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The absorption and fluorescence of anthracene in the near ultra-violet Katagiri, Seiko 1964

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THE ABSORPTION AND FLUORESCENCE OF ANTHRACENE IN THE NEAR ULTRA-VIOLET by SEIKO KATAGLRI B. En., The U n i v e r s i t y of N i i g a t a , Japan, 1962 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF M. Sc. i n the Department of Chemistry We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1964 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y ' o f • B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study,, I f u r t h e r agree that per m i s s i o n f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood t h a t : c o p y i n g or p u b l i  c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n p e r mission. Department of The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada v i i ABSTRACT The fluorescence and absorption spectra of anthracene i n the near u l t r a - v i o l e t were i n v e s t i g a t e d i n n-heptane, flu o r e n e , biphenyl and n-hexane matrices at low temperature. The assignment of the e x c i t e d e l e c t r o n i c s t a t e as lBiu was confirmed. In the ground e l e c t r o n i c s t a t e eight Q^and f i v e |>^, and i n the 'B|V upper e l e c t r o n i c s t a t e seven and f i v e b j | fundamentals were assigned. I t was deduced that the p o t e n t i a l surfaces of the ' A | and the '6,u s t a t e s were s i m i l a r i n shape as there was an approximate agreement be tween the values of corresponding fundamental v i b r a t i o n s i n the two e l e c t r o n i c s t a t e s . The p o t e n t i a l surfaces were un u s u a l l y harmonic f o r a polyatomic molecule, at l e a s t along the normal co-ordinates a v a i l a b l e t o t h i s study. No evidence f o r the presence of anharmonicity was found i n even the highest overtone (the t h i r d ) measured, although s e v e r a l p o s s i b l e examples of Fermi resonance between v i b r a t i o n a l modes were observed both i n fluorescence and i n absorption. The Fermi resonances were assigned p r i m a r i l y on the basis of i n t e n s i t y t r a n s f e r between l i n e s r a t h e r than l i n e s h i f t s . . The presence of a weaker long-axis p o l a r i z e d t r a n s i t i o n ( 'B^u*- '/^  ) i n anthracene p r e d i c t e d by theory was not detected. v i i i The lowest energy e l e c t r o n i c t r a n s i t i o n i n fluorene was found to be p o l a r i z e d along the long a x i s of t h i s molecule. \ ACKNOWLEDGMENT I am deeply g r a t e f u l to Dr. Alan V. Bree f o r h i s guidance and encouragement i n every phase of t h i s work; h i s assistance has developed my i n t e r e s t and understanding i n the work. I wish t o express my a p p r e c i a t i o n to Miss V.V.B. V i l k o s f o r her help i n many ways, and a l s o to the te c h n i c i a n s i n t h i s department f o r the preparation of some equipment. i i i CONTENTS Page SURVEY OF PREVIOUS WORK 1 T h e o r e t i c a l P r e d i c t i o n s 1 E l e c t r o n i c States of Anthracene 1 V i b r a t i o n a l States of Anthracene 3 Mixed C r y s t a l Phenomena 5 Previous "Experimental Work 6 EXPERIMENTAL ARRANGEMENT 8 Preparation of the Samples . . 8 Measurement of the Spectra 10 Apparatus 10 Measurement of the Lines 11 RESULTS 14 DISCUSSION 31 Fluorescence Spectra 31 Fundamental Modes . . . 31 Fermi Resonance 3# Other Features 3§ Absorption Spectra 38 Fundamental Modes of the ' B t u u P P e r State 38 Comparison of the Fundamentals on the ' A|and on the E l e c t r o n i c States • 3§ i v Page Fermi Resonance . 41 Other Lines . . . . . . . . . 43 S h i f t of the Origins of tke 'B.^'A, T r a n s i t i o n i n the D i f f e r e n t Matrices . 49 BIBLIOGRAPHY 50 V TABLES Table Page 1 Character Table of P*h and the Axis Convention of the Anthracene Molecule : 2 2 A Summary of Some C a l c u l a t i o n s on the E l e c t r o n i c States, of Anthracene i n the Near E l t r a - V i o l e t . 2 3 0^ and t>>^  Fundamentals Observed i n Anthracene . 7 4 Fluorescence Spectra of Anthracene i n Various Matrices 17 5 Absorption Spectra of Anthracene i n Various Matrices . 24 6 Absorption Spectrum of Fluorene at 4.2°K 31 7 P o s s i b l e Examples of Fermi Resonance i n the Fluorescence of Anthracene 34 8 The Fundamentals of Anthracene i n the Ground and the ' Upper State 40 9 P o s s i b l e Examples of Fermi Resonance i n the Absorption of Anthracene 41 10 S i m i l a r i t y of the Stru c t u r e around Some Strong Absorption Lines 44 v i FIGURES Figure Page 1 Low Temperature Sample C e l l s 12 2 The Fluorescence Spectrum of Anthracene i n n- Heptane at 4.20K 15 3 The Fluorescence Spectrum of Anthracene i n Fluorene at 4.2°K 15 4 R e l a t i v e I n t e n s i t i e s of the Lines i n F l u o r e s  cence (a) Anthracene i n n-Heptane at 4.2°K (b) Anthracene i n Fluorene at 4.2°K . . . . 16 5 Absorption Spectrum of Anthracene In n-Heptane at 4.20K fZky 6 Absorption Spectrum of Anthracene i n Fluorene at 4.2°K .vi ''.'2£. 7 R e l a t i v e I n t e n s i t i e s of the Lines i n Absorption (a) Anthracene i n n-Heptane at 4.2°K (b) Anthracene i n Fluorene at 4.2°K . . . . SURVEY OP PREVIOUS WORK Th e o r e t i c a l P r e d i c t i o n s E l e c t r o n i c States of Anthracene Group theory may be u s e f u l l y applied to the c a l c u l a  t i o n of the molecular o r b i t a l s (MO's) of an anthracene molecule us i n g as a basis set the atomic 2p* f u n c t i o n s centred on each carbon nucleus. Anthracene possesses PaW molecular symmetry and i t s character t a b l e and a x i s convention are shown below. I t can be shown (1) that the one-electron MO's are /\o, B I J , B a j and &30 y i e l d i n g the c o n f i g u r a t i o n s A9, Bi U » B»u and B a * . According to Weissman (2) antisymmetric s p i n func t i o n s have symmetry, so s i n g l e t T T - e l e c t r o n c o n f i g u r a t i o n s r e t a i n the symmetry given above. Thus the only allowed t r a n s i t i o n s a r i s i n g from the ground s t a t e are to & i 0 and fc8Uexcited s t a t e s p o l a r i z e d along the long and short a x i s of the molecule, r e s p e c t i v e l y . Many c a l c u l a t i o n s ( 3 ) - ( l 3 ) have been c a r r i e d out on the energies of the e l e c t r o n i c t r a n s i t i o n s of anthracene and the corresponding o s c i l l a t o r strengths ( f ) i n d i f f e r e n t approximations (e.g. a l l o w i n g f o r the i n t e r a c t i o n between many c o n f i g u r a t i o n s , the i n c l u s i o n of many-centred i n t e g r a l s 2 Table 1 Character Table of D2h and the Axis Convention of the Anthracene Molecule * V * yt xx xy D2h E C2 C2 C2 i rj f tf" T R z(M> Ag 1 1 1 1 Au 1 1 1 1 B i g 1 - 1 - 1 1 B l u 1 -1 -1 1 B2g 1-1 1 - 1 B2u 1-1 1 - 1 B3g 1 1 - 1 - 1 B3u 1 1 - 1 - 1 i n the s e c u l a r equation, e t c . ) . A l l c a l c u l a t i o n s put only Diu and ' B w l e v e l s i n the region of the 3800 A system. Only one c a l c u l a t i o n (9) found the ' B I U l e v e l lower than 'feto* T t l e o s c i l l a t o r s trength of the Vljf t r a n s i t i o n was much higher than that of the '^ iy - - ' / , } i n every approximation, and f o r the l a t t e r P a r i s e r (8) and Mataga (10) ca l c u l a t e d zero. Table 2 A Summary of Some C a l c u l a t i o n s on the E l e c t r o n i c States of Anthracene i n the Near U l t r a - V i o l e t 1 1 1 1 1 -1 -1 -1 1 -1 -1 1 Rz 1 1 1 -1 Tz 1 -1 • 1 -1 Ry 1 1 -1 1 Ty 1 1 -1 -1 Rx 1 -1 1 1 Tx r e f . • Blu (HI) f 'B lut f lO V.B. 3 3.07 M.O. 4 0.836 Jr 1.261 Jr X =resonance integral 5 0.11 0.005 Modified 6,7 3-72 <v 0.10 MO Methods 8 3.6 0.4 3 Table 2 continued r e f . '&1 («M) f * B i (BL) f „ ev ev Modified 9 3.6 3.2 MO Methods 10 3.48 0.39 3.91 0.00 11 3.15 12 3.44 0.283 3.51 0.116 TBX Approximation 3.44 0.265 3.61 0.063 IRX Approximation 3.23 0.395 3.51 0.162 TBM Approximation 3.15 0.290 3.63 0.087 IRM Approximation 