberg

Theodore P r o v d e r

B r i a n M. H a r n e y

Charles N . Satterfield

W . Jeffrey H o w e

D e n n i s Schuetzle

Herbert D . K a e s z

Davis L . Temple, Jr.

Marvin

Charles S. Tuesday

Margoshes

D o n a l d E. M o r e l a n d

C. Grant

Willson

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

FOREWORD T h e A C S SYMPOSIUM SERIES was founded i n 1974 to provide

a medium for publishing symposia quickly i n book form. T h e format of the Series parallel IN CHEMISTRY SERIES except that i n order to save time the papers are not typeset but are reproduced as they are submitted b y the authors i n camera-ready form. Papers are reviewed under the supervision of the Editors w i t h the assistance of the Series Advisory B o a r d and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. B o t h reviews and reports of research are acceptable since symposia may embrace both types of presentation.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

PREFACE I T H A S B E E N F O U R Y E A R S since the previous A C S symposium on M u l t i ­ channel Image Devices was held and its proceedings subsequently published ( A C S Symposium Series No. 102). A great deal of progress has since been made, both in technology and in the acceptance of multichannel image devices by spectroscopists. Progress in optoelectronic image detectors (OIDs) has been due predom­ inantly to the rapid advancemen technology. This progress has enabled the manufacture of large-format selfscanned O I D s with scientific-grade performance in good yields and at a rela­ tively affordable cost and a more reliable delivery schedule. Self-scanned silicon photodiode arrays with up to 4096 individual light sensing elements have emerged as the linear O I D of choice for most spectrometric applications. For two-dimensional area array OIDs, charge-coupled devices and charge-injection devices with element (pixel) packing of up to 800 χ 800 seem to provide a better overall performance. Similar progress has been achieved in microchannel plate ( M C P ) image intensifier technology, particu­ larly in respect to long-term stability. As a result, self-scanned solid-state O I D s with and without M C P image intensifications have rapidly replaced O I D s based on vidicon (second genera­ tion) television pickup tube technology. Moreover, indications are that in the near future, the progress in silicon-based L S I devices will expand to more exotic materials such as InSb, InAs, HgCdTe, and PbSnTe. This expansion will allow for the development of IR-sensitive OIDs that could eventually replace mechanical multiplex detection systems based on mathematical trans­ formations such as Fourier and Hadamard. Acceptance of O I D s by scientists has been a direct result of their success in the field, i.e., their unique performance characteristics and long-term reli­ ability. As more instrumentation based on these devices enters the scientific marketplace, a steady improvement in performance can be expected. Even more important, feedback from an ever increasing number of users should help convince O I D manufacturers of the commercial viability of the scientific market, which may result in devices specifically designed for, rather than adapted for, scientific usage. These proceedings are not intended to present a rigorous and expansive analysis of either the fundamental operation or the scientific applicability of ix In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

multichannel image devices. Instead, a few selected works are presented in this rapidly growing field. We hope to succeed in demonstrating the potential of this detection technique and to persuade others to probe into its use in their studies. YAIR T A L M I

Princeton Instruments, Inc. P.O. Box 2318 Princeton, N J 08540 August 1983

χ In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1 Guidelines for the Selection of Four Optoelectronic Image Detectors for Low-Light Level Applications YAIR TALMI Princeton Instruments, Inc., P.O. Box 2318, Princeton, NJ 08540 KENNETH W. BUSCH Department of Chemistry, Baylor University, Waco, TX 76798

The performance characteristics of four optoelectronic image d e t e c t o r s (OIDs) are discussed. The detectors discussed are the s i l i c o n intensified target v i d i c o n (SIT), the i n t e n s i f i e d SIT, the i n t e n s i f i e d s i l i c o n photodiode array detector (ISPD), and the self-scanned photodiode array detector. The main objective of the paper i s to provide research workers interested in applying OIDs to a particular application with comparative performance information so that the best detector for a particular application may be selected. O p t o e l e c t r o n i c image d e t e c t o r s (OIDs) are c u r r e n t l y of g r e a t i n t e r e s t to a v a r i e t y of r e s e a r c h w o r k e r s whose work i n v o l v e s the d e t e c t i o n of d i f f e r e n t portions of the electromagnetic spectrum. The great d i v e r s i t y of OIDs makes i t d i f f i c u l t f o r researchers who are not n e c e s s a r i l y experts i n the area of o p t o e l e c t r o n i c image d e t e c t i o n to s e l e c t the best image detector f o r a p a r t i c u l a r a p p l i c a t i o n . For example, a detector whose p e r f o r m a n c e i s s a t i s f a c t o r y f o r a p p l i c a t i o n s i n v o l v i n g h i g h - l i g h t l e v e l s may be t o t a l l y u n s a t i s f a c t o r y when used t o detect l o w - l i g h t l e v e l s . The main o b j e c t i v e of t h i s paper i s to provide a framework f o r comparison of a number of OIDs which are expected to have a r e a l impact on spectroscopic d e t e c t i o n i n the foreseeable f u t u r e . The scope of t h i s d i s c u s s i o n w i l l be l i m i t e d t o t h o s e spectroscopic a p p l i c a t i o n s where low l i g h t l e v e l s are involved. I t i s i n t h e s e a p p l i c a t i o n s where most of the p r o b l e m s o c c u r , and where an i n - d e p t h knowledge of d e t e c t o r p e r f o r m a n c e i s r e q u i r e d i n s e l e c t i n g a detector f o r a p a r t i c u l a r a p p l i c a t i o n . By d e f i n i t i o n , i n t h i s c a t e g o r y a r e i n c l u d e d a l l "photons t a r v e d " a r e a s of s p e c t r o m e t r y . I t i s i n s t r u c t i v e t o c o n s i d e r some of the causes of photon s t a r v a t i o n i n spectroscopy.

0097-6156/83/0236-0001 $08.50/0 © 1983 A m e r i c a n C h e m i c a l S o c i e t y

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2

MULTICHANNEL IMAGE DETECTORS

L o w - l e v e l s i g n a l s can r e s u l t f r o m a v a r i e t y of c a u s e s . Among them are: 1. I n e f f i c i e n c y of the s p e c t r o m e t r i c phenomena, e.g., various chemiluminescence, bioluminescence, or thermoluminescence processes. 2. I n e f f i c i e n t s p e c t r a l s o u r c e s or a s s o c i a t e d o p t i c s as e n c o u n t e r e d w i t h the f l u o r e s c e n c e of microscopic samples or i n l a s e r microprobe atomic emission where there i s i n s u f f i c i e n t energy i n the e x c i t a t i o n source. 3. Loss of l i g h t due to high requirements of f i l t e r i n g as i n resonance Raman spectroscopy where a high degree of f i l t e r i n g i s necessary to exclude the resonance e x c i t i n g l i n e . A n o t h e r common r e a s o n f o r p h o t o n s t a r v a t i o n i n v o l v e s s p e c t r a l phenomena of a t r a n s i e n t n a t u r e . The s h o r t e r the measured p u l s e , the more d i f f i c u l t i t i s to keep i t at a s u f f i c i e n t l y high energy l e v e l to be measured. In t h i s category b e l o n g a g r e a t number of l a s e r s p e c t r o s c o p i c s t u d i e s of l i f e t i m e , i.e., s t u d i e t r a n s i t i o n states, forbidde r a d i a t i o n and r a d i a t i o n l e s s mechanisms, etc. Another example of a spectroscopic technique involving transient signals is picosecond l a s e r spectroscopy which permits d i f f e r e n t i a t i o n of phenomena that would be otherwise unresolved, e.g., s c a t t e r i n g and f l u o r e s c e n c e . Ultra-short spectroscopy also allows measurements to be made i n the p r e s e n c e of e x c e e d i n g l y h i g h c o n t i n u o u s - w a v e (cw) b a c k g r o u n d l e v e l s as e n c o u n t e r e d i n the measurement of plasma temperatures by Thompson s c a t t e r i n g , or measurement of fluorescence of o i l s l i c k s i n d a y l i g h t . In t h i s c a s e , the d i s c r i m i n a t i o n a g a i n s t the h i g h b a c k g r o u n d l e v e l i s performed by synchronizing data a c q u i s i t i o n with pulse t i m i n g . The performance of these procedures can be improved by averaging many i n d i v i d u a l p u l s e s , o b t a i n e d e i t h e r s y n c h r o n o u s l y or asynchronously· L i m i t e d periods of i l l u m i n a t i o n or e x c i t a t i o n of the sample r e p r e s e n t s t i l l a n o t h e r cause of p h o t o n s t a r v a t i o n . Brief p e r i o d s of i l l u m i n a t i o n a r e o f t e n n e c e s s a r y i n a number of spectroscopic a p p l i c a t i o n s to a v o i d d e l e t e r i o u s e f f e c t s . For example, prolonged i r r a d i a t i o n of v a r i o u s f l u o r e s c i n g compounds can o f t e n r e s u l t i n bleaching. S i m i l a r l y , microsample a n a l y s i s by Raman s p e c t r o m e t r y can e a s i l y cause s t r u c t u r a l and c o m p o s i t i o n a l damage t o the sample upon l o n g e x p o s u r e to the intense e x c i t a t i o n r a d i a t i o n . F i n a l l y , the a v a i l a b i l i t y of very l i m i t e d amounts of sample can r e s u l t i n a low r a d i a n t f l u x when samples a r e a n a l y z e d by emission spectroscopy. C l i n i c a l analyses i n v o l v i n g samples from e i t h e r c h i l d r e n or s m a l l animals are an example of a s i t u a t i o n where the amount of sample a v a i l a b l e i s o f t e n very l i m i t e d .

Detectors for Low-Light Level Operation

light

There are a v a r i e t y of OIDs that are s a t i s f a c t o r y f o r lowl e v e l operation. Here, however we w i l l d i s c u s s only the

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1.

TALMI AND BUSCH

Low-Light

Level

3

Applications

d e t e c t o r s most a v a i l a b l e c o m m e r c i a l l y , or those t h a t expected to have a r e a l impact i n the foreseeable f u t u r e .

are

S i l i c o n I n t e n s i f i e d T a r g e t (SIT) V i d i c o n and I n t e n s i f i e d (ISIT)

SIT

These are s i l i c o n v i d i c o n (generic name) TV-type d e t e c t o r s t h a t have been e x t e n s i v e l y d e s c r i b e d i n the s p e c t r o s c o p i c l i t e r a t u r e . B a s i c a l l y they c o n s i s t of a t w o - d i m e n s i o n a l (2D) sensing area made of an array of s i l i c o n photodiodes that a l s o s e r v e as t e m p o r a r y s t o r a g e d e v i c e s , and an e l e c t r o n beam s c a n n i n g a r r a n g e m e n t t o r e a d the s i g n a l o f f t h i s t a r g e t * The h i g h s e n s i t i v i t y of the SIT i s a c h i e v e d t h r o u g h an image i n t e n s i f i c a t i o n s t a g e w i t h an i n t e r n a l g a i n o f a p p r o x i m a t e l y 2500. An image, e.g., a s p e c t r u m , i s f i r s t c o n v e r t e d by the photocathode to an equivalen focusing section, accelerate r e t a i n i n g i t s f i d e l i t y , r e s u l t i n g i n i n c i d e n t e l e c t r o n s of a p p r o x i m a t e l y 8-9 KeV. S i n c e each p h o t o e l e c t r o n p r o d u c e s an a d d i t i o n a l e l e c t r o n / h o l e p a i r i n the s i l i c o n t a r g e t f o r each increase of 3.6 eV, the i n t e r n a l g a i n i s approximately 2500 f o r 9 KeV e l e c t r o n s . I f the r e a d o u t n o i s e i s m a i n t a i n e d a t approximately 2500 electrons/scan/channel, then a S/N = 1 should theoretically be achieved for a signal of 1 photoelectron/channel· The I S I T c o n s i s t s of a f i r s t - g e n e r a t i o n t r i o d e image i n t e n s i f i e r w i t h an o p t i c a l g a i n o f 50-100 w h i c h i s o p t i c a l l y coupled to the photocathode of an SIT tube. The main advantages of an I S I T o v e r an SIT a r e : Gain. T h e o r e t i c a l l y , the ISIT can produce a s i g n a l of at l e a s t 5 d i g i t a l c o u n t s per p h o t o e l e c t r o n . I n f a c t , most of t h i s assumed advantage i s l o s t i n the d i g i t i z a t i o n p r o c e s s and, of course, the maximum S/N f o r a s i g n a l of 1 photoelectron cannot be b e t t e r than 1. However, by l o w e r i n g the g a i n of the p r e a m p l i f i e r to o b t a i n a readout noise l e s s than 1 count rms, an improvement i n S/N i s obtained f o r low l e v e l s i g n a l s (less than 1 photoelectron/scan) when on-target i n t e g r a t i o n i s u t i l i z e d . S p e c t r a l Response. For r e a s o n s stemming from d i f f e r e n c e s i n production p h i l o s o p h i e s , r a t h e r t h a n f u n d a m e n t a l d i f f e r e n c e s , commercial image i n t e n s i f i e r s have more e f f i c i e n t photocathodes, p a r t i c u l a r l y i n the r e d . I n the UV, a U V - t o - v i s i b l e c h e m i c a l converter i s s t i l l necessary. Gat i n g Rat i o . The on/off gating r a t i o i s higher by a p p r o x i m a t e l y 2 o r d e r s o f m a g n i t u d e , making the I S I T a b e t t e r detector f o r pulse spectroscopy.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4

MULTICHANNEL IMAGE DETECTORS

Format, Larger format image i n t e n s i f i e r s , e.g., 25 and 40 mm i n d i a m e t e r , a r e more a v a i l a b l e and a f f o r d a b l e t h a n SITs. Such i n t e n s i f i e r s can be o p t i c a l l y coupled to an SIT through a format reducing o p t i c a l - f i b e r coupler, e.g., 40 to 18 mm (SIT standard format). This method w i l l a l l o w a wider s p e c t r a l coverage w i t h l i t t l e , i f any, l o s s i n r e s o l u t i o n . The main d i s a d v a n t a g e s of the I S I T a r e : 1. S u b s t a n t i a l l y c o s t l i e r . 2. Less r e l i a b l e ; s h o r t e r l i f e t i m e . 3. More d i f f i c u l t to o p e r a t e ; more o p e r a t i o n a l p a r a m e t e r s t o c o n t r o l . 4. L e s s immune t o sudden " l i g h t " shocks. 5. Worse p i n ­ cushion d i s t o r t i o n ; worse r e s o l u t i o n and more n o n l i n e a r wavelength c a l i b r a t i o n . 6. More d i f f i c u l t i e s on c o o l i n g , i.e., more l a g a t l o w e r t e m p e r a t u r e s . T h e r e f o r e , the I S I T i s even l e s s amenable than the SIT to long term s i g n a l i n t e g r a t i o n (on-target). MCP/SPD ( I S P D - I n t e n s i The ISPD or i n t e n s i f i e d s i l i c o n photodiode array d e t e c t o r c o n s i s t s of a m i c r o c h a n n e l p l a t e (MCP) image i n t e n s i f i e r o p t i c a l l y c o u p l e d t o a s i l i c o n p h o t o d i o d e a r r a y (SPD) as the m u l t i c h a n n e l r e a d o u t / s t o r a g e d e v i c e . The MCP (1_) i s a d i s k shaped c o n t i n u o u s dynode e l e c t r o n m u l t i p l i e r imager. It c o n s i s t s of m i l l i o n s of hollow microchannels formed i n a g l a s s substrate, where each channel, which i s 12-25 y m i n diameter, i s s e m i - c o n d u c t i n g . Each c h a n n e l a c t s as an i n d i v i d u a l e l e c t r o n m u l t i p l i e r w i t h an absolute geometric r e g i s t r a t i o n between the input and output p l a t e s of the device. An o p t i c a l gain of 10-50 χ 10 i s t y p i c a l of MCPs. C a s c a d i n g a few MCPs w i t h c h a n n e l s a r r a n g e d i n a c h e v r o n o r i e n t a t i o n can produce g a i n s o f 10 , s u f f i c i e n t f o r m u l t i c h a n n e l photon c o u n t i n g . An MCP image i n t e n s i f i e r c o n s i s t s of an input photocathode f o l l o w e d by an MCP wafer which i s i n turn f o l l o w e d by a phosphor output. Focusing of the e l e c t r o n image produced by the photocathode can be done by p r o x i m i t y , e l e c t r o s t a t i c , and m a g n e t i c methods. Only the f i r s t two a r e a v a i l a b l e c o m m e r c i a l l y at an a f f o r d a b l e c o s t . This d i s c u s s i o n w i l l concentrate on the MCP/SPD, although other OIDs can be used as readout devices Q ) . Self-Scanned S i l i c o n Photodiode Array

(SPD)

At t h i s time, the self-scanned s i l i c o n photodiode array i s t h e o n l y c o m m e r c i a l l y a v a i l a b l e d e v i c e t h a t has been s p e c i f i c a l l y designed as a p a r a l l e l s p e c t r o m e t r i c detector. The performance c h a r a c t e r i s t i c s o f t h i s d e t e c t o r and i t s a p p l i c a b i l i t y t o s p e c t r o p h o t o m e t r i c and s p e c t r o f l u o r o m e t r i c measurements have been r i g o r o u s l y d i s c u s s e d e l s e w h e r e (.2-4.). The self-scanned photodiode array i s a m o n o l i t h i c s i l i c o n wafer c o n s i s t i n g of an a r r a y of d i o d e s , each a c t i n g as a l i g h t - t o charge t r a n s d u c e r and a s t o r a g e d e v i c e . Two s h i f t r e g i s t e r s

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1.

TALMI AND BUSCH

Low-Light

Level

5

Applications

r e a d out the c h a r g e s i g n a l s s t o r e d i n each of the odd and even diodes* Low n o i s e and, c o n s e q u e n t l y , h i g h dynamic range a r e a c h i e v e d t h r o u g h an a p p r o p r i a t e d e s i g n o f t h e readout p r e a m p l i f i e r and s i g n a l p r o c e s s i n g e l e c t r o n i c s e c t i o n * One c o m m e r c i a l l y a v a i l a b l e d e v i c e , an S-type s i l i c o n p h o t o d i o d e array made by EG&G Reticon, i s arranged so that each i n d i v i d u a l d i o d e i s 2*5 mm h i g h and 25 μ m wide* T h i s 100:1 a s p e c t r a t i o i s a compromise design corresponding to t y p i c a l aspect r a t i o s of conventional spectrometer entrance s l i t s .

The Ideal Multichannel (Parallel) Detector The s e l e c t i o n of a p a r t i c u l a r OID f o r a g i v e n a p p l i c a t i o n i n v o l v e s a number of important c o n s i d e r a t i o n s . In e v a l u a t i n g the performance of these devices, i t i s i n s t r u c t i v e to compare t h e i r a c t u a l performanc h y p o t h e t i c a l i d e a l OID d e t e r m i n e the e x t e n t t o w h i c h a g i v e n d e t e c t o r compares i n performance to an " i d e a l " OID. Table I l i s t s some of the major c h a r a c t e r i s t i c s of an i d e a l OID. These c r i t e r i a a r e d i v i d e d i n t o two c a t e g o r i e s : operation and performance c h a r a c t e r i s t i c s , and s p e c t r o m e t r i c performance* In the d i s c u s s i o n which f o l l o w s , each detector under c o n s i d e r a t i o n w i l l be evaluated against the c r i t e r i a l i s t e d i n Table I. To be most u s e f u l to the reader i n l o c a t i n g i n f o r m a t i o n about a p a r t i c u l a r performance f e a t u r e of a g i v e n d e t e c t o r , the f o l l o w i n g d i s c u s s i o n w i l l be o r g a n i z e d around the format developed i n Table I. Thus, the reader who i s i n t e r e s t e d i n a p a r t i c u l a r c h a r a c t e r i s t i c — immunity t o l i g h t shocks, f o r e x a m p l e — can r e f e r to that s e c t i o n f o r a d i s c u s s i o n of t h i s feature f o r the d e t e c t o r s under consideration*

Operation and Performance Characteristics Cost and

Availability

S I T &. I S I T * C o m m e r c i a l l y a v a i l a b l e from manufacturers at a moderate cost*

a variety

of

ISPD* C o m m e r c i a l l y a v a i l a b l e i n 18, 25, and 40 mm diameters (MCP i n t e n s i f i e r s ) . SPDs a r e a v a i l a b l e as 64, 128, 512, 1024, and 4096 element arrays. SPD* SPDs a r e c o m m e r c i a l l y a v a i l a b l e , a l t h o u g h due t o production y i e l d c o n s i d e r a t i o n s t h e i r p r i c e i s s t i l l r e l a t i v e l y high. R e l i a b i l i t y and R e p e a t a b i l i t y SIT & ISIT.

Reasonably good.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

6

MULTICHANNEL IMAGE DETECTORS Table

I . The

Ideal Spectrometric Multichannel ( P a r r a l e l ) Detector

O p e r a t i o n and 1.

Low

Performance

c o s t and w i d e c o m m e r c i a l

Characteristics

availability.

2.

High

3.

Long s h e l f l i f e , l o n g o p e r a t i o n l i f e , Day-to-day r e p r o d u c i b i l i t y .

d e v i c e - t o - d e v i c e performance r e l i a b i l i t y

4.

A v a i l a b i l i t y i n a l a r g e v a r i e t y of formats, s i z e s , p i x e l ( p i c t u r e c e l l ) d i m e n s i o n s , and a r r a y d e n s i t y . B o t h l i n e a r area a r r a y s are necessary.

and

and

repeatability.

l o n g term

stability.

and

5.

S i m p l i c i t y o f o p e r a t i o n , m i n i m a l number o f a d j u s t a b l e paramet e r s t h a t a f f e c t the performance of the d e t e c t o r

6.

M e c h a n i c a l and e l e c t r o n i a b i l i t y under h o s t i l e environments, i . e . , h e a t , h u m i d i t y , m a g n e t i c f i e l d s , RF, e t c . Minimum m i c r o p h o n i c e f f e c t s , i . e . , s t a b i l i t y o f r e a d o u t under v i b r a t i o n and o t h e r p r e s s u r e wave oscillations.

7.

Immunity t o sudden " l i g h t s h o c k s , " i . e . , r a p i d r e c o v e r y f r o m sudden e x p o s u r e t o h i g h l i g h t l e v e l s . N o n - d e s t r u c t i v e under v e r y h i g h l i g h t l e v e l s as l o n g as e x c e e d i n g l y h i g h t e m p e r a t u r e s are not reached.

8.

Low

9.

High t o l e r a n c e f l a t n e s s a c r o s s the e n t i r e a r r a y to a v o i d l o s s of r e s o l u t i o n at the f l a t f o c a l p l a c e of the s p e c t r o m e t e r . A v a i l a b i l i t y of curved a r r a y s to f i t curved f o c a l p l a n e s .

weight,

low power c o n s u m p t i o n , e a s y and

Spectrometer

adjustable cooling,

Performance

1.

Wide s p e c t r a l r e s p o n s e : x - r a y t o IR. At l e a s t i t i s d e s i r a b l e to have d e t e c t o r s c o v e r i n g t h e UV t o NIR, x - r a y t o UV, and NIR t o medium IR.

2.

Arrays with a variable s p a t i a l (spectral) resolution. This can be a c h i e v e d by v a r y i n g e l e c t r o n i c a l l y t h e d e n s i t y o f t h e array. Good r e s o l u t i o n .

3.

Random a c c e s s r e a d o u t o f i n d i v i d u a l p i x e l s . Pseudo random access, i . e . , fact access, i s possible with l i n e a r arrays. R e a l random a c c e s s i s p o s s i b l e w i t h a v a r i e t y o f 2D a r r a y s . To enhance t h e S/N o f low l e v e l s i g n a l s i n t h e p r e s e n c e o f h i g h l e v e l s i g n a l s , t h i s a c c e s s s h o u l d be p o s s i b l e w i t h o u t a d e s t r u c t i v e readout.

4.

U l t r a r a p i d e l e c t r o n i c s h u t t e r i n g ( g a t i n g ) f o r measurements o f v e r y f a s t t r a n s i e n t phenomena. H i g h o n / o f f g a t i n g r a t i o s are necessary.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1.

TALMI AND

BUSCH

Low-Light

Level

Table

I

1

Applications

(Continued)

5.

Minimum number o f b l e m i s h e s and d e f e c t s . Most d e f e c t s b e l o n g to e i t h e r o f two c a t e g o r i e s ; s e v e r e l y r e d u c e d ( o r no) r e s p o n s e , and u n u s u a l l y h i g h d a r k c h a r g e .

6.

Minimum v a r i a t i o n s i n d a r k - c h a r g e and r e s p o n s e a c r o s s t h e array. A l s o , minimum v a r i a t i o n i n s p e c t r a l r e s p o n s e , i . e . , c o n s t a n c t response ( w i t h h i g h e f f i c i e n c y ) w i t h wavelength. T h i s e x c l u d e s , by d e f i n i t i o n , " p h o t o n s e n s o r s . "

7.

Low d a r k - c h a r g e t o e n a b l e l o n g s i g n a l i n t e g r a t i o n and s t o r a g e p e r i o d s and f o r l o n g - t e r m s t a b i l i t y . Dark c h a r g e n o i s e s h o u l d be n e g l i g i b l e compared w i t h e i t h e r p r e a m p l i f i e r / r e a d o u t o r photon shot n o i s e .

8.

Low p r e a m p l i f i e r n o i s e v e r y low l i g h t l e v e techniques. Readou time.

9.

V e r y w i d e dynamic r a n g e . Usually a transfer characteristic c u r v e ( i n p u t - t o - o u t p u t ) w i t h a s l o p e ( ) o f 1 i s i d e a l . However, a c a p a b i l i t y t o a l t e r t h i s s l o p e e l e c t r o n i c a l l y c a n be a d v a n t a g e o u s when v e r y h i g h dynamic r e s e r v e s a r e n e c e s s a r y w i t h p i x e l s of l i m i t e d c a p a c i t a n c e .

and v e r

high gain (

10^)

to detect

10. Long s i g n a l i n t e g r a t i o n p e r i o d s f o r enhancement o f S/N. f l e x i b i l i t y i n a l t e r i n g the l e n g t h of these p e r i o d s .

High

11. Long s i g n a l s t o r a g e p e r i o d s . P a r t i c u l a r l y u s e f u l i n 2D OIDs where a l a r g e amount o f d a t a must be t e m p o r a r i l y s t o r e d b e f o r e b e i n g t r a n s f e r r e d t o memory. 12. N e g l i g i b l e r e a d o u t l a g , i . e . , i n c o m p l e t e r e a d o u t o f s i g n a l . This i s s p e c i f i c a l l y important f o r t r a n s i e n t spectrometry. 13. N e g l i g i b l e b l o o m i n g , i . e . , p i x e l - t o - p i x e l c r o s s t a l k , due t o c h a r g e s i g n a l o v e r s p i l l t o a d j a c e n t p i x e l s . 14. Long t e r m e l e c t r i c a l , r a d i o m e t r i c and g e o m e t r i c (wavelength c a l i b r a t i o n ) . 15. Minimum d i s t o r t i o n o f image ( s p e c t r a l Constant m a g n i f i c a t i o n .

mostly

stability

l i n e s ) a c r o s s the a r r a y .

16. E q u a l m o d u l a t i o n t r a n s f e r f u n c t i o n (MTF) a c r o s s t h e a r r a y . This w i l l r e s u l t i n equal r e s o l u t i o n c h a r a c t e r i s t i c s . 17. C a p a b i l i t y t o v a r y s p e c t r a l d i m e n s i o n s e i t h e r o p t i c a l l y ( w i t h o p t i c a l f i b e r c o u p l e r s ) o r e l e c t r o n i c a l l y (by d e m a g n i f i c a t i o n ) to e n s u r e c o m p a t i b i l i t y w i t h f o c a l p l a n e f o r m a t . 18. C a p a b i l i t y o f v a r y i n g s c a n

time.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

MULTICHANNEL IMAGE DETECTORS

8

ISPD. Reasonably good, although they vary widely with respect to photocathode s e n s i t i v i t y (yA/lumen) and u n i f o r m i t y . SPD. D e v i c e - t o - d e v i c e r e l i a b i l i t y i s g e n e r a l l y good, although s u b s t a n t i a l v a r i a t i o n s i n dark charge are s t i l l s i g n i f i c a n t . Long Term S t a b i l i t y SIT &. ISIT.

Reasonably

satisfactory.

ISPD. E l e c t r o s t a t i c a l l y f o c u s e d MCPs a r e b e t t e r i n t h i s r e s p e c t . P r o x i m i t y - f o c u s e d MCP i n t e n s i f i e r s show an a v e r a g e g a i n r e d u c t i o n of 40-50% a f t e r 2-3,000 hours of n o n - s t o p e x p o s u r e t o v a r y i n g low l i g h t l e v e l s . T h i s i s e q u i v a l e n t t o approximately 1-1.5 years of continuous o p e r a t i o n (4 hours/day). A l s o , the gain can be r a i s e of 6,000-8,000 hours. SPD. The SPD, t y p i c a l of other i n t e g r a t e d c i r c u i t devices, has good s h e l f and o p e r a t i o n a l l i f e . Day-to-day r e p r o d u c i b i l i t y of p e r f o r m a n c e i s good. A p o t e n t i a l p r o b l e m , not y e t t h o r o u g h l y i n v e s t i g a t e d , i s v a r i a t i o n i n response a f t e r prolonged exposure to high l e v e l UV r a d i a t i o n . At what energy dose that phenomenon becomes s i g n i f i c a n t and whether o r n o t a n n e a l i n g can be p e r f o r m e d i s not y e t f u l l y u n d e r s t o o d . However, when used i n conventional UV-spectrometric measurements, t h i s p r o b l e m does not appear to be serious. Formats SIT & I S I T . A v a i l a b l e i n a few f o r m a t s , e.g., 18diameter, but non-standard ones are expensive.

and 25-mm

ISPD. MCP wafers are a v a i l a b l e i n a v a r i e t y of formats, shapes, and s i z e s , e.g., c i r c u l a r w a f e r s of up t o 13 cm i n d i a m e t e r and r e t a n g u l a r w a f e r s of up t o 8 χ 10 cm a r e r o u t i n e l y f a b r i c a t e d . MCPs a r e a l s o a v a i l a b l e w i t h i n d i v i d u a l c h a n n e l s b i a s e d a t a s p e c i f i c angle. This i s very u s e f u l f o r grazing incidence VUVX-ray spectroscopy. MCPs can be stacked, i n tandem, to achieve h i g h g a i n s , or o t h e r w i s e the i n d i v i d u a l c h a n n e l s a r e bent i n e i t h e r a " J " - or "C"-shape t o a l l o w o p e r a t i o n a t h i g h v o l t a g e s (2500-3000 V). At t h e s e v o l t a g e s , a s i n g l e MCP may a c h i e v e enough g a i n f o r photon c o u n t i n g (1_). MCP i n t e n s i f i e r s a r e c o m m e r c i a l l y a v a i l a b l e o n l y i n 18, 25, and 40 mm diameter formats. SPD. S-type SPDs a r e a v a i l a b l e i n f o r m a t s of 64, 128, 512, and 1024 e l e m e n t a r r a y s . A new 4096 e l e m e n t a r r a y SPD d e t e c t o r ( a s p e c t r a t i o 40:1) i s now a v a i l a b l e . A r e a a r r a y s w i t h up t o 200 χ 200 e l e m e n t s a r e a l s o a v a i l a b l e . I t i s our o p i n i o n (as

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1.

TALMI AND BUSCH

Low-Light

Level

9

Applications

w e l l as others), however, that charge-coupled devices (CCDs) and c h a r g e - i n j e c t i o n type i m a g e r s (CIDs) h o l d g r e a t e r p r o m i s e f o r two-dimensional measurements. S i m p l i c i t y of

Operation

SIT &. ISIT. No. Magnetic c o i l parameters, cathode v o l t a g e , image s e c t i o n must a l l be c a r e f u l l y c o n t r o l l e d .

and

ISPD. Very simple operation with minimum v a r i a b l e parameters. MCP photocathode v o l t a g e i s a d j u s t a b l e from approximately 160240 V, but i s t y p i c a l l y f i x e d . Phosphor v o l t a g e i s a d j u s t a b l e f r o m 5-6 KV, and i s a l s o t y p i c a l l y f i x e d . MCP v o l t a g e can be v a r i e d t o a d j u s t the g a i n . As a r u l e of thumb, a 50 V i n c r e a s e i n v o l t a g e w i l l d o u b l e the g a i n . W i t h r e s p e c t t o SPDs, the phases and p r e a m p l i f i e Scan time i s u s u a l l y f i x e d SPD. The SPD i s r a t h e r s i m p l e to o p e r a t e . Adjustments are mainly those of phases and odd-even p r e a m p l i f i e r gain s e t t i n g . The a r r a y does not p r o v i d e any a m p l i f i c a t i o n g a i n (as SIT or MCP/SPD d e v i c e s do). Scan r a t e can a l s o be a d j u s t e d f r o m a few KHz to a few MHz. Ruggedness SIT ISIT. Rather good mechanical and e l e c t r i c a l ruggedness. These d e v i c e s a r e s u s c e p t i b l e to m a g n e t i c f i e l d s , and s u f f e r from microphonic e f f e c t s . ISPD. R a t h e r compact and r e s i l e n t e l e c t r o n i c a l l y rugged. Magnetic MCPs and SPDs are non-microphonic, under high humidity and temperature

d e v i c e s . M e c h a n i c a l l y and f i e l d s have l i t t l e e f f e c t . and are designed to operate conditions.

