Construction and focusing of the U.S.C. normal incidence: 1 meter vacuum spectograph

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CONSTRUCTION AND FOCUSING OF THE U. S. C. NORMAL INCIDENCE, 1

METER VACUUM SPECTROGRAPH

A Thesis Presented to the Faculty of the Department of Physics University of Southern California

In Partial Fulfillment of the Requirements for the Degree Master of Science

by Arthur Wayne Ehler June

19^0

UMI Number: EP63351

All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion.

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This thesis, w ritte n by

Arthur Wayne Ehler under the guidance of h .X 3 ... F a c u lty C o m m ittee, and app ro ved by a l l its members, has been presented to and accepted by the C o u n cil on G ra d u ate S tudy and Research in p a r t ia l f u l f i l l ­ ment of the requirements f o r the degree of

Master of Science

Date.

Faculty Committee

Chairman

ACKNOWLEDGMENTS

I am indebted to Professor G. L. Weissler for his guidance throughout the progress of this work, and to the Office of Naval Research for a twelve months research assistant ship.

TABLE OF CONTENTS

CHAPTER

PAGE

I. INTRODUCTION ................................

1

II. GENERAL DESIGN FEATURES OFTHE NORMAL INCIDENCE VACUUM SPECTROGRAPH........................

5

III. THE VACUUM S Y S T E M ...........................

13

IV, DETAILED DESIGN FEATURES OFCOMPONENT PARTS OF THE SPECTROGRAPH..........................

l6

The Grating and its H o l d e r ........... . . .

16

The Plateholder . .

20

. . ..............

The S l i t .................................

25

V. FOCUSING PROCEDURE AND RESULTS ..............

2?

Focusing Procedure ..................... Results............ VI.

.

27 28

SUMMARY AND CONCLUSIONS......................

32

BIBLIOGRAPHY.....................................

35

LIST OP FIGURES FIGURE 1. Main Spectrograph T u b e ........................

PAGE

9

2. Spectrograph and Pumping System.............. ..

1

3.

17

Grating Support Assembly .................... .

Ij.. Grating Support Assembly......................

18

5. Plate Holder Assembly .........................

21

6. Plate Holder Assembly .........................

22

7. Focusing P l a t e .............................

29

CHAPTER

I

INTRODUCTION Various mechanisms contribute in greater or lesser extents to the formation and maintenance of many types of electrical discharges through gases, such as sparks, coronas, Geiger counter action, Geissler discharges, lightning, and even the conductiong layers in the upper atmosphere, and many others too numerous to mention, A group of such mechanisms is concerned with the actions of shortwave length photons as they interact with matter in either a gaseous or solid state.

Very little is

known at present about such processes as their absorption in gases, their photoionization cross sections in gases, and their photo-electric efficiencies for some metal surfaces. Attempts are underway to measure some of these quantities by various ingenious experiments involving micropulsetechniques in specific types of discharges, such as the excellent work In progress at the Bell Telephone Labora­ tories, at Westinghouse, at Northwestern University, at the 1 University of California in Berkeley, and others.

1 See, for instance, the abstracts of contributed papers to the ’’Conference on Gaseous Electronics,11 in Pitts burgh, November, 19^4-9• Private communication from Dr. D.

2

A long term project is underway at the University of Southern California to make measurements on the interaction of such photons with matter in a direct manner employing vacuum spectroscopic techniques*

This method has the ad­

vantage of gaining data which are not dependent on numerous other gas-discharge paremeters as for instance in the work cited*

It is therefore more generally applicable and also

open to use by researchers outside the field of gaseous electronics• In following this type of program, a grazing incidence vacuum spectrograph has been constructed with a 15,000 lines per inch 2 meter radius of curvature grating, and measurements on the absorption coefficient in N2 in the wavelength range of from 600 to 1300 A* U* have been completed*

2

In order to obtain data on the above mentioned quanti­ ties such as photoelectric efficiencies of metal surfaces and photoionization cross sections in gases, each with photons in the vacuum U. V* range, a normal incidence vacuum spectro­ graph seemed to be most apt*

For mechanical reasons an

Alpert, Westinghouse Research Laboratories. ^ Technical Report No. 1 (inpreparation), Office of Naval Research, Contract No. N6onr-238, Task Order VI, (Spring, 1950).

