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The infrared absorption spectrum of carbon disulphide Edwards, Thomas Harvey 1948

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^ £ 3 By THE INFRARED ABSORPTION SPECTRUM ' OF CARBON DlSULPHIDE by THOMAS HARVEY EDWARDS 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 of MASTER OF ARTS i n the department Of PHYSICS THE UNIVERSITY OF BRITISH COLUMBIA Ap r i l , 1948 ABSTRACT T h i s paper d e a l s with the problem of s e t t i n g up an i n f r a r e d spectrometer and r e c o r d e r under s u i t a b l e c o n d i t i o n s , and i n a p p l y i n g the instrument to the a b s o r p t i o n spectrum of CS 2 i n the vapor phase. S i x a b s o r p t i o n bands, corresponding t o the fundamental v i b r a t i o n V ^  at 1535 cm""1", the d i f f e r e n c e band - V ^ at 877 cm" 1, and the f o u r combination bands'^ ^+ at 2185 cm - 1, V +2V> a t 2332 cm - 1, V,+2 V. at 2838 cm" 1, and 3 <= 3 J-_-| V. •+- + 2 V _ at 2959 cm have been examined. 1 3 2 U s i n g t h i s v a l u e f o r . V.,, a b e t t e r agreement between 3 the f o r c e constant of the C S bond, c a l c u l a t e d from V , w i t h 3 that c a l c u l a t e d from V i s obtained. The work i s t o be continued. ACKNOWLEDGMENT. The author i s plea s e d t o express h i s g r a t i t u d e t o Dr. A. M. Crooker, under whose a h l e s u p e r v i s i o n t h i s work has been c a r r i e d out. In a d d i t i o n , the c o n t r i b u t i o n s o f the f o l l o w i n g are g r a t e f u l l y acknowledged:-, Messrs. A l e c F r a s e r , E a r l P r i c e , and W i l l i a m Pye, of the U n i v e r s i t y P h y s i c s Shop. Messrs. M. Mit c h n e r and E. Rogers were co-workers on the p r o j e c t . T h i s r e s e a r c h was c a r r i e d out w i t h the a i d of a Bursary from the N a t i o n a l Research coun-c i l of Canada. INDEX Page, I INTRODUCTION 1 I I THEORY 4 I I I EXPERIMENTAL WORK. 2 1 . (A) C o n s t r u c t i o n of a s u i t a b l e room and con-t r o l of humidity, temperature, and v i b r a -t i o n t h e r e i n ' 2 1 (B) C o n s t r u c t i o n of a u x i l i a r y equipment t o c o n t r o l c o n d i t i o n s of temperature and pressure of our samples i n the one-meter gas c e l l . 26 (C) Experimental arrangement of the i n s t r u -ments and t h e i r c o n t r o l 29 (D) C a l i b r a t i o n of the spectrometer 39 (E) A b s o r p t i o n spectrum of CS 2 43 IV RESULTS • 4 5 V CONCLUSIONS 47 VI BIBLIOGRAPHY 48 FIGURES: to face page I. Molecular Energy Levels 3 II Normal Vibrations of a Linear Symmetrical YX Molecule 12 2 III Psychrometric Chart 25 IV Temperature Control Circuit 26 V CS 2 Generator and Controls 28 VI Block Diagram of Apparatus 29 VII Plan View of Equipment 29 VIII Circuit Diagram of Control Panel 30 IX Spectrometer 32 X . NH F i l l i n g System 40 3 PHOTOS: I Humidity Control Apparatus 22 II Front View of Temperature Control 26* III Apparatus with Shielding 28 IV Apparatus without Shielding 31 V-VIII Calibration Charts • 40 ' IX-XI. Calibration Graphs 43 XII The Combination Bands V, + »V, . v, *V, 45 XIII The Combination Band V , + a V , 45 XIV The Combination Band V , + V , 45 XV Structure of the V 3 Band 45 XVI The Difference Band V 3 - V, 45 I INTRODUCTION I t has been our g o a l to set up the P e r k i n -Elmer i n f r a r e d spectrometer and a s s o c i a t e d instruments under s u i t a b l e c o n d i t i o n s o f temperature, humidity, and v i -b r a t i o n a l c o n t r o l ; t o c a l i b r a t e them, and apply them to the ab s o r p t i o n spectrum of CS 2 i n the vapor phase. The i n f r a r e d a b s o r p t i o n spectrum o f CSg has been measured by s e v e r a l o b s e r v e r s , ( r e f s . 1,4,10), both in. the l i q u i d and vapor phases. I t i s s i g n i f i c a n t t h a t these observers do not agree on the f r e q u e n c i e s i n cm."''" at which c e r t a i n bands appear. F u r t h e r , the observed v a l u e s d i f f e r o c c a s i o n a l l y from the t h e o r e t i c a l v a l u e s . We d e s i r e t o check those f r e q u e n c i e s which f a l l i n the r e g i o n over which our instrument i s c a l i b r a t e d . The t h i r d normal frequency i s the most important of t h e s e , s i n c e c a l c u l a t i o n of f o r c e constants and c a l c u l a t i o n s of the f r e q u e n c i e s of combina-* t i o n bands depend on i t . A c c o r d i n g t o Herzberg, ( r e f . 8 ) , CSg has c e r t a i n bands i n i t s a b s o r p t i o n spectrum which, assuming t h a t the CSg molecule i s l i n e a r , should not be pres e n t . Since these bands appear most s t r o n g l y i n the l i q u i d , i f a t a l l i n the vapor phase, i t i s b e l i e v e d t h a t the degree of a s s o c i a t i o n of the molecules i s r e s p o n s i b l e f o r the presence of these bands. T h e i r presence r e p r e s e n t s a breakdown of selection rules. In addition, bands involving \)3occur at longer wavelengths in the liquid state. It was hoped that the study of the absorption spectrum under independent variation of pressure and temp-erature would provide f r u i t f u l data for further theoretical study of the CSg molecule and of the intermolecular forces involved in chemical association. The general theory of molecular spectra and mol-ecular structure i s well presented in Herzberg's two vol-umes, (refs. 7,8), and Wu, (ref. 15), presents a good treatise on vibrational spectra with special reference to the linear symmetrical YXg molecule, to which type we be-lieve GSg belongs. The spectra of molecules appear in three more or less distinct regions of the spectrum; the ultra-violet or visible, the near infrared, and the far infrared, in order to account for these three types of band spectra and their fine structure, i t i s postulated that the internal energy of a molecule i s essentially of three kinds,- namely -electronic, vibrational, and rotational, each of which i s quantized. The order of magnitude of these energies i s about 5 ev., .1 ev., and .oo5 ev. respectively; hence the frequencies associated with them are in the regions 40.,000. cm"1, 800 cm"1, and 40 cm"1. " '-. ~ The structure of the electronic bands indicates'; that vibrational and rotational transitions may accompany v i b r a t i o n a l t r a n s i t i o n s whioh occur w i t h i n a gi v e n e l e c t r o n i c l e v e l , u s u a l l y the ground l e v e l . I t i s w i t h these v i b r a -t i o n r o t a t i o n bands t h a t we s h a l l be concerned. On the b a s i s of these t h r e e energy t y p e s , the ener-gy l e v e l s o f a molecule can t h e r e f o r e be r e p r e s e n t e d as i n FIGURE I . The wid e l y spaced l e v e l s are e l e c t r o n i c and to each of these corresponds a set of v i b r a t i o n a l l e v e l s . F i n a l l y , each v i b r a t i o n a l l e v e l has a set of r o t a t i o n a l l e v -e l s a s s o c i a t e d with i t . The diagram i s not to s c a l e . In g e n e r a l , t r a n s i t i o n s do not occur f o r which there i s no change i n the e l e c t r i c d i p o l e moment. However, weak l i n e s do occur where the quadropole moment or magnetic d i p o l e moment change. With t h i s i n t r o d u c t i o n then we w i l l now examine some of the most u s e f u l p a r t s of the theory of v i b r a t i o n a l s p e c t r a , some of which was d i s c u s s e d i n the seminars h e l d by the group working on molecular s t r u c t u r e . 3 F l G r U R E X E N E - R C r Y L E V E L S ELECTRONIC VIBRATIONAL *»TrvnoNm. L E V E L S L E V E L S s ofl-LEVELS A*. II THEORY An e s s e n t i a l step i n the study of the s p e c t r a o f a polyatomic molecule i s the mechanical problem of v i b r a ~ t i o n a l modes. A molecule c o n s i s t i n g of n atoms has, i n g e n e r a l , 3n-cT degrees of freedom, but s i n c e t h e r e i s one degree of f r e e i n t e r n a l r o t a t i o n i n l i n e a r m o lecules, they have 3n-5 v i b r a t i o n a l degrees of freedom. Representing t h i s number by s then, we may choose c o o r d i n a t e s q]_,....q 0, g i v i n g the displacements of the atoms from t h e i r p o s i t i o n s of e q u i l i -brium. The p o t e n t i a l energy V can now be expanded i n a T a y l o r s e r i e s of the form -where the zero s u b s c r i p t s r e f e r to v a l u e s taken at the e q u i -l i b r i u m p o s i t i o n s . Choosing V Q — 0, and seeing t h a t ^J=o we have t h e r e f o r e -For s m a l l v i b r a t i o n s , terms beyond the q u a d r a t i c may be n e g l e c t e d and we have -S i m i l a r l y the k i n e t i c energy may he w r i t t e n -2 -77  AtJ p %i ( 4 ) where the a's are f u n c t i o n s of the masses of the atoms. The corresponding Lagrangian equations of motion are » or f o r these T and V / i r ) _ M where k - 1,2,... a E v a l u a t i o n of these equations y i e l d s s equations of the form which have g e n e r a l s o l u t i o n s of the form : #/t- =" rf« ***** (J* + <*) (8) an equation of wave motion w i t h amplitude A k, phase constant eC, and frequency given by * TT~ — >^ (9) S u b s t i t u t i o n o f (8) i n (7) y i e l d s s l i n e a r homogeneous simultaneous equations Z (10> which have n o n - t r i v i a l s o l u t i o n s f o r the A f e i f the s e c u l a r determinant of the sth degree i s zero; i . e . . . (11) 5 This equation yields s values for X in terms of the a's and k 1 s. As shown by Whittaker,(ref. 14), i t i s always possible to make a linear transformation of coordinates; v l z . -a« = H ^ ( 1 2 ) such that a l l cross-product terms are eliminated in the expressions for the potential and kinetic energy (3) and (4). These may now be written -and r = i fi* ( w ) This procedure i s called normal coordinate transformation, and yields the following Lagrangian equation of motion -<L(*Z) + 1 * = o ( 1 5 ) Since T and'V contain no cross terms, and the a's are unity, equation (7) becomes -with solutions p « /f£ ^<*, ( ~ t ^ft) ^7) The here are the same as before and further, the B r*s are related to the A^'s through equation ( 1 2 ) , so that -a. flA= &L (i 8-) Hence the c's can be determined to enable us to transform from the q^ to the normal coordinates Q r„ 6 S u b s t i t u t i o n of (17) i n (12) y i e l d s $* = ^ ( . j ( 1 9 ) f o r k = 1,2,...s For c e r t a i n v i b r a t i o n s a l l the B r except one, say B r t are zero; then %M (^') =- 6^, (Jy^' * ( 2 0 ) •with k - 1,. .. s which i n d i c a t e s t h a t the nucleus undergoes simple harmonic motion of frequency )£/related to A* ' i as shown b e f o r e 'by = X with a l l v i b r a t i o n s i n the same phase. 