SPD. E l e c t r o n i c a l l y and m e c h a n i c a l l y very rugged. Tolerant of h i g h t e m p e r a t u r e , h u m i d i t y , v i b r a t i o n , and e l e c t r i c a l or magnetic f i e l d s . L i g h t Shocks SIT & ISIT. Can t o l e r a t e a r e a s o n a b l e l i g h t f l u x w i t h o u t s u b s t a n t i a l l y adverse effects. However, p r e c a u t i o n s necessary to avoid a c t u a l damage to the photocathode.

any are

ISPD. MCPs s h o u l d not be exposed t o l i g h t l e v e l s above 4x10"^ f o o t - c a n d l e f o r l o n g p e r i o d s of t i m e . P r e c a u t i o n s s h o u l d be s i m i l a r to those taken w i t h PMTs. Exposure to high l i g h t l e v e l s w i l l t e m p o r a r i l y r a i s e the n o i s e l e v e l of the d e t e c t o r . To p r o t e c t the MCP and warn the u s e r , ISPD d e t e c t o r s a r e u s u a l l y

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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equipped w i t h an automatic b r i g h t - c o n t r o l c i r c u i t r y that shuts o f f the p h o t o c a t h o d e v o l t a g e when p h o t o c u r r e n t o r phosphor c u r r e n t s above a t h r e s h o l d l e v e l a r e r e a c h e d . A l s o , a b e e p e r alarm warns the user of t h i s s i t u a t i o n (5). SPD. H i g h immunity t o " l i g h t shocks". R e c o v e r s r a p i d l y f r o m exposure to very h i g h l i g h t l e v e l s . Not adversely a f f e c t e d by l i g h t e x c e p t when l e v e l s a r e s u f f i c i e n t l y h i g h t o h e a t the s i l i c o n wafer above i t s m e l t i n g point. Low Weight and Power Consumption SIT & I S I T . No. ISPD. MCP i n t e n s i f i e r s , t y p e , a r e v e r y compac f o r o p e r a t i o n and are eas

particularly

the p r o x i m i t y f o c u s e d

SPD. S m a l l s i z e and low w e i g h t . Uses v e r y l i t t l e power, and can be cooled to l i q u i d n i t r o g e n temperature. Flatness SIT & I S I T . A r r a y i s f l a t . Curved f o r m a t s a r e not a v a i l a b l e . E l e c t r o s t a t i c focusing causes pin-cushion d i s t o r t i o n and a l o s s of geometric r e s o l u t i o n at the detector's edges. ISPD. Both MCPs and SPDs a r e v e r y f l a t and have e x c e l l e n t g e o m e t r i c (wavelength) r e g i s t r a t i o n . E l e c t r o s t a t i c focusing, however, causes p i n - c u s h i o n d i s t o r t i o n and t h e r e f o r e a l o s s of g e o m e t r i c r e g i s t r a t i o n , p a r t i c u l a r l y a t the d e t e c t o r ' s edges. The p r o x i m i t y MCP produces a near p e r f e c t geometric r e g i s t r a t i o n and does not s u f f e r f r o m r e d u c e d s e n s i t i v i t y a t the edges ( s h a d i n g ) . Concave-shaped MCPs a r e a v a i l a b l e f o r non-photon a p p l i c a t i o n s (see below). SPD. Very f l a t a r r a y s w i t h e x c e l l e n t g e o m e t r i c t y p i c a l of i n t e g r a t e d c i r c u i t technology.

resolution,

Spectrometries Performance S p e c t r a l Response SIT & I S I T . S p e c t r a l r e s p o n s e depends on the s e l e c t i o n o f the p h o t o c a t h o d e and can be o p t i m i z e d f o r the b l u e o r the n e a r IR. T y p i c a l p h o t o c a t h o d e s a r e S-20 and S-20R (extended r e d m u l t i a l k a l i ) which a l l o w an adequate response up to 900 nm. Because the e l e c t r o s t a t i c a l l y focused image s e c t i o n r e q u i r e s a s p e c i a l o p t i c a l - f i b e r f a c e p l a t e window ( c u r v e d i n s i d e t o match t h e f o c u s i n g f i e l d c o n t o u r s ) made out of g l a s s , the c u t - o f f

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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wavelength f o r SITs i s approximately 350 nm. To overcome t h i s problem, a t h i n l a y e r of a proper o r g a n i c s c i n t i l l a t o r ( f l u o r e s c i n g agent) i s deposited on the o p t i c a l - f i b e r f a c e p l a t e . With such a s c i n t i l l a t o r an o v e r a l l quantum e f f i c i e n c y (QE) of 1-2% can be a c h i e v e d t o 100 nm or l o w e r . The e f f e c t i v e QE o f S I T d e t e c t o r s i s l o w e r t h a n t h a t o f PMTs because of t r a n s m i s s i o n losses through the f i b e r f a c e p l a t e . ISPD* The MCP i n t e n s i f i e r o f f e r s an u n u s u a l l y wide s p e c t r a l response, unmatched by any other OID. Each s p e c t r a l region w i l l be discussed i n turn. In the U V - V i s i b l e . the e l e c t r o s t a t i c MCP i n t e n s i f i e r has the same s p e c t r a l c h a r a c t e r i s t i c s as the SIT, i . e . , S-20 o r S20R p h o t o c a t h o d e s , and r e q u i r e s a c h e m i c a l c o n v e r t e r f o r UV measurements. The p r o x i m i t y focused MCP i n t e n s i f i e r has a good response i n the 200-85 In the VUV, two p o s s i b i l i t i e windows can be used w i t h a v a r i e t y of photocathodes, mostly of the s o l a r - b l i n d type. Second, the photocathode m a t e r i a l can be d i r e c t l y deposited on the input of the MCP. For instance, C s l , KBr, and CsTe p h o t o c a t h o d e s a r e good t o a p p r o x i m a t e l y 100 nm, whereas L i F , BaF > M g F 2 , and CaF2 respond below 100 nm. Third, the channels themselves respond d i r e c t l y to photons of energies below approximately 100 nm. MCPs have been used e x t e n s i v e l y f o r d i r e c t measurement of p a r t i c l e s : f o r example, ions i n the 3 eV-20 KeV range, e l e c t r o n s i n the 100 eV to 100 KeV r a n g e , p o s i t r o n s , and m e t a s t a b l e s and n e u t r a l s such as He, Ne, A r , K r , Xe, e t c . T h e r e i s a s u b s t a n t i a l l i t e r a t u r e on t h i s mode of d e t e c t i o n , but i t s c o m p l i l a t i o n here i s beyond the scope of t h i s paper. 2

SPD. SPDs have good r e s p o n s e i n the s p e c t r a l r e g i o n f r o m 185 (or l e s s ) t o 1100 nm (2.). F o r use i n the VUV and EVUV r e g i o n s , i t i s p o s s i b l e t o d e p o s i t a f l u o r e s c i n g agent d i r e c t l y on the d i o d e s w h i c h w i l l a c t as a w a v e l e n g t h - u p c o n v e r t e r , t h e r e b y e n s u r i n g a good r e s p o n s e to w a v e l e n g t h s l e s s t h a n 185 nm. In the x-ray s p e c t r a l region, i n o r g a n i c phosphors can be deposited on the o p t i c a l - f i b e r window c o u p l e r ( S F - t y p e SPDs). W i t h an a p p r o p r i a t e t h i n n i n g of the p r o t e c t i v e S1O2 o v e r c o a t , the SPD can d i r e c t l y respond to h i g h energy p a r t i c l e s such as e l e c t r o n s . Some a d v e r s e s i d e e f f e c t s , m a i n l y due t o an i n c r e a s e i n d a r k charge under continuous bombardment, have been reported. Also, there i s a s i g n i f i c a n t a b s o r p t i o n of e l e c t r o n s w i t h e n e r g i e s below 6 KeV, even w i t h an o v e r c o a t i n g o f S1O2 t h a t i s 1 Pm thick. T h i s a b s o r p t i o n makes the d e v i c e i n s e n s i t i v e t o e l e c t r o n s at these energies.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Resolution SIT & I S I T . The r e s o l u t i o n of SIT d e t e c t o r s can be d e f i n e d i n s e v e r a l w a y s — a) p o s i t i o n ( s p a t i a l ) r e s o l u t i o n , b) R a y l e i g h r e s o l u t i o n , and c) modulation t r a n s f e r f u n c t i o n (MTF). S p a t i a l r e s o l u t i o n i s one channel which i s t y p i c a l l y 25 μ m wide. S p e c t r a l r e s o l u t i o n i s the p r o d u c t of the c h a n n e l w i d t h and the r e c i p r o c a l d i s p e r s i o n of the s p e c t r o m e t e r . For example, a s p e c t r o m e t e r w i t h a f o c a l l e n g t h of 0.25 m and g r a t i n g of 152.5 grooves/mm t y p i c a l l y p r o d u c e s a r e c i p r o c a l l i n e a r d i s p e r s i o n of 25 nm/mm. Therefore a 25 μηι channel w i l l c o v e r 0.64 nm. A 305 g/mm g r a t i n g u s e d w i t h t h e same spectrometer would produce a r e s o l u t i o n of 0.32 nm/channel. R a y l e i g h r e s o l u t i o n i s the s e p a r a t i o n r e q u i r e d f o r two images to be recognized as such. With OIDs t h i s w i l l t y p i c a l l y mean t h a t s e p a r a t i o n i 50%. I f zero cross t a l c h a n n e l i s n e c e s s a r y between the two l i n e s . Under i d e a l c o n d i t i o n s , cross t a l k between SIT channels i s approximately 40 to 50%, so that Rayleigh r e s o l u t i o n i s about 50 Pm (2 channels). I f a n a r r o w s l i t i s used w i t h a h i g h q u a l i t y s p e c t r o m e t e r , Rayleigh r e s o l u t i o n i n wavelength u n i t s becomes 50 μπι times the reciprocal dispersion. A s i m p l i f i e d v e r s i o n of w a v e l e n g t h r e s o l u t i o n which w i l l be used throughout t h i s manuscript i s the number of channels (or p i x e l s ) at FWHM of a s p e c t r a l l i n e . For an SIT, t h i s r e s o l u t i o n i s a p p r o x i m a t e l y 2-3 c h a n n e l s . The r e s o l u t i o n , however, i s not u n i f o r m a c r o s s the d e t e c t o r and worsens when a p p r o a c h i n g t h e edges. The r e a s o n f o r t h i s i s a pin-cushion geometric d i s t o r t i o n , t y p i c a l of e l e c t r o s t a t i c image sections. A d i s t o r t i o n o f 5% i s t y p i c a l and w i l l r e d u c e r e s o l u t i o n and cause e r r o r s i n the wavelength c a l i b r a t i o n curve of the d e t e c t o r . The l a t t e r , however, can be c o m p u t e r corrected. A n o t h e r means of e x p r e s s i n g the f i d e l i t y of an o p t i c a l s y s t e m i s i n terms of i t s m o d u l a t i o n t r a n s f e r f u n c t i o n . The m o d u l a t i o n t r a n s f e r f u n c t i o n d e s c r i b e s the a b i l i t y of the o p t i c a l system to a c c u r a t e l y reproduce an object whose p a t t e r n of luminance v a r i e s i n à s i n u s o i d a l manner. An o p t i c a l system w h i c h can p r e c i s e l y d u p l i c a t e the m o d u l a t i o n p a t t e r n o f t h e object i n the modulation p a t t e r n of the image has a modulation t r a n s f e r f u n c t i o n equal to 1.0; t h i s represents the performance of a p e r f e c t system. The g r e a t e r the d i f f e r e n c e between the m o d u l a t i o n p a t t e r n i n the image compared w i t h t h a t i n the object, the l o w e r the m o d u l a t i o n t r a n s f e r f u n c t i o n . In p r a c t i c e , the modulation t r a n s f e r p a t t e r n i s u s u a l l y evaluated w i t h a square-wave p a t t e r n produced by a p e r i o d i c a r r a y of lines. The modulation t r a n s f e r f u n c t i o n approaches zero as the s p a t i a l f r e q u e n c y of the l i n e s i n c r e a s e s . The limiting r e s o l u t i o n i n terms of t h e m o d u l a t i o n t r a n s f e r f u n c t i o n i s

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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determined by the number of l i n e p a i r s per m i l l i m e t e r which give a modulation t r a n s f e r f u n c t i o n which approaches zero. When two c l o s e l y spaced s p e c t r a l l i n e s of equal i n t e n s i t y s a t i s f y the Rayleigh c r i t e r i o n , the v a l l e y between the l i n e s i s approximately 19% of the peak i n t e n s i t y . This corresponds to an MTF o f about 0.10. Based on the d a t a p r e s e n t e d under t h e d i s c u s s i o n of R a y l e i g h r e s o l u t i o n , a MTF of 0.10 would be expected to r e s u l t from a p a t t e r n w i t h about 20 lines/mm. The r e s o l u t i o n of t h e SIT i s d e f i n i t e l y v a r i a b l e ; any number of adjacent channels can be grouped together and read out as a s i n g l e r e s o l u t i o n element. A l s o , the number of diodes read out i n each channel can be v a r i e d from four (approximately 100 ym) t o 400 ( a p p r o x i m a t e l y 10 mm). Thus, the t w o - d i m e n s i o n a l target can be d i v i d e d i n t o a number of h o r i z o n t a l tracks, each of w h i c h can c o n t a i n any number of r e s o l u t i o n e l e m e n t s (max. 512) depending on the groupin ISPD. W i t h p r o x i m i t y MCP i n t e n s i f i e r s the r e s o l u t i o n i s p r a c t i c a l l y constant across the e n t i r e array. By our s p e c t r a l r e s o l u t i o n d e f i n i t i o n (see SIT), the r e s o l u t i o n i s 3-4 diodes at FWHM. Near f i e l d s t r a y l i g h t , however, widens t h e wings of s p e c t r a l l i n e s . There are a few phenomena b e l i e v e d to cause t h i s apparent s t r a y r a d i a t i o n energy ( v e i l i n g g l a r e ) . These i n c l u d e : (a) L i g h t t r a n s m i t t e d through the semi-transparent photocathode i s r e f l e c t e d from the MCP wafer back to the photocathode. This process i s enhanced by the closeness of the photocathode to the i n p u t f a c e p l a t e of the MCP, i . e . , 200 ym. (b) H i g h e n e r g y photoelectrons c o l l i d i n g w i t h the dead area between MCP channels (10,000 V/cm f i e l d ) can p r o d u c e s e c o n d a r y e l e c t r o n s t h a t a r e c o l l e c t e d by c h a n n e l s w i t h i n a p p r o x i m a t e l y a 200 ym r a d i u s (8 diodes). With e l e c t r o s t a t i c i n t e n s i f i e r s t h i s s t r a y energy (estimated at approximately 3% of photoelectron energy) can be c o l l e c t e d by a s p e c i a l g r i d e l e c t r o d e p l a c e d between t h e p h o t o c a t h o d e and the MCP. T h i s i s i m p o s s i b l e t o do w i t h proximity intensifiers. (c) S e c o n d a r y e l e c t r o n s a r e s p r e a d during m u l t i p l i c a t i o n across the channel. This w i l l reduce the r e s o l u t i o n w i t h both types of i n t e n s i f i e r s . S i n c e the r e s o l u t i o n w i t h the proximity-focused phosphor i s approximately i n v e r s e l y p r o p o r t i o n a l to the gap between the phosphor and the o u t p u t f a c e p l a t e o f t h e MCP, and s q u a r e r o o t i n v e r s e l y p r o p o r t i o n a l to the v o l t a g e between them, i t could be somewhat i m p r o v e d by r e d u c i n g t h e gap and/or r a i s i n g t h e v o l t a g e . However, corona discharge problems set a p r a c t i c a l l i m i t on t h i s approach. The e f f e c t of n e a r f i e l d s t r a y r a d i a t i o n on t h e s p e c t r a l l i n e p r o f i l e i s shown i n F i g u r e 1. SPD. The s p e c t r a l r e s o l u t i o n of an SPD i s governed by the same f a c t o r s d i s c u s s e d f o r t h e S I T , i . e . , i t d e p e n d s on t h e g r a t i n g / s p e c t r o m e t e r c o m b i n a t i o n . The s p a t i a l r e s o l u t i o n i s degraded by a degree of d i o d e - t o - d i o d e c r o s s - t a l k , i.e., i t i s

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

MULTICHANNEL IMAGE DETECTORS

Figure 1. The effect of near field stray radiation on the spectral line profile (546-nm Hg emission line). The detector was a microchannel-plate image intensified diode array (Reticon model RL-512SF, Princeton Instruments, Inc. model IRY-512). Key: a, full line profile; b, upper half (FWHM) of line; and c, lower portion of line, 0-10% relative emission intensity.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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v e r y d i f f i c u l t t o c o n t a i n the e n t i r e energy of a v e r y n a r r o w l i n e w i t h i n the b o u n d a r i e s o f a s i n g l e d i o d e . Typically, however, 2 d i o d e s or l e s s can c h a r a c t e r i z e the FWHM o f a spectral line. The c r o s s - t a l k of SPDs i s a c t u a l l y an advantage because i t r e d u c e s a l i a s i n g p r o b l e m s (2). This " a n t i - a l i a s i n g " e f f e c t of c r o s s - t a l k i s p a r t i c u l a r y i m p o r t a n t f o r h i g h resolution applications. A d e m o n s t r a t i o n of the e x c e l l e n t r e s o l u t i o n of SPDs i s shown i n F i g u r e 2. The s p a t i a l and s p e c t r a l r e s o l u t i o n of the SPD can be e l e c t r o n i c a l l y v a r i e d by summing a d j a c e n t d i o d e s i n groups o f 2,4,6,8, 10 e t c . The d i o d e s a r e r e a d out ( r e c h a r g e d ) a t v e r y h i g h r a t e s , e.g., 2 MHz, but o n l y the group-summed t o t a l i n t e g r a t e d s i g n a l i s d i g i t i z e d , and a t a r e g u l a r s c a n r a t e , e.g., 60 KHz. Diode grouping i s u s e f u l i n k i n e t i c s a p p l i c a t i o n s where s e q u e n t i a l scans must be r a p i d l y s t o r e d , but where s p e c t r a l r e s o l u t i o n ca in experimental results (low-pass f i l t e r i n g ) the odd/even g a i n v a r i a t i o n p a t t e r n t y p i c a l of SPDs, and thus making t h e i r r e a l - t i m e d i s p l a y more amenable to user i n t e r p r e t a t i o n . Random Access C a p a b i l i t y SIT £ ISIT. Real random-access i s p o s s i b l e because the e l e c t r o n r e a d o u t beam can be r a n d o m l y d i r e c t e d t o any p o r t i o n o f t h e target, skipping a l l others. T h i s mode of o p e r a t i o n i s extremely u s e f u l i n that i t a l l o w s a ( d e s t r u c t i v e ) readout of v e r y i n t e n s e s p e c t r a l l i n e s , t h e r e b y a v o i d i n g severe blooming problems, while p e r m i t t i n g weak s i g n a l s to i n t e g r a t e f o r long p e r i o d s of t i m e , on t a r g e t , to improve t h e i r S/N r a t i o s . The random-access c a p a b i l i t y of the SIT makes i t i d e a l f o r a v a r i e t y o f 2D a p p l i c a t i o n s , e.g., é c h e l l e s p e c t r o m e t r y , total l u m i n e s c e n c e s p e c t r o m e t r y , e t c . The m a i n l i m i t a t i o n on t h i s mode of o p e r a t i o n i s s e t by dark c h a r g e b u i l d u p t h a t , u n l e s s reduced by c o o l i n g , w i l l l i m i t the time a v a i l a b l e f o r on-target s i g n a l integration. ISPD. Random access c a p a b i l i t y depends only on the p r o p e r t i e s of the readout device. With the SPD only a pseudo-random, f a s t access (skip) readout i s p o s s i b l e (2,3). SPD. The d i o d e s i n an SPD must be s e q u e n t i a l l y scanned by the s h i f t r e g i s t e r s and t h e r e f o r e a r e a l random a c c e s s cannot be a c c o m p l i s h e d . However, a pseudo-random a c c e s s ( f a s t a c c e s s ) readout mode i s p o s s i b l e . In t h i s mode, diodes that c o n t a i n no r e l e v a n t s p e c t r a l i n f o r m a t i o n are skipped (not read) at a high scan r a t e , e.g., 2 MHz, whereas t h o s e c o n t a i n i n g d e s i r e d i n f o r m a t i o n a r e d i g i t i z e d a t a r e g u l a r s c a n r a t e , e.g., 60 KHz. T h i s o p e r a t i o n p e r m i t s a f a s t a r r a y scan without s a c r i f i c e i n r e s o l u t i o n , as grouping does.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

MULTICHANNEL IMAGE DETECTORS

(a) CHART SPEED 0.2 IN/MIN SCAN SPEED 1 nm/MIN DOUBLE BEAM CONVENTIONALSPECTROPHOTOMETER

ω ο ζ

< CD cr ο < / > œ


4000 can be achieved w i t h such s i g n a l s . Indeed, SPD d e t e c t i o n systems are a b l e t o a c h i e v e p h o t o m e t r i c p r e c i s i o n l e v e l s b e t t e r t h a n 10"" AU. Dynamic Range SIT &. ISIT. Under the d e f i n i t i o n of dynamic range as the r a t i o of the l a r g e s t readable s i g n a l to the rms noise of the d e t e c t i o n system, there are a c t u a l l y at l e a s t four kinds of dynamic range values. We w i l l confine our d i s c u s s i o n to the f o l l o w i n g three types of dynamic range. S i n g l e c h a n n e l dynamic range i s u s u a l l y e q u a l t o the dynamic range of the A/D converter used, since readout n o i s e i s a r b i t r a r i l y set equal to one d i g i t a l count. A range of 16,384:1 (14 b i t s ) i s e a s i l y achieved f o r t h i s range which describes the maximum and minimum s i g n a l each channel can measure. I n t r a s p e c t r a l dynamic range i s the r a t i o of the l a r g e s t and s m a l l e s t input s i g n a l s that can be simultaneously measured i n a s i n g l e s c a n ( r e g a r d l e s s of t h e l e n g t h of s i g n a l i n t e g r a t i o n t i m e ) . T h i s d e f i n i t i o n i s one of the l e a s t u n d e r s t o o d . T h e r e are a v a r i e t y of r e a s o n s why t h i s range i s s u b s t a n t i a l l y l e s s than 1 6 , 3 8 4 : 1 ; f o r example: (a) S t r a y l i g h t r e f l e c t e d f r o m the i n t e n s i f i e r e n c l o s u r e can o b s c u r e low l e v e l s i g n a l s , (b) H i g h d a r k c u r r e n t and/or background s i g n a l s upon w h i c h the a n a l y t e s i g n a l i s s u p e r i m p o s e d can r e d u c e t h i s f o r m of dynamic range. Even though b o t h a r t i f a c t s can be a c c u r a t e l y s u b t r a c t e d , the noise a s s o c i a t e d w i t h them can cause obscuration of the s i g n a l . For example: i f an a n a l y t e s i g n a l w i t h an i n t e n s i t y of 9 p h o t o e l e c t r o n s / s c a n i s s u p e r i m p o s e d on a l i n e wing of an adjacent l i n e w i t h a s i g n a l magnitude of 91 photpelectrons/scan, the S/N f o r t h e a n a l y t e s i g n a l i s 9 / ( 9 1 + 9 ) = 0.9. In the absence of the l i n e wing, the net S/N = 3. Memory dynamic range depends on the s i z e of the a v a i l a b l e memory. Of c o u r s e , v e r y wide d i g i t a l memories can be used t o s t o r e v e r y l a r g e s i g n a l v a l u e s , i.e., t h e r e s u l t of o n - t a r g e t and in-memory r e a d o u t modes. Depending on the n o i s e of the s i g n a l and detector, the o p t i m a l memory width can be s e l e c t e d , but a minimum range of 10 :1 i s u s e f u l . 1 / 2

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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ISPD. The ISPD p r o v i d e s a l i n e a r dynamic range (1-3%) of a t l e a s t 16,384:1 Ç14 b i t A/D c o n v e r t e r ) . A t i l l u m i n a t i o n l e v e l s above 4 χ 10 foot candle (with a corresponding output b r i g h t n e s s of approximately 1 foot lambert), the MCP saturates and t h e r e a f t e r has a n o n - l i n e a r t r a n s f e r characteristic. However, t h i s l e v e l i s way above the A/D c o n v e r t e r s a t u r a t i o n l e v e l and has no i m p l i c a t i o n s on low l i g h t l e v e l spectroscopy. SPD. The dynamic range o f SPDs w i l l be d i s c u s s e d i n terms o f t h e i r s i n g l e c h a n n e l dynamic range and t h e i r i n t r a s p e c t r a l dynamic range. S i n g l e channel dynamic range i s p r a c t i c a l l y l i m i t e d by the range o f a f f o r d a b l e A/D c o n v e r t e r s t h a t can o p e r a t e a t the p r o p e r scan r a t e s . W i t h 14 b i t A/D c o n v e r t e r s ( 1 6 , 3 8 4 : 1 ) , l i n e a r i t y i s a c h i e v e d t o w i t h i n a t l e a s t 2-3% o v e r the e n t i r e range. The V a r i a b l e (2.) g r e a t l y increases the p r a c t i c a v a r y i n g the on-target i n t e g r a t i o n time periods so that h i g h and low i n t e n s i t y s i g n a l s can be r e a d w i t h c o m p a r a b l e S/N r a t i o s . The r e c i p r o c i t y , i . e . , the p r o d u c t of s i g n a l i n t e n s i t y and s i g n a l i n t e g r a t i o n time i s l i n e a r w i t h i n a count range exceeding 10 : 1 . The VIT t e c h n i q u e a l l o w s d e t e c t i o n of low l i g h t l e v e l s i g n a l s w h i c h would o t h e r w i s e be d e t e c t a b l e o n l y by h i g h g a i n d e t e c t o r s , e.g., PMTs o r I S P D s . A good example of t h i s c a p a b i l i t y i s g i v e n elsewhere i n t h i s book (4). The i n t r a s p e c t r a l dynamic range of the SPD i s a f f e c t e d by a c e r t a i n degree of s t r a y l i g h t , although s u b s t a n t i a l l y l e s s than the ISPD. F u t h e r m o r e , s i n c e d e t e c t o r heads have t h e i r own sealed quartz windows, i t i s p o s s i b l e to use the SPD without i t s own q u a r t z window and t h u s r e d u c e the degree of i n t e r n a l reflection. A d d i t i o n a l l y , i n t e r n a l r e f l e c t i o n s between t h e s i l i c o n target and the detector quartz window can be reduced by using o f f - a x i s o p t i c s . S i g n a l Storage C a p a b i l i t y SIT ISIT. S t o r a g e t i m e of SITs depends o n l y on the d a r k c h a r g e l e v e l , w h i c h depends on the d e t e c t o r t e m p e r a t u r e . At room temperature, a storage time of 1-2 seconds i s t y p i c a l . At d r y - i c e temperature, 20-50 minutes of storage time are p o s s i b l e . ISPD. S t o r a g e t i m e depends on c o o l i n g t e m p e r a t u r e . F o r l o n g s t o r a g e p e r i o d s t h e MCP m u s t a l s o be c o o l e d t o r e d u c e spontaneous photoemission. However, c o o l i n g of the photocathode w i l l reduce the red response of the photocathode. SPD. to

S i g n a l storage time i s s m a l l unless the detector i s cooled below -50 °C (30-60 minutes).

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Lag SIT & I S I T . V i d i c o n d e t e c t o r s r e q u i r e some t i m e to a d j u s t to l a r g e v a r i a t i o n s i n s i g n a l magnitude because of readout d i s c h a r g e l a g , i.e., i n c o m p l e t e r e a d o u t of s i g n a l i n a s i n g l e scan. A c t u a l l y , b e t t e r t h a n t h e o r e t i c a l (based on shot n o i s e ) S/N p e r f o r m a n c e i s a c h i e v e d i n c v ( c o n t i n u o u s wave) o p e r a t i o n because lag causes s i g n a l averaging. However, discharge l a g i s a d i s t i n c t d i s a d v a n t a g e i n s i n g l e shot measurements. The p r e s e n c e of l a g means t h a t a t l e a s t 5-10 scans a r e r e q u i r e d t o read a l l the charge o f f the t a r g e t . This i s not a problem when high r e p e t i t i o n r a t e p u l s e s , i.e., a few per second or more, are measured. ISPD. The SPD r e a d o u t d e v i c e i s p r a c t i c a l l y d i s c h a r g e l a g f r e e . Any l a g of the ISP the output phosphor (althoug the e l e c t r i c a l r e s p o n s e t i m e o f the c h a n n e l s becomes an important f a c t o r ) . As explained w i t h the SIT (under discharge l a g ) , slow phosphor decay t i m e w i l l have l i t t l e e f f e c t on the a c c u r a c y o f m e a s u r e m e n t s o f s i n g l e p u l s e s or w i t h h i g h r e p e t i t i o n r a t e p u l s e s or cw. I t becomes a p r o b l e m , however, with low r e p e t i t i o n r a t e pulses where each pulse i s superimposed on the decay t a i l o f the p r e c e d i n g p u l s e . F o r t u n a t e l y , the phosphor decay t i m e d e c r e a s e s s i g n i f i c a n t l y as the e x c i t a t i o n (measured) pulse becomes shorter. Decay time i s a l s o a f f e c t e d by the energy l e v e l of the p u l s e and by the g r a n u l a r i t y of the phosphor. I t seems, however, t h a t f o r s h o r t p u l s e s < 1 U s , decay time may not become an accuracy l i m i t i n g f a c t o r . SPD. When high q u a l i t y devices are used, readout l a g of SPDs i s very s m a l l f o r l i g h t l e v e l s below the capacitance s a t u r a t i o n of the d i o d e s , i . e . , a p p r o x i m a t e l y 2 χ 10 photons/diode. Above t h a t l i g h t l e v e l , c h a r g e d i f f u s e s t o the s u b s t r a t e and a few a r r a y scans may be n e c e s s a r y t o f u l l y r e a d i t o f f the t a r g e t . I t i s i n t e r e s t i n g t o n o t e t h a t the v i d i c o n l a g i s due to incomplete readout of s m a l l s i g n a l s , whereas the reverse i s true f o r SPDs. Blooming SIT &, ISIT. Blooming w i l l occur when diodes are saturated, i.e., s i g n a l w i l l s p i l l o v e r t o adacent d i o d e s . T h i s i s p a r t i a l l y r e d u c e d by the e l e c t r i c a l i s o l a t i o n between a d j a c e n t d i o d e s . A n o t h e r phenomenon w i t h s i m i l a r r e s u l t s i s h a l a t i o n o r l i g h t r e f l e c t e d back to the p h o t o c a t h o d e f r o m the t a r g e t (the photocathode i s semi-transparent). T h i s i s , i n essence, a s t r a y l i g h t phenomenon t h a t p r o d u c e s an h a l o around h i g h i n t e n s i t y spectral l i n e s .

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ISPD. B l o o m i n g p r o b l e m s a r e v e r y s m a l l and s i m i l a r to t h o s e discussed f o r the SPD (see below), i.e., blooming i s l i m i t e d to charge o v e r s p i l l to adjacent diodes only, once the capacitance l e v e l ( a p p r o x i m a t e l y 8 χ 10 e l e c t r o n s ) i s exceeded. Stray energy r a d i a t i o n problems have been p r e v i o u s l y discussed. SPD. W i t h q u a l i t y d e v i c e s , b l o o m i n g seems t o be c o n f i n e d t o adjacent diodes, i.e., upon s a t u r a t i o n of a diode, s i g n a l charge w i l l p a r t i a l l y s p i l l o v e r to the n e a r e s t u n s a t u r a t e d d i o d e . L o c a l i z e d blooming i s e s s e n t i a l i n order to m a i n t a i n an adequate s p a t i a l r e s o l u t i o n across the array and to e f f i c i e n t l y u t i l i z e the VIT r e a d o u t t e c h n i q u e , i.e., the s m a l l e r the b l o o m i n g , the s m a l l e r the e f f e c t of h i g h l e v e l s i g n a l s on n e a r - b y low l e v e l signals. Long Term S t a b i l i t y SIT &. ISIT. Long-term s t a b i l i t y i s p o s s i b l e , but d i f f i c u l t , to a c h i e v e . I n s t a b i l i t i e s w i l l o c c u r as a r e s u l t of t e m p e r a t u r e variations. Examples of p a r a m e t e r s a f f e c t e d by environmental c o n d i t i o n s and whose v a r i a b i l i t y w i l l d e g r a d e d e t e c t o r performance are: t a r g e t dark charge, c o i l o p e r a t i o n , magnetic i n t e r f e r e n c e w i t h c o i l operation, i n s t a b i l i t i e s i n high v o l t a g e s u p p l y w h i c h cause v a r i a t i o n s i n f o c u s i n g and d i s t o r t i o n , v a r i a t i o n s i n c a t h o d e v o l t a g e w h i c h a f f e c t r e a d o u t and l a g p a r a m e t e r s , e t c . In g e n e r a l , the l a r g e r the number of s e t - u p parameters, the greater the i n s t a b i l i t y . ISPD. Good l o n g t e r m s t a b i l i t y p r o v i d e d t h a t the MCP has not been p r e v i o u s l y exposed to high l e v e l i l l u m i n a t i o n and that the SPD i s a c c u r a t e l y thermostated. SPD. Long t e r m r a d i o m e t r i c s t a b i l i t y i s good. Geometric s t a b i l i t y i s u n a f f e c t e d by e x t e r n a l p e r t u r b a t i o n s , e.g., m a g n e t i c f i e l d , and i s a l w a y s the same. E l e c t r i c a l s t a b i l i t y depends on the design of the p r e a m p l i f i e r and s i g n a l processing e l e c t r o n i c s and i s u s u a l l y not a problem. In the past, long i n memory i n t e g r a t i o n periods r e s u l t e d i n o u t p u t s e p a r a t i o n i n t o f o u r s p e c t r a ( f o r each phase); t h i s has not been o b s e r v e d w i t h the new (SPD) d e t e c t o r head (Princeton Instruments, Inc., Model RY-512). Long t e r m b a s e l i n e s t a b i l i t y depends on c o o l i n g and p r e c i s i o n thermostating, and can be maintained at or below the l e v e l of photon-shot noise. Distortion SIT &. ISIT.

