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On the ortho - para enhancement of molecular iodine by selective laser excitation Booth, James L. 1985

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ON THE ORTHO - PARA ENHANCEMENT OF MOLECULAR IODINE BY SELECTIVE LASER EXCITATION by JAMES L. BOOTH B . S c , M c G i l l U n i v e r s i t y , 1983 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES Deptartment of P h y s i c s We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA October 15, 1985 © James L. Booth, 1985 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 r e q u i r e m e n t s f o r an advanced degree a t the The U n i v e r s i t y of B r i t i s h C o l u m b i a , I agree t h a t 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 r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g 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 g r a n t e d by t h e Head of my Department or by h i s o r her r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r 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 g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department of P h y s i c s The U n i v e r s i t y of B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date: O c t o b e r , 1985 ABSTRACT Based upon the r e s u l t s r e p o r t e d by B a l y k i n , V . S. r L e t o k h o v , V . S., Mish e n , V . I . , and Semchishen, V . A., Chem. Phys., 17, 111 (1 9 7 6 ) , an attempt was made t o s h i f t t he o r t h o - i o d i n e t o p a r a - i o d i n e r a t i o by (1) s e l e c t i v e l y p r e d i s s o c i a t i n g the o r t h o - i o d i n e m o l e c u l e s w i t h the 5145 A argon i o n l a s e r l i n e and (2) by r e a c t i n g the s e l e c t i v e l y e x c i t e d o r t h o m o l e c u l e s w i t h the s c a v e n g e r s , 2-hexene, a c e t y l e n e , n i t r i c o x i d e , n i t r o s y l c h l o r i d e , and e t h y l i o d i d e . In both c a s e s the o r t h o t o para r a t i o was m o n i t o r e d v i a t he f l u o r e s c e n c e induced by a s c a n n i n g dye l a s e r beam. N e i t h e r t a c t i c prooved e f f e c t i v e , i n c o n t r a d i c t i o n w i t h the a f o r e m e n t i o n n e d a u t h o r s . In the case of systems c o n t a i n i n g low p r e s s u r e s ' of i o d i n e ' vapour a l o n e , time dependent d i f f e r e n t i a l q uenching of the v i b r a t i o n a l B 3 I1q + u x 1 ^ g t r a n s i t i o n s was o b s e r v e d a t t r i b u t e d t o o u t g a s i n g of the t e s t c e l l s over the c o u r s e of the experiment and may be the cause of the apparant s h i f t c l a i m e d by Letokhov e t a l . In m i x t u r e s of i o d i n e and sc a v e n g e r , the n u l l r e s u l t was p r o b a b l y due t o the f o r m a t i o n of r a d i c a l c h a i n s d u r i n g t h e p h o t o - i n d u c e d r e a c t i o n s c a p a b l e of r a p i d l y r e l a x i n g any s h i f t t h a t had o c c u r r e d . i i T a b l e of C o n t e n t s ABSTRACT . . . i i L i s t of T a b l e s v L i s t of F i g u r e s v i i Acknowledgements x I . The B a s i c Idea 1 I I . G e n e r a l T h e o r e t i c a l Remarks 5 A. The N u c l e a r W a v e f u n c t i o n 7 B. The E l e c t r o n i c C o n t r i b u t i o n 10 C. Symmetry P r o p e r t i e s of E l e c t r o n i c W a v e f u n c t i o n s 13 1. u and g Symmetry 14 2. R e f l e c t i o n Through the I n t e r n u c l e a r A x i s ....14 3. P a r i t y of the W a v e f u n c t i o n 15 4. I n t e r c h a n g e of the N u c l e i 16 5. F i n e S t r u c t u r e 17 D. H y p e r f i n e S t r u c t u r e 21 1. Ortho and Par a C h a r a c t e r 21 E. The E f f e c t s of H y p e r f i n e I n t e r a c t i o n s 23 1. R a d i a t i v e Decay 29 2. C o l l i s i o n a l l y Induced O r t h o - P a r a T r a n s i t i o n s 31 I I I . P r e v i o u s R e s u l t s 36 A. Or t h o - and P a r a - Hydrogen 36 1. Hyrdrogen Atom Exchange 39 2. Paramagnetic C o n v e r s i o n 41 B. Or t h o and Para Enhancement i n I o d i n e 43 1. The Ex p e r i m e n t s of V. S. Letokhov e t a l 46 IV. E x p e r i m e n t a l R e s u l t s 61 i i i A. I o d i n e Vapour E x p e r i m e n t s 62 1. S i n g l e L a s e r Beam I r r a d i a t i o n Technique .....62 2. Dual Beam I r r a d i a t i o n Technique 72 B. I o d i n e and Scavengers 107 1. I o d i n e and 2-Hexene 108 2. I o d i n e and A c e t y l e n e 124 3. I o d i n e and N i t r i c Oxide 134 4. I o d i n e and N i t r o s y l C h l o r i d e 136 5. I o d i n e and E t h y l I o d i d e 139 V. C o n c l u s i o n s and D i s c u s s i o n 144 A. Comments on T h i s R e s e a r c h 144 B. S u g g e s t i o n s f o r F u t u r e Work 149 BIBLIOGRAPHY 1 52 APPENDIX A' : ORDER OF MAGNITUDE ESTIMATE OF THE HYPERFINE ORTHO-PARA COUPLING 1 54 i v L i s t of T a b l e s 2.1 E s t i m a t e of the Energy of the O f f - D i a g o n a l Terms of the H a m i l t o n i a n of M o l e c u l a r I o d i n e ". 18 2.2 V a l u e of the P r e d i s s o c i a t i o n C o n s t a n t s f o r t h e V i b r a t i o n a l L e v e l s 9<v'<22 f o r the B 3 n 0 + U s t a t e ....20 3.1 Parahydrogen t o Orthohydrogen R a t i o vs Temperature ...38 3.2 P a r a - I o d i n e t o O r t h o - I o d i n e R a t i o vs Temperature 44 4.1 E x p e r i m e n t a l C o n d i t i o n s f o r a 3.75 mto r r I o d i n e C e l l U s i n g the S i n g l e Beam Technique 65 4.2 14* — 1" P(77),R(82) and Argon Ion Induced F l u o r e s c e n c e Decay i n a 3.75mtorr I o d i n e C e l l vs Time 67 4.3 Ortho and Para I o d i n e Peak H e i g h t s and r a t i o vs Time..67 4.4 E x p e r i m e n t a l C o n d i t i o n s f o r a 3.00 mto r r I o d i n e C e l l U s i n g the Dual Beam Technique 75 4.5 l 5 ' - 0 " P(25):R(30) and l 5 ' - 0 " P ( 2 5 ) : 1 8 ' - 1 " P(84) Peak H e i g h t R a t i o s vs Time Observed i n a 3.00 mto r r C e l l 79 4.6 Argon Ion L a s e r Induced F l u o r e s c e n c e v s Time i n a 3.00 mtor r Test C e l l 81 4.7 E x p e r i m e n t a l C o n d i t i o n s f o r a Test C e l l E v a c u a t e d t o 5 x l 0 " 6 t o r r P r i o r t o Use (Run #1) 87 4.8 l 5 ' - 0 " P ( 2 5 ) : R ( 3 0 ) , l 5 ' - 0 " P(25):1B'-1" P ( 8 4 ) , and l 5 ' - 0 " P ( 2 5 ) : 1 9 ' - 1 " P(121) Peak H e i g h t R a t i o s vs Time f o r Run #1 89 4.9 Argon Ion L a s e r Induced F l u o r e s c e n c e v s Time d u r i n g Run #1 92 4.10 E x p e r i m e n t a l C o n d i t i o n s f o r a Test C e l l E v a c u a t e d t o l . 6 x l 0 " 5 t o r r P r i o r t o Use (Run #2) 95 4.11 15'-0" P(25):R(30) and l 5 ' - 0 " P ( 2 5 ) : 1 8 ' - 1 " P(84) Peak H e i g h t R a t i o s vs Time f o r Run #2 97 4.12 Argon Ion L a s e r Induced F l u o r e s c e n c e vs Time d u r i n g Run #2 99 4.13 E x p e r i m e n t a l C o n d i t i o n s f o r a Test C e l l E v a c u a t e d t o 9 . 5 x l 0 ' s t o r r P r i o r t o Use (Run #3) 101 4.14 15'-0" P(25):R(30) and l 5 ' - 0 " P ( 2 5 ) : 1 8 ' - 1 " P(84) Peak H e i g h t R a t i o s vs Time f o r Run #3 103 v 4.15 Argon Ion L a s e r Induced F l u o r e s c e n c e vs Time d u r i n g Run #3 105 4.16 E x p e r i m e n t a l C o n d i t i o n s f o r a Test C e l l C o n t a i n i n g 2.24 m torr of I o d i n e and 1 t o r r of 2-Hexene 112 4.17 19'-1" P(95) Induced F l u o e s c e n c e i n a C e l l C o n t a i n i n g 2.24 mtorr of I o d i n e and 1 t o r r of 2-Hexene 115 4.18 18 * — 1" P(37):R(42) Peak H e i g h t R a t i o vs Time f o r a C e l l C o n t a i n i n g 2.24 mtorr of I o d i n e and 1 t o r r of 2-Hexene ..116 4.19 E x p e r i m e n t a l C o n d i t i o n s f o r a T e s t C e l l C o n t a i n i n g 30 m torr of I o d i n e and 2.3 t o r r of 2-Hexene 118 4.20 15'~0" P(25):R(30) Peak H e i g h t R a t i o vs Time i n a C e l l C o n t a i n i n g 30 mtorr of I o d i n e and 2.3 t o r r of 2-Hexene ..120 4.21 Argon Ion L a s e r Induced F l u o r e s c e n c e vs Time i n a C e l l C o n t a i n i n g 30 mtorr of I o d i n e and 2.3 t o r r of 2-Hexene ..122 4.22 E x p e r i m e n t a l C o n d i t i o n s used by V. Kushawaha f o r an I s o t o p i c S e p a r a t i o n of I o d i n e 124 4.23 E x p e r i m e n t a l C o n d i t i o n s f o r a Test C e l l C o n t a i n i n g C o n t a i n i n g 27 mtorr of I o d i n e and 30 t o r r of A c e t y l e n e ..130 4.24 15'~0" P(25):R(30) and 15'-0 B P ( 2 5 ) : 1 8 ' - 1 " P(84) Peak R a t i o s vs Time i n a C e l l C o n t a i n i n g 27 m t o r r of I o d i n e and 30 t o r r of A c e t y l e n e 132 4.25 E x p e r i m e n t a l C o n d i t i o n s f o r a T e s t C e l l C o n t a i n i n g 17 m torr of I o d i n e and 7 t o r r of A c e t y l e n e 133 4.26 l 5 ' - 0 " P(25):R(30) and l 5 ' - 0 " P ( 2 5 ) : 1 8 ' - 1 " P(84) Peak 30 t o r r . of A c e t y l e n e 133 4.27 E x p e r i m e n t a l C o n d i t i o n s f o r a T e s t C e l l C o n t a i n i n g 150 m t o r r of I o d i n e and 3 t o r r of N i t r o s y l C h l o r i d e 137 4.28 E x p e r i m e n t a l C o n d i t i o n s f o r a T e s t C e l l C o n t a i n i n g 30 m t o r r of I o d i n e and 13 t o r r of E t h y l I o d i d e 141 4.29 IS'-O" P(25):R(30) and 15'~0" P ( 2 5 ) : 1 8 ' - 1 " P(84) Peak R a t i o s vs Time i n a C e l l C o n t a i n i n g 30 m t o r r of I o d i n e and 17 t o r r of E t h y l I o d i d e 143 v i L i s t of F i g u r e s 3.1 A P l o t of the F l u o r e s c e n c e of the O r t h o - I o d i n e and P a r a - I o d i n e Induce by the 5145 A and 5017 A Argon Ion L a s e r L i n e s R e s p e c t i v e l y (from r e f . 2 ) 49 3.2 F l u o r e s c e n c e Induced by the 5145 A and 5017 A Argon Ion L a s e r L i n e s i n a C e l l C o n t a i n i n g 5 mtorr of I o d i n e Vapour as R e p o r t e d by Letokhov e t a l 3 51 3.3 F l u o r e s c e n c e Decay i n I o d i n e C e l l s as a F u n c t i o n of I n i t i a l I o d i n e P r e s s u r e , (from r e f . 3) ..51 3.4 Enrichment C o e f f i c i e n t f o r O r t h o - t o P a r a - I o d i n e as a F u n c t i o n of I n i t i a l I o d i n e P r e s s u r e . ( f r o m r e f . 3) 51 3.5 Enrichment C o e f f i c i e n t Dependence Upon I n i t i a l I o d i n e P r e s s u r e i n Systems of I o d i n e and 2-Hexene Reported by Letokhov e t a l 3 57 3.6 The Dependence of the I n i t i a l Rate of Change of O r t h o - I o d i n e C o n c e n t r a t i o n , L a s e r I n t e n s i t y , and the Root of the 2-Hexene C o n c e n t r a t i o n 57 4.1 E x p e r i m e n t a l Arrangement f o r the Single.Beam I r r a d i a t i o n Technique 64 4.2 T y p i c a l 14 * - 1 " P(77) and R(82) s p e c t r a l l i n e s Observed D u r i n g the I r r a d i a t i o n of a 3.75 mtorr I o d i n e C e l l 66 4.3 Argon Ion L a s e r Induced F l u o r e s c e n c e Decay and 15'-0 B P(25):R(30) Peak H e i g h t R a t i o Observed i n a 3.75 mtorr I o d i n e C e l l 68 4.4 F l u o r e s c e n c e Spectrum of I o d i n e E x c i t e d by the 5145 A Argon Ion L a s e r L i n e 70 4.5 E x p e r i m e n t a l Arrangement f o r the D u a l Beam I r r a d i a t i o n Technique 74 4 . 