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The raman effect in carbon disulfide MacDonald, John Campbell Forrester 1948

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THE RAMAN EFFECT IN CARBON DISULFIDE by John Campbell F o r r e s t e r MacDonald A t h e s i s submitted i n p a r t i a l f u l f i l m e n t o f the requirements f o r the degree o f MASTER OF ARTS i n the Department o f PHYSICS The U n i v e r s i t y o f B r i t i s h Columbia A p r i l , 19AS ACKNOWLEDGEMENTS The author i s p l e a s e d to express h i s g r a t i t u d e t o Dr. A. M. Crooker f o r h i s c o n t i n u a l a d v i c e , a s s i s t a n c e and encouragement. In a d d i t i o n , he wishes t o acknowledge the work o f Mr. A. W . Pye i n con n e c t i o n w i t h the many gla s s b l o w i n g prob-lems i n v o l v e d i n the r e s e a r c h . The work was c a r r i e d out under a r e s e a r c h grant from the. N a t i o n a l Research C o u n c i l o f Canada. The author i s the h o l d e r o f a Studen t s h i p from the C o u n c i l . INDEX Page I . INTRODUCTION A . Objec ts 1 B . H i s t o r y 1 C . Theory 3 I I . APPARATUS A . The Raman s p e c t r o g r a p h . . . 8 B . The l i g h t source . 18 •0; The Raman tubes 22 I I I . EXPERIMENTAL A . Tes ts - 29 B . I n v e s t i g a t i o n of l i q u i d CS2 30 C. I n v e s t i g a t i o n of , s o l i d CS2 • • 31 D. Measurements and c a l c u l a t i o n s . . . . . . 31 IV . . RESULTS A . I n v e s t i g a t i o n o f the l i q u i d 33 B . I n v e s t i g a t i o n o f the s o l i d 3A V . DISCUSSION . • • 35 V I . CONCLUSION 37 V I I . BIBLIOGRAPHY : 39 ILLUSTRATIONS PLATES Page I. The Raman spectrograph - side view. . .preceding 8 II. « » " - camera . . . . « 8 III. " « " - collimator . . " 8 IV. " " - plate holder . » 8 V. The Apparatus for the investigation of liquid CS 2 . " 25 VI. The Apparatus for the investigation of solid CS2 " 28 VII. Iron arc spectrum " 30 VIII. Transmission of NaN02 " 30 IX. Raman Spectrum of CCI4,. « 30 X. Raman Spectrum of liq u i d CS 2 - two hours " 33 XI. " » " 0 " - eight hours « 33 FIGURES 1. Normal vibrations of CS 2 molecule. . . . . " A t 2. Principal infrared and Raman transitions of CS 2 " A 3. Raman spectrograph 11 8 A. Circuit diagram - automatic tickler.. . . . 21 5. " " pov/er supply 22 6. Geometry of the Raman tube for the liquid. 23 7. Raman tube for the investigation of liquid CS 2 preceding 25 8. Raman tube for the investigation of solid CS 2 » 28 9. Geometry of the Raman tube for the solid . 27 THE RAMAN EFFECT IN CARBON DISULFIDE I. INTRODUCTION A. OBJECTS The objects of this research were twofold: 1. To develop apparatus and experimental techniques to shorten the exposure times in the study of the vibrational Raman effect of liquids and liquids in the solid state. 2. To use the above in the investigation of the vibra-tional Raman spectra of the liquid and solid states of Carbon Disulfide. Concurrently, other workers were investigating the infrared absorption spectrum of CS 2. The two projects, when integrated, should give a reasonably complete basis for the description of the CS2 molecule. B . HISTORY The study of the Raman spectrum of a substance in conjunction with that of the infrared absorption spectrum has been found to be of great importance in the study of i t s mole-cular structure. Since the prediction of the Raman Effect in .1923 hy Smekal (32), and of i t s discovery in 1928 by Raman (27), 2 some thousands of articles dealing with i t have appeared in the literature. In the intervening time the theory has been built up to such an extent that, in many cases, an' Interpreta-tion of vibration-rotational Raman and infrared absorption spectra of a molecule now leads to an unambiguous description of i t . Details of structure, inter-nuclear distances, normal vibration and rotation frequencies, moments of inertia and force constants can be determined in this way. The particular molecule under investigation in this case is that of C S 2 . Principally because the CSg molecule i s triatomic with a relatively simple structure> i t s Raman and infrared absorption spectra have been rather thoroughly ob-served by previous investigators. Very early iri the develop-ment of Raman research, a number of workers (17), (28), (24), (30) found a strong vibration-rotation band at 656 cm"1 and a weaker one in the neighbourhood of "SpO . cm""1. Later Krishna-murti (19) discovered a faint companion to the' 656'^fcnr""1 band at approximately 647 cm"1. With improved technique and a spectrograph of higher dispersion, Mesnage (21) resolved the 800 cm"1 band into two components. Later Bhagavantam (12) detected the presence of bands at 400, 1229, and 1577 cm - 1. Although the forme'r was subsequently confirmed at 395 cm"1 by Ventkateswaran (3$), the bands at 1229 and 1577 have not been found by other workers. In 1934* Langseth, Sorensen and Niel-son (20) did a very complete study of the molecule in the liquid state with much improved apparatus. Paralleling these investigations, the infrared 3 absorption spectrum was being studied, by other workers in order that a complete theory of structure could be developed. Dennison and Wright (15) and Bailey and Cassie (11) were the principal investigators of this aspect of the problem. The interpretation of the results led to the assump-tion of a linear symmetrical structure. This assumption was confirmed in 1935 by the work of Sanderson (29) on the rota-tional structure of the infrared hand at 2183 • 9 ' cm""1. Further, Placzek (26) has shown that this assumption checks theoreti-cally. C. THEORY 1. Theory of Apparatus The theory connected with the development of appara-tus i s so~closely linked with design that i t w i l l he discussed under "Apparatus." 2. Theory of CS? Molecule - General The mathematical theory of the linear symmetxic t r i a -tomic molecule has been dealt with by Placzek (25) and i s f u l l y outlined by Herzberg (4). It w i l l not be discussed here. Since the apparatus used in this research was.de-signed primarily for the observation of vibrational Raman spectra, the discussion w i l l be restricted to this type only, to the exclusion of vibration-rotational spectra. 3. Normal Vibrations The possible normal vibrations of a linear and sym-metric triatomic molecule are indicated in Figure 1. A 0£ — •% Qg. 1 - normal VibraNona of fhe CS, Molecule. OZ'O i 0*1 00*1 03'0 / I'O OZ'O I 0*0 • W nu uej-t OI'O flftiaw X X Fig. 2 - Principal Infrared ond Raman Transitions of CS.. c o n s i d e r a t i o n o f the "zero p o s i t i o n " o f the v i b r a t i o n s i n d i -c a tes a net d i p o l e moment of v e r y n e a r l y z e r o . T h i s i s con-f i r m e d by experiment. With r e g a r d t o the normal v i b r a t i o n s , i t may be noted: (a) )){ i s a symmetrical expansion and c o n t r a c t i o n , w i t h no change i n p o s i t i o n s o f the p o s i t i v e and n e g a t i v e c e n t r e s o f charge, i . e . no change i n d i p o l e moment throughout the c y c l e . (b) l>i i s a doubly degenerate v i b r a t i o n , i n that as the d i r e c t i o n o f v i b r a t i o n i s a t r i g h t angles t o the i n f i n i t y -f o l d a x i s o f symmetry, th e r e e x i s t two p e r p e n d i c u l a r d i r e c t i o n s i n t o which the v i b r a t i o n can be r e s o l v e d . I t i s accompanied by a p e r i o d i c change i n d i p o l e moment. (c) }J3 i s a l s o o f changing d i p o l e moment, but the d i r e c t i o n o f change i s along the symmetry a x i s . 