'Such a v i b r a t i o n , where the n u c l e i , v i b r a t e w i t h the same frequency and are i n phase, Is c a l l e d a normal v i b r a t i o n of the molecule. In g e n e r a l there are s normal modes of v i -b r a t i o n given by V — JZ^p where the A's are the s s o l u -t i o n s of the s e c u l a r determinant. Knowing the a ^ j and bjLj we can evaluate the normal v i b r a t i o n f r e q u e n c i e s of the mole-c u l e . I f two. normal mo l e c u l a r v i b r a t i o n s have the same f r e -quency, and hence energy, they are degenerate. Such degen-eracy i s u s u a l l y due to m o l e c u l a r symmetry. In g e n e r a l , any complex motion of the n u c l e i can be t r e a t e d as e q u i v a l e n t to the s u p e r p o s i t i o n of the s, s e p a r a t e , r e l a t i v e l y simple, normal v i b r a t i o n s of the molecule. T h i s i s c a l l e d a L i s s a j o u s motion. We wish to connect v i b r a t i o n a l energy with these normal v i b r a t i o n s . The wave equation of the motion of the n u c l e i where "V i s the n u c l e a r wave f u n c t i o n and m^ i s the mass of the i t h nucleus. In terms o f the displacement c o o r d i n a t e s q k ( 2 1 ) becomes -Transforming t o normal c o o r d i n a t e s we have -Expressing7"= faJ?* fii "• ?V^VAe can separate equation ( 2 3 ) i n t o s independent equations of the form each b e i n g a s s o c i a t e d with one of the normal c o o r d i n a t e s . These equations are of the same form as those of the har-monic o s c i l l a t o r f o r which the ei g e n v a l u e s are known to be Ey = fa + i) * (25) where Ej i s the v i b r a t i o n a l energy v j i s the v i b r a t i o n a l quantum number and V j i s the frequency i n wave numbers of the j t h normal v i b r a t i o n . Assuming no i n t e r a c t i o n , the energi e s are a d d i t i v e ; and hence ^, , , , , „ , p _ £ E". = £ (26) where the s e l e c t i o n r u l e i s ^ Y ^ 3 3 - / , s i n c e the motions are assumed harmonic. I t i s i n s t r u c t i v e to see j u s t how s e l e c t i o n r u l e s are c a l c u l a t e d . I t i s known from quantum mechanics t h a t the 8 p r o b a b i l i t y of a t r a n s i t i o n between two s t a t e s tn and n, whose wave f u n c t i o n s ave'f^ a n d ^ n , which i s accompanied by ab-sorption, or emission of d i p o l e r a d i a t i o n , i s determined by the m a t r i x element P„ , where mn * . ^ ( 2 7 ) where/4 i s the d i p o l e moment. In g e n e r a l a l l P m n f o r which the i n t e g r a l v a n i s h e s , r e p r e -sent f o r b i d d e n t r a n s i t i o n s . A simple harmonic o s c i l l a t o r has a p o t e n t i a l of the form ^ = «i X* , where x i s the displacement and «C i s a p o s i t i v e constant. The Schroedinger equation i s then -li2f -h t&r / £ XX)y= O (28) f o r which the s o l u t i o n s are the e i g e n f u n c t i o n s 7^ c o n t a i n -i n g the Hermite Polynomials and are g i v e n by 7* = (V r where ^ _ J~^Zlc and the are orthonormal; i . e . ?/r ~~ ^ ( 3 0 ) For o s c i l l a t i o n s along the x a x i s s u b s t i t u t i o n y i e l d s t since t h e r e f o r e I n t e g r a t e onoe hut the "Y*a are o r t h o g o n a l by ( 3 0 ) such, t h a t u n l e s s k - j . t h e r e f o r e the s e l e c t i o n r u l e i s A*r ( 3 2 ) We note here t h a t anharmonicity r e q u i r e s the i n c l u s i o n of terms of h i g h e r degree i n the p o t e n t i a l and k i n e t i c energy e x p r e s s i o n s . I n c l u s i o n of the cubic term g i v e s ^ * / " - - * / - ^ as a d d i t i o n a l s e l e c t i o n r u l e p o s s i b i l i t i e s . T h i s i s accom-p l i s h e d through an a l t e r a t i o n i n the e i g e n f u n c t i o n s . U s u a l l y we d e s i r e to know what the motions are which are a s s o c i a t e d with the normal f r e q u e n c i e s . We proceed by w r i t i n g out the e x p r e s s i o n s f o r the k i n e t i c and p o t e n t i a l energies i n terms of the displacements of the n u c l e i from t h e i r e q u i l i b r i u m p o s i t i o n s . S o l v i n g the s e c u l a r determin-ant and c a r r y i n g out the subsequent c a l c u l a t i o n s we d e t e r -mine the c o e f f i c i e n t s c k r by which the displacement c o o r d i -nates q^ transform to the normal c o o r d i n a t e s Q r. By per-forming the i n v e r s e t r a n s f o r m a t i o n we f i n d t h a t the c o e f f i -and i n c l u s i o n of the q u a r t i c term g i v e s Air 10 c i e n t a c' of = (33) can be e v a l u a t e d , thus determining the forms of the d i s -placements. For the l i n e a r symmetrical YXg molecule t h e r e are a c t -u a l l y f o u r normal v i b r a t i o n s and the determinant w i l l be of the f o u r t h o r d e r , although two of the s o l u t i o n s are i d e n t i -c a l due to degeneracy. D i r e c t s o l u t i o n s of normal c o o r d i n -ate problems are d i f f i c u l t t o o b t a i n due to the s i z e of the determinants i n v o l v e d . F o r t u n a t e l y the a p p l i c a t i o n of group theory makes i t p o s s i b l e to use the symmetry p r o p e r t i e s o f molecules, to e f f e c t the s e p a r a t i o n of both T and V i n t o terms i n v o l v i n g no cross-terms between c o o r d i n a t e s i n v o l v e d i n one p a r t with t h o s e ' i n any other. These c o o r d i n a t e s are c a l l e d symmetry c o o r d i n a t e s . The c o o r d i n a t e s i n v o l v e d i n each p a r t have the same symmetry - c h a r a c t e r i s t i c s , which are d i f f e r e n t from those i n other p a r t s . Consequently the secu-l a r equation i s immediately f a c t o r e d i n t o a number of equa-t i o n s of lower degree, each i n v o l v i n g only one set of sepa-r a t e d c o o r d i n a t e s with the same symmetry. F o r t u n a t e l y , f o r l i n e a r symmetric molecules such as CSg, each normal v i b r a -t i o n belongs to a d i f f e r e n t c l a s s . In t h i s case there are only three constants t o be evaluated and there are three normal f r e q u e n c i e s from which to do:'it. Herzberg, ( r e f . 8, pages 153-154), performs t h i s c a l -c u l a t i o n and a r r i v e s at <£77""'>i4 = °~" */» (34) (35) (36) 11 In equations ( 3 4 ) , ( 3 5 ) , and (36"), a-^ i s the f o r c e constant of the X-Y hond; a-^ i s the i n t e r a c t i o n constant of the two bonds; and a-33 i s the f o r c e constant f o r the b i n d i n g of the molecule. We note t h a t the degenerate v i b r a t i o n V 2 depends only on a ^ , whereas the non-degenerate v i b r a t i o n s and depend on a-^ and a-j^* The normal v i b r a t i o n s of the l i n e a r Y X 2 molecule are shown i n FIGURE I I . corresponds to a motion i n which the S atoms o s c i l l a t e symmetrically with r e s p e c t to the C atom. Since the e q u i l i -brium and dynamic c o n f i g u r a t i o n s are both symmetric the d i p o l e moment i s always zero and hence V-^  i s i n f r a r e d i n a c t i v e . V 2 c o n s i s t s of a motion of the C atom a g a i n s t the S atoms i n a l i n e p e r p e n d i c u l a r to the a x i s of symmetry of the mole-c u l e . The motion i s doubly degenerate s i n c e t h e r e are two d i r e c t i o n s , p e r p e n d i c u l a r to t h i s a x i s , along which the f o r c e s are i d e n t i c a l . V 2 i s i n f r a r e d a c t i v e with a f i n e s t r u c t u r e of u n i f o r m l y spaced l i n e s and with a s t r o n g zero (Q) branch. V, i s an o s c i l l a t i o n o f the G atom with r e s p e c t to the 3 S atoms, along the symmetry a x i s . T h i s v i b r a t i o n possesses a changing e l e c t r i c moment and i s s t r o n g l y i n f r a r e d a c t i v e . The f i n e s t r u c t u r e a s s o c i a t e d with t h i s band c o n s i s t s of P and R branches but no Q branch. In v*± the carbon atom does not move, whereas i n V g and the s e p a r a t i o n of the sulphur atoms i s f i x e d . As was mentioned b e f o r e , V 2 i s degenerate, and hence we can take a l i n e a r combination of V~ and V and o b t a i n 12 F I G r U R E H NORMAL VIBRATIONS OF A LINEAR, Sy/^WETfMCRL X o X © X O—» another motion of the same frequency, hut where not a l l atoms move i n phase or i n s t r a i g h t l i n e s . F or Instance, to r o t a t e i n a c i r c l e about the a x i s . T h i s i s c a l l e d nuta-t i o n and r e s u l t s i n a constant angular momentum about the a x i s . We s p e c i f y t h i s angular momentiim by the quantum num-ber 1. As a r e s u l t o f t h i s new concept we r e d e f i n e normal v i -b r a t i o n s t o be those f o r which a l l atoms have the same f r e -quency and the C a r t e s i a n c o o r d i n a t e components of t h e i r d i s -placements perform simple harmonic motion. I t i s important to note t h a t both c l a s s i c a l e l e c t r o -dynamics and the quantum theory r e q u i r e t h a t the f r e q u e n c i e s of the emitted or absorbed r a d i a t i o n be the same as the f r e -quency of the mechanical v i b r a t i o n s . There are s e v e r a l approximate methods f o r deter m i n i n g the normal f r e q u e n c i e s , i n c l u d i n g the method of c e n t r a l f o r c e s , the method of extreme f i e l d s , and the va l e n c e f o r c e method. The l a s t mentioned method works w e l l f o r l i n e a r Y X 2 molecules. The b a s i c i d e a of the valence f o r c e treatment i s th a t displacements of atoms along the d i r e c t i o n of t h e i r bond b r i n g s r e s t o r i n g f o r c e s much g r e a t e r than those due to changes i n angles between valence bonds. On pages 19 and 20 Wu a r r i v e s at the expr e s s i o n s ?[, = (^7) V„ -f- V where V i s 90°out of phase causes each nucleus 2a K2b 2b (38) (39) 13 which may be shown to be i d e n t i c a l w i t h our p r e v i o u s expres-sions ( 3 4 ) , ( 3 5 ) , and ( 3 6 ) , where a-,-, =• k-, ; a ==• 0 ; and 2 ~ k f c . Herzberg g i v e s f o r CS 2 1 pr k 1 = 8 . 