6 The 17408 cm" 1 S p e c t r a l Region of M o l e c u l a r I o d i n e 76 4.7 T y p i c a l S p e c t r a Near the 17408 cm* 1 Region Observed i n a 3.00 m t o r r I o d i n e C e l l D u r i n g I r r a d i a t i o n 78 4.8 P l o t of the l 5 ' - 0 " P ( 2 5 ) : R ( 3 0 ) , 15'-0 B P ( 2 5 ) : 1 8 ' - 1 " P(84) Peak H e i g h t s vs Time i n a 3.00 mtorr I o d i n e C e l l ...80 4.9 Argon Ion L a s e r Induced F l u o r e s c e n c e Decay Observed i n a 3 m t o r r I o d i n e C e l l 82 4.10 The 17408 cm" 1 S p e c t r a l Region of M o l e c u l a r I o d i n e as Observed i n a H i g h P r e s s u r e and Low P r e s s u r e I o d i n e v i i C e l l 84 4.11 T y p i c a l S p e c t r a l L i n e s Observed i n a C e l l E vacuated t o 5 x l 0 " 6 t o r r P r i o r t o Use (Run #1) 88 4.12 P l o t of the 15'-CT P ( 2 5 ) : R ( 3 0 ) , 15'-0 B P ( 2 5 ) : 1 8 * - 1 " P ( 8 4 ) , and, 15*-0" P ( 2 5 ) : 1 9 ' - 1 " P(121) Peak H e i g h t s vs Time f o r Run #1 90 4.13 Argon Ion L a s e r Induced F l u o r e s c e n c e Decay Observed D u r i n g Run #1 9 1 4.14 T y p i c a l S p e c t r a l L i n e s Observed i n a C e l l E vacuated t o 1 . 6 x l 0 ' 5 t o r r P r i o r t o Use (Run #2) 96 4.15 P l o t of the 15'-0" P ( 2 5 ) : R ( 3 0 ) , l 5 ' - 0 " P ( 2 5 ) : 1 8 ' - 1 " P ( 8 4 ) , and, 15'~0" P ( 2 5 ) : 1 9 ' - 1 " P(121) Peak H e i g h t s vs Time f o r Run #2 98 4.16 Argon Ion L a s e r Induced F l u o r e s c e n c e Decay Observed D u r i n g Run #2 1 00 4.17 T y p i c a l S p e c t r a l L i n e s Observed i n a C e l l E v a c u a t e d t o 9 . 5 x l 0 ' 5 t o r r P r i o r t o Use (Run #3) 102 4.18 P l o t of the l 5 ' - 0 " P ( 2 5 ) : R ( 3 0 ) r l 5 ' - 0 " P ( 2 5 ) : 1 8 ' - 1 " P ( 8 4 ) , and, 15'-0" P ( 2 5 ) : 1 9 ' - 1 " P(121) Peak H e i g h t s vs Time D u r i n g Run #3 : 1 04 m 4.19 Argon Ion L a s e r Indycsed F l u o r e s c e n c e Decay Observed D u r i n g Run #3 1 06 4.20 The 17476 cm" 1 . S p e c t r a l Region of M o l e c u l a r I o d i n e 109 4.21 Arrangement f o r E x p e r i m e n t s I n v o l v i n g M i x t u r e s of I o d i n e and 2-Hexene * 111 4.22 T y p i c a l S p e c t r a l L i n e s Observed i n a C e l l C o n t a i n i n g 2.24 m t o r r of I o d i n e and 1 t o r r or 2-Hexene 113 4.23 P l o t of the 19'-1" P(95) Induced F l u o r e s c e n c e Decay and t h e 18'-1" P(37):R(44) Peak H e i g h t R a t i o Observed i n a C e l l C o n t a i n i n g 2.24 m t o r r of • I o d i n e and 1 t o r r of 2-Hexene 114 4.24 T y p i c a l I o d i n e S p e c t a Observed i n a C e l l C o n t a i n i n g 30 m t o r r of I o d i n e and 2.3 t o r r of 2-Hexene 119 4.25 P l o t of the Argon Ion L a s e r Induced F l u o r e s c e n c e Decay and t h e l 5 ' - 0 " P(25):R(30) Peak H e i g h t R a t i o vs Time i n a C e l l C o n t a i n i n g 30 m t o r r of I o d i n e and 2.3 t o r r of 2-Hexene „ 121 v i i i 4.26 Mass S p e c t r o g r a p h s P u b l i s h e d by V. K u shawaha 3 0 on the I s o t o p i c S e p a r a t i o n of I o d i n e 126 4.27 Arrangement f o r E x p e r i m e n t s I n v o l v i n g M i x t u r e s of I o d i n e and A c e t y l e n e 129 4.28 P l o t of the 15*-0" P ( 2 5 ) : R ( 3 0 ) , 15'-0" P(25):18'-1" P(84) Peak H e i g h t s vs Time Observed i n a C e l l C o n t a i n i n g 27 mtorr of I o d i n e and 30 t o r r of 2-Hexene 131 4.29 T y p i c a l 14'-1 n P(77) and R(82) S p e c t r a l L i n e s Observed i n a C e l l C o n t a i n i n g 3 t o r r of N i t r i c Oxide and 3 mtorr of I o d i n e 135 4.30 F l u o r e s c e n c e Near t h e 17476 cm" 1 Region i n a C e l l C o n t a i n i n g 150 mtorr of I o d i n e and 3 t o r r of N i t r o s y l C h l o r i d e 138 4.31 P l o t of the 15'-0" P ( 2 5 ) : R ( 3 0 ) , 15'-0" P ( 2 5 ) : 1 8 ' - 1 " P ( 8 4 ) , and, l 5 ' - 0 " P ( 2 5 ) : 1 9 ' - 1 " P(121) Peak H e i g h t s vs Time Observed i n a C e l l C o n t a i n i n g 30 mtorr of I o d i n e and 13 t o r r of E t h y l I o d i d e 142 A .1 The C o o r d i n a t e System Used t o D e s c r i b e t h e P o s i t i o n s of the E l e c t r o n s and N u c l e i i n Appendix A 155 i x Acknowledgements I would l i k e t o thank my s u p e r v i s o r , Dr. F. W. Da l b y , f o r h i s h e l p and i n f u s i o n of common sense when i t was most needed. I would a l s o l i k e t o thank him f o r the u s e f u l d i s c u s s i o n s and s u g g e s t i o n s he p r o v i d e d throughout the co u r s e of t h i s r e s e a r c h . In a d d i t i o n I wi s h t o ext e n d my g r a t i t u d e t o Dr. S. Parmar who c o n t r i b u t e d g r e a t l y t o the p u r i f i c a t i o n of t h e c h e m i c a l s p e c i e s and the p r e p a r a t i o n of the t e s t c e l l s used i n t h i s r e s e a r c h , and who, w i t h K. Mah, o f f e r e d many h e l p f u l comments and made the environment more p l e a s a n t . F u r t h e r , I would l i k e t o e x p r e s s my a p p r e c i a t i o n to Dr. J . V a n d e r l i n d e f o r h i s e n t h u s i a s m and guidance d u r i n g h i s b r i e f s t a y s a t the U n i v e r s i t y of B r i t i s h Columbia. F i n a l l y , I would l i k e t o thank Dr. N. Basco f o r the l o a n of some of h i s equipment and Mr. E. M c W i l l i a m s f o r the g l a s s b l o w i n g work he p r o v i d e d . T h i s t h e s i s i s d e d i c a t e d t o J . C. x I . THE BASIC IDEA The i d e a f o r t h i s p r o j e c t o r i g i n a t e d w i t h my s u p e r v i s o r , Dr. F. W. Dalby , and was proposed t o me when I f i r s t a r r i v e d a t the U n i v e r s i t y of B r i t i s h Columbia. As i s the case w i t h most i n t e r e s t i n g i d e a s , t h i s one had a somewhat c o l o r f u l h i s t o r y . In 1972 Dr. Dalby had taken a s a b b a t i c a l a t the E c o l e Normale i n P a r i s . D u r i n g t h i s t i m e , among o t h e r t h i n g s , he attempted t o s e l e c t i v e l y d e p l e t e the o r t h o - i o d i n e p o p u l a t i o n i n a sample of o r d i n a r y m o l e c u l a r i o d i n e and t h e r e b y c r e a t e a s h i f t i n the u s u a l o r t h o t o para r a t i o . A f t e r some p r e l i m i n a r y e x p e r i m e n t s which d i s p l a y e d no e v i d e n c e f o r such a s h i f t the attempt was abandonned . Some y e a r s l a t e r , back a t the U n i v e r s i t y of B r i t i s h Columbia , he uncovered a paper w r i t t e n by V. S. Letokhov e t a l 1 c o n c e r n i n g t h i s v e r y s u b j e c t p u b l i s h e d i n 1973. T h i s paper c l a i m e d t h a t , indeed, such a s h i f t had been obser v e d i n m o l e c u l a r i o d i n e . T h i s r e v i t a l i z e d Dr. D a l b y ' s i n t e r e s t e s p e c i a l l y as the t e c h n i q u e .employed c o u l d e a s i l y be d u p l i c a t e d and even improved upon u s i n g the equipment a v a i l a b l e i n h i s own r e s e a r c h l a b o r a t o r y . In o r d e r t o d e s c r i b e the method proposed t o produce the d e s i r e d e f f e c t , i t i s f i r s t n e c e s s a r y t o d e s c r i b e a few of the p r o p e r t i e s of m o l e c u l a r i o d i n e . I o d i n e i s a d i a t o m i c m o l e c u l e composed of two i d e n t i c a l atoms each of h a v i n g 5 n u c l e a r s p i n ^- Hence the net n u c l e a r s p i n of any g i v e n m o l e c u l e , I , may range from z e r o t o f i v e i n i n t e g r a l s t e p s . F u r t h e r , m o l e c u l a r i o d i n e has been ob s e r v e d t o obey Fermi 1 2 D i r a c s t a t i s t i c s and, t o a good a p p r o x i m a t i o n , a consequence of t h i s i s t h a t , i n the ground e l e c t r o n i c s t a t e , the even r o t a t i o n a l l e v e l s a r e c o u p l e d t o the even n u c l e a r s p i n s t a t e s and the odd r o t a t i o n a l l e v e l s a r e c o u p l e d t o the odd n u c l e a r s p i n s t a t e s . To t h e same a p p r o x i m a t i o n , the energy of a g i v e n r o t a t i o n a l - v i b r a t i o n a l s t a t e i s independent of the n u c l e a r s p i n , I , t h e r e b y l e a d i n g t o a degeneracy of each r o t a t i o n a l s t a t e g i v e n by ^1 I« 0,1,4 ( l . i b ) 9 i I* ».o.5 Those s t a t e s w i t h h i g h e r n u c l e a r degeneracy a r e c a l l e d o r t h o s t a t e s ( i . e . the odd r o t a t i o n a l s t a t e s ) and those w i t h lower s t a t i s t i c a l n u c l e a r weight (t h e even r o t a t i o n a l s t a t e s ) a r e c a l l e d p a r a s t a t e s . At a f i x e d t e m p e r a t u r e , T, the e q u i l i b r i u m r a t i o of the o r t h o - i o d i n e t o p a r a - i o d i n e i s N° which a t room temperature r e d u c e s t o N* ~ 1-4 j,.dd (i. z) £ is (2j+i) e ^ 3 t h e n a t u r a l l y o c c u r r i n g r a t i o . To change t h i s r a t i o one may t a k e advantage of the f a c t t h a t the m o l e c u l a r i o d i n e has a w e l l - s t u d i e d dense spectrum i n the v i s i b l e r e g i o n w i t h many s p e c t r a l l i n e s l e a d i n g t o a phenomenon known as p r e d i s s o c i a t i o n . P r e d i s s o c i a t i o n o c c u r s when two e l e c t r o n i c s t a t e s , one s t a b l e and t h e o t h e r d i s s o c i a t i v e , o v e r l a p . Upon making a t r a n s i t i o n from one s t a b l e s t a t e t o a r e g i o n near t h i s o v e r l a p t h e r e i s a f i n i t e p r o b a b i l i t y t h a t the m o l e c u l e w i l l f o l l o w the u n s t a b l e e l e c t r o n i c c u r v e and break up r a t h e r than g o i n g i n t o the s t a b l e s t a t e . T h i s d i s s o c i a t i o n of the m o l e c u l e i s c a l l e d p r e d i s s o c i a t i o n . Hence, by s e l e c t i n g e i t h e r an even or odd R or P t r a n s i t i o n i n m o l e c u l a r i o d i n e l e a d i n g t o p r e d i s s o c i a t i o n one may, i n t h e o r y , s e l e c t i v e l y d e s t r o y e i t h e r the o r t h o or para m o l e c u l e s . Of c o u r s e t h i s i s not the whole s t o r y as t h e r e a r e bound t o be problems w i t h t h i s method, namely, c o l l i s i o n s w i t h paramagnetic c o n t a m i n a n t s and w i t h i o d i n e atoms w i l l p r o b a b l y be a b l e t o cause t r a n s i t i o n s from o r t h o t o p a r a s t a t e s ( and v i c e - v e r s a ) i n the r e m a i n i n g m o l e c u l e s . Whether a net s h i f t of the o r t h o - p a r a r a t i o i s f e a s i b l e w i l l depend s t r o n g l y upon the a b i l i t y of the i o d i n e atoms t o scramble the r e m a i n i n g m o l e c u l e s . I f one i s not a b l e t o produce any s h i f t i n m o l e c u l a r i o d i n e a l o n e , or i f one wishes t o i n c r e a s e the s h i f t a c h i e v e d , then one needs o n l y add a s p e c i e s t o the system t h a t w i l l r e a c t p r e f e r e n t i a l l y w i t h e i t h e r the i o d i n e atoms or t h e e x c i t e d m o l e c u l e s . 