4 . S e l e c t i o n Rules i n the I n f r a r e d Although the exact d e r i v a t i o n o f the s e l e c t i o n r u l e s f o r t he fundamental f r e q u e n c i e s i n the i n f r a r e d would r e q u i r e the methods o f quantum mechanics, i t happens t h a t t h i s i s a case where the Correspondence P r i n c i p l e i s a p p l i c a b l e , and a p u r e l y c l a s s i c a l treatment w i l l l e a d t o c o r r e c t r e s u l t s . Only those v i b r a t i o n s which give r i s e t o a changing d i p o l e moment can i n f l u e n c e , o r he i n f l u e n c e d hy, the a l t e r n a t i n g e l e c t r o -magnetic f i e l d s o f r a d i a t i o n , and so he a c t i v e i n a b s o r p t i o n . In the case o f a t r i a t o m i c molecule w i t h a centre o f symmetry, those v i b r a t i o n s which are symmetric w i t h r e s p e c t t o t h a t c e n t r e w i l l not he observed i n the i n f r a r e d . Thus w i l l 5 not appear i n the i n f r a r e d a b s o r p t i o n spectrum, w h i l e U. and l£ - w i l l . The above i s the o n l y g e n e r a l r u l e governing the appearance o f v i b r a t i o n a l f r e q u e n c i e s i n i n f r a r e d a b s o r p t i o n . A l l other r u l e s depend on the symmetric p r o p e r t i e s o f i n d i v i -d u a l c l a s s e s o f molecules, and w i l l n ot be d i s c u s s e d here." 5. S e l e c t i o n Rules i n V i b r a t i o n a l Raman Sp e c t r a I t i s convenient t o c o n s i d e r t h a t the occurence o f a Raman l i n e i n v o l v e s a double t r a n s i t i o n between t h r e e energy l e v e l s o f the molecule, two o f which are known completely w h i l e the t h i r d ( h y p o t h e t i c a l ) l e v e l i s some unobseryable upper s t a t e . The s t r i c t t h e o r y f o r the d e r i v a t i o n o f the s e -l e c t i o n r u l e s i s then o f n e c e s s i t y v e r y c o m p l i c a t e d . P l a c z e k (25) has shown, however, t h a t a s e m i - c l a s s i c a l treatment may be employed whereby these r u l e s a re deduced s o l e l y from the symmetry p r o p e r t i e s o f the"two known s t a t e s , without r e f e r e n c e to the unknown t h i r d s t a t e . H i s method i s to i n v e s t i g a t e the dependence o f the p o l a r i z a b i l i t y o f the molecule on i t s n u c l e a r energy s t a t e s , and on these a l o n e . For t h i s t o be p o s s i b l e , t h e r e must be no a p p r e c i a b l e p e r t u r b a t i o n o f the n u c l e a r s t a t e s by e l e c t r o n i c or i n c i d e n t r a d i a t i o n f r e q u e n c i e s . J u s t as we have seen t h a t the appearance o f a frequency i n i n f r a r e d absorp-t i o n i s determined c l a s s i c a l l y by the change i n d i p o l e moment a s s o c i a t e d w i t h i t , so now we determine the appearance o f a frequency i n the Raman spectrum from the change i n the induced moment a s s o c i a t e d w i t h the v i b r a t i o n . C o n s i d e r i n g the CS2 molecule w i t h t h i s i n mind, i t 6 i s seen t h a t )/j w i l l he "allowed" i n the Raman spectrum, s i n c e the p o l a r i z a b i l i t y w i l l he d i f f e r e n t at the extreme phases o f the v i b r a t i o n s . In the cases J£ and , however, the p o l a r i -z a b i l i t y (not b e i n g 1 a d i r e c t e d q u a n t i t y ) w i l l not change p e r i o d i c a l l y , and so f r e q u e n c i e s j/ and w i l l not appear i n the Raman spectrum. . I t i s important t o note t h a t , f o r a molecule w i t h a centre o f symmetry, c e r t a i n o f i t s v i b r a t i o n s cannot be a c t i v e i n both i n f r a r e d a b s o r p t i o n and Raman s c a t t e r i n g . T h i s empha-s i z e s the complementary n a t u r e o f the two s t u d i e s . 6 . Combination Frequencies In the f i r s t approximation used i n molecular t h e o r y , i t i s customary t o c o n s i d e r t h e vibratdcns o f the molecule t o be simply l i n e a r l y harmonic. Then the t o t a l energy o f the molecule would be given by: where fUj^ i s the v i b r a t i o n a l quantum number. The s e l e c t i o n r u l e s f o r i n f r a r e d a b s o r p t i o n would be o f the form: where Z U j r e p r e s e n t s the change i n quantum number Si n c e every molecule e x h i b i t s overtones g i v e n by: . and combination bands g i v e n by: the a n a l y s i s must be a m p l i f i e d t o i n c l u d e a degree o f anhar-m o n i c i t y . The t o t a l energy may then be gi v e n i n the g e n e r a l form: £ ^ ^  ^ fe + tJ + ^  £ fe +±Jfe^J 7 •where the second term introduces the anharmonicity. The changes in quantum numbers ^ w i l l not now be so restricted, and the frequency of the most general band may be given by: V = XT** ^ f S i ^ Although this anharmonicity and interaction of the vibrations apparently allows a l l possible Aflfk symmetry considerations w i l l limit very greatly the number of allowed overtones and combination bands. From considering a Raman frequency as the result of two;transitions, one of which must be an absorption, we see that certain combination frequencies w i l l appear in the Raman effect as a result of the extensions and restrictions men-tioned above. These combination hands are in most cases less intense than the fundamentals. 7. Fermi Resonance When two energy levels of the same symmetry l i e close together, the resulting large perturbation causes the appearance of a Raman hand which i s normally forbidden. In this way Fermi (16) explained the appearance of the two nearly equally intense hands in the Raman spectrum of CG^. Since the CSg molecule i s assumed to be of the same point group (J^j, ) as CO2, the weaker hand at 796 cm - 1, indicated in Figure 2 , is considered to be due to the accidental coincidence of j / and 2^. c fe a P l a t e I I The Raman Spectrograph - Camera w i t h L i d Removed. (a) Camera lens tube. (b) Prisra. (c) C o l l i m a t o r l e n s . P l a t e I I I The Raman Spectrograph - C o l l i m a t o r w i t h L i d Removed. (a) S l i t supporting b l o c k . (b) B a f f l e . (c) C o l l i m a t o r l e n s . Plate 17 The Raman Spectrograph'- The Plate Holder. In the operating p o s i t i o n , the pins i n the curved hacking (a) f i t into the holes i n the f o c a l plane blocks (b), and the lightproof door (c) i s closed. I I . APPARATUS A. THE RAMAN SPECTROGRAPH 1. General The requirements of a good Raman spectrograph are great light-gathering power and high speed, in combination with a reasonable dispersion and resolving power over the required range. The problem of f i l l i n g such a spectrograph with light has been dealt with by Neilson (23), and his re-sults have been extended hy Newton (22) to slits, of f i n i t e dimensions. The construction of the spectrograph here described was begun in 194-5 hy Mr. T. E. Whittemore. It was redesigned and rebuilt hy the author in 1 9 4 - 7 . Plates I, II, III and IV show the general construc-tion, and Figure 3 i s a side elevation and plan. A l l refer-ences in the following discussion are to Figure 3. 