Subject

to p i n cushion d i s t o r t i o n (see above).

ISPD. E l e c t r o s t a t i c i n t e n s i f i c a t i o n has a maximum d i s t o r t i o n (pin-cushion) of approximately 5%. P r o x i m i t y i n t e n s i f i e r s have

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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no g e o m e t r i c d i s t o r t i o n . M a g n i f i c a t i o n r a t i o s (input/output) can vary from detector to detector i n the 0.9 to 1.04 range. SPD. SPD a r r a y s have no g e o m e t r i c d i s t o r t i o n . Variationi n r e s o l u t i o n , r e s p o n s e , dark c h a r g e , p a t t e r n n o i s e , e t c . a r e random across the array. Modulation T r a n s f e r Function SIT &. I S I T . P i n c u s h i o n d i s t o r t i o n ( s e e above) r e s u l t s i n a v a r i a t i o n of the modulation t r a n s f e r f u n c t i o n across the t a r g e t of t h e d e t e c t o r w i t h a maximum d i s t o r t i o n p r e s e n t i n t h e periphery. ISPD. pair):

T y p i c a l MTF v a l u e s

f o r MCPs a r e as f o l l o w s ( l p = l i n e

lp/mm

proximity

electrostatic

2.5

86%

90%

7.5

58%

60%

15

20%

25%

The MTF of the SPD i s s u b s t a n t i a l l y b e t t e r than that of the MCP i n t e n s i f i e r s and t h e r e f o r e c o n t r i b u t e s l i t t l e to the o v e r a l l MTF of t h e ISPD. V a r i a b l e Dimensions SIT

ISIT.

The dimensions of the SIT and ISIT are f i x e d .

ISPD. A 40 mm MCP can be coupled to a 25 mm SPD (1024 elements) through a 40/25 mm format reducing o p t i c a l - f i b e r coupler and thereby r e s u l t i n a p r o p o r t i o n a l increase i n the s p e c t r a l window of the detector. SPD. Format reducing o p t i c a l - f i b e r couplers, e.g., 40 mm to 25 mm, can be used as s p e c t r u m d e m a g n i f i e r s . Such d e v i c e s can increase the s p e c t r a l coverage of an SPD at the s a c r i f i c e of UV response. U V - t o - v i s i b l e chemical converters can be deposited on the i n p u t o p t i c a l - f i b e r f a c e p l a t e , b u t a c e r t a i n degree o f l a t e r a l c r o s s - t a l k could result i n a d e t e r i o r a t i o n i n resolution. F l e x i b l e bundles of coherent (imaging) o p t i c a l f i b e r s can be used to bind together (side by side) any number of SPDs i n a contiguous manner. This can allow an a d a p t a b i l i t y of SPDs to any f o c a l plane and any s p e c t r a l coverage required.

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V a r i a b l e Scan Time SIT &. ISIT, V a r i a t i o n of s c a n t i m e i s p o s s i b l e w i t h i n a reasonable range, i.e., lowest p o r t i o n l i m i t e d by discharge l a g , highest p o r t i o n by dark charge l e v e l . V a r i a t i o n s between 20200 ys are e a s i l y achieved. ISPD. Rather than vary the scan time, i t i s p r e f e r a b l e to vary the o n - t a r g e t i n t e g r a t i o n t i m e . T h i s can be c o n v e n i e n t l y a c c o m p l i s h e d over the range f r o m 16 ms to 120 seconds ( w i t h c o o l i n g to -10 ° C ) . SPD. Scan t i m e can be v a r i e d f r o m a few KHz to a few MHz, but v a r i a t i o n of scan t i m e t h r o u g h a d j u s t m e n t of e x p o s u r e t i m e i s advantageous.

Conclusions The p r i m a r y purpose of t h i s paper has been to p r o v i d e a c o n v e n i e n t r e f e r e n c e f o r r e s e a r c h e r s who a r e i n t e r e s t e d i n a p p l y i n g OIDs f o r l o w - l i g h t l e v e l s p e c t r o s c o p y and who need a s o u r c e of d e t a i l e d i n f o r m a t i o n about t h e s e d e v i c e s to e n a b l e them to s e l e c t the proper detector f o r a p a r t i c u l a r a p p l i c a t i o n . T h i s paper has p r e s e n t e d an i n - d e p t h c o m p a r i s o n o f f o u r o p t o e l e c t r o n i c image devices w i t h 25 performance c r i t e r i a . In using the g u i d e l i n e s presented i n t h i s paper f o r the s e l e c t i o n of a p a r t i c u l a r OID f o r l o w - l i g h t l e v e l a p p l i c a t i o n s , one should d i v i d e the v a r i o u s c r i t e r i a discussed i n t o two c a t e g o r i e s based on the p a r t i c u l a r a p p l i c a t i o n under c o n s i d e r a t i o n . The f i r s t category should i n c l u d e those c r i t e r i a w h i c h a r e of paramount importance f o r the p a r t i c u l a r a p p l i c a t i o n . The other category should i n c l u d e those c r i t e r i a which are of secondary importance i n the p a r t i c u l a r a p p l i c a t i o n . Care s h o u l d be t a k e n a t t h i s stage to avoid mutually e x c l u s i v e s i t u a t i o n s . For example, i t w i l l be d i f f i c u l t to f i n d a detector which i s s e n s i t i v e to very l o w - l i g h t l e v e l s and i s simultaneously immune to l i g h t shocks. Based on the p a r t i c u l a r c r i t e r i a i d e n t i f i e d i n the f i r s t c a t e g o r y , a p a r t i c u l a r OID may emerge as b e i n g the most s a t i s f a c t o r y f o r the g i v e n a p p l i c a t i o n . A l t e r n a t i v e l y , no c l e a r - c u t winner may emerge from the comparison which may mean t h a t the s e l e c t i o n of a p a r t i c u l a r OID may not be c r i t i c a l t o the given a p p l i c a t i o n or that the current s t a t e - o f - t h e - a r t OIDs are unable to meet the requirements of the experiment. I t should be c l e a r from the d i s c u s s i o n of the performance c r i t e r i a of OIDs t h a t no one d e t e c t o r i s b e s t s u i t e d f o r a l l p o s s i b l e s p e c t r o s c o p i c a p p l i c a t i o n s . Proper selection, t h e r e f o r e , depends on an i n - d e p t h u n d e r s t a n d i n g o f d e t e c t o r c h a r a c t e r i s t i c s as discussed i n t h i s paper. F o r t u n a t e l y , there are a l r e a d y a d i v e r s i t y of OIDs a v a i l a b l e and more a r e b e i n g developed. T h i s i s a r a p i d l y changing a r e a and second-

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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generation devices with improved performance s p e c i f i c a t i o n s are being c o n t i n u a l l y developed. Therefore, i f current s t a t e - o f - t h e a r t devices are not s a t i s f a c t o r y f o r a given a p p l i c a t i o n , i t may not be much longer before improved devices are developed.

Literature Cited 1.

2. 3. 4.

5. 6.

Siegmund, Oswald H. W.; Malina, Roger F. in "Image Devices in Spectroscopy," Talmi, Y., Ed.; ACS SYMPOSIUM SERIES No. ACS:Washington, D.C., 1983. Talmi, Y.; Simpson, R. W. Appl. Opt. 1980, 19, 1401. Talmi, Y. Appl. Spectrosc. 1980, 36, 1. Grabau, F.; Talmi, Y. i n "Image Devices i n Spectroscopy," Talmi, Y., Ed.; ACS SYMPOSIUM SERIES No. 102 ACS:Washington, D.C., 1983. "Model IRY Detecto Inc., 1982. Talmi, Y.; Baker, D. C.; Jadamec, J. E.; Saner, W. A. Anal. Chem. 1978, 50, 936A.

R E C E I V E D July 26, 1983

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2 The Silicon-Intensified Target Vidicon Detector Operation, Characterization, and Application in Atomic Spectroscopy Research J O H N W. O L E S I K 1 and J O H N P. WALTERS2 University of Wisconsin-Madison, Madison, WI 53706

In this paper w the s i l i c o n intensified target (SIT) vidicon, describing what we conclude to be the spectroscopically most important properties of the detector. We provide experimental evaluation of the SIT vidicon characteristics including two-dimensional image fidelity; channel-to-channel and pixel-to-pixel response as a function of position on the detector; and temporal resolution and gating. We illustrate how these unique properties enhance our ability to make spectroscopic measurements while at the same time impose limitations on the use of the SIT vidicon. We describe a variety of experiments which employed an SIT vidicon detection system for atomic spectroscopy with various degrees of spatial and temporal resolution.

The s i l i c o n i n t e n s i f i e d t a r g e t ( S I T ) v i d i c o n h a s a number o f u n i q u e p r o p e r t i e s w h i c h make i t a v a l u a b l e d e t e c t o r f o r a t o m i c spectroscopy. The S I T v i d i c o n p r o v i d e s t w o - d i m e n s i o n a l p h o t o e l e c t r i c d e t e c t i o n w i t h h i g h s e n s i t i v i t y and r a p i d s i g n a l readout. Time r e s o l u t i o n c a n be o b t a i n e d i n a t i m e - r e s o l v e d ( r e a l t i m e ) mode o n t h e m i l l i s e c o n d s c a l e and i n a t i m e - g a t e d ( e q u i v a l e n t t i m e ) mode o n t h e s u b m i c r o s e c o n d s c a l e . C o u p l i n g an S I T v i d i c o n d e t e c t o r w i t h a n o n - l i n e c o m p u t e r for d e t e c t o r c o n t r o l , experiment s y n c h r o n i z a t i o n , data a c q u i s i t i o n and d a t a p r o c e s s i n g r e s u l t s i n a p o w e r f u l and f l e x i b l e d e t e c t i o n s y s t e m . The f l e x i b i l i t y r e s u l t s f r o m t h e 1Current address: Department of Chemistry, Indiana University, Bloomington, IN 47405. 2Current address: Department of Chemistry, St. Olaf College, Northfield, MN 55057.

0097-6156/ 83 / 0236-0031 $06.00/ 0 © 1983 A m e r i c a n C h e m i c a l S o c i e t y

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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MULTICHANNEL IMAGE DETECTORS

s o f t w a r e c o n t r o l o f t h e speed o f s i g n a l a c q u i s t i o n , s p e c t r a l r e s o l u t i o n , photometric accuracy, exposure time, s p a t i a l r e s o l u t i o n , and t e m p o r a l r e s o l u t i o n . C o m p l e t e c o m m e r c i a l s y s t e m s i n c l u d i n g an S I T v i d i c o n d e t e c t o r , d e t e c t o r c o n t r o l l e r , h i g h v o l t a g e p u i s e r f o r t i m e g a t i n g , and a c o m p u t e r , i n c l u d i n g s o f t w a r e , a r e a v a i l a b l e . The P r i n c e t o n A p p l i e d R e s e a r c h C o r p o r a t i o n (PARC) OMA I I s y s t e m was u s e d i n t h e s e s t u d i e s , mounted i n t h e f o c a l p l a n e o f a one m e t e r M c P h e r s o n model 2051 monochromator. W h i l e t h e S I T v i d i c o n d e t e c t i o n s y s t e m p r o v i d e s u n i q u e and exciting spectroscopic detection c a p a b i l i t i e s , inherent l i m i t a t i o n s of v i d i c o n d e t e c t i o n prevent i t from r e p l a c i n g other d e t e c t o r s such as p h o t o g r a p h i c e m u l s i o n s , p h o t o m u l t i p l i e r tubes, or l i n e a r p h o t o d i o d e a r r a y s . R a t h e r , t h e SIT v i d i c o n i s c o m p l e m e n t a r y t o t h e more t r a d i t i o n a l s p e c t r o s c o p i c d e t e c t o r s . Operation

of t h e SI

A l t h o u g h t h e o p e r a t i o n o f v i d i c o n d e t e c t o r s h a s been d e s c r i b e d e l s e w h e r e (_l-_4), we b r i e f l y s u m m a r i z e i t h e r e s i n c e t h e v i d i c o n components and o p e r a t i o n a f f e c t t h e c h a r a c t e r i s t i c s o f t h e S I T v i d i c o n as a s p e c t r o s c o p i c d e t e c t o r . S i m i l a r to other i m a g i n g d e t e c t o r s , t h e s i l i c o n v i d i c o n i s made up o f t h r e e m a i n components. A t r a n s d u c e r c o n v e r t s t h e p h o t o n s p e c t r a l image to i t s corresponding e l e c t r i c a l analog. I n the s i l i c o n v i d i c o n the s i l i c o n t a r g e t performs t h i s f u n c t i o n w h i l e i n t h e s i l i c o n i n t e n s i f i e d t a r g e t v i d i c o n , shown i n F i g u r e 1, t h e s c i n t i l l a t o r and p h o t o c a t h o d e a c t as t h e t r a n s d u c e r . The s i l i c o n t a r g e t a c t s as a s t o r a g e d e v i c e f o r t h e e l e c t r i c a l a n a l o g o f t h e s p e c t r a l image. The t a r g e t i s b u i l t on an η-type s i l i c o n w a f e r w h i c h s e r v e s as a common c a t h o d e . P - t y p e s e m i c o n d u c t o r anodes a r e grown on t h e w a f e r , a p p r o x i m a t e l y e i g h t m i c r o n s apart. The v i d i c o n s i g n a l i s a c q u i r e d by an e l e c t r o n beam, t w e n t y - f i v e m i c r o n s i n d i a m e t e r , w h i c h a c t s as t h e r e a d i n g device . The o p e r a t i o n o f a s i l i c o n v i d i c o n c o n s i s t s o f p r e p a r a t i o n , e x p o s u r e , d e v e l o p m e n t and r e a d i n g s t e p s . The t a r g e t i s r e s t o r e d t o an e q u i l i b r i u m p o t e n t i a l by s c a n n i n g t h e e l e c t r o n beam t o charge the p-type i s l a n d s t o a negative p o t e n t i a l , e r a s i n g p r e v i o u s l y s t o r e d i m a g e s . The η-type s u b s t r a t e i s h e l d a t ground p o t e n t i a l , r e s u l t i n g i n a r e v e r s e d - b i a s e d diode condition. T h i s p r e p a r e s t h e t a r g e t f o r e x p o s u r e by p r o d u c i n g a d e p l e t i o n l a y e r w h i c h a c t s as a s t o r a g e c a p a c i t o r . P h o t o e l e c t r o n s imaged on t h e t a r g e t p r o d u c e e l e c t r o n - h o l e p a i r s . The h o l e s d i f f u s e t h r o u g h t h e d e p l e t i o n l a y e r i n t o t h e p - t y p e i s l a n d s , d i s c h a r g i n g t h e c a p a c i t o r s . The d e v e l o p m e n t s t e p i s p e r f o r m e d by s c a n n i n g t h e t a r g e t w i t h t h e e l e c t r o n beam, t h e v i d i c o n analog t o a photographic developer. The change i n c u r r e n t due t o r e c h a r g i n g o f e a c h c a p a c i t o r i s a m p l i f i e d t o become t h e v i d e o s i g n a l . Since t h i s current i s proportional to

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. IMAGE SECTION

HIGH VELOCITY PHOTOELECTRON BEAM IMAGED ON TARGET

ANOOE

SCANNING SECTION

^SILICON TARGET LOW VELOCITY I SCANNING BEAM

FIELD MESH GUN FOCUSING GRID (#4) GRID (#3)

CONTROL GRIO (#1)

ELECTRON GUN

CATHODE

ACCELERATING GRID (#2)

Figure 1. Illustration of the silicon-intensified target (SIT) vidicon. (Reproduced with permission from Ref. 8. Copyright 1978, Princeton Applied Research Corporation.)

FIBER OPTIC FACEPLATE

LIGHT

IMAGE FOCUS GRIDS-, PHOTOCATHODE

34

MULTICHANNEL IMAGE DETECTORS

t h e amount o f c a p a c i t o r r e c h a r g i n g n e c e s s a r y t o r e a t t a i n t h e e q u i l i b r i u m p o t e n t i a l , and t h e c a p a c i t o r s a r e d i s c h a r g e d i n p r o p o r t i o n t o t h e number o f p h o t o e l e c t r o n s , t h e r e c h a r g i n g c u r r e n t i s p r o p o r t i o n a l t o t h e i n c i d e n t l i g h t i n t e n s i t y . The v i d e o s i g n a l i s d i g i t i z e d and s t o r e d i n t h e c o m p u t e r random a c c e s s memory a n d / o r d i s p l a y e d on a CRT. The i n t e n s i f i e r s t a g e i n c r e a s e s t h e s e n s i t i v i t y o f t h e S I T v i d i c o n o v e r an u n i n t e n s i f i e d s i l i c o n v i d i c o n d e t e c t o r . The p h o t o e l e c t r o n s a r e a c c e l e r a t e d and e l e c t r i c a l l y f o c u s e d o n t o t h e t a r g e t by an e l e c t r o s t a t i c f i e l d o f s e v e n t o n i n e k i l o v o l t s . A p p r o x i m a t e l y 1500 e l e c t r o n - h o l e p a i r s a r e p r o d u c e d f o r e a c h e l e c t r o n which s t r i k e s the s i l i c o n t a r g e t . However, b e c a u s e t h e p h o t o c a t h o d e h a s a much l o w e r quantum e f f i c i e n c y t h a n t h e s i l i c o n t a r g e t , t h e p h o t o s i g n a l g a i n o f t h e SIT v i d i c o n i s o n l y a p p r o x i m a t e l y two h u n d r e d t i m e s t h a t o f t h e u n i n t e n s i f i e d s i l i c o n v i d i c o n (_3) a n region. The s e n s i t i v i t has b e e n compared (_5) t o a 1P28 p h o t o m u l t i p l i e r t u b e i n t e g r a t e d s i g n a l . The SIT v i d i c o n s e n s i t i v i t y was f o u n d t o be s i m i l a r t o a 1P28 p h o t o m u l t i p l i e r t u b e f r o m 400 t o 600 nm, somewhat more s e n s i t i v e f r o m 600 t o 800 nm and a p p r o x i m a t e l y t w e n t y t i m e s l e s s s e n s i t i v e f r o m 200 t o 400 nm. W i t h . t h e new s c i n t i l l a t o r i n t h e OMA I I s y s t e m , t h e d i f f e r e n c e i n s e n s i t i v i t y i n the u l t r a v i o l e t r e g i o n i s f i v e t o t e n times l e s s than the 1P28 PMT. The s c i n t i l l a t o r i s n e c e s s a r y b e c a u s e o f t h e o t h e r w i s e p o o r r e s p o n s e b e l o w 400 nm, due m a i n l y t o l o s s e s i n t h e f i b e r optic faceplate. The f i b e r o p t i c f a c e p l a t e i s n e c e s s a r y b e c a u s e a curved surface i s required f o r e l e c t r o s t a t i c imaging i n the i n t e n s i f i e r s e c t i o n ( 6 0 o f t h e SIT v i d i c o n tube. The SIT v i d i c o n d e t e c t i o n c a n be g a t e d by c o n t r o l l i n g t h e i n t e n s i f i e r s t a g e i n a manner s i m i l a r t o a t r i o d e t u b e i n t h e g r o u n d e d g r i d c o n f i g u r a t i o n (7_) . A n e g a t i v e v o l t a g e p u l s e (-500 t o -1200 v o l t s ) i s a p p l i e d t o t h e p h o t o c a t h o d e t o t j i r n t h e t u b e o n . The o n / o f f g a t i n g r a t i o o f t h e d e t e c t o r i s 10 t o 10 . C o n t r o l of t h e SIT v i d i c o n . Regions of the v i d i c o n target t o be r e a d o u t , and t h e r e f o r e t h e s p e c t r a l a n d / o r s p a t i a l r e g i o n s t o be o b s e r v e d , a r e c o n t r o l l e d by s c a n n i n g o f t h e e l e c t r o n beam. I n t h e PARC OMA I I s y s t e m t h e e l e c t r o n beam s c a n n i n g i s u n d e r s o f t w a r e c o n t r o l . A 500 χ 500 a r r a y o f p o s i t i o n s on t h e v i d i c o n t a r g e t ( p i x e l s , o r p i c t u r e e l e m e n t s ) c a n be r a n d o m l y a d d r e s s e d and r e a d . I n d i v i d u a l r e g i o n s o f t h e t a r g e t t o be r e a d o u t , c a l l e d t r a c k s , a r e d e f i n e d by a n u m b e r o f c h a n n e l s ( c o l u m n s of p i x e l s ) o f a c h o s e n h e i g h t , y. A f r a m e c o n s i s t s o f one r e a d o u t s c a n o f t h e s e l e c t e d number o f t r a c k s . The e x p o s u r e t i m e i s d e t e r m i n e d b y t h e c h a n n e l t i m e , t h e t i m e t h e e l e c t r o n beam t a k e s t o r e a d e a c h c h a n n e l , t h e number o f c h a n n e l s r e a d o u t , t h e number o f c h a n n e l s g r o u p e d t o g e t h e r a s one r e s o l u t i o n e l e m e n t , a n d t h e n u m b e r o f s c a n s w h i c h a r e

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2.

OLESIK A N D WALTERS

Silicon-Intensified

Target Vidicon

Detector

35

accumulated. The e x p o s u r e t i m e c a n a l s o be c o n t r o l l e d b y a s o f t w a r e - c o n t r o l l e d d e l a y b e f o r e t h e t a r g e t i s r e a d . However, c o o l i n g o f t h e v i d i c o n i s r e q u i r e d f o r d e l a y t i m e s o f more t h a n 100 ms a s d i s c u s s e d b e l o w . V i d i c o n Components

Effect

on D e t e c t i o n

The most o b v i o u s l i m i t a t i o n o f v i d i c o n d e t e c t o r s i s t h e s m a l l d e t e c t o r a r e a . T y p i c a l l y , t h e SIT v i d i c o n d e t e c t o r a r e a i s 12.5 by 12.5 mm. However, t h e u s e f u l d e t e c t o r a r e a may be f u r t h e r l i m i t e d by d i s t o r t i o n w h i c h i s most s e v e r e n e a r t h e edges o f t h e d e t e c t o r , a s w i l l be d i s c u s s e d b e l o w . I fa p o l y c h r o m a t o r w i t h a d i s p e r s i o n o f 0.83 nm/mm i s u s e d , a w a v e l e n g t h r a n g e o f 10 nm c a n be m o n i t o r e d when t h e f u l l w i d t h o f t h e v i d i c o n i s used. Silicon target. The o f t h e e l e c t r i c a l s i g n a l image r e s u l t s i n a n i n t e g r a t i n g d e t e c t o r s e n s i t i v e t o e n e r g y r a t h e r t h a n power. The t a r g e t i s t h e m a i n s o u r c e o f d a r k c u r r e n t and l i m i t s t h e o n - t a r g e t s t o r a g e t i m e t o a p p r o x i m a t e l y one h u n d r e d m i l l i s e c o n d s a t room t e m p e r a t u r e ( V ) . The s t o r a g e t i m e c a n be i n c r e a s e d t o up t o two h o u r s by c o o l i n g t h e d e t e c t o r (_9 ,_10 ) . H o w e v e r , c o o l i n g i s i n c o n v e n i e n t due t o the s i z e o f t h e v i d i c o n d e t e c t o r ( i n comparison t o a photodiode a r r a y , f o r example). Some c o m m e r c i a l l y a v a i l a b l e c o o l e r s a r e c o n s t r u c t e d so t h a t t h e f o c a l p l a n e o f t h e p o l y c h r o m a t o r used must be t h r e e i n c h e s o u t s i d e o f where t h e c o o l e r and v i d i c o n a r e attached t othe polychromator housing o f t e n r e q u i r i n g a s p e c i a l l y designed polychromator o r a d d i t i o n a l o p t i c s . This i s due t o t h e need t o i n s u l a t e t h e v i d i c o n c o o l e r and p r e v e n t f r o s t i n g o f t h e window b e t w e e n t h e p o l y c h r o m a t o r and t h e vidicon. Cooling also results i n substantially increased l a g , w h i c h i s d i s c u s s e d below. Because t h e t a r g e t r a t h e r than t h e photocathode i s t h e main dark c u r r e n t source, t h e dark c u r r e n t s i g n a l i s independent o f the gate w i d t h used. The random n o i s e component o f t h e d a r k current can l i m i t the d e t e c t a b i l i t y o f l i g h t s i g n a l s . Multiple s c a n s o r a s y n c h r o n o u s a v e r a g i n g i n c r e a s e s d e t e c t a b i l i t y and s i g n a l - t o - n o i s e (_10 a s s h o w n i n F i g u r e 2. Blooming. M i g r a t i o n o f c h a r g e o n t h e s i l i c o n t a r g e t a n d , more i m p o r t a n t l y , u s e o f e l e c t r o n beam r e a d o u t r e s u l t s i n b l o o m i n g , o r c r o s s t a l k , b e t w e e n c h a n n e l s . The c r o s s t a l k i n t h e PARC s u p p l i e d S I T v i d i c o n i s s p e c i f i e d ( 8 ) t o be l e s s t h a n 60% b e t w e e n a c e n t r a l c h a n n e l and e a c h o f i t s two n e i g h b o r s . T a b l e I shows t h e measured b l o o m i n g f r o m a c e n t r a l c h a n n e l t o nearby channels. Blooming l i m i t s t h e r e s o l u t i o n o b t a i n a b l e w i t h v i d i c o n detectors. F o r e x a m p l e , i f a n a t o m i c l i n e w i t h a 0.02 nm l i n e w i d t h i s o b s e r v e d t h r o u g h a p o l y c h r o m a t o r w i t h 0.83 nm/mm

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

36

MULTICHANNEL IMAGE DETECTORS A

Β

C

D

©

Figure 2. Vidicon detected, copper hollow cathode spectra showing an increase in signalto-noise ratio as the detection time and number of readout scans is increased. Key: A, signalfrom a single readout scan (detection time, 16.6 ms); B, signalfrom 10 readout scans (detection time, 166 ms); C, signalfrom 100 readout scans (detection time, 1.66 s); and D, signalfrom 1000 readout scans (detection time, 16.6 s). The Y axes for all spectra are in arbitrary intensities units. The spectral lines observed are: 1, 329.30 nm; 2, 329.77 nm; 3, 330.80 nm; and 4, 331.13 nm. Detector conditions were: 80 μ$/channel readout; 500 channels read out; 10 prep scans before each spectrum was obtained; and delta Y - 100.

Table I .

Channel center

+ + + + + + + -

0 1 1 2 2 3 3 4 4 5 5 6 6 7 7

from

Blooming

Intensity

19935 10435 11133 2481 2360 624 622 251 399 92 346 62 185 43 99

i n SIT v i d i c o n . *

Percentage of center channel i n t e n s i t y 100.0% 52.3 55.8 12.4 11.8 3.1 3.1 1.3 2.0 0.4 1.7 0.0 0.1 0.0 0.0

* B l o o m i n g was m e a s u r e d by u s i n g an i r o n h o l l o w c a t h o d e lamp s o u r c e , a 20 m i c r o n e n t r a n c e s l i t on a M c P h e r s o n 1 m e t e r C z e r n y T u r n e r p o l y c h r o m a t o r u s e d i n f i r s t o r d e r . The 352.126 nm i r o n I l i n e was c e n t e r e d on a s i n g l e c h a n n e l (X = 2 3 0 ) . F i v e h u n d r e d c h a n n e l s , e a c h 80 p i x e l s h i g h (Y = 210 t o 290) were m o n i t o r e d i n PARC t i m i n g mode 1 and s c a n mode 1 ( n o r m a l ) . One h u n d r e d r e a d o u t s c a n s w e r e p e r f o r m e d a f t e r two p r e p a r a t i o n s c a n s w i t h a 20 m i c r o s e c o n d / c h a n n e l r e a d o u t t i m e . (Other channel times of 40, 6 0 , 8 0 , 100, 120 and 140 m i c r o s e c o n d s were a l s o u s e d w i t h s i m i l a r blooming r e s u l t i n g . )

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Detector

37

d i s p e r s i o n , t h e measured h a l f - w i d t h o f t h e l i n e w i l l be a p p r o x i m a t e l y 0.06 nm, due t o t h e 6 0 % c r o s s t a l k t o n e i g h b o r i n g c h a n n e l s . T h i s e f f e c t w i l l be much l e s s s e v e r e f o r s i g n a l s w i t h larger linewidths. B l o o m i n g o c c u r s i n b o t h d i m e n s i o n s so t h a t i f t h e second dimension o f t h e v i d i c o n d e t e c t o r i s used f o r s p a t i a l r e s o l u t i o n , i t t o o w i l l be l i m i t e d by s i g n a l b l o o m i n g . The e f f e c t s o f b l o o m i n g a r e most s e v e r e when a weak s i g n a l o f i n t e r e s t i s o b s e r v e d n e a r an i n t e n s e s i g n a l o r when l a r g e changes i n i n t e n s i t y o c c u r o v e r s h o r t image d i s t a n c e s . Lag due t o i n c o m p l e t e s i g n a l r e a d o u t f r o m t h e t a r g e t . When t h e e l e c t r o n beam s c a n s o v e r t h e t a r g e t , t h e s i g n a l i s n o t c o m p l e t e l y r e a d o u t i n a s i n g l e p a s s . T h i s phenomenon, c a l l e d d i s c h a r g e l a g , i n a s i m p l i s t i c v i e w , i s due t o t h e r e s i s t a n c e o f t h e r e a d i n g e l e c t r o n beam, c a p a c i t a n c e o f t h e s i l i c o n t a r g e t , and the c a p a c i t i v e recharg depends o n t h e s i g n a l i n t e n s i t c a p a c i t a n c e i s a f u n c t i o n o f t h e c h a r g e p r e s e n t on i t . The percentage of t h e s i g n a l read out i n each pass i s a f u n c t i o n o f t h e d w e l l t i m e , o r t h e t i m e t h e r e a d o u t beam s p e n d s a t e a c h p i x e l , t h e e l e c t r o n beam c u r r e n t ( L 2 ) , and t h e r e l a t i o n s h i p between t h e t a r g e t and beam v o l t a g e s . S i n c e t h e s c a n r a t e i s c o n t r o l l e d by t h e r e a d o u t t i m e p e r c h a n n e l , t h e d w e l l t i m e i s a f u n c t i o n o f t h e c h a n n e l t i m e and t h e number o f p i x e l s m a k i n g up each channel : d w e l l time = channel time / p i x e l s per channel. The l a g was measured f o r a number o f d i f f e r e n t d w e l l t i m e s . The r e s u l t s a r e shown i n F i g u r e 3. L a g l i m i t s t h e r a t e a t w h i c h t h e v i d i c o n t a r g e t c a n be r e a d o u t , and t h e r e f o r e , t h e t i m e r e s o l u t i o n a t t a i n a b l e i n t h e t i m e - r e s o l v e d , o r r e a l - t i m e , mode. A p p r o x i m a t e l y 30 s p e c t r a o f 500 c h a n n e l s e a c h c a n be a c q u i r e d i n a o n e - s e c o n d p e r i o d . S p e c t r a c a n be o b t a i n e d more r a p i d l y i f t h e s i z e o f t h e t a r g e t r e g i o n s c a n n e d , and t h e r e f o r e t h e w a v e l e n g t h r a n g e m o n i t o r e d , are reduced. With pulsed or t r a n s i e n t i l l u m i n a t i o n , l a g also r e s u l t s i n a n o n l i n e a r d e t e c t o r response t o i n c i d e n t l i g h t i n t e n s i t y . The e f f e c t s o f l a g on t h e l i n e a r i t y o f d e t e c t o r r e s p o n s e c a n be r e d u c e d by u s i n g m u l t i p l e r e a d o u t s c a n s , l o n g d w e l l t i m e s , b o t h r e s u l t i n g i n l o n g e r r e a d o u t t i m e s , o r o t h e r schemes (13,14, I D

· The v i d i c o n c a n be c o o l e d i n o r d e r t o r e d u c e t h e t h e r m a l l y generated dark c u r r e n t , a l l o w i n g long i n t e g r a t i o n times t o m o n i t o r weak s i g n a l s . However, l a g i s g r e a t l y i n c r e a s e d when t h e d e t e c t o r i s c o o l e d . As a r e s u l t , e r a s u r e o f p r e v i o u s images and t a r g e t p r e p a r a t i o n becomes e x t r e m e l y i m p o r t a n t . The e f f e c t s o f l a g c a n be m i n i m i z e d by u s i n g p r e i l l u m i n a t i o n . I n commercial c o o l e d h o u s i n g s , a l i g h t e m i t t i n g d i o d e (LED) i s p o s i t i o n e d n e a r

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

MULTICHANNEL IMAGE DETECTORS

6000 5000 4000 3000 2000 1000 J

L 10

NUMBER OF READOUT SCANS Figure 3. Relative integrated signal showing the effect of lag as a function of dwell time controlled by channel time. Each channel consisted of500 pixels (delta Y- 500). Signal is due to a single pulse of a Xe strobe lamp (General Radio, 1539-A Stroboslave). Light was detected in zero order with a 20^m entrance slit. Dwell time: o, 40 ns; •, 80 ns; A, 160 ns; and o, 240 ns.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2.