4 V.S.Letokhov e t al1»2»3 have c l a i m e d t o have produced a s u b s t a n t i a l change i n the o r t h o t o p a r a r a t i o i n systems c o n t a i n i n g m o l e c u l a r i o d i n e a l o n e and i n m i x t u r e s of i o d i n e and 2-hexene by employing p r e c i s e l y the a f o r e m e n t i o n e d t e c h n i q u e . W i t h t h e s e p r o m i s i n g c l a i m s , my t h e s i s p r o j e c t was launc h e d w i t h the aim of r e p r o d u c i n g , and perhaps even i m p r o v i n g upon the r e s u l t s of Letokhov et a l and then s t u d y i n g the r e l a x a t i o n p r o c e s s of any s h i f t produced c a t a l y s e d by v a r i o u s paramagnetic s p e c i e s . The f i n d i n g s of th e s e i n v e s t i g a t i o n s a r e r e p o r t e d h e r e , d i v i d e d i n t o f o u r main c h a p t e r s . Chapter two c o n t a i n s some g e n e r a l quantum m e c h a n i c a l d e t a i l s used t o d e s c r i b e a homonuclear d i a t o m i c m o l e c u l a r system and then a d d r e s s e s the o r t h o and p a r a symmetry p r o p e r t i e s of such a system. S p e c i f i c a l l y , t h e m e a n i n g f u l n e s s of t h e s e l a b e l s i s a d d r e s s e d and some o r d e r of magnitude e s t i m a t e s of an o r t h o t o para c o n v e r s i o n ( or v i c e - v e r s a ) mediated v i a c o l l i s i o n s w i t h p a r a m a g n e t i c p a r t i c l e s or r a d i a t i o n a r e d i s c u s s e d . The t h i r d c h a p t e r c o n t a i n s a b r i e f h i s t o r y of some of the r e s e a r c h t h a t has been undertaken i n t o the p r o p e r t i e s of o r t h o - and para-hydrogen and then p r e s e n t s and d i s c u s s e s the r e s u l t s of Letokhov e t a l . Ch a p t e r s f o u r and f i v e d e s c r i b e t h e i n v e s t i g a t i o n s and r e s u l t s of t h i s work i n d e t a i l and then p r e s e n t some g e n e r a l c o n c l u s i o n s drawn from the s t u d y , e n d i n g w i t h a few s u g g e s t i o n s f o r f u t u r e e x p e r i m e n t s . I I . GENERAL THEORETICAL REMARKS A c c o r d i n g t o quantum mechanics, each of the p o s s i b l e s t a t e s of a p h y s i c a l system a re c h a r a c t e r i z e d by a w a v e f u n c t i o n , |*>. I t i s p o s t u l a t e d t h a t the modulus squared of t h i s w a v e f u n c t i o n i s the p r o b a b i l i t y of f i n d i n g the system i n such a s t a t e |*>. I T I " * « ( 2 1 ) F u r t h e r , a l l o b s e r v a b l e s a r e r e p r e s e n t e d by l i n e a r Hermetian o p e r a t o r s whose ( r e a l ) e i g e n v a l u e s a r e the p o s s i b l e v a l u e s of t h e i r c o r r e s p o n d i n g o b s e r v a b l e . I n a n a l o g y w i t h c l a s s i c a l mechanics, one i s a b l e t o w r i t e down the H a m i l t o n i a n of the system and, i n the absense of e x t e r n a l f o r c e s , the H a m i l t o n i a n r e p r e s e n t s the t o t a l energy of the system. H(?i,%*) = The S c h r o d i n g e r e q u a t i o n i s d y n a m i c a l e v o l u t i o n of the system, ihdlVy = H I 4 R > a t then used t o s o l v e the 5 6 where the c l a s s i c a l q u a n t i t i e s such as p o s i t i o n , q, a n g u l a r momentum, T, e t c . a r e r e p l a c e d by t h e i r c o r r e s p o n d i n g quantum m e c h a n i c a l o p e r a t o r s . I n the case of a d i a t o m i c m o l e c u l e w i t h Z,+Z2 e l e c t r o n s the H a m i l t o n i a n may be e x p r e s s e d , t o f i r s t o r d e r , as H = TLEl • -" iZ ie* - z W • Hi* • £ £ (2-4) where P. i s the l i n e a r momentum of n u c l e u s i , p. i s the l i n e a r momentum of n u c l e u s j , M. i s the mass of n u c l e u s i , mj i s t h e mass of an e l e c t r o n , r ^ i s the d i s t a n c e between n u c l e u s I and e l e c t r o n j , R 1 2 i s the. i n t e r n u c l e a r d i s t a n c e , and r ^ j i s the d i s t a n c e between e l e c t r o n s i and j . [Note t h a t the t o t h i s a p p r o x i m a t i o n , a l l the f i n e s t r u c t u r e and h y p e r f i n e s t r u c t u r e terms have been o m i t t e d ] A l t e r n a t e l y , t h i s may be g e n e r a l i z e d a s , W « 2 PL + + V e + Vn * V„«. (2 .5) 2Mr 2mj An e x a c t s o l u t i o n t o e q u a t i o n (2.5) i s not p o s s i b l e . However, owing t o t h e f a c t t h a t t h e e l e c t r o n s move much f a s t e r than the n u c l e i , one may app r o x i m a t e the w a v e f u n c t i o n of the system as l*> = I V ' V C2.6) 7 T h i s , t h e Born - Oppenheimer a p p r o x i m a t i o n , s t a t e s t h a t the e l e c t r o n s take up 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 i m m e d i a t e l y f o r any g i v e n n u c l e a r p o s i t i o n . M a t h e m a t i c a l l y , t h i s means t h a t f i r s t and second o r d e r d e r i v a t i v e s of the e l e c t r o n i c w a v e f u n c t i o n w i t h r e s p e c t t o i n t e r n u c l e a r d i s t a n c e may be n e g l e c t e d . F o l l o w i n g H e r z b e r g * , the S c h r o d i n g e r e q u a t i o n i s se p a r a t e d i n t o Z^rt + 2 m ( E e - V e - V n e ) % - O (2 . 7a) ' ' -n1 Z l l V j K * ( E - E e - V n ) f t = O (2 . 7 b) E q u a t i o n (2.7a) i s s o l v e d n u m e r i c a l l y f o r v a r i o u s i n t e r n u c l e a r d i s t a n c e s t o y i e l d a p l o t of E e ( R 1 2 ) . These r e s u l t s may be r e p l a c e d i n (2.7b) and t h i s e q u a t i o n s o l v e d f o r t he t o t a l energy of t h e system, E. A. THE NUCLEAR WAVEFUNCTION For p r a c t i c a l p u r p o s e s , E e + V n i s u s u a l l y r e p l a c e d by an e f f e c t i v e p o t e n t i a l , U ( r - r ) . Working i n t h e n u c l e a r c e n t e r of mass c o o r d i n a t e s , (2.7b) f i n a l l y becomes i I f + _ U IfsmBl^ + _±_ l M + . r z 3 r l d r l r l s in6 ? 8 l 38/ r xsin l6 9 ^ J ^ 2 8 ) 2* ( E > U ( r - r c ) ) ] K = O T2- ) 8 where r = R 1 2 i s the i n t e r n u c l e a r d i s t a n c e and u i s the reduced mass of the n u c l e i . Whether or not the e q u a t i o n i s a n a l y t i c a l l y s o l u b l e depends upon the c h o i c e of e f f e c t i v e p o t e n t i a l . For example, i f one models the n u c l e i as a s i m p l e h a r m o n i c . o s c i l l a t o r , then one may s e p a r a t e the n u c l e a r motion i n t o a v i b r a t i o n a l term and a r o t a t i o n a l t e r m 5 which y i e l d s the energy of a g i v e n r o t a t i o n a l - v i b r a t i o n a l s t a t e , w i t h the r o t a t i o n a l e i g e n f u n c t i o n s b e i n g s p h e r i c a l harmonics and the v i b r a t i o n a l e i g e n f u n c t i o n s a r e Hermite p o l y n o m i a l s of the argument | where u)here E(v,R) = BR(R+1) + a) e(v + 0.5) - DR 2(R+1) 2 E R = BR(R+1) - DR 2(R+1) 2 E y = a> e(v+0.5) (2.10) C2.10a) (z.iOt) 9 One o b s e r v e s t h a t the energy of the system i s dependent upon the quantum numbers v and R so t h a t we may r e p r e s e n t the s t a t e as |v,R> - 7 ^ v ( r - r e ) ^ R CZ.IZ) The above a p p r o x i m a t i o n h o l d s w e l l f o r low v i b r a t i o n a l and r o t a t i o n a l s t a t e s of the m o l e c u l e but i t b r e a k s down f o r h i g h e r s t a t e s . As the v i b r a t i o n a l quantum number, v, i n c r e a s e s so does the i n t e r n u c l e a r d i s t a n c e t o such a p o i n t t h a t e v e n t u a l l y the m o l e c u l e s h o u l d f a l l a p a r t which i s o b v i o u s l y not the case f o r the s i m p l e harmonic o s c i l l a t o r model. A l s o , f o r h i g h r o t a t i o n a l s t a t e s , the n u c l e i a re s u b j e c t t o c e n t r i f u g a l f o r c e s which t e n d t o s t r e t c h them a p a r t . Hence, s e p a r a t i o n of the n u c l e a r w a v e f u n c t i o n as a r o t a t i o n a l p a r t and a v i b r a t i o n a l p a r t i s not r i g o r o u s l y p o s s i b l e . To t a k e t h e s e two d i f f i c u l t i e s i n t o account a n h a r m o n i c i t y and c e n t r i f u g a l d i s t o r t i o n terms may be added t o t h e e f f e c t i v e p o t e n t i a l , U ( r - r e ) , t o u l t i m a t e l y y i e l d an energy of t h e s t a t e |v,R> E v , R = uep ~ V e " 2 + " (2.13) + [B -a v]R(R+1) + [D -B v ] R 2 ( R + 1 ) 2 +... e e e e (where v - (v+0.5) here) and the t o t a l energy of t h e system 10 may be r e s o l v e d as E = E g + G(v) + F(v,R) where E g i s the e l e c t r o n i c energy of the s t a t e , F(v,R) i s the r o t a t i o n a l energy, and G(v) i s the v i b r a t i o n a l energy. B. THE ELECTRONIC CONTRIBUTION In r e a l i t y the e l e c t r o n s a r e a l s o r o t a t i n g about the n u c l e i and g i v e a nonzero c o n t r i b u t i o n t o the net a n g u l a r momentum of the system. A d i a t o m i c m o l e c u l e may be t r e a t e d a symmetric t o p i n analogy w i t h c l a s s i c a l mechanics e x p r e s s i n g the H a m i l t o n i a n i n terms of the a n g u l a r momenta, the moments of i n e r t i a of the system r e f e r r e d t o the p r e f e r r e d c o o r d i n a t e s a t t a c h e d t o the m o l e c u l e , i . e . 