13 3 .31-3.47 V i b r a t i o n a l States of Anthracene The anthracene molecule has 66 fundamental v i b r a t i o n a l modes c l a s s i f i e d as | 2 ft|, S Qu , , II bm , bb*j , II bau , II and 6 bju . Among them only Q) and bt^ modes are expected to be b u i l t on the allowed &w and B»u o r i g i n s by v i b r a t i o n a l p e r t u r b a t i o n of the e l e c t r o n i c t r a n s i t i o n s . No c a l c u l a t i o n s of the energies of the fundamental v i b r a t i o n s have been reported. However, and ^ f u n d a  mentals are a c t i v e i n Raman spectra and so any a v a i l a b l e data may be consulted to a i d i n the assignment of the v i b r a  t i o n a l s t r u c t u r e i n fluorescence. The energy d e v i a t i o n of combination bands from t h e i r harmonic value can occur due to anharmonicity of the p o t e n t i a l f i e l d i n the molecule. For a c c i d e n t a l l y degenerate or very close l y i n g v i b r a t i o n a l l e v e l s the anharmonicity gives r i s e to a Fermi resonance (14) which causes a s p l i t t i n g of the two degenerate l e v e l s , or a f u r t h e r separation of two l e v e l s of the same symmetry. These two e f f e c t s (anharmonicity and 4 Fermi resonance) are mentioned here because they might he expected to appear i n the observed spectra. The matrix element of the d i p o l e moment operator M i s defined as (15) where m = JVef M ^ t l • T i l e i n i t i a l and f i n a l v i b r a t i o n a l wave fun c t i o n s S"<*<wc| and f ^ o t l are s t a t i o n a r y s t a t e functions of a many-dimensional , ' a s c i l l a t o r . The Franck-Condon p r i n c i p l e s t a t e s that m does not depend on the coordinates of the n u c l e i . At a s u f f i c i e n t l y low temperature the molecule normally e x i s t s i n i t s v i b r a t i o n l e s s ground s t a t e , and since only those t r a n s i t i o n s are p o s s i b l e f o r which the overlap i n t e g r a l ^ (Tnucl € n u c l o i l n u c l does not vanish, t o t a l l y symmetric v i b r a t i o n s are a c t i v e i n the upper s t a t e . For anthracene these are Q| fundamentals or any odd overtone. 2 Although the i n t e n s i t y of a l i n e i s given by M , i t cannot be predicted because the overlap i n t e g r a l depends on the change i n geometry of the molecule between the ground and the e x c i t e d s t a t e s which i s not known. Conversely from the.observed i n t e n s i t i e s of members of a v i b r a t i o n a l progression, changes i n molecular dimensions may be d i s - cussed, (16),(17). 5 Mixed C r y s t a l Phenomena I f the so l u t e molecule does not i n t e r a c t w i t h the surrounding solvent molecules that make up the host c r y s t a l l a t t i c e , then the s o l u t e molecules may be regarded as an "oriented gas". The solvent molecules would then only serve to hold the guest molecules i n a f i x e d o r i e n t a t i o n i n space and the observed spectrum would be i d e n t i c a l w i t h the f r e e molecule spectrum observed i n the vapour phase. However, various m o d i f i c a t i o n s on the f r e e molecule spectrum are found i n the mixed c r y s t a l spectrum and these a r i s e from the perturbations caused by the surrounding solvent molecules (18) ( 1 9 ) . These are ( i ) a s h i f t of the e n t i r e spectrum to the red or t o the blue and ( i i ) a change i n the i n t e n s i t i e s of the i n d i v i d u a l l i n e s i n the spectrum due to i n t e n s i t y s t e a l i n g from other nearby systems. E f f e c t ( i ) i s d i f f i c u l t to p r e d i c t and only one c a l c u l a t i o n has been made ( 2 0 ) ; c a l c u l a t i o n s of e f f e c t ( i i ) have been made usi n g second- order p e r t u r b a t i o n theory f o r some systems ( 2 1 ) . S h p o l ' s k i i (22) (23) has shown that w e l l - r e s o l v e d s p e c t r a of organic molecules may be obtained i n normal p a r a f f i n s o l i d s o l u t i o n at 77°K. This method provides an abundance of p r e c i s e data concerning the v i b r a t i o n a l s t r u c t u r e of e l e c t r o n i c s t a t e s . A t h e o r e t i c a l treatment of the S h p o l ' s k i i e f f e c t has been presented by Rebane and Khizhnyakov (24) ( 2 5 ) . 6 Previous Experimental Work A l l previous workers have i n t e r p r e t e d the 3800 A 0 absorption system of anthracene as a r i s i n g s o l e l y from a 'Al t r a n s i t i o n . The predicted 'A% t r a n s i t i o n has not been observed. The system has been analysed i n the vapour (26), s o l u t i o n (27), s o l i d s o l u t i o n (28) and c r y s t a l (29) at various temperatures as low as 4*2°K. At 20°K s e v e r a l ftj fundamentals were resolved i n the mixed c r y s t a l s of naphthal||ne and phenanthrene both i n absorption and i n fluorescence spectra (30). In a r i g i d s o l u t i o n of n-heptane at 77°K B©lj©tnikova a l s o resolved many frequencies (28). Some Raman (31) and IR (32) (33) data are a v a i l a b l e f o r anthracene. In t a b l e 3 the a v a i l a b l e data concerning (k% and k*§ fundamentals i n the and 'Bio e l e c t r o n i c s t a t e s are summarized. The aim of the present experimental i n v e s t i g a t i o n i s t o analyse the v i b r a t i o n a l and e l e c t r o n i c s t a t e s of the o molecule i n the 3800 A re g i o n and to search f o r the o r i g i n of '&zo *— ' A ^ t r a n s i t i o n w i t h i t s associated v i b r a t i o n a l s t r u c t u r e . 7 Table 3 <Xj and bjj Fundamentals Observed i n Anthracene data from anthra- anthra cene cene i n pure naph- c r y s t a l thalene 4 .20K 20OK (29) (30) absorption spectra anthra- anthra- anthra cene i n cene i n cene i n phenan- MeoH-F*oH n- threne 90°K heptane 20°K (27) 77°K (30) (28) Data from Raman spectra (31) anthra- anthra cene cene pure s o l u - c r y s t a l t i o n 350 c m 399 c m 393 c m 400 c m 1170 1400 1164 1401 739 1031 1159 1389 1450 - 1 I A - 415 403 390 397 C * 400 C k 475 474 522 522 - 606 655 652 b„ 757 752 749 (?) 745 (?)"- *% 1009 1012 bn 1163 1165 1165 H 1165 1180 b H 1264 1264 1265 1261 (?) 1262(?) 1407 1416 1407 1403 1397 1413 1439 1444 bis 1481 1481 1559 1567 1567 1555 1551 ft* 1645 1632 1631 EXPERIMENTAL ARRANGEMENT Prepar a t i o n of the Samples 3 S c i n t i l l a t i o n grade anthracene obtained from R e i l l y Tar and Chemical Corporation was subjected to fourteen passes i n a F i s h e r zone r e f i n e r . S o l u t i o n s of the p u r i f i e d anthracene w i t h concentrations ranging from 0.73 x 10 ^ M to 5.0 x 10 ^"M were prepared i n s p e c t r o q u a l i t y n-heptane and n-hexane supplied by Matheson Coleman & B e l l . A l l s o l u t i o n s were stored i n darkness to avoid photo-oxidation of anthracene. Mixed c r y s t a l s of anthracene i n fluorene and i n biphenyl were grown i n an evacuated pyrex tube u s i n g a Bridgeman furnace (34). Eastman red l a b e l biphenyl was used without f u r t h e r p u r i f i c a t i o n . The anthracene impurity con tained i n a s o l u t i o n of Eastman red l a b e l fluorene d i s s o l v e d i n petroleum ether was extracted i n t o concentrated s u l f u r i c a c i d . The e x t r a c t i o n procedure was repeated u n t i l the s u l f u r i c a c i d l a y e r remained c o l o u r l e s s (about twelve ti m e s ) . The p u r i f i e d fluorene was recovered and was passed f o r t y - s i x times through a zone r e f i n e r . Ingots about 10 cm long and 0.8 cm diameter were grown over a period of about 24 hours i n a Bridgeman furnace. Mono c r y s t a l l i n e portions of the ingots were i s o l a t e d using a p o l a r i z i n g microscope. Selec t i o n of a s i n g l e c r y s t a l sample was made a f t e r checking f o r 9 complete e x t i n c t i o n i n orthoscopic p r o j e c t i o n under a L e i t z - WetzLar p o l a r i z i n g microscope. The desired c r y s t a l face was found a f t e r l o c a t i n g the c r y s t a l axes under conoscopic examination. The