2. Optical Elements • (a) Prism The prism at hand was one made at Research Enterprises according to the specifications of Dr. A. M. Crooker. It was designed especially for incorporation into just such a Raman spectrograph. It i s a 60° prism of face 9 15.5 cm by 11 cm high, made of very transparent f l i n t glass of type EDF 649338. The great size of this prism (c) makes possible a correspondingly large optical system, with great light-gathering power as a result. (b) Collimator Since the speed of the spectrograph i s inde-pendent of the f/number of the collimator, we are.not res-tricted in our choice of a collimator lens by this considera-tion. We, therefore, choose a lens of angular aperture smaller than that of the camera lens, though of large enough aperture to completely u t i l i z e the area of the prism presented, to i t . . We dp this since a lens of smaller angular aperture is usually less expensive, and even more important, is l i k e l y to have less spherical aberration and coma than one of larger angular aperture. Further, since the dimensions of the spec-t r a l line image are to those of the s l i t as the ratio of the focal lengths of the camera and collimator lenses, a larger s l i t may be used to obtain the same size line image. The collimatD r lens (d) incorporated into the spectrograph under discussion i s a flint-crown achromat of focal length 167 cm and aperture 11.5 cm made by Carl Zeiss of Jena. The method of mounting. and adjustment w i l l be dis-cussed later. (c) Camera The speed of a spectrograph is inversely pro-portional to the square of the f/number of the camera, and i s independent of the f/number of the collimator. Accordingly, '10 s i n c e we wish . a high-speed spectrograph, we choose a camera l e n s of l a r g e a p e r t u r e compared to i t s f o c a l l e n g t h , w i t h good imagery i n as f l a t a f o c a l plane.as p o s s i b l e . The i d e a l spec-t r o g r a p h camera l e n s should be c o r r e c t e d f o r chromatic and s p h e r i c a l a b e r r a t i o n , coma, astigmatism and c u r v a t u r e o f f i e l d . T h i s i d e a l i s i m p o s s i b l e to r e a l i z e , however, and a working compromise must be adopted. I t i s obvious that a good photographic l e n s would a l s o be s u i t a b l e i n a s p e c t r o g r a p h i c camera. S p h e r i c a l a b e r r a t i o n and coma should a t a l l c o s t s be minimized, but the e l i m i n a t i o n o f chromatic a b e r r a t i o n i s not so important. An achromatic l e n s , c o r r e c t e d f o r two c o l o r s , w i l l show "secondary chromatic a b e r r a t i o n , " or v a r i a t i o n o f f o c a l l e n g t h f o r other c o l o r s . Such a l e n s , i f used i n a s p e c t r o g r a p h i c camera, w i l l u s u a l l y g i v e a f o c a l curve convex toward the prism. T h e t i l t o r c u r v a t u r e o f the image s u r f a c e which r e s u l t s from the use o f such a l e n s g i v e s an i n c r e a s e i n the s p e c t r a l d i s p e r s i o n , but does not g i v e a corresponding i n -crease i n the r e s o l v i n g power s i n c e each l i n e image i s wider i n the same r a t i o . The camera l e n s (b) used i n t h i s s p e c t r o g r a p h i s a 1/1 t e l e s t i g m a t l e n s o f f o c a l l e n g t h 100 cm made by Bausch and Lomb f o r t h e K-16 a e r i a l camera. T h i s l e n s i s eminently s a t i s f a c t o r y f o r the purpose, a l t h o u g h the above-mentioned f o c a l plane c u r v a t u r e n e c e s s i t a t e d the a d o p t i o n o f f i l m i n p l a c e o f p l a t e s , w i t h the accompanying inconvenience i n camera l o a d i n g . (d) S l i t The s l i t ( f ) i s a standard H i l g e r u n i l a t e r a l D- . 11 with b u i l t - i n shutter, mounted on a tube of length 4«5 cm. It i s so designed as to give widths of 0 to 1 . 0 mm in scale divisions of 0 . 0 2 mm. The undiaphramed s l i t length i s 7 mm. Ways are provided before the s l i t to permit the use of dia-phragms. This s l i t i s unsatisfactory for the purpose, for reasons to be discussed later. 3 . Construction (a) In order to find'the angle between the camera and collimator mounts of the spectrograph, we must calculate the angle of minimum deviation for a number of wavelengths over the desired range. From the well-known relation: S±±sL sin where: ©x = angle of minimum deviation for ~X. o(. = apex angle of prism NA= index for A. N^ = sin 2 we get: whence: \ - 2 jarcsin (N xsin * Line X . £ F g h 4 8 6 1 . 3 4 3 5 8 . 6 4 0 4 6 . 8 52°29' 5 3 * 4 0 ' 5 4 ° 4 2 ' (b) The r i g i d mount., (h) of the spectrograph con-sists of two pieces of 5 " by 5 " (18.9 lbs/ft) H-section steel of lengths 218 cm and 128 cm, welded together at an angle of 127 degrees. The upper surface i s finished f l a t to 1 mm. Bolted securely to the beam are the 1^" mahogany base of the camera and prism tube and the 5-ply base of the collimator 12 tube. The s i d e s o f the tubes are o f f and 5-ply, s t r o n g l y braced wherever p o s s i b l e . The top i s o f the same m a t e r i a l and i s i n two s e c t i o n s , one c o v e r i n g the c o l l i m a t o r and the o t h e r c o v e r i n g the camera and p r i s m . The s e c t i o n s make a snug, l i g h t -proof f i t which a l l o w s , however, f o r removal when n e c e s s a r y . The mountings f o r the l e n s e s , stops, s l i t and p l a t e h o l d e r en-sure r i g i d i t y o f the whole assembly.' (c) The S l i t Mount At the s l i t end o f the c o l l i m a t o r tube i s a s t r e n g t h e n i n g frame t o which i s a t t a c h e d 3 - l i " mahogany blocks which p r o v i d e a r i g i d end to the tube as w e l l as a s o l i d s l i t mount. The i n s i d e o f these b l o c k s was turned out on a l a t h e i n t o the shape o f a cone i n order t o ensure no r e f l e c -t i o n o f f the w a l l s . F i r m l y b o l t e d t o the o u t s i d e b l o c k i s a brass tube which accomodates the tube o f the H i l g e r s l i t ( f ) , p e r m i t t i n g a s l i t t r a v e l o f approximately 3.5 cm along the a x i s . B o l t e d t o the beam beyond the end b l o c k s i s a wooden t a b l e (g) on which l i g h t , sources can be supported. (d) The C o l l i m a t o r Lens Mount The l e n s (d) i s mounted by f i x i n g i t s tube i n t o a h o l e i n a brass p l a t e which can be b o l t e d to a mahogany-block mounted i n the c o l l i m a t o r tube p e r p e n d i c u l a r t o the a x i s . The b l o c k i s cut out i n the c e n t r e t o a l l o w the passage o f l i g h t . The p o s i t i o n o f the p l a t e can be a d j u s t e d to a l i g n the l e n s i n the o p t i c a l system. (e) The,Prism Mount The prism (c) i s mounted on a t r i a n g u l a r t a b l e , " 13 so p l a c e d t h a t p a r a l l e l l i g h t from the c o l l i m a t o r l e n s f i l l s a maximum area o f the a d j a c e n t f a c e o f the pris m a t minimum d e v i a t i o n . Once s e t f o r minimum d e v i a t i o n , the p r i s m can be clamped i n p l a c e by p r e s s u r e on the t o p . I t s edges are accu-r a t e l y p e r p e n d i c u l a r t o the a x i s o f the spectrograph. No p r o v i s i o n i s made f o r the r o t a t i o n o f the prism, s i n c e the wave-length range i s predetermined. • ( f ) The Camera Mount Since the camera l e n s was o r i g i n a l l y used i n an a e r i a l camera and was mounted i n a r i g i d tube (b) f o r the purpose, i t was decided to i n c o r p o r a t e the whole tube assembly i n t o the sp e c t r o g r a p h . Two p i e c e s o f f " 5 - p l y are i n s e r t e d i n the camera box w i t h h o l e s i n them l a r g e enough to accomodate the tube, i n such a way t h a t by the t i g h t e n i n g o f a f i n a l screw the tube i s ' f i x e d as reg a r d s l o n g i t u d i n a l as w e l l as t r a n s v e r s e p o s i t i o n . The h o l e s a r e so l o c a t e d as t o permit the camera l e n s to r e c e i v e a maximum o f l i g h t from the prism. The l e n s a x i s i s p a r a l l e l t o the di r e c t i o n o f the r e f r a c t e d l i g h t from the mercury 4358 AU l i n e . (g) The whole o f t h e . i n s i d e o f the spectrograph ( e x c l u d i n g the o p t i c a l elements) i s p a i n t e d w i t h a matt b l a c k p a i n t made o f a s u i t a b l e mixture o f lampblack, s h e l l a c and methyl a c e t a t e . A b a f f l e (e) i s l o c a t e d i n the c o l l i m a t o r tube. 4. Alignment (a) S e t t i n g o f the c o l l i m a t o r f o r p a r a l l e l l i g h t through the prism. The f i r s t requirement was to ensure t h a t 14, the plane