3

an instrument of this type is most adaptable for this kind of work, particularly if constructed in such a way as to make possible its use as a vacuum monochromater with an exit slit, A considerable number of normal incidence vacuum spectrographs have been described in the literature and only some of the more pertinent references are given here.3 Only one vacuum monochromater which has been recently con1,

structed has come to our a t t e n t i o n , a n d this instrument can only be used as such, not as a spectrograph. It is the purpose of this report to describe in detail an instrument which embodies all of those features of design that have been recognized by past experiences as most desirable in spectrographs of this type, including monochromator action,

3 J, C, Boyce, Rev. of Kod. Phys, 13,1 (194-1) Harrison, Lord and Loofbourow, Practical Spectroscopy (New York: Prentice-Hall, Inc,, 1§4-^)f Chap, 19> P^ 5>31• R. A. Sawyer, Experimental Spectroscopy (New York: PrenticeHall, Inc., 191j4) * Hans Bomke, Vacuumspectroscopie (Leipzig: Verlag von Johann Ambrosius Barth, 19371 also Ann Arbor: Edwards Brothers, Inc., 19l}i^).

h

3

violet*

T. Lyman, The Spectroscopy of the Extreme Ultra

(New York:

Longmans, Green, and Company, 192b)•

By private communication with Baird Associates, Cambridge, Massuchetts: John Richardson, Ultraviolet Vacuum Monochromator, October, 28, 19^7> (Description and blue prints)•

CHAPTER

II

GENERAL DESICN FEATURES OF THE NORMAL INCIDENCE VACUUM SPECTROGRAPH Due to the astigmatism of the concave grating (15,000 lines per inch and 1 meter radius of curvature), must decide on an angle of incidence must be determined for which the astigmatic image of the primary slit is a minimum.

This is done so that the loss of intensity of

the diffracted light from excessive spread of the image, is held to a minimum. £

Using the tables and data worked

out by Dieke and Beutler

A

an angle of incidence of 13-5

(near normal incidence) was chosen.

This resulted in an

astigmatic image of about !+mm in length at 1000 A. U. for a point source.

With the short length of the primary slit

used in work involving a Lyman continuum, this proved about right in order to get a uniform exposure of that width on the photographic plate.

A schematic diagram of

the spectrograph is presented in Figure 1.

The main tank

^ G. H. Dieke, Jour. Opt. Soc. Am., 23,271+ (1933)5 see also Harrison, Lord and Loofbourow, op. cit.,p. 91* ^ (19^-5)*

H. G. Beutler, Jour. Opt. Soc. Am., 35? 32l+

n

6

containing grating and plateholder was constructed from round, seamless copper tubing of 12 inches inside diameter and I;, feet length*

This large diameter was chosen pur­

posely because it seemed to yield very definite advantages over smaller tubing*

The two most important ones are;

(a)

that the spectrograph could be focussed visually since the wavelength range in the first order extended from the central image into the blue visible region or about l^OO A.# U*

Since the plate factor of this instrument (also

called the reciprocal of the linear dispersion)

is very

nearly uniform and about 17 A* U. per millimeter, a wave­ length range of lf5>00 a . U. Corresponds to about 26*5 cm or 10*5 inches thus giving us a comfortable margin of 3/ij. inch each between the position of the central image and the tank well and between the !|500 A. U. position and the tank well.

This extension of the wavelength range to Ij.500 A. U.

is not only important for rapid alignment and focussing, but it also makes it possible to link the results obtained in the vacuum u. v. with the work of earlier investigators in the quartz u. v. and visible range of the spectrum, (b) the second important advantage of such a large diameter spectrograph tank is that a more complete and deeper lightbaffling system could be installed in order to reduce the

7 intensity of scattered light. In addition, of course, the bigger size makes it much easier to work on mechanical installations Inside the tube.

Thus the increased cost of a larger tube seemed

amply justified by the outlined advantages, particularly since the machine shop work was not judged to be increased in this case. The spectrograph is equipped with two important features which afford a considerable saving of time:

the

plateholder, Pig. 1G, (capable of eight exposures of [jjnm width each, to be described in detail later)

can be iso­

lated from the rest of the spectrograph by means of a vac­ uum tight flap-valve, Fig. IK.

This means that successive

plates or films can be taken without having to break the vacuum in the main tank.

All that needs to be done once a

plate has been completely exposed is that the flap-valve be closed, tne plateholder volume be let down to atmospheric pressure and the tank end-plate-door be opened to insert a new plate.