OLESIK AND

WALTERS

Silicon-Intensified

Target Vidicon

Detector

39

t h e v i d i c o n f a c e . The LED i s f l a s h e d t o c o m p l e t e l y d i s c h a r g e t h e s i l i c o n diodes i n o r d e r t o t o t a l l y erase p r e v i o u s images. Readout scans a r e then used t o p r e p a r e the t a r g e t f o r exposure. A l t e r n a t i v e l y , t h e e l e c t r o n beam p o t e n t i a l c a n be s h i f t e d between t h e p r e p a r a t i o n and r e a d i n g p o t e n t i a l l e v e l s t o reduce t h e e f f e c t s of l a g ( 13, 14,15). E l e c t r o s t a t i c image intensifier» The i n t e n s i f i e r s t a g e o f t h e SIT v i d i c o n i n c r e a s e s s e n s i t i v i t y t h r o u g h e l e c t r o n g a i n and provides time g a t i n g c a p a b i l i t y . V a r i a b l e g a i n i s p o s s i b l e by a d j u s t m e n t o f t h e c a t h o d e v o l t a g e . However, e l e c t r o s t a t i c f o c u s i n g o f t h e i n t e n s i f i e d image i n t r o d u c e s l o s s e s i n r e s o l u t i o n and s e n s i t i v i t y n e a r t h e t a r g e t edges due t o p i n c u s h i o n d i s t o r t i o n (JJ>_,r7) o f t h e i m a g e . The d i s t o r t i o n a l s o r e s u l t s i n a n o n l i n e a r w a v e l e n g t h a x i s when an SIT v i d i c o n i s placed at the f o c a l plan n o n l i n e a r i t y of 2 to 3 e q u a t i o n g e n e r a t e d r e l a t i n g t h e c h a n n e l number t o w a v e l e n g t h . The s o f t w a r e f o r g e n e r a t i o n o f t h e c h a n n n e l t o w a v e l e n g t h f u n c t i o n i s i n c l u d e d i n t h e PARC OMA I I s y s t e m . Image f i d e l i t y when t h e SIT v i d i c o n i s u s e d i n t h e c o n t i n u o u s mode was t e s t e d by i m a g i n g a o n e - h u n d r e d t w e n t y - f i v e m i c r o n s c r e e n , b a c k l i t by a c o p p e r h o l l o w c a t h o d e lamp, on a 0.5 mm w i d e e n t r a n c e s l i t o f t h e o n e - m e t e r C z e r n y - T u r n e r monochromator. The t w e n t y - f i v e c e n t i m e t e r f o c a l l e n g t h q u a r t z i m a g i n g l e n s was p o s i t i o n e d f o r one t o one m a g n i f i c a t i o n a t 327.4 nm. The r e s u l t i n g image a t t h e f o c a l p l a n e was d e t e c t e d p h o t o g r a p h i c a l l y and w i t h t h e SIT v i d i c o n as shown i n F i g u r e 4. D i s t o r t i o n near the c e n t e r of the v i d i c o n t a r g e t i s l e s s than f i f t y m i c r o n s (2 c h a n n e l s ) . However as l a r g e r c h a n n e l h e i g h t s a r e u s e d d i s t o r t i o n i n c r e a s e s as shown i n F i g u r e 5 and T a b l e I I . The d i s t o r t i o n i s due t o a change i n m a g n i f i c a t i o n which causes a skewing of s p e c t r a l l i n e s toward the o u t s i d e of t h e t a r g e t a t l a r g e c h a n n e l h e i g h t s , as shown i n F i g u r e 6. I n t h e t i m e - g a t e d mode t h e d i s t o r t i o n due t o t h e e l e c t r o n i m a g i n g i s more s e v e r e . F o c u s i n g o f t h e p h o t o e l e c t r o n s i n t h e SIT v i d i c o n i s a f u n c t i o n o f t h e g a t e p u l s e w i d t h and v o l t a g e . T h e r e f o r e , f o r e a c h new t i m e g a t e - w i d t h t h e g a t e p u l s e v o l t a g e must be r e o p t i m i z e d . F o c u s i n g i s a l s o s e n s i t i v e t o t h e i n c i d e n t light i n t e n s i t y l e v e l f o r short gate widths (7,18). The r a t i o of photocathode v o l t a g e t o gate v o l t a g e c o n t r o l s the e l e c t r o n image f o c u s i n g and m a g n i f i c a t i o n . As a r e s u l t , s e v e r e p i n c u s h i o n d i s t o r t i o n o c c u r s when t h e g a t e t i m e i s o f t h e same m a g n i t u d e as t h e r i s e and f a l l t i m e s o f t h e g a t e p u l s e o r when t h e g a t e p u l s e has r i n g i n g o r a s l o p i n g v o l t a g e (7). The e f f e c t on t h e o b s e r v e d s p e c t r u m w i l l be most s e v e r e i f t h e i n c i d e n t l i g h t p u l s e i s l o n g e r t h a n t h e g a t e w i d t h . The e f f e c t at s h o r t g a t e t i m e s c a n be m i n i m i z e d by m o d i f i c a t i o n o f t h e g a t i n g c i r c u i t (7_) o r c o m b i n i n g a n o p t i c a l g a t e , s u c h as a P o c k e l s c e l l , w i t h t h e e l e c t r o n i c g a t e o f t h e SIT v i d i c o n ( 1 8 ) .

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1

' 50

' 347

' 1

100

349 1

Γ

150

35Ι

'

200

' 353

1

χ

x

(fim)

CHANNEL

Figure 4. SIT vidicon detected signalfrom a 250 750-μιη portion of the image of a backlit 125-pm screen imaged with unity magnification at the focal plane. The SIT vidicon signal was obtained with the maximum attainable detector spatial resolution, with each picture element, or with pixel, 25 μm in diameter.

0

345

INTENSITY

(X)

2.

OLESIK AND

Silicon-Intensified

WALTERS

ι

1

0

100

Target Vidicon

1

1

200

300

r-

400

Detector

41

1

500

CHANNEL (X) Figure 5. Continuous (nongated) of the image of emission lines from an iron hollow cathode

Table I I .

lamp.

D i s t o r t i o n o f t h e SIT v i d i c o n d e t e c t e d image i n t h e c o n t i n u o u s ( n o n g a t e d ) mode.* X

(Channel)

29

111

157

265

374

414

468

-5 -2 0 -4 -8 -13

-2 0 +1 -1 -5 -9

-2 0 0 -2 -4 -7

-1 0 0 -1 -1 -2

+1 0 0 0 0 +1

+3 0 0 -1 0 +1

+5 +1 +1 0 +1 +2

Y 75-125 125-175 175-225 325-375 375-425 425-475

A one m e t e r * An i r o n h o l l o w c a t h o d e lamp s o u r c e was u s e d . monochromator ( M c P h e r s o n 2051) was used w i t h a 20 m i c r o n E a c h s p e c t r u m was a c q u i r e d w i t h a s c a n h e i g h t entrance s l i t . ( d e l t a Y) of 50. B e l o w are l i s t e d t h e d i f f e r e n c e s b e t w e e n o b s e r v e d c h a n n e l of peak i n t e n s i t y and t h e c h a n n e l o f peak i n t e n s i t y f o r the c e n t e r e d (Y = 225-275) s c a n .

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

MULTICHANNEL IMAGE DETECTORS

Ο Ο Ο

IlJlL

ILLLJ 100

200

300

CHANNEL

Ο Ο Ο

400

500

(X)

ο ~ ο

CO LU

100

200

300

CHANNEL

400

500

(Χ)

Ο Ο Ο

CO

U U

UJJI ιοο

200

300

CHANNEL

400

500

(Χ)

Figure 6. The effect of pincushion distortion on detected spectra. Key: A, spectrum obtained reading out the center 2.4 mm (delta Y- 100) of the vidicon target; B, spectrum obtained reading out the center 7.2 mm (delta Y- 300); and C, spectrum obtained reading out the complete 12.5 mm (delta Y - 500).

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2.

OLESIK AND

WALTERS

Silicon-Intensified

Target Vidicon

Detector

43

W h i l e t h e s e v e r e p i n c u s h i o n d i s t o r t i o n was e x p e c t e d a t s h o r t g a t e t i m e s (100 ns o r l e s s ) , and has b e e n r e p o r t e d e a r l i e r ( 1 8 ) , we a l s o o b s e r v e d s e v e r e p i n c u s h i o n d i s t o r t i o n a t l o n g e r g a t e t i m e s . T h i s d i s t o r t i o n was o b s e r v e d w i t h a s p a r k d i s c h a r g e l i g h t s o u r c e w i t h a d u r a t i o n of a p p r o x i m a t e l y one-hundred m i c r o s e c o n d s and t o a l e s s e r e x t e n t w i t h a h e l i u m - n e o n l a s e r s o u r c e . The d i s t o r t i o n was most s e v e r e w i t h a o n e - m i c r o s e c o n d g a t e and l e s s s e v e r e a t l o n g e r and s h o r t e r g a t e w i d t h s , as shown i n F i g u r e 7. A f t e r c o n s i d e r a b l e e f f o r t and i n v e s t i g a t i o n we have b e e n u n a b l e t o i d e n t i f y t h e c a u s e o f t h i s d i s t o r t i o n o r t o reduce i t . U s i n g t h e PARC OMA I I s y s t e m t h e p u i s e r d u t y c y c l e i s l i m i t e d t o 0.1% i n t h e 10 n s e c t o 99.9 m i c r o s e c o n d r a n g e . The d u t y c y c l e i s l i m i t e d t o 1.0% i n t h e 1 m i c r o s e c o n d t o 999 microsecond range. T h e r e f o r e , when a o n e - h a l f m i c r o s e c o n d g a t e w i d t h i s u s e d , t h e maximu one-half m i l l i s e c o n d gat be on o n l y t w i c e p e r s e c o n d . Channel-to-channel and p i x e l - t o - p i x e l r e s p o n s e . Variation i n c h a n n e l - t o - c h a n n e l and p i x e l - t o - p i x e l r e s p o n s e was m e a s u r e d u s i n g an Ε G & G model 590 c a l i b r a t e d , c o n t i n u o u s s o u r c e lamp system. W h i l e c h a n g e s i n r e s p o n s e a c r o s s t h e t a r g e t were w e l l w i t h i n t h e +/- 10% r a n g e s p e c i f i e d by PARC w i t h i n t h e c e n t e r 10 mm χ 10 mm a r e a (400 c h a n n e l s ) o f t h e t a r g e t , t h e r e was a d e f i n i t e dependence o f b o t h s p e c t r a l r e s p o n s e and c h a n n e l - t o c h a n n e l r e s p o n s e p r e c i s i o n on s p a t i a l p o s i t i o n , as s e e n i n F i g u r e 8. P i x e l - t o - p i x e l v a r i a t i o n was m e a s u r e d a t χ = 150 and 250 on t h e SIT t a r g e t among 10 p i x e l s a t e a c h χ p o s i t i o n . Relative s t a n d a r d d e v i a t i o n s i n t h e i n t e n s i t i e s w e r e f o u n d t o be 3.3% and 3.5% r e s p e c t i v e l y . A t r a d e o f f between t h e s e n s i t i v i t y i n t h e c e n t e r o f t h e d e t e c t o r and t h e u n i f o r m i t y a c r o s s t h e d e t e c t o r c a n be made (14) by a d j u s t i n g t h e i m a g e i n t e n s i f i e r f o c u s v o l t a g e and t h e e l e c t r o n gun f o c u s v o l t a g e (gun f o c u s i n g g r i d G3 and image f o c u s i n g g r i d s as shown i n F i g u r e 1 ) . F o r t h e measurements shown i n F i g u r e 8 t h e f o c u s i n g v o l t a g e s w e r e a d j u s t e d f o r an optimum c o m b i n a t i o n o f s e n s i t i v i t y and u n i f o r m i t y o f s e n s i t i v i t y . Any c h a n g e s i n réponse f r o m c h a n n e l - t o - c h a n n e l c a n be c o m p e n s a t e d by n o r m a l i z i n g e a c h s p e c t r u m a c q u i r e d u s i n g a s t a n d a r d s p e c t r u m s t o r e d i n t h e computer memory. As a r e s u l t , c h a n g e s i n r e s p o n s e o f t h e o p t i c a l s y s t e m , s u c h as g r a t i n g e f f i c i e n c y o r d e t e c t o r r e s p o n s e c a n be c o m p e n s a t e d i f known o r calibrated. Examples o f

E x p e r i m e n t s E m p l o y i n g SIT

Vidicon

Detection

We h a v e u s e d t h e SIT v i d i c o n d e t e c t i o n s y s t e m i n a number o f separate experiments. D e t e c t i o n requirements i n these

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

MULTICHANNEL IMAGE DETECTORS

44

Figure 7. Distortion detected for the gated SIT-vidicon spark-excited spectra at gate times near 1 μ$. Key to detection gate width: A, 6.0 μ*; Β, 1.0 μ*; and C, 0.1 μ*.

100

200

300

400

CHANNEL (Χ)

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

500

2.

OLESIK A N D WALTERS

Silicon-Intensified

Target

Vidicon

45

Detector

4500 r-

4000

UJ

È

3500

UJ

3000 STARTING CHANNEL

UJ CO

ζ ο

CL CO UJ

2.3

U

18

μ

oc


* A / V e x c i t a t i o n beam —

entrance

slit

hi/

to data ->

acquisition system

photocathode

concave holographic grating

photodiode array phosphor

microchannel plate, electron multiplier

Figure 1. Instrumental

configuration for fluorescence diode array.

measurements with an intensified

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

7. INGLE AND RYAN

Luminescence

Measurements

159

The begin scan s i g n a l i s used to s t a r t the DA diode i n t e r r o ­ g a t i o n and to c o n t r o l the i n t e g r a t i o n time o f a spectrum. The array i s c l e a r e d with one begin scan s i g n a l , the photon s i g n a l i s i n t e g r a t e d , and then another begin scan s i g n a l i s used f o r s i g n a l readout. The time between begin scan s i g n a l s determines the i n t e g r a t i o n time. The diode a r r a y s i g n a l s are output on a video l i n e . T h i s was connected to a f a s t sample-and-hoId and then a f a s t (2 ps) 12 b i t ADC. T h i s allows the peak s i g n a l v o l t a g e from each diode to be sampled and converted to a 12 b i t d i g i t a l r e p r e s e n t a t i o n to be stored i n computer memory. S e q u e n t i a l memory l o c a t i o n s are used f o r the s i g n a l from each diode with the s a t u r a t i o n s i g n a l corresponding to 1 0 * o r 4096. 2

Software. The software was t a i l o r e d to f i t our p a r t i c u l a r a p p l i c a t i o n . Some o f th 1. Successive 512 poin of memory where the i n t e g r a t i o n time ( t ) , time between s p e c t r a , and a number o f spectra are user chosen. 2. Under user c o n t r o l , s number o f s u c c e s s i v e scans can be summed i n the same memory l o c a t i o n s f o r s i g n a l averaging. 3. To save memory space, the data from the f i r s t η diodes can be ignored (not s t o r e d ) . 4. To save memory space and time, the l a s t m diodes can be clocked at a f a s t e r c l o c k rate and the data ignored. 5. A f t e r s p e c t r a are a c q u i r e d , they can be d i s p l a y e d on a graphics t e r m i n a l . Overlapping o f d i f f e r e n t s p e c t r a and s c a l e expansion allow easy comparison o f s p e c t r a . 6. The s i g n a l from one diode or the sum o f s i g n a l s from η diodes can be d i s p l a y e d . 7. The sum o r d i f f e r e n c e o f any two s p e c t r a o r o f s i g n a l s from one o r more diodes can be c a l c u l a t e d o r d i s p l a y e d . The d i f f e r e n c e o p t i o n can be used to c a l c u l a t e a r a t e o r subtract a blank. 8. The mean and standard d e v i a t i o n o f the s i g n a l s from any group o f diodes f o r a s e r i e s o f e q u i v a l e n t runs can be c a l c u l a t e d and d i s p l a y e d . 9. A wavelength c a l i b r a t i o n program allows readout o f s i g n a l information i n terms o f nanometers r a t h e r than diode numbers. Performance

Characteristics

Dark Signal C h a r a c t e r i s t i c s . The dark s i g n a l c h a r a c t e r i s t i c s are measured with no l i g h t impingent the IDA and are important because the dark s i g n a l l i m i t s the maximum i n t e g r a t i o n time and can l i m i t the S/N at low l i g h t l e v e l s . Without c o o l i n g , the DA reaches s a t u r a t i o n i n about 10 s, while i t takes about 50 s with c o o l i n g to about -10°C. Normally the heat from the high tempera­ ture side o f the P e l t i e r c o o l e r i s d i s s i p a t e d by ambient a i r movement. Considerable dark s i g n a l d r i f t i s observed throughout

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a day as the a i r temperature changes. We found i t e s s e n t i a l to use a c o o l i n g c o l l a r based on c i r c u l a t i o n o f tap water to d i s s i pate the heat. This lowered the dark s i g n a l by over a f a c t o r o f two, but more s i g n i f i c a n t l y i t lowered the d r i f t i n the dark s i g n a l . B e t t e r thermostating c o n t r o l of the P e l t i e r c o o l e r would reduce the d r i f t problem a l s o . It was determined that a f t e r a l a r g e dark s i g n a l had accumul a t e d , s e v e r a l scans were r e q u i r e d to reach a constant dark s i g n a l l e v e l f o r successive scans. This problem was a l l e v i a t e d by c o n t i n u a l l y scanning ( p r o v i d i n g begin scan p u l s e s ) every 20 ms between spectra to keep the dark s i g n a l accumulation at a small v a l u e . I n t e n s i f i e r C h a r a c t e r i s t i c s . The i n t e n s i f i e r gain can be v a r i e d between about 200 and 10^. Intensification i s essential for luminescence measurement dark and readout n o i s noise s i t u a t i o n s ( s i g n a l shot or f l i c k e r n o i s e ) i s independent of the i n t e n s i f i e r g a i n . The i n t e n s i f i e r degrades the r e s o l u t i o n of the DA by about a f a c t o r o f 2 to 3.5 diode elements. At the lowest i n t e n s i f i e r g a i n s e t t i n g the dark s i g n a l c h a r a c t e r i s t i c s are e q u i v a l e n t to those obtained with the i n t e n s i f i e r b i a s voltage o f f . At f u l l i n t e n s i f i e r gain, the dark s i g n a l i s increased about 10% and the dark s i g n a l n o i s e i s doubled. T h i s i s suspected to be due to thermal e l e c t r o n s generated at the photocathode that r e c e i v e the same i n t e n s i f i e r gain as photoelectrons. When l i g h t i s removed from the photocathode o f the i n t e n s i f i e r , a decaying l i g h t s i g n a l i s observed due to r e s i d u a l phosphorescence from the phosphor used i n the i n t e n s i f i e r . I t appears to be composed o f two components with h a l f - l i f e s o f tens of m i l l i s e c o n d s and a few seconds. This behavior d i d not s i g n i f i c a n t l y a f f e c t our luminescence measurements. Because i n t e g r a t i o n times were t y p i c a l l y 1 s or g r e a t e r , the s h o r t e r l i v e d component response time i s i n s i g n i f i c a n t . In other experiments (not i n v o l v i n g luminescence) w i t h pulsed l i g h t sources, the phosphor l i f e t i m e d i d d i s t o r t s i g n a l s modulated a t frequencies greater than 10 Hz. The l o n g e r - l i v e d decay component e f f e c t was i n s i g n i f i c a n t i n our time sequence experiments because the r e l a t i v e change i n s i g n a l between s e q u e n t i a l spectra was a f r a c t i o n o f f u l l s c a l e . S i g n a l and S/N R e l a t i o n s h i p s . As a f i r s t approximation the f o l l o w i n g c h a r a c t e r i s t i c s are expected to apply: n,

(1) (2)

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Luminescence

7. INGLE AND RYAN n.

n. α L

Measurements

(3)

c, t , s

c

1/2

, t

161

1/2

, s

1/2

(4)

where o f f s e t s i g n a l ( f i x e d p a t t e r n "noise") dark s i g n a l

CT

R

% σ

τ c

light

signal

total

signal

= readout n o i s e dark c u r r e n t n o i s e

= l i g h t signal nois t o t a l noise number o f channels added together

t

= i n t e g r a t i o n time

s

= number o f scans added together

Equations 1 and 2 j u s t i n d i c a t e that the t o t a l s i g n a l o r n o i s e i n counts i s due to the sum o f three independent e f f e c t s : the o f f s e t or readout process, the dark s i g n a l , and the l i g h t s i g n a l . R e l a t i o n s h i p 3 i n d i c a t e s that i f we add the s i g n a l s from two channels together, i n t e g r a t e twice as long, or sum the s i g n a l s from two scans, the dark and l i g h t s i g n a l s should double. For r e l a t i o n s h i p 4, i t i s assumed that s i g n a l l e v e l s are low enough that fundamental shot n o i s e i s l i m i t i n g , which i s p r o p o r t i o n a l to the square root o f the s i g n a l and hence the square root o f f a c t o r s d i r e c t l y p r o p o r t i o n a l to the s i g n a l . The above r e l a t i o n s h i p s were tested by v a r y i n g c, t , and s over a l a r g e range (14). F i r s t dark s i g n a l measurements were made and then measurements were made with the IDA photocathode i l l u m i n a t e d with uniform white l i g h t . The c o n t r i b u t i o n s o f dark and readout s i g n a l and n o i s e were subtracted from the t o t a l s i g n a l to o b t a i n the c o n t r i b u t i o n from only the l i g h t s i g n a l . Most of the above r e l a t i o n s h i p s are shown to be true and S/N s o f 500 were obtained i n some cases. The dependence o f s i g n a l s and noise on the number of channels added together i s approximate due to diode to diode v a r i a t i o n s i n dark s i g n a l and diode r e s p o n s i v i t y . The dark s i g n a l was not p r o p o r t i o n a l to t above about 25% o f DA s a t u r a t i o n ; a negative d e v i a t i o n was observed. The same a f f e c t was observed f o r the l i g h t s i g n a l i f the dark s i g n a l component reaches 25% s a t u r a t i o n i n the i n t e g r a ­ t i o n time. Hence i t i s c r i t i c a l that the dark s i g n a l subtracted from the t o t a l s i g n a l to o b t a i n the net l i g h t i s measured with 1

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the same i n t e g r a t i o n time. In t h i s case, the net l i g h t s i g n a l i s p r o p o r t i o n a l to the i n c i d e n t l i g h t i n t e n s i t y . The l i g h t s i g n a l noise (στ,) was found not to be p r o p o r t i o n a l to the square root o f the number of channels unless the diodes chosen were separated by 9 diodes. The noise στ, increased f a s t e r than f a l l separations because the s i g n a l s from adjacent and near diodes are not independent. This i s b e l i e v e d to be due to the r e s o l u t i o n degradation caused by the i n t e n s i f i e r . Photons s t r i k i n g a 50 μη spot on the i n t e n ­ s i f i e r photocathode cause an eventual burst of photons over s e v e r a l hundred micrometers. Thus the s i g n a l s from s e v e r a l diodes can t r a c k together the s i g n a l caused by photons a r r i v i n g at one spot on the photocathode. o

r

s m

D e t e c t i o n L i m i t C o n s i d e r a t i o n s . For low l i g h t l e v e l measure­ ments, the S/N i s l i m i t e n o i s e (o%) i s about 3. and with f u l l i n t e n s i f i e r g a i n , the dark s i g n a l noise becomes l i m i t i n g . The noise generated by the data a c q u i s i t i o n c i r c u i t r y i s i n s i g n i f i c a n t compared to the readout noise on the video l i n e generated by the IDA and i t s e l e c t r o n i c s . Since i n our system, 1 readout count i s e q u i v a l e n t to about 5000 e's or about 1-2 photoelectrons at f u l l i n t e n s i f i e r gain (^ 6000), the readout n o i s e i s about 1.7 χ 10^ e l e c t r o n s . Thus our system can detect the presence o f 4-8 photoelectrons i n one diode channel, and hence i s near the photon counting l i m i t . A p p l i c a t i o n s o f the

IDA

Several chemical systems were studied with the s p e c t r o f l u o rometer with IDA d e t e c t i o n to l e a r n about the p o t e n t i a l advantages provided by multichannel d e t e c t i o n . A d e t e c t i o n l i m i t o f 0.2 ng/mL was obtained f o r measurements o f the steady s t a t e fluorescence o f quinine s u l f a t e (12). Here an 8 s i n t e g r a t i o n time was employed and the s i g n a l s from the 6 diodes (7.5 nm range) centered at the wavelength o f peak emission were averaged. The low d e t e c t i o n obtained i n d i c a t e s that the IDA i s a powerful q u a n t i t a t i v e t o o l f o r t r a c e fluorescence measure­ ments. The d e t e c t i o n with the IDA i s only about ten times higher than obtained with the same spectrofluorometer with monochromator wavelength s e l e c t i o n and PMT d e t e c t i o n (the s l i t width was about 4 l a r g e r than used with the IDA) · Although only information from 6 o f the 512 diodes was used f o r q u a n t i t a t i v e information i n t h i s case, the complete emission spectrum was obtained i n a matter o f seconds. T h i s i s about two orders o f magnitude l e s s than the time necessary to scan a spectrum with a conventional spectrofluorometer. The a b i l i t y to acquire s p e c t r a r a p i d l y provides a c o n s i d e r a b l e time savings, p a r t i c u l a r l y i n s i t u a t i o n s where i t i s necessary to survey the fluorescence c h a r a c t e r i s t i c s o f a number o f compounds. The

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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INGLE AND RYAN

Luminescence

Measurements

163

wavelengths of emission maxima and r e l a t i v e fluorescence i n t e n s i t i e s at a number of d i f f e r e n t e x c i t a t i o n wavelengths can be e s t a b l i s h e d i n a r e l a t i v e l y short time. For measurement of steady-state f l u o r e s c e n c e , the r a p i d a c q u i s i t i o n of s p e c t r a i s a convenience but not e s s e n t i a l . However, the r a p i d a c q u i s i t i o n of s p e c t r a l information i s e s s e n t i a l f o r measurement of t r a n s i e n t species or where the luminescence s i g n a l c o n t i n u a l l y changes with time. This w i l l be demonstrated f i r s t f o r k i n e t i c - b a s e d luminescence measurements and then chemiluminescence measurements. Fluorescence Kinetic-Based Measurements. Our s t u d i e s of the r e a c t i o n r a t e determination of thiamine (vitamin B l ) w i l l be used to demonstrate the unique c a p a b i l i t i e s of r a p i d a c q u i s i t i o n of s p e c t r a i n k i n e t i c measurements. The k i n e t i c method i s based on the o x i d a t i o n of thiamin h i g h l y f l u o r e s c e n t thiochrom the change i n fluorescence s i g n a l at 444 nm that occurs i n a f i x e d time a f t e r mixing the sample and reagents, i s d i r e c t l y p r o p o r t i o n a l to the thiamine c o n c e n t r a t i o n . Figure 2 shows emission s p e c t r a o f a s o l u t i o n c o n t a i n i n g thiamine and r i b o f l a v i n (vitamin B2) taken with the IDA at v a r i o u s times. Curve A i s the spectrum of the o r i g i n a l v i t a m i n sample s o l u t i o n a c i d i f i e d to pH 2 with H g added. Under computer c o n t r o l t h i s i n i t i a l spectrum i s acquired i n 2 s and then pH 12.2 b u f f e r i s i n j e c t e d i n t o the sample c e l l to r a i s e the r e a c t i o n mixture pH to i n i t i a t e the thiochrome r e a c t i o n . Curves B, C, and D are the emission s p e c t r a obtained at 16, 40, and 60 s a f t e r i n i t i a t i o n of the r e a c t i o n with a 2 s i n t e g r a t i o n time. These s p e c t r a are now used to i l l u s t r a t e a number o f important p o i n t s . 1. The growth of the thiochrome peak centered at 444 nm i s obvious· 2. The d i f f e r e n c e i n the sum o f the s i g n a l s from the 10 diodes (533-545 nm) centered at the wavelength of peak emission o f thiochrome i s a u t o m a t i c a l l y c a l c u l a t e d from the stored spectra to give a number p r o p o r t i o n a l to the r a t e . The d e t e c t i o n l i m i t f o r thiamine i s 6 χ 10"^ M and i l l u s t r a t e s the d e t e c t i v i t y of the IDA. This i s only a f a c t o r of 2 greater than the d e t e c t i o n l i m i t obtained with an e q u i v a l e n t system based on PMT d e t e c t i o n . 3. The f i r s t order and second order (not shown) R a y l e i g h s c a t t e r i n g peaks of the 365 nm e x c i t a t i o n l i g h t are obvious. The d e t e c t i v i t y of the IDA i s a l s o i l l u s t r a t e d by the d i s t i n c t water Raman band at 417 nm. 4. The i n i t i a l fluorescence peak centered at 550 nm i s due to the i n t r i n s i c fluorescence o f r i b o f l a v i n . Its intensity i s decreased when the pH i s increased. This i l l u s t r a t e s the use o f the IDA as a d i a g n o s t i c t o o l s i n c e p o t e n t i a l s p e c t r a l i n t e r f e r e n c e s i n a r e a l sample can be r e a d i l y i d e n t i f i e d 2 +

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

MULTICHANNEL IMAGE DETECTORS

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

7.