o f the l e n s was p e r p e n d i c u l a r to and c e n t r e d on the a x i s of the c o l l i m a t o r . T h i s was done as f o l l o w s . A plywood c o l l a r was made which f i t t e d snugly over the s l i t end o f the l e n s tube. Two narrow brass s t r i p s were f i x e d on the c o l l a r i n such a way as t o c r o s s i n f r o n t o f the c e n t r e o f the l e n s when the c o l l a r was i n p l a c e . To t h i s c e n t r e p o i n t was a f f i x e d a t h r e a d whose other end was passed through the top o f the s l i t and p u l l e d t a u t . Thus when l i g h t was passed through the s l i t by means of a r i g h t - a n g l e d prism, the f o c a l p o i n t o f the r e f l e c t i o n o f f the back (prism) f a c e o f the l e n s c o u l d be made to l i e on the t h r e a d by shimming the mounting p l a t e from be-h i n d . When the mounting p l a t e was f a s t e n e d a f t e r the f o r e g o i n g procedure, l i g h t f n n the s l i t through the l e n s l e f t the l e n s p a r a l l e l . (b) The p r i s m was next s e t f o r minimum d e v i a t i o n f o r the 4358 fl. l i n e i n the mercury spectrum by the u s u a l method and s e c u r e d . (c) In the mounting o f the tube '(b) c o n t a i n i n g the camera l e n s , the supports were so p l a c e d as t o ensure a maximum of l i g h t ' from the p r i s m through the camera l e n s . The tube was, however, l e f t so t h a t i t c o u l d be moved along i t s a x i s d u r i n g the f i x i n g of the p o s i t i o n of the f o c a l s u r f a c e . The determina-t i o n o f t h i s f o c a l s u r f a c e was the l a s t and most d i f f i c u l t s t e p . .The o r i g i n a l p l a t e - h o l d e r (a) was one adapted from a s m a l l H i l g e r spectrograph, and took standard 3i x 41" p l a t e s . An i r o n a r c was a c c u r a t e l y placed on the a x i s of the spectrograph and a condensing l e n s so p l a c e d as to image 15 the arc on the collimator lens through the s l i t , in such a way as to get a maximum of light on the lens. The s l i t was then set at the optimum calculated width (Baly ( l ) , Vol. I, p. 285), i.e.: s = 2 f X f = focal length of lens A . A « eff. aperture » " ,«-7 A= wavelength 2 x 167.5 x 4360 x 10"' 7 » 0.02 mm Using a two step Hartmann diaphragm and blocking off f i r s t one side of the optical system and then the other, a series of focus plates corresponding to different positions and degrees of t i l t of the plate-holder were taken (See Sawyer (8), p. 100), From the interpretation of the data obtained, i t was determined that the focal surface.was convex toward the camera lens with a radius of 150 mm, and the red end was t i l t e d 8 degrees toward the camera lens. Since i t was impossible to bend a plate to conform to such a radius, film 'had to be adopted. The plate-holder had to be rebuilt to take 35 mm movie film and to hold i t accurately on the focal surface. The method of so doing i s obvious from an inspection of Plate IV. Once the film was so positioned, minor adjustments of the s l i t position were enough to bring the whole of the visible range into perfect focus. Plate VII shows a sample Iron arc spectrogram of double linear enlargement, exposure one-half second. 5. Constants of the Spectrograph (a) Dispersion From the spectrogram in Plate VII, a large 16 number o f l i n e s over the v i s i b l e range were measured, and from t h i s data the d i s p e r s i o n f o r d i f f e r e n t p a r t s o f the range was c a l c u l a t e d . I t was found to v a r y from 59 A/mm at 5590 A to 12.6 A/mm a t 3850 A , w i t h v a l u e s o f 26 and 22.5 a t 4500 A and A358 A r e s p e c t i v e l y . The t h e o r e t i c a l l i n e a r d i s p e r s i o n may be ob-t a i n e d from the r e l a t i o n : dX » dA dN do where: ds dN d.9 ds dX i s o b t a i n e d from d i f f e r e n t i a t i n g the Hartmann i n t e r p o l a t i o n dN f o r m u l a : N = N Q - C i . e . : dX = ( X - A Q ) 2 dN C where f o r t h i s p r i s m : A 0 - 2047 . 9 A and C - 146.57 A. dQ depends on the f o c a l l e n g t h o f the.camera l e n s and the ds angle of t i l t o f the p l a t e pi where l i g h t o f the w a v e - l e n g t h cons idered s t r i k e s i t . That i s : d9 - cos <t> We l e t jzJ = 0 ° at 4358 A . ds F dN i s ob ta ined by a c o n s i d e r a t i o n o f S n e l l ' s Law a p p l i e d t o d9 the p r i s m at minimum d e v i a t i o n , 'and f o r a 60° p r i s m g i v e s : dN = / 1-NV4 d9 y . Hence: d ^ = ( A - A Q ) 2 X . / l - N^/4 x cos &5 ds. ' F =36450 x 0.548 x ' l 1000 o = 20 A/mm a t 4358 A . w h i c h i s i"n~ reasonable agreement w i t h the e x p e r i m e n t a l l y 17 determined v a l u e . (b) R e s o l v i n g Power A c c o r d i n g t o R a y l e i g h , the t h e o r e t i c a l R . P . o f a p r i s m i s g i v e n b y : R . P . • t dN where t - e f f e c t i v e t h i c k n e s s o f p r i s m dX = 15.5 cm h e r e , s i n c e a l l o f p r i s m i s u s e d . Then, t h e o r e t i c a l l y , at 4358 A : R . P . = 15.5 x 1 0 8 x 1 36^450 . w 43,000 The p r a c t i c a l R . P . u s u a l l y d i f f e r s q u i t e w i d e l y from t h i s t h e o r e t i c a l v a l u e due to the e f f e c t o f such f a c t o r s as the r e l a t i v e i n t e n s i t i e s o f the two ad jacent l i n e s , the w i d t h and method of i l l u m i n a t i o n o f the s l i t , and the g r a i n i n e s s or c o n -t r a s t o f the photographic e m u l s i o n . The p r a c t i c a l R . P . of f a s t spectrographs such as t h i s one i s l i m i t e d hy d i s p e r s i o n and the c h a r a c t e r i s t i c s o f the photographic e m u l s i o n . C o n s i d e r i n g the l i n e a r R . P . o f the f i l m used as 50 l i n e s / m m , t h a t i s , 0 .02 mm, and w i t h a v a l u e of the d i s p e r s i o n o f 22 ; 5 A/mm a t 4358 A , we see t h a t '.a dA d i f f e r e n c e o f (22.5 x 0 .02) o r 0.45 A should be r e s o l v e d at 4358 A . T h i s g i v e s : R . P . = _X = A358 - 9700 dA 9 .45 (c) I n t e r p o l a t i o n Formula From the measurements r e f e r r e d t o i n ( a ) , the constants \ , C and d Q of the Hartmann i n t e r p o l a t i o n f o r m u l a : d Q - d 18 are calculated for interpolation in the range A000 to 5000 A i , i by the method outlined on page 230 of Sawyer (8). \3 =4936.691 d3 58.887 - 4531.155 *2 — 46.371 Xi - 4134*684 dl = 28.798 * i = 396.471 <*2- dl - 17.573 Ai 802.007 d 3- dl - 30.089 d 3- d 2 = 12.516 d2-dx = a - .04^32 d3~dl = b = .03751 A 2 " A l Ay-X£ a - b = .00681 d3" d2 = C - 1837.89 a-h d Q ^o=\ ~ £ = .2296.79 0 d0= a (A2-X>) = a (2234-37) = 99.027 C - C x d Q - 182,000 d 0 d Q = d 0 •+ d x - 127.825 Then: A = /\0 + C - 2296.79 + 182000 d 0 - d 127.825 - d which i s the interpolation formula for the range indicated. B.. THE LIGHT SOURCE 1. Considerations In the Rayleigh scattering of light, the intensity 19 o f the s c a t t e r e d l i g h t i s p r o p o r t i o n a l t o the f o u r t h power o f the f r e q u e n c y . T h i s holds In g e n e r a l f o r Raman s c a t t e r i n g . A c c o r d i n g l y , the best source f o r Raman e x c i t a t i o n would be a monochromatic source o f very s h o r t wavelength and h i g h i n t e n -s i t y . S i nce the s i z e o f the o p t i c s i n the sp e c t r o g r a p h used p r e c l u d e d the use o f quar t z , the a v a i l a b l e wavelengths had a lower l i m i t , namely the lower t r a n s m i s s i o n l i m i t o f g l a s s . Thus the i d e a l source would be one which emits a str o n g mono-chromatic r a d i a t i o n i n the neighbourhood o f 4 0 0 0 AU. In the absence o f such a source, attempts are u s u a l l y made to approach t h i s i d e a l w i t h a s u