3 3 l i e s a l o n g the a x i s of symmetry of the m o l e c u l e and 3 , = 3 2 complete the o r t h o g o n a l c o o r d i n a t e system. HROT * * £ * 2 * = I . * ^ . Z * w t ) + I s T ° 3 (2.15) 21, 21* 2 I 3 As per u s u a l , the b o d y - f i x e d system may be r e l a t e d t o a s p a c e - f i x e d one v i a the E u l e r a n g l e s 6 4>,x,6. w, = 0 s i n x _ tfsinflcosx u>2 - 0cosx + <>sin0sinx (2.1 fed} u3 = 4>cosd + x 11 R e p l a c i n g t h e s e i n the f o r m u l a (2.15) one u l t i m a t e l y o b t a i n s 7 21, 2.1 a U.vrtQ Z I , s i n l 8 I . s i n B (2.17) One r e p l a c e s the p^(where £ = 6,<t>,\) w i t h t h e i r quantum m e c h a n i c a l o p e r a t o r s , t o f i n a l l y o b t a i n an e x p r e s s i o n f o r the energy of a g i v e n s t a t e , E = BJ(J+1 ) + (A-B)A 2 (2.19) where J i s the t o t a l a n g u l a r momentum of t h e system e x c l u d i n g s p i n , A i s the p r o j e c t i o n of the a n g u l a r momentum a l o n g the i n t e r n u c l e a r a x i s , and A, B a r e c o n s t a n t s p r o p o r t i o n a l t o the moments of i n e r t i a of the d i f f e r e n t axes of the body. More i n t u i t i v e l y , a d i a t o m i c m o l e c u l e has an i n t r i n s i c c y l i n d r i c a l symmetry. Hence the t o t a l a n g u l a r momentum of the system i s not co n s e r v e d but o n l y the p r o j e c t i o n of the 1 2 i n t e r n u c l e a r a x i s , A. The e l e c t r o n i c s t a t e s a r e l a b e l l e d by t h i s q u a n t i t y i n ana l o g y w i t h a tomic s t a t e s , L (A=0), IT (A=1), A (A=2), e t c . N e g l e c t i n g a l l f i n e s t r u c t u r e and h y p e r f i n e i n t e r a c t i o n s the energy of the system i s independent of t h e net e l e c t r o n i c s p i n § . T h i s r e s u l t s i n s p i n degeneracy, 2S+1, f o r each s t a t e A. The t o t a l e l e c t r o n i c a n g u l a r momentum a l o n g the i n t e r n u c l e a r a x i s i s = |A+I| (zzo) where Z i s t h e p r o j e c t i o n of t h e s p i n a l o n g the a x i s , and the s t a t e i s l a b e l l e d by 2S+1 (2.zi) A | A + t l S e v e r a l a n g u l a r momenta have thus f a r been mentionned, 5, the t o t a l e l e c t r o n i c s p i n , L, the t o t a l e l e c t r o n i c o r b i t a l a n g u l a r momentum, and, the r o t a t i o n a l a n g u l a r momentum of the n u c l e i . The problem a r i s e s t h a t one must c o u p l e t o g e t h e r these q u a n t i t i e s t o produce the c o n s t a n t s of mo t i o n , i f any e x i s t . V a r i o u s schemes were e x p l o r e d by Hund, of which t h e r e l e v a n t case f o r i o d i n e i s Hund's case ( a ) . T h i s case i s c h a r a c t e r i z e d b y 8 13 where the f i r s t term i s the d i f f e r e n c e between e l e c t r o n i c energy s t a t e s , the second i s the f i n e s t r u c t u r e s p l i t t i n g , and t h e t h i r d i s the r o t a t i o n a l s t a t e s e p a r a t i o n . The c o u p l i n g i s a c c o m p l i s h e d as f o l l o w s : the i n t e r a c t i o n of e l e c t r o n i c and n u c l e a r motion i s v e r y weak and t h e e l e c t r o n i c motion i s s t r o n g l y c o u p l e d t o the i n t e r n u c l e a r a x i s , making A , I , f i good quantum numbers. The v e c t o r L*+S=R i s formed and added t o R t o form the r e s u l t a n t , 3. T h i s c o u p l i n g l e a d s t o the r o t a t i o n a l energy of the form, E_ = B [ J ( J + l ) - n 2 ] (2.220 x\ V and t h e e i g e n f u n c t i o n s may be r e p r e s e n t e d by the quantum numbers, |*> = | S , I , A , f i , v , J > (2.24) Such a scheme i s the case predominant f o r heavy m o l e c u l e s . such as i o d i n e . ( One may f u r t h e r s p e c i a l i z e t h i s f o r m u l a f o r c a s e s i n which Q = 0, the c a s e s of i n t e r e s t f o r t h i s t h e s i s . Hence, 3 = S . ) C. SYMMETRY PROPERTIES OF ELECTRONIC WAVEFUNCTIONS I n a d d i t i o n t o the above quantum numbers, e l e c t r o n i c s t a t e s a r e d e s c r i b e d by t h e i r symmetry p r o p e r t i e s . For 14 d i a t o m i c homonuclear m o l e c u l e s , the m i d p o i n t of the two n u c l e i i s a c e n t e r of symmetry f o r t h e m o l e c u l e which s h a l l be denoted " c s " . 1. U AND G SYMMETRY C o n s i d e r tO g, an o p e r a t o r which r e f l e c t s t he e l e c t r o n i c c o o r d i n a t e s a t the c s . As [6 e,H e3 = 0, then 6 g i s an e i g e n o p e r a t o r of the e l e c t r o n i c w a v e f u n c t i o n . F u r t h e r , by a p p l y i n g t h i s o p e r a t i o n t w i c e t o a g i v e n e l e c t r o n i c s t a t e one must o b t a i n the o r i g i n a l s t a t e , hence the e i g e n v a l u e s of t h i s o p e r a t o r must be +1 or - 1 . Those s t a t e s w i t h e i g e n v a l u e +1 a r e known as g s t a t e s w h i l e those w i t h e i g e n v a l u e -1 are u s t a t e s . 2. REFLECTION THROUGH THE INTERNUCLEAR AXIS L e t 6 2 be an o p e r a t o r which r e f l e c t s the molecule i n a p l a n e p a s s i n g through t h e i n t e r n u c l e a r a x i s . As above, t h i s o p e r a t i o n i s an e i g e n o p e r a t o r of the e l e c t r o n i c w a v e f u n c t i o n h a v i n g e i g e n v a l u e s +1 and - 1 . S t a t e s h a v i n g the e i g e n v a l u e +1 a r e l a b e l l e d " + " and t h o s e h a v i n g the v a l u e - 1 , Thus a s t a t e i s symmetric w i t h r e s p e c t t o both of the above symmetry o p e r a t i o n s ( e i g e n v a l u e s = + 1 ) , has S=0, A=0, and 0=0. 15 3. PARITY OF THE WAVEFUNCTION P a r i t y i s the o p e r a t o r which t a k e s a l l the c o o r d i n a t e s of the p a r t i c l e s of the system t o t h e i r i n v e r s e , i . e . t o - r . and R" • t o -5 .. As the H a m i l t o n i a n c o n t a i n s o n l y even e i n j n j J powers of the c o o r d i n a t e s t h i s o p e r a t o r commutes w i t h the H a m i l t o n i a n and, by the same argument as b e f o r e , one e x p e c t s t o have e i g e n v a l u e s ±1. A T h i s o p e r a t o r , P, may be e x p r e s s e d as P = 0 R ( 2- 2 5> z where R i s a r o t a t i o n of the m o l e c u l e by n about an a x i s a p e r p e n d i c u l a r t o the i n t e r n u c l e a r a x i s t h r o u g h the c s and 6*2 i s a r e f l e c t i o n of the m o l e c u l e i n a p l a n e p a s s i n g t h r o u g h the i n t e r n u c l e a r a x i s and p e r p e n d i c u l a r t o the a x i s a. The e l e c t r o n i c e i g e n f u n c t i o n remains unchanged by the A o p e r a t i o n of R as i t i s d e f i n e d as f u n c t i o n of the e l e c t r o n i c c o o r d i n a t e s r e l a t i v e t o the n u c l e a r p o s i t i o n s . F u r t h e r , the v i b r a t i o n a l w a v e f u n c t i o n depends o n l y upon the r e l a t i v e d i s t a n c e between the n u c l e i and i s t h e r e f o r e not A a f f e c t e d by R e i t h e r . The r o t a t i o n a l e i g e n f u n c t i o n f o r s t a t e s w i t h 0=0 ( i . e . 3=5) may be a p p r o x i m a t e d as 16 (2.27) and R | » R^> = (-D J|^ R> u.z*> F i n a l l y the r e f l e c t i o n , 6 , a c t s o n l y upon the e l e c t r o n i c e i g e n f u n c t i o n so t h a t the net r e s u l t becomes, P|*> = X z ( - 1 ) J | * > ( M ) where X i s +1 f o r + e l e c t r o n i c s t a t e s and -1 f o r - s t a t e s , z 4.. INTERCHANGE OF THE NUCLEI . The f i n a l symmetry o p e r a t i o n t o be c o n s i d e r e d f o r homonuclear d i a t o m i c m o l e c u l e s i s the i n t e r c h a n g e of the A n u c l e i , N. T h i s o p e r a t i o n may be performed by f i r s t a p p l y i n g a p a r i t y o p e r a t i o n f o l l o w e d by a r e f l e c t i o n of the e l e c t r o n i c c o o r d i n a t e s t h r o u g h the cs ( t h e u-g symmetry A A A o p e r a t i o n ) : N = ° e p * N|*> = X z X u _ g ( - 1 ) J|*> (£.301 To t h i s p o i n t a l l t h e terms c o u p l i n g n u c l e a r and e l e c t r o n i c motion have been n e g l e c t e d t h e r e b y a l l o w i n g one t o a f f e c t a s e p a r a t i o n of t h e two. In r e a l i t y some of thes e terms p l a y an i m p o r t a n t r o l e i n the e x p e r i m e n t s which were 17 p e r f o r m e d , and t h u s , r e q u i r e c l o s e r i n s p e c t i o n . 5. FINE STRUCTURE The f i n e s t r u c t u r e H a m i l t o n i a n may be e x p r e s s e d as a sum of terms c o u p l i n g the e l e c t r o n i c motion and the n u c l e a r r o t a t i o n . M. B r o y e r 8 has s t u d i e d t h i s i n d e t a i l and he e x p r e s s e s i t as H, = H +H +H +H (2.3 1) f s so ss r o r s where H g o i s the e l e c t r o n i c s p i n - o r b i t a l c o u p l i n g , H g s i s the e l e c t r o n i c s p i n - s p i n c o u p l i n g , H i s the c o u p l i n g between t h e e l e c t r o n i c o r b i t a l motion and the n u c l e a r r o t a t i o n , and H r g i s the n u c l e a r r o t a t i o n and e l e c t r o n i c s p i n c o u p l i n g . M.Broyer and J.Vi g u e have both l o o k e d a t t h i s H a m i l t o n i a n i n some d e t a i l 8 ' 9 . T a b l e 2.1 below, taken from r e f e r e n c e 8, page 21, summarizes the s e l e c t i o n r u l e s f o r the f i n e s t r u c t u r e H a m i l t o n i a n and compares the o r d e r of magnitude of each of the terms i n the case of i o d i n e . 18 T a b l e 2.1 E s t i m a t e of the energy of the o f f - d i a g o n a l terms of t h e f o r m o l e c u l a r i o d i n e . TERM |AJ| |AA| |AS| |AZ| STRENGTI (cm- 1) Hv 0 0 0 0 0 200 Hso 0 0,1 0,1 0,1 0 5000 0 0,1,2 0,1,2 0,1,2 0 <1 H S R 0 0,1 0,1 0,1 0,1 10-"J H*o 0 0,1 0 0 0,1 10-"J 0 0 0 0 0 0.03 tf(L5)/jjr* 0 0 0,1 0,1 0 0.03 -tf t J - S j / f j r 1 0 0 0 0,1. 0,1 0.06 0 0,1 0 0 0,1 0.06 and u «« /»• g, 0* 0~. C l e a r l y the f i n e s t r u c t u r e components which a r e the most i m p o r t a n t a r e H g o and H g s . However they can c o u p l e o n l y s t a t e s w i t h the same v a l u e of 0. As su c h , they a r e not of much i n t e r e s t f o r the purposes of t h i s t h e s i s as t h i s means t h a t they can c o u p l e t o s t a t e s s e v e r a l thousand cm" 1 h i g h e r i n energy than e i t h e r the fi3nQ+u or the X'Zg of m o l e c u l a r i o d i n e , t h e p r i n c i p a l s t a t e s of i n t e r e s t i n t h i s s t u d y . Terms l i k e 3*(L*+S) from the r o t a t i o n a l p a r t of the H a m i l t o n i a n , however, may c o u p l e s t a t e s w i t h AO=0,±1. Hence the 19 1 I T U a d i s s o c i a t i v e s t a t e , may c o u p l e t o the B 3 I 1 q + u s t a t e t h i s way and l e a d t o a n a t u r a l p r e d i s s o c i a t i o n . The r o t a t i o n a l and v i b r a t i o n a l dependence of the p r e d i s s o c i a t i o n of the B 3 I I q + u s t a t e was s t u d i e d i n d e t a i l by J . V i g u e 9 . H i s r e s u l t s a r e summarized i n Table 2.2 and i n the e q u a t i o n s f o r the p r e d i s s o c i a t i o n r a t e below. r o r t h o = C V 2 J ( J + 1 ) + 6.8a v 2 (Z3Z-) P C V 2 J ( J + 1 ) + 4 . 7 a y 2 fZ.33) (taken from r e f e r e n c e 9 page 222.) 