INGLE AND RYAN

Luminescence

Measurements

165

simultaneously as the primary a n a l y t i c a l information i s obtained. In t h i s case there i s a small s p e c t r a l overlap o f the t a i l o f the r i b o f l a v i n peak with the maximum of thiochrome f l u o r e s c e n c e . 5. The computer was programmed to not only c a l c u l a t e the r a t e o f the thiochrome r e a c t i o n but to subtract a p r e v i o u s l y stored blank spectrum from spectrum A to o b t a i n a s i g n a l p r o p o r t i o n a l to the r i b o f l a v i n c o n c e n t r a t i o n . A detection l i m i t of 7 χ Ι Ο " ^ M was obtained f o r r i b o f l a v i n . Thus the m u l t i p l e wavelength c a p a b i l i t y o f the IDA was used f o r simultaneous determination of vitamins i n one vitamin p i l l sample to provide a s i g n i f i c a n t time savings over using two a n a l y t i c a l procedures on two separate samples. The d i a g n o s t i c c a p a b i l i t i e s of IDA d e t e c t i o n were f u r t h e r revealed when applying the thiochrome r a t e method to other matrices such as urin samples can be r a p i d l wavelengths. S h i f t s i n the maxima of emission spectra under these d i f f e r e n t c o n d i t i o n s r e v e a l the presence o f a d d i t i o n a l f l u o r e s c e n t s p e c i e s . Spectra of p o t e n t i a l i n t e r f e r e n t s can be compared to the spectrum o f the sample to confirm the presence of c e r t a i n s p e c i e s . In the case o f u r i n e samples, a f l u o r e s c e n t compound was formed at a r a t e comparable to the thiochrome r e a c t i o n when the pH was increased. The appearance o f t h i s fluorescence peak was r e a d i l y observed when the d i f f e r e n c e i n spectra taken at d i f f e r e n t times was c a l c u l a t e d and d i s p l a y e d . This information could not be obtained with a conventional scanning spectrofluorometer. C l e a r l y by o b t a i n i n g the complete fluorescence spectrum of sample, unique or unexpected i n t e r f e r e n c e s or problems can be i d e n t i f i e d which might a f f e c t the q u a l i t y o f the measurement. D i f f e r e n c e s i n the magnitude of the Rayleigh s c a t t e r peak or the general background b a s e l i n e are o f t e n i n d i c a t i v e o f s c a t t e r i n g components i n the sample. This can e x p l a i n a decrease i n measurement p r e c i s i o n due to an increase i n background n o i s e . Chemiluminescence Measurements. For chemiluminescence measurements, the IDA i s used as a fundamental and d i a g n o s t i c t o o l rather than a q u a n t i t a t i v e a n a l y s i s t o o l . This i s because the l i g h t l e v e l s i n CL are very low, and normally no wavelength s e l e c t i o n i s employed. Even with a monochromator and a PMT d e t e c t o r , d e t e c t i o n l i m i t s are s e r i o u s l y degraded. For most chemiluminescence r e a c t i o n s , a multichannel spectrometer i s e s s e n t i a l to o b t a i n s p e c t r a information because with d i s c r e t e sampling ( i n j e c t i o n of the f i n a l reagent to i n i t i a t e the r e a c t i o n ) , the CL r e a c t i o n l a s t s only at most a few seconds. Some chemiluminescence r e a c t i o n s proceed f o r minutes or even hours, but s t i l l there are u s u a l l y s i g n i f i c a n t i n t e n s i t y changes over a minute such that a d i s t o r t e d spectrum would be

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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obtained with a scanning monochromator. We have used the IDA to o b t a i n s p e c t r a during s i x d i f f e r e n t CL r e a c t i o n s (12,15). Knowledge o f the CL spectrum, and hence wavelength o f peak emission provides the f o l l o w i n g advantages. 1. F o r l a t e r q u a n t i t a t i v e a n a l y s i s with a PMT, the PMT photocathode response can be chosen f o r maximum detectability. 2. The CL spectrum can be compared to the absorption s p e c t r a o f r e a c t a n t s , products, and suspected i n t e r f e r i n g species to determine i f s p e c t r a l absorption i n t e r f e r e n c e s o r i f self-absorption i s significant. 3. The CL spectrum can be compared to fluorescence spectra o f intermediates o r products to i d e n t i f y the e x c i t e d species r e s p o n s i b l e f o r CL, and hence provide mechanistic information. The spectrum shown i n Figure 3 i s f o r the r e a c t i o n o f 0C1~ wit 703 nm c l e a r l y i n d i c a t r e s p o n s i b l e f o r CL. 4. The CL spectrum o f the blank r e a c t i o n can be compared to the CL spectrum obtained with the analyte to e s t a b l i s h i f the analyte i s an a c t i v a t o r that increases the r a t e o f the blank CL r e a c t i o n o r induces a new CL r e a c t i o n t o occur. For CL s p e c t r a we found i t c r i t i c a l to o b t a i n a "dark" spectrum immediately before automated i n i t i a t i o n o f the r e a c t i o n by i n j e c t i o n o f the f i n a l reagent and a c q u i s i t i o n o f the CL spectrum. S u b t r a c t i o n o f the dark spectrum from the CL spectrum compensates f o r dark s i g n a l d r i f t . A l s o , s i g n a l averaging o f many s p e c t r a (one from each r e a c t i o n run) i s o f t e n e s s e n t i a l to achieve a reasonable S/N. Conclusions Our s t u d i e s have demonstrated that a molecular luminescence spectrometer with IDA multichannel d e t e c t o r i s a powerful q u a n t i t a t i v e , fundamental, and d i a g n o s t i c t o o l . F o r molecular fluorescence i t allows survey spectra to be obtained q u i c k l y . The simultaneous wavelength coverage i s e s s e n t i a l f o r o b t a i n i n g fluorescence s p e c t r a during k i n e t i c r e a c t i o n s . F o r steady s t a t e o r k i n e t i c measurements, complete fluorescence s p e c t r a l information can be c r i t i c a l i n i d e n t i f y i n g s p e c t r a l o v e r l a p problems i n r e a l samples which can go unnoticed i n conventional measurements where only one narrow wavelength range i s monitored. F o r CL measurements, the s p e c t r a l information i s u s e f u l f o r diagnosis o f s p e c t r a l absorption problems, as w e l l as f o r p r o v i d i n g mechanistic information. A molecular absorption spectrophotometer based on diode a r r a y d e t e c t i o n has been a v a i l a b l e f o r many years. Although some commercial molecular fluorescence spectrometers have p r o v i s i o n f o r a t t a c h i n g a multichannel d e t e c t o r , i t i s

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INGLE AND

RYAN

Luminescence

Measurements

Figure 3. Chemiluminescence spectrum of H 0 - OCT singlet oxygen reaction. duced from Ref. 15. Copyright 1981, American Chemical Society.) 2

2

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(Repro-

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s u r p r i s i n g that none have been designed t o t a l l y around an i n t e n s i f i e d diode a r r a y . T h i s should occur i n the f u t u r e . I t i s n a t u r a l to compare the performance o f an optimized spectrofluorometer with PMT d e t e c t i o n and photon counting s i g n a l processing to a spectrofluorometer with IDA d e t e c t i o n f o r r o u t i n e q u a n t i t a t i v e measurements. Here the sum o f the s i g n a l s from a few diodes d e f i n e s a s p e c t r a l bandpass equivalent to that defined by a monochromator with PMT d e t e c t i o n . With our system, some l o s s o f d e t e c t a b i l i t y occurs i n c e r t a i n s i t u a t i o n s . D e t e c t a b i l i t y i s sometimes l i m i t e d by the readout n o i s e , the dark s i g n a l n o i s e , o r the s i z e o f the i n d i v i d u a l diode a r r a y d e t e c t o r elements. For short i n t e g r a t i o n times (< 1 s ) , read­ out noise can be l i m i t i n g f o r very weak s i g n a l s . Others (17) have demonstrated that with proper design o f the e l e c t r i c a l r e a d ­ out c i r c u i t r y , the readout noise can be reduced a f a c t o r o f 10 compared to our system be achieved. This woul ments. Often f o r FL measurements a short i n t e g r a t i o n time i s used because diode s a t u r a t i o n would occur with longer i n t e g r a ­ t i o n times. Thus the present magnitude o f the readout noise i s o f t e n not l i m i t i n g because photon c a r r i e d noise i s dominant. For longer i n t e g r a t i o n times used with weaker s i g n a l s ( o r i f the readout noise was reduced), dark s i g n a l noise can be limiting. In our system, the dark s i g n a l corresponds to about 80 photoelectrons/s o r about a f a c t o r o f 10 g r e a t e r than achieved with a good cooled PMT system. T h i s means the r e l a t i v e dark s i g n a l noise i s greater with the IDA than a PMT. Reduction of the dark s i g n a l r a t e , e s p e c i a l l y the component from the i n t e n s i f i e r photocathode, would h e l p . Cooling the i n t e n s i f i e r photocathode i s p o s s i b l e but would be inconvenient. T h i s again would be most c r i t i c a l f o r CL measurements. We have observed many s i t u a t i o n s where a CL s i g n a l was e a s i l y observed with a PMT but was not d e t e c t a b l e with the IDA. F o r many s i t u a t i o n s with fluorescence measurements, the background fluorescence or s c a t t e r i n g s i g n a l count r a t e i s greater than the dark s i g n a l count r a t e such that dark s i g n a l noise i s not l i m i t i n g . The readout and dark s i g n a l noise problems are aggravated by the small s i z e o f the diode d e t e c t o r element compared to a PMT photocathode. In some instances the PMT photocathode views about 100 times more photons/s than a diode d e t e c t o r element ( f a c t o r i n g out the i n t e n s i f i e r g a i n ) . T h i s i s the p r i c e paid for m u l t i p l e wavelength coverage. This disadvantage can, o f course, be reduced by u s i n g a l a r g e r spectrograph entrance s l i t width and summing the s i g n a l s from η diodes. In the l a t e r case, the readout o r dark s i g n a l noise increases by νίΓ. As p r e v i o u s l y mentioned, the S/N near the d e t e c t i o n l i m i t i s o f t e n l i m i t e d f o r complex r e a l samples by background s i g n a l n o i s e , and above the d e t e c t i o n l i m i t , where the analyte f l u o r e s ­ cence s i g n a l i s g r e a t e r than the background s i g n a l , by a n a l y t i ­ c a l s i g n a l n o i s e . I f shot n o i s e i s l i m i t i n g , the smaller detec-

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t o r element s i z e can s t i l l cause a S/N disadvantage since the S/N i s p r o p o r t i o n a l to the number o f photons viewed per u n i t time. The S - s e r i e s Reticon DA has diodes about 5 times t a l l e r (2.5 mm) than the DA we used, and thus should help t h i s problem. Looking t o the f u t u r e , i t would be u s e f u l to have diode arrays with d e t e c t o r elements 5-10 mm high and with only 100 diodes per inch (250 pm s p a c i n g ) . T h i s would i n c r e a s e the number o f pho­ tons impingent on a diode a r r a y element t o be more n e a r l y comparable to that impingent on a PMT. The l a r g e r diode width would reduce r e s o l u t i o n f o r the same wavelength coverage but i n molecular luminescence high r e s o l u t i o n i s not o f t e n r e q u i r e d and t y p i c a l l y we sum the s i g n a l s from 5-10 diodes f o r q u a n t i t a t i v e measurements. H o p e f u l l y t h i s new diode array could be c o n s t r u c t ­ ed without a s i g n i f i c a n t i n c r e a s e i n the dark s i g n a l accumula­ t i o n r a t e and with a l a r g e F i n a l l y , i t should b c e r t a i n samples, background o r analyte photon s i g n a l fluxes a r e l a r g e enough that measurements are f l i c k e r noise l i m i t e d (noise p r o p o r t i o n a l to the s i g n a l ) . In t h i s case, the IDA already provides d e t e c t i o n l i m i t s e q u i v a l e n t t o that obtained with PMT detection. Acknowledgment s Acknowledgment i s made t o the NSF (Grant No. CHE-76-16711 and CHE-79-21293) f o r p a r t i a l support o f t h i s r e s e a r c h , and one o f us (M. A. R.) g r a t e f u l l y acknowledges an NSF graduate f e l l o w s h i p and an American Chemical S o c i e t y , D i v i s i o n o f A n a l y t i c a l Chemistry Summer F e l l o w s h i p sponsored by General Motors Research Laboratory. Literature

Cited

1. Johnson, D. W.; C a l l i s , J . B.; Christian, G. D. Anal. Chem. 1977, 49, 747A. 2. Kohen, E.; Kohen, C.; Salmon, J. M. Mickrochim. Acta. 1976, II, 195. 3. F i l l a r d , J . P.; deMurcia, M.; Gasiot, J . ; Chor, S. J . Phys. E. 1975, 8, 993. 4. Jadamec, J . R.; Saner, W. Α.; Talmi, Y. Anal. Chem. 1977, 49, 1316. 5. Conney, R. P.; Vo-Dinh, T.; Winefordner, J . D. Anal. Chim. Acta. 1977, 89, 94. 6. Conney, R. P.; Vo-Dinh, T.; Walden, G.; Winefordner, J . D. Anal. Chem. 1977, 49, 939. 7. Steinhart, H.; Sandmann, J . Anal. Chem. 1977, 49 950. 8. Johnson, D. W.; C a l l i s , J . B.; Christian, G. D., "Higher Order Strategies for Fluorescence Analysis Using an Image Detector", Talmi, Y. Ed.; ACS Symposium Series No. 102, ACS: Washington, DC, 1979; p. 97.

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9. Jadamec, J . R.; Sanner, W. Α.; Sager, R. W., "Versatility of an Optical Multichannel Analyzer as an HPLC Detector", Talmi, Y. Ed.; ACS Symposium Series No. 102, ACS: Washington, DC, 1979; p. 115. 10. Hirschberg, J. G.; Wouters, A. W.; Kohen, E.; Kohen, C; Thorell, B.; Eisenberg, B.; Salmon, J. M.; Ploem, J . S., "A High Resolution Grating Microspectrofluorometer with Topographic Option for Studies i n Living Cells", Talmi, Y. Ed.; ACS Symposium Series No. 102, ACS: Washington, DC, 1979; p. 263. 11. Talmi, Y.; Baker, D. C.; Jadamec, J. R.; Saner, W. A. Anal. Chem. 1978, 50, 936A. 12. Ryan, Μ. Α.; Miller, R. J.; Ingle, J. D., J r . Anal. Chem. 1978, 50, 1772. 13. Ryan, Μ. Α.; Ingle, J . D., J r . Talanta 1981, 28, 225. 14. Ryan, Μ. Α.; Ingle 15. Marino, D. F.; Ingle 16. Ryan, Μ. Α.; Ingle, J . D., Jr. Anal. Chem. 1980, 52, 2177. 17. Simpson, R. W. Rev. S c i . Instrum. 1979, 50, 730. R E C E I V E D May 23, 1983

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

8 Time-Resolved Resonance Raman Spectroscopy of Radiation-Chemical Processes G. N. R. TRIPATHI Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556

A tunable pulsed laser Raman spectrometer for time resolved Raman studie cesses i s described state of art optical multichannel detection and analysis techniques for data acquisition and electron pulse radiolysis for i n i t i a t i n g the reactions. By using this technique the resonance Raman spectra of intermediates with absorption spectra i n the 248-900 nm region, and mean lifetimes > 30 ns can be examined. This apparatus can be used to time resolve the vibrational spectral overlap between transients absorbing i n the same region, and to follow their decay kinetics by monitoring the well resolved Raman peaks. For kinetic measurements at millisecond time scale, the Raman technique i s preferable over optical absorption method where low frequency noise i s quite bothersome. A time resolved Raman study of the pulse radiolytic oxidation of aqueous tetrafluoro-hydroquinoneand p-methoxyphenol i s b r i e f l y discussed. Time r e s o l v e d e l e c t r o n i c a b s o r p t i o n and emission s p e c t r o s copy has been e x t e n s i v e l y used f o r s o l u t i o n phase k i n e t i c s t u d i e s of f a s t chemical processes i n i t i a t e d by i o n i z i n g r a d i a t i o n . A wealth o f i n f o r m a t i o n on r a t e parameters and r e a c t i o n mechanisms on a v a r i e t y of chemical r e a c t i o n s has been obtained by t h i s t e c h nique. As v a l u a b l e as these techniques are, they have l i m i t a tions. In p a r t i c u l a r , the e l e c t r o n i c spectra i n s o l u t i o n a r e o f ten broad and f e a t u r e l e s s and o f f e r l i t t l e s t r u c t u r a l information. As a consequence, the i d e n t i f i c a t i o n of a r e a c t i o n intermediate i s based on chemical i n t u i t i o n and not on i t s s p e c t r a l c h a r a c t e r i s t i c s . Moreover, when more than one t r a n s i e n t i s present i n the system with overlapping e l e c t r o n i c absorption, the k i n e t i c monit o r i n g of the i n d i v i d u a l c o n c e n t r a t i o n becomes d i f f i c u l t . Vibra0097-6156/ 83/0236-0171S06.00/0 © 1983 A m e r i c a n C h e m i c a l S o c i e t y

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t i o n a l s p e c t r a , on the other hand, provide " f i n g e r - p r i n t " i n f o r mation on molecular s t r u c t u r e . However, they have been d i f f i c u l t to generate on r e a c t i v e s p e c i e s i n s o l u t i o n s due t o low concent r a t i o n s g e n e r a l l y a v a i l a b l e f o r observation and due t o t h e i r transient existence. With advent of tunable l a s e r s resonance Raman spectroscopy has developed as a powerful t o o l f o r o b t a i n i n g the s o l u t i o n phase v i b r a t i o n a l s p e c t r a of molecular species i n low c o n c e n t r a t i o n . Ordinary Raman s c a t t e r i n g has very poor s e n s i t i v i t y and a l s o lacks s e l e c t i v i t y . The Raman b a n d - i n t e n s i t i e s of i n d i v i d u a l components i n a mixture l a r g e l y depend on t h e i r r e l a t i v e concentrat i o n s . Resonance Raman occurs when e x c i t a t i o n i s made i n the e l e c t r o n i c a b s o r p t i o n of a molecule. Then the s c a t t e r i n g c r o s s s e c t i o n i s enhanced by s e v e r a l orders of magnitude (2,3). By t a k i n g advantage of t h i s resonance enhancement, the Raman spectrum of a low c o n c e n t r a t i o i n t e r f e r e n c e from t h e solvent^Rama t r a of r a d i c a l s as low as 10 M i n c o n c e n t r a t i o n a r e reported (3). By comparing the s p e c t r a recorded i n resonance and o f f r e s onance regions, the resonant s c a t t e r e r and solvent Raman bands can be e a s i l y d i s t i n g u i s h e d . I f s e v e r a l species a r e present i n the system, the Raman s p e c t r a of each one can be e x c i t e d s e l e c t i v e l y by tuning the e x c i t a t i o n wavelength i n t o t h e i r e l e c t r o n i c a b s o r p t i o n bands. The resonance Raman spectra e x h i b i t s e l e c t i v e resonance enhancement of only those fundamental v i b r a t i o n s which couple with the e l e c t r o n i c t r a n s i t i o n i n resonance, and t h e r e f o r e , a r e simple and e a s i e r t o i n t e r p r e t . Furthermore, the i d e n t i f i c a t i o n of the resonance enhanced modes can lead t o a d e f i n i t e assignment of the resonant e l e c t r o n i c t r a n s i t i o n . For time r e s o l v e d s t u d i e s i n s o l u t i o n s the Raman s p e c t r a must be generated by short l a s e r p u l s e s , and advantage of t h e resonance e f f e c t must be taken t o s e l e c t i v e l y enhance the Raman i n t e n s i t y of the t r a n s i e n t . Recent developments i n the time r e solved resonance Raman spectroscopy (TRRS) are mostly based on the short pulse l a s e r e x c i t a t i o n and o p t i c a l multichannel detect i o n . Bridoux and Delhaye (4) were t h e f i r s t t o introduce t h i s technique i n Raman spectroscopy. The f i r s t t r a n s i e n t Raman spectrum (p-terphenyl anion) was observed at Riso N a t i o n a l Laboratory, Denmark, i n 1976 by generating r a d i c a l s by a s i n g l e e l e c t r o n p u l s e obtained from a Febetron a c c e l e r a t o r ( r e p e t i t i o n r a t e , 0.01 Hz) ( 5 ) . The e a r l y developments (1976-1978) i n TRRS, with a p p l i c a t i o n s i n photochemistry and photobiology, a r e discussed by M.A. El-Sayed (6). In the l a s t four years (1978-1982) improvements i n the multi-channel photon d e t e c t o r and low j i t t e r short pulse l a s e r technology have g r e a t l y expanded the scope of s t u d i e s by TRRS. Recently, t h i s technique has been extended t o a wide range of a p p l i c a t i o n s i n v o l v i n g chemical and biochemical r e a c t i o n s , p h y s i c a l r e l a x a t i o n phenomena, s u r f a c e and m i c e l l a r react i o n s , combustion d i a g n o s t i c s , a n a l y t i c a l m i c r o a n a l y s i s and electro-chemistry (7).

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E l e c t r o n pulse r a d i o l y s i s i s a w e l l e s t a b l i s h e d method f o r i n i t i a t i n g f a s t chemical r e a c t i o n s i n c o n t r o l l e d c o n d i t i o n s , and f o r producing a high concentration of t r a n s i e n t species i n a short time, f a c i l i t a t i n g t h e i r observation by TRRS technique. However, very few l a b o r a t o r i e s have demonstrated c a p a b i l i t y to c a r r y out such experiments (5,8). A s i n g l e pulse Raman e x p e r i ment such as used at Riso, Denmark, r e q u i r e s a high l a s e r pulse energy f o r s c a t t e r i n g . This can induce unwanted photochemical and nonlinear e f f e c t s . An important advantage i n o p t i c a l m u l t i channel d e t e c t i o n l i e s i n i t s c a p a b i l i t y t o i n t e g r a t e the data from a l a r g e number of pulses i n a reasonable time to improve s i g n a l / n o i s e r a t i o . However, i n s i n g l e pulse experiments t h i s advantage i s l o s t . By using an e l e c t r o n a c c e l e r a t o r which can be operated at a high r e p e t i t i o n r a t e , the Raman spectra generated by a l a r g e number of low energy l a s e r pulses can be i n t e g r a t e d and averaged i n a short developing a time resolve be described here (11). In the time resolved Raman measurements on radiation-chemic a l systems, o p t i c a l multichannel d e t e c t i o n o f f e r s some d i s t i n c t advantages over the photon counting techniques. The intense Cerenkov pulse a s s o c i a t e d with the e l e c t r o n pulse i s intense enough t o saturate a p h o t o m u l t i p l i e r tube (PMT). In an o p t i c a l multichannel detector, the Cerenkov pulse can be e f f e c t i v e l y gated o f f by t u r n i n g the detector on w i t h i n a few nanoseconds a f t e r the e l e c t r o n pulse i s over. Apart from t h i s , such spectra are f r e e from the v a r i a t i o n i n e l e c t r o n or l a s e r pulse i n t e n s i t y u n l i k e the spectra obtained by s i n g l e channel devices. D e s c r i p t i o n of the Apparatus F i g . 1 shows a schematic of the time resolved resonance Raman apparatus f o r r a d i a t i o n chemical s t u d i e s . The e s s e n t i a l components of the experimental set up are (a) a pulsed e l e c t r o n r a d i a t i o n source f o r inducing the r e a c t i o n s , (b) a tunable pulsed l a s e r to probe the Raman s c a t t e r i n g , (c) a Spex double monochromator f o r a n a l y z i n g the s c a t t e r e d l i g h t , and (d) a gated detector f o r r e c o r d i n g the s p e c t r a . The e l e c t r o n a c c e l e r a t o r employed has been an AS 2000 Van de Graaff a c c e l e r a t o r manufactured by High Energy Corporation, Burl i n g t o n , Mass. The e l e c t r o n i c c o n t r o l c i r c u i t has been modified c o n s i d e r a b l y , and at present, t h i s a c c e l e r a t o r i s capable of del i v e r i n g 30 ns to 5 ys e l e c t r o n pulses of 2 - 2.5 MeV energy and ^ 1.2 A current at a r e p e t i t i o n r a t e of 60Hz or l e s s . The a c c e l e r a t o r can a l s o be operated i n the continuous beam mode. The f o cussed beam has a diameter of 0.3 cm at the output window. A n i t r o g e n or excimer pumped dye l a s e r has been used as the e x c i t a t i o n source. To avoid unwanted photochemical e f f e c t s , the l a s e r p u l s e energy used f o r s c a t t e r i n g has been l e s s than 0.5 mJ. The Van de Graaff a c c e l e r a t o r and l a s e r are t r i g g e r e d e x t e r n a l l y

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

studies. resonance Raman apparatus for radiation-chemical of the time-resolved Figure 1. Schematic

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by TTL pulses d e r i v e d from a common p u l s e generator f o r synchronized o p e r a t i o n . The e l e c t r o n i r r a d i a t i o n i s done from the s i d e of the Raman c e l l , and the l a s e r e x c i t a t i o n from the bottom. The l a ser beam i s passed through t h e i r r a d i a t e d r e g i o n w i t h i n 1 mm of the e x i t window of the Raman c e l l . The s c a t t e r e d l i g h t i s c o l l e c t e d normal t o t h e plane of the e l e c t r o n and l a s e r beam by a lens of short f o c a l length, and focussed on the spectrometer s l i t by another lens of f / n comparable t o that of the spectrometer ( f / 7 . 8 ) . A p o l a r i z a t i o n scrambler i s i n s e r t e d i n the o p t i c a l path before t h e s l i t t o compensate f o r the p o l a r i z a t i o n e f f e c t s of the spectrometer g r a t i n g s . For p o l a r i z a t i o n s t u d i e s of the Raman bands a p o l a r i z e r i s introduced i n the o p t i c a l path before the scrambler. A Spex 1402 spectrometer (0.85 m, Czerny Turner) with holographic g r a t i n g s (1800 groves/mm) has been used as the d i s p e r s i v e element. The Raman s c a t t e r i n g i s detected by an o p t i c a l multi-channel a n a l y s e r 121 microchannel p l a t e i n t e n s i f i e With the d e t e c t o r place plan stag the. spectrometer, approx. 450 cm of the spectrum can be r e corded on the 700 i n t e n s i f i e d channels, and averaged i n the memory or d i r e c t l y on t a r g e t . The d e t e c t o r i s operated i n gated mode a t gate pulse width i n the range of 5-20 ns and i t s operat i o n i s synchronized with the Raman s i g n a l . By g a t i n g the detector the Raman s p e c t r a produced a few nanoseconds a f t e r the t e r m i n a t i o n of e l e c t r o n p u l s e can be obtained. The time r e s o l u t i o n i s achieved by d e l a y i n g the l a s e r pulse e l e c t r o n i c a l l y with respect to the e l e c t r o n p u l s e . A PMT d e t e c t o r mounted at the e x i t s l i t of the second stage of the Spex spectrometer i s used p r i m a r i l y f o r allignment pur^ poses. The PMT s i g n a l i s sampled by a 1S1 T e k t r o n i x sampling o s c i l l o s c o p e gated a t the peak of the l a s e r p u l s e . The output of the sampling u n i t i s d i g i t i z e d and processed by an LSI 11/2 microcomputer i n t e r f a c e d t o a PDP 11/55 computer t o s t o r e the data files. The LSI 11/2 can be used t o r o t a t e the g r a t i n g s with p r e c i s i o n , scan a spectrum, c o n t r o l the experiments and c a r r y out the s i g n a l averaging task. PMT d e t e c t i o n i n pulse r a d i o l y s i s experiments has been used only f o r l o n g - l i v e d species (10). By use of an a p p r o p r i a t e l y synchronized f a s t e l e c t r o m e c h a n i c a l shutter, i t i s p o s s i b l e t o b l o c k the Cerenkov p u l s e , while a l l o w i n g the Raman s i g n a l t o reach the spectrometer. However, even with t h i s arrangement, i t i s d i f f i c u l t t o employ PMT d e t e c t i o n f o r s t u d i e s at times shorter than m i l l i s e c o n d s . The Raman c e l l used i n these s t u d i e s has t o be s m a l l i n v o l ume t o i n s u r e i t s uniform i r r a d i a t i o n . A c y l i n d r i c a l S u p r a s i l quartz c e l l , 0.1 - 0.2 ml i n volume with o p t i c a l l y f l a t bottom window, with an i n l e t on the s i d e near the bottom and an o u t l e t at the top, has been found q u i t e s u i t a b l e . To a v o i d product a c cumulation and bubble formation due t o l o c a l i z e d h e a t i n g , the s o l u t i o n i s flowed c o n t i n u o u s l y and a l s o precooled with i c e d water b e f o r e e n t e r i n g the Raman c e l l . For deoxygenation, the

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s o l u t i o n i s bubbled with a mixture of two gases instead of one (N^O + N« or Ar + N^) t o avoid s a t u r a t i o n . This i s necessary t o prevent bubble formation. T h i s apparatus i s s u i t a b l e f o r t r a n s i e n t Raman studies of r a ^ i a j i o n j c h e m i c a l intermediates with a b s o r p t i o n spectra (ε > 10 M cm ) l y i n g i n the r e g i o n 248-900 nm, and mean l i f e t i m e s > 30 ns. Application Using t h i s technique, we have examined the resonance Raman spectra of a number of phenoxyl (eg. F i g . 2) and semiquinone r a d ­ i c a l s produced i n the e l e c t r o n pulse r a d i o l y s i s of corresponding phenols and hydroquinones (9-15). These r a d i c a l s absorb i n the 350-500 nm r e g i o n and e x h i b i t only modejate^resonance enhancement (extinction coefficient obtained e x c e l l e n t Rama decay k i n e t i c s . We c i t e here two examples which demonstrate the q u a l i t y of the r e s u l t s obtained, and a l s o the advantages of t h i s technique over the o p t i c a l a b s o r p t i o n method. E l e c t r o n i c a b s o r p t i o n spectrum of tetrafluoro-p-benzosemiquinone (TFPBSQ) anion (13) i s d i f f i c u l t t o d i s t i n g u i s h from i t s protonated counterpart (p-benzosemiquinone; PBSQ) ( 9 ) . We p r e ­ sent i n F i g . 3 the resonance Raman spectra of TFPBSQ and PBSQ r a d i c a l s using o p t i c a l m u l t i c h a n n e l d e t e c t i o n . The semiquinone r a d i c a l s were produced by 1 ys e l e c t r o n pulse r a d i o l y s i s of a 2mM hydroquinone aqueous s o l u t i o n (N^O saturated, pH ^ 11) and were measured 10 y s a f t e r the e l e c t r o n pulse. These spectra were obtained by averaging over approx. 2200 i n d i v i d u a l spectra, accu­ mulated at a r a t e of 7.5 s p e c t r a per second. The S/N r a t i o f o r the most intense peak i s b e t t e r than 100. The primary o x i d i z i n g s p e c i e s i n t h i s case has been the OH r a d i c a l . Secondary oxida­ t i o n of hydroquinone by B r , produced i n ^ 0 saturated, 0.1 M KBr s o l u t i o n s , y i e l d i d e n t i c a l Raman s p e c t r a . The Raman spectra of the two r a d i c a l s show c h a r a c t e r i s t i c m a n i f e s t a t i o n of t h e i r s t r u c t u r a l d i f f e r e n c e s i n c o n t r a s t t o the s i m i l a r i t y i n t h e i r _^ e l e c t r o n i c a b s o r p t i o n s p e c t r a . The prominents peaks a t 1620 cm (CC s t r e t c h ) and 1435 cm (CO s t r e t c h ) i n PBSQ a r e s h i f t e d t o 1677 cm and 1556 cm r e s p e c t i v e l y i n TFPBSQ. The TFPBSQ spec­ trum shows no Raman band-in the frequency r e g i o n 500-1500 cm , although one at 1161 cm (assigned t o CH bend) i s observed i n the PBSQ spectrum. TFPBSQ r a d i c a l i s observed t o decay at ms times i n time r e ­ solved Raman and o p t i c a l a b s o r p t i o n experiments (13). However, i n t h i s time domain, the low frequency noise i s q u i t e bothersome i n k i n e t i c measurements by the l a t t e r method. The Raman measure­ ments a r e f r e e from such d i s t u r b a n c e s . F i g . 4 d e p i c t s the decay of the Raman s i g n a l (1677 cm band) of TFPBSQ ( i n i t i a l concen­ t r a t i o n , approx. 50 yM) measured at d i f f e r e n t time delays of the probe l a s e r pulse (437 nm) a f t e r the e l e c t r o n beam. While the 2

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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150 RAMAN

SHIFT

(cm ) 1

Figure 2. Resonance Raman spectrum of phenoxyl radical excited at 399 nm (excimerpumped dye laser), 0.5 μ$ after the electron pulse. Radical concentration is approxi­ mately 10~ M . The S/N ratio for the 1505-cmr band is better than 50/1. 4

1

C H 06

C

1000

1200

1400 CM

4

6 4°2 F

1800

1600

1

Figure 3. Resonance Raman spectrum (1000-1800 cm' ) of tetrafluoro-p-benzosemiquinone radical anion fXe = 437.8) observed 10 μ£ after its creation by 1 μ$ electron pulse radiolysis of a basic solution of tetrafluorohydroquinone (2 χ 10~ M;pH~ 11) in water. The dotted line shows the Raman spectrum of p-benzosemiquinone anion obtained under similar conditions. 1

3

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

MULTICHANNEL IMAGE DETECTORS

Figure 4. Decay of the Raman signal of tetrafluoro-p-benzosemiquinone anion (1677cmr band) as a function of time (At) between the irradiation pulse and the probe laser. 1

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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i n i t i a l decay i s q u i t e r a p i d , t h e k i n e t i c s a r e obviously compli­ cated s i n c e the r e s i d u a l s i g n a l magnitude a t 128 ms delay (20%) i s a p p r e c i a t e l y greater than 2-3% magnitude expected i f the l i m ­ i t i n g decay r a t e represents a second order process. This r a d i c a l , very probably, decays by d i s p r o p o r t i o n a t i o n . 2SQ

v

^

HQ + BQ

(1)

which e x p l a i n s the p e r s i s t e n c e of i t s s i g n a l at 128 ms. The r a t e constant f o r d i s p r o p o r t i o n a t i o n was estimated at approx. 10 Μ β . From a study of t h e e f f e c t of hydroquinone (HQ) concen­ t r a t i o n on semiquinone (SQ) decay, the e q u i l i b r i u m constant f o r t h i s r e a c t i o n (eq. 1) i s estimated a t approx. 100. The e l e c t r o n r a d i o l y t i c o x i d a t i o n of para s u b s t i t u t e d phe­ nols i n b a s i c aqueous s o l u t i o n s produces a phenoxyl r a d i c a l along with ρ-benzοsemiquinon attack. The absorption (400-460 nm). The time resolved resonance Raman technique can be used t o o b t a i n the spectra of both the r a d i c a l s and a l s o f o r e s ­ t i m a t i n g the r e l a t i v e y i e l d s - o f the two r e a c t i o n s . In F i g . 5 the Raman spectra (1400-1700 cm ) obtained at d i f f e r e n t time i n t e r ­ v a l s a f t e r the pulse r a d i o l y s i s of a 2 mM aqueous s o l u t i o n of p-methoxylphenol (^0 saturated; pH ^ 11) a r e depicted. The Raman e x c i t a t i o n wavelength was 437.8 nm. The Raman spectra ob­ tained between 100 ys and 10,000 ys do not show any observable v a r i a t i o n i n i n t e n s i t y or peak p o s i t i o n s and a r e i d e n t i f i e d with p-benzosemiquinone anion. The spectrum shown i n F i g . 5d has ap­ proximately one f i f t h the i n t e n s i t y of a spectrum obtained under i d e n t i c a l c o n d i t i o n s i n the pulse r a d i o l y s i s of a 2mM hydroqui­ none s o l u t i o n . Therefore, approx. 20% of the OH r a d i c a l s , pro­ duced by the r e a c t i o n of e l e c t r o n pulse with ^ 0 saturated water, are estimated t o a t t a c k at the OCH^ s i t e . The Raman spectra ob­ t a i n e d between 100 ns - 10 ys c l e a r l y show the presence of an­ other species with a decay time at the microsecond range along with t h e long l i v e d species (PBSQ). A f t e r s u b t r a c t i n g d i g i t a l l y the PBSQ c o n t r i b u t i o n ( F i g . 5d) from the 100 ns data ( F i g . 5 a ) the Raman spectrum of the short l i v e d species, i d e n t i f i e d as pmethoxy-phenoxyl r a d i c a l , i s obtained ( F i g . 5 e ) . Since the PBSQ r a d i c a l i s r e l a t i v e l y l e s s r e a c t i v e , i t s Raman bands can be used as an i n t e r n a l standard marker f o r f o l l o w i n g the k i n e t i c behavior of ρ -methoxyphenoxyl r a d i c a l . From the observed s p e c t r a l i n t e n ­ s i t i e s t h e Raman s c a t t e r i n g c r o s s - s e c t i o n of PBSQ v i b r a t i o n s i s estimated t o be approx. 2-5 times higher than that of p-methoxyphenoxyl v i b r a t i o n s , at 437.8 nm. In summary, we have combined s t a t e of the a r t o p t i c a l m u l t i ­ channel a n a l y s i s techniques w i t h the w e l l e s t a b l i s h e d method of e l e c t r o n pulse r a d i o l y s i s t o construct a pulsed l a s e r Raman spec­ trometer f o r time resolved s t u d i e s of t r a n s i e n t intermediates i n s o l u t i o n . This apparatus can be a p p l i e d t o time r e s o l v e the v i ­ b r a t i o n a l s p e c t r a l overlap between t r a n s i e n t s decaying at d i f f e r -

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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180

A

At = IO,000/iS

At = 100ns -J I I I 1400 1500 1600 1700 CM*

1

Figure 5. Resonance Raman spectra (1400-1700 cm' ) of transient intermediates pro­ duced in the pulse radiolysis of a 2 m M aqueous solution of p-methoxyphenol (N 0 saturated; ρ H ~ 11). Key: a-d, the spectra obtained at different times (At) after the electron pulse; d, the spectrum of p-benzosemiquinone anion (marked A); and e, the spectrum of p-methoxyphenoxy radical (marked B) obtained after subtracting dfrom a. This figure illustrates the use of time-resolved resonance Raman spectroscopy to timeresolve the spectral overlap between transients decaying at different time scales. 1

2

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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181

ent time s c a l e s , i d e n t i f y them on the b a s i s of t h e i r c h a r a c t e r i s ­ t i c Raman bands, and t o i l l u c i d a t e t h e i r s t r u c t u r e as manifested i n t h e i r v i b r a t i o n a l s p e c t r a . T h e i r r e a c t i o n dynamics can be followed by monitoring the temporal changes of the w e l l r e s o l v e d Raman peaks. The resonance Raman band i n t e n s i t i e s c o n t a i n u s e f u l i n f o r m a t i o n f o r a s s i g n i n g the "poorly r e s o l v e d " e l e c t r o n i c spec­ t r a of intermediates i n s o l u t i o n . For example, f o r the 400450 nm e l e c t r o n i c a b s o r p t i o n i n p-benzοsemiquinone anions con­ f l i c t i n g assignments have been proposed ( •> B^ and B^ ·> A^) by the quantum chemists. However, on the b a l l s o¥ the res§nance Raman s t u d i e s of t h i s r a d i c a l * we have c l e a r l y shown the e l e c ­ t r o n i c t r a n s i t i o n t o be B^ -> (9,10). The time r e s o l v e d resonance Raman spectroscopy, i s no doubt, a powerful new technique for i n v e s t i g a t i n g t h e r a d i a t i o n induced processes i n chemical, b i o l o g i c a l and p h y s i c a l systems. Acknowledgment I wish t o thank P r o f e s s o r Robert H. Schuler, D i r e c t o r of the Notre Dame R a d i a t i o n Laboratory, f o r h i s advice and encouragement i n the development o f t h i s technique. The TRRS a p p l i c a t i o n s c i t e d i n t h i s paper a r e r e s u l t s of our c o l l a b o r a t i v e work.