chosen samples were chopped up along c l e a v  age planes u s i n g t h i n r a z o r blades, and polished by hand t o the,,required t h i c k n e s s . f i r s t on f i n e emery-paper and then on Kleenex t i s s u e s or lens t i s s u e s soaked i n ethanol water mixture (1:1). The c r y s t a l thickness and the concentration of anthracene were adjusted so that the o p t i c a l , d e n s i t y of the 389 cm~"^ " fundamental mode i n k p o l a r i z a t i o n was about 0.5 - 1.5 at room temperature. This range of the o p t i c a l d e n s i t y was chosen to detect the various l i n e s of d i f f e r e n t i n t e n s i t y . The concentrations of the mixed c r y s t a l s were 0.993 - 8.00 x 10~4M/M and the f u l l thickness range a v a i l - able (about 0.2 mm to 2 mm) *was3 used. The t h i n n e r c r y s t a l s were prepared by mounting a l a r g e r s i n g l e c r y s t a l with correct a x i s alignment i n a brass r i n g packed w i t h p l a s t e r of P a r i s . The samples were c a r e f u l l y ground and polished a f t e r the p l a s t e r of P a r i s had s e t . Before the f i n a l p o l i s h the packing around the t h i n c r y s t a l protected i t from breakage. This method produced c r y s t a l s of about the same thickness as the r i n g . Large s i n g l e c r y s t a l s of fluorene were e a s i e r t o grow than biphenyl c r y s t a l s . 10 Measurement of the Spectra Apparatus I t was important to work at a s u f f i c i e n t l y low temperature to resolve the v i b r a t i o n a l s t r u c t u r e . L i q u i d helium (4.2°K) and l i q u i d n i t r o g e n (63°K and 77°K) were used as r e f r i g e r a n t s . The biggest problem i n t a k i n g spectra at low temperature i s to ensure good thermal contact between sample and r e f r i g e r a n t . Some l i q u i d cements or n a i l p o l i s h (35) have been recommended f o r t h i s purpose. S i l i c o n e grease, rubber cement ( 3 6 ) , n a i l p o l i s h and OE 7031 cement were used i n the work at 4.2°K. GE 7031 cement gave the best r e s u l t s s ince the l i n e s were sharpest (IQLdth 4 cm""'" f o r an average l i n e i n n-heptane). For the work at 4 .2°K the c r y s t a l was attached to a copper d i s c w i t h GE cement and the d i s c was secured f i r m l y to the inner helium can. The brass s o l u t i o n c e l l (Figure 1) f o r n-heptane and n-hexane were attached with, b o l t s and GE 7031 cement to ensure a good thermal con t a c t between the c e l l holder and copper helium can. The n-heptane and n-hexane s o l u t i o n s were a l s o studied at l i q u i d n i t r o g e n temperatures using the c e l l s shown i n Figure 1. The s o l u t i o n was syringed i n t o the c e l l through a small hole that was l a t e r sealed w i t h a small lead b a l l held under pressure against the opening by a s p r i n g s t r i p . The two quartz windows were sealed w i t h indium 0 - r i n g s . 11 Temperatures lower than 77°K were obtained by pumping on the l i q u i d n i t r o g e n , the temperature bding estimated by measur ing the n i t r o g e n vapour pressure. The temperature was reduced i n t h i s way to about 63°K, the t r i p l e point of n i t r o g e n , and t h i s temperature was maintained f o r about 50 minutes before the n i t r o g e n was completely pumped o f f . Some spectra at 77°K were obtained u s i n g the apparatus shown i n Figure 1. The sample was placed i n the spade-shaped inner s i l i c a c e l l and frozen by immersion i n l i q u i d n i t r o g e n . Resistance wire was wound i n a coarse s p i r a l around a s i l i c a dewar to avoid f r o s t i n g . In t h i s arrangement the l i g h t had to t r a v e r s e both the p o l y c r y s t a l l i n e sample and the l i q u i d n i t r o g e n around the c e l l . Measurement of the Lines A l l low temperature spectra i n t h i s t h e s i s were obtained u s i n g a H i l g e r and Watts E 201 l a r g e L i t t r o w spectro graph. The source f o r absorption and emission s p e c t r a was a high pressure Xenon lamp C&sram XBO 162). Kodak 103 a-0, 103-F and I I I - F spectroscopic p l a t e s were subjected to a wide range of exporsures t o b r i n g out a l l the l i n e s i n optimum contrast and were processed i n the manner recommended by the manufacturers. The p l a t e s were enlarged by a f a c t o r of about ten on to high contrast photographic paper ( I l f o r d Bromide - B 3 26 K and Kodabromide A5) and the s p e c t r a l l i n e s were measured from FIG I LOW TEMPERATURE SAMPLE CELLS 13 the p r i n t s by i n t e r p o l a t i o n or e x t r a p o l a t i o n using nearby i r o n standard l i n e s (37). Distances between l i n e s were meas ured to an accuracy of about 0.1 mm by means of a p r e c i s e l y engraved r u l e r or a t r a v e l l i n g microscope. An e r r o r of about 1 cm~l was introduced by these measuring methods f o r even the sharpest l i n e s . Kayser's t a b l e (38) was used to convert the wavelengths i n a i r to the wave numbers i n vacuum. RESULTS In Figures 2 to 7, o r i g i n a l p r i n t s used f o r l i n e measurement and sketches roughly i n d i c a t i n g the r e l a t i v e l i n e i n t e n s i t y are shown both i n n-heptane (4.2°K) and i n fluorene matrices. More p r e c i s e energy values are tabulated i n Tables 4 and 5 f o r the fluorescence and absorption spectra, r e s p e c t i v e l y . The numbering i n the f i g u r e s c o r r e l a t e with those i n the t a b l e s , s eparately f o r the fluorescence and absorption data. Some dotted l i n e s i n Figure 4 i n d i c a t e s p e c t r a l l i n e s which were found only i n s p e c i a l samples and whose i n t e n s i t i e s r e l a t i v e to other l i n e s are not known. Not a l l the impurity l i n e s f o r the fluorene matrix are shown; these extra l i n e s probably a r i s e from fluorene i t s e l f and/or some impurity such as carbazole or phenanthrene. Table 6 shows absorption l i n e s a r i s i n g from the fluorene matrix at 4.20K. (a) fluorene tlb(M) lb) n-heptane (c) fluorene llc(L) FIGS 2 AND3 THE FLUORESCENCE SPECTRA OF ANTHRACENE IN n-HEPTANE AND FLUORENE AT 4 . 2 °K H II i —J—LLJ L n. i i h l.n 1.. J . Ii. 1 1 .,,1,1 .. 1 i 1 II fl 1 II . 1 1 1 1 . i i 1 • 1 . I 9 1—LIU 1—i_i—1—I 1—| u 1 1 1 l. 1 • 1 i . i . . (a) Anthracene in n-heptane at 4.2°K ||b or fla i l l l l . • • l . i ' n — i — r 36 » « « 55 62 m T782 87 93 98 KB «» r n — T T - -I 1 1 1 , , l_ 120 E3 |27 I (b) Anthracene m fluorene at 42°K FIG 4 RELATIVE INTENSITIES OF THE LINES IN FLUORESCENCE 17 Table 4 Fluorescence Spectra of Anthracene i n Various Matrices ? ^ h / ^ f 1 , a , 4 ? 2 ? K n ~ fluo r e n e , 4 . 2 0 K n-heptane remarks b. 11M (b) 1 1 1 (c) hexane 11M (b,a) 1 1 L (c) 6 3 , 7 7 0 £ 4 . 2 0 K 770K 1 -2744 -2744 2 -2446 -2446 3 -2154 -2154 4 -1814 -1814 5 -1600 -1600 6 - 809 - 809 7 - 282 - 282 8 - 226 - 226 9 - 164 10 - 131 11 26056 - 46 12 26056 26498 25975 25975 13 16 14 51 15 136 136 148 16 184 184 196 17 18 406 406 396 398 19 429 20 466 466 471 21 532 532 527 22 2 3 24 25 620 620 621 625 26 675 675 670 670 27 28 754 754 755 755 29 30 795 795 794 794 792 3 1 828 32 880 880 870 33 894 34 905 917 35 962 962 950 36 1025 1025 1019 1022 37 1052 38 1130 39 1173 1173 1177 1175 1175 398 627 763 792 1021 1044 - 166 .-„c 26247 26211 0-0, o r i g i n 179 2 1 4 3 6 8 3 9 4 - 2 5 3 9 4 3 9 4 , ag 510 553 575 589 629 734 759 778 787 874 510, ag? impurity 629, ag 759, ag 2x394-1 911 911, b3g 1020 1045 1141 1169 1163 1 0 2 0 , ag 1 0 4 5 , b 3 g ? 3 9 4 + 7 5 9 - 1 2 . FR d 1 1 6 3 , ag 18 Table 4 continued biphenyl,a.4.20K n- Fluorene. 