20 TABLE 2.2 The Value of the Predissociation Constants Cv,2 and a v, 2 for the Vi b r a t i o n a l States 9<v'<22. (This table o r i g i n a l l y appeared in reference 9 page 264) a , 2 C , 2 v v' 10-s" 1 s" 1 v' r r a d ( v ' } I0 6s 6 e- 1 9 10 1 1 12 13 1 4 14 16 17 18 19 20 21 22 0.9±0.15 0.82±0.12 0.79±0.10 0.7210.07 0.7410.05 0.7410.05 0.7010.06 0.7110.07 0.7010.08 0.8110.10 0.7610.10 0.7710.10 0.7310.15 0.7210.15 138135 103135 52125 24110 81 6 41 3 61 4 1811 1 30114 39116 57125 62128 73135 79145 86135 59120 40118 22116 31 3 31 3 91 8 151 9 2011 0 3411 5 40117 51120 60130 21 D. HYPERFINE STRUCTURE 1. ORTHO AND PARA CHARACTER I f one n e g l e c t s the c o u p l i n g between n u c l e a r s p i n , I , and any o t h e r q u a n t i t i e s i n the m o l e c u l e one may r e p r e s e n t the n u c l e a r w a v e f u n c t i o n as l*n> = I V I V , x ( I , ' I a ) > { 2 - 3 + ) where |x(11»I 2)> i s the n u c l e a r s p i n w a v e f u n c t i o n . A D e f i n i n g the o p e r a t o r X j , which i n t e r c h a n g e s the n u c l e a r s p i n c o o r d i n a t e s , one e x p e c t s t h a t the w a v e f u n c t i o n of the m o l e c u l e w i l l be e i t h e r symmetric or a n t i s y m m e t r i c w i t h r e s p e c t t o t h i s i n t e r c h a n g e as t h i s o p e r a t o r w i l l commute w i t h t h e H a m i l t o n i a n i f one n e g l e c t s the h y p e r f i n e terms. Indeed, u s i n g the same arguments as b e f o r e , one ex p e c t s t h a t i t s e i g e n v a l u e s w i l l be ±1. In f a c t , from r e f e r e n c e 8 page 27, * i | I i , l 2 > - ( - l ) I j + I r I | I , , I 2 > The consequences of whether the n u c l e i behave l i k e f e r m i o n s or bosons under i n t e r c h a n g e i s p r o f o u n d . Suppose one i n t e r c h a n g e s the n u c l e i as w e l l as t h e i r n u c l e a r s p i n s . A T h i s i s a c c o m p l i s h e d by a p p l y i n g the o p e r a t o r N which, as p r e v i o u s l y d e f i n e d , i n t e r c h a n g e s t h e n u c l e a r c o o r d i n a t e s A f o l l o w e d by X . T h i s r e s u l t s i n H|*>= XIN|*>= X 2 X u _ g ( - 1 ) J ( - D I , + I r I | * > 22 where I i s the t o t a l n u c l e a r s p i n of the system and I , and I 2 a r e the n u c l e a r s p i n s of atoms 1 and 2 i n the m o l e c u l e . Hence i f the n u c l e i behave l i k e bosons then the e i g e n v a l u e on the r i g h t hand s i d e of e q u a t i o n (2.37) w i l l be +1, and i f they behave l i k e f e r m i o n s then the e i g e n v a l u e w i l l be - 1 . In the case of i o d i n e i n which the atoms have 5 n u c l e a r s p i n -j i t ^ a s been o b s e r v e d t h a t the n u c l e i behave as f e r m i o n s . Thus one has = v „ - 9 < - " J ' H 5 U ' 3 7 ) As t h e s e e x p e r i m e n t s d e a l t p r i m a r i l y w i t h the X 1Z* and B 3 n n + u s t a t e s , these w i l l be c o n s i d e r e d e x p l i c i t l y . For the X 1Z* s t a t e , a f t e r one p l a c e s the a p p r o p r i a t e e i g e n v a l u e s i n y e q u a t i o n ( 2 . 3 7 ) , one o b s e r v e s t h a t I+J must be even. T h i s i m p l i e s t h a t the even r o t a t i o n a l s t a t e s (J=R=even) a r e c o u p l e d t o the even n u c l e a r s t a t e s (1=0,2,4) w h i l e the odd n u c l e a r s t a t e s (1=1,3,5) a r e c o u p l e d t o the odd r o t a t i o n a l s t a t e s . One r e f e r s t o each of t h e s e m o d i f i c a t i o n s as p a r a and o r t h o l e v e l s r e s p e c t i v e l y . For the B 3 I1q + u s t a t e t h i s c o u p l i n g i s r e v e r s e d ( i . e . the even r o t a t i o n a l s t a t e s a r e c o u p l e d t o the odd n u c l e a r s t a t e s and v i c e - v e r s a ) owing t o the change i n the v a l u e of u-g The e x p e c t e d w e i g h t i n g of o r t h o l e v e l s t o para l e v e l s i n the absence of any h y p e r f i n e i n t e r a c t i o n s ( i . e . the energy of a g i v e n s t a t e i s independent of the n u c l e a r s p i n 23 of t h a t s t a t e ) i s r e a d i l y d e t e r m i n e d by p e r f o r m i n g t h e sum 1(21+1) over odd n u c l e a r s p i n v a l u e s , ( 1 , 3 , 5 ) , f o r o r t h o and over the even p o s s i b l e n u c l e a r s p i n v a l u e s , ( 0 , 2 , 4 ) , f o r para and d i v i d i n g the two. The r e s u l t i s , o b v i o u s l y , 7:5. E. THE EFFECTS OF HYPERFINE INTERACTIONS A b a s i c premise i n t h i s r e s e a r c h i s t h a t the o r t h o and para c h a r a c t e r of the i o d i n e m o l e c u l e i s w e l l d e f i n e d i n the two e l e c t r o n i c s t a t e s of i n t e r e s t , a t l e a s t f o r low r o t a t i o n a l and v i b r a t i o n a l s t a t e s . T h i s i s not imme d i a t e l y apparant as the h y p e r f i n e i n t e r a c t i o n s , w h i c h have t h u s f a r been n e g l e c t e d , do not a l l commute w i t h the o r i g i n a l A H a m i l t o n i a n . T h e r e f o r e the o p e r a t o r , Z, i s not r i g o r o u s l y an e i g e n o p e r a t o r of the system and some m i x i n g of. the o r t h o and para l e v e l s i s a l l o w e d . The h y p e r f i n e i n t e r a c t i o n s f o r the case of i o d i n e have been s t u d i e d i n d e t a i l by M.Broyer e t a l 1 0 and i s b r i e f l y o u t l i n e d below. H h f " "mD + HEQ + HMO + HEH + * ' * C 2 ^ where H ^ i s a magnetic d i p o l e term, H £ q i s an e l e c t r i c g u a d r u p o l e term, i s a magnetic o c t u p o l e , H E H i s an e l e c t r i c h e x a d e c a p o l e , e t c . These i n t e r a c t i o n s c o u p l e t h e n u c l e a r s p i n s t o the n u c l e a r r o t a t i o n , e l e c t r o n i c r o t a t i o n and s p i n . One r e s u l t of t h i s i s t h a t the a n g u l a r momentum of the m o l e c u l e , 3, and 24 the t o t a l n u c l e a r s p i n , I , a r e no l o n g e r good quantum numbers but a r e c o u p l e d t o form, I+J'=F. The wavef unct i o n s used t o d e s c r i b e the s t a t e s now become, |*> = | S,Z,A,n,v,J,I,F,M p > •<2.33> The h y p e r f i n e i n t e r a c t i o n s a r e a r e l a t i v e l y minor c o n t r i b u t i o n t o the t o t a l energy of the system so t h a t one may t r e a t as a p e r t u r b a t i o n on the system. The e i g e n s t a t e s of the t o t a l H a m i l t o n i a n , i n c l u d i n g h y p e r f i n e terms, can be c o n s t r u c t e d of s t a t e s l i k e those i n e q u a t i o n ( 2 . 3 9 ) . \%fy =- u , i > + z <»'i'i h m u i > u - r > C 2 4 , 0 ) F u r t h e r m o r e , each of the terms i n e q u a t i o n (2.38) may be s u b d i v i d e d a s , . H. = H.(1) + H.(2) +H.(1,2) i i l l where i = MD,EQ,MO,EH,etc. and H ^ ( j ) i s the i n t e r a c t i o n between n u c l e u s j and the e l e c t r o n s , and H^(1,2) i s the i n t e r a c t i o n between the two n u c l e i . The o r d e r of magnitude of each term was e s t i m a t e d by B r o y e r e t a l f o r the case of m o l e c u l a r i o d i n e and, a c c o r d i n g t o t h e s e a u t h o r s , the major c o n t r i b u t i o n s come from the magnetic d i p o l e and e l e c t r i c q u a d r u p o l e terms. 25 For the purpose of t h i s t h e s i s the .terms which c o u p l e s t a t e s w i t h A l = ±1,±3, e t c . a r e c o n s i d e r e d as t h e s e are the terms t h a t w i l l mix o r t h o and p a r a l e v e l s . To d e t e r m i n e e x a c t l y which terms w i l l do t h i s one needs c o n s i d e r the symmetry p r o p e r t i e s of each term of the h y p e r f i n e H a m i l t o n i a n , H ^ , e x p l i c i t l y . (a) M a gnetic D i p o l e I n t e r a c t i o n The magnetic d i p o l e i n t e r a c t i o n may be e x p r e s s e d as ( i n g a u s s i a n u n i t s ) (2.42) ( U ) (2.43) end (b) E l e c t r i c Quadrupole 26 Following Broyer again, « r « = zn-tr Q ^ ( P ) y . V ( * > < 2 + 4 ° V 5" ft! However, i t i s more instru c t i v e to rewrite the quadrupole interaction following Ramsey 1 1 I; (2 I,*»»)( 2 J-OC 2 7+3) * 3Z* n t t r' A V F i n a l l y , H Eq (l,Z) - Z l ' l ) m f QrnCD V i . (1.2) + Q 8 mU)V-i(2,i)j where Q M ( i ) and V* M(1,2) are defined by equations (2.44a) and (2.44b) respectively. (c) Symmetry Properties The symmetry properties of each of the terms in the hyperfine Hamiltonian are discussed in d e t a i l in reference 8 pages 34-36. To summarize, 27 t+t' <*»' , u , I ' | H h f (2 ) | J > , U , I > = l<v' , u , I ' |H h f (1 )\v,u,l> (2.4-/) <u' , g , I ' |H h f (2) | K , U , I > = ( - 1 ) I + I + 1 < v ' , g F I ' | H h f ( l ) | ^ , u , I > (i.4-8) <j>',u , I'|H h f(1,2)|v,u,l> = (-1) 1 , u , I ' | H h f ( \ , 2 ) \ v , u , l > U41) , g , I ' |H h f (1 ,2) | * , u , I > = 0 <a.so) From e q u a t i o n s (2.47) t o (2.50) one may observe t h a t the p a r t of the h y p e r f i n e H a m i l t o n i a n which a c t s between the n u c l e i , (H^{},2)), can c o u p l e o n l y u t o u s t a t e s o r g t o g s t a t e s and not mix o r t h o and para l e v e l s . On the o t h e r hand, the h y p e r f i n e terms a c t i n g between the i n d i v i d u a l n u c l e i and the e l e c t r o n s can c o u p l e o r t h o and para s t a t e s i n s e p a r a t e 'u and g e l e c t r o n i c s t a t e s . ( i . e . an o r t h o l e v e l i n a "u" e l e c t r o n i c s t a t e can mix w i t h a par a l e v e l i n a "g" e l e c t r o n i c s t a t e . ) The s t r e n g t h of t h i s m i x i n g due t o the magnetic d i p o l e and e l e c t r i c q u a d r u p o l e terms i s e s t i m a t e d i n Appendix A y i e l d i n g the r e s u l t ^ \ u , I ' I H j ^ ' P + H E ° " p | v , g , I > - I O " 2 ™ - 1 <z-*l> so t h a t e q u a t i o n (2.40) becomes 28 For the ground s t a t e of i o d i n e , X 1!