Literature Cited 1. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This i s Document No. NDRL-2433 from the Notre Dame Radiation Labora­ tory. 2. Champion, P.M.; Albrecht, A.C. Ann. Rev. Phys. Chem. 1982,33, 353. Tang, J; Albrecht, A.C. "In Raman Spectroscopy"; Szymanski, H., Ed.; Plenum: New York, 1970, Vol.II, p.33. 3. Tripathi, G.N.R. Chem. Phys. Lett. 1981, 81, 375. 4. Bridoux, M.; Delhaye, M. i n "Advances i n Infrared and Raman Spectroscopy"; Clark, R.; Hester, R.E., Eds.; Hyden, Vol.II, p. 140. 5. Pagsberg, P; Wilbrandt, R.; Hansen, K.B.; Weisberg, K.V. Chem. Phys. Lett. 1976, 39, 538. Hansen, K.B.; Wilbrandt, R.; Pagsberg, P. Rev. Sci. Instrum. 1979, 50, 50. 6. El-Sayed, M.A. i n "Multichannel Image Detectors"; Talmi, Υ., Ed.; ACS SYMPOSIUM SERIES No. 102, ACS: Washington, D.C. 1979; p. 215. 7. Proceeding of the "International Conference on Time Resolved Vibrational Spectroscopy", Lake Placid, New York, Aug. 16-21, 1982; G. Atkinson, (Department of Chemistry, Syracuse Univer­ s i t y ) , Ed.. The references on most of the time resolved vi­ brational spectroscopic applications between 1976-82 can be obtained from the papers presented i n this conference.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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8. Dallinger, R.F.; Farquharson, S.; Woodruff, W.H.; Rogers, M.A.J. J. Am. Chem. Soc. 1981, 103, 7033; Lee, P.C.; Schmidt, K.; Gordon, S.; Meisel, D. Chem. Phys. Lett. 1981, 80, 242. 9. Tripathi, G.N.R. J. Chem. Phys. 1981, 74, 6044. 10. Tripathi, G.N.R.; Schuler, R.H. J. Chem. Phys. 1982, 76, 2139. 11. Tripathi, G.N.R.; Schuler, R.H. J. Chem. Phys. 1982, 76, 4289. 12. Tripathi, G.N.R.; Schuler, R.H. Chem. Phys. Lett. 1982, 88, 253. 13. Tripathi, G.N.R.; Schuler, R.H. J . Phys. Chem. (in press). 14. Tripathi, G.N.R.; Schuler, R.H. Chem. Phys. Lett. (in press). 15. Schuler, R.H.; Tripathi, G.N.R.; Prabenda, M.; Chipman, D.M. J. Phys. Chem. (in press). R E C E I V E D June 7, 1983

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

9 Picosecond Emission Spectroscopy with Intensified Photodiode Arrays P. F. BARBARA and A. J. G. STRANDJORD Department of Chemistry, University of Minnesota, Minneapolis, MN 55455 A spectrometer for recording time and wavelength resolved fluorescence data i s described i n d e t a i l The instrument employ niques and photodiod emission spectra at variable delay times after molecular excitation. Examples are given of the u t i l i z a t i o n of the apparatus in the study of the excited state relaxation of aromatic molecules. Experiments to evaluate the accuracy of the technique are also presented. Following photo e x c i t a t i o n a s o l u t i o n sample r e t u r n s to thermal e q u i l i b r i u m by a v a r i e t y of photochemical and photop h y s i c a l processes. The f a s t e r processes, e.g. v i b r a t i o n a l r e l a x a t i o n and s o l v e n t r e l a x a t i o n , have only r e c e n t l y begun to be s t u d i e d by d i r e c t k i n e t i c methods (1.-5.). Picosecond emission spectroscopy has been e s p e c i a l l y u s e f u l i n e l u c i d a t i n g these u l t r a f a s t processes (1,_3,5). As e l e c t r o n i c a l l y e x c i t e d molecules r e l a x , t h e i r f l u o r e s c e n c e spectrum shows time dependence that r e f l e c t s the r e l a x a t i o n processes. Many f e a t u r e s of the emission spectrum can show time dependence, i n c l u d i n g the s p e c t r a l shape 0l>.3,6-9), the peak i n t e n s i t y , the l i n e a r p o l a r i z a t i o n (10) and, i n p r i n c i p l e , the c i r c u l a r p o l a r i z a t i o n (11). In extreme cases, the emission spectrum can a c t u a l l y have two separate f l u o r e s c e n c e bands from two d i f f e r e n t isomers of the e l e c t r o n i c a l l y e x c i t e d molecules (12-15). For molecules w i t h t h i s behavior, i t i s p o s s i b l e to determine the k i n e t i c s of e x c i t e d s t a t e i s o m e r i z a t i o n by t r a n s i e n t emission spectroscopy. If one ignores p o l a r i z a t i o n e f f e c t s , the i d e a l experimental data f o r the t r a n s i e n t emission spectrum of a molecule would be a high r e s o l u t i o n three dimensional p l o t of e m i s s i o n - i n t e n s i t y versus emission-wavelength and delay time a f t e r molecular e x c i t a t i o n . Most published t r a n s i e n t emission data, however, are of the form i n t e g r a t e d emission i n t e n s i t y over a l l emissionwavelengths versus delay time. These data, o b v i o u s l y , have l e s s u s e f u l information than the multidimensional approach.

0097-6156/83/0236-0183$06.00/0 © 1983 American Chemical Society In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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In the present paper we d e s c r i b e an apparatus f o r r e c o r d ­ ing t r a n s i e n t emission s p e c t r a that y i e l d s data which approach the i d e a l multidimensional case. We emphasize i n the d i s c u s s i o n the advantages of multichannel d e t e c t i o n f o r t r a n s i e n t emission data. We a l s o b r i e f l y compare our approach to a l t e r n a t i v e methods f o r r e c o r d i n g time and wavelength r e s o l v e d f l u o r e s c e n c e data on the picosecond time s c a l e . Several experimental methods can be found i n the l i t e r a t u r e f o r r e c o r d i n g picosecond time and emission-wavelength r e s o l v e d f l u o r e s c e n c e data. They can be c l a s s i f i e d according to how many data p o i n t s , I(t^,,X.), are acquired f o r each pulse of the e x c i t a t i o n l a s e r , where I(t.,λ.) i s a s i n g l e p o i n t on the multidimensional r e p r e s e n t a t i o n o f time and wavelength r e s o l v e d emission data. The most common technique i s to r e c o r d , f o r each l a s e r pulse, the f u l l k i n e t i c s a t a s i n g l e narrow emission wave­ l e n g t h r e g i o n with a streak-camera/tw channel analyzer. T h i s i . e . I(t,X=constant), but i s not w e l l s u i t e d f o r r e c o r d i n g emission s p e c t r a a t f i x e d delay time, i . e . I(t=constant,X). A few r e p o r t s have appeared on combining the streak camera temporal d i s p e r s i o n with polychromators and three dimensional o p t i c a l multichannel d e t e c t i o n . This approach y i e l d s three dimensional f l u o r e s c e n c e data f o r each l a s e r pulse. With the present t e c h n o l o g i c a l l i m i t a t i o n s of three dimensional d e t e c t o r s and s t r e a k cameras, however, data of t h i s type s u f f e r from low wavelength r e s o l u t i o n . As d e t e c t o r and streak camera technology improve, t h i s technique may become the method of choice f o r time and wavelength r e s o l v e d emission s p e c t r a . An e x c e l l e n t a l t e r n a t i v e to the s t r e a k camera approach i s f l u o r e s c e n c e time r e s o l u t i o n by the up-conversion method, which we d e s c r i b e i n d e t a i l below. In the simplest form of t h i s technique, n o n - l i n e a r o p t i c a l methods a r e employed to e s s e n t i a l l y construct a picosecond " s h u t t e r " or "gate". In most a p p l i c a t i o n s a s i n g l e emission datum i s acquired f o r each l a s e r pulse, i . e . I(t=constant, X=constant). By r e p e a t i n g the experiment a t d i f f e r e n t delay times, k i n e t i c t r a c e s can be acquired that a r e of comparable q u a l i t y t o those obtained by s t r e a k camera methods (16-17). A l t e r n a t i v e l y , i f delay time i s held constant but X i s scanned, high q u a l i t y emission s p e c t r a can be obtained ( 3 ) . In t h i s paper we d e s c r i b e a novel apparatus that combines the "upconversion" method with multichannel o p t i c a l d e t e c t i o n . This instrument records, f o r each l a s e r pulse, a h i g h - r e s o l u t i o n emission spectrum .at a chosen delay time, i . e . I(t=constant, X). The paper i s organized i n the f o l l o w i n g way. F i r s t , a b r i e ' i n t r o d u c t i o n to f l u o r e s c e n c e upconversion and other n o n - l i n e a r processes i s given. T h i s i s followed by a d e s c r i p t i o n of an apparatus f o r making up converted f l u o r e s c e n c e measurements, and experimental examples f o r t h i s instrument. The f o l l o w i n g t e x t d e s c r i b e s the advantages o f o p t i c a l multichannel d e t e c t i o n s f o r J

J

f!

n

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these types of measurements, and compares multichannel d e t e c t i o n to s i n g l e channel measurements. F i n a l l y , some f u r t h e r measure­ ments on the performance o f the apparatus are presented. Apparatus Background. The d i s c u s s i o n begins with a b r i e f review of how non-linear o p t i c a l c r y s t a l s are p r a c t i c a l l y u t i l i z e d i n laser spectrometer systems. The most common use of these m a t e r i a l s i s the production of l a s e r r a d i a t i o n a t d i f f e r e n t frequencies than that of the l a s e r fundamental, W . For example, second harmonic generation i s o f t e n u t i l i z e d to °create r a d i a t i o n a t 2W from the fundamental l i g h t W . Another technique o f t e n used i s sum frequency mixing i n n o n - l i n e a r o p t i c a l m a t e r i a l s . For example, l i g h t of freqency 3W can be generated e f f i c i e n t l y by mixing W l i g h t with 2W l i g h t ? The 2W l i g h t i s prepared independently by second harmonic generation can be employed to produc two l i g h t beams of a r b i t r a r y frequency W and W^, i . e . W = + W^. In p r a c t i c e , the e f f i c i e n c y of the mixing process depends on many parameters which we d e s c r i b e below. Over 15 years ago Duquay and Hansen (18) suggested an a l t e r n a t i v e use of sum-frequency generation, namely picosecond o p t i c a l sampling. This technique has become a popular method f o r measuring the d u r a t i o n o f u l t r a - s h o r t l i g h t p u l s e s . I t a l s o has been used r e c e n t l y i n time r e s o l v e d molecular f l u o r e s c e n c e experiments, the subject of t h i s paper. In t h i s a p p l i c a t i o n , sum frequency mixing i s u s u a l l y c a l l e d upconversion. The f l o u r e s c e n c e r a d i a t i o n of i s mixed (upconverted) i n a n o n l i n e a r c r y s t a l with a picosecond d u r a t i o n "gate" pulse (W ) to produce upconverted l i g h t a t W- + W . The i n t e n s i t y ^of the upconverted l i g h t i s l i n e a r l y related** to the i n t e n s i t y of the f l u o r e s c e n c e that a r r i v e s a t the c r y s t a l a t the same time as the gate pulse. The i n c o r p o r a t i o n of conversion i n a p r a c t i c a l l y u s e f u l f l u o r e s ­ cence spectrometer i s d e s c r i b e d i n the next s e c t i o n . 0

&

g

Overview. The experimental apparatus we d e s c r i b e ( F i g . 1) i s a time and wavelength r e s o l v e d f l u o r e s c e n c e spectrometer which was constructed i n our l a b o r a t o r y i n 1981. Two types of picosecond experimental data can be acquired with the apparatus: 1) timer e s o l v e d f l u o r e s c e n c e t r a c e s a t narrow (15 nm) emission wave­ length regions (method A) and 2) wavelength-resolved "gated" f l u o r e s c e n c e s p e c t r a of f i x e d delay times a f t e r molecular e x c i t a t i o n (method B). In p r i n c i p l e , both techniques can y i e l d the same i n f o r m a t i o n by r e p e a t i n g method A at s e v e r a l emission wavelengths or by r e p e a t i n g method Β a t d i f f e r e n t delay times. In p r a c t i c e , however, method A i s b e t t e r s u i t e d f o r determining q u a n t i t a t i v e k i n e t i c data, w h i l e method Β i s more a p p r o p r i a t e f o r studying s u b t l e time-dependent f l u o r e s c e n c e s p e c t r a l shape and band p o s i t i o n v a r i a t i o n s .

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Sample

H

Ρ I

7f

Upconverting crystal

energy monitor

1064nm 534nm 356nm THQ ^mm—*

SHQ

1

Nd* YAQ Amplifier

PAR |" photodiode! array I

1064nm 30psec lOmjoule

A ( n p

Integrator &

Hold

micro komputerl

1

Quantel Nd* YAQ Oscillator

"jja—^ I

controller & interface

Pulsed power supply

The Glan polarizer combines fluorescence light and a portion of the 1064-nm radiation. The combined beam impinges on the upconverting crystal. The polarizer also splits a portion of thefluorescencefor detection by the reference channel of the spectrometer. Key: PH, pin hole; OC, output coupler; SHG, second-harmonic generator; THG, third-harmonic generator; BS, dichroic beam splitter; F, colored glassfilter; PA R, Princeton Applied Research; PMT, photomultiplier tube; A MP, analog electronic amplifier; A D, analog to digital converter; and OMA, optical multichannel detector.

Figure 1. A diagram of a time- and wavelength-resolvedfluorescence apparatus that employs the upcon­ version method for time resolution.

Reference monochromator

UC signal monochromator

H — 0 EEL • ; 0 0 C_E>^-D

Time delay stage

Monitor

Ε

Q-Switch dye cell

2

si 23

m n

m H

O

m

S

m r

χ >

η

a

9.

BARBARA AND STRANDJORD

Picosecond

Emission

Spectroscopy

187

The apparatus employs a p a s s i v e l y mode locked Nd/YAG l a s e r o s c i l l a t o r , a Pockel c e l l p u l s e e x t r a c t o r , and Nd/YAG l a s e r a m p l i f i e r to produce ^ 10 mjoule, 30 psec l a s e r pulses a t 1064 nm. Non-linear c r y s t a l s convert ^ 30% of t h i s l i g h t to 355 nm, which i s used f o r e x c i t a t i o n of the sample. The o p t i c a l path length of the 355 nm l i g h t i s v a r i e d by a computer-controlled time delay stage. The sample fluorescence i s time resolved by the upconversion method. The fluorescence i s upconverted i n a Type I I KDP c r y s t a l by a p o r t i o n of the 1064 nm l i g h t which t r a v e l s a f i x e d path that i s portrayed i n Figure 1. The "upconverted" l i g h t i n t e n s i t y (λ^

=

+ (1064 nm) "*") from the KDP upconverting c r y s t a l i s

measured as a f u n c t i o n of d e l a y stage length t o e x t r a c t the fluorescence time dependence. This approach, y i e l d s accurate k i n e t i c traces w i t h e x c e l l e n Experimental Examples Method A. For Method A we determine the upconverted l i g h t pulse i n t e n s i t y w i t h a p h o t o m u l t i p l i e r tube (pmt) which i s i n t e r f a c e d by i n t e g r a t o r / h o l d and a n a l o g / d i g i t a l e l e c t r o n i c s to our Cromemco computer. Data c o l l e c t i o n i s automated to enhance accuracy and the s i g n a l / n o i s e r a t i o by i ) s i g n a l averaging over s e v e r a l l a s e r pulses, i i ) c o r r e c t i n g the data f o r l a s e r energy f l u c t u a t i o n , and i i i ) r e j e c t i n g data from poorly mode-locked pulses. The r e p e t i t i o n r a t e o f the l a s e r i s 10 pulses per second. The d i g i t a l l y recorded s i g n a l i s the r a t i o of upconverted l i g h t i n t e n s i t y to t o t a l time i n t e g r a t e d fluorescence i n t e n s i t y as a f u n c t i o n of delay stage l e n g t h . This r a t i o i s e s p e c i a l l y i n ­ s e n s i t i v e to l a s e r i n t e n s i t y f l u c t u a t i o n s and other sources of n o i s e and e r r o r (16). Method A i s p a r t i c u l a r l y u s e f u l f o r studying the dynamics of d u a l f l u o r e s c e n t systems, i n c l u d i n g excimer formation and e x c i t e d s t a t e i s o m e r i z a t i o n . For example, we have studied the dynamics of the e x c i t e d s t a t e i s o m e r i z a t i o n of 3-hydroxyflavone (3HF). The s t a b l e ground-state form of 3HF i s the "normal" (N) isomer. O p t i c a l e x c i t a t i o n of Ν y i e l d s N*, which r a p i d l y rearranges to give a d i f f e r e n t e l e c t r o n i c a l l y e x c i t e d isomer, namely the "tautomer" form ( T * ) . The fluorescence maxima f o r the N* and T* forms a r e 413 nm and 543 nm, r e s p e c t i v e l y .

Ο Ν

+

OH

Τ

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

188

MULTICHANNEL IMAGE DETECTORS

In a recent paper (12) we showed that the species r e ­ s p o n s i b l e f o r the green (543 nm) f l u o r e s c e n c e of 3HF was formed k i n e t i c a l l y from blue (413 nm) e m i t t i n g species by two k i n e t i c components. We r a t i o n a l i z e d t h i s complex behavior by a model (Scheme 1) i n which there are two pathways f o r e x c i t e d s t a t e i s o m e r i z a t i o n , i . e . , a slow and a f a s t component. k

AC

"AB

k

A aA

+ k

A

nrA

k

rB

+k

„ nrB

k

rC

+k

„ nrC

Scheme 1 Here A i s the i n i t i a l l y s t a t e which i s the source of the f a s t component of the blue emission; Β i s a " r e l a x e d form of A which f l u o r e s c e s to give the slower blue component; CJ i s the s o l e source of green emission, k and k „ are the n o n - r a d i a t i v e nrA nrB decay r a t e s f o r A and B, and k . and k _ are the r a d i a t i v e r a t e s — — rA rB f o r these s p e c i e s . represents the T^L tautomer and A and Β are two d i f f e r e n t forms of N*_, see Strandjord, et a l . (12), f o r further details, k and k are the " f a s t " and "slow" r a t e 11

A

constants f o r ESIPT, r e s p e c t i v e l y . Figure 2 portrays time i n t e g r a t e d f l u o r e s c e n c e spectra of 3HF that r e v e a l the dual f l u o r e s c e n c e bands of t h i s molecule (12-15). Spectra are presented f o r methanol s o l v e n t and Di-Methanol solvent f o r which the 3HF a l c o h o l i c proton i s ex­ changed with a deuteron. K i n e t i c t r a c e s recorded by method A for 3HF are shown i n F i g u r e 3. Each t r a c e was acquired i n ^ 500 sec. of instrument time, i . e . , 5000 l a s e r p u l s e s . The "blue" emission decays with a " s h o r t " and "long" f i r s t order time constant, τ and τ , r e s p e c t i v e l y . The green emission, i n t u r n , i s formed with these same time constants. In many environments, however, i s much l e s s than our instrument time r e s o l u t i o n (^ 30 psec), see below. In these cases, the blue emission appears to e x h i b i t a s i n g l e decay time, τ . Accordingly* Β

the green emission formation k i n e t i c s shows an "immediate" com­ ponent and a "delayed" component a s s o c i a t e d with τ and τ , Δ

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

BARBARA AND STRANDJORD

Picosecond

Emission

Spectroscopy

Figure 2. Time-integrated fluorescence spectra (297 K) of 10~ M solutions of 3 H F in methanol (—) and MeOD (—). The spectra are plotted on the same relative intensity scale. 5

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

190

MULTICHANNEL IMAGE DETECTORS

T=240K

T=240K

I

I

0

200 Time

I

400

(psec)

Figure 3. Time-resolvedfluorescence traces of 3 H F in MeOH and MeOD acquired by Method A (see text). The 413-nm emission was indirectly monitored by upconversion at 297 nm, and the 543-nm emission was monitored by upconversion at 360 nm. The points in thefigure are experimental data, and the solid lines are computerfits to this data. (The fitting algorithm has been described elsewhere (I).)

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

9.

BARBARA AND STRANDJORD

Picosecond

Emission

Spectroscopy

191

r e s p e c t i v e l y , see F i g u r e 3. E i t h e r the blue or green emission can, t h e r e f o r e , be used to determine τ (12). Β We have been a b l e to study the experimentally determined time constants, e.g. τ , f o r 3HF fluoresence as a f u n c t i o n of Β

s o l v e n t , temperature and i s o t o p i c s u b s t i t u t i o n of the s o l u t e and s o l v e n t . These data, when combined with other s p e c t r o s c o p i c measurements have been employed i n the i n v e s t i g a t i o n of the i s o m e r i z a t i o n mechanism of 3HF. Method B. Very few molecules e x h i b i t dramatic f l u o r e s e n c e dynamics of the type observed f o r 3HF. In most cases the observed e f f e c t s are more s u b t l e , i n v o l v i n g a s l i g h t timedependent v a r i a t i o n i n the f l u o r e s c e n c e band p r o f i l e . E f f e c t s of t h i s type can be due to v i b r a t i o n a l r e l a x a t i o n , solvent r e l a x ­ a t i o n and t o r s i o n a l r e l a x a t i o n We employ method Β mode, our apparatus y i e l d s r e l a t i v e h i g h - r e s o l u t i o n f l u o r e s c e n c e spectra at d i f f e r e n t time windows a f t e r e x c i t a t i o n of the sample by the 355 nm pulse. The s p e c t r a are acquired by the upconversion method. The upconverted f l u o r e s c e n c e spectrum i s recorded simultaneously a t a l l monitored wavelengths by an o p t i c a l multichannel analyzer. I t i s constructed from a p o l y ­ chromator (HR320 Instruments SA) and an i n t e n s i f i e d s i l i c o n photodiode a r r a y d e t e c t o r (Princeton Applied Research Model 1412). The d e t e c t o r i s i n t e r f a c e d to our Cromemco computer. We have used t h i s approach to i n v e s t i g a t e the e x c i t e d s t a t e r e l a x a t i o n dynamics of 2-(2 -hydroxyphenyl)benzothiazole, HBR (1). The f l u o r e s c e n c e p r o p e r t i e s of t h i s molecule are w e l l described by a k i n e t i c scheme o u t l i n e d i n F i g u r e 4. Barbara, Brus and Rentzepis r e c e n t l y reported that the r a p i d l y formed (< 4 psec) keto (2) f l u o r e s c e n c e spectrum of HBT e x h i b i t e d a band-shape e v o l u t i o n f o r ^ 10 psec a f t e r e x c i t a t i o n of (1) (8). These authors, who monitored the f l u o r e s c e n c e k i n e t i c s at d i f f e r e n t 10 nm wide r e g i o n s , observed that the high energy edge (475 nm) of the keto f l u o r e s c e n c e s u f f e r e d a r a p i d bathochromic s h i f t f o r ^ 10 psec. Our time-resolved k i n e t i c t r a c e s a l s o e x h i b i t evidence f o r a s h o r t - l i v e d broadened blue edge ( 1 ) . We have observed that the 460±10 nm emission e x h i b i t s a < 10 psec r i s e t i m e and two l i f e t i m e s , one 20 ± 10 psec and the other > 2000 psec. The 490 nm k i n e t i c s , i n c o n t r a s t , e x h i b i t s a s i n g l e > 2000 psec l i f e t i m e , but two r i s e t i m e s . One of these i s < 10 psec, our instrument r e s o l u t i o n with d e i o n v o l u t i o n , and the other i s 30 ± 10 psec. The dual r i s e t i m e k i n e t i c s at 480 nm are c l e a r l y d i s t i n g u i s h a b l e from the response of our apparatus to emission with a s i n g l e < 5 psec r i s e t i m e . We assume that the < 10 psec formation component represents k ~ l i n Figure 4, see r e f e r e n c e ( 1). In an attempt to f u r t h e r c h a r a c t e r i z e t h i s i n t e r e s t i n g phenomenon, we have i n v e s t i g a t e d the f l u o r e s c e n c e of HBT i n argon by method Β (1). We observe that the e a r l y (y 50 psec) ?

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

192

MULTICHANNEL IMAGE DETECTORS

f l u o r e s c e n c e spectrum of HBT i s indeed d e t e c t a b l y d i f f e r e n t from s p e c t r a recorded from 100 psec - 2000 psec, see Figures 5 and 6. The 50 psec spectrum as compared to the 250 psec spectrum, on the same r e l a t i v e emission i n t e n s i t y s c a l e , a c t u a l l y shows more i n t e n s i t y at 465 nm but l e s s at 485 nm. This i s supported by the experiments described i n the previous s e c t i o n . The f l u o r e ­ scence band-shape shows much l e s s e v o l u t i o n during the p e r i o d 100 - 2000 psec. F i g u r e 5 shows s p e c t r a i n the r e g i o n of the onset of 2*_ emission. Detectable v a r i a t i o n s between the ^ 50 psec and > 100 psec spectrum are a l s o present i n the > 490 nm r e g i o n , but we have not i n v e s t i g a t e d t h i s wavelength r e g i o n i n d e t a i l . The wavelength s c a l e s f o r the upconverted s p e c t r a i n Figures 5 and 6 were c a l c u l a t e d from the formula

fl

+

-1 1064 nm

the λ values are the experimentall above. I t i s i n t e r e s t i n g to note that the "gated s p e c t r a are h i g h l y r e p r o d u c i b l e . This i s demonstrated i n the lower panel of F i g u r e 5 where t r a c e s from two separate determinations of the 115 psec spectrum are p l o t t e d on the same i n t e n s i t y s c a l e . The s p e c t r a i n Figure 5 have not been c o r r e c t e d f o r the wavelength dependent s e n s i t i v i t y of our apparatus i n the "gated ' mode. We d i s c u s s t h i s subject i n d e t a i l below. This c o r r e c t i o n , which should be i d e n t i c a l f o r a l l time values, i s , however, small as judged by a comparison of the non-time-resolved emission spectrum (Figure 6a) to the l a t e time-gated spectrum which should be r e s p o n s i b l e f o r more than 99% of the non-time-resolved f l u o r e s e n c e (Figure 6b). The time e v o l u t i o n of the f l u o r e s c e n c e spectrum of HBT shown i n F i g u r e 5 i s e n t i r e l y c o n s i s t e n t with the previous proposal that 2*^ i s i n i t i a l l y created with excess v i b r a t i o n a l energy that d i s s i p a t e s on the 10-30 psec time s c a l e (8). The broadened and s h i f t e d high-energy edge of _2* band as compared to the relaxed (> 250 psec) spectrum, seems to be analogous to other examples of f l u o r e s c e n c e band-shape time dependence due to v i b r a t i o n a l r e l a x a t i o n i n the e x c i t e d - e l e c t r o n i c s t a t e (2,6). We have concluded, t h e r e f o r e , that i s created from the 1* 2*_ process with an i n i t i a l v i b r a t i o n a l excess. 11

1

Photodiode Array Versus P h o t o m u l t i p l i e r D e t e c t i o n . The advan­ tages of photodiode array d e t e c t i o n , PDA, as compared to photo­ m u l t i p l i e r tube, pmt, d e t e c t i o n f o r emission spectroscopy are w e l l known (19). These advantages are e s p e c i a l l y important f o r the s p e c i f i c examples we d i s c u s s here, namely, upconverting emission spectroscopy. T h i s i s d r a m a t i c a l l y demonstrated i n F i g u r e 7 where we compare s i n g l e channel pmt versus multichannel PDA d e t e c t i o n of a small p o r t i o n of the upconverted f l u o r e s c e n c e spectrum of coumarin 520 i n ethanol solvent at room temperature.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

9.

Picosecond

BARBARA AND STRANDJORD

Emission

193

Spectroscopy

2

ENOL

KETO

Figure 4. A representation of the phototautomerization mechanism of phenyl)benzothiazole (HBT) in nonpolar solvents.

1

1

465

1

1

2-(2-hydroxy-

1

475 WAVELENGTH

485

Figure 5. Time-resolved fluorescence spectra of HBT in an argon matrix at 12 Κ acquired by Method B. The time labels on the spectra represent different delay times with respect to excitation by the 355-nm, 30-ps laser pulse. The wavelength scale, k , has been calculated from the formula n

^

~ Κ!

+

0064

nmf

1

where k . is the actual observation wavelength of the upconvertedfluorescence. duced from Ref. 1. Copyright 1983, American Chemical Society.) lH

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

(Repro­

194

MULTICHANNEL IMAGE DETECTORS

WAVELENGTH Figure 6. A time-integrated fluorescence spectrum of HBT (top) under the sample conditions described in Figure 5. The bottom panel portrays a spectrum of the type described in Figure 5. (Reproduced from Ref 1. Copyright 1983, American Chemical Society.) Upconverted Wavelength (nm) 340

345

350

Multi Channel Detection

^ 200 psec Delay

^^^NA^

^--200 psec Delay

Single Channel Detection

200 psec Delay

/

500

510

1 520

y

\

/

530

Actual Wavelength (nm) Figure 7. A comparison of photodiode array multichannel detection to photomultiplier tube single channel detection for the upconverted emission spectrum of coumarin 520.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

9.