1 x 1 0 dynes / cm from V 1 5 k =r 6 . 9 x- 1 0 dynes / cm from y/ 5 k& . 2 3 4 x 1 0 dynes / cm from y 2 the q u a l i t y of the agreement between k^ from and i n -d i c a t e s the q u a l i t y of the valence f o r c e f i e l d approximation. The p r e v i o u s l y accepted v a l u e of was 1 5 2 3 cm""*'. The - 1 value which we measured was 1 5 3 5 cm . R e c a l c u l a t i n g k^ from as we measured i t , by equation ( 3 9 ) , one o b t a i n s the v a l u e ^ s 7.1 x 10 dynes / cm. T h i s I n d i c a t e s an even c l o s e r ap-proximation than was p r e v i o u s l y found. On the b a s i s of t h i s v a l e nce f o r c e f i e l d . Mecke d e v i s e d a n o t a t i o n f o r c l a s s i f y i n g normal v i b r a t i o n s . He employs the symbol ^ f o r v a l e n c e ( s t r e t c h i n g ) v i b r a t i o n s , and S" f o r de-formation (bending) v i b r a t i o n s . For a l i n e a r molecule which has ( 3 n - 5 ) normal v i b r a t i o n s , there are ( n - 1 ) v a l e n c e v i b r a -t i o n s , and ( 2 n - 4 ) deformation v i b r a t i o n s . F u r t h e r , when the d i p o l e moment changes, i t may change i n d i r e c t i o n s p a r a l l e l or p e r p e n d i c u l a r to the symmetry axis,,hence an a d d i t i o n a l c l a s s i f i c a t i o n i s p o s s i b l e . P a r a l l e l changes are l a b e l l e d TT» p e r p e n d i c u l a r changes are l a b e l l e d t ( f o r the German "par-a l l e l " and "senkrecht" r e s p e c t i v e l y ) . In t h i s n o t a t i o n , f o r a l i n e a r Y X 2 molecule one sees t h a t 14 V, i s a \H*1 v i b r a t i o n V,is a t(r)vibration V ^ i s a \j(ir) v i b r a t i o n . In a d d i t i o n , when a molecule possesses a c e n t r e of sym-metry, v i b r a t i o n s which do not a l t e r any symmetry property of the molecule w i t h r e s p e c t to i n v e r s i o n at the centre of symmetry, are l a b e l l e d g , ( f o r the German gerade - even). V i b r a t i o n s which are anti-symmetric.are l a b e l l e d U , ( f o r the German ungerade). On t h i s b a s i s we can formulate a r u l e of mutual e x c l u -s i o n f o r molecules with a c e n t r e of symmetry. Since we get a changing d i p o l e moment only f o r t r a n s i t i o n s g**u - - i . e . between s t a t e s of o p p o s i t e symmetry and s i n c e f o r the Raman e f f e c t t r a n s i t i o n s occur only between s t a t e s of the same sym-metry g«-»g or u*-»u. We say -therefore t h a t t r a n s i t i o n s which are allowed i n the i n f r a r e d are f o r b i d d e n i n the Raman spec-trum and v i c e - v e r s a . In the l i n e a r symmetric YX 2 molecule the d i p o l e moment i s zero f o r V/ , at a l l times; hence i t i s not i n f r a r e d a c t i v e but i s Raman a c t i v e . V 2 and i n v o l v e changing d i p o l e mo-ments however, and hence appear i n the i n f r a r e d though not i n the Raman spectrum. In a d d i t i o n to the normal v i b r a t i o n s , overtone combina-t i o n and d i f f e r e n c e bands may occur, where the s e l e c t i o n r u l e s w i l l depend on the molecular symmetry. R e c a l l i n g equation (26) (26) 15 we f i n d t h a t these may be r e p r e s e n t e d approximately by y/ = n.v, + n,V* + r\*V 3 ( 4 o ) where the n's may have i n t e g r a l v a l u e s -4- or — s o long as i s a p o s i t i v e number. I f on l y one n i s not zero then we have an overtone band; i f a l l the n's are p o s i t i v e we have a com-b i n a t i o n band; and i f some n i s negative we have a d i f f e r e n c e band. The f o u r t h quantum number 1, which i n d i c a t e s the ang-u l a r momentum a s s o c i a t e d w i t h the degenerate V2 must be given to completely s p e c i f y the s t a t e . The f o u r numbers n-^, n 2 , 1, and n^ completely s p e c i f y the s t a t e , w r i t t e n i n t h a t o r d e r -f o r example - 1 , 2 , 2 , 0 means n-^  = 1 ; n 2 « 2 ; 1 « 2 ; n^=srO. The p o s s i b l e v a l u e s of 1 are r e s t r i c t e d so t h a t 1 can on l y have v a l u e s n 2 , n 2 - 2 , n 2 - 4 , e t c . I f a molecule has a centr e of symmetry then t o t a l l y sym-metric v i b r a t i o n s have no overtones at a l l . For CS 2 t h i s means n e i t h e r V-^  nor any overtone of V-^ w i l l appear i n the i n f r a r e d . Dennison ( r e f . 5) has shown t h a t 1 . t r a n s i t i o n s i n v o l v i n g p e r p e n d i c u l a r change i n d i p o l e mo-ment (V 2) may occur i f and only i f (a) n 2 i s odd, (b) n i s even, 3 (c) 41= ± l , where n-^  may have any v a l u e . 2 . t r a n s i t i o n s i n v o l v i n g a p a r a l l e l change i n d i p o l e moment (V^) may occur i f and only i f (a) n 2 i s even, (b) n, i s odd, 3 to) Al = 0 , where n^ may have any v a l u e . 16 These c o n c l u s i o n s depend on the symmetry p r o p e r t i e s of the k i n e t i c and p o t e n t i a l f u n c t i o n s and not on the a p p r o x i -mations employed. As b e f o r e , writing*v=f l iV i+ruV»+ n^V3 f or the rough l o c a t i o n of an overtone band where the n's are p o s i t i v e such bands occur f o r which ng-* n^ i s odd. From 1. and 2. above we see th a t I f n 2 i s even, the change i n e l e c t r i c mo-ment i s p a r a l l e l , and i f n 2 i s odd, the change i s perpendic-u l a r . For three c o l i n e a r symmetrical atoms the sum of the f r e -quencies to two observed bands (overtones, combination bands, or fundamentals) w i l l not be a c t i v e . For i f V and V ' are two a c t i v e f r e q u e n c i e s , the new n 2 ' s and n^'s w i l l both be odd, and hence t h e i r sum cannot be odd, as r e q u i r e d f o r r a d -i a t i o n . One way of r e c o g n i z i n g which f r e q u e n c i e s are to be asso-c i a t e d w i t h observed v i b r a t i o n a l bands i s by t h e i r observed r o t a t i o n a l s t r u c t u r e . For l i n e a r molecules the r o t a t i o n a l energy I s given by p _ 1*2 T(T + l) where J i s the r o t a t i o n a l quantum number and I i s the common value of the two non-zero components o f - t h e moment of i n e r t i a . The v i b r a t i o n a l energy f o r a given normal v i b r a t i o n may be where d i s the degeneracy of the v i b r a t i o n , d =1 f o r a l i n -ear o s c i l l a t o r , but f o r V 2 of CS 2, d=-2. As an approximation, the v i b r a t i o n a l and r o t a t i o n a l e n e r g i e s are a d d i t i v e , hence or n e g a t i v e i n t e g e r s , so l o n g as N) i s p o s i t i v e , then only w r i t t e n (42) 1? we w r i t e f L-f-%)/,cV + J L T{T+0 (43) For the simple case o f 4 v * t l as a s e l e c t i o n r u l e , we t r e a t the two types of v i b r a t i o n , p a r a l l e l and p e r p e n d i c u l a r , sep-a r a t e l y . P a r a l l e l bands may be shown to have a s e l e c t i o n ruledJ= - 1 . F o r d J ^ - l we get the n e g a t i v e or P branch of r o -t a t i o n a l f i n e s t r u c t u r e , and f o r d J ^ * * " ! we get the p o s i t i v e or R branch. The Q, branch i s m i s s i n g . P e r p e n d i c u l a r bands may be shown to have t r a n s i t i o n s i n -v o l v i n g A J - 0 i n a d d i t i o n t o 4 J = i l . A J - 0 i s allowed when-ever n u t a t i o n o c c u r s ; t h e r e f o r e we o b t a i n , i n a d d i t i o n to the P and R branches, a strong Q branch. I t i s the presence of t h i s Q, branch at the centre of a perpendicular, v i b r a t i o n band which d i s t i n g u i s h e s i t from p a r a l l e l bands, e x h i b i t e d by fun-damental f r e q u e n c i e s of a l i n e a r polyatomic molecule. Since we have i d e n t i f i e d y 2 a s a P e r p e n d i c u l a r band, and as a p a r a l l e l band, the two w i l l be d i s t i n g u i s h a b l e from t h e i r s t r u c t u r a l shapes. The s e p a r a t i o n of the r o t a t i o n a l • f t f i n e s t r u c t u r e l i n e s should be a"x c » and i s too f i n e f o r our instrument to r e s o l v e . As a matter of f a c t , n u c l e a r s p i n e f f e c t s t h i s v a l u e . Symmetrical l i n e a r molecules e x h i b i t a l -t e r n a t i n g i n t e n s i t i e s i n l i n e s . In t h i s case the spins of the two X n u c l e i determine the r e l a t i v e i n t e n s i t i e s . Since the spi n of the sulphur atoms i s zero, one s e r i e s w i l l be absent and the s e p a r a t i o n becomes ^£]pc • w e are unable to d e t e c t i t . Three more e f f e c t s are of i n t e r e s t i n d e t e r m i n i n g the p o s i t i o n s of v i b r a t i o n a l bands. The f i r s t of these i s a c c i -18 d e n t a l resonance, n o t i c e d by Fermi i n 1 9 3 1 ; the second i s an i s o t o p e e f f e c t ; and the t h i r d i s molecular a s s o c i a t i o n . Fermi-resonance occurs when two v i b r a t i o n a l l e v e l s b e l o n g i n g to d i f f e r e n t v i b r a t i o n s ( o r combinations) have n e a r l y e q u a l energy. Such resonance o c c u r s only f o r l e v e l s of the same symmetry. Fermi-resonance of Vc. with 2 Vy i s l a r g e and i s accomplished through cubic combinations of the e a r l y terms i n the e x p r e s s i o n f o r the p o t e n t i a l , e q u a t i o n ( 3 ) , whereas resonance between K:and 3 Vj or h i g h e r overtones i s accom-p l i s h e d through q u a r t i c combinations. • When two l e v e l s are a c c i d e n t a l l y degenerate they r e p e l each o t h e r , g i v i n g an i n c r e a s e d s e p a r a t i o n between them. The two s t a t e s share t h e i r wave-functions and two new f u n c t i o n s are formed g i v i n g bands at d i s p l a c e d f r e q u e n c i e s . Both bands now behave l i k e fundamentals and t h e i r i n t e n s i t i e s are more n e a r l y e q u a l . In the case of CSg, and 2 l ^ are resonant but the resonance i s loose and has l i t t l e e f f e c t . The i s o t o p e e f f e c t i s the change i n normal (and other) v i b r a t i o n s due to l s o t o p i c changes. Replacement of an atom or atoms i n a molecule by i s o t o p e s l e a v e s the p o t e n t i a l f i e l d unchanged, as i s expected, but the change i n mass a l t e r s the f r e q u e n c i e s of the normal v i b r a t i o n s and may lower the sym-metry i n the molecule. In the case of CS 2 replacement of only one sulphur atom d e s t r o y s the symmetry, and "v may ap-pear i n the i n f r a r e d . . Wu ( r e f . 