*, the n e a r e s t s t a t e w i t h which i t may c o u p l e an o r t h o and para l e v e l i s the 3 n i u or a 3 n n u which l i e r o u g h l y t e n thousand cm 1 above the ground s t a t e , making the s t r e n g t h of the c o u p l i n g of the o r d e r of 1 0 - 6 . |*> =* |f,g,I> + 10-6|*>',u,I + 1 > + l O - 6 | f * , u , I - 1 > (2.52) Hence the o r t h o and p a r a c h a r a c t e r of the m o l e c u l a r ground s t a t e i s w e l l p r e s e r v e d even when the h y p e r f i n e i n t e r a c t i o n s a r e taken i n t o a c c o u n t . In t he case of the B 3 I I n , u e l e c t r o n i c s t a t e , however, t h i s i s not so o b v i o u s e s p e c i a l l y near the d i s s o c i a t i o n l i m i t , v'=87. In t h i s • r e g i o n many e l e c t r o n i c s t a t e s l i e c l o s e t o g e t h e r so t h a t the energy d i f f e r e n c e i n the denominator of e q u a t i o n (2.40) becomes much s m a l l e r and the Franck-Condon o v e r l a p i n t e g r a l s may be l a r g e e n a b l i n g a s i g n i f i c a n t u—g o r t h o and para l e v e l m i x i n g . In f a c t , J . P i q u e e t a l 1 2 have r e c e n t l y r e p o r t e d such h y p e r f i n e i n d u c e d symmetry b r e a k i n g near the B s t a t e d i s s o c i a t i o n l i m i t i n i o d i n e . In t h i s work o n l y low l y i n g v i b r a t i o n a l l e v e l s were used ( v' < 43 ). For t h e s e s t a t e s the n e a r e s t g s t a t e s t h a t may c o u p l e a r e the 3 n n g or the 1HQg which a r e r o u g h l y f o u r thousand cm' 1 above the s t a t e s of i n t e r e s t . Hence the s t r e n g t h of t h e c o u p l i n g here i s of the o r d e r of 10' 5. Again t h i s i s n e g l i g i b l e and the o r t h o and para c h a r a c t e r i s w e l l 29 m a i n t a i n e d . | * > * | P , U , I > + 1 0~ 5 | »>' ,g, 1 + 1 > +10" 5 | vr ,g,I-1> ("2.53) 1. RADIATIVE DECAY Another method of g o i n g from an o r t h o s t a t e t o a p a r a s t a t e i s t h r o u g h the spontaneous e m i s s i o n of a photon r e s u l t i n g i n such a decay. In the absense of h y p e r f i n e i n t e r a c t i o n s t h i s p r o c e s s i s s t r i c t l y f o r b i d d e n i n s i d e a g i v e n e l e c t r o n i c s t a t e v i a a d i p o l e i n t e r a c t i o n . However, as has been seen, the h y p e r f i n e H a m i l t o n i a n weakly c o u p l e s u and g o r t h o and p a r a s t a t e s and t h e r e b y a l l o w s t h e s e t r a n s i t i o n s . I t i s e x p e c t e d t h a t the p r o b a b i l i t y of such a t r a n s i t i o n i s e x t r e m e l y low owing t o the weakness of the o r t h o and para c o u p l i n g and t h i s i s indeed the case as i s o u t l i n e d below. R a i c h and G o o d 1 3 a d d r e s s e d t h i s problem i n d e t a i l f o r the case of m o l e c u l a r hydrogen. T h e i r c a l c u l a t i o n i s used as a g u i d e t o o b t a i n an e s t i m a t e of t h e t r a n s i t i o n p r o b a b i l i t y i n the case of a ground s t a t e i o d i n e m o l e c u l e . (Only the ground s t a t e i s c o n s i d e r e d as d u r i n g an experiment any g i v e n m o l e c u l e i s i n the B 3 I I Q + u s t a t e f o r a s h o r t p e r i o d of time 10" S seconds per e x c i t a t i o n . ) The t r a n s i t i o n p r o b a b i l i t y i s g i v e n by 3 30 where M i s the d i p o l e t r a n s i t i o n moment between a ground s t a t e o r t h o l e v e l and para l e v e l , and lieu i s the energy of the e m i t t e d photon. Thus one e s t i m a t e s u as t = 1 0- ^ X 1 ! * , J , I IZer. I v' , J ' ,1 ' > + 9 1 (2.55) l 0 - 6 < f " , J " , I B | I e ? i | X ' 2 * , J , I > where I',I"=I±1. T h i s l e a d s t o t - 4 . l 0 L 6 ( 5 d i P ° l e ) ( 2 5 f e ) Thus, M*M - 1 6 . l O - 1 2 | R d i P ° l e | 2 (2.5-/) The v a l u e of |g^1P°-1-e|2 may be e s t i m a t e d from the mean l i f e t i m e of t r a n s i t i o n s between the X t o B s t a t e s by E s 3 h - L2.se) where v i s the t r a n s i t i o n from s t a t e n t o m i n cm" 1. With nm t h i s e q u a t i o n (2.54) becomes 3 frc3 22.^ Vnm^> (2.59) W/f * 8 By* (16 ^  to'") 31 U s i n g the v a l u e s i> I0'*cm _ 1, and T « 1 0 - 6 seconds nm from V i g u e 9 , and B v =* 0.0375cm - 1 from H e r z b e r g * one u l t i m a t e l y o b t a i n s f o r the t r a n s i t i o n p r o b a b i l i t y , j 3 W i f a ' ( 4 . 7 X l 0 1 - y r ) ^ 2 b o ) Thus the p r o b a b i l i t y of a t r a n s i t i o n from an o r t h o - i o d i n e s t a t e t o a p a r a - i o d i n e s t a t e ( o r v i c e - v e r s a ) v i a d i p o l e r a d i a t i o n i n the X 1!* e l e c t r o n i c s t a t e i s 9 c o m p l e t e l y n e g l i g i b l e over the c o u r s e of an e x p e r i m e n t . 2. COLLISIONALLY INDUCED ORTHO-PARA TRANSITIONS F i n a l l y , a homonuclear d i a t o m i c m o l e c u l e p o s s e s s i n g o r t h o and p a r a m o d i f i c a t i o n s may t r a n s i t between the two o n l y i f the n u c l e a r s p i n of one of the n u c l e i can be r e o r i e n t e d w i t h r e s p e c t t o the o t h e r . T h i s i s d i f f i c u l t t o do but may be a c c o m p l i s h e d by b r i n g i n g the m o l e c u l e i n c o n t a c t w i t h a magnetic or e l e c t r i c f i e l d g r a d i e n t a c r o s s t h e n u c l e i . Such a c o n d i t i o n o c c u r s when the m o l e c u l e c o l l i d e s w i t h a paramagnetic s p e c i e s . In f a c t , the para t o o r t h o r e l a x a t i o n i n m o l e c u l a r hydrogen has been s t u d i e d by A . F a r k a s 1 * i n j u s t such a manner. The t h e o r e t i c a l b a s i s f o r t h i s e f f e c t was i n t r o d u c e d by W i g n e r 1 5 and r e f i n e d by K a l k e r and T e l l e r 1 6 c i r c a 1934. F o l l o w i n g t h e s e a u t h o r s , the e f f e c t may be viewed as f o l l o w s : 32 Upon c o l l i s i o n w i t h a paramagnetic p a r t i c l e , the n u c l e i of the d i a t o m i c m o l e c u l e f e e l an inhomogenous magnetic f i e l d s e t up by the magnetic moment, uQ, of the c o l l i d i n g s p e c i e s . T h i s , t r e a t e d as a p e r t u r b a t i o n , may be w r i t t e n as V = M p[Ii-H c(r 1) + I 2 . f l c ( r 2 ) ] where ^ c ^ j ) * s the magnetic f i e l d s e t up by the paramagnetic c o l l i d i n g s p e c i e s a t n u c l e u s " i " . The p o t e n t i a l , V , may be symmetrized and a n t i s y m m e t r i z e d as V ' = u [ ( T 1 + T 2 ) . ( H ( r 1 ) + H ( r 2 ) ) ] * T p V.' = u [ (I i - T 2 ) • ( H ( r , ) - H ( r 2 ) ) ] ( 2 . u b ) j P The a n t i s y m m e t r i c p a r t a l o n e i s c a p a b l e of p r o d u c i n g an o r t h o - p a r a t r a n s i t i o n and, t h u s , t h i s i s the p a r t which w i l l be c o n s i d e r e d h e r e . As T,-T 2 = 2 I - I , , and knowing t h a t o n l y the 21, p a r t may c o u p l e s t a t e s w i t h AI=±1, i . e . o r t h o s t a t e s t o para s t a t e s , then the e f f e c t i v e p o t e n t i a l , U, may be w r i t t e n as U = M p [ l , - ( H ( r 1 ) - H ( r 2 ) ) ] (.2.(0 3 "> Expanding the ( H ( r , ) - H~(r 2)) term as and l e t t i n g 3 H , (2.4 S ) 33 one f i n a l l y o b t a i n s U = 3 M p Mc [ I ^ + J ^ y j L l - O ^ ) + I l t X * ] Ubb) where X^ = ( x , - x 2 ) . . F o r the purposes of an o r d e r of magnitude c a l c u l a t i o n one may approximate the p o t e n t i a l as u =* q p p | j c R M J 1 z cos e (2.67) where R N i s the i n t e r n u c l e a r d i s t a n c e , and 6 i s the a n g l e between a s p a c e - f i x e d a x i s and the i n t e r n u c l e a r a x i s . U s i n g the p o t e n t i a l i n (2.67) as a time dependent p e r t u r b a t i o n and r e p r e s e n t i n g the w a v e f u n c t i o n of the system as |*> = I a n ( t ) K n > E X P - ( i i r ) <**8) where t h e \^n> are the time independent s o l u t i o n s t o the u n p e r t u r b e d H a m i l t o n i a n . S o l v i n g the S c h r o d i n g e r e q u a t i o n l e a d t o the w e l l known s o l u t i o n f o r the a ^ ( t ) (tiu-t#y.-34 and, t h u s , the p r o b a b i l i t y of t r a n s i t i o n from a s t a t e l ^ | c > t o a s t a t e |\^ > i s I f l i t t J l 1 - 4|<JiTl U 1 f t ) ! ' sin-fcJut) , In the case of i o d i n e the u n p e r t u r b e d s t a t e s have the form, | S, L,A,Ji,v , J , I ,F jMp, > so t h a t the m a t r i x element, M, becomes M = d<S/,A',fi,,v',J/,l',F',M^,| I 1 z c o s 0 |S,Z,A,S2,v,J,I ,F,MF> where d = (9MpMcR)/r* E v a l u a t i n g t h i s w i t h the h e l p of Appendix A one o b t a i n s M a« dI 1{j5(J',J±1)6(I',I±1)<v'|v> (2-72) where £j i s the m a t r i x element, <J' ,S2' | cos0 | J ,S2> . Thus, a i t i T - rftf Rj- it ^ t )<v'l*>l* sm (AE T ' T * 't) (3.73) The most p r o b a b l e t r a n s i t i o n s w i l l o c cur i n ca s e s where the energy d i f f e r e n c e between the two l e v e l s i s the l e a s t . T h i s means a t r a n s i t i o n between d i f f e r e n t r o t a t i o n a l l e v e l s a l o n e so t h a t I v'=v I z | * « ; | - 8 1 hp »pc* R „ a i > * ^ , & i n * ( z e ^ J t \ C 2 7 4 0 35 From e q u a t i o n (2.74) i t i s i m m e d i a t e l y r e c o g n i z e d t h a t t h e p r o b a b i l i t y of an o r t h o t o p a r a t r a n s i t i o n upon c o l l i s i o n w i t h a paramagnetic p a r t i c l e i s v e r y s e n s i t i v e t o the p o i n t of impact, r , and t o the s p a c i n g of the r o t a t i o n a l energy l e v e l s , B v . For example, a l l o t h e r f a c t o r s b e i n g e q u a l , e x c e p t f o r n u c l e a r s p i n , the d i f f e r e n c e of the r o t a t i o n a l s p a c i n g between hydrogen and i o d i n e , and the r o t a t i o n a l quantum numbers, one o b s e r v e s t h a t the r a t i o of t r a n s i t i o n p r o b a b i l i t i e s f o r each of t h e s e m o l e c u l e s i s 2 5 < R < t x / o 5 U 7 5 ) As the s p a c i n g of the r o t a t i o n a l l e v e l s of m o l e c u l a r i o d i n e i s q u i t e s m a l l , one e x p e c t s i t t o be q u i t e s e n s i t i v e t o such c o l l i s i o n a l l y i n d u c e d t r a n s i t i o n s . In e f f e c t , the h y p e r f i n e m i x i n g of o r t h o and para l e v e l s i n the X ' l g and B 3 n Q + u s t a t e s ( a t l e a s t f o r low l y i n g v i b r a t i o n a l l e v e l s ) and the r a d i a t i v e c o u p l i n g of such s t a t e s i s c o m p l e t e l y n e g l i g i b l e . Paramagnetic c a t a l y s e d o r t h o t o p a r a t r a n s i t i o n s i n i o d i n e , however, may p l a y a v e r y i m p o r t a n t r o l e i n s c r a m b l i n g any o r t h o - and para - i o d i n e s e p a r a t i o n a t t e m p t e d . I I I . PREVIOUS RESULTS T h i s c h a p t e r i s d i v i d e d i n t o two p a r t s ; f i r s t , p a s t e xperiments i n v o l v i n g o r t h o - and para-hydrogen w i l l be reviewed and t h e i r r e l e v a n c e as r e g a r d s m o l e c u l a r i o d i n e i s p r e s e n t e d v . Second , some r e c e n t e x p e r i m e n t s c a r r i e d out by V.S.Letokhov e t a l on o r t h o - and p a r a - i o d i n e w i l l be summarized . A. ORTHO ~ AND PARA - HYDROGEN As has been d i s c u s s e d p r e v i o u s l y , homonuclear d i a t o m i c m o l e c u l e s p o s s e s s o r t h o and para m o d i f i c a t i o n s . In the case of both hydrogen and i o d i n e , whose ground e l e c t r o n i c s t a t e s