4.2°K n-heptane remarks b. 11M (b) 11L (c) hexane 11M (b,a) ILL (c) 63,77°K 4.2°K ~ 77°K - ~ - 40 41 42 43 1244 44 45 46 47 48 1414 49 1454 50 51 52 1516 53 54 55 1559 56 1620 57 58 59 60 61 62 1810 63 64 65 1877 66 67 68 69 1962 70 71 2039 72 73 • 76 1193 1244 1269 1414 1454 1516 1559 1620 1810 1877 1962 2039 1272 1268 1305 1356 1415 1411 1442 1484 1501 1567 1562 1598 1699 1640 1809 1963 2037 1806 1826 1848 1877 1906 1922 1957 2030 2061 2160 1186 1268 1305 1356 1411 1442 1484 1562 1597 1640 1675 1706 1806 1957 2030 2061 2100 1194 1267 1413 1568 1643 1808 1963 2037 1180 1233 1257 1267 1283 1305 1340 1383 1409 1431 1516 1538 1652 1568 1639 1660 1715 1736 1781 1803 1888 1910 1924 1960 1996 .2033 2049 2072 2124 2163 •1180, b3g 2x616 (b2u) + 1? 2x629-1,FR . 1267, ag 394+911 1409-25-1 1409, ag 629+2x394 +14,FR ? e 2x759-2 1568-25-5 394+1163+5 1568, ag 1639, b3g 1660, b3g 2x394+911-16 629+1163-10; 759+1020+2; 394+1409-25 +3 394+1409 629+1267-8 394+2x759-2 759+H63-2 394+1568-2 ? 394+1639 394+1660-5 510+1568-6,? 759+1409-5 19 Table 4 continued biphenyl,a.4 . 2 0 K 11M (b) 11L (c) n- hexane 77°K Fluorene. 11M (b,a) 4.2°K 11L (c_) n-heptane 63,77°K 4.2°K remarks b. 77 2202 2202 2193 2198 2204 2195 2x394+1409-2 78 2237 79 2270 629x1639+2 80 2286 2290 1020+1267+3 81 2322 2329 759+1568; 2x1163+3 82 2340 2340 2352 2354 2358 2x394+1568+2 83 2399 759+1639+1 84 2435 2435 2427 2427 2435 2426 1020+1409-3 85 2502 86 2523 2523 2523 2x1267-11 87 2587 2587 2577 2580 2580 2572 2571 1163+1409-1 88 2594 2591 1180+1409+2 89 2637 1020+1639-22 90 2681 2681 2671 2677 2669 1267+1409-7 91 2737 2729 2730 1163+1568-1 92 2819 2753 1180+1568+5 93 2828 2828 2821 2815 2815 2819 2818 2x1409 94 2831 1267+1568-4 95 2906 2906 2897 2897 2894 • 1267+1639-12 96 2905 97 2923 1267+1660-4; 2x759+1409-4 394+2x1267-5 98 2978 2978 2967 2966 2965 2973 2969 1409+1568-8; 394+1163+ 1409+3 99 2999 2999 100 3047 3047 3050 3050 3041 3043 1409+1639-5 101 3064 394+1267+ 1409-3 102 3084 3084 3090 3090, ag? 103 3122 3122 3131 3131 3136 3129 2x1568-7 104 3220 3199 1568+1639 105 3205 . 3211 3212 3208 394+2x1409-4 106 3273 3273 3271 2x1639-7 107 3303 3301 629+1267+ 1409-4 108 3359 3359 3365 3363 394+1409+ 1568-8; 2x394+1163+ 1409+3 109' 3425 3425 3434 3440 3450 3438 629+2x1409-9 394+1409+ . 1639+4 20 Table 4 continued biphenyl,a.4.2°K n- Fluorene 4.20K n-heptane remarks b. 11M (b) 11L (c) hexane 11M (b,a) 11L (c) 63,77°K 4.20K 77°K ~ " 110 3492 3492 3521 3526 111 3579 3579 3598 3598 3596 2x394+2x1408 -11; 629+1409+ 1568-11; 2x394+1163+ 1409+4 112 3732 759+1409+ 1568-4; 911+2x1409+3 394+759+1163 +1409+9 113 3778 3756 3758 3754 2x394+1409+ 1568-13 114 3841 3838 3838 3833 1020+2x1409- 5 115 3935 3912 116 3992 3984 3978 1163+2x1409 -3» ?» 1020+1409+ 1568-19 117 4087 4080 4075 1267+2x1409 -6 118 1 4125 394+1409+ 2x1163-4 119 4164 4138 4154 120 4246 4222 4228 4224 3x1409-3 121 4301 4297 122 4398 4372 4379 4372 1568+2x1409 -14 123 4450 4449 1639+2x1409 -8 124 4524 4524 4524 1409+2x1568 -21 125 4600 4605 394+3x1409 -16 126 4692 4681 4696 394+1267+ 1409+1639+7 127 4765 4757 4767 394+1568+ 2x1409-13; 2x1568+1639 -8 2x394+1163+ 2x1409-2 128 4832 21 Table 4 continued a. C r y s t a l axes are shown i n brackets while M and L show molecular short and long axes, r e s p e c t i v e l y . b. Assignments are made usi n g the data from the n-heptane spectrum. c. The o r i g i n s i n the d i f f e r e n t matrices are given i n cm""'", and a l l the other e n t r i e s i n the t a b l e show d i f f e r e n c e s from the o r i g i n . d. (FE.) Fermi resonance. e. Doubtful i n i t s appearance. fa) fluorene || b IM) (b) n-heptane (c) fluorene It c (L) F I G S 5 A N D 6 T H E A B S O R P T I O N S P E C T R A O F A N T H R A C E N E IN n - H E P T A N E A N D F L U O R E N E A T 4.2 ° K ill.••il..i.lLl.,iill .JL,IJ,II11IIIII,J. JH.11.II1.IL •••• | l ' ,1 INI l l l i l - .U I I..i. i. - l . - i . 30 «3 SO 58 60 71 173 IN IW N) IM 2 0 (a) Anthraoene in n-heptane at 4.2°K lb or lis _ L u u I .M.I_lLi J i i i i l Il I II I.I 1_ JlUL 30 43 SO 56 65 71 89 03 KM IO 123 133 138 148 135 166 171 HO 186 193 199 205 -rr-rr (b) Anthracene in fluorene at 4.2°K FIG 7 RELATIVE INTENSITIES OF THE LINES IN ABSORPTION Table 5 Absorption Spectra of Anthracene i n Various Matrices biphenyl n- f l u o r e n e , n-heptane a. 4 . 2 0 K hexane 4 . 2 0 K 11M 11L 77°K 11M 11L 77°K 63°K 4 .2°K (b) (o) (b,a) (c) 1 -154 2 -136 -144 3 -127 4 -112 5 - 92 6 -:-80 7 -73 - 73 8 ~ - 43 c 9 26056 26498 25975 26247 26239 26221 0-0, origin'' 10 29 24 24 25 25, l a t t i c e 11 47 2x25-3?; 47? 12 70 13 Q 5 14 105 15 118 16 133 17 146 18 178 19 187 20 219 211 21 229 22 236 23 253 254 24 262 25 273 279 26 299 27 317 316 28 340 29 351 30 391 385 389 392 385 387 389 389 389, ag 31 422 410 389+25-4 , 32 420 2x211-2 FR ? 33 436 389+2x25-3? 34 457 35 464 36 474 37 484 38 523 521 39 531 40 539 41 553 42 566 43 583 581 585 590 589 590 590, ag Table 5 continued 25 biphenyl n- flu o r e n e , n-heptane a. 4.2°K hexane 4.2°K remarks 11M 11L 77°K 11M 11L 77°K 63°K 4.2°K (b) (c) (b,a)(c) 44 610 616 590+25+1 45 628 4 6 652 658 663 663, ag? 47 687 48 703 4 9 709 50 735 729 733 740 740 744 744. ag 51 778 773 780 777 780 779 2x389+1 52 812 798 2x389+25-5 5 3 809 389+2x211-2 FR? 54 823 55 836 840 56 859 857 57 871 58 882 891 890 892 894 894,b3g 59 918 60 927 926 61 941 62 955 63 ( 977 979 389+590 64 1005 2x211-7 FR? 65 1024 1023 1028 1024 1027 1030 1030, ag 66 1055 1055 1062 1057 389+663+5, FR? 67 1084 68 1096 1102 69 1126 70 1143 1 1 3 5 389+744+2 71 1158 1152 1169 1155 1160 1157 1157, ag 72 1166 1166 1166, b3g 73 1190 74 1199 1199 1199 11L:1166+24+9 75 1213 76 1 2 3 9 1247 1247, ag? 77 1266 78 1285 1283 389+894 79 1311 1297 80 1304 81 1327 1318 2x663-8? 82 1338 590+744+4, FR? 83 1 3 5 4 84 1374 590+2x389+6, FR 85 1391 1386. 1396 1396 1397 1399 1399, ag 86 1422 1418 389+1030-1, FR 26 Table 5 continued biphenyl a - f l u o r e n e , n-heptane a. 4 . 2 0 K hexane 4 . 2 o K 11M 111 77°K 11M 11L 77°E 6 3 ° K 4 . 2 0 K (b) (c) (b,a) (c) remarks b. 87 88 89 90 9 1 1496 92 9 3 1 5 4 1 94 95 96 97 9 8 9 9 1 0 0 1 0 1 1 0 2 1 0 3 1 0 4 1 0 5 106 107 108 1 0 9 1 1 0 1 8 9 0 1 1 1 1 1 2 1 9 3 5 1 1 3 1 1 4 1979 1 1 5 116 2048 1 1 7 118 1 1 9 2128 1 2 0 2 1 7 8 1 2 1 1 2 2 1 2 3 2 2 4 3 124 1 2 5 126 2 3 2 2 127 1491 1554- 1779 1880 1 9 3 2 2 0 3 5 2171 2 2 7 6 2 3 2 1 1427 1431 1399+25+7, FR 1443 1447 1467 1462 1458 1463 1464 111:1464, b3g 1495 1482 1480 590+894-4, FR 1498 1498 1504 1503 1503, ag 1532 1533 744+2x389+11, FR; 1503+25+5 1555 1547 1551 1547 389+1157+1 1560 1559 389+1166+4 1578 389+1157+25+7 1601 1604 1619 590+1030-1 1641 389+1247+5? 1658 1591 1699 1700 663+1030+7? 1720 389+2x663+5? 1727 1749 590+1157+2 1785 2x894-3 1783 1786 1789 1788 389+1399 1809 2x389+1030+1; 389+1399+25-4 1814 1822 663+H57+2, FR? 1834 663+3x389+4? 1860 1853 1853 1860 1855 389+1464+2 1892 1887 1893 1885 389+1503-7 1917 1917 1941 1936 2x389+1157+1 1959 1971 1963 1978 1986 1990 1993 590+1399+4 2006 2008 2056 2052 2051 2051 2x1030-9 2094 2071 663+1399+9? 2112 2x389+2x663+8? 2145 2141 2141 744+1399-2 2176 1177 2181 2178 2x389+1399+1 2202 2220 2225 2220 389+663+1157+6 2248 2256 2254 744+1503+7 2287 2287 2285 2x389+1503+4 2286 2296 894+1399+3 2319 2x1157+5 2336 2336 2331 2329 1157+1166+6 27 Table 5 continued biphenyl h- fluorene, a. 4.2°K hexane 4.2°K 11M 11L 77°K 11M 111 (b) (c) (b,a) (c) n-heptane 77°K 63°K 4.2°K remarks 128 2348 2347 129 2378 130 2404 131 2416 2422 2425 2426 2428 2428 132 2555 2449 2458 2457 133 2481 2482 2490 134 2508 135 2533 136 2560 2548 2561 2561 2560 2562 2559 137 2623 2607 2617 2620 138 2637 2637 2638 139 2648 140 2650 2665 2668 2655 2665 2660 141 2710 2715 2701 142 2729 2720 143 2751 144 2728 2730 2734 145 2783 2796 2800 2798 146 2816 2818 147 2832 148 2855 2861 2863 2864 149 2893 2886 2890 2897 2901 2900 150 2941 2947 2945 2945 2947 2955 2954 151 2964 2978 2954 152 3007 3010 3004 153 3022 3020 154 3039 3053 3057 3052 3048 155 3081 156 3100 157 3126 3119 158 3152 159 3174 3178 3176 3191 3187 3189 3193 160 3212 389+590+1399 1157+1247?; 894+1503+7? 11M:1030+1399-1? 389+663+1399+6? 1030+1464-4 389+663+1464-8? 1030+1503 11M:1157+1399+3; 111:1166+1399 1157+1464-1 11M:389+744+1503+2; 111:3x389+1464-7? 1247+1399+2? 11M:1503+1157 111:1503+1167+1 389+1157+1166+8 590+744+1399+2, FR 2x1399 389+1030+1399; 2x1399+25-5 663+2x389+1399+8?; 2x389+894+1157+4? 