BARBARA AND STRANDJORD

Picosecond

Emission

Spectroscopy

195

The f l u o r e s c e n c e spectrum of t h i s molecule i s broad and s t r u c t u r e l e s s i n the wavelength r e g i o n of F i g u r e 7. With PDA o p t i c a l multichannel d e t e c t i o n we observe, as expected, no s i g n a l a t negative delay time with r e s p e c t to the a r r i v a l time of the 355 nm pulse at the sample, and at 200 psec a f t e r the pulse a r r i v e s , the broad s t r u c t u r e l e s s spectrum of coumarin 520 i s observed. The experimental parameters were as f o l l o w s : 1.0 mm entrance s l i t s on the Instruments SA HR 320 s p e c t r o meter, t o t a l accumulation time 60 s e c , a l a s e r r e p e t i o n r a t e of 10 pulses-per-second, and c e r t a i n PDA s e t t i n g s which are d e s c r i b e d below. S i n g l e channel pmt d e t e c t i o n of the same spectrum w i t h the same HR 320 spectrometer e x h i b i t s a t l e a s t on order of magnitude lower s i g n a l to n o i s e r a t i o . For the pmt measurements, entrance and e x i t s l i t s of 1.0 mm were employed and the HR 320 wavel e n g t h was scanned d i g i t a l l y the same f o r the PDA an Why does multichannel PDA d e t e c t i o n e x h i b i t such improved s i g n a l to n o i s e over s i n g l e channel pmt d e t e c t i o n ? A major f a c t o r i s c l e a r l y that the s i g n a l averaging i n t e r g r a t i o n time of each of 170 r e s o l u t i o n elements i n the PDA case i s the f u l l a c q u i s i t i o n time, i . e . , 60 seconds, w h i l e the a c q u i s i t i o n time f o r the PMT experiments i s a small f r a c t i o n , 1/20, of the t o t a l a c q u i s i t i o n time due to the n e c e s s i t y of scanning f o r s i n g l e channel d e t e c t i o n . A second very important f a c t o r i s a s s o c i a t e d with p u l s e - t o pulse f l u c t u a t i o n s of the l a s e r i n t e n s i t y , which i n our system can be as great as V30%. The PDA experiment f o r time r e s o l v e d upconverted spectra i s e s p e c i a l l y i n s e n s i t i v e to i n t e n s i t y f l u c t u a t i o n s of the l a s e r i n t e n s i t y because the spectrum i s acquired simultaneously and the f l u c t u a t i o n s of the d i f f e r e n t channels a r e c o r r e l a t e d . In c o n t r a s t , f o r s i n g l e channel det e c t i o n these f l u c t u a t i o n s a r e u n c o r r e l a t e d and c o n t r i b u t e n o i s e to the spectrum. As mentioned above, we have attempted i n the s i n g l e channel case to c o r r e c t f o r these f l u c t u a t i o n s by r a t i o metry u s i n g the time i n t e g r a t e d f l u o r e s c e n c e i n t e n s i t y as a measure o f the l a s e r f l u c t u a t i o n . But our experience i s that the c o r r e l a t i o n o f upconverted l i g h t i n t e n s i t y with the i n t e g r a t e d f l u o r e s c e n c e i n t e n s i t y i s not p e r f e c t l y l i n e a r , as expected from i d e a l c o n s i d e r a t i o n s , and a c t u a l l y e x h i b i t s l a r g e f l u c t u a t i o n s about the expected l i n e a r r e l a t i o n s h i p . The n o n i d e a l i t y may be due to p u l s e - t o - p u l s e f l u c t u a t i o n s of the s p a t i a l c h a r a c t e r i s t i c s of the l a s e r which could vary the e f f i c i e n c y of upconversion. A t h i r d and f i n a l f a c t o r t h a t , i n p r i n c i p l e , can be an advantage of PDA d e t e c t i o n i s a s s o c i a t e d with s i g n a l averaging. For the PDA measurements the s i g n a l averaging f o r F i g u r e 7 was accomplished p r i m a r i l y by analog i n t e r g r a t i o n of the s i g n a l on the photodiode a r r a y . In d e t a i l , a 20 second delay was i n c o r porated between three read c y c l e s of the PDA, and the s i g n a l was d i g i t i z e d only three times, i . e . each read c y c l e . For the pmt

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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measurements, i n c o n t r a s t , the s i g n a l i s d i g i t i z e d f o r each l a s e r pulse and d i g i t i z a t i o n and readout noise can c o n t r i b u t e s i g n i f i c a n t l y to the data. In p r i n c i p l e , an analog i n t e r g r a t i o n scheme might a l s o be incorporated with the pmt d e t e c t o r . In p r a c t i c e , commercially a v a i l a b l e long time (>1 sec) i n t e g r a t o r s f o r pmt d e t e c t o r s are subject to e l e c t r i c a l d r i f t which cont r i b u t e s to the o v e r a l l noise c h a r a c t e r i s t i c s of the system. Perhaps the important c o n c l u s i o n here i s that commercially a v a i l a b l e PDA detectors l i k e PAR 1420 have superbly constructed s i g n a l processing e l e c t r o n i c s that allow f o r near i d e a l analog s i g n a l averaging, but comparable equipment f o r pmt d e t e c t o r s are not, to our knowledge, a v a i l a b l e . Instrument Performance Wavelength S e n s i t i v i t y . The o v e r a l l wavelength s e n s i t i v i t y of emission spectrometers play accuracy of the recorde lengths that can e f f e c t i v e l y be studied by a p a r t i c u l a r appa r a t u s . The upconversion method adds a source of wavelength dependent s e n s i t i v i t y that i s both u s e f u l and problematic. On the p o s i t i v e s i d e , upconversion allows e f f i c i e n t det e c t i o n of red wavelengths (> 650 nm) where d i r e c t d e t e c t i o n spectrometers are i n s e n s i t i v e . For example, f o r our spectrometer 700 nm f l u o r e s c e n c e i s upconverted to 422 nm. This l a t t e r wavelength i s i n a r e g i o n where our pmt and array detectors are quite sensitive. A disadvantage of the upconversion approach, however, i s that the e f f i c i e n c y of n o n - l i n e a r mixing can be a strong f u n c t i o n of the wavelengths of each of the beams and t h e i r o r i e n t a t i o n r e l a t i v e to the d i r e c t i o n of the o p t i c a l a x i s of the n o n - l i n e a r c r y s t a l . T h i s phenomenon i s a r e s u l t of the phase-matching requirement f o r sum frequency generation. A d e t a i l e d d i s c u s s i o n of phase matching i n f l u o r e s c e n c e upconv e r s i o n can be found elsewhere (16). P r a c t i c a l l y speaking, we have found that i t i s p o s s i b l e to upconvert an ^ 75 nm wide r e g i o n of v i s i b l e fluorescence with a l e s s than 80% drop i n e f f i c i e n c y toward the edges of the region. This i s accomplished by simultaneously s a t i s f y i n g the phase match requirement at s e v e r a l wavelengths by f o c u s s i n g the fluorescence l i g h t i n the Type I I KDP c r y s t a l . The c r y s t a l i s t i l t e d to maximize the conversion e f f i c i e n c y at the center of the d e s i r e d r e g i o n . This approach y i e l d s ^ 75 nm wide spectra throughout the v i s i b l e , near u l t r a v i o l e t and near i n f r a r e d r e g i o n , depending on the c r y s t a l angle and the spectrometer settings. Accurate f l u o r e s c e n c e s p e c t r a can be produced from the raw upconverted data by s u i t a b l e data processing on our Cromemco computer. T y p i c a l l y , we c o r r e c t f o r the channel dependent gain of the photodiode array and the wavelength dependent e f f i c i e n c y of the upconversion process which we determine independently.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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The wavelength axes o f the s p e c t r a a r e a l s o c a l i b r a t e d by the f l u o r e s c e n c e p r o c e s s i n g software. L i n e a r i t y . A common source of e r r o r f o r picosecond time-resolved emission spectrometers i s a n o n - l i n e a r response of the d e t e c t o r to emission i n t e n s i t y . Streak camera temporal d i s p e r s o r s , f o r example, e x h i b i t a l i m i t e d dynamic range, which, i n unfavorable cases, can lead to severe experimental a r t i f a c t s (20-21). The apparatus we d e s c r i b e i n t h i s paper has two p o t e n t i a l sources of e r r o r s due to n o n - l i n e a r i t y , namely, the upconversion process and the a c t u a l d e t e c t o r , i . e . photodiode a r r a y or pmt. The upconversion process can, i n p r i n c i p l e , be n o n - l i n e a r at high f l u o r e s c e n c e l i g h t i n t e n s i t y . We have examined the l i n e a r i t y of upconversion f o r our apparatus by studying the upconverted l i g h t i n t e n s i t y versus f l u o r e s c e n c e i n t e n s i t y at f i x e d delay time. The observed dependence i n F i g u r e 8 demons t r a t e s that the upconversio l i n e a r range f o r the experimenta voke. The f l u o r e s c e n c e i n t e n s i t y i n F i g u r e 9 was v a r i e d by c a l i b r a t e d n e u t r a l d e n s i t y f i l t e r s . The l i n e a r i t y of the pmt and a s s o c i a t e d e l e c t r o n i c s was v e r i f i e d independently. We have a l s o studied the d i g i t a l s i g n a l i n t e n s i t y versus l i g h t i n t e n s i t y behavior o f the photodiode a r r a y d e t e c t o r used i n method B. Non-linear e f f e c t s have been observed f o r V i d i c o n multichannel analyzers when employed i n measuring the i n t e n s i t y of picosecond d u r a t i o n l i g h t . Photodiode a r r a y s , i n c o n t r a s t , have r a r e l y been employed i n picosecond spectrometer, and, we are not aware of a previous study of the l i n e a r i t y of these devices f o r the d e t e c t i o n of high i n t e n s i t y picosecond d u r a t i o n light. The d i g i t a l s i g n a l i n t e n s i t y from the photodiode a r r a y that was described above versus i n t e n s i t y of ^ 50 psec d u r a t i o n l i g h t i s shown i n F i g u r e 9 . A l i n e a r behavior i s , i n f a c t , observed.

0

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Intensity Figure 8. A plot of measured upconverted signal intensity (PMT voltage) cence intensity at fixed delay time. The data were acquired using Method fluorescence intensity was varied with neutral density filters. PMT voltage tored by integrator and hold electronics followed by an analog to digital

vs. fluoresA, and the was moniconverter.

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Intensity Figure 9. An evaluation of the response of the photodiode arrays signal as a function of light intensity of— 50 ps duration. filter. The light was created by ~ 30-ps, 355-nm light pulses. This molecule is known to have a fluorescence lifetime of 6 + 3 ps in these environments.

Acknowledgement Acknowledgement i s made f o r p a r t i a l support of t h i s r e s e a r c h to t h e f o l l o w i n g agencies: 1) the Donors of the Petroleum Research Fund (administered by the American Chemical S o c i e t y ) , 2) the Research Corporation and 3) the Graduate School of the U n i v e r s i t y o f Minnesota.

Literature Cited 1.

Ding, Κ., Courtney, S.H., Strandjord, A.J.G., Flom, S., Friedrich, D., and Barbara, P.F., J. Phys. Chem., 1983, 87(7), 1184. 2. Barbara, P.F., Rentzepis, P.M., and Brus, L.E., J. Chem. Phys., 1980, 72, 6802. 3. Choi, Κ., Boczar, B.P., and Topp, M.R., Chem. Phys., 1981, 57, 415. 4. Doany, F.E., Greene, B.I., Liang, Y., Negus, D.K., and Hochstrasser, R.M., i n "Picosecond Phenomena", Springer, 1980. 5. Okamura, T., Sumitani, M., Yoshihara, K., Chem. Phys. Lett., 1983, 94(3), 339. 6. Barbara, P.F., Rand, S.D., and P.M. Rentzepis, J. Amer. Chem. Soc., 1981, 103, 2156. 7. Barbara, P.F., Rentzepis, P.M., and Brus, L.E., J. Amer. Chem. Soc., 1980, 102, 2786. 8. Barbara, P.F., Brus, L.E., and Rentzepis, P.M., J. Amer. Chem. Soc., 1980, 102, 5631.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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9. 10. 11. 12. 13.

14. 15. 16. 17. 18. 19. 20.

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Barbara, P.F., Brus, L.E., and Rentzepis, P.M., Chem. Phys. Lett., 1980, 69, 447. Waldeck, D.H., Lotshaw, W.T., McDonald, D.B., and Fleming, G. R., Chem. Phys. Lett., 1982, 88(3), 297. Turner, D.H., Tinoco, I., and Maestre, M., J. Amer. Chem. Soc., 1974, 96(13), 4340. Strandjord, A.J.G., Courtney, S.H., Friedrich, D.M., and Barbara, P.F., J. Phys. Chem., 1983, 87(7), 1125-1133. Itoh, M., Tokumura, K., Tanimoto, Y., Okada, Y., Takeuchi, H., Obi, Κ., and Tanaka, I., J. Amer. Chem. Soc., 1982, 104, 4146. Sengupta, P.K., and Kasha, Μ., Chem. Phys. Lett., 1979, 68(2,3), 382. Woolfe, G.J., and Thistlewaite, P.J., J. Amer. Chem. Soc., 1981, 103, 6916. Hallidy, L. Α., and Topp M.R. Chem Phys Lett. 1977 46, 8. Beddard, G.S., Doust, , , , Phillips, , J. Photochem., 1981, 17, 427. Duguay, M.A., and Hansen, J.W., Appl. Phys. Lett., 1968, 13, 178. Talmi, Υ., Image Devices i n Spectroscopy, this journal. Lieber, Α., i n "Picosecond Phenomena", Springer Series i n Chemical Physics, eds. Shank, C.V., Ippen, E.P., and Shapiro, S.L., (Springer, Berlin, 1976) p. 178. Yu, W., i n "Picosecond Phenomena", Springer Series i n Chemical Physics", eds. Shank, C.V., Ippen, E.P., and Shapiro, S.L., (Springer, Berlin, 1978) p. 347.

R E C E I V E D August 12, 1983

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

10 Picosecond Spectroscopy and Applications to Chemical and Biological Systems E. F. HILINSKI, S. V. MILTON, and P. M. RENTZEPIS Bell Laboratories, Murray Hill, NJ 07974 The mechanisms of a number of photochemical reactions and photophysical hav been elucidated sion, absorption, and Raman spectroscopy. Picosecond spectroscopy enables one to observe ultrafast events in great detail as a reaction evolves. Most picosecond laser systems currently rely on optical multichannel detectors (OMCDs) as a means by which spectra of transient species and states are recorded and their formation and decay kinetics measured. In this paper, we describe some early optical detection methods used to obtain picosecond spectroscopic data. Also we present examples of the application of picosecond absorption and emission spectroscopy to such mechanistic problems as the photodissociation of haloaromatic compounds, the visual transduction process, and intermolecular photoinitiated electron transfer. Picosecond spectroscopy provides a means of studying u l t r a f a s t events which occur i n p h y s i c a l , chemical, and b i o l o g i c a l processes. Several types of l a s e r systems are c u r r e n t l y a v a i l a b l e which possess time r e s o l u t i o n ranging from l e s s than one picosecond to s e v e r a l picoseconds. These systems can be used to observe t r a n s i e n t s t a t e s and species involved i n a r e a c t i o n and to measure t h e i r formation and decay k i n e t i c s by means of picosecond absorption, emission and Raman spectroscopy. T e c h n o l o g i c a l advances i n l a s e r s and o p t i c a l d e t e c t i o n systems have permitted an i n c r e a s i n g number of photochemical r e a c t i o n s to be studied in. greater d e t a i l than was p r e v i o u s l y p o s s i b l e . Several recent reviews (1-4) have been w r i t t e n which d e s c r i b e these picosecond l a s e r systems and s e v e r a l a p p l i c a t i o n s of them 0097-6156/ 83/ 0236-0201 $06.00/0 © 1983 A m e r i c a n C h e m i c a l S o c i e t y

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t o i m p o r t a n t b i o l o g i c a l and c h e m i c a l p r o b l e m s . H e r e we w i l l b r i e f l y p r e s e n t s e v e r a l o f t h e e a r l i e r a s w e l l a s more r e c e n t o p t i c a l t e c h n i q u e s w h i c h h a v e been u s e d t o s t u d y t h e t r a n s i e n t s p e c i e s and s t a t e s g e n e r a t e d by p i c o s e c o n d l a s e r e x c i t a t i o n . We s h a l l a l s o d i s c u s s some e x p e r i m e n t s p e r f o r m e d utilizing p i c o s e c o n d s p e c t r o s c o p y which p r o v i d e examples of the t y p e s o f p r o b l e m s w h i c h c a n be s t u d i e d and t h e d e t a i l e d i n f o r m a t i o n w h i c h c a n be o b t a i n e d . Methods o f O p t i c a l D e t e c t i o n When t h e g e n e r a t i o n o f p i c o s e c o n d l a s e r p u l s e s f i r s t became p o s s i b l e i n t h e l a b o r a t o r y , methods o f e l e c t r o n i c d e t e c t i o n w h i c h c u r r e n t l y a r e i n s u c h w i d e s p r e a d u s e f o r measurements p e r f o r m e d on t h e p i c o s e c o n d t i m e s c a l e , e.g. s t r e a k cameras and two-dimensional photodiod the experimenter. Technique measure a c c u r a t e l y t h e k i n e t i c s o f p h o t o i n i t i a t e d e v e n t s w h i c h o c c u r on t h i s u l t r a s h o r t t i m e s c a l e , b u t more i m p o r t a n t l y t o m o n i t o r t h e w i d t h and shape o f t h e l a s e r p u l s e t o e n s u r e r e l i a b l e and r e p r o d u c i b l e g e n e r a t i o n o f t h e s e p u l s e s . I n i t i a l l y a two p h o t o n f l u o r e s c e n c e (TPF) t e c h n i q u e (_5,6) was u t i l i z e d t o m o n i t o r b o t h c h e m i c a l k i n e t i c s and l a s e r p u l s e s . The TPF a p p a r a t u s o r d i n a r i l y c o n s i s t s o f a few o p t i c a l components w h i c h d i r e c t t h e l a s e r p u l s e s i n t o an o p t i c a l c e l l c o n t a i n i n g a s u i t a b l e fluorescent chemical.(7) The l a s e r p u l s e i s t h e n a l l o w e d t o t r a v e r s e t h e c e l l and r e f l e c t b a c k upon i t s e l f (Figure l a ) . Another arrangement i s a t r i a n g u l a r c o n f i g u r a t i o n of m i r r o r s i n w h i c h a beam s p l i t t e r d i v i d e s t h e l a s e r p u l s e i n t o two e q u a l p u l s e s w h i c h a r e t h e n r e d i r e c t e d by two m i r r o r s t o i n t e r s e c t w i t h i n t h e sample c e l l ( F i g u r e l b ) . A photograph i s made o f t h e r e s u l t i n g f l u o r e s c e n c e . The p h o t o g r a p h i s t h e n analyzed w i t h a densitometer i n order to determine the f l u o r e s cence i n t e n s i t y v e r s u s p o s i t i o n . The f i n a l d e n s i t o m e t e r t r a c e i s a n a l y z e d to e x t r a c t temporal i n f o r m a t i o n about the l i g h t pulses. A n o t h e r e a r l y method u s e d t o m o n i t o r t h e l a s e r p u l s e was t h e t h r e e p h o t o n f l u o r e s c e n c e (3PF) t e c h n i q u e . ( 8 , 9 ) The a d v a n t a g e o f 3PF o v e r TPF i s t w o - f o l d : t h e c o n t r a s t r a t i o i s 10:1 f o r 3PF a s opposed t o 3:1 f o r TPF; and i n a d d i t i o n t o t e m p o r a l i n f o r m a t i o n a b o u t t h e p u l s e s a v a i l a b l e by TPF, 3PF a l s o p r o v i d e s p u l s e shape i n f o r m a t i o n . ( 1 0 - 1 3 ) This a d d i t i o n a l information i s o b t a i n e d because the t h i r d - o r d e r c o r r e l a t i o n f u n c t i o n which r e l a t e s t h e 3PF i n t e n s i t y t o t h e p u l s e i n t e n s i t y i n c l u d e s d e p e n d e n c e upon t h e p h a s e o f t h e p u l s e f r e q u e n c y c o m p o n e n t s . ( 1 1 ) A g a i n , t h e r e s u l t i n g f l u o r e s c e n c e i s p h o t o g r a p h e d , and a d e n s i t o m e t e r t r a c e i s made t o d e t e r m i n e f l u o r e s c e n c e i n t e n s i t i e s . A z u l e n e i s an e x a m p l e o f a m o l e c u l e w h i c h h a s been s t u d i e d q u i t e extensively.(14-20) T y p i c a l d a t a a r e shown i n F i g u r e 2.

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FILTERS

Figure 1. (A) Apparatus used for time-resolved picosecond absorption spectroscopy which utilizes the TPF technique. A set of photodiodes and oscilloscopes (PI, P2, P3, and P4) are used to measure the intensity of the laser pulse before and after each pass through the cell. (Reproduced with permission from Ref 15. Copyright 1969, North-Holland Publishing Company.) (B)An alternative TPF experimental arrangement consisting of two TPF triangles, one contains the sample being studied and the other contains a standard chemical used for TPF. Key: BS, beam splitter; F, optical filter; R, reflector; L, lens; PD, photodiode; potassium dihydrogenphosphate (KDP) second-harmonic generating crystal (Reproduced with permission from Ref. 20. Copyright 1978, Weizmann Science Press of Israel.)

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2 mm

2 mm

.35H

Figure 2. Typical experimental data for consecutive TPF of azulene in solution excited at 530 nm. The upper microdensitometer trace corresponds to the azulene pattern; the lower trace is the TPF pattern of a BBOT dye solution obtained simultaneously with the azulene data. (Reproduced with permission from Ref 20. Copyright 1978, Weizmann Science Press of Israel.)

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Applications

205

A l t h o u g h t h e u s e o f TPF a n d 3PF methods p r o v e d t o be h i g h l y s u c c e s s f u l i n some c h e m i c a l s y s t e m s , t h e y were n o t a p p l i c a b l e t o m o s t . O t h e r e x p e r i m e n t a l t e c h n i q u e s h a d t o be d e v e l o p e d . The o p t i c a l m u l t i c h a n n e l d e t e c t o r (OMCD) p l a y e d a n i m p o r t a n t r o l e i n t h e d e v e l o p m e n t o f new t e c h n i q u e s . R e c o r d i n g d a t a i n a p o i n t b y - p o i n t f a s h i o n , i . e . u s i n g a probe l i g h t o f a s i n g l e frequency and a t a p a r t i c u l a r d e l a y t i m e a f t e r e x c i t a t i o n w i t h one l a s e r s h o t , makes d a t a g a t h e r i n g i n t r i n s i c a l l y more d i f f i c u l t t o c o r ­ r e l a t e w i t h c o n d i t i o n s o f the e x c i t a t i o n process. With t h e a d v e n t o f m u l t i c h a n n e l p h o t o d i o d e a r r a y s , one c a n now m o n i t o r an e n t i r e s e q u e n c e o f e v e n t s o c c u r r i n g a f t e r e x c i t a t i o n b y e a c h l a s e r p u l s e . Coupled w i t h a polychromator, e c h e l o n , o r s t r e a k c a m e r a , t h e amount o f i n f o r m a t i o n a v a i l a b l e p e r s h o t i s o r d e r s of magnitude g r e a t e r than w i t h a s i n g l e p h o t o m u l t i p l i e r tube (PMT) o r p h o t o d i o d e (PD) a r r a n g e m e n t . L i s t e d i n T a b l e I a r e some a d v a n t a g e s a n d d i s a d v a n t a g e A l t h o u g h we s t i l l do u s t i o n s i n o u r e x p e r i m e n t s , we b e l i e v e t h a t t h e a d v a n t a g e s o f t h e OMCD f a r s u r p a s s i t s d i s a d v a n t a g e s . Concurrent w i t h the development o f s u i t a b l e photodiode a r r a y s , t e c h n i q u e s were b e i n g d e v e l o p e d f o r g e n e r a t i n g d i s p e r s i o n i n b o t h t h e t i m e a n d w a v e l e n g t h d o m a i n s . I n 1970 i t was o b s e r v e d (21) t h a t f o c u s i n g i n t e n s e p i c o s e c o n d p u l s e s i n t o some s o l i d s and l i q u i d s produced superbroadened c o n t i n u a w i t h bandw i d t h s o f > 1 0 ^ c m . I t was a l s o shown (22) t h a t t h e s e " w h i t e l i g h t " p u l s e s had d u r a t i o n s on the o r d e r o f the p i c o s e c o n d p u l s e s w h i c h p r o d u c e d them. These c o n t i n u u m p u l s e s have p r o v e n t o be very useful f o r time-resolved spectroscopy. Time d i s p e r s i o n c a n be a c h i e v e d b y u s e o f a n e c h e l o n ( 2 3 ) , a stepped o p t i c a l delay which p r o v i d e s d i s c r e t e temporal d i s ­ p e r s i o n by s u b j e c t i n g t h e c r o s s s e c t i o n o f t h e i n t e r r o g a t i n g p u l s e t o a s e r i e s o f d i f f e r e n t o p t i c a l p a t h l e n g t h s . An e c h e l o n c a n b e b u i l t i n a number o f d i f f e r e n t ways; o n e o f w h i c h i s w i t h a set o f o p t i c a l l y contacted g l a s s o r quartz p l a t e s which i m p r e s s e s t h e t i m e l a g a l o n g t h e c r o s s s e c t i o n o f t h e beam d e p e n d i n g upon t h e t h i c k n e s s o f g l a s s t h a t t h e l i g h t h a s t o traverse. The a d v a n t a g e s o f t h i s method a r e s u b s t a n t i a l when c o u p l e d t o a n OMCD. U s i n g t h i s t e c h n i q u e , one c a n m e a s u r e t h e k i n e t i c s o f a chemical event w i t h picosecond time r e s o l u t i o n . When a n OMCD i s u s e d i n c o n j u n c t i o n w i t h a p o l y c h r o m a t o r , t h e changes i n l i g h t a b s o r p t i o n a s a f u n c t i o n o f time and wavelength w h i c h a r e e x h i b i t e d by t h e m o l e c u l e u n d e r i n v e s t i g a t i o n c a n be monitored, A c r i d i n e h a s been s t u d i e d b y means o f t h e e c h e l o n technique.(24,25) The c h a n g e s i n a b s o r p t i o n (ΔΑ) w e r e m e a s u r e d w i t h a d o u b l e beam p i c o s e c o n d s p e c t r o m e t e r s i m i l a r t o t h e o n e shown i n F i g u r e 3. The d i f f e r e n c e a b s o r p t i o n s p e c t r u m a t o n e s e l e c t e d d e l a y t i m e a f t e r e x c i t a t i o n was c a l c u l a t e d a c c o r d i n g t o -1

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

T y p i c a l l y S-20 r e sponse, -300-800 nm range, w h i c h i s more than adequate f o r most o f t h e work.

Automatically takes care of shot-to-shot fluctuations i n energy ; t h e r e f o r e fewer shots a r e needed f o r averaged data. Also the data a r e much more r e a d i l y interpreted.

S i m u l a t a n e o u s measurements over extended range

Advantages

Advantages

High i n i t i a l

cost

Somewhat more d i f f i c u l t t h a n PMTs o r PDs t o u s e due t o i n creased complexity and e x t r a n e c e s s a r y hardware

A PMT o r PD w i t h particular spectral r e s p o n s e c a n be chosen depending on usage requirements.

Comparatively lower cost

Very easy t o u s e and a l i g n

Large q u a n t i t i e s of accumulated data a r e needed i n o r d e r t o o b t a i n comparable i n f o r m a t i o n o f an OMCD. A l s o d a t a a r e not as r e a d i l y interpreted.

C a p a b l e o f o n l y one measurement a t a time

Disadvantages

PMT o r PD

a n d D i s a d v a n t a g e s o f OMCDs a n d PMT/PDs

Disadvantages

Advantages

Near photon-countLower l i g h t s e n s i ing c a p a b i l i t y t i v i t y ( c a n be e n hanced w i t h t h e u s e of m u l t i - c h a n n e l i n tensifier plates. With the i n t e n s i f i e r plate i nfront of the d e t e c t o r near photoncounting c a p a b i l i t y may be a c h i e v e d . )

OMCD

Table I .

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

-Λ>

A>

-Aw

N M

P U L S E

3 5 0

NM

N M

P U L S E S

3+

4

Figure 3. A double-beam picosecond laser system that utilizes a silicon-intensified target (SIT) vidicon. Key: 1, mode-locking dye cell; 2, Nd .glass laser oscillator; 3, Glanpolarizer; 4, Pockets cell; 5', translatable 90° polarization rotator; 6, fixed Glan polarizer crossed relative to 3; 7, laser amplifier; 8, second-harmonic (530 nm) generating crystal (KDP Type I); 9, third-harmonic generating crystal (KDP Type II); 10, 20-cm CCl cell for generating a continuum; 11, quartz echelon for producing picosecond optical delays; 12, polarizer; 13, sample cell; R, 100% reflector; PR, partial reflector; BS, beam splitter; and OMA, optical multichannel analyzer. (Reproduced with permission from Ref. 24. Copyright 1977, American Institute of Physics.)

< κ < tOO

INTERROGATION

3 5 0

( C O M P U T E R

208

MULTICHANNEL IMAGE DETECTORS ί βχ(λ) )

( ηοβχ(λ))

ς

ς

e

x

e

x

where ΔΑ i s t h e change i n a b s o r b a n c e , g ^) j R ^ r e the i n t e n s i t i e s a s a f u n c t i o n o f w a v e l e n g t h o f t h e sample p r o b e a n d t h e r e f e r e n c e p r o b e , r e s p e c t i v e l y , when t h e s a m p l e i s s u b j e c t e d to e x c i t a t i o n b y t h e l a s e r p u l s e . s > a n d Rnoex(X) p r o b e l i g h t i n t e n s i t i e s when t h e s a m p l e i s n o t s u b j e c t e d t o e x c i t a t i o n by t h e l a s e r p u l s e . A s i n g l e p i c o s e c o n d p u l s e a t 1060 nm was e x t r a c t e d f r o m a m o d e - l o c k e d t r a i n o f a n Nd r g l a s s l a s e r . A f t e r subsequent a m p l i f i c a t i o n s o f t h e ^ 6 - p s FWHM l a s e r p u l s e , p e a k powers o f V50 g i g a w a t t s a r e e a s i l y g e n e r a t e d . P a r t o f t h e f u n d a m e n t a l l i g h t i s t h e n c o n v e r t e d i n t o t h e s e c o n d h a r m o n i c ( 5 3 0 nm) b y means o f a KDP c r y s t a l then f r e q u e n c y mixed w i t d u c e t h e t h i r d h a r m o n i c (355 nm). The r e m a i n i n g 1060-nm a n d 530-nm p u l s e s a r e u s e d t o p r o d u c e t h e p i c o s e c o n d " w h i t e l i g h t " c o n t i n u u m p u l s e i n C C l ^ . The c o n t i n u u m i s t h e n d i s p e r s e d i n time t h r o u g h an e c h e l o n and f o c u s e d onto t h e sample. One o f t h e f i r s t segments o f l i g h t o u t o f t h e e c h e l o n i s a l w a y s t i m e d f o r a r r i v a l a t t h e s a m p l e c e l l a t a known t i m e r e l a t i v e t o e x c i t a ­ tion. T h i s t i m i n g i s a c h i e v e d e i t h e r by t h e b l e a c h i n g o f a dye s u c h a s DODCI, w h i c h u n d e r h i g h power d i s p l a y s a r e c o v e r y t i m e of a f e w p i c o s e c o n d s , o r by means o f a K e r r e f f e c t o p t i c a l s h u t t e r , f o r example a C S c e l l . The 355-nm p u l s e i s u s e d t o e x c i t e t h e s a m p l e , a n d t h e c o n t i n u u m segments ( p r o b e p u l s e s a r e u s e d t o m o n i t o r t h e r e s u l t i n g ΔΑ. The p r o b e p u l s e s , a f t e r p a s s i n g t h r o u g h t h e s a m p l e a r e t h e n f o c u s e d i n t o a monochromator w h i c h h a s mounted to i t s o u t p u t a two d i m e n s i o n a l s i l i c o n i n t e n s i f i e d t a r g e t p h o t o d i o d e a r r a y c o n n e c t e d t o a c o m p u t e r . W i t h t h e monochromator s e t a t 430 nm, S u n d s t r o m e t . a l . ( 2 4 ) d i r e c t l y m e a s u r e d t h e p o p u l a t i o n growth r a t e o f t h e lowest t r i p l e t s t a t e o f a c r i d i n e , w h i c h when c a l c u l a t e d f r o m t h e d a t a i l l u s t r a t e d i n F i g u r e 4 i s 3 χ 10 sec- . Another example o f t h e u s e o f o p t i c a l m u l t i c h a n n e l d e t e c ­ t i o n i s the picosecond spectroscopic study o f a c r i d i n e , s - t e t r a z i n e , and rhodamine Β by B a r b a r a e t . a l . ( 2 6 ) I n t h i s t y p e of s t u d y , a s a m p l e c o n t a i n i n g t h e compound o f i n t e r e s t i s p l a c e d i n t h e p a t h o f a n ^ 6 - p s FWHM l a s e r p u l s e . The l a s e r p u l s e i s f o c u s e d onto t h e sample c e l l , and t h e e m i t t e d l i g h t i s c o l l e c t e d and d i r e c t e d , by a n a s s e m b l y o f l e n s e s , i n t o a s t r e a k camera which i s capable o f time-resolving the emission. Processing and a n a l y s i s o f t h e s t r e a k camera d a t a a r e a c c o m p l i s h e d b y means of a n a s s e m b l y c o n s i s t i n g o f a t w o - d i m e n s i o n a l p h o t o d i o d e a r r a y a

n

(

a

n o e x ( X

a

2

1

0

1

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

r

e

t h i 2

10.

HILINSKI ET AL.

Picosecond

Spectroscopy

and

209

Applications

L2LO08 06 Q402 0-

20

AO

Ï2Ô"

tS

5&

ÏOÔ

Î2ÎS

iïS

$B5 5δΓ

T(PSEC)

Or "700

72Ô

740

760

780

βοδ

r(PSEC) Figure 4. Absorption changes in acridine monitored at 430 nm from 0 to 800 ps after excitation at 355 nm. The polarizations of exciting and probing light pulses were parallel to each other. (Reproduced with permission from Ref. 24. Copyright 1977, American Institute of Physics.)