Then this volume can be roughed out with a fore­

vacuum pump, (Fig. IN),

the flap-valve opened, and a new

series of exposures can be started. Secondly, a It inch bronze gate valve, Fig. IE, with rubber gasket and Wilson seal for high-vacuum use, was installed between the primary slit, Fig. IB, and the

8

grating, Fig* 1M.

This makes it possible to either clean

the slit jaws of dust or change light sources, Fig* 1A, again without having to break the vacuum in the main tank* A pump-out, Fig* 1C, between slit and valve connected temporarily to the exhaust of the light source enables one to rough out both the source and the volume behind the slit before opening the valve to resume operation*

The valve

also serves as an optical shutter during the break-in period of the light source. In addition to a 6 inch, flanged T-joint connected to the main exhaust line, Fig* 1L, three others like it, Fig. 1J, were screwed and soldered to the main tank in order to provide for cold traps, pressure gauges, gasleaks, etc*

The entire inside of the spectrograph, in­

cluding the si it-tube, Fig* IS, and the light-baffling system, Fig. ID, but excluding the grating and plate-holder, was painted with a dull flat black paint which was made by mixing appropriate amounts of ordinary lamp-black with clear glyptal. lamps.

All painted surfaces were baked with infra-red The pumping speed of the diffusion pump was such

that before painting an ultimate vacuum of 2 x 10

-6

^

Hg

was reached, whereas after painting and in about the same / length of time an ultimate vacuum of 5 x 10 mm Hg was

To p

V iew

S ide

View

MAiN A

Sc

SPECTROGRAPH a l e

TUBE

10 obtained. On flanges, doors, and other vacuum joints, double gaskets with pumpouts in between were used uniformly in order to facilitate leak hunting.

It is interesting to

note in connection with the above cited figures on ultimate vacua, obtained with several carefully calibrated gauges at the tank, that the total number of gasket joints for the entire system, excluding valves and Wilson seals, is about twenty. It is contemplated to install into the plateholder, just in front of the film emulsion, an exit slit of about 1/10 to 1/2 mm width.

The wavelength range passed by this

exit slit can be determined accurately by placing a film into the plateholder and taking an exposure showing emis­ sion lines within a centimeter on either side of the blacken­ ing due to the radiation passing through the exit slit,

A

tube with a second slit is then mounted in the end plate of the spectrograph tank and placed closely to the exit slit and carefully aligned with it.

It will then be possible

to mount in the region P, Pig. 1, either a positive ion 7 space charge detector for measurements of photo-ionization

7 Technical Report No. 3, ONR Contract N6onr-238, Task Order VI, February 1, 195>0.

11 cross sections in gases as a function of wavelengths or a photomultiplier arrangement for the measurements on photo­ electric efficiencies of surfaces.

In conjunction with

each one of these experiments one must use of course a sensitive and absolute radiation wattage indicator in order to obtain the total number of photons per second, which passes through the

exit slit. The wavelength of the exit

slit or, in true monochoinator

fashion keepingthat slit

fixed, by rotating the grating about a vertical axis (to be described in detail later). For the photo-ionization work it will be necessary to fill the region

F, Fig. 1, with the gas under investi-

gation, say N£, at

a pressure of roughly 10

mm Hg.

Even though the exit slit will be relatively narrow, it must be remembered that under such conditions gas will enter the main spectrograph tube from two sources:

(a)

from the light-source through the primary slit, and (b) from region F through the exit slit, in addition there will be considerable de-gassing from the black painted surfaces.

It is for these reasons that a high-speed, 11

inch, 2-stage oil-diffusion pump is used in conjunction with a manifold of an average internal diameter of 6 inches. The free end of the slit-tube has been provided with a strong flange equipped with concentric double gaskets

12

in grooves.

To this can be bolted either a glass light-

source chamber with a Pyrex pipe flange on one end, such aa a Lyman source, or a source chamber constructed out of metal, such as a high-current carbon arc operated in a vacuum in a magnetic field* The entire spectrograph is rigidly mounted on a strong, cast-iron table which is bolted to the concrete floor.

With all pumps in operation no vibrations can be

felt and the spectral lines are sharp.

CHAPTER

III

THE VACUUM SYSTEM A. photograph of the spectrograph (with the plate­ holder end toward the observer) and associated pumps and manifolds is reproduced in Fig, 2.