1399+1464+1 1399+1503-2 111:1464+1503-11; 11M:389+1399+1157+7 2x744+1503, FR 2x1503-2 389+1157+1464+10 11M:389+1157+1503+1 111:389+1166+1503-1 3x1030-9; 2x389+2x1157-11 2x389+1157+1166-1 C-H s t r e t c h , b3g? 590+1157+1399+6 11M:389+2x1399+6 111:389+590+744+1464 +6 2x389+1030+1399+5 Table 5 continued 28 biphenyl n _ fluorene, n-heptane a. 4 . 2 0 K hexane 4 .2°K 11M ILL 77°K IIM H L 77°K 63°K 4 . 2 0 K remarks & (£> (£,a) (c) 161 3227 389+590+744+1503+1 162 3248 3254 3258 3264 389+1399+1464+12 163 3283 3277 3280 3288 3292 3293 389+1399+1503+2 164 3333 3339 3340 3342 3345 3348 2x389+1164+1399+7? 165 3365 3389 3384 590+2x1399-4 166 3394 389+2x1503-1 167 3416 3414 744+1157+1503+7 168 3433 3443 3446 3439 3442 2x389+1157+1503+4 169 3476 3460 1399+2x1030+1 170 3565 3569 3564 3573 3580 3574 2x389+2x1399-3 171 3588 1030+1157+1399+2 172 3602 3609 590+2x1503+13 173 3634 e 174 3668 3654 3648 744+1399+1503+2 175 3666 3678 3688 3678 2x398+1399+1503 176 3718 3718 3714 1399+2x1157+1 177 3750 3723 3737 3731 3x389+1157+1399+8 178 3813 3804 3823 3823 3826 1030+2x1399-2; 1503+2x1157+9? 179 3838 3858 3857 3853 389+663+2x1399+3? 180 3886 1030+1399+1464-7? 181 3903 1157+1247+1503-4? 894+2x1503+3? 182 3922 1030+1399+1503-10; 389+744+2x1399-9? 183 3955 3944 3948 3968 3955 1157+2x1399 184 3985 389+1030+1166+1399+1; 389+1030+1157+1399+10 185 4013 4017 1157+1399+1464-3 186 4035 4030 1030+2x1503-6 187 4042 4066 4058 4073 4055 1157+1399+1503-4 188 4097 4112 4100 389+1399+2x1157-2 189 4108 4117 4118 4134 4118 1157+1464+1503-6 190 4149 4160 1157+2x1503-1 191 4177 4180 4172 4197 4199 4195 3x1399-2 192 4198 4214 389+1030+2x1399-3; 389+2x1157+1503+8 193 4227 663+2x389+2x1399-12? 194 4225 4262 4261 4265 1464+2x1399+3 195 4280 4278 4280 4294 4301 4297 1503+2x1399-4 196 4328 4339 4327 4349 4352 4351 389+1157+2x1399-7 197 4368 392+1166+2x1396+18 198 4388 4380 1399+1464+1503+14 Table 5 continued 29 biphenyl n- fluorene, n-heptane a. 4 . 2 0 K hexane 4 . 2 0 K 11M 111 77°K 11M 111 77°K 63°K 4 . 2 0 K remarks (b) (c) (b,a) (c) 199 4433 4438 4447 4446 389+1157+1399+1503+2; 389+1166+1399+1503-7? 200 4515 4513 4509 3x1503 201 4552 4545 389+1157+2x1503-7 202 4572 4582 4585 4589 4584 389+3x1399-2 203 4630 4630 4699 4656 4655 389+1464+2x1399+4 204 4668 4667 4684 4691 4683 389+1503+2x1399-7 205 4723 4700 4739 4741 4739 2x389+1157+2x1399+6; 2x389+1166+2x1399-3 206 4829 4832 2x389+1157+1399+1503-5 207 4890 389+3x1503-8; 590+1503+2x1399-1 208 4973 2x389+3x1399-2 209 5047 2x389+1464+2x1399+7; 744+1503+2x1399+2 210 5076 2x389+1503+2x1399-3 211 5126 2x1157+2x1399+14?; 1157+1166+2x1399+5 212 5227 1030+3x1399; 1030+1503+2x1399-4; 1399+1503+2x1157+11? 1157+1166+1399+1503+2^ 213 5366 1157+3x1399+12; 1166+3x1399+3 214 5417 1157+1464+2x1399-2; 215 5437 1247+3x1399+7? 216 5458 1157+1503+2x1399 217 5522 1157+1399+1464+1503-1 218 5567 1464+3x1399+6; 1157+1399+2x1503+5? 219 5596 4x1399 220 5672 1157+3x1503+6; 1464+3x1399+11 221 5684 1503+3x1399-16 222 5741 389+1157+3x1399-2; 389+1166+3x1399-11? 223 5825 389+1166+1464+2x1399+8 389+1157+1464+2x1399+ 17 224 5846 389+1157+1503+2x1399-1 225 5910 1399+3x1503+2 226 5983 389+4x1399-3 227 6053 389+3x1399+1464+3 30 Table 5 continued biphenyl n- flu o r e n e , n-heptane ( a 4 . 2 °K hexane 4.2 OK im 111 77°K 11M 11L 77°K 63°K 4.20K remarks ( b f (c) (b,a) (c)  228 6084 389+1503+3x1399-5 229 6135 2x389+1157+3x1399+3; 2x389+1166+3x1399-6 a. C r y s t a l axes are shown i n brackets while M and L show molecular short and long axes, r e s p e c t i v e l y . b. Assignments are made using the data from the n- heptane spectrum. c. The o r i g i n s i n the d i f f e r e n t matrices are given i n cm-"1", and a l l the other e n t r i e s i n the t a b l e show d i f f e r e n c e s from the o r i g i n . d. (FR) Fermi resonance. e. This l i n e i s d o u b t f u l . 31 Table 6 Absorption Spectrum of Fluorene at 4.2°K 11M ( b ) b * 11L (c) 11M (b) 111 (c) 11M (b) 111 {'o) 1 31062 35 31665 69 32219 2 31080 36 31666 31668 70 32230 3 31130 37 31695 31696 71 32246 4 31141 38 31716 72 32254 5 31157 39 31738 73 32262 6 31182 40 31742 74 32274 7 31190 41 31750 75 32298 8 31211 42 31795 76 32319 9 31230 43 31809 77 32323 10 31256 31255 44 31813 78 32359 11 31264 45 31835 79 32367 12 31290 31290 46 31844 80 32381 13 31299 47 31857 31857 81 32385 14 31318 31319 48 3186 3 31860 82 32394 15 31350 49 31892 83 32398 16 31363 50 31902 84 ' 32420 117 31377 31379 51 31919 85 32433 18 31409 31410 52 31928 86 32455 32455 19 31417 53 31956 87 32481 32480 20 31439 54 31959 31961 88 32494 21 31473 55 31967 89 32502 22 31478 56 31984 90 32526 123 31499 57 31992 91 32543 24 31512 58 32005 .32001 92 32569 25 31517 59 32017 93 32590 26 31520 60 32049 94 32603 27 31524 31524 61 32066 95 32619 28 31550 31548 62 32086 32082 96 32644 29 31584 63 32109 32110 97 32672 30 31651 64 32130 98 32687 31 31619 65 32148 99 32706 32 31623 66 32154 100 32731 33 31636 67 32161 101 32752 32755 34 31656 68 32206 32203 102 32761 103 32766 105 32805 107 32829 104 32787 106 32815 108 32858 a. The frequency of each l i n e i s given - 1 i n cm b. C r y s t a l axes are shown i n brackets while M and L show molecular short and long axes, r e s p e c t i v e l y . DISCUSSION Fluorescence Spectra Fundamental Modes S p e c t r a l l i n e s i n fluorescence may a r i s e from anthra cene molecules, molecules of the matrix or some other im p u r i t y molecule. Unknown i m p u r i t i e s present a r e a l problem i n fluorescence spectroscopy since a very small t r a c e of impurity (as low as 10 M) can make a l a r g e c o n t r i b u t i o n to the o v e r a l l emission... Lines due to fundamental modes of anthracene may be d i s t i n g u i s h e d from other emission l i n e s since only these form combinations b u i l t on the o r i g i n and the o r i g i n can be a s s i g n  ed from the absorption spectrum. On the basis of t h e i r i n t e n s i t i e s , p o l a r i z a t i o n and a b i l i t y to form combinations eight 0^ fundamental modes of the e l e c t r o n i c s t a t e were assigned: 394, 629, 759, 1020, 1163, 1267, 1409 and 1568 cm"1. T h e o r e t i c a l l y twelve fundamentals are pre d i c t e d f o r anthracene and among them three due to C-H str e t c h e s appear i n the region of 2900 - 3100 cm - 1 (39). So below 2000 cm" 1 nine (i^  modes should be found. From t h e i r i n t e n s i t y and p o l a r i z a t i o n behaviour e i t h e r 510, 874 or 1340 cm-1 may be se l e c t e d as t h i s n i n t h fl| fundamental. Among these 510 and 1340 cm - 1 modes appeared one and four times r e s p e c t i v e l y i n combination w i t h known A3 modes while 874 cm d i d not appear at a l l . From t h i s point of view, the n i n t h fundamental i s most probably the l i n e at 1340 cm - 1 w i t h the l i n e at 510 cm""'" pre f e r r e d next. However, 1340 cm"1 d i d not appear i n biphenyl while the other two d i d . Further, Raman data (31) shows 522 cm - 1 as ag i n anthracene c r y s t a l and i n s o l u t i o n which i s probably close enough to our 510 cm"1. No l i n e s corresponding to the other two v i b r a t i o n s were found i n the Raman. Thus although no d e f i n i t e assignment could be made f o r the n i n t h £)g fundamental, the l i n e at 510 cm"1 seems to be the most probable contender i f emphasis i s placed on i t s appearance i n the Raman spectra. The other two must be i n t e r p r e t e d as impurity l i n e s , or b j | belonging to the e l e c t r o n i c s t a t e i f f o r some reason fluorescence appears from a 1 o r i g i n . 