1 5 ) t r e a t s the problem f o r the l i n e a r symmetrical YX molecule on pages 41 and 42. 19 In g e n e r a l f o r CS 2 the percentage of molecules of o t h e r 12 3 2 i s o t o p e s than C S 2 I s s m a l l and hence the bands due to these i s o t o p e s are weak. The side-band to V, i s a t t r i b u t e d t o an 3 i s o t o p i c s h i f t . The most n o t i c e a b l e e f f e c t of m o l e c u l a r a s s o c i a t i o n i s the decrease i n frequency of v i b r a t i o n s observed i n the l i q u i d s t a t e of c e r t a i n molecules from the v a l u e s i n the u n - a s s o c i -ated vapour s t a t e . In of CS 2 t h i s amounts to about 2 5 cm. The work i s to be continued examining the e f f e c t of pressure and temperature on t h i s s h i f t . 2 0 ' I l l ' EXPERIMENTAL' WORK The experimental work may.be d i v i d e d i n t o f i v e p a r t s : (A) C o n s t r u c t i o n of a s u i t a b l e room and c o n t r o l of hum i d i t y , temperature, and v i b r a t i o n t h e r e i n . (B) C o n s t r u c t i o n of a u x i l i a r y equipment to c o n t r o l c o n d i -t i o n s of temperature and pressure of our samples i n the one-meter gas c e l l . (C) E x perimental arrangement of the ..Instruments and t h e i r c o n t r o l . (D) C a l i b r a t i o n o f the spectrometer. (E) A b s o r p t i o n spectrum of CSg. ( T h i s work i s to be c a r r i e d on throughout- the summer by myself and next term by o t h e r s . Now t h a t the apparatus i s p r o p e r l y a d j u s t e d and c a l i b r a t e d , r e s u l t s w i l l be forthcoming more r a p i d l y ) . (A) F o r optimum performance of t h i s f i n e apparatus i t i s ne-cessary t h a t the instruments be l o c a t e d where there i s 1. low r e l a t i v e humidity - t o minimize h y g r o s c o p i c a c t i o n which damages NaCl and KBr p a r t s i n humid l o c a t i o n s . 2. constant temperature - to prevent e x c e s s i v e d r i f t i n the instrument. 3. steady p o s i t i o n - to minimize v i b r a t i o n s which superpose " n o i s e " on the r e a d i n g s . 21 In order t o s a t i s f y these c o n d i t i o n s , a room measuring 9' x 8' x 6 1 was c o n s t r u c t e d , t h e r m a l l y lagged, and equipped with a i r - l o c k doors. The problem o f steady p o s i t i o n was e l i -minated by l o c a t i n g the room on the basement, f l o o r i n Room 109. The q u e s t i o n of humidity c o n t r o l r e q u i r e d c o n s i d e r a b l e experimental work; f o r the humidity i n U n i v e r s i t y l a b o r a t o r -i e s approsimates 75% f o r f i v e summer months and 50% f o r f i v e months i n winter. E a r l y attempts t o dry a i r wi t h t r a y s of C a C l 2 and c i r c u l a t i n g fans showed a n e g l i g i b l e e f f e c t i n an o r d i n a r i l y c l o s e d but unsealed room. The manufacturers of the spectrometer suggested t h a t most s a t i s f a c t o r y i n s t a l l a -t i o n s were set up i n a s e a l e d room w i t h humidity c o n t r o l . I t was de c i d e d to f o l l o w t h i s a d v i c e ; but s t i l l no commercial f i r m i n Vancouver had had experience on such u n i t s . The Can-adian General E l e c t r i c f i n a l l y agreed t o undertake the i n s -t a l l a t i o n , and the sealed room was c o n s t r u c t e d t o s p e c i f i c a -t i o n s by the U n i v e r s i t y c a r p e n t e r s . The f i n a l room has now been o p e r a t i n g s a t i s f a c t o r i l y f o r f i v e months, a f t e r c o n s i d -erable experimentation and a l t e r a t i o n s . The experimental arrangement of our a i r - c o n d i t i o n i n g u n i t i s as f o l l o w s : (see photo I) - the r e f r i g e r a t i o n c o i l and fan enclosed i n a c a b i n e t measuring 2' x 2' x 2' was mounted i n the south east corner of Room 109 w i t h the con-densing u n i t p l a c e d o u t s i d e the room i n order to reduce v i -b r a t i o n s . The compressor motor i s a G.E. -jsH.P. 110V 6 0 c y c l e u n i t , model 6M365U, and the c o o l i n g agent i s Freon 12 ( d i -22 c h l o r o d i f l u o r o m e t h a n e ) . T h i s apparatus i s not normally used as a d e h u m i d i f i e r . In order to make i t such i t was necessary to add and modify s e v e r a l p a r t s . Since a constant temperature i s r e q u i r e d i n the room, an extension measuring approximately 1 ' x 1 ' x 2 ' 6 " i n the form of an a i r t u n n e l , was a f f i x e d to the c o i l mounting, through which the a i r passes a f t e r b e i n g c o o l e d by the c o i l . In the end s e c t i o n of t h i s t u n n e l are l o c a t e d three 6 0 0 watt cone-heaters to reheat the a i r b e f o r e i t r e t u r n s to the room. I n the c e n t r e zone, three f i n e mesh wire screens are l o c a t e d t o prevent r a d i a n t heat from r e a c h i n g the c o i l . T h i s was found necessary as the f i n a l s e c t i o n of the c o i l rose 6°C under t h i s r a d i a t i o n . A f t e r the screens were put i n the r i s e was l e s s . t h a n 2°C. Since the r a t e at which a i r passes over the c o i l a f f e c t s the time taken to reach the u l t i m a t e humidity o b t a i n a b l e , a t h r o t t l e was c o n s t r u c t e d to c o n t r o l the flow of a i r . F i n a l l y , the drainage system was changed i n o r d e r to f a c i l i t a t e the r e -moval of water from the room, once i t i s c o l l e c t e d by the c o i l . A " d r i p pan" wi t h steep, c a r e f u l l y waxed s i d e s was s u b s t i t u t e d f o r the shallow one s u p p l i e d with the c o o l i n g u n i t . C o n t r o l of humidity and temperature by t h i s a i r - c o n d i t -i o n i n g u n i t i s e f f e c t e d by the f o l l o w i n g system of c o n t r o l s : a w a l l thermostat i s used to c o n t r o l the cone-heaters and hence the room temperature, i n the customary manner. The hu-mi d i t y c o n t r o l s are standard with t h i s c o i l , and operate as f o l l o w s : 2 3 1. the t h r o t t l e v a l v e , a v a r i a b l e c o n t r o l at the c o i l , which c o n t r o l s the r a t e of flow of the f l u i d i n p u t to the c o i l ' s evaporator.. 2 . the back-pressure c o n t r o l , on the condenser u n i t , which stops the pump when the r e t u r n p ressure of the r e f r i g e r -ant i s too low. 3. the s o l e n o i d v a l v e , on the f l u i d i n p u t l i n e , a p o s i t i v e o n - o f f f l u i d s w i t c h , operated by the h u m i d i s t a t , which corresponds i n the c o n t r o l of humidity to a thermostat. When the h u m i d i s t a t measures a room humidity lower than i t i s set f o r , i t actuates the s o l e n o i d v a l v e , s t o p p i n g the flow of l i q u i d r e f r i g e r a n t to the c o i l s . Consequently, the r e t u r n pressure drops r a p i d l y and the back-pressure c o n t r o l stops 'the condenser pump. E i t h e r the motor switch or the back-pressure c o n t r o l can stop the pump. The back-pressure can be actuated by the hu m i d i s t a t when the humidity of the room i s below s e t t i n g , or by the temperature, and consequently the p r e s s u r e , of the r e t u r n i n g r e f r i g e r a n t being too low. I f the l a t t e r case oc-curs o f t e n , a g r e a t e r mass r a t e of flow of a i r may be passed over the c o i l t o gi v e b e t t e r performance. As a s a f e t y f e a t u r e of the o p e r a t i o n , we have two r e l a y s which are actuated when the pump motor i s o f f . One of these r e l a y s t u r n s o f f the cone-heaters and prevents t h e i r over-h e a t i n g the room or the c o o l i n g c o i l s . The oth e r r e l a y stops the fan and prevents the warm, dry a i r from p a s s i n g over the c o i l , p i c k i n g up moisture and r e d i s t r i b u t i n g i t about the room. 2 4 The theory of d e h u m i d i f y i n g c o i l a i s w e l l t r e a t e d by Brown and Marco ( r e f . 3 ) ; and the method of G. L. Tuve ( r e f . 13), which i s b r i e f l y o u t l i n e d below, p r o v i d e s a simple, d i -r e c t method of c a l c u l a t i n g the f i n a l humidity o b t a i n e d from a r e f r i g e r a t i o n c o i l system. T h i s i t does by means of a non-l o g a r i t h m i c psychrometric c h a r t such as G. E. puts out. T h i s method i s p o s s i b l e s i n c e Tuve has shown, by mathematical an-a l y s i s and by t e s t s , t h a t the behaviour of a wet c o i l i a sub-s t a n t i a l l y the same as t h a t of the same c o i l when dry, but with i t a surface at the dew-point temperature of the e n t e r i n g a i r . FIGURE I I I repreaenta a p o r t i o n of a n o n - l o g a r i t h m i c psychrometric c h a r t . The p o i n t "a" r e p r e a e n t a a aet of poaa-i b l e s t a r t i n g p o i n t c o n d i t i o n a , namely- a conatant room d r y -bulb temperature of 70°F and i n i t i a l R.H. of 60%. The l i n e "ab" repreaenta a p o a s i b l e l i n e of c o o l i n g and d e h u m i d i f y i n g on paasage of a i r over the c o i l . I t a alope depends upon the r a t i o of l a t e n t heat removed to t o t a l heat removed i n the pro-cesa. The l i n e "be" repreaenta subsequent r e h e a t i n g to 70°F, with a f i n a l drop i n humidity at t h i a temperature. Over a p e r i o d of a e v e r a l auch c y c l e a the room reachea a minimum hu-m i d i t y , depending a o l e l y upon the d r y - b u l b temperature of the room, and the dew-point temperature maintained by the c o i l . At the present t h r o t t l e s e t t i n g theae temperaturea are 70°F and 40°F r e s p e c t i v e l y (meaaured), and hence the minimum ob-t a i n a b l e humidity i a 35.5$. T h i a c a l c u l a t i o n i s confirmed by v a l u e s given i n the Handbook of Phys lea, and Chemi atrv,. 