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

210

MULTICHANNEL IMAGE DETECTORS

( v i d i c o n o r r e t i c o n ) , o p t i c a l m u l t i - c h a n n e l a n a l y z e r (OMA), and a computer. We s h a l l u s e t h e p i c o s e c o n d l a s e r s y s t e m d e s c r i b e d by B a r b a r a e t . a l . (26) a s an example o f an a p p a r a t u s w h i c h can be u s e d f o r e m i s s i o n s t u d i e s and i n c l u d e a d i s c u s s i o n o f t h e p r o b ­ lems and l i m i t a t i o n s t h a t must be c o n s i d e r e d and c o r r e c t e d i n order to o b t a i n meaningful, r e l i a b l e data. They u s e d a p a s s i v e l y mode-locked N d ^ : g l a s s o s c i l l a t o r to generate a t r a i n o f 1060-nm, 6-ps FWHM p u l s e s . S i n g l e p u l s e s were e x t r a c t e d e l e c t r o - o p t i c a l l y f r o m t h e p u l s e t r a i n w h i c h emerges f r o m t h e l a s e r c a v i t y and a m p l i f i e d t o ^100 mJ by means ο f t h r e e N d : Y A G amplifiers. Samples o f a c r i d i n e were e x c i t e d w i t h t h e t h i r d h a r m o n i c (355 nm) o f t h e l a s e r f u n d a m e n t a l w a v e l e n g t h . The s e c o n d h a r m o n i c (532 nm) was u s e d f o r e x c i t a t i o n o f s - t e t r a z i n e and r h o d a m i n e B. The e x c i t a t i o n l i g h t p u l s e was f o c u s e d t o a 2-mm s p o t on t h e s a m p l e p u l s e was d i r e c t e d i n t t i m i n g marker which preceded the e x c i t a t i o n p u l s e . F i g u r e 5 i l l u s t r a t e s a t y p i c a l s t r e a k image o f t h e t i m e d e p e n d e n t f l u o r e s c e n c e w h i c h r e s u l t s by e x c i t a t i o n o f r h o d a m i n e Β a t 530 nm w i t h a 0.1 mJ l a s e r p u l s e . The p r e p u l s e i s o b s e r v e d a t 50 ps b e f o r e t h e a p p e a r a n c e o f t h e t i m e - d e p e n d e n t f l u o r e s ­ cence s i g n a l . D i s t o r t i o n s o f t h e s t r e a k image a r e t h e r e s u l t o f a number o f c o n t r i b u t i n g c a u s e s w h i c h i n c l u d e v i g n e t t i n g i n t h e o p t i c s r e l a y i n g t h e e m i t t e d l i g h t f r o m t h e sample t o t h e s t r e a k camera s l i t , c h a n n e l - d e p e n d e n t s e n s i t i v i t y w i t h i n t h e OMA, n o n - l i n e a r i t y i n t h e t i m e b a s e o f t h e s t r e a k c a m e r a , and c h a n g e s i n g a i n a l o n g the s u r f a c e of the photocathode of the s t r e a k camera. C a l i b r a t i o n s o f t h e t i m e b a s e can be made o p t i c a l l y by u s e o f étalons w h i c h s p l i t an i n c o m i n g l a s e r p u l s e and d e l a y t h e s p l i t p u l s e s s p a t i a l l y , and t h e r e f o r e t e m p o r a l l y , by known amounts o f t i m e . These t e m p o r a l l y s p l i t p u l s e s a r e then d e t e c t e d by means o f t h e s t r e a k e a r n e r a / v i d i c o n / O M A / c o m p u t e r s y s t e m . V a r i a t i o n s i n s i g n a l i n t e n s i t y a l o n g t h e t i m e a x i s can be c o r r e c t e d by measurement o f i n t e n s i t y v s . t i m e f o r a l o n g - l i v e d e m i s s i o n a t s e l e c t e d s t r e a k s p e e d s . T h i s measurement p r o v i d e s a g a i n p r o f i l e f o r t h e s t r e a k c a m e r a . The g a i n p r o f i l e o f t h e s t r e a k camera and t h e t i m e - b a s e c o r r e c t i o n can be a p p l i e d as c o r r e c t i o n s t o raw s t r e a k d a t a a t t h e t i m e o f d a t a p r o c e s s i n g . F i g u r e 5 i l l u s t r a t e s t h e change i n a p p e a r a n c e o f d a t a o b t a i n e d f o r the t i m e - r e s o l v e d f l u o r e s c e n c e of rhodamine Β a f t e r c o r r e c ­ t i o n s a r e made w h i c h compensate f o r o p t i c a l d e t e c t i o n e r r o r s . The a p p a r e n t f l u o r e s c e n c e d e c a y w h i c h i s q u i t e o b v i o u s i n F i g u r e 5 i s t h e r e s u l t o f d i s t o r t i o n c a u s e d by t h e method o f s i g n a l d e t e c t i o n . Once c o r r e c t e d a p r a c t i c a l l y c o n s t a n t f l u o r e s ­ c e n c e i n t e n s i t y i s o b s e r v e d i n t h e 120-350 ps r a n g e o f t i m e a f t e r excitation. T h i s s h o u l d be t h e c a s e s i n c e r h o d a m i n e Β e x h i b i t s a f l u o r e s c e n c e l i f e t i m e i n the range of nanoseconds. +

3+

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

10.

HILINSKI ET AL.

Picosecond

Spectroscopy

and

211

Applications

SAMPLE FLUORESCENCE STREAK CAMERA

INITIAL D A T A - 1 SCAN

>(c/> ζ UJ I-

OPTICAL STREAK IMAGE OMA CORRECTED D A T A - 1 S C A N

DIGITAL DATA COMPUTER CORRECTIONS i. non L I N E A R S W E E P RATE i i . VIGNETTING iij. TRIGGER J I T T E R Ο

J

50

I

100

I

150

I

200

L

250

300

350

t (psec) Figure 5. The data processing system with actual time-resolved fluorescence measured for a 5 χ 10~ M rhodamine Β in ethanol (1-mm pathlength) excited by a 0.1-mJ, 530-nm laser pulse. The data obtained initiallyfrom the OMA are corrected by computer. In these data, the average time between points (individual OMA channels) is 0.8 ps. (Reproduced with permission from Ref. 26. Copyright 1980, North-Holland Publishing Company.) 4

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

212

MULTICHANNEL IMAGE DETECTORS

T y p i c a l l y , i n measurements of time-resolved luminescence i n the time regime of tens of picoseconds, data obtained from 10 to 20 l a s e r shots are averaged to improve the s i g n a l - t o - n o i s e r a t i o and to minimize the e f f e c t s of shot-to-shot v a r i a t i o n s i n the l a s e r pulse energy and shape. Once the r e l i a b i l i t y of the data has been ensured by a p p l i c a t i o n of the c o r r e c t i o n s described above and made necessary by detector-induced d i s t o r t i o n s , the time-resolved f l u o r e s c e n c e data i s analyzed i n terms of a k i n e t i c model which assumes that the emitting s t a t e i s formed with a r i s e t i m e , T , and a decay time, τ-p. Deconvolution of the e x c i t a t i o n pulse from the observed molecular f l u o r e s c e n c e i s performed n u m e r i c a l l y . The shape of the e x c i t a t i o n pulse to be removed from the s t r e a k camera data i s assumed to be the same as the prepulse shape, and t h e r e f o r e the prepulse i s g e n e r a l l y used f o r the deconvolution procedure. Figure 6 i l l u s t r a t e s the q u a l i t y of the f i t of th can be achieved. In t h e i r study of the f l u o r e s c e n c e of a c r i d i n e i n hexane induced by a 0.1 mJ, 355-nm l a s e r p u l s e , Barbara e t . a l . (26) demonstrated the s e n s i t i v i t y of t h i s technique f o r the measure­ ment of T and Tp,. The best f i t of the data, i n t h i s case, i s a r i s e t i m e , T , which i s pulse-width l i m i t e d and a decay l i f e ­ time, Tp, of 38 ps. These data which are i l l u s t r a t e d i n Figure 6 i n d i c a t e that r i s e t i m e s as short as 3 ps can be a c c u r a t e l y determined, and decay l i f e t i m e s can be f i t t e d with an accuracy of MOL R

R

r

A p p l i c a t i o n s of Picosecond

Spectroscopy

P h o t o d i s s o c i a t i o n of Haloaromatic Compounds. Picosecond emission spectroscopy was r e c e n t l y employed (27,28) to study the photod i s s o c i a t i o n of haloaromatics and the subsequent f l u o r e s c e n c e of the formed r a d i c a l s . An e a r l i e r paper by Huppert e t . a l . (29) d e s c r i b e s the p r e d i s s o c i a t i v e pathways of the haloaromatics f o l l o w i n g e x c i t a t i o n at 266 and 355 nm. What makes the more recent work d i f f e r e n t i s t h a t , a f t e r the i n i t i a l e x c i t a t i o n by a 266-nm 25-ps FWHM p u l s e , a second 355-nm 25-ps pulse i r r a d i a t e s the same area of the sample. With t h i s two-color f l u o r e s c e n c e technique they were able to observe c l e a r l y the r e a c t i o n dyna­ mics of the r a d i c a l s which were formed by the i n i t i a l l i g h t pulse. The compounds which were i n v e s t i g a t e d by means of the twoc o l o r f l u o r e s c e n c e technique were l-(bromomethyl)naphthalene, 1- (chloromethyl)naphthalene, 2-(bromomethyl)naphthalene, and 2- (chloromethyl)naphthalene. Fluorescence which r e s u l t e d when a p a r t i c u l a r (halomethyl)naphthalene i n hexane was e x c i t e d at 266 nm alone or at 266 and 355 nm was monitored i n e i t h e r of two ways. Emission i n t e n s i t y v s . time f o r a p a r t i c u l a r range of wavelengths was measured by means of an Imacon s t r e a k camera and

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

10.

HILINSKI ET AL.

Picosecond

Spectroscopy

and

Applications

213

P r i n c e t o n A p p l i e d R e s e a r c h (PAR) 1205A OMA d e t e c t i o n s y s t e m . E m i s s i o n i n t e n s i t y v s . w a v e l e n g t h was measured w i t h o u t t i m e r e s o l u t i o n by means o f a p o l y c h r o m a t o r a n d PAR 1205A d e t e c t i o n system. When e x c i t e d a t 266 nm, a l l f o u r compounds e x h i b i t e d a b r o a d b l u e f l u o r e s c e n c e c e n t e r e d a t ^ 0 0 nm w i t h l i f e t i m e s r a n g i n g f r o m 100 t o 3500 p s . E x c i t a t i o n w i t h a s e c o n d 355-nm l a s e r p u l s e w h i c h was d e l a y e d r e l a t i v e t o t h e f i r s t 266-nm p u l s e p r o d u c e d a f l u o r e s c e n c e c e n t e r e d a t ^ 6 0 0 nm w i t h a l i f e t i m e o f VL0 n s . S i n c e t h e e m i s s i o n s p e c t r a ( F i g u r e 7) o b t a i n e d by e x c i t a t i o n o f l - ( c h l o r o m e t h y l ) n a p h t h a l e n e and l - ( b r o m o m e t h y l ) n a p h t h a l e n e a r e s u p e r i m p o s a b l e i n t h e r e g i o n f r o m 550 t o 750 nm, t h i s r e d f l u o r e s c e n c e was a t t r i b u t e d t o t h e 1 - n a p h t h y l m e t h y l radical. I n a s i m i l a r manner, t h e r e d f l u o r e s c e n c e r e s u l t i n g from t w o - c o l o r e x c i t a t i o n o f 2 - ( c h l o r o m e t h y l ) n a p h t h a l e n e and 2-(bromomethyl)naphthalen radical. This two-colo t i o n w i t h OMCDs h a s p r o v e n t o be a p o w e r f u l t o o l i n t h e s t u d y o f p h o t o d i s s o c i a t i o n o f h a l o a r o m a t i c compounds. The V i s u a l T r a n s d u c t i o n P r o c e s s . Picosecond absorption spectrosc o p y w h i c h u t i l i z e s OMCDs a l s o h a s p r o v i d e d i m p o r t a n t m e c h a n i s t i c i n f o r m a t i o n t h a t p r e v i o u s l y was n o t a v a i l a b l e b y means o f o t h e r techniques. D e t a i l e d p a t h w a y s o f a number o f r e a c t i o n s w h i c h a r e important from a p h y s i c a l , c h e m i c a l , and/or b i o l o g i c a l v i e w p o i n t h a v e been e l u c i d a t e d b y means o f t h i s t e c h n i q u e . A recent p i c o s e c o n d s p e c t r o s c o p i c s t u d y by S p a l i n k e t . a l . (30) h a s d e m o n s t r a t e d t h a t a n e x p e r i m e n t a l c r i t e r i o n , w h i c h h a s been u s e d t o s u p p o r t t h e h y p o t h e s i s t h a t c i s - t r a n s i s o m e r i z a t i o n (31) i s the p r i m a r y event i n the v i s u a l t r a n s d u c t i o n p r o c e s s , i s not true. T h i s c r i t e r i o n i s b a s e d on t h e commonly o c c u r r i n g s t a t e m e n t t h a t both the n a t u r a l l y o c c u r r i n g 1 1 - c i s - r h o d o p s i n and t h e s y n t h e t i c 9 - c i s - r h o d o p s i n l e a d t o t h e same p r i m a r y p h o t o c h e m i c a l product, bathorhodopsin. Of c o u r s e , t h e e x i s t e n c e o f a common i n t e r m e d i a t e generated from e i t h e r 1 1 - c i s - o r 9 - c i s - r h o d o p s i n w o u l d s u p p o r t t h e commonly p r o p o s e d mechanism o f c i s - t r a n s i s o m e r i z a t i o n a s t h e primary event i n the v i s u a l t r a n s d u c t i o n process. However, t h e d a t a o b t a i n e d b y S p a l i n k e t . a l . ( 3 0 ) i n d i c a t e t h a t a common i n t e r m e d i a t e i s n o t g e n e r a t e d f r o m b o t h rhodopsins. Using a mode-locked Nd :YAG l a s e r system t o generate p i c o s e c o n d sample e x c i t a t i o n p u l s e s and p i c o s e c o n d p r o b i n g c o n t i n u u m p u l s e s i n t h e i r d o u b l e beam s p e c t r o m e t e r , S p a l i n k e t . a l . (30) were a b l e t o m e a s u r e d i f f e r e n c e a b s o r p t i o n s p e c t r a of i r r a d i a t e d samples o f 1 1 - c i s - r h o d o p s i n and 9 - c i s - r h o d o p s i n a t s e l e c t e d t i m e s a f t e r e x c i t a t i o n b y means o f a PAR OMA-2 o p t i c a l m u l t i c h a n n e l d e t e c t i o n s y s t e m . The d i f f e r e n c e a b s o r p t i o n s p e c t r a l d a t a were o b t a i n e d o v e r t h e e n t i r e s p e c t r a l r a n g e f r o m 410 nm t o 650 nm a t o n e t i m e w i t h a n OMCD a s o p p o s e d t o t h e 3+

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

214

MULTICHANNEL IMAGE DETECTORS

t(psec) Figure 6. Time-resolved fluorescence ofa 1.1 10~ M acridine in hexane excited by a 0.1-mJ, 355-nm laser pulse. The experimental points, which are averagedfrom nine laser shots, are displayed with the computer-generated curves that were calculated to fit fluoresence risetimes, t , and fluorescence decay times, t , to the experimental data. (Reproduced with permission from Ref. 26. Copyright 1980, North-Holland Publishing Company.) x

r

3

f

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

10.

Picosecond

HILINSKI ET AL.

:

Spectroscopy

215

and Applications

v

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>

< St :

—I

500

L-

\

Λ

w

\

1

600

1

(b) Τ ^1

J-J

I

i

L

_

J

I

»

~

70

Figure 7. Two-color fluorescence spectra of the 1-naphthylmethyl and 2-naphthylmethyl radicals. Spectra were produced by excitation of the sample with a 25-ps, 266-nm laser pulse followed by a 25-ps, 355-nm pulse delayed by ~ 60 ps. Key to (halomethylnaphthalenes: a, l-(chloromethyl)naphtha1ene; b, 2-(chloromethyl)naphthalene; c, l-(bromomethyl)naphthalene; and d, 2-(bromomethyl)naphthalene. Note: the fluo­ rescence in the blue region of Spectrum c is due to impurities that contaminated the sample of l-(bromomethyl)naphthalene (28).

o l d e r , tedious wavelength-by-wavelength method. The d i f f e r e n c e absorption spectrum at one s e l e c t e d delay time a f t e r e x c i t a t i o n was c a l c u l a t e d according to equation 1 and i s the average of data obtained from at l e a s t 55 e x c i t a t i o n s by l a s e r p u l s e s . Figure 8 i l l u s t r a t e s the d i f f e r e n c e spectra obtained 85 ps a f t e r e x c i t a t i o n of samples of 9-cis-rhodopsin and 1 1 - c i s rhodopsin at room temperature with 25-ps, 532-nm l a s e r p u l s e s . The time of 85 ps a f t e r e x c i t a t i o n was s e l e c t e d to ensure that only the batho-intermediate was observed.(30) I n s p e c t i o n of both spectra reveals two important d i f f e r e n c e s that had p r e v i ­ ously gone undetected. F i r s t , the s p e c t r a l bands of p o s i t i v e ΔΑ and of negative ΔΑ and the i s o s b e s t i c point which r e s u l t from e x c i t a t i o n of 9-cis-rhodopsin are s h i f t e d bathochromically by VL0 nm r e l a t i v e to the corresponding p o s i t i o n s i n the d i f f e r e n c e spectrum measured 85 ps a f t e r e x c i t a t i o n of 11-cis-rhodopsin. This 10-nm s h i f t to longer wavelengths i s s i m i l a r to the s h i f t observed f o r the ground-state absorption maximum of 9 - c i s rhodopsin r e l a t i v e to 11-cis-rhodopsin (Figure 8 ) . The second d i f f e r e n c e i s t h a t , although both 11-cis-rhodopsin and 9 - c i s rhodopsin e x h i b i t the same amount of decreased a b s o r p t i o n corresponding to ground-state depopulation, the r a t i o of t h e i r absorption maxima i s not 1:1, as should be expected f o r the generation of a common intermediate f rom both p r e c u r s o r s , but 1.5:1. Computer s u b t r a c t i o n of the two negative s t a t i c a b s o r p t i o n spectra from the two d i f f e r e n c e absorption spectra obtained v i a

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

216

MULTICHANNEL IMAGE DETECTORS

li-cis

Rhodopsin

U

< < Oo

0.0

1.2

< *

0J6

0.0

1.2

<
10~ torr) in the channels and gas molecules released from the channel walls during multiplication (22, 23, 28). The ions travel back up the channel and may strike the channel wall near the M C P input face, releasing electrons, which results in a secondary electron avalanche and afterpulsing. 4

4

5

5

5

6

6

The problem of ion feedback has been circumvented in a number of ways. First, if the M C P ' s are baked in vacuum, 'scrubbed' by running them at high current, and not reexposed to air, the problem of desorbed gas from the channel walls is greatly reduced. G o o d results are achieved in this way for sealed tube devices. A t high gains, however, the baking and scrubbing are insufficient to suppress ion feedback, so various geometrical modifications of M C P ' s have been devised. One method is to use M C P ' s with curved channels ((L/d) typically in the range 80:1 —• 140:1), so that ions strike the channel walls closer to their point of origin than in a straight channel M C P . This reduces the size of the secondary electron pulses associated with ion feedback to acceptable levels, allowing gains of up to 2 x 10 to be used. A t these high gains, the pulse height distribution is peaked with a Gaussian type shape and a F W H M on the order of 30% - 60% (30-32, 34). The narrow pulse height 6

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

13.

SIEGMUND AND

MALINA

Detection

of Extreme

257

UV and Soft X-Rays

distribution is a consequence of the limiting of the gain when 'saturated' mode, as described previously.

operated in the

Ion feedback may also be reduced by using a tandem (V) or triple (Z) stacking arrangement of M C P ' s . These configurations are oriented such that there is an angle of about 15° —20° between the channels at the stack interfaces. Ions generated at the stack output face cannot proceed past this bend, and thus produce only small secondary electron pulses. Tandem and Ζ arrangements have operational gains as high as 10 -10 ( L 28> 33), with a gain/voltage curve shape similar to that of single M C P ' s (Figure 4). A pair of 40:1 L/d M C P ' s separated by 25 μ m —• 150 μ m is a commonly used tandem configuration (23, 28). 7

8

The pulse height distributions for a tandem of two 40:1 L/d M C P ' s in this arrangement are poor (100%-200% F W H M ) (23, 28, 31) compared to curved channel M C P ' s . However, by increasing the L/d ratio of each of the M C P ' s to 80:1, or greater, and using each M C P in a near saturated mode, pulse height distributions of 30% — 50% F W H M have been achieved (1_, 25, 27, 33). Figure 5 demonstrates the progressive narrowing of the pulse height two 80:1 (5) M C P ' s placed back to back (no gap). Recent results (27) also indicate that reducing the channel diameter improves the pulse height distribution. The configuration of the interface between the two M C P ' s of a tandem is an important factor in determining the gain/pulse height performance. For a back to back tandem pair of M C P ' s , electrons from the first M C P can enter only a maximum of 3 channels (35) in the second M C P , assuming flat M C P surfaces. If a small gap between the M C P ' s is introduced, the electrons from the first M C P can spread into many more channels of the second plate. This results in an increase of the gain, since more chan­ nels contribute; however, the pulse height distribution is degraded because the channels of the second plate are not as highly saturated. If a field is applied across the gap (23, 2 7 , 36) between the M C P ' s , the gain drops and pulse height F W H M is found to improve. This results because the electrons from the first plate are accelerated into fewer channels of the second M C P . The configuration used in any application depends on the desired gain/pulse height characteristics. It should be noted that the gain of M C P ' s is not constant with accumulated dosage. Lifetests 0 , 12, 25, 29) of M C P ' s have shown that there is an initial sharp drop in the gain, associated with outgassing of the channel walls, followed by a plateau region of stable operation. The behavior of M C P parameters after re-exposure to air (37, 38) indicates reversible increases in gain attributable to adsorption of gas in the channels. For this reason, it is advisable to 'scrub' (or 'burn in') M C P ' s until the pla­ teau region is reached before use. Another frequently-used technique is to bake out (29) M C P ' s in vacuum; however, this can cause an additional permanent drop of the M C P gain. It has been suggested (13) that, in addition to driving off adsorbed gases and water vapour, the bake causes desorption of the alkali metals which are responsible for the secondary emission properties of the M C P . Desorption of alkali metals under electron bombardment may also play a part in determining the overall lifetime of M C P ' s , although there is, at present, insufficient information to characterize M C P life­ time properties completely. Unlike conventional photomultiplier tubes, M C P ' s can be subjected to high mag­ netic fields (>100 G ) without significant changes in gain and pulse height distribution (39). This is attributable to the small size and length of the M C P channels and the high electric fields within the channels. Magnetic fields are, however, a more serious concern in the readout system located behind the M C P .

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

MULTICHANNEL IMAGE DETECTORS

258

Voltage Applied (Each MCP)

Figure 4. Gain / voltage curve for a tandem pair of mi L/d MCPs with d = 12.5 μτη.

A

a.

Figure 5. Pulse height distribution of a tan­ dem pair of MCPs each having L/d = 80.1 anda = 12.5 μm. Conditions (top): gain, 10 ; and FWHM, 58ψ . Conditions (middle): gain, 1.9 χ 10 ; and FWHM, 41%. Con­ ditions (bottom): gain, 4.6 10 ; and FWHM, 32%). 6

0

6

x

6

Number of Events

1

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

13.

SIEGMUND AND

Detection

MALINA

of Extreme

UV and Soft X-Rays

259

Noise characteristics The number of background noise events Q 8 , 23, 29) from both single and tan­ dem M C P ' s is generally very low (< 1 count/cm /sec), but this can still be a significant limitation in applications such as X-ray and E U V astronomy, where very low count rates are experienced. Lower thresholds must be carefully chosen to exclude the low gain exponential tail responsible for a large fraction of the background counts. Most manufacturers now produce M C P ' s which have, on average, dark count rates of < 0.5 counts/sec/cm , and we have found that selected plates can have rates as low as 0.1 - 0.2 counts/sec/cm . Investigation Q ) of the sources of background events sug­ gests that, at sea level, cosmic ray events, intrinsic radioactivity, and thermionic emis­ sion contribute less than 5% to the overall background rate. It has been suggested CO that defects in the M C P channel walls are responsible for the background events through field emission. This correlates with the non-saturated pulse height distribu­ tions obtained for background events using a tandem, indicating that background events are generated all along the channel walls (1). 2

2

2

Other noise phenomena 'switched on' channels, 'hot spots', 'dead' channels, and field emission from mounting hardware. Due to improved selection and production techniques, the incidence of 'dead' channels and 'switched o n ' channels is now relatively low. However, hot spots, producing relatively large numbers of spurious events, are a more common occurrence. Currently this effect is thought to be associated with dust or damage on the front sur­ face of M C P ' s . Certainly our experiences suggest that, if care is taken with cleanliness and handling of M C P ' s , the occurrence of hot spots can be kept very low. T i m e Response and Count Rate Performance A single event electron pulse emerging from a M C P can be extremely short (< 1 ns) in duration (23, 30, 40-42). Due to the high fields and short distances, the transit time of the electrons through a M C P is calculated to be on the order of 250 ps (40:1 L/d M C P ) and the transit time spread to be about 50 ps (42). Experimental results with M C P photomultiplier tubes (PMT's) indicate (23, 30, 40, 41) that pulse widths of 200 - 300 ps can be achieved with single M C P ' s . Fast M C P P M T ' s with Ζ M C P configurations have longer (23, 40, 41) pulse widths (300 - 500 ps), due to the extra length of the channels. Slowing-down effects, caused by the reduction of electron ener­ gies in the saturated mode also contribute to this. The recovery time of a single channel after an event is determined by the rate at which the extracted charge can be replaced. For a typical 1 m m thick M C P of resis­ tance 300 m f t , the channel recovery time t is of the order 20 ms (21, 23), where r — RC (R and C are the effective channel resistance and capacitance, respec­ tively). So, the maximum event rate per channel is —50 counts/sec, and if this rate is exceeded, the channel does not recover fully between events and the gain drops as a consequence. Since an M C P has on the order of 10 - 10 channels, the overall count­ ing rate can be very high ( > 1 0 counts/sec), provided no channel experiences more than —50 counts/sec. Some manufacturers (5, 6) provide M C P ' s with low channel resistance to reduce the channel time constant in applications which require high local­ ized counting rates. c

c

C

C

c

c

5

6

6

For a tandem M C P configuration, a number of channels of the second M C P are excited by a single event in the front M C P . Therefore, neighboring channels of the first M C P will, in effect, cause some of the same channels of the second M C P to be

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

260

MULTICHANNEL IMAGE DETECTORS

excited. So, although the recovery time for a single event is similar to that for a single MCP,

the overall counting rate for the tandem is lower, by a factor equivalent to the

number of channels which a single event excites in the second M C P . Photocathode Materials Photocathode materials are used routinely to enhance the detection efficiency of M C P ' s at soft X-ray and E U V wavelengths. Unlike detectors with optical photocathodes, most M C P detectors for this wavelength range operate in a 'windowless' mode, because there are no suitable window materials sufficiently strong to hold back 1 atm. Since either production of photocathodes in situ or transport of unsealed devices under ultra high vacuum is usually impractical in space instrumentation, photocathodes which are stable under exposure to air are normally selected. Since the photon energies of interest are in excess of 10 eV, photocathode materials with relatively high work functions (a few eV) can be used. Widely used materials include M g F , L i F and C s l . Many other alkali halides ar since many of these compound 2

Photocathodes may be used in two configurations, opaque (or reflection) photocathodes and transmission photocathodes. Detailed calculations of the performance of both types of photocathode may be found in the articles of Henke (43-45). Opaque photocathodes usually are deposited directly onto the input face of the M C P such that the photocathode material penetrates a short distance into the channels. Thus incident radiation which enters the channels strikes the photocathode material, resulting in enhanced detection efficiency. A s in the case of a bare M C P , radiation striking the interchannel web is not normally detected, so electron deflection grids in front of the M C P are sometimes used to further the enhancement of detection efficiency. Transmission photocathodes for E U V and soft X-ray applications can be deposited on very thin substrates such as parylene, polypropylene, lexan, aluminum, or even fine mesh grids. This photocathode assembly is normally used in a proximity-focussed configuration mounted close to the M C P input (300 μ,ηι —• 1000 μ,ιτι). Electrons emit­ ted from the photocathode are then accelerated to the M C P by a high electric field (500 — 1000 V/mm). One of the limitations of this configuration, however, is that the pho­ toelectrons spread laterally in the photocathode/MCP gap, resulting in a practical limita­ tion of —50 μπγ F W H M for the position resolution (33, 46). In general, transmission photocathodes also show lower quantum yields (electrons/incident photon) than opaque photocathode configurations. Cesium Iodide Cesium iodide has received much attention recently as a potential photocathode material (17, 47-55) by virtue of its high quantum efficiency in the soft X-ray and E U V regions. A compilation of C s l efficiency data in the range 10 A - 2000 A for a number of different configurations is given in Figure 6. (Grazing angle of incidence is indi­ cated). The measurements of Fraser 0 7 ) , Martin et ai 0 5 ) , and McClintock (47) were taken with opaque C s l coatings on the surface of M C P ' s . However, the Q D E ' s meas­ ured by Fraser refer to efficiency of C s l in the channels alone, since the electrons emit­ ted from the interchannel web were not collected. O n the other hand, in the measure­ ments of McClintock and Martin et al. electrons from the interchannel web were col­ lected with an electric field. In the latter case 0 4 ) , the channels were found to contri­ bute little (—20%) to the overall quantum efficiency. This was because the C s l coating inside the channels was too thin (»FM-TM

Figure 5. (a) A schematic diagram of the rocket-borne spectrometer designedfor measuring the rotational profile of the Α-band absorption spectrum of O, molecules. Key: II, image intensifier; IS, photodiode array; CG, clock generator; CC, clock controller; and P D, photodiode. (Reproduced with permission from Ref. 11. Copyright 1983, American Geophysical Union.) (b) A photograph of the rocket-borne spectrometer. Key: 1, polychromator; 2, airtight chamber; 3, electronics; 4, position of the entrance slit of the polychromator; and 5, entrance of the optical system that introduces the solar light into the polychromator.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

306

MULTICHANNEL IMAGE DETECTORS

Figure 5. (c) The spectrometer, window for the polychromator;

set in the nose cone of the rocket. Key: I, entrance and 2, entrance window for the triggering optical system.

Figure 6a. Photograph

of the S-310-11 rocket.

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

15.

Rocket,

MATSUZAKI ET AL.

Balloon,

Figure 6b. Photograph

I_i 755

I

ι

ι

•

• 760

and Spacecraft

Experiments

of the S-310 rocket launch.

•

•

ι

ι

ι 765

ι

ι

ι

υ

λ / nm Figure 7. Examples of the solar radiation spectra measured in the S-310-11 rocket experiment. Key: 1, measured at 24.8 km; and 2, 190 km. (Reproduced with permission from Ref. 11. Copyright 1981, American Geophysical Union.)

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

307

MULTICHANNEL IMAGE DETECTORS

308

λ" / 10" .nm" 4

U

A

Figure 8. Plot of the [ln{\ (kj/lfk,)} - //ifl/XJ/I/X, )}] value as a function of the (λ; X ) value. Here, Ifkjand \/X)are the signal intensities of Spectra 1 and 2 of Figure 7, respectively, at a wavelength λ = 755 nm. (Reproduced with permission from Ref 11. Copyright 1983, American Geophysical Union.) 4

4

Figure 9. The absorbance spectrum obtained from the plot in Figure 8. with permission from Ref. 11. Copyright 1983, American Geophysical

(Reproduced Union.)

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

15.

MATSUZAKI ET AL.

Rocket,

Balloon,

and Spacecraft

309

Experiments

c a l c u l a t i o n has been p r e v i o u s l y d e s c r i b e d (11). An example o f the spectra obtained from these t h e o r e t i c a l c a l c u l a t i o n s i s shown i n Figure 10. To achieve a best f i t between the observed and c a l c u l a t e d spectra the absorptions o f t h e i r peaks were compared. An example to such a comparison, f o r a temperature o f 220 Κ i s shown i n Figure 11. The b e t t e r the c a l c u l a t e d spectrum repro­ duces the experimental one the more c l o s e l y the v a r i o u s peaks a l l i g n e d with the l i n e a r r e l a t i o n s h i p curve. S i m i l a r p l o t s were produced f o r v a r i o u s temperatures and standard d e v i a t i o n values 1. e., σ=/Σ[ ( f ^ - f ) , where (£*i-f) are the d e v i a t i o n s from the l i n e a r i t y , were c a l c u l a t e d by u s i n g the l e a s t squares method. Evaluated standard d e v i a t i o n s as f u c t i o n o f temperature are shown i n Figure 12. From the minimum value o f standard d e v i a t i o n , the r o t a t i o n a l temperature o f "spectrum l i n Figure 2 i s c a l c u l a t e d t o be 218±1.5 K. The temperature obtaine 21 km and agrees with tha plements, 1966, as compiled i n the "Handbook o f Geophysics and Space Environments". T h i s means, that t h i s a l t i t u d e , the 0 2 ( Σ ^ ) molecules are i n thermal e q u i l i b r i u m with ground s t a t e N 2 mole­ c u l e s ; the c o l l i s i o n a l d e a c t i v a t i o n i s f a s t e r than a c t i v a t i o n o f oxygen photochemical processes (12). Thus, we were able to confirm that the present spectrometer measures the e f f e c t i v e spectrum necessary to determine r o t a t i o n a l temperatures with a s a t i s f a c t o ­ ry accuracy. The r o t a t i o n a l temperatures o f the 0 (^Σσ) molecule, at other a l t i t u d e s were s i m i l a r l y obtained, fhe f i n a l report w i l l be presented elsewhere (13). z

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2. S t r a t o s p h e r i c Aerosol Measurements Using the CCD-Based Spectrometer Balloon experiments can be e f f e c t i v e f o r studies o f photo­ chemical r e a c t i o n s o f s t r a t o s p h e r i c molecules. Spectrometer 3, Table I, was used to obtain a l t i t u d e p r o f i l e s o f s c a t t e r i n g coef­ f i c i e n t s o f s t r a t o s p h e r i c a e r o s o l s . The spectrometer i s shown s c h e m a t i c a l l y i n F i g u r e 13. C o r r e c t s o l a r l i g h t i l l u m i n a t i o n o f the spectrometer was controled by combination o f azimuth and zenith c o n t r o l l e r s . A J o b i n Yvon H20IR monochromator (F/3.5, f=200 mm, 600g/mm g r a t i n g ) without i t s e x i t s l i t was used as the polychromator. Wavelength c a l i b r a t i o n was performed using a He-Ne l a s e r and a Hg lamp as r e f e r e n c e sources. The multichannel imager was a 2048-element l i n e a r self-scanned charge-coupled device, NEC model yPD 792. Video s i g n a l was d i g i t i z e d (8 b i t converter), temporarily stored i n memory and then t r a n s m i t t e d to ground, at a r a t e o f one spectrum per second, by an FM (frequency modulation) analog telemeter. The scanning r a t e o f the CCD was 204.8 elements/ms. The b a l l o o n borne spectrometer, b a l l o o n B15-IOO, was launched from the Sanriku Balloon Center (SBC, 39° 09.5 »N, 141°49.5'E) at 6:30 a.m. JST on May 31, 1979. The balloon a l t i t u d e was 22 km when i t was i n l e v e l f l i g h t . The experiment

In Multichannel Image Detectors Volume 2; Talmi, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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