are 1 I * and whose n u c l e i b oth obey F e r m i - D i r a c s t a t i s t i c s , g the even r o t a t i o n a l l e v e l s a r e c o u p l e d t o the even n u c l e a r s p i n s t a t e s (para s t a t e s ) w h i l e the odd n u c l e a r s p i n s t a t e s ( o r t h o s t a t e s ) a r e c o u p l e d t o the odd r o t a t i o n a l l e v e l s . By adding up the t o t a l number o f s t a t e s p o s s i b l e f o r o r t h o m o d i f i c a t i o n s and p a r a m o d i f i c a t i o n s f o r a n u c l e u s w i t h s p i n I , , one o b t a i n s the f a c t t h a t the i n t e n s i t i e s of an o r t h o r o t a t i o n a l t r a n s i t i o n t o a para t r a n s i t i o n s h o u l d f o l l o w R = Jl Jl I, + } Z z. (3.1) 36 37 For hydrogen the e x i s t e n c e of o r t h o and para forms was p o s t u l a t e d by H e i s e n b e r g 1 8 and H u n d 1 9 and such i n t e n s i t y a l t e r n a t i o n was obs e r v e d . In e q u i l i b r i u m , the r a t i o of ortho-hydrogen t o para-hydrogen may be w r i t t e n u s i n g the p a r t i t i o n f u n c t i o n , NT = where E ( v , j ) i s the energy of the r o t a t i o n a l and v i b r a t i o n a l s t a t e c h a r a c t e r i z e d by t h e quantum numbers v, and J , g° o r ^ i s the n u c l e a r s p i n degeneracy of the o r t h o or para s t a t e s ( 3 and 1 r e s p e c t i v e l y f o r hydrogen ) and g j = 2J+1 i s the r o t a t i o n a l degeneracy of the s t a t e . T a b l e 3.1 shows some v a l u e s of the o r t h o t o p a r a r a t i o a t v a r i o u s t e m p e r a t u r e s , ( t a k e n from r e f e r e n c e 14 page 14) 38 TABLE 3.1 Parahydrogen t o Orthohydrogen R a t i o a t V a r i o u s Temperatures. Temperature Parahydrogen P e r c e n t (K) Orthohydrogen Parahydrogen 20 544.8 99.82 50 3.327 76.89 80 0.9377 48.39 100 0.6262 38.51 150 . 0.3994 25.72 210 0.3463 25.54 273 0.3357 25.13 0.3333 25.00 From th e s e r e s u l t s i t would appear t h a t a s i m p l e way t o enhance the para t o o r t h o r a t i o i n hydrogen i s t o s i m p l y c o o l the m o l e c u l e s down from room t e m p e r a t u r e . T h i s f o l l o w s one's i n t u i t i o n t h a t , as the hydrogen c o o l s , the m o l e c u l e s a r e e x p e c t e d t o f a l l i n t o t h e i r l o w e s t energy s t a t e , J=0, a parahydrogen s t a t e . U n f o r t u n a t e l y , when t h i s was f i r s t t r i e d no such enhancement was o b t a i n e d owing t o the nonmixing of o r t h o and para s t a t e s d i s c u s s e d i n c h a p t e r 2. An a l t e r n a t e t e c h n i q u e used by H a r t e c k and B o n h o e f f e r 2 V 1»2-2»23 was t o c o o l t h e hydrogen over c h a r c o a l . As the gas was adsorbed o r t h o and p a r a c h a r a c t e r c o u l d be changed p r o b a b l y due t o some pa r a m a g n e t i c s u r f a c e e f f e c t s . Hence, a sample c o n t a i n i n g a 39 s p e c i f i e d f r a c t i o n of parahydrogen c o u l d be p r e p a r e d and, c o u r t e s y of the nonmixing of the two s p e c i e s , c o u l d be s t o r e d f o r l o n g p e r i o d s even a t room t e m p e r a t u r e . Much work was c a r r i e d out on the c o n v e r s i o n of o r t h o h y d r o g e n t o parahydrogen and v i c e - v e r s a c i r c a 1930. Two methods of t h i s c o n v e r s i o n w i l l be c o n s i d e r e d h e r e , namely by exchange of a hydrogen atom, H + o-H 2 -*-*-p-H2 + H ( a * ) and v i a c o l l i s i o n s w i t h paramagnetic s p e c i e s X + 0-H 2 p-H 2 + X (3.4.) 1. HYRDROGEN ATOM EXCHANGE Bo n h o e f f e r and Harteck 2 1» 2 2 i n v e s t i g a t e d the p o s s i b i l i t y of o r t h o t o p a r a c o n v e r s i o n i n hydrogen due r a d i a t i o n by f i l l i n g a c o n t a i n e r w i t h 200 t o r r of normal hyrogen ( r a t i o of 3:1) and p l a c i n g i t i n l i q u i d a i r . A b s o l u t e l y no c o n v e r s i o n was o b s e r v e d over the c o u r s e of f o u r weeks showing t h a t the r a d i a t i v e c o n v e r s i o n does not occur r a p i d l y , i f a t a l l . Next some hydrogen was p l a c e d i n a b r a s s c o n t a i n e r and kept a t 150 atmospheres p r e s s u r e a t l i q u i d a i r t e m p e r a t u r e . C o n v e r s i o n t o parahydrogen was observed here and a t t r i b u t e d t o a c a t a l y t i c w a l l r e a c t i o n by F a r k a s . In f a c t s e v e r a l 40 d i f f e r e n t m a t e r i a l s were t e s t e d by B o n h o e f f e r and F a r k a s 1 9 and i t i s n o t a b l e t h a t they found t h a t even g l a s s v e s s e l s c a t a l y s e d the c o n v e r s i o n . ( a t 2 atmospheres p r e s s u r e and -185°C a sample went from 25 t o 30 p e r c e n t parahydrogen i n 6 days) A.Farkas s t u d i e d the r e a c t i o n of hydrogen atoms on parahydrogen m o l e c u l e s by p r e p a r i n g a c e l l w i t h 47 p e r c e n t parahydrogen and then p l a c i n g the sample i n an oven t o e l e v a t e the te m p e r a t u r e t o between 700°C and 800°C. T h i s r e s u l t s i n spontaneous p r o d u c t i o n of hydrogen atoms and t h e i r e f f e c t upon the o r t h o t o par a r a t i o was s t u d i e d . The time dependence of the parahydrogen c o n c e n t r a t i o n , p f c, was obs e r v e d t o f o l l o w , <P t " P-) = <Po - P - e ) E X P - ( k [ H ] t ) (3-5) where p 0 i s the o r i g i n a l parahydrogen c o n c e n t r a t i o n , and p.. i s the e q u i l i b r i u m c o n c e n t r a t i o n . T h i s f o r m u l a a g r e e s w i t h the mechanism d e s c r i b e d i n e q u a t i o n ( 3 . 3 ) . Indeed, i t may be deduced t h a t the v a l u e f o r k i s f o u r t i m e s the o r t h o t o p a r a c o n v e r s i o n r a t e . From F a r k a s ' r e s u l t s k may be w r i t t e n as k « 1 . 1 8 6 - 1 0 V T EXP-(5500/RT) C-3-O measured i n l i t e r s per mole per second. E x p e r i m e n t s performed by G e i b and H a r t e c k 2 5 i n whi c h hydrogen atoms were i n t r o d u c e d i n t o a system of e n r i c h e d 41 parahydrogen y i e l d e d v a l u e s f o r k of t h e same o r d e r of magnitude as t h a t g i v e n above. U s i n g t h e s e r e s u l t s (as the ex p e r i m e n t s of Ge i b and H a r t e c k were performed i n a temperature range of 10° t o 100°C) one may e s t i m a t e the h a l f - l i f e of the c o n v e r s i o n as X k[M] Z73K [Hi 1 2. PARAMAGNETIC CONVERSION F a r k a s and S a c h s s e 2 6 a l s o i n v e s t i g a t e d t h e homogeneous c o n v e r s i o n of parahydrogen t o normal hydrogen c a t a l y s e d by v a r i o u s , paramagnetic i m p u r i t i e s such as oxygen and n i t r i c o x i d e . The t h e o r e t i c a l t r e a t m e n t of t h i s " e f f e c t has a l r e a d y been g i v e n i n c h a p t e r 2 and the r e s u l t s of the e x p e r i m e n t a l i n v e s t i g a t i o n s a r e g i v e n below. S t a r t i n g w i t h a g i v e n amount of o r t h o h y d r o g e n , o, and parahydrogen, p, the a c t i o n of a paramagnetic s p e c i e s , X, on hydrogen i s d e s c r i b e d by e q u a t i o n ( 3 . 4 ) . Indeed, an i d e n t i c a l time dependence f o r the parahydrogen i s o b t a i n e d as g i v e n i n e q u a t i o n ( 3 . 5 ) , ( P t ~ P-) » (Po - p . j E X P - ( k [ X ] t ) In t h e case of oxygen as the paramagnetic s p e c i e s , F a r k a s and Sachsse d e t e r m i n e d t h a t the o r d e r of magnitude of t h i s c o n v e r s i o n , k, was about 9.2 l i t e r s per mole per minute t o y i e l d a h a l f l i f e of <± = <2L£ - ^50 "*>»e-s C3.8) 4 2 N i t r i c o x i d e y i e l d e d v e r y s i m i l a r r e s u l t s t o t h e s e , however, i n the case of n i t r i c o x i d e o n l y those m o l e c u l e s which a r e i n the e l e c t r o n i c s t a t e 2 n 3 a r e paramagnetic. Hence e q u a t i o n (3.5) s h o u l d r e a d , ( P t " P<«) = (Po " p^)EXP-(k[NO*]t) * where [NO ] i s t h e c o n c e n t r a t i o n of paramagnetic n i t r i c o x i d e . A g a i n , one may e s t i m a t e the r a p i d i t y of the c o n v e r s i o n t h r o u g h the h a l f l i f e , KCMO*] Cmo*] i -By comparing the t h r e e v a l u e s f o r 1 t o r r of each s u b s t a n c e one has a i a-Hence t h e a b i l i t y of hydrogen atoms t o scramble o r t h o and parahydrogen f a r outweighs the a b i l i t y of the o t h e r two. Hence, i f one assumes a s i m i l a r b e h a v i o u r i n i o d i n e i t would seem u s e f u l t o p e r f o r m e x p e r i m e n t s i n which as few f r e e atoms a r e produced as p o s s i b l e and a l s o t o use t e s t c e l l s t h a t a r e as f r e e from c o n t a m i n a t i o n (by a i r ) as p o s s i b l e . 43 B. ORTHO AND PARA ENHANCEMENT IN IODINE I o d i n e , as hydrogen, d i s p l a y s o r t h o and para m o d i f i c a t i o n s . U n f o r t u n a t e l y , u n l i k e hydrogen, the f r a c t i o n of p a r a - i o d i n e i s not enhanced by s i m p l y c o o l i n g m o l e c u l a r i o d i n e over c h a r c o a l . T h i s becomes c l e a r i f one c o n s i d e r s the r a t i o of p a r a - i o d i n e t o o r t h o - i o d i n e as a f u n c t i o n of tempe r a t u r e as shown i n Ta b l e 3.2. 