25 F l G t O « £ 3 i r PSYCH ROMETRK <^5>- _ / ' U n n T h i s value i s w e l l below the humidity r e q u i r e d ; under normal c o n d i t i o n s the a i r - c o n d i t i o n i n g u n i t e a s i l y m a i n t a i n s a R.H. of hO%±l%, and a temperature of 70°Fi-l°F, even when s e v e r a l persons are i n the room. (B) The a u x i l i a r y equipment c o n s i s t s of two temperature con-t r o l s f o r the gas a b s o r p t i o n c e l l . One of these i s a t h y r a -t r o n type temperature c o n t r o l , and the other i s a s e l f - b a l -ancing b r i d g e type temperature c o n t r o l . In g e n e r a l , observers of the a b s o r p t i o n of CSg i n the. vapour phase have made l i t t l e e f f o r t to c o n t r o l the tempera-tu r e and pressure of t h e i r samples d u r i n g a n a l y s i s . For ex-ample, P l y l e r and Humphreys ( r e f . 10) merely p l a c e d a s m a l l q u a n t i t y of l i q u i d GS 2 i n the c e l l and allowed i t t o stand f o r two hours t o reach c o n d i t i o n s of e q u i l i b r i u m . Since they were i n t e r e s t e d p r i m a r i l y i n the appearance or f a i l u r e t o appear of c e r t a i n bands, i t was necessary f o r them to know the exact temperature and pressure of the gaseous CSg. On the other hand, the v a r i a t i o n i n i n t e n s i t y of a b s o r p t i o n o f the bands i s one of the t h i n g s which we hope to observe. As a r e s u l t , we have b u i l t these temperature c o n t r o l s . The s e l f - b a l a n c i n g wheatstone b r i d g e type temperature c o n t r o l i s shown i n photo I I and FIGURE IV. The g e n e r a l p r i n c i p l e ^ o f the s e l f - b a l a n c i n g b r i d g e type thermo-regulator ( r e f s . 11 and 12) i s as f o l l o w s : the h e a t i n g c o i l i t s e l f i s made to be one arm of the b r i d g e c i r c u i t , w i t h the b r i d g e - c u r r e n t i n t h i s . a r m s e r v i n g as the h e a t i n g c u r r e n t . 26 PHOTO I FRONT VIEW OF TEM PERATURE CONTROL F l t V U R E l Z "TV l i e R - R E S I S T A N C E . R h - R H E O S T A T R c R E L A Y Aft - A D D I T I O N A L RES 1 S T A N C E S - S T O P COCK Siv - S W I T C H 6 - 8 0 U . B S P - C O N T A C T P O I N T C - C O N S T R I C T I O N Tr -T - T U B E : T h i s branch i s made of n i c k e l with a h i g h temperature c o e f -f i c i e n t of r e s i s t a n c e , and the other branches are made of substances with low c o e f f i c i e n t s . The temperature i s regu-l a t e d by keeping the r e s i s t a n c e o f the h e a t i n g element.con-s t a n t , and the c o n t r o l a c t s on a change i n r e s i s t a n c e of .5$. T h i s type of r e g u l a t o r has three important advantages: 1. i t s o p e r a t i o n i s comparatively simple. 2. there i s no temperature l a g between r e g u l a t o r and h e a t i n g elements.' 3. i t takes up no u s e f u l space. T h i s i s important i n our work s i n c e the c l e a r a n c e s between the gas a b s o r p t i o n c e l l and spectrometer are small and the c o i l cannot o b v i o u s l y be p l a c e d i n s i d e the c e l l , where i t would be i n the o p t i -c a l path. The U n i v e r s i t y glass-blower, Wm. Pye., d i d an e x c e l l e n t job of the g l a s s work', u s i n g Strong's diagram as a model'. The e s s e n t i a l f e a t u r e s o f t h i s d e v i ce are the f o l l o w i n g : (a) the two bulbs are of equal volume. (b) they c o n t a i n i d e n t i c a l h e a t i n g c o i l s , 6*0 m i l . nichrome wire of r e s i s t a n c e 15 ohms. .(c) the tube i s c o n s t r i c t e d at C i n o r d e r to prevent r a p i d o s c i l l a t i o n of the mercury, which causes unnecessary " c h a t t e r " i n the r e l a y o p e r a t i o n . The e l e c t r i c connections i n FIGURE IV are obvious and the o p e r a t i o n a l procedure i s simple. The bu l b s are a i r -f i l l e d at about 1 A.P. and the f i x e d r e s i s t a n c e R (advance) i s a d j u s t e d through T, by a d d i t i o n or removal of mercury 27 so that the s u r f a c e of the l e f t hand column of mercury i s j u s t at the c o n t a c t P. We now have e q u i l i b r i u m c o n d i t i o n s . I f the temperature of the f e e l e r r e s i s t a n c e becomes too h i g h or too low, the h e a t i n g i n the two bulbs i s unequal, and the r e s u l t i n g change i n p r e ssure i n the bulba opens or c l o s e s the mercury c o n t a c t . T h i s i n t u r n operates a r e l a y a c t u a t i n g the h e a t i n g and bridge c u r r e n t . In e f f e c t , the r i g h t hand bulb i s a measure of v o l t a g e of the h e a t i n g element, and s i n c e the r a t i o of the f i x e d r e s i s t a n c e R to t h a t of the l e f t hand bulb i s c o n s t a n t , t h i s b u l b i s a measure of the h e a t i n g c u r r e n t . E s s e n t i a l l y we are b a l a n c i n g the two. In order t h a t the temperature of the c e l l may be con-t r o l l e d by the means mentioned, we have wrapped our one meter c e l l w ith a h e a t i n g c o i l . I t has been i n s u l a t e d with a l a y e r of asbestos f o l l o w e d by a l a y e r of f e l t . The r e s i s t a n c e t h e r -mometer i s l o c a t e d as shown i n FIGURE V. The system d e v i s e d to supply CSg to the c e l l at a d e f -i n i t e p r e s s u r e , and the method of measuring the pressure i s shown i n FIGURE V and photo I I I . The pressure i s t o be con-t r o l l e d by means of an open connection t o the f i l l i n g - s y s t e m which i s kept at a f i x e d temperature (lower than t h a t of the c e l l ) i n the bath, the temperature chosen so t h a t the vapour pressure at t h a t temperature i s the d e s i r e d p r e s s u r e i n the c e l l . As t h i s p a r t of the apparatus has not been needed f o r the work performed to t h i s d a t e , i t s u s e f u l n e s s cannot y e t be assessed. 28 H or Ui 2 3 O h Z o C z < s H - HEATERS VV.B. S — S T O P COCKS C - CONOEMSER T " THEflMoSTAT W.S. " WATER BATH TT. " TH 15TL.E TU6E. RX " RESiSfftNtE THERMOMETER TEMPERATURE. s 4 HYVAC POMP (C) Experimental .arrangement of the apparatus: our equip-ment c o n s i s t s of the f o l l o w i n g u n i t s -1. Perkin-Elmer (P-E) model B i n f r a r e d spectrometer. 2. one-meter gas a b s o r p t i o n c e l l , 10 cm. gas a b s o r p t i o n c e l l , and l i q u i d c e l l s of a s s o r t e d t h i c k n e s s e s . 3. P-E model 53 (G.M.) 75 c y c l e D.C. breaker a m p l i f i e r . 4. Brown e l e c t r o n i c r e c o r d e r . 5. P-E model 54 power supply. . 6. Supreme e l e c t r i c powerstat. 7. C o n t r o l p a n e l o'f our own d e s i g n . 8. Synchronous wave-drive apparatus of our own making. 9. Weston model 310 wattmeter to measure power used by the g l o b a r . I t i s customary to use the P-E c a b i n e t , c o n t r o l p a n e l , and P-E synchronous wave-drive u n i t s w i t h t h i s spectrometer. A saving of approximately $800 was e f f e c t e d by e l i m i n a t i o n of these p a r t s . R e d i s t r i b u t i o n of the components e l i m i n a t e d the n e c e s s i t y of a s p e c i a l c a b i n e t . A m o d i f i e d c o n t r o l p a n e l of our own making was found s a t i s f a c t o r y , and a constant speed A.C. motor combined with a v a i l a b l e gears g i v e s us f o u r speeds f o r the wave ?d*"ive sweeps of the spectrum. These speeds a r e -1' 02"; 2' 04"; 5' 12"; 10 f 24" per r e v o l u t i o n of the wave length d r i v e s h a f t , which r e p r e s e n t s 1000 d i v i s i o n s on the wave-drive micrometer. FIGURE VI g i v e s a b l o c k diagram of the apparatus, and FIGURE V I I shows a p l a n view of the room and equipment. The l i n e v o l t a g e of approximately 110 V i s s u p p l i e d through the 29 B L O C K D l f l G r R A M . OF APPARATUS F I G U R E 3ZH PLAN VIEW OF EQUIPMENT S O L A "TRANSFORMER WATT P f t N E l , TEMPERATURE tONT«OL SPECTROMETER W\PL»FlfiK MoToP, S H I E L D AIR LOCK RECORDER DOORS / power switch on the c o n t r o l p a n e l , t o the poweratat and to the r e c o r d e r mechanisms. The powerstat s u p p l i e s a constant 110 Y i n p u t to the a m p l i f i e r ' s power supply and a v a r i a b l e v o l t a g e f o r the g l o b a r , our "black-body" type source of r a d -i a t i o n . The wattmeter i s used to c o n t r o l the power used by the g l o b a r i n order t h a t i t s r a d i a t i o n curve may be d u p l i c -ated. The power supply sends a h i g h - v o l t a g e D.C. in p u t t o the a m p l i f i e r , and i n a d d i t i o n s u p p l i e s 80 v o l t s A.C. f o r the c i r c u i t breaker. The a m p l i f i e r c o n v e r t s minute D.C. i n -put v o l t a g e s from the thermocouple to A.C, by means of a mo-t o r - d r i v e n c i r c u i t breaker, a m p l i f i e s t h i s s m a l l A.C. v o l t -age, r e c t i f i e s i t by means of a second synchronous c i r c u i t breaker, and passes i t through a f i l t e r to t h e . r e c o r d e r pen mechanism. FIGURE V I I I i s the c i r c u i t diagram of the c o n t r o l p a n e l and a f f e c t e d p a r t s of .the instruments; the o p e r a t i o n of the c o n t r o l s i s c l e a r - before any pa r t o f the apparatus can be operated the power switch P must be tu r n e d "on". I t i s eas-i l y seen t h a t , even i f the master s w i t c h M i s " o f f " , c l o s i n g of the switch marked "Pen" j o i n s t e r m i n a l s A and B i n the power t e r m i n a l b l o c k of the r e c o r d e r , which a c t u a t e s the pen. In a s i m i l a r manner, c l o s i n g of switches Ch ( c h a r t ) , or W.D. (wave-drive motor), operates these p a r t s r e s p e c t i v e l y . . In normal o p e r a t i o n o f the instrument, d u r i n g the pro- . cess of scanning the spectrum, the c h a r t and pen are r e q u i r e d only when the wave-drive motor i s "on". Consequently, a master switch M i s provided t o operate these t h r e e p a r t s sim-30 FiCruREinzr CIRCUIT PlA<lR«fA Of CONTROL PANEL ll-V f\ " MASTER SutlTCH R * R E L A Y SWITCH Wt) - WAVE 0<21U£ floToR SU>»TCH P C - PEN SuilTCH Ch - CHART S W I T C H P - Pou>££ SWITCH IP - /MCRO-^ ftlve SAFETY SWITCH * L - AHCRO-UtTT«oUi<SAFEry SWITCH AS - AUXo SToP SWITCH 0A\ " DRIVEr*OTOfc u l t a n e o u s l y . When M i s c l o s e d , the r e l a y i s a c t u a t e d , c l o s -i n g s w i t c h R, p r o v i d e d t h a t M.D. i s c l o s e d , as i t w i l l nor-mally he. As a r e s u l t , the three c i r c u i t s are now completed. In the normal course o f o p e r a t i o n s R i s c l o s e d , M.L I s c l o s e d , M.D. i s c l o s e d , and A . S . i s - o p e n . The l a s t t hree are s a f e t y switches. The purpose of the M i c r o - L i t t r o w s w i t c h M.L. i s to make c e r t a i n t h a t under no circumstances w i l l the motor f o r c e the L i t t r o w m i r r o r past i t s maximum safe p o s i t -i o n . The d r i v e - m i c r o switch i s p r o v i d e d t o stop the spectrum sweep a f t e r each completed r o t a t i o n of the wave-length d r i v e arm, which corresponds to 1000 d i v i s i o n s . I t opens the r e l a y switch R, e q u i v a l e n t to opening the master s w i t c h , and t h e r e -by stops the motor, pen, and chart s i m u l t a n e o u s l y . T h i s makes i t p o s s i b l e to adjust the s l i t width, g a i n , e t c . , as may be expedient i n r e g i o n s of d i f f e r e n t wave-length. C l o s -i n g of the auto-stop switch A . S . f o r s u f f i c i e n t time to a l -low the m i c r o - d r i v e switch t o c l e a r , recompletea the c i r c u i t , and the next c y c l e o f scanning i s under way. I n s t r u c t i o n b o o k l e t s p r o v i d e d with the apparatus show c l e a r l y how the spectrometer shouldSbe assembled, s p e c i a l care b e i n g given to the NaCl and KBr p a r t s i n v o l v e d . Photo IV shows p a r t s of the apparatus, the spectrometer, c e l l , am-p l i f i e r , and r e c o r d e r , b e f o r e f i n a l s h i e l d s and i n s u l a t i o n were i n p l a c e . Photo I I I shows the present arrangement w i t h s h i e l d i n g and I n s u l a t i o n . The only p r e c a u t i o n necessary t o in s u r e proper o p e r a t i o n I s t o keep the power supply w e l l away from the a m p l i f i e r and r e c o r d e r . T h i s was achieved by a i t u -31 a t i n g the power supply on the f l o o r . D e s c r i p t i o n and o p e r a t i o n of the spectrometer and asso-c i a t e d apparatus: FIGURES VII and IX and photo I I I show the arrangement of the equipment, which may be d i v i d e d f u n c t i o n -a l l y i n t o s e v e r a l s e c t i o n s -1. a source of continuous i n f r a r e d r a d i a t i o n . 2. a condensing u n i t which focuses t h i s heterogeneous "white" l i g h t on the entrance s l i t . 3. sample c e l l s . w h i c h c o n t a i n the m a t e r i a l of which the a b s o r p t i o n i s to be examined, p l a c e d i n the path of t h i s "white" l i g h t . 4. . the monachromator which d i s p e r s e s the white l i g h t from the entrance s l i t and s e l e c t s a s m a l l range of wave-lengths to f a l l on the e x i t s l i t . 5. the r e c e i v e r which takes t h i s r a d i a t i o n and focuses i t on the thermocouple. 6. the d i r e c t c u r r e n t a m p l i f i e r which a m p l i f i e s the minute D.C. thermocouple v o l t a g e s t o c u r r e n t s which are l a r g e enough to be recorded. 7. the Brown e l e c t r o n i k r e c o r d e r which measures the energy f a l l i n g on the thermocouple and r e c o r d s i t . The source of i n f r a r e d r a d i a t i o n I s a 2" by 3/8" c y l i n d r i c a l rod of carborundum, which approximates black-body r a d i a t i o n , except at long wave-lengths. I t o p e r a t e s . a t about 1000°C, on 200 watts. In cases where g r e a t e r i l l u m i n a t i o n i s needed we have operated up to 280 watts, which i s approximately the maximum power output of our s o l a t r a n s f o r m e r . 32 c SAMPLE CELL - GrLOlSAft - P L A N E MIRRORS 1.1V, V, VI — S P H E R I C A L M I R R O R S a , — O F F - A X I S PARASOLft — S L I T M I C R O M E T E R — WAVE LENfrTH M I C R O M E T E R P — P R I S M s — SHUTTERS s, — E N T R A N C E SLIT — E X I T S L I T Tc — T H E R M O C O U P L E D — TEMPERATURE C O M P E N S A T O R D.S. — . D R I V £ SHAFT In order to prevent chromatic a b e r r a t i o n , which would be present i f l e n s e s were used, f r o n t s u r f a c e a l u m i n i z e d m i r r o r s are used throughout. T h i s makes i t p o s s i b l e to f o -cus the apparatus with v i s i b l e l i g h t and have i t a l s o i n focus f o r the i n f r a r e d r e g i o n . MI and MVI are plane mir-r o r s , known as d i a g o n a l f l a t s , which are f i x e d i n p o s i t i o n . T h e i r purpose i s to change the d i r e c t i o n of the l i g h t i n o r -der to reduce the bulk of the instrument. M i l i s a s p h e r i -c a l m i r r o r . M i l l i s an o f f - a x i s p a r a b o l a (18") of f o c a l length 2 7 cm and aperture f 4 . 5 . The r e s o l v i n g power of the instrument depends g r e a t l y on the accuracy of the sur-face of t h i s m i r r o r and i t s c o r r e c t adjustment. MIV i s a plane L i t t r o w m i r r o r operated by the wave-drive mechanism. MY i s a plane m i r r o r mounted on the prism t a b l e i n such a way as t o be r o t a t e d f o r temperature compensation. MVII I s a s p h e r i c a l m i r r o r . The entrance s l i t SI i s 12 mm h i g h , and i s curved t o compensate f o r curvature i n the s l i t image caused by the prism. The e x i t s l i t SII i s a l s o 12 mm h i g h , and i s opera-ted s imultaneously with SI; both open b i l a t e r a l l y , are of equal width, and are read d i r e c t l y In thousandths of a mm on the s l i t micrometer. Three mounts permit the use of f i l t e r s or s h u t t e r s . G e n e r a l l y one uses -1 . L i F f i l t e r - used from 5 to 9 . 5 / * t o reduce e f f e c t s of s c a t t e r e d l i g h t , e s p e c i a l l y of s h o r t e r wave-lengths. 33 2 . Glass f i l t e r - used from 9 . 5 to 15>*< . 3 . Opaque s h u t t e r of g l a s s a l u m i n i s e d on both, s i d e s . .. -The prism c o n s i s t s i n our case of a s i n g l e c r y s t a l of rock s a l t (NaCl), of which the apex angle i s 60° and face 60 by 75 mm. It" i s f i x e d i n p o s i t i o n , and consequently does not always operate i n the p o s i t i o n of minimum d e v i a t i o n . The i n -strument i s designed t o operate with any of the f o l l o w i n g three prisms, which are s p e c i a l l y mounted on separate t a b l e s to f a c i l i t a t e i n t e r c h a n g e a b i l i t y . 1 . NaCl used from 2 . 5 to 15/<. 2 . KBr used from 1 0 to 25/H. 3 . L i F used from 2 . 5 to 5 . 5 / K As the temperature c o e f f i c i e n t s o f the r e f r a c t i v e i n d i -ces of the prisms are l a r g e , MV, mounted on the prism t a b l e , i s r o t a t e d by means of a b i m e t a l l i c s t r i p t o make the drum s e t t i n g f o r a wave-length i n v a r i a n t w i t h temperature. Each prism has I t s own t a b l e and m i r r o r with t h e . c o r r e c t s t r i p or element mounted on the t a b l e . The thermocouple i s a h i g h vacuum type of thermocouple with a response time of one second. I t i s extremely s e n s i -t i v e , and i s 95% compensated f o r temperature d r i f t . I t i s accompanied by a carbon " g e t t e r b u l b " t o m a i n t a i n a h i g h vacuum, which i n c r e a s e s s e n s i t i v i t y . Mw i s the wave-drive micrometer which r o t a t e s the L i t t -row m i r r o r 9 0 of a r c f o r 2 0 0 0 d i v i s i o n s . D i s the b i m e t a l l i c s t r i p temperature compensator mentioned above. C i s the c e l l f o r gas a b s o r p t i o n . The windows are of NaCl throughout, as 3 4 g l a s s , q u a r t z , e t c . absorb s t r o n g l y i n the " i n f r a r e d . O p t i c a l Path: R a d i a t i o n from the g l o b a r f a l l s on MI, i s r e f l e c t e d on to Mil,. and focused through the sample c e l l G on the entrance s l i t . The s l i t image then f a l l s on M i l l , the o f f - a x i s p ara-b o l a , i s c o l l i m a t e d by i t , d i s p e r s e d by the prism onto the L i t t r o w m i r r o r , which r e t u r n s i t through the prism onto Para-b l i c M i l l , which focuses i t , a f t e r r e f l e c t i o n by MV, as a spectrum on the e x i t s l i t . T h i s e x i t s l i t s e l e c t s a c e r t a i n range of wave-lengths depending on I t s s e t t i n g , and with the a i d of b a f f l e s prevents s c a t t e r e d l i g h t from p a s s i n g onto the thermocouple. The l i g h t now f a l l s on MVII, and i s focused on one of two opposed thermocouple j u n c t i o n s . The purpose of these two j u n c t i o n s i s to compensate f o r temperature d r i f t by o p p o s i t e l y d i r e c t e d emfs. The t o t a l path l e n g t h i s 195 cms. A m p l i f i c a t i o n : The emf generated by the thermocouple i s l e d by a c a b l e , which i s s h i e l d e d e l e c t r i c a l l y , m a g n e t i c a l l y , and t h e r m a l l y , to the i n p u t side of our G.E. a m p l i f i e r . T h i s a m p l i f i e r con-Ve r t s minute D.C. i n p u t v o l t a g e s from, the thermocouple to A.G. by means of a motor-driven c i r c u i t b r e a k e r , a m p l i f i e s t h i s small A.C. v o l t a g e , r e c t i f i e s i t by means of a second syn-chronous c i r c u i t b r eaker, and passes i t through a f i l t e r t o the r e c o r d e r mechanism. Recording: Our r e c o r d i n g u n i t i s a Brown e l e c t r o n i k r e c o r d e r , con-s i s t i n g of a constant speed c h a r t , ( t h i s speed i s a d j u s t a b l e 35 by changing gear r a t i o s ) , w i t h r e c o r d i n g pen operated by a s e l f - b a l a n c i n g wheatstone b r i d g e . The s c a l e i s l i n e a r i n energy f a l l i n g on the thermocouple. Adjustment: • The Perkin-Elmer b o o k l e t s recommend a r a t h e r long pro-cedure f o r the assembly, adjustment, and o p t i c a l alignment of the spectrometer. However, i n view of the f a c t t h a t the instrument i s p r o p e r l y a d j u s t e d before l e a v i n g the f a c t o r y , a more simple procedure i s , i n g e n e r a l , p o s s i b l e , and i s ad-v i s a b l e under o r d i n a r y circumstances. Recommended Procedure: 1. Mount a l l NaCl windows and the s h u t t e r s p r o v i d e d with great care, to prevent water vapour, marks, e t c . , from s p o i l i n g , t h e i r s u r f a c e s . 