3090 cm"1 may be assigned as an fundamental due to C-H s t r e t c h i n g since i t does not analyse as a combination l i n e , i t agrees w i t h previous e m p i r i c a l data (39) and i t has the expected p o l a r i z a t i o n . However, 3526 cm - 1 i s probably too high to be assigned as an a<j C-H s t r e t c h i n g frequency. F i v e b ^ fundamentals were found f o r the e l e c t r o n i c ground s t a t e at 911, 1045, 1180, 1639 and 1660 cm - 1. They were u s u a l l y weaker i n i n t e n s i t y than the modes and so d e t e c t i o n of combinations was d i f f i c u l t . However, combina t i o n s i n v o l v i n g a l l modes except 1045 cm"! were found. The b j |assignment r e s t s p r i m a r i l y on p o l a r i z a t i o n data ( i . e . , a l l these l i n e s appeared more s t r o n g l y i n the c d i r e c t i o n of 34" the fluorene m a t r i x ) . The modes at 1180 and 1639 cm - 1 agreed c l o s e l y with Raman data (31). Although the 1045 cm - 1 mode d i d not combine w i t h other fundamentals, i t i s t e n t a t i v e l y assigned as a fundamental since i t i s close to the 1012 cm"-1 fundamental observed i n the Raman spectrum (31). Fermi Resonance Three p o s s i b l e examples of Fermi resonance were observed i n fluorescence (Table 7 ) . A l l of these occured near strong Q| fundamentals as expected. In set (a) energy s h i f t s of those two l i n e s were found, i n set (b) i n t e n s i t y t r a n s f e r was more s i g n i f i c a n t , while i n set (c) both i n t e n s i t y and energy were a f f e c t e d s t r o n g l y . Table 7 P o s s i b l e Examples of Fermi Resonance i n the Fluorescence of Anthracene set l i n e No. i n n-heptane i n fluorene i n biphenyl 38 1141=394+759-12 1130=396+755-21 (a) 39 1163 ag 1175 ag 42 1257=2x629-1 (b) 43 1267 ag (c) 48 1409 ag 1411 ag 1414 ag 49 1431=2x391+629+14 1442=2x396+621+29 1454=2x406+620+22 35 Other Features I f e r r o r s of measurement due to l i n e broadening (see l i n e s 59, 86 and 95) are taken i n t o account, the combinations suggest the us u a l diatomic type of p o t e n t i a l curve, which i s anharmonic i n high quantum regi o n . Combinations of four fundamentals represented the most complex l i n e s observed i n our spec t r a and these were perhaps beginning, to show an- harmonicity of the 'A^ p o t e n t i a l surface. However, the p o t e n t i a l surface was s u r p r i s i n g l y harmonic f o r such a l a r g e polyatomic molecule. The o r i g i n and the 25 cm"1 l a t t i c e mode b u i l t on the o r i g i n d i d not appear i n fluorescence and t h i s can be explained i n terms o f reabsorption of the emission. Both of these l i n e s appeared s t r o n g l y i n absorption. T h e o r e t i c a l l y , fundamental modes of any symmetry species may combine provided that the combination has the symmetry or . In p r a c t i c e , combinations of t h i s general type do not appear. However, l i n e 41 at 1233 cm"1 might be in t e r p r e t e d as the overtone of the b i u mode at 616 cm - 1 observed i n the i n f r a - r e d (32); no combination of the observed Gl^ and b^can account f o r t h i s l i n e , although the p o s s i b i l i t y of a r i s i n g from impurity must be considered. Some l i n e s appeared, which could not be assigned i n terms of the observed fl^ and baj. fundamentals. These may be separated i n t o two kinds: ( i ) l i n e s which appeared i n only one of the four matrices, and ( i i ) l i n e s which appeared i n 36 more than one matrix. The l i n e s belonging to ( i ) may a r i s e from i m p u r i t i e s i n the matrix or from the host molecule. Further i f we assume a "solvent' 1 s h i f t of the 'A| o r i g i n d i f f e r e n t from that of the 'Aj o r i g i n , then the unexplained l i n e s could be i n t e r p r e t e d as ground s t a t e v i b r a t i o n a l modes from a lBao upper e l e c t r o n i c s t a t e . Phenanthrene, carbazole and a c r i d i n e are p o s s i b l e i m p u r i t i e s i n the matrices, fluorene and biphenyl because they are isomorphic w i t h those i m p u r i t i e s . Phenanthrene (41) (42) and carbazole (43) f l u o r e s c e near the o r i g i n of anthracene. Since a c r i d i n e does not f l u o r e s c e i n i t s c r y s t a l s t a t e or i n organic solvent (44) at l e a s t at room temperature and since i t s fluorescence spectrum i s to the red of the anthracene spectrum (44), the presence of a c r i d i n e i s not important i n the a n a l y s i s of fluorescence. In general i f a l i n e appears only i n a s p e c i a l " m a t r i x and i f the i n t e n s i t y r e l a t i v e to the other common l i n e s d i f f e r s over a number of samples, i t may be taken as an impurity l i n e . In t h i s sense only l i n e 22 at 533 cm""1 apparently arose from some impurity. Since the s p e c t r a of phenanthrene and carbazole measured under the same experimental c o n d i t i o n s as here are not a v a i l  a b le, the p r e c i s e assignment of i m p u r i t i e s i s not p o s s i b l e at present. Fluorene f l u o r e s c e s to the blue of the o r i g i n of anthracene (43) and the l i n e s 1 and 2 probably form part of the fluorene fluorescence spectrum because they agree c l o s e l y w i t h the data taken at 77°K i n n-heptane (43). Biphenyl does 3'7 not f l u o r e s c e i n t h i s region (27) and n e i t h e r do n-heptane nor n-hexane. The remaining p o s s i b l e i n t e r p r e t a t i o n of the l i n e s ( i ) involved a t r a n s i t i o n from a ' B a y upper s t a t e . I f t h i s i s t r u e , a somewhat s i m i l a r i n t e n s i t y and energy separa t i o n to v i b r a t i o n a l modes from the 1 system should be ob served. However, t h i s was not so and t h i s l a s t p o s s i b i l i t y may be excluded. For the l i n e s belonging to ( i i ) three p o s s i b l e i n t e r p r e t a t i o n s may e x i s t : i m p u r i t i e s i n anthracene, l i n e s from the l i g h t source, and v i b r a t i o n a l modes due to f l u o r e s  cence from the 'feay s t a t e assuming no "solvent" s h i f t . As im p u r i t i e s i n anthracen anthraquinone must be considered i n a d d i t i o n to carbazole and phenanthrene, f o r the o x i d a t i o n of anthracen could occur e s p e c i a l l y i n the presence of l i g h t and oxygen. Anthraquinone vapour (45) f l u o r e s c e s i n the region 20,000 - 23,000 cm"1. Thus the l i n e s 115 and 121 could be due to anthraquinone, and the l i n e s near the o r i g i n (9, 15, 16, 20 and 35) might a r i s e from e i t h e r carbazole or phenanthrene, but again p r e c i s e assignment i s impossible f o r the l a c k of data. I f emission from the source appeared, the l i n e must show the same energy independent of the matrix. From t h i s point of view no l i n e s arose from the common source xenon. Thus to account f o r the presence of the unassigned l i n e s i n the fluorescence spectra, the existence of some im p u r i t i e s must be claimed. 3?" Absorption Spectra Fundamental Modes of the ' & t p Upper State From a p r e l i m i n a r y examination of t h e i r p o l a r i z a t i o n , i n t e n s i t y and appearance i n combinations, f i f t e e n i n t e r v a l s may be chosen as fundamentals with energy l e s s than 2000 cm - 1; e.g. eleven &% fundamentals: 389, 590, 744, 1030, 1157, 1399, 1503, 663, 1057., 1247 and 1338 cm"1, and four fundamentals 894, 1166, 1464 and 926 cm"1. Since there can be only twelve Q$ fundamentals i n a l l and three are expected i n the region of C-H s t r e t c h i n g frequencies near 3000 cm"1 there must be only nine fl^ fundamentals below 2000 cm"1. The f i r s t seven fundamentals and the f i r s t three fundamentals are assigned with c e r t a i n t y . Thus two more ft| fundamentals must be selected from the l a s t four l i s t e d . The i n t e r v a l 3119 cm"1 i n n-heptane might be added as a kjjc-H s t r e t c h i n g frequency. The l i n e at 663 cm"1 may be assigned as an fl^ funda mental w i t h some c e r t a i n t y although i t d i d not appear i n n- hexane or i n biphenyl. No a l t e r n a t i v e explanation f o r i t was p o s s i b l e , and many l i n e s could be best i n t e r p r e t e d as combina t i o n s i n v o l v i n g 663 cm"1 as an ft^ fundamental. The l i n e at 1057 cm"1 was s l i g h t l y stronger than 663 cm"1 and appeared i n n-heptane at 77°K while 663 cm~l d i d not. However, i t was not so u s e f u l as 663 cm"1 i n i n t e r p r e t i n g combinations