The exhaust flange of

the main tank, Fig, 1L, is bolted to a manifold, Fig. 2A, 6 inches in diameter and two feet in length which contadns at its center a cold trap, Fig. 2B.

This manifold connects

then to a right angle valve, Fig. 2C, with an opening of 6 inches which can isolate the diffusion pump from the spectrograph.

The valve, in turn, sits on top of a baff­

le tower, lLj_ inches high and 12 inches in diameter, Fig. 2D.

The baffles are a series of copper disks and annular

plates with copper tubing soldered to them.

A t H, P.

motor driven refrigeration unit cools these baffles to about -i|.50C using Freon-22 as refrigerant.

This arrange­

ment proved to be very effective in preventing backstreaming of oil vapors from the 11 inch throat, 2-stage diffusion pump, Fig. 2E. backing unit.

A small Kinney pump, Fig. 2F, is used as It is possible to isolate the diffusion

pump, by two valves, Fig. 2C and 2G, and rough-out the tank through a by-pass valve, Fig. 2H.

The fore-pump sits

PUM PING

on foam rubber pads and is secured to the floor with rubber-clad bolts thus minimizing the transmission of vibration to the floor and thereby to the spectrograph* For the same reasons a flexible metal bellows is inserted between the Kinney and the diffusion pump.

Measurements

were taken on the pumping speeds as a function of pressure at two points in the system and were found to be more than satisfactory for our purposes*

A conventional, separate

vacuum system protects the instrument, gauges, etc. in case of power failure, vacuum leaks, and water cooling breakdown*

CHAPTER

IV

DETAILED DESIGN FEATURES OF COMPONENT PARTS OF THE SPECTROGRAPH The grating and its holder.

The grating was manu­

factured by the Department of Physics of Johns Hopkins University,

It is ruled on aluminum deposited on a concave,

Ij. inch diameter glass blank of 100.2 cm radius of curvature. The ruled surface is about 1.5 x 3*6 inches with l5>000 lines per inch.

It is an ordinary grating, not lightly

ruled, and it does not concentrate light in a given order for a given wavelength range. For the sake of easy focusing and general versatility such as monochromator use, a rather elaborate grating holder was constructed along the general lines suggested in the Q

literature.

The design presented here, Fig. 3 and I}.,

includes six degrees of freedom, three of translation and three of rotation.

Translation was provided in the direct­

ion of the radius of the Rowland circle as seen in Fig. 3D

®

Harrison, Lord and Loofbourow, op. cit., p. 109* H. G. Beutler, op. cit., p. 3 H * R. A. Sawyer, op. cit., p. 291* R. A. Sawyer, J*_ JD. S^ 15 > 305 (1927).

r

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Q

Vie w

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i l it 11 11_ 1 H H

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G qatin Q

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-El M m

Secti on

Fig. 3

A-A Ge

a t in g

Su ppo et A Scaue : Fuu_ &2H

s s e m b ly

Re f e r e n c e : P m y s ic a .2 .1 2 5 -1 4 3 ,1 9 3 5

owe

3

t

ii

G r a t in g

Support

A ssem.

F ig.4-

19

and IpD, in the direction tangential to the Rowland circle, Fig. 3E and IpE, and perpendicular to the plane of the Rowland circle, Fig. 3F.

Rotation about three mutually

perpendicular axes was arranged such that the single inter­ section of these axes fell within 0.1 mm of the center of the grating.

A ten degree control was provided to rotate

the grating about an axis normal to its face in order to make the ruling parallel to the primary slit and perpendi­ cular to the plane of the Rowland circle, Fig. 3A and IpA. Rotation about an axis in the plane of the grating, parallel to the plane of the Rowland circle was necessary to make the grating surface perpendicular to the Rowland circle plane, Fig. 3C and IpC.

Finally a rotation about an axis

in the plane of the grating but perpendicular to the Rowland circle, Fig. 3B and 1]_B, makes it possible to adjust the position of the central image such as to coincide with the axis of rotation of the plateholder, Fig. 5A and 6a. This last rotational motion can also be used within a wavelength range of about IpOO A. U. for monochromator action: with a fixed exit slit, one of the control-screws, Fig. 3B and Ij.B, is connected to a shaft which passes through a Wilson seal in the end plate of the main spectrograph tank and can then be driven slowly from the outside.