i t a b l e a r c e m i t t i n g - a l i n e spectrum, i n c o n j u n c t i o n w i t h a f i l t e r . The mercury a r c g i v e s a spectrum o f reasonably few w e l l - s e p a r a t e d l i n e s . I t was thought a t f i r s t t h a t t h i s source c o u l d be used u n f i l t e r e d , and, indeed, i t was so used i n ob-t a i n i n g the t e s t C C l ^ spectrograms. However, CS2 i s decomposed by u l t r a - v i o l e t r a d i a t i o n o f wavelength s h o r t e r than 3660 ang-stroms, and t h i s n e c e s s i t a t e s the use of a f i l t e r to e l i m i n a t e . o a l l t r a c e s o f t h i s r a d i a t i o n . The 4046 A l i n e i s reduced i n o i n t e n s i t y by such- a f i l t e r , but the 4358 A l i n e i s not appre-c i a b l y weakened, and so can be used f o r the e x c i t i n g r a d i a t i o n . The 46OO cm""1 " c l e a r space" on the Stokes (red) s i d e o f 4358 i s adequate f o r the purpose without the n e c e s s i t y o f f i l t e r i n g out l i n e s o f l o n g e r wavelength. " • 2. D e s c r i p t i o n In o r d e r t o get the maximum l i g h t onto the l i q u i d , i t was de c i d e d to b u i l d the a r c i n the form o f a h e l i x . 20 The form o f the a r c f i n a l l y adopted f o r the i n v e s t i g a t i o n i s as f o l l o w s . A 100 cm l e n g t h o f 10 mm. i n s i d e diameter Pyrex t u b i n g i s bent i n t o the form o f a h e l i x o f I.D. 5.5 cm and o f A j t u r n s over a l e n g t h o f 11 cm. To the. ends o f the h e l i x are a f f i x e d anode and cathode bulbs w i t h a p p r o p r i a t e o r i e n t a t i o n . The cathode bulb c o n t a i n s a po o l o f mercury which forms the cathode e l e c t r o d e , w h i l e the anode i s a s p i r a l o f heavy tung-sten w i r e . The mercury i n the cathode i s p u r i f i e d , w i t h the g r e a t e s t c a r e ; sprayed through 10% HNO3, t n e n through d i s t i l l e d water, and f i n a l l y d i s t i l l e d t w i c e . B e f o r e i n t r o d u c i n g the mercury, the whole assembly i s cleaned i n s i d e w i t h hot chromic c l e a n i n g s o l u t i o n , d i s t i l l e d water, hot concentrated HNO3, d i s -t i l l e d water and f i n a l l y d r i e d on a vacuum pump. A f t e r the mercury i s i n t r o d u c e d , a Hyvac pump i s a t t a c h e d to the anode bulb through a connecting tube. The whole assembly, w i t h the ex c e p t i o n o f the pump, I s mounted on a s u i t a b l e stand which can be f a s t e n e d t o the s l i t t a b l e . (See P l a t e V) The system i s then evacuated-by means o f the Hyvac pump and the] mercury and gl a s s degassed by h e a t i n g when the pump i s o p e r a t i n g . 3. O p e r a t i o n The method o f s t a r t i n g the a r c i s as f o l l o w s . Con-nect i t to a 220 VDC source through a 22 ohm, A.A,amp s l i d e wire r h e o s t a t and an i r o n cored c o i l o f the order o f 50 mh i n -ductance. (The r h e o s t a t serves to c o n t r o l . t h e c u r r e n t , w h i l e the c o i l opposes any tendency f o r the a r c to stop once i t i s op e r a t i n g . ) S t a r t the pump, and heat the h e l i x s t r o n g l y w i t h a bunsen burner, being c a r e f u l not t o a l l o w any p a r t to get 21 r e d h o t . Switch the burner to the cathode p o o l and, at the same time, a p p l y the f u l l v o l t a g e o f a T e s l a c o i l t o the s t a r t i n g e l e c t r o d e . W ith a c e r t a i n amount o f experience, the arc can be made to f i r e immediately. The r h e o s t a t should be used to a d j u s t the c u r r e n t to an o p e r a t i n g value o f 4*5 - 5 . 0 amps. T h i s optimum o p e r a t i n g v a l u e v a r i e s d i r e c t l y w i t h the c r o s s e c t i o n a l area o f the tube. T h i s type of a r c has been made to operate without the r a t h e r cumbersome pump arrangement; i . e . when evacuated and s e a l e d o f f . Such an a r c i s , however, extremely u n r e l i a b l e both i n s t a r t i n g - a n d o p e r a t i o n . A c c o r d i n g l y , the above type was adopted. . 4 . As an a d d i t i o n a l safeguard t o prevent the a r c s t o p -ping' w h i l e 'unattended d u r i n g l o n g , . MAC. exposures, a r e l a y i s a t t a c h e d to the 50 mh c o i l i n the 220 v o l t DC c i r c u i t and w i r e d i n t o the 110 v o l t AC supply c i r c u i t o f the T e s l a c o i l i n such a way t h a t , should the a r c go out, the F i g u r e 4 « . T e s l a c o i l w i l l i n s t a n t a n e o u s l y " t i c k l e 1 ' i t , causing i t t o f i r e a g a i n . See f i g u r e 4 -5. The l i g h t source d e s c r i b e d above i s eminently s u i t -a b l e f o r the i n v e s t i g a t i o n o f the l i q u i d . I t s e f f i c i e n c y c ould p r o b a b l y be i n c r e a s e d by surrounding i t w i t h a c y l i n d r i -c a l r e f l e c t o r , hut c o o l i n g problems would he much i n c r e a s e d . T h i s type o f a r c i s u s e l e s s f o r experiments i n v o l v i n g degree o f d e p o l a r i z a t i o n o f Raman l i n e s . Cotu D.C. CmcoiT 22 6. The a r c f o r use i n the i n v e s t i g a t i o n o f the Raman spectrum of s o l i d C S 2 i s e s s e n t i a l l y the same as the one des-c r i b e d above, except t h a t i t i s designed to be operated i n a v e r t i c a l p o s i t i o n . See P l a t e VI. I t i s o f 130 cm. e l e c t r o d e -t o - e l e c t r o d e l e n g t h , and has a p o t e n t i a l drop o f 170 v o l t s DC a c r o s s i t when o p e r a t i n g . 7. Power Supply Si n c e the arcs r e q u i r e more than the 120 VDC a v a i l -a b l e , a 3 phase 220 VAC to 120 VDC motor generator i s n e c e s -s a r y to i n c r e a s e the v o l t a g e t o the r e q u i r e d v a l u e . T h i s i s so connected (see F i g u r e 5) t h a t v o l t a g e s up to 275 VDC are a v a i l -a b l e . An a d d i t i o n a l c o n n e c t i o n sup-\ p l y i n g the b u i l d i n g 110 VDC alone i s p r o v i d e d f o r the i r o n a r c used i n + GMCMBR + | L o b t a i n i n g comparison s p e c t r a . The 'w output of the system i s d i s t r i b u t e d F i g u r e 5. through a s u i t a b l e switchboard i n c o r p o r a t i n g an ammeter. C. THE RAMAN TUBES 1. General The methods o f c o n t a i n i n g the substance under i n -v e s t i g a t i o n are almost as numerous as the i n v e s t i g a t o r s . The d e s i g n most f r e q u e n t l y used, and which i s here adopted, i s a simple Pyrex tube w i t h a p l a n e - p a r a l l e l window c l o s i n g one end. The other end i s drawn o f f i n t o a " R a y l e i g h horn" which, when blackened on the o u t s i d e , serves to prevent l i g h t being A r 1 23 r e f l e c t e d from t h a t end o f the tube i n t o the s l i t . T h i s d e sign was f i r s t Used by Wood (36), as adopted from the work of R a y l e i g h . I t i s important t h a t no l i g h t r e f l e c t e d o f f the si d e s o f the tube enter the s l i t , s i n c e we wish to study o n l y the l i g h t s c a t t e r e d o f f the molecules o f the substance con-t a i n e d t h e r e i n . The dimensions o f the tube, then, w i l l depend on the f o c a l r a t i o o f the c o l l i m a t o r l e n s , the q u a n t i t y o f the substance a v a i l a b l e , and the method o f mounting the tube be-f o r e the s l i t o f the spectrograph. Since the problems o f d e s i g n and c o n s t r u c t i o n d i f f e r v e ry w i d e l y f o r the tubes used i n t h e study o f the l i q u i d and the s o l i d CS2, they w i l l be d i s c u s s e d s e p a r a t e l y . 