and 1057 cm could i t s e l f be i n t e r p r e t e d as the combination (590+663) cm-1 e s p e c i a l l y when the p o s s i b i l i t y of Fermi resonance between 39 t h i s combination and the strong fundamental 1030 cm - 1 was considered. Thus 1057 cm"1 was taken as a combination r a t h e r than an ft^ fundamental. Both 1247 and 1338 cm"1 could be taken as fundamentals or as the combinations 1247 = 590+663-6 and 1338 + 590+744+4; although the l i n e s appeared to be too intense to be simple combinations. The l i n e at 1338 cm"1 was s u f f i c i e n t l y c l o s e to the strongest l i n e i n the spectrum at 1399 cm"1 f o r i t s i n t e n s i t y as a combination to be accounted f o r . However, the l i n e at 1247 cm"1 was more i s o l a t e d from other strong l i n e s so t h a t , i n t h i s case, a Fermi resonance could not r e a d i l y be assumed. Again the l i n e 1247 cm"1 appeared i n the fluorene matrix while the other d i d not. Thus, on these grounds, 663 and 1247 cm"1 are t e n t a t i v e l y assigned as ftj fundamentals and added to the previous l i s t of seven. As a bj| fundamental 926 cm"1 i s quite d o u b t f u l , how ever the a l t e r n a t i v e explanation (894+25+7) i s a l s o d o u b t f u l , and the l i n e at 2428 em"1 could be accounted f o r as a combina t i o n of 926 and 1503 cm"1 (long a x i s p o l a r i z e d ) . The combina- 1 1 t i o n of 926 cm w i t h the stronger fundamental at 1399 cm could not be found since i t was hidden beneath the combination 2329 = 1157+1166+6. Comparison of the Fundamentals on the 'Aft and on the ' B i u  E l e c t r o n i c States A l l p o s s i b l e fundamentals of anthracene i n the ground and i n the excit e d s t a t e 't^mare summarized i n Table 8,. 40 For the fundamentals which have been assigned with c e r t a i n t y , a correspondence between each fundamental at the two e l e c t r o n i c states have been observed both i n energy value and i n t e n s i t y . This i n d i c a t e s that the p o t e n t i a l energy surfaces of the ground and the e x c i t e d e l e c t r o n i c s t a t e are s i m i l a r i n these normal coordinates at l e a s t . This leads to the expectation that there w i l l be a correspondence f o r a l l the i n t e r v a l s , and the two (X^ fundamentals t e n t a t i v e l y assigned (663 and 1247 cm - 1) might be added to the s i x c e r t a i n ones to account f o r the nine fundamentals as p r e d i c t e d . Further there i s a tendency that the fundamentals i n the 'Aj s t a t e have higher energy than those i n the 1 s t a t e . This behaviour has been a l s o observed i n naphthalene (46). Table 8 The Fundamentals of Anthracene i n the '/\| Ground and the ' ^ i u Upper State Remark o r * " 1 394 c m 3 8 9 c m c e r t a i n 510 590 probable 629 663 probable 759 744 c e r t a i n 874 d o u b t f u l 1020 1030 c e r t a i n 1057 389+663+5, FR'/' ? 1163 c e r t a i n 1267 .1247 probable ^  1340 1338 590+744+4: i n 'B,u s t a t e only 1409 1399 c e r t a i n 1568 1503 c e r t a i n 3018 C-H s t r e t c h " ? 911 894 c e r t a i n 1045 926 p o s s i b l e 1180 1166 c e r t a i n 1639 1464 c e r t a i n 1660 3119 c e r t a i n C-H s t r e t c h ? ? 401 Fermi Resonance P o s s i b l e examples of Fermi resonance i n absorption are summarized i n Table .9. Table 9 P o s s i b l e Examples of Fermi Resonance i n the Absorption of Anthracene set l i n e No. i n n-•heptane, at 4.2°K i n fluorene, at 4.2°K 65 1030, ag 1028, ag C- a 66 1057 = 389+663+5 1062 = 387+657+18 82 1338 = 590+744+4 V, 84 1374 = 590+2x389+6 u 85 1 3 9 9 , ag 1 3 9 6 ,ag 86 1418 = 389+1030-1 1422 = 392+1028+2 89 1464, b3g 1462, b3g C 90 1480 = 590+894-4 1495 = 585+891+19 A 105 1788 = 389+1399 1783 = 392+1396-5 u. 107 1822 = 663+1157+2 1814 = 652+1169-7 144 2734 = 590+744+1399+2 e 145 2798 = 2x1399 150 2954 = 389+1157+1399+7 2445 = 392+1169+1396-12 1 151 2991 = 2x744+1503 2978 = 2x733+1498+14 A l l of these examples show a more pronounced Fermi resonance i n the fluorene matrix; t h i s was the tendency i n 42 fluorescence a l s o . The sets i n the fluorene matrix a, c and f show good examples of the e f f e c t both i n terms of the energy s h i f t and i n t e n s i t y t r a n s f e r . The other sets i n fluorene and a l l the sets i n n-heptane show only an i n t e n s i t y t r a n s f e r . In set b the strong fundamental at 1399 cm"*1 seems to share the i n t e n s i t y among .'e.Everal nearby combinations. However, since there must be some er r o r i n estimating i n t e n s i t i e s from the photographic p r i n t s , p a r t i c u l a r l y near the strong l i n e at 1339 cm - 1, a c t u a l Fermi resonance might occur only between the l i n e s 85 and 86. Although anharmonicity increases i n the higher energy region, no Fermi resonances were i d e n t i f i e d because of the decreasing i n t e n s i t y of the l i n e s w i t h a consequent increase i n the measurement e r r o r due to l i n e broadening. In the other two matrices a lower r e s o l u t i o n of the spectra d i d not a l l o w Fermi resonance to be i d e n t i f i e d . From the measurement of the energies of the funda mentals and t h e i r various combinations and of the r e l a t i v e i n t e n s i t y d i s t r i b u t i o n amongst them, i t i s seen that the p o t e n t i a l energy surface of the e l e c t r o n i c s t a t e i s s u r p r i s i n g l y harmonic. This f a c t may a l s o account f o r the small numbers of examples of Fermi resonance i n the anthra cene s p e c t r a . Other Lines The l i n e s which could not be assigned i n terms of the observed and fundamentals may be separated i n t o the f o l l o w i n g types: 1. l i n e s which appeared i n only one of the four matrices, (a) i n n-heptane, and (b) i n fluorene 2. l i n e s which appeared i n more than one matrix. Type 1 ( a ) : i n n-heptane. Some weak l i n e s were grouped around the o r i g i n and other strong CX| modes of the 1 &io e l e c t r o n i c s t a t e , as summarized i n Table 10. This s t r u c t u r e might be found i n the higher quantum region, but i t s i d e n t i f i c a t i o n i s impossible because of the appearance of much stronger combinations of the fundamentals. These weak l i n e s have two p o s s i b l e i n t e r p r e t a t i o n s . F i r s t l y these may be a number of s p e c i a l s i t e s i n the l a t t i c e , each site:; having a d i f f e r e n t environment and g i v i n g r i s e to a d i f f e r e n t "solvent s h i f t " . Thus f o r each d i f f e r e n t environ ment a separate s h i f t e d spectrum should be observed; the i n t e n s i t y of each " s h i f t e d " spectrum would depend on the number of anthracene molecules occupying that type of s i t e . The d i f f e r e n t p o s s i b l e s i t e s that might be considered are s u b s t i t u t i o n a l s i t e s , i n t e r s t i t i a l s i t e s , s i t e s next to a vacancy, or next to other anthracene molecules ( e i t h e r one or more) (47). T a b l e 10 S i m i l a r i t y of the S t r u c t u r e Around Some S t r o n g A b s o r p t i o n L i n e s 127 - A cra-1 112 :. ,92 80 73 k-3 a V9 cra-1 0-0 MS + 4 cm~l 25 14,7 70 85 133 H4-6 178 d e 153 236 135 127 262 110 279 90 - 73 k-9 299 316 3k0 389 vvS 21 h7 75 95 132 l k 2 16k 177 klO k36 k6k k8k 521 531 553 5 6 6 A e l & 133± "~ 116 k% 69 51 521 539 590 S 26 - 73 97 - - - 616 663 687 d e i 5 i 628 ~* 116± 663 92 76 70 - 687 703 709 779 MS 19 kk 78 92 1391' l k 7 f 162 176 798 823 857 871 918 926 9 k l 955 153 131+ 127 I l k 91 78 71 h8 23 k6 7k 92 135 1^5 163 177 w w: VW-7 vvw W VVW vvw w vw c. S MW w W VW VW VW W a. the c e n t e r s o f t h e s t r u c t u r e : the o r i g i n and ag fundamentals b. average energy d i f f e r e n c e f r o m c. s: s t r o n g , MW: medium weak, W: weak, VW: v e r y weak, VVW: v e r y v e r y weak, MS: medium s t r o n g , VVS: v e r y v e r y s t r o n g d. t h i s row shows t h e d i f f e r e n c e of energy v a l u e from V» i n cm"*-*- e. t h i s row shows the d i f f e r e n c e o f energy v a l u e from the o r i g i n i n c m ~ l 4§ Another p o s s i b i l i t y which must he considered i s the formation of c l a t h r a t e compounds. I t has already been sug gested by C i a i s (48) and S h p o l ' s k i i (49) that saturated normal p a r a f f i n molecules form a cage about the solute molecule. Evidence favouring t h i s point of view i s found i n the f o l l o w i n g experiment (50). 