This ro­

tation of the grating will change the wavelength of radiation

20

passing through the exit slit* All the above mentioned motions can be locked in order to prevent slipping due to accidental vibrations. The various ways, screws, and shafts were lubricated with Litton molecular lubricant, type C or Apiezon oil.

Both

oils have a very low vapor pressure and are also used in diffusion pumps. The plateholder.

Drawings and a photograph of the

plateholder are shown in Fig. 5 and 6 respectively.

The

assembly is mounted on a 2 inch wide copper ring, B, split at the top, which can be slipped into the main spectrograph tube and secured in the desired place by pressing the ring against the walls using screw, G.

Fastened solidly to the

ring are two pieces, N on the left, and P on the right, which support the main plateholder-support assembly, J. This assembly has two ways in which slides the actual plateholder, F, up and down in order to obtain eight dif­ ferent exposures on the same plate.

The plateholder-sup­

port, J, pivots about a shaft, A, which is an integral part of support post, P. support, J, rests on N. point, the first one:

The moving end of the plateholderTwo screws are provided at this

to make J rotate through small angles

about A in order to make the precision milled surfaces, C,

F ig 5

PLATE HOLDER ASSEMBLY FULL

SIZE

OWS. 5

23

of the actual plateholder, coincide with the Rowland circle, and the second one:

to lock the support, J, in place once

the correct position for sharp focus has been attained. The above referred to surfaces, C, which define the position of the photographic emulsion, are located in such a manner that the axis, A, just touches G (Pig. 5).

This is the

place where the central image of the primary slit should appear sharply in focus provided that all three:

slit,

grating and axis, A, are tangent to the Rowland circle. The plateholder surfaces, C, have a radius of curvature equal to that of the Rowland circle:

$0.1 cm, so that only

a small rotation of J (and taereby F and C)

about A is

necessary in order to have the plate or film-defining surfaces, C, coincide with the Rowland circle.

As a con­

sequence, the entire spectral range of the instrument will be sharply in focus. The bottom surface, C, has a ledge on it so as to facilitate the placement of the film or plate.

Since it is

necessary to hold the film accurately in position, a backing plate, B, made of aluminum and also milled to fit the Row­ land circle, presses the film against C.

The plate, B, has

J inch holes, K, drilled into it to make it lighter and swings like a door about a pin, L; it can be taken off the

2k

assembly by simply pulling out the pin. Screw, R, can rotate in a bearing in the bottom part or the plateholder-support, J, but it can not move longitud­ inally.

The upper portion of this screw is inside a long

nut which is an integral part of actual plateholder, F. When the screw turns,

F will move up or down on the ways

inside J andenable one to take several plate*

exposures on one

The lower end of the screw terminates in a slotted

disk which is coupled to a similar disk, with a pin in it, on the upper

end of a shaft.

Wilson seal,

Fig. IP, to the outside of the spectrograph

and can be turned from there.

The shaft passes through a

The slotted and pinned disks,

H, are necessary to transmit rotation from the shaft to the screw since their respective axes may be displaced parallel to each other by as much as i inch.

In this way one ac­

complishes a total vertical motion of the plateholder, F, of If inch, sufficient for 8 exposures of Ipnm width each. This width is not only limited by the astigmatism (not uniform over the length of the plate)

but more so by a

diaphragm, S, placed in front of the plateholder and at­ tached to the plateholder-support, J, which remains in a fixed position after focusing has been completed. A black-body box can be attached to diaphragm, S, in order to absorb the large amount of radiation within

25

the central image and avoid scattered light reaching the photographic plate* The slit*

Various designs of slits are used in the

many spectroscopy laboratories through out the world* However, it still seems worthwhile to point out some of the special features which have proven themselves to be desirable in the vacuum spectrograph* In order to adjust more easily for the correct angle of Incidence it helps to be able to move the center of the slit by small amounts to either side.

This is accomplished

by having one slit jaw slide independent of the other one in accurately machined and lapped ways.

The screw controll­

ing the motion of each jaw has a calibrated disk attached to it such that each division on the disk indicates a trans­ lation of the jaw by 0.01 mm.

Occasionally greater accuracy

makes it necessary to estimate between such divisions which is facilitated by a Vernier scale giving a theoretical change of 0.001 mm or 1 micron per Vernier division.