2. Raman Tube f o r I n v e s t i g a t i n g L i q u i d CS? (a) Design In o r d e r to ensure t h a t a maximum o f s c a t t e r e d l i g h t e nters the s l i t , the tube i s mounted w i t h the plane win-dow immediately i n f r o n t o f t h e s l i t . Then the r a t i o o f r a d i u s to l e n g t h i s e a s i l y c a l c u l a t e d from the geometry o f the system. I t i s seen from a c o n s i d e r a t i o n o f F i g u r e 6 t h a t : LENS F i g u r e 6 2 A a 4- h = r - h F I hence: r = h + (a 4- h ) / or /= F ( r - h) F • a 4- h Since the s l i t . i s r e c t a n g u l a r , i t i s obvious t h a t the r a d i u s r e q u i r e d f o r a g i v e n l e n g t h w i l l be l e s s i n the h o r i z o n t a l d i r e c t i o n than i n the v e r t i c a l . I t i s o n l y i n cases where the amount o f l i q u i d a v a i l a b l e i s s m a l l t h a t t h i s need be c o n s i d e r e d , s i n c e a l l o w i n g f o r i t means t h a t the tube w i l l no l o n g e r have the r e a d i l y a v a i l a b l e c y l i n d r i c a l shape. An a l t e r n a t i v e method o f overcoming t h i s d i f f i c u l t y i s by r e d u c i n g the l e n g t h of the s l i t . (b) C o n s t r u c t i o n The tube i s b u i l t as i n d i c a t e d i n F i g u r e 7 . The window (w) i s f u z e d to the end o f the tube (a) w i t h a p o i n t flame and any r e s u l t i n g s t r a i n s are removed by a n n e a l i n g . The r e s u l t i s a window which i s plane and s t r a i n - f r e e i n the c e n t r a l zone immediately i n f r o n t o f the s l i t . O v e r a l l l e n g t h i s 2 1 cm and i n s i d e diameter i s 2 . 2 cm. In o r d e r to prevent chemical decomposition o f the C S 2 by u l t r a - v i o l e t r a d i a t i o n , a f i l t e r (d) i s n e c e s s a r y . T h i s i s a concentrated s o l u t i o n o f N a N 0 2 . i n water, as suggested by Hibben ( 5 ) and Bhagavantam ( 2 ) . A H i l g e r medium-quartz spectrogram o f the t r a n s m i s s i o n o f a 7 mm l a y e r o f N a N 0 2 i s shown i n P l a t e V I I I . The f i l t e r j a c k e t i s of i n s i d e diameter 3 . 9 -cm and i s f i t t e d to the Raman tube by means of rubber stoppers waxed on. The f i l t e r i s n o t c i r c u l a t e d , c o o l i n g being e f f e c t e d as d e s c r i b e d below. Side tubes (n) are p r o v i d e d f o r f i l l i n g . A l l p a r t s o f t h e assembly P l a t e V The Apparatus f o r the I n v e s t i g a t i o n of the L i q u i d CS The l e t t e r s r e f e r to the t e x t . I n a d d i t i o n : (t) i s the arc h e l i x . (u) i s the cathode bulb. (v) i s the vacuum connection to the anode, (w) i s the connection to the a i r supply, (x) i s the arc mounting t a b l e . 25 i n d i c a t e d by heavy l i n e s are blackened. T h i s l e a v e s a c l e a r l e n g t h o f 11 cm. f o r r e c e i v i n g the i n c i d e n t r a d i a t i o n . (c) Mounting S i n c e heat from a Bunsen flame i s n e c e s s a r y to s t a r t the a r c , i t must be removed from i t s o p e r a t i n g p o s i -t i o n w h i l e b e i n g s t a r t e d to prevent the flame damaging the s l i t . Once s t a r t e d , i t can be moved i n t o p o s i t i o n and the Raman tube i n s e r t e d through the c e n t r e of the h e l i x t o a p o s i -t i o n w i t h the'window (w) b e f o r e the s l i t . The method o f sup-p o r t i s i n d i c a t e d i n F i g u r e 7. A mahogany, b l o c k i s d r i l l e d through to. r e c e i v e the tube and then s p l i t i n f o two unequal p a r t s (gj.) and (g£). To the f r o n t o f (g£) and h o r i z o n t a l l y a c r o s s the exact c e n t r e o f the h o l e i s f i x e d a b r a s s bar (h) of the exact s i z e t o f i t the Hartmann s l o t Q ) o f the s l i t mount ( s ) . In the c e n t r e of the bar,, and so p l a c e d as to be e x a c t l y o p p o s i t e the s l i t when the b l o c k i s i n p o s i t i o n , a 3 mm. h o l e i s d r i l l e d . Thus when the tube (a) i s i n s e r t e d i n -to the b l o c k s , the window (w) i s e x a c t l y o p p o s i t e the s l i t , whose e f f e c t i v e l e n g t h i s now reduced to 3 mm. Thumbscrews on each s i d e clamp the tube r i g i d l y i n t o the b l o c k s . A c o l l a r (e) r e s t i n g on. a b l o c k ( f ) supports the horn end o f the tube-.1 See P l a t e V. (d) Alignment To a l i g n the tube along- the a x i s o f the c o l l i -mator, an incandescent lamp i s shone i n t o the e x i t s l o t o f the camera, and a p o i n t e r i s p l a c e d a t the exact c e n t r e o f the c o l -l i m a t o r l e n s on the s l i t s i d e . On l o o k i n g through a p i n h o l e 26 a t (p) i n the d i r e c t i o n o f the s l i t , t h e . c o l l i m a t o r l e n s can -be seen w i t h the p o i n t e r i n the centre of i t i f the tube i s p r o p e r l y a l i g n e d . (e) C o o l i n g The p r o x i m i t y o f the arc to the tube assembly causes the C S 2 to reach i t s b o i l i n g p o i n t o f 4-6.5°C v e r y r a p i -d l y . T h i s i s prevented hy a system o f a i r c o o l i n g . A p i e c e of J i n c h copper t u b i n g (m), c l o s e d at one end and connected to a compressed a i r supply a t the o t h e r , i s bent i n t o a c i r c l e o f a p p r o p r i a t e s i z e and f i x e d t o the b l o c k ( g 2 ) . H o l e s , 0 .5 cm. i n d i a m e t e r , are e q u a l l y spaced around the c i r c l e so as to d i r e c t a stream of c o o l a i r a l o n g the f i l t e r j a c k e t , '^he a i r i s p r e c o o l e d and f r e e d o f o i l - by a s u i t a b l e f i l t e r . 3 . Raman Tube f o r I n v e s t i g a t i n g S o l i d CS? (a) Design S ince the f r e e z i n g p o i n t o f C S 2 i s - 1 1 1 ° C , the Raman tube must he surrounded by some' such substance as l i q u i d a i r throughout the exposure . t i m e . A c c o r d i n g l y , a d e s i g n as shown i n F i g u r e 8 was adopted. T h i s i s an a d a p t a t i o n of t h a t used by S u t h e r l a n d , Lee and Wu (34) and S u t h e r l a n d (33) as d i s c u s s e d by K o h l r a u s c h ( 6 ) . S ince i n t h i s case the window (wx) cannot be p l a c e d immediate ly before the s l i t , a condensing system must be d e s i g n e d . The problem i s (See F i g u r e 9 ) , by s u i t a b l y p l a c i n g a l e n s o f c a l c u l a t e d f o c a l l e n g t h f , t o image a p lane P a t plane Y w i t h m a g n i f i c a t i o n s / d , and to- image plane Q a t plane £ w i t h m a g n i f i c a t i o n A / d . 27 y Figure 9. The equations of condition are: 1 ( I = ) _ 1 _ f L f u + 1 = 1 + v u- F + v (d =. ) (L + u)s = uA v F + v Solving, and substituting the fixed values: F = 167 cm A • 11 cm S = 0.3 cm L m 15 cm we get: f = 6.9 cm d = A.45 cm u = 7.2 cm v = II .64. cm By this method (referring to Figure 8), a single lens (l) focusses the front window (w^) of the Raman tube (a) on the collimator lens, and at the same time focusses the back end of the tube on the s l i t , (b). Construction In the sectional diagram of Figure 8, (d) i s a NaN02 f i l t e r which i s f i l l e d at ( f ) , (c) i s a specially con-structed Dewar flask to contain the liquid a i r at (b), and (a) Plate VI The Apparatus f o r the Investigation of the S o l i d C 3 2 The l e t t e r s r e f e r to the text. In addition: (t) i s the arc h e l i x . (u) i s the anode bulb. (v) i s the vacuum connection to the arc. (w) i s the connection to the a i r supply, (x) i s the mounting table. I <j t b u b e d I ! Flg.8 28 i s the Raman tube proper. The assembly, w i t h the e x c e p t i o n o f t h e . f i l t e r j a c k e t , i s a s i n g l e u n i t o f Pyrex. O v e r a l l l e n g t h i s 24 cm and o u t s i d e diameter ( i n c l u d i n g the f i l t e r ) i s 6 cm. The g l a s s i s blackened w i t h Duco where i n d i c a t e d by heavy l i n e s . The assembly i s supported at the c a l c u l a t e d d i s t a n c e before the s l i t by a c i r c u l a r b l o c k (g) c o n t a i n i n g ' a b r a s s tube (h) s u p p o r t i n g the 45 degree prism'(p) and the l e n s ( l ) . The d i s t a n c e u as c a l c u l a t e d above i s t h a t from (w]_) to ( l ) , t a k i n g i n t o account the index o f the p r i s m . The d i s t a n c e v i s t h a t from ( l ) to the s l i t ( s ) . An a i r b l a s t (m) i s p r o v i d e d as p r e v i o u s l y d e s c r i b e d t o c o o l the assembly and to prevent f r o s t from c o a t i n g the top where the l i q u i d a i r i s b o i l i n g away. The v e r t i c a l h e l i c a l a r c ( p r e v i o u s l y d e s c r i b e d ) i s mounted so as to s l i d e downward over the tube assembly when o p e r a t i n g . Arc and assembly mounting are f i x e d t o a r i g i d t a b l e . See P l a t e VI. (c) Alignment The method o f alignment i s s i m i l a r t o t h a t d e s c r i b e d f o r the tube o f F i g u r e 7. 