3.4 - benzpyrene i n c y c l o - hexane gave a d i f f u s e spectrum at 77°K. A d d i t i o n of 10% n-octane was s u f f i c i e n t to produce the sharp spectrum at 77°K t y p i c a l of the S h p o l ' s k i i e f f e c t . I f the assumption i s true that c l a t h r a t e compounds are formed, then the molecule may undergo f r e e or hindered r o t a t i o n . This allows an a l t e r n a t e explanation f o r the c l o s e l y spaced weak l i n e s to the blue of the o r i g i n . However there are c l o s e l y spaced l i n e s of about the same i n t e n s i t y to the red of the o r i g i n , and to e x p l a i n the presence of these l i n e s i t i s necessary to assume that the molecules r o t a t e i n the ground s t a t e . This i s not p o s s i b l e at the low temperature used and so t h i s explanation i s not p r e f e r r e d . The stronger l i n e s about 200 cm"1 to the blue of the o r i g i n are hard to account f o r . However, i f the symmetry of the c r y s t a l f i e l d at the s i t e occupied by the anthracene molecule i s lower than the molecular point group then i n t r a  molecular v i b r a t i o n s other than ftj or 1>SJ modes may appear. The i n t e n s i t i e s of these extra l i n e s would depend on the strength of the coupling between the molecule w i t h i t s environment. This coupling i s weak i n such a molecular 46 c r y s t a l and so pe r t u r b a t i o n theory may be a p p l i e d . The i n t e r a c t i o n s between solvent and s o l u t e molecules are of two kinds. There i s a mixing of e l e c t r o n i c wave functions of solvent and sol u t e molecules g i v i n g r i s e to a solvent s h i f t (20); we are not concerned w i t h t h i s e f f e c t here. There i s a l s o an i n t e r a c t i o n between the v i b r a t i o n a l states of the anthracene molecule w i t h the v i b r a t i o n a l states of the environment which occurs i n the f o l l o w i n g way. The anthracene molecule i s s l i g h t l y bigger i n the ex c i t e d s t a t e ; t h i s behaviour has already been observed i n benzene (51) and naphthalene (46). The evidence i n support of t h i s con c l u s i o n i s that the o r i g i n i s not the strongest l i n e i n the spectrum, r a t h e r the l i n e at 1399 cm"1 i s . That i s , the Franck-Condon overlap f a c t o r i s greatest to the excite d e l e c t r o n i c s t a t e w i t h one quantum of the 1399 cm - 1 funda mental and so there i s an expansion i n the corresponding normal coordinate. However, the expansion of the anthra cene molecule i s f e l t by the surrounding molecules, l a t t i c e mations r a t h e r than in t r a m o l e c u l a r v i b r a t i o n s of the solute molecule tend to be e x c i t e d , because the r e s t o r i n g forces are much weaker between the molecules of the l a t t i c e than between the s t r o n g l y bonded atoms of the molecule. Thus there i s an i n t e r a c t i o n between the i n t e r n a l v i b r a t i o n s of anthracene and l a t t i c e v i b r a t i o n s . A p p l i c a t i o n of f i r s t order p e r t u r b a t i o n theory leads d i r e c t l y to the r e s u l t that the mixing of the states i s greatest when the energy separa-4? t i o n between them i s s m a l l e s t . But the frequencies of l a t t i c e modes are u s u a l l y l e s s than 100 cm - 1. Hence the low energy v i b r a t i o n a l s t a t e s of anthracene w i l l be most e f f e c t e d . In t h i s way low energy anthracene fundamentals of symmetry other than or may appear. Naphthalene has low energy fundamentals of symmetry b i j , baj and bsu (52). The l i n e at 420 cm" 1 i s i n t e r p r e t e d as the overtone of the 211 cm""1 fundamental gaining i n t e n s i t y from the strong (Xgfundamental at 389 cm - 1 by Fermi .resonance. Type 1 (b) i n flu o r e n e . Most of the l i n e s i n t h i s spectrum can be r e a d i l y i n t e r p r e t e d i n terms of the anthra cene fundamentals and t h e i r combinations. In fluorene the l i n e s are broader than i n n-heptane even at 4.2°K. This i s e s p e c i a l l y true i n higher energy regions where c l o s e l y spaced combinations have not been resolved. No evidence f o r the presence of i m p u r i t i e s ( l i k e carbazole which i s known to form a s o l i d solution;: i n fluorene (40)) has been found. The absorption spectrum of fluorene i t s e l f was observed above 31000 cm - 1. While the d e t a i l of t h i s spectrum i s not understood, i t i s apparent that the f i r s t group of l i n e s i s p o l a r i z e d along C a x i s . Thus, the low energy t r a n s i t i o n i n fluorene i s p o l a r i z e d along the long a x i s of the molecule. Type 2. The low frequency i n t e r v a l s (about 25 cm - 1), b u i l t on a l l strong l i n e s , are common to the spectra i n a l l matrices. These are due to l a t t i c e modes which couple to the i n t e r n a l modes of anthracene as explained e a r l i e r . I t 48 i s not unexpected to f i n d the l a t t i c e modes of d i f f e r e n t molecular c r y s t a l s being about the same energy, since the frequency (V) of a v i b r a t i o n i s r e l a t e d to the forc e con stant (k) and the reduced mass of the system (/*) by V = w ( < / / 0 * The fhrce constant (k) i s d i r e c t l y r e l a t e d to bonding be tween the molecules which i n t u r n i s given by the heat of sublimation. The heats of sublimation f o r molecular c r y s t a l s are of the same order of magnitude, e.g. f o r fluorene (53) i t i s 19.8 kcal/mole; biphenyl (54), 17.9 keal/mole; naphthalene (53), 17.3 kcal/mole; n-octadecane (55), 36.8 kcal/mole. No data i s a v a i l a b l e f o r the heat of sublimation of n-heptane but we w i l l assume that i t i s about the same as f o r n-octadecane. The heat of sublimation i s temperature dependent and the values given above were measured around room temperature. I f i t i s assumed that the heats of sublimation at 4.20K f o r a l l the matrices are n e a r l y equal (and l a r g e r than the room temperature v a l u e ) , then we can •9, conclude that the f a r c e constants are al s o w i t h i n the same order of magnitude. Because the molecules considered have about the same molecular weight (fluorene, 166; biphenyl, 154; naphthalene, 128; anthracene, 178; n-heptane, 100), the reduced masses of the systems are nea r l y enough the same. Therefore the frequencies of the l a t t i c e modes should be very s i m i l a r (fluorene, 29 cm - 1; n-heptane, 25 cm - 1; naphthalene ( 3 0 ) , 26 cm - 1). Thus these low frequency 49 i n t e r v a l s are assigned as l a t t i c e modes. In one sample of anthracene i n fluorene a weak l i n e at 73 cm"1 t o the red of the o r i g i n was observed. This i s taken to represent the presence of a ground state.phonon. The i n t e n s i t y of t h i s l i n e was about one ten t h that of the o r i g i n and so the temperature of t h i s sample was about 50°K. The l i n e was measured i n an e a r l i e r spectrum i n which the c r y s t a l was cemented to the helium can with s i l i c o n e grease. This i s f u r t h e r evidence that s i l i c o n e grease provides poor thermal contact at low temperature. S h i f t of the Ori g i n s of ' f lf The energy of the *- 7*3 t r a n s i t i o n showed a red s h i f t as the matrix was changed from n-hexane, n-heptane, biphenyl to fluorene. And a s h i f t was a l s o observed i n n- heptane as the temperature was lowered as seen i n Table 5. 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