Since

the jaws can be moved from the outside without breaking the vacuum of the light source chamber, this control device may also be used in density versus intensity calibration of the photographic plate:

similar to the operation of a step-

slit used for such calibrations in prism spectrographs one

26

can vary here the relative intensities of radiation passing through the slit by changing its width by known amounts, The slit jaws were made of stellite, a very hard tungsten-alloy steel, so that they could be cleaned with­ out damaging their sharp edges« In order to make the slit perpendicular to the plane of

the Rowland circle one should provide for rotation

about an

axis normal to the plane of the slit jaws*

This

was accomplished by means of a collar which is attached to the rotating part of the slit and carries two screws opposing

each other* These screw up against a metal stop

attached to the part of the slit assembly which does not rotate*

Thus, by loosening one screw and tightening the

other, one can rotate the slit through small angles in either direction. It is an additional convenience to be able to move the slit in a controllable fashion by small amounts toward or away from the grating.

This was effected by means of a

knurled disk which acts against a fixed collar around the moveable slit tubing.

A full clockwise rotation of this

knurled disk will move the slit jaws longitudinally toward the grating by 0*025 inches. (out of a total of 25 ) inches.

Rotating it only through one

division translates the slit 0.001

CHAPTER

V

FOCUSING PROCEDURE AND RESULTS FOCUSING PROCEDURE From the theory of the concave grating it follows that the subsequent requirements must be fulfilled in order that the spectrograph be in focus over the entire wave­ length range: a) The center of the grating, the slit, cassette (plateholder)

and the

must lie on the Rowland

circle• b) The distance from the pivot point of

the cassette

to the center of the grating must be equal to the distance from the center of the grating to the slit, c) The grating normal should lie in the plane of the Rowland circle, and pass through its center. d) The lines of the grating, the length of the slit, and the photographic plate (or the cassette sur­ faces)

must be perpendicular to the plane of the

Rowland circle. Conditions (a) to (d)

were realized, not necessarily in

sequence, in the following way:

the spectrograph tank was

28

first leveled and the grating normal was made horizontal and in approximately the right direction by using a cathetometer.

Then an aluminum template was used to line up the

slit and the plateholder so as to lie on the Rowland circle. This template was J inch thick, about four inches wide, and about 19 inches long and curved.

One of its long sides was

accurately milled to the radius of curvature of the Rowland circle with two sharp lines on it to indicate the positions of the slit and the central image, Fig. 5A, as calculated for an angle of incidence of 13*5°.

Then it was only neces­

sary to obtain a sharp central image at the pivot of the plateholder by fine-adjustment of the grating. was observed with a 10-power magnifier.

This image

A similar procedure

was used on the lines in the visible region of the spectrum of a Hg-arc.

The final adjustment for sharpest focus was

made photographically by turning the screw, Fig. 5D, and this was checked over the entire length of vertical plate­ holder travel. RESULTS Fig. 7 is the reproduction of a series of exposures taken with a quenched A. C. spark discharge through a cap­ illary filled with helium at about one mm of Hg.

The lines

were identified by comparing the ratio of the separation

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of the lines on the focusing plate in mm, as measured by a Hilger precision comparator, to the ratio of separation in o A. U. of the lines in a standard wavelength table.7 Only lines of possible elements were considered.

When tie

ratios of four or five lines on the plate compared to the ratios of the wavelengths of four or five lines in the tables then these lines were tentatively identified.

As

more lines were checked with the tentative identification as the standard, the reliability of the identification was checked.

Identification in the short wavelength region

was simplified because of fewer lines and because of lack of molecular bands.

It was more difficult to identify

lines in the long wavelength region for two reasons:

(a)

because of the complexity of the lines superposed bv a large variety of molecular emission bands and (b) of a low plate factor (16.9 A. U. /mm).

because

Since one is

interested mainly in the vacuum ultraviolet, it was not

9 J. C. Boyce, J. T. Moore, "Provisional Wavelength Identification Tables for the Vacuum Ultraviolet,” (by private communication from the George Eastman Research Laboratory of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, 19^-1)* J. C. Boyce, H. A.. Robinson, Jour Opt. Soc. Am., 26, 133 (1930). ---

31

considered necessary at this time to work in this region except to make sure of sharp focusing.