29 III. EXPERIMENTAL A. TESTS During the course of the construction and alignment of the Raman tubes, numerous test plates were taken, using Carbon Tetrachloride as the scattering substance. CCl^ was used in preference to CS2 for the following reasons: 1. It is inexpensive, easy to handle and readily avail-able in a reasonably pure form. 2.. It has a Boiling Point.of 76.8°C. as compared with that of A6.3°C. for CS2, and hence i s better for t r i a l s i n -volving methods of cooling. 3. The vapor in small concentrations is. non-poisonous, while that of CS2 is quite poisonous. A. Most important, CCI4 has a number of very strong Raman bands. Hence suitable test plates may be obtained-w-ith rea-sonably short exposures. Plate IX shows the Raman bands of CCI4 which appeared with an exposure of 30 minutes. The test plates showed up a.number of errors in the design which were subsequently corrected. T n e o n e flaw which could not be completely eliminated was a ring which appeared Plate VII: Iron Arc Spectrum. Exposure of One-Half Second. Plate VIII: H i l g e r Medium quartz Spectrograms: Bottom: Iron Arc Spectrum. Top: Transmission Through a 7 mm Layer of NaN02. Plate IX: Raman Spectrum of Carbon Tetrachloride Excited by the 4358 A Line of Mercury, Exposure 30 Minutes. 30 on the plate around each strong exciting line on long exposure. This ring was due to reflection from the internal components of the s l i t tube. Attempts made to eliminate i t included modi-fications to s l i t components, baffling, and f i n a l l y restriction of the s l i t length. The result was a definite reduction in the intensity of the ring, but not a complete elimination. On Plate XI at (a) part of the ring around the 4-358 line appears, while at (b) the corresponding part around the 54-61 line ap-pears. In order to eliminate this ring altogether, a complete redesign of the s l i t would be necessary. This has not beeh attempted. B. INVESTIGATION OF THE LIQUID CS? The samples used were of 'commercially available Baker CP. Carbon Disulfide, Lot Number 12134-6. Since this brand i s •highly purified in manufacture, no attempts were made to purify i t further. Every precaution was.taken to ensure cleanliness of the Raman tube before f i l l i n g . The Sodium Nitrite f i l t e r solution was freed of a l l suspended matter before insertion into the f i l t e r jacket. A l l exposures were taken with the CS2 sample held at a temperature of 37.0 £ 0.5°C. Ansco 35 mm panchromatic Ultra Speed film was used for a l l exposures. Development was for 5 minutes in a tray using Kodak D-19 developer. Useful exposures were obtained as follov/s: 1. Exposure 2 hrs. S l i t ¥idth .02 mm (See Plate X) 2. Exposure 3 h r s . S l i t Width .04 mm (See P l a t e XI) One-half second I r o n a r c comparison s p e c t r a were a p p l i e d - t o each p l a t e w i t h s l i t w i d t h the same as t h a t used f o r the Raman exposure. C. INVESTIGATION OF THE SOLID CS? The CS2 sample used was from the same l o t . number as 1 was used i n the i n v e s t i g a t i o n o f the l i q u i d . L i q u i d n i t r o g e n was used as the c o o l a n t . U n f o r t u n a t e l y , before a u s e f u l p l a t e c o u l d he o b t a i n e d , the Dewar f l a s k assembly s h a t t e r e d , and no time was a v a i l a b l e t o r e b u i l d i t . I t i s f e l t t h a t the assem-b l y d e s i g n was n o t a t f a u l t , but r a t h e r t h a t s m a l l undetected s t r a i n s were p r e s e n t i n the g l a s s and the extreme v a r i a t i o n i n temperature caused the s h a t t e r i n g . D. MEASUREMENTS AND CALCULATIONS 1. A l l measurements were made w i t h a H i l g e r moving t a b l e comparator; The average o f f i v e i n d i v i d u a l measurements was taken f o r each* l i n e . the Stokes s i d e of the 4358 A mercury l i n e . In a d d i t i o n , a number o f l i n e s o f known wavelength i n the i r o n comparison spectrum were measured and the a p p r o p r i a t e Hartmann i n t e r p o l a -t i o n formula c a l c u l a t e d for. the range i n v o l v e d . Using the f o r -Observable Raman bands were measured p r i n c i p a l l y on mula the wavelengths o f a l l ' i n v o l v e d line's were c a l c u l a t e d . For the i r o n l i n e s o f known wavelength, the q u a n t i t y 32 was determined and plotted against wavelength. From this curve the appropriate corrections were determined and applied to the unknown lines. Finally, the wavelengths of - the unknown lines were converted to their wave-number equivalents, and hence the wave-number differences for the Raman lines were determined. > 2. Sample Calculation for Plate XI (a) Hartmann formula for range 413A.68A - 4531.155 A ^ = X0-t__C_ = Xo(dn-d) - C = do-d (d Q-d) = 2359.482 (141.523 - d) - 170602 A (HI.523 -d) (b) Method of Tabulation Line d d 0-d ^ calc AX ^correct y AJ/ 413A.68A Fe 45.420- 96.103 4134.692 + .002 4134 . 6 8 4 4V=648(?) Raman 61.254 80 .269 4484.860 + .073 4484.933 22296.9 6 4 7 . 6 P l a t e X: Raman Spectrum of L i q u i d Carbon D i s u l f i d e . Exposure Two Hours. I r o n A r c Comparison Spectrum, P l a t e X I: Raman Spectrum of L i q u i d Carbon D i s u l f i d e . Exposure E i g h t Hours. I r o n A r c Comparison Spectrum. 33 IV. RESULTS A.. INVESTIGATION OF THE LIQUID The observed Raman lines are tabulated and compared with those reported by Langseth, Sorensen and Nielsen (20). The intensities as reported by these workers are also l i s t e d . LS&N Intensity Ay observed 383 395 0.05 -403 — — 640 - -648.3 4.2 • 648.2 656.5 18.9 656.8 787.7 0.01 • — 796.0 1.5 798.2 8 O 4 . 9 0.5 808.1 812.2 0.1 — No evidence of the other lines reported by the above-mentioned workers was found. The 656 cm"1 line was measured as an a n t i r Stokes displacement from 4358 A and checked within experimen-t a l error. In addition, two previously unreported lines were measured on an eight hour plate. One of these lines was in the halation from the 4358 line at 140.8 cm"1, while, the other formed a definite boundary to the halation at 222.7 cm"1. The 656 and 798 lines are estimated to be accurate to within ± 1.0 cm"1. The accuracy of measurement of the 1 weaker lines i s somewhat less, because of their proximity to the s t r o n g l i n e s and t h e i r s m a l l e r r e l a t i v e i n t e n s i t y . B. INVESTIGATION OF THE SOLID No r e s u l t s a re a v a i l a b l e f o r the s o l i d . 35 -1 V. DISCUSvSION The frequencies obtained for the observed l i n e s agree, within the combined experimental error, with those found by the previous investigators. The 656.8 l i n e has been interpreted by Herzberg (4),' page 277, asithe Raman fundamental a r i s i n g from the t r a n s i - ' t i o n between the states (00 a0) and (10°0). The remaining observed l i n e s are attributed to transitions as follows: 648 cm"1: (Ol 3^) to (ll 1©) (00°0) to (02°0) (Ol^-O) to (03 10) A l l these t r a n s i t i o n s are indicated i n Figure 2. 