2. A d j u s t the s l i t s with p r e c i s i o n , so t h a t they are p a r a l -l e l and c l o s e t o zero s i m u l t a n e o u s l y , then a d j u s t the . s l i t micrometer to read zero at t h i s p o s i t i o n . 3. Check t h a t the source image i s focused on the entrance s l i t . 4. - Check t h a t the entrance s l i t image f a l l s d i r e c t l y on the e x i t s l i t . 5. Check t h a t the e x i t s l i t image i s focused on one of the thermocouple J u n c t i o n s . I f at any p o i n t the apparatus f a i l s t o s a t i s f y one of these c o n d i t i o n s , i t i s necessary to check the Perkin-Elmer pro-cedure from then on,, a v o i d i n g , wherever p o s s i b l e , the d i f f -i c u l t adjustment of the c o i l i m a t i n g and f o c u s i n g m i r r o r s , 36* e s p e c i a l l y the o f f - a x i s p a r a b o l a , upon whose, performance the u l t i m a t e r e s o l u t i o n of the instrument depends. The f u l l P e r -kin-Elmer procedure i s not given here s i n c e i t i s r e a d i l y a v a i l a b l e and q u i t e lengthy. Perkin-Elmer suggests t h a t when the Instrument i s prop-e r l y a d j u s t e d , i t w i l l e a s i l y r e s o l v e the 4.2 /< C0 2 band i n t o a n i c e d o u b l e t . That our instrument does so can be seen from photo V I I . The 4.2/4 band i s the sharp minimum on the f a r l e f t of the photo, o c c u r r i n g at a drum r e a d i n g of about 1640. The f o l l o w i n g procedure i s recommended i n u s i n g the I n -f r a r e d spectrometer, to o b t a i n a c h a r t of energy ve r s u s on the r e c o r d e r . 1. Decide on the r e g i o n t o be scanned. 2. Decide what compromise between r e s o l u t i o n and s t a b i l i t y i s t o be made. Set the gain c o n t r o l a c c o r d i n g l y . 3. S e l e c t the d e s i r e d speed f o r the wave-length d r i v e , r e -membering t h a t the thermocouple response time i s one sec-ond, and t h a t r e s o l u t i o n i s i n v e r s e l y p r o p o r t i o n a l to the speed, s e n s i t i v i t y i s i n v e r s e l y p r o p o r t i o n a l to the square of the speed. Set the wave-drive gear s h i f t a c c o r d i n g l y . 4. S e l e c t the proper s h u t t e r f o r the r e g i o n . 5. Set the s l i t - w i d t h t o such a v a l u e t h a t upon scanning the r e g i o n by hand, the s e p a r a t i o n of the maximum and m i n i -mum r e c o r d e r readings i s about 4/5 of f u l l s c a l e d e f l e c -t i o n . 6". Set the zero balance so t h a t n e i t h e r the mazimum nor the minimum i s o f f the s c a l e . 37 7. Record a l l s e t t i n g s . 8. S t a r t o p e r a t i o n by means of the master s w i t c h . 9. Mark s u i t a b l e wave-drive s e t t i n g s on the c h a r t by means of the t e s t - m i c r o v o l t switch on the a m p l i f i e r d u r i n g the o p e r a t i o n . T h i s i s known as a f i d u c i a l wave-length mark-er , as i t p r o v i d e s us w i t h a f i x e d b a s i s of comparison. In g e n e r a l , one may wish to scan the whole r e g i o n .in steps, t o l o c a t e bands i n a substance's spectrum, or one may wish to know the shape of the a b s o r p t i o n v e r s u s \) curve. For the former purpose the auto-stop switch i s l e f t on, and w i l l stop pen, c h a r t , and motor a f t e r each 1000 d i v i s i o n s on the micro-wave-drive drum. T h i s a l l o w s f o r adjustments i n s l i t - w i d t h , e t c . , .as may be' expedient. The p r e c e d i n g procedure i s e n t i r e l y s a t i s f a c t o r y f o r wave-length or frequency r e a d i n g s and checks, p r o v i d e d t h a t the bands do not f a l l among strong bands of atmospheric water and 00^, which form sharp maxima and minima on our black-body r a d i a t i o n curve. In cases where o p t i c a l d e n s i t y or q u a n t i t a t i v e i n t e n s i -t i e s of energy t r a n s m i t t e d are d e s i r e d t o be measured, i t i s necessary to compare the d e f l e c t i o n s w i t h and without the sample i n the beam, without d i s t u r b i n g any of the s e t t i n g s . The treatment of these r e a d i n g s w i l l be given i n the next s e c t i o n . The power s e t t i n g of the g l o b a r I s important i n cases where performances are t o be repeated, i n t h a t the tempera-ture of the g l o b a r determines both the shape of the b l a c k -38 body r a d i a t i o n curve and the i n t e n s i t y i n any g i v e n wave-length r e g i o n . T h i s i s p a r t i c u l a r l y t r u e near the peak of the curve. (D) C a l i b r a t i o n of the spectrometer: A f t e r the spectrometer was p r o p e r l y a d j u s t e d and t e s t e d f o r performance, the next s t e p was to c a l i b r a t e i t , i n such a way as to a s s o c i a t e a wave-drive drum s e t t i n g w i t h a f r e -quency i n cm - 1. j The g e n e r a l procedure i n v o l v e d r e q u i r e d a scanning of the spectrum from 3 t o 1 5 / V, w i t h s u i t a b l e absorbers i n the o p t i -c a l path. These absorbers have a b s o r p t i o n bands of which the g e n e r a l appearance and f r e q u e n c i e s of a b s o r p t i o n maxima are well.known by v i r t u e of p r e v i o u s work with g r a t i n g s and spec-trometers of comparable r e s o l v i n g power. T h i s i s important i n order t h a t f r e q u e n c i e s given be of use f o r our c a l i b r a t i o n . We used the a b s o r p t i o n bands of H 2 0 , C 0 2 , and NH^; and i d e n t -i f i c a t i o n was made from R.S.I. 1 £ , 5 1 5 , 1 9 4 2 , ( r e f . 9 ) , where curves are shown f o r g r a t i n g and spectrometer of comparable r e s o l v i n g power. TABLE I shows the c o n d i t i o n s under which the c a l i b r a t i o n c h a r t s were re c o r d e d . SUBSTANCE REG-ION CELL LENGTH PRESSURE SLIT WIDTH GAIN PHOTO C 0 2 1 5 - * l 4 / « 40 cm 1 5 0 mm 0 . 1 5 mm 1 0 V NH^ 13-^8/w 10 cm 100 mm 0 . 1 5 mm 9 . 6 VI c8°) 8 —>»4At i n atmosphere 0 . 0 5 0 m m 1 0 VII NH, AO cm 3 0 0 mm juu m » 3 — » 2 . 5 / j i n atmosphere j 0.020mm 10 V I I I « ~ 2 40 cm low 3 9 For the H"20 bands, t h a t water present i n the atmosphere at about 40$ R.H. was s u f f i c i e n t throughout. The C0 2 was i n t r o d u c e d by means o f a s m a l l t r a y of dry i c e i n the f i r s t housing, which has a path l e n g t h of 40 cm. The pressure of the C0 2 would be low but i s not c r i t i c a l f o r these r e a d i n g s . In o r d e r to examine the NH^ a b s o r p t i o n bands, a g a s - f i l l -i n g apparatus was c o n s t r u c t e d , c o n s i s t i n g of a high-vacuum pump, a manometer, a c e l l - f i l l i n g o u t l e t , a CaClg d r y i n g tube, and a NH^ generator. FIGURE X shows t h i s apparatus. The 10 cm gas a b s o r p t i o n c e l l was f i t t e d w i t h s u i t a b l e i n l e t and o u t l e t tubes and Joined to the system, the whole of which was then evacuated. The pump was then cut o f f and the NH^OH warmed u n t i l there was NH^ at the d e s i r e d pressure i n the c e l l . The c e l l was then removed from the system and p l a c -ed i n i t s proper p o s i t i o n on the spectrometer. I t i s to be noted t h a t some of the c o n d i t i o n s of p r e s s u r e , s l i t - w i d t h , g l o b a r power, a m p l i f i e r g a i n , e t c . , may be v a r i e d from the v a l u e s we used i f s u i t a b l e adjustments of the o t h e r s are made. G e n e r a l l y speaking, the s l i t - w i d t h s used are the s m a l l e s t which permit reasonable e n e r g i e s t o r e a c h the thermo-couple, so t h a t the s i g n a l t o n o i s e r a t i o may be h i g h . Four c h a r t s were obtained f o r s e c t i o n s i n the r e g i o n from 3 to 15/4 u s i n g the procedure o u t l i n e d , c a r e f u l l y marking wave d r i v e s e t t i n g s every 25 d i v i s i o n s by means of our f i d u c i a l wave-length marker. These c h a r t s are shown i n photos Y, V I , V I I , and V I I I , 40 K H 3 FILLING, SYSTG/^N s s p H3 6 c L 5 P - HWPic Pur^P C ' CELL. D " PRY/NCr T i»6£ Gr " & £ N E R . A T O R . T " T/UBE To REPLENISH NH^OK B " gUNSEN 8URNER S ~ S T O P COCKS CRLtBRRTlO/v CHRRT FOP PE IVFRR-REP5PECTPOH1E TEpAfoOEi. /IB SE*I/>L **• 2/S W/THA/OCC Pp/SM irVTH£ /?£.&/OA LiOC-Ti to ~ts= 7Z.OCm' if A= to /S/A C PLI BR PT/OK FROAA TH£ /* S^jf COx PPSORPTlW R.S.I /?42 SLIT WIDTHo-zcmm. TEMP ?O*F GLOBPR Pov/ER loowPTTS MP*/AAVAA GO//V IO Hv/Aipny 3s~ °/o WRVE-DR/VE SPEED tl PfESSO/x'E /so sr,r7? #g. '3 " SEA? XE V CEU Tfy1 4o eye ve S}*E~ wtver- jr/fe /?f <vo;/v<; s 1 PHOTO 3 £ P H O T O m CALtBRATV<"W C/f/t/VT Fo* P£ iNr**- fee* SpecfKomefc* HOO»L /2,0 O W M / / I V X uLftT,JtfM*, Sur Utern oos*> mm fir** 7 ° ' * ceo*** f*u*EH /so u/*m • st/ur/nt"** 6*1*4 to Hun/p/Ty 90 7' 8 A tow CV*VC MB eft. ••••• MMM i *t . 1 . 1 \ t / / V C / 1 r r r r r r r r t r f t t r t r r r r T r t t t t t f t P H O T O m CRLIBRRRTION CHART foff PE. iNFRR RED S P E C T R O M E T E R M O O C L /X8 S E X / R L # 2 1 3 IV ' tT/ f AftCl PlflSSI /# r#£ flfl/v&E V—Hoo <-rr>-'TO 3 Poo r f r ' ( r ^ W - r o . ^ CF>£/8R/)T/O/Y F R O M TH£ /Yf^COjltyO BAA'DS. PERFORMED A I A J H C H II. y?*i lei/vr/r/ew/a*FRO* , _ 51—, .—J———-j—I 1-— i ~i 1 j / g K ST/z,s/s-,/??i. SI/TWWTM TEMP 70°r Ol08RRPovyei> Joe w/ifTS 5»/* « HUM I a i ry 3 tX Mi/iz/mw* Wnve o/r/v^ speeo /I'lt" rj?x<rfv H,Om Wvnos IWCtiE CO, CE it JE^HT/V *o em. /VfSS^ffe />-OT C A-rT/'c ff s~ -/* o>re 5 ae^o^/ serve * v.*. A .rs ot*£ v- <-'^ % s~ PHOTO 2DL TABLE I I ahowa drum re a d i n g s measured from these c h a r t s , - 1 versus f r e q u e n c i e s i n cm DRUM READING FREQUENCY i n cm - 1 SUBSTANCE 517 589 748 765 798.. 807 812 814 843 855 885 893 947 >55 ?84 1049 1055 1082 1111 1131 1136 1155 1161 . 1177 1184 1198 1207 1218 1228 • 1236 1248 1253 1268 . 1284 1298 1313 1325 1337 1355.7 1361.5 1389 1391.6 1403 1415 1420.3 1427.5 1430.7 6 6 7 . 0 7 2 0 . 7 8 0 7 . 7 812.4 828.2 830.9 833.0 8 3 4 . 8 848.0 8 5 4 . 0 8 6 8 . 2 8 7 2 . 7 888.. 3 9 9 2 . 4 9 0 8 . 4 • 948.8 9 5 2 . 0 9 7 2 . 4 9 9 2 . 8 1 0 0 7 . 2 1 0 1 2 . 6 1 0 2 7 . 3 1 0 3 3 . 8 1046.9 1054.4 1 0 6 6 . 3 1 0 7 5 . 8 1085.1 1 0 9 6 . 1 1104.1 1117.4 1 1 2 3 . 0 1141.3 1 1 5 9 . 7 1177.9 1 1 9 5 . 9 1 2 1 3 . 4 1 2 3 1 . 