It can be seen

that from Figure 7 that the entire wavelength range of this instrument (I|Jj.OO A. U. to 370 A. U. ) is simultan­ eously in focus. The light source used in this exposure was a dis­ charge in helium (at one mm Hg)

through a quartz capillary,

two mm I. D. and 25mm long, viewed end on by the slit. Power was supplied to the tube electrodes over a series air gap of eight mm separation by 15,000 volts A. C. with a .017 microjarad condenser across the secondary winding of the high voltage transformer. The first exposure in Figure 7 (top)

was of eight

minutes duration which overexposed the visible and near ultraviolet regions but showed sharpness of focus In the far ultraviolet region.

The last exposure (bottom)

lasted

only fifteen seconds which was not enough to bring out the vacuum ultraviolet spectrum but sufficient to show the visible lines in focus.

The intermediate exposures demon­

strate sharp focus for the wavelength ranges in between these extremes.

It is concluded from this that the instru­

ment performs satisfactorily over the entire spectral region for which it was designed.

CHAPTER

VI

SUMMARY AND CONCLUSION The general design features of a normal incidence vacuum spectrograph are described with particular reference to the use of this instrument on problems in the field of gaseous electronics*

A grating with 15>000 lines per Inch

and a 1 meter radius of curvature is employed*

The wave­

length range in the first order extends up to \\$00 A. U* which makes it possible to focus the instrument in the visible and also provides for a link between new results obtained in the vacuum u* v* and older ones In the quartz, u* v* or visible regions.

Convenient vacuum valves have

been incorporated in order to separate the slit and the photographic plateholder from the main spectrograph tank. This allows one to work in air on those parts without having to break the vacuum In the tank.

A plateholder has

been constructed with surfaces milled accurately to fit the Rowland circle.

It is capable of providing for 8

separate exposures on a plate, each Ipnm wide.

The grating

holder has six degrees of freedom, three of translation in directions mutually perpendicular and three of rotation about axes also mutually perpendicular.

The rotational

motion about that axis which is perpendicular to the plane

33

of the Rowland circle can be used for monochromator action of the instrument in conjunction with a fixed exit slit. A slit with hard stellite jaws is described which has three accurately controllable motions:

each jaw can be individ­

ually moved, the slit can be rotated about an axis perpend­ icular to its plane, and the slit can be moved longitudin­ ally toward or away from the grating.

Detailed design

features of the plateholder, slit, and grating holder are given.

The focusing procedure is described and the operat­

ion of the instrument is demonstrated with a reproduction of a line emission spectrum.

BIBLIOGRAPHY A.

BOOKS

Bomke, Hans, Vacuumspectroscopie» Leipzig! Verlag von Johann Ambrosius Barth, 1§3? $ also Ann Arbor: Edward Brotriers, Inc., 19^1* 2I4.8 pp. Harrison, G. L*, R. C. Lc d, J. R. Loofbourow, Practical Spectroscopy. New York: Prentice-Hall, Inc•, 605 PP* Lyman, T., The Spectrosopy of the Extreme Ultraviolet. New York: Longmans, Green, and Company^ 1928 . 135 PP* Sawyer, R. A., Experimental Spectroscopy. Prentice-Hall, Inc., l^IjljT. 323 PP* B.

New York:

PERIODICAL ARTICLES

Beutler, H. G., Journal of the Optical Society of America, 35, 32J4. (19l4.Fr: Boyce, J. C., Reviews of Modern Physics, 13, 1 (19^4-1 )• Boyce, J. C., H. A. Robinson, Journal of the Optical Society of America, 26, 133 (193&)* **" Dieke, G. H., Journal of the Optical Society of America, 23, 27k- d W T Sawyer, R. A., Journal of the Optical Society of America, 15, 305 (19277: C.

OTHER PUBLICATIONS

Technical Report Nos. 1 and 3, ONR contract N6onr--238, Task Order VI, February 1, 1950.

36

D.

UNPUBLISHED MATERIALS

Abstracts of contributed papers to the "Conference on Gaseous Electronics" in Pittsburgh, November, 19^1-9• Private communication from Dr. D. Alpert, Westinghouse Research Laboratories. Boyce, J. C., and J. T. Moore, "Provisional Wavelength Identification Tables for the Vacuum Ultraviolet," (by private communication from the George Eastman Research Laboratory of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, 191-1-1) • Richardson, J., "Ultraviolet Vacuum Monochromator," October, 28, I9I4.7 (Description and blue prints). Private com­ munication from Baird Associates, Cambridge, Massachu­ setts .