2V2 (=793.4) l i e s close enough to V-± (-656) for there to exist a Fermi resonance e f f e c t between them, and hence 796 cm"*1 appears, although weakly compared to 656 cm""1. 648 and 808 cm"1 are' due to transitions between harmonics and combination l e v e l s as indicated, and hence have very small i n t e n s i t y . • The V2 (396.7s cm"1) frequency, observed by previous investigators, is. forbidden i n the rigorous theory of the i s o l a t e d CS2 molecule. Herzberg (loc c i t ) att r i b u t e d i t s 798 cm 808 cm"1 36 appearance to a perturbation due to the close association of the molecules in the liquid state. It should not appear in the Raman spectrum of the gas. The-non-appearance of this line in the present i n -vestigation may be explained in one of two ways: 1. Insufficiently long exposures. This appears unlikely because of the speed and great light-gathering power of the spectrograph. Further work is to be done in this' regard, however. 2. If Herzberg's assumption is correct, then, since the degree of association of the molecules i s less at temperatures close to the temperature of vaporization, this line should be weaker at these temperatures. Langseth, Sorens.en and Nielsen (20) give no information as to t emperature of their samples during exposure. If the temperature of their sample was con-siderably lower than the 37°C. of the present investigation, then the absence of the 396.7 line possibly may be explained in this way. The lines at 14-0.8 and 222.7 cm - 1 have not been re-ported by other investigators and there appears to he no theoretical explanation for their presence. They must, there-fore, be put down to halation effects u n t i l such time as further work i s done on them. 37 VI. CONCLUSION A spectrograph has been b u i l t f o r the i n v e s t i g a t i o n o f the Raman e f f e c t , and u s i n g i t the Raman spectrum o f l i q u i d CS2 has been i n v e s t i g a t e d . The r e s u l t s check c l o s e l y w i t h those o f pre v i o u s i n v e s t i g a t o r s . The i n v e s t i g a t i o n o f the s o l i d as o u t l i n e d i s to he done i n the immediate f u t u r e . To f u t u r e i n v e s t i g a t o r s , , the author suggests the f o l l o w i n g m o d i f i c a t i o n s and improvements: 1. The s l i t o f the spe c t r o g r a p h should be completely r e d e s i g n e d or r e p l a c e d . i n o r d e r t o e l i m i n a t e the appearance of the r i n g p r e v i o u s l y mentioned. 2. Because o f the extremely l a r g e o p t i c a l elements con-t a i n e d i n the c e n t r a l p a r t o f the spectrograph, i t i s f e l t t h a t some form o f temperature c o n t r o l f o r t h i s p a r t should he b u i l t . 3. Some b e t t e r method f o r temperature c o n t r o l o f the sample d u r i n g exposure should be developed. The a i r b l a s t method d e s c r i b e d h e r e i n i s too approximate, and has the f u r t h e r disadvantage o f c o a t i n g the tube w i t h o i l from the a i r supply d u r i n g l o n g exposures. 4 . Apparatus and techniques f o r measuring the i n t e n s i t y and degree o f d e p o l a r i z a t i o n o f a Raman l i n e should he deve-l o p e d . A W o l l a s t o n prism mount has a l r e a d y been b u i l t f o r d e p o l a r i z a t i o n measurements. 39. T i l , BIBLIOGRAPHY A. BOOKS (1) B a l y , E. C. C (2) Bhagavantam, S. (3) Herzberg, G. (4) Herzberg, G. (5) Hibben, J . H. (6) Kohlrausch, K. W. P. (7) Kohlrausch, K. W. 2P. (8) Sawyer, R. A. (9) Sutherland, G. B. B. M. (10) Wood, R. W. "Spectroscopy" (3 v o l s ) 3rd E d i t i o n (1929), - Longmans, London. : " S c a t t e r i n g o f L i g h t and the Raman E f f e c t " (1940), -Andhra U n i v e r s i t y , W a l t a i r , I n d i a . "Molecular S p e c t r a and M o l e c u l a r S t r u c t u r e (Diatomic Mole-c u l e s ) " (1939), - P r e n t i c e -H a l l , New York. " I n f r a r e d and" Raman S p e c t r a " (1943), - Van Nostrand, New York. "The Raman E f f e c t and i t s Chemi-c a l A p p l i c a t i o n s " (1939), -R e i n h o l d , New York. "Der Smekal-Raman-Effekt" (193D, - J u l i u s S p r i n g e r , B e r l i n . "Der Smekal-Raman-lffekt" (Erganzungsband 1931-1937)" (1938), - J u l i u s S p r i n g e r , B e r l i n . "Experimental Spectroscopy" (1941), - P r e n t i c e - H a l l , New York. " I n f r a r e d and Raman S p e c t r a " (1935), - Metheun, London. " P h y s i c a l O p t i c s " (1934), -MacMillan, New York. 4-0. . PAPERS 11) B a i l e y , C. R. and C a s s i e , A . B . D . Bhagavantam, S . Dennison , D . M . Dennison , D . M. Dennison , D . M . and W r i g h t , N . F e r m i , E . Ganesan, A . S. and Ventekateswaran, S . G l o c k l e r , G. K r i s h n a m u r t i , D . Langseth , A . Sorensen, J . U . and N i e l s e n , J . R . Mesnage Newton, T . E, N i e l s e n , J . R . F e t r i a k a l n , A . and Hoohberg, J . P l a c z e k , G. P l a c z e k , G. Proceedings of the R o y a l S o c i e t y , A 132, 236 (1931). N a t u r e , 126, 995 (1930). P h y s i c a l Review, 4 1 , 304 (1932). Reviews o f Modern P h y s i c s , 3» 280 (1931). P h y s i c a l Review, 3 8 , 2077 ( 1 9 3 D . Z e i t s c h r i f t f u r P h y s i k , . 7 0 , 84 (1931) . N a t u r e , 124, 5 7 (1929) . Reviews o f Modern P h y s i c s , 13", 112 (1943) . I n d i a n J o u r n a l of P h y s i c s , 3, 103 (1930). J o u r n a l o f Chemical P h y s i c s , 2 , 402 (1934). J o u r n a l de Phys ique e t l e Radium, 2, 403 (1931). T h e s i s f o r the Degree o f Master of A r t s ; U n i v e r s i t y o f B . C. (1941). J o u r n a l of the O p t i c a l S o c i e t y o f A m e r i c a , 20, 701 (1930). Z e i t s c h r i f t f u r P h y s i k a l d s c h e Chemie, B3, 217, 405 (1929). Z e i t s c h r i f t f u r P h y s i k , 70, 84 (1931). Handbuch der R a d i o l o g i e , Band 6, T i e l 2, 205 (1934). 41 (27) Raman, G. V. (28) Raman, C. V. and K r i s h n a n , K. S. (29) Sanderson, J". A. (30) Schaefer, M a t o z z i and ..Alderhold (3D S i r k a r , S. 0. (32) Smekal, A. (33) Sutherland, G. B. B . M. (34) Sut h e r l a n d , Lee and Wu (35) Ventekateswaran, S. (36) Wood, R. W. I n d i a n J o u r n a l o f Physics,-2, 387 (1928). Nature, 122, 882 (1928). P h y s i c a l Review, 50, 209 (1935). P h y s i k a l i s c h e Z e i t s c h r i f t , 30, 581 (1929). I n d i a n J o u r n a l o f P h y s i c s , 10, 189 (1936). D i e Naturwissenschaften, 11, 873 (1923). Proceedings o f the R o y a l S o c i e t y , 141, 535 (1933). Proceedings o f the Royal S o c i e t y , 176, 484, (1940). P h i l o s o p h i c a l Magazine, 15, 263 (1933). P h y s i c a l Review, 36, 1421 (1930). ABSTRACT A high-speed s i n g l e p r i s m g l a s s s p e c t r o g r a p h o f great l i g h t g a t h e r i n g power has been b u i l t i n o r d e r t o r e -duce the exposure times necessary i n Raman E f f e c t i n v e s t i -g a t i o n s . Two low-pressure h e l i c a l mercury a r c s o f over one meter e l e c t r o d e - t o - e l e c t r o d e l e n g t h have a l s o been b u i l t f o r use as h i g h i n t e n s i t y sources o f e x c i t i n g r a d i a t i o n . By means o f these the v i b r a t i o n a l Raman spectrum o f l i q u i d CS2 has been obtained w i t h an exposure o f e i g h t hours. The 648, 656, 796 and 804 cm" 1 l i n e s as r e p o r t e d by p r e v i o u s i n v e s t i g a t o r s have been confirmed w i t h i n e x p e r i -mental e r r o r . No evidence o f the 397 cm"*1 l i n e has been found. 

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