1 1261 1272 1314 1320 1340 1363 1376 1388 1396 PHOTO CHART GRAPH C0r> Y _ co; V IX NH, VI IX n3 11 II 11 t i It 11 11 II 11 tt II 11 11 II 11 it It 11 it II 11 11 ti-11 tt l l t i 11 II it 11 II 11 11 II n it II 11 11 II 11 11 t l 11 11 t l 11 11 II 11 11 II 11 i t II tt 11 II it 11 II 11 11 II t i tt II 11 11 II 11 t i II 11 11 II 11 11 II 11 11 t l t i 11 II 11 11 It 11 11.. II tt 11 t l 11 tt II 11 it IX ,x 11 11 II II H o 0 VII II II 2 II t l It 11 II X 11 II II 11 II II 11 II II it t l II it II t l 11 tt II cont'd 41 I TABLE I I continued - 1 DRUM READING FREQUENCY i n cm SUBSTANCE 1435 1406 1441.7 1420 d 11 1447 1431 " 1449 1437 11 1454.2 1449 it 1458 1459 11 1461 1466 11 1464.4 1474 it 1470.5 1491 ti 1473.5 1498 11 1477 1508 11 tt 1480.3 1518 1481.8 1523 11 1486.4 1535 11 1489 1542 it 1495 1560 1498.5 1571 11 1501.4 1578 11 1506.8 1596 ti 1514 1618 " 1516 1624 11 1519.4 1637 11 1524 1649 11 1527.3 1664 11 1529.5 1671 11 1533 1685 11 1537 1700 tt 1541.5 1718 it 1545.7 1736 - 11 tt 1549 1751 1554.5 1774 it 1559.5 1794 ti 1562.5 1812 it 1 5 6 7 1830 11 1570 1846 11 1575 1870 it 1579 1 8 9 1 11 1582.5 1911 11 1585 1 9 2 2 11 1588.5 1944 11 1592.3 1 9 6 8 11 1 5 9 7 1993 11 1640 2336 C?,2 1644 2 3 6 7 it*-1 7 0 3 3217 1708 3337 • O 1711 3434 it 1718 3617 1722 3741 d. 11 1 7 2 6 3 8 8 2 11 PHOTO CHART GRAPH VII X it 11 11 11 11 it it . 11 11 tt it 11 ti 11 11 11 11 11 it 11 11 11 11 it ti 11 11 11 11 ti V I I I X,XI XI II tl II II II II II 42 We have p l o t t e d these r e a d i n g s on three graphs f o r the regions 7 0 0 to 1 3 0 0 om"1; 1 2 0 0 to 2 0 0 0 cm"'; and 1800 to 3 9 0 0 cm - 1. Photos IX, X, and XI show these graphs. From them we can o b t a i n d i r e c t l y the frequency i n c m - 1 of any a b s o r p t i o n maximum which we may observe at any wave-drive s e t t i n g i n the reg i o n c a l i b r a t e d . I t I s g r a t i f y i n g t o note t h a t a rec e n t check i n d i c a t e d t h a t the c a l i b r a t i o n has not s h i f t e d . An attempt was made to f i t a c u b i c equation t o the cen-t r a l r e g i o n of the c a l i b r a t i o n curve by the method of l e a s t squares, but the r e s u l t i n g equation was not accurate enough to be a c c e p t a b l e . Such an a n a l y t i c a l e x p r e s s i o n would be de-s i r a b l e s i n c e our probable e r r o r would be s m a l l e r and more, a c c u r a t e l y known. (E) The a b s o r p t i o n spectrum of CS 2: From the p o i n t of view of experimental d i f f i c u l t i e s , C S 2 is'one of the more troublesome m a t e r i a l s to work with. I t i s a h i g h l y v o l a t i l e poisonous substance, which d i s s o l v e s ' most greases, waxes, rubber, and other components used i n f i l l i n g systems and connections. A f t e r c o n s i d e r a b l e experimentation with greases we found s i l i c o n e grease was useable i n stop-cocks, although i t must be r e p l a c e d o c c a s i o n a l l y . The only wax we were able to f i n d which was i n s o l u b l e i n GS 2 was h i g h - p y s e a l , which i s very b r i t t l e and f a i r l y d i f f i c u l t to handle. We used i t f o r a l l g l a s s to metal s e a l s . Most rubbers, n a t u r a l and s y n t h e t i c , are a f f e c t e d by CS 2. They s w e l l to many times t h e i r n a t u r a l s i z e . Neoprene was the l e a s t a f f e c t e d i n t e s t s made by us, 4 3 PHOTO I £ P H O T O X PHOTO X L and therefore we used i t for gaskets and flexible connections. The method used to determine the frequencies of the CS 2 absorption bands i s as follows:- successive "sweeps" of the entire region from 15 /* to 2/f were made with and without CS2 in the 10 cm c e l l . Comparison of the charts'obtained in-dicated one very strong and two medium bands. A similar pro-cedure with the one meter c e l l indicated three more bands which are f a i r l y weak. Each of these bands was then examined carefully, using minimum useable slit-width, maximum power, maximum gain, and minimum speed, to give the best possible results. For each band the instrument was run with shutter closed, both before and after the small region to be examined. The line joining these "end-traces" was then used as our base line from which to measure the intensity of radiation f a l l i n g on the thermocouple in a given region. For for instance, the machine was run from drum-reading 1465 to 1475 with shut-ter closed, from 1475 to 1495 with shutter open, and f i d u c i a l marks were made at 1481 and 1491 divisions by means of the test-microvolt switch on the amplifier. From charts taken with the c e l l evacuated and then f i l l e d with CS 2 > we measured the energy transmitted (which i s linear on the chart), for every \ wave-drum division. The % transmission for CS2 i s given by CS 2'reading x inn ^ o r e a o h °^ bands we plotted vacuo rdg. % transmission versus drum readings, which are convertible to frequencies by our calibration graphs. 44 IV RESULTS Six bands due to CS 2 were found between 12 and 3/< Photos XII to XVI show the graphs of % transmission versus wave-drive reading for a l l six. Various pressures and c e l l -lengths were used, from a "traoe" in the 10 cm c e l l for to approximately 30 cm Hg in the one meter c e l l for - V 1 # The band at 6.52 ft i s by far the strongest, and hence may be expected to be one of the fundamentals. As ^  i s i n -active in the infrared, this band must be either V 2 or y^ . However, according to our theory (pages 16, 17, 18), » 2 i a a perpendicular band with a strong central Q branch, and y^ i s a parallel band with the Q branch missing; hence V-^  w i l l appear as a doublet. On this basis the band at 6,5/4 i s V^, and the central peak transmission corresponds to the centre of the band. The side-band i s attributed to the isotope effect. A l l of the other bands can be written as simple combina-tion or difference bands. TABLE III gives the six bands ob-served in CS2 vapor. BAND DRUM READING METHOD FREQUENCY PROBABLE ERROR 3.38/* 1688.5 2 rdgs. 2 9 5 9 cm"1 ±6 cnT 1 3.5V* 1681.0 2 rdgs. 2838 cm'1 ±6 cm"1 4.29>« 1640.0 1 rdg. 2 3 3 2 cm"1 +5 cm"1 4.58/* 1623.45 4 rdgs. 2185 cm"1 ±5 cm"1 6.52/ 1846.75 6 rdgs. 1535 cm - 1 12 cm"1 11.4./* 928.0 1 rdg. 8877 cm'1 . t l cm"1 45 1 6 9 5 1 6 9 0 1 6 8 5 1 6 8 0 1 6 7 5 PHOTO XIC IG43 164a | G 4 f 1640 1639 1638 1637 Wavelength Drut^i 1618 \61Q 1622 1624 I62<3 I6a8 1630 " Wdvel ewq+1^  Drcwi > PHOTO PHOTO X Z PHOTO ; TABLE IV g i v e s these s i x bandsj t o g e t h e r with the two other fundamentals \)^ and "Vg. LOWER UPPER BEST VALUES 1 9 4 7 PLYLER EDWARDS BAND TERM STATE STATE t o 1 9 4 5 & HUMPHREYS MITCHNER n n n 0 l n , n-,n0ln-, HERZBERG- obs. c a l o . & ROGERS 1 2 3 1 2 3 obs. V, 0 0 0 0 1 0 0 0 656.5 -. 655* 0 0 0 0 0 1 1 0 396.7 - 3 9 7 * 0 0 0 0 0 0 0 1 . 1 5 2 3 1 5 3 5 1 5 3 5 * 1 5 3 5 1 0 0 0 0 0 0 1 8 7 8 8 7 9 880 8 7 7 0 0 0 0 1 0 0 1 , 2 1 8 3 . 9 2 1 8 4 2 1 9 0 2185 0 0 0 0 0 2 2 1 2 3 2 9 2 3 3 6 2 3 2 9 2 3 3 2 0 0 0 0 2 0 0 1 - 2857 2845 2 8 3 8 0 0 0 0 1 2 0 i - 2 9 5 9 2 9 8 4 2 9 5 9 * these v a l u e s used to c a l c u l a t e f r e q u e n c i e s of combination and d i f f e r e n c e bands. The c a l c u l a t e d v a l u e s of P l y l e r & Humphreys are obtained by assuming t h a t OSg-is so heavy a molecule t h a t anharmonicity e f f e c t s are sm a l l . The v a l u e s we obtained agree very w e l l w i t h the best data o b t a i n a b l e . The one exc e p t i o n i s our va l u e of 2 8 3 8 c m - 1 f o r V , + 2 V . . The experimental e r r o r l s i n s u f f i c i e n t t o ao-• - 1 count f o r the d i f f e r e n c e of 19 cm between our va l u e and th a t of P l y l e r & Humphreys. 46 V CONCLUSIONS We have successfully set up the Perkin-Elmer spectrometer and associated instruments, calibrated i t , and applied i t to CS 2 in the vapor phase. We have verified the frequencies at which six major v i -brational bands of CS2 occur in the vapor, as given by Plyler & Humphreys'. Of particular importance i s our confirmation of their value for As shown on page 14, this ^ - 3 " 1535 cm - 1 gives a better value for the force constant k than i s obtained by Herzberg from V^=1523 c m . The new value of k ^ T . l x l O 3 dynes/cm i s in better agreement with the value obtained from y ^ , and shows that.the valence force approximation method Is justifiably applied to CS2. The work remaining to be done includes a search for ninfe. more bands which f a l l in our region and were found by Plyler & Humphreys in the liquid only. In addition to these bands there should be side-bands due to isotope effects which could be measured. Finally, the work at various pressures and temperatures remains to be done to determine the effects of association on the frequencies at which the bands appear. 47 VI BIBLIOGRAPHY (1) B a i l e y , C.R. and C a s s i e , A.B.D. - Proc. Roy. Soc. London 1^2, 2 3 6 , 1931 ( 2 ) Barnes, R.B., McDonald, R.S. , - J o u r . Aop. Phys. W i l l i a m s , Van Z., and Kennard, R.F. 16, 7 7 , 1945 ( 3 ) Brown and Marco - I n t r o d u c t i o n to Heat T r a n s f e r (McGraw H i l l Co., New York) 1942 ( 4 ) Dennison, D.M. and Wright, N. - Phys. Rev. 3J3, 2 0 7 7 , 1931 (5) Dennison, D.M. - Rev. Mod. Phys. 2, .280, 1931 (6) Dennison, D.M. - Rev. Mod. Phys. 1 2 , 1 7 5 , 1940 ( 7 ) Herzberg, G. - M o l e c u l a r Spectra and M o l e c u l a r S t r u c t u r e l Diatomic Molecules ( P r e n t i c e - H a l l , N.Y) 1939 ( 8 ) Herzberg, G. - M o l e c u l a r Spectra and M o l e c u l a r S t r u c t u r e l l I n f r a r e d and Raman Spectra (D. Van Nostrand, N.Y.) 1945 ( 9 ) O e t j e n , R.A. , Chao Lan Tao, - R.S.I., 1 .^, 5 1 5 , 1942 and R a n d a l l , R.M. ( 1 0 ) P l y l e r , E.K. and Humphreys, C.J. - Jour. Research, N.B.S. IE. 5 9 , R.P. 1814 1947 (11) P r o c t o r , R.F. and Douglas, R.W. - J . S . I . 2 , 1 9 2 , 1932 (12) Strong, S. - Procedures i n Experimental P h y s i c s ( P r e n t i c e - H a l l , I n c . , N.Y.) 1946 (13) Tuve, G.L. and S e i g e l , L . J . - Trans. A.S.H.V.E. 4 4 , 5 2 3 , 1938 (14) Whittaker, E.T. - A n a l y t i c a l Dynamics (Dover, N.Y.) 1944 (15) Wu, T.Y. - V i b r a t i o n Spectra and S t r u c t u r e of Polyatomic Molecules- ( N a t i o n a l U n i v e r s i t y of Peking, Kun Ming, China) 1939 

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