44 TABLE 3.2 P a r a - I o d i n e t o O r t h o - I o d i n e v e r s u s Temperature Temperature (K) p a r a - i o d i n e o r t h o - i o d i n e p e r c e n t p a r a - i o d i n e 0.2 51.800 98.1 0.4 3.517 77.9 0.6 1.464 59.4 0.8 0.9924 50.2 1.0 0.8282 45.3 10.0 0.7143 41.7 50.0 0.7143 41.7 100.0 0.7143 41.7 300.0 0.7143 41.7 0.7143 41.7 (These v a l u e s have been c a l c u l a t e d u s i n g the r o t a t i o n a l and v i b r a t i o n a l c o n s t a n t s s u p p l i e d by H e r z b e r g * and p e r f o r m i n g the summation over s t a t e s J"=0 t o 101 and v" = 0 t o 5.) As the r o t a t i o n a l l e v e l s p a c i n g i s much c l o s e r i n i o d i n e as compared t o hydrogen one must go t o much lower t e m p e r a t u r e s t o o b t a i n a s u b s t a n t i a l p a r a - i o d i n e enhancement. However, a t such low t e m p e r a t u r e s m o l e c u l a r i o d i n e forms a s o l i d and has no vapour p r e s s u r e a t a l l . ( i . e . a t -70°C the vapour p r e s s u r e of i o d i n e i s r o u g h l y 10" 6 t o r r . ) Hence, i f i o d i n e forms a m o l e c u l a r s o l i d t h e n , because of the nonmixing of o r t h o and p a r a s t a t e s , the system w i l l not s p o n t a n e o u s l y go over i n t o the e q u i l i b r i u m 45 r a t i o f o r t h a t temperature and i f i o d i n e forms an at o m i c s o l i d , then upon h e a t i n g up the sample enough t o o b t a i n s u b s t a n t i a l vapour p r e s s u r e , the vapour w i l l i m m e d i a t e l y t a k e on the e q u i l i b r i u m r a t i o f o r t h a t t e m p e r a t u r e which w i l l be, f o r a l l i n t e n t s and p u r p o s e s , the u s u a l 7:5. Another method of s e p a r a t i o n i s r e q u i r e d , t h e n . The f i r s t p e o p l e t o c a r r y out an experiment which h i n t e d a t the p o s s i b i l i t y of a s e p a r a t i o n of o r t h o - and p a r a - i o d i n e were R. M. Badger and J . W. U r m s t o n 2 7 . In 1930 t h e s e men c a r r i e d out an experiment on m o l e c u l a r i o d i n e i n the p r e s e n c e of 2-hexene. T h e i r i d e a was t o p r e f e r e n t i a l l y e x c i t e the o r t h o - i o d i n e m o l e c u l e s w i t h the 5461 A mercury l i n e and have th e s e m o l e c u l e s r e a c t w i t h a s u i t a b l e a g e n t , 2-hexene. The mercury 5461 A l i n e was shone i n t o a c e l l c o n t a i n i n g 170 mtorr of i o d i n e and 6 t o r r of 2-hexene f o r 24 h o u r s . A f t e r t h i s time the a u t h o r s c l a i m e d t h a t 50% of the i o d i n e had r e a c t e d . The c o n t e n t s of the t e s t c e l l were t e s t e d f o r any o r t h o t o para enchacement by h o l d i n g a second c o n t a i n e r f i l l e d w i t h the same amount of 2-hexene b e s i d e the t e s t c e l l and f i l l i n g i t w i t h i o d i n e vapour u n t i l an o b s e r v e r d e c i d e d t h a t the c o l o r i n the two c e l l s matched. Next , w h i t e l i g h t from a t u n g s t e n lamp was passed t h r o u g h the two c e l l s and t h e i r f l u o r e s c e n c e i n t e n s i t i e s compared. The w h i t e l i g h t f l u o r e s c e n c e was judged t o be t h e same i n the two c e l l s w h i l e the 5461 A mercury l i n e ' s f l u o r e s c e n c e was judged t o be weaker i n the t e s t c e l l . From the s e r e s u l t s , ( a l b e i t q u a l i t a t i v e ) i t was c o n c l u d e d t h a t some 46 enhancement had been o b t a i n e d . However, no f u r t h e r work on t h e m a t t e r was f o r t h c o m i n g t o the b e s t of my knowledge. 1. THE EXPERIMENTS OF V. S. LETOKHOV ET AL A f t e r a f o r t y year i n t e r l u d e and t h e advent of the l a s e r the s u b j e c t was t a k e n up once a g a i n by V. S. Letokhov e t a l . In a paper p u b l i s h e d i n 1973 1 i t was r e p o r t e d t h a t a p a r t i a l o r t h o - t o p a r a - i o d i n e r a t i o enhancement had been a c h i e v e d . The t e c h n i q u e employed was the f o l l o w i n g : a c y l i n d r i c a l t e s t c e l l of l e n g t h 6 cm and volume 20 cm 3 was f i l l e d w i t h 250 mtorr of i o d i n e vapour. T h i s c e l l was then i r r a d i a t e d w i t h the 5145 A l i n e of an argon i o n l a s e r which e x c i t e s the B 3 n Q — X 1!* ( 4 3 ' - 0 n ) P(13) and R(15) t r a n s i t i o n s of m o l e c u l a r i o d i n e . Both o f . t h e s e t r a n s i t i o n s i n v o l v e o r t h o s t a t e s and l e a d t o some p r e d i s s o c i a t i o n . Hence, by s e l e c t i v e l y e x c i t i n g and b r e a k i n g up the o r t h o - i o d i n e m o l e c u l e s L etokhov et a l were a b l e t o o b t a i n a s h i f t o r t h o t o p a r a r a t i o . E x p e r i m e n t a l l y , the a u t h o r s o b s e r v e d the argon i o n i n d u c e d f l u o r e s c e n c e i n t e n s i t y d e c r e a s e over a 10 t o 20 minute p e r i o d and then l e v e l o f f . I t was c l a i m e d t h a t the t o t a l number of i o d i n e m o l e c u l e s remained c o n s t a n t as they were m o n i t o r e d w i t h the 5461 A l i n e a mercury a r c lamp w i t h a 4 c m - 1 l i n e w i d t h and the f l u o r e s c e n c e induced remained c o n s t a n t . Hence a s h i f t i n the r a t i o had o c c u r r e d . The dominant r e a c t i o n s were c l a i m e d t o be 47 o - 1 2 — - 21 w I + O-I 2 — — p-I 2 + 1 X 21 — o - 1 2 o r p-I 2 k which l e a d s t o t h e e q u i l i b r i u m c o n d i t i o n s , o-Zxt*>) = T C f I +g - p(£ l* |U1 (3 .VZ) 24 L Z * p - l . t * ) = C + i 5 / - l C [ - p + /pTT I2, t s.,3) Z ,tK 24 L 2 V 4 J I C o ) = /^_C^/1 + yi^ Tj (3-1+) p = / t ? / IT + 1 \ r v/ T24C I X fc / and assuming t h a t the exchange r a t e , X, i s much l e s s than the r e c o m b i n a t i o n r a t e , k, one has, * c = * ( § ) However, t h e r e a r e two main d i f f i c u l t i e s w i t h t h i s p a per; f i r s t , t h e vapour p r e s s u r e of the i o d i n e r e p o r t e d i n 48 t h i s paper c o r r e s p o n d s t o a temperature of about 22°C, v e r y c l o s e t o room t e m p e r a t u r e . T h i s i s u n d e s i r a b l e as some c r y s t a l s of i o d i n e may form i f the temperature drops. Such c r y s t a l s c o u l d a c t b o t h as a s o u r c e of i o d i n e and as a s i t e f o r s c r a m b l i n g of o r t h o and p a r a i o d i n e b o t h of which are bad from the p o i n t of view of an enhancement. Second, i f one uses the data p r o v i d e d by the a u t h o r s and s u b s t i t u t e s t h i s i n t o the f o r m u l a f o r the f l u o r e s c e n c e i n t e n s i t y of the o r t h o - i o d i n e a t t=0 t o t=» one o b t a i n s , l l L l S J - (3.16) Lf£ a t a l a s e r power of 0.4 W i n c o n t r a d i c t i o n w i t h the observed v a l u e of 1.7. Another paper by V. S. Letokhov and V. A. S e m c h i s h e n 3 9 appeared i n 1974 i n which the work of Badger and Urmston was b u i l t upon. A t e s t c e l l 4 c e n t i m e t e r s i n l e n g t h and 3 cm 3 i n volume was p r e p a r e d w i t h 0.2 t o r r of i o d i n e vapour and 2.8 t o r r of 2-hexene. As b e f o r e , t h e r e a c t i o n was d r i v e n by the 5145 A l i n e of an argon i o n l a s e r . T h i s t i m e , p e r i o d i c a l l y d u r i n g the e x p e r i m e n t , the argon i o n 5017 A l i n e was sent t h r o u g h the t e s t c e l l t o m o n i t o r the p a r a - i o d i n e c o n c e n t r a t i o n as t h i s f r e q u e n c y e x c i t e s the B ^ g T j ^ X 1 ^ (62'-0") P(26) t r a n s i t i o n . The r e s u l t s of t h i s experiment ar e shown i n f i g u r e 3.1.(taken from r e f e r e n c e 2) N o t i c e t h a t the o r t h o - i o d i n e f l u o r e s c e n c e decreased over a p e r i o d of an hour t o r o u g h l y 5 t o 6 % of the o r i g i n a l 49 tK K to V e id. nits 1 z i V l * 1 F i g u r e 3.1 The above p l o t of the f l u o r e s c e n c e induced by the 5145 A , ( 1 ) , and the 5017 A , ( 2 ) , argon i o n l a s e r l i n e s i n a m i x t u r e of 0.2 t o r r of i o d i n e and 2.8 t o r r of 2-hexene i s t a k e n from r e f e r e n c e 2. The 5145 A l i n e e x c i t e s p r i m a r i l y o r t h o - i o d i n e w h i l e the 5017 A e x c i t e s p r i m a r i l y p a r a - i o d i n e . 50 s i g n a l w h i l e the p a r a - i o d i n e f l u o r e s c e n c e remained c o n s t a n t d u r i n g t h i s t i m e . The mechanism proposed f o r the r e a c t i o n was a d i r e c t a d d i t i o n of an e x c i t e d i o d i n e m o l e c u l e t o the 2-hexene and no r a d i c a l c h a i n f o r m a t i o n . I f , in d e e d , a r a d i c a l c h a i n were the p r o c e s s by which t h e two s p e c i e s r e a c t e d then i o d i n e atoms would be formed and by analogy w i t h the case of hydrogen, th e s e atoms would be e x p e c t e d t o e f f i c i e n t l y s c ramble the m o l e c u l e s . Hence, based on these r e s u l t s , an enrichment f a c t o r of almost 100% c o u l d be r e a l i z e d by t h i s t e c h n i q u e . A f i n a l paper on t h i s s u b j e c t was produced by V. I . B a l y k i n , V. S. L e t o k h o v , V. I . M i s h i n , and, V. A. Sernchishen 3 i n 1976 which r e v i e w e d the p r e v i o u s r e s u l t s and i n some i n s t a n c e s c o n t r a d i c t e d p r e v i o u s f i n d i n g s . (For example, the c l a i m t h a t a s h i f t i n the o r t h o t o para r a t i o i s o b s e r v a b l e i n a c e l l f i l l e d w i t h 250 mtorr of i o d i n e i s r e f u t e d and, i n the case of i o d i n e p l u s 2-hexene, the o b s e r v a t i o n t h a t the p a r a - i o d i n e f l u o r e s c e n c e remained c o n s t a n t was a l s o c o n t r a d i c t e d . ) The t e c h n i q u e i n v o l v e d i n the ex p e r i m e n t s have p r e v i o u s l y been d e s c r i b e d and r e l i e d upon s e l e c t i v e l a s e r a c t i o n t o d r i v e the p r o c e s s . In t h e s e e x p e r i m e n t s a molybdenum g l a s s c e l l 12.5 cm l o n g w i t h 6 cm 3 volume, and 20 cm 2 s u r f a c e a r e a was e v a c u a t e d t o about IO'* t o r r and then f i l l e d w i t h the d e s i r e d c o n s t i t u e n t s . 51 P(Mro»w» F i g u r e 3.3 The f l u o r e s c e n c e decay of i o d i n e c e l l s as a f u n c t i o n of i n i t i a l i o d i n e p r e s s u r e . ( f r o m r e f e r e n c e 3) THEORY 100 190 TO-fiM.stl F i g u r e 3.2 F l u o r e s c e n c e i n d u c e d by the 5145 A and 5017 A argon l a s e r l i n e s r e p o r t e d by Letokhov et a l i n r e f e r e n c e 3 o b s e r v e d i n a c e l l c o n t a i n i n g 5 mt o r r of i o d i n e vapour. PiMlOflR) F i g u r e 3.4 The Enrichment c o e f f i c i e n t f o r o r t h o - t o p a r a - i o d i n e as a f u n c t i o n of i n i t i a l i o d i n e p r e s s u r e . N o t i c e t h a t a t low p r e s s u r e s enrichment c o e f f i c i e n t s of up t o 2 are c l a i m e d by Letokhov e t a l w h i l e above about 80 mtorr no enrichment i s o b s e r v e d , (from r e f e r e n c e 3) 52 The r e s u l t s of the i o d i n e vapour a l o n e i n the c e l l a r e summarized c o n c i s e l y on f i g u r e s 3.2 t o 3.5 ta k e n from r e f e r e n c e 3. The f l u o r e s c e n c e of the o r t h o and para s i g n a l s both decay but by d i f f e r e n t amounts. An enrichment c o e f f i c i e n t , E, d e f i n e d by, o - !».(«>) t p - I t co) where o - I 2 ( t ) and p - I 2 ( t ) a r e the o r t h o - i o d i n e and p a r a - i o d i n e c o n c e n t r a t i o n s a t time " t " . As can be seen, above 80 mtorr of i n i t i a l i o d i n e p r e s s u r e no enrichment i s . o bserved and • the enrichment i n c r e a s e s as one d e c r e a s e s the i n i t i a l i o d i n e vapour p r e s s u r e . However, as p r e v i o u s l y s t a t e d , t h i s i s i n c o n t r a d i c t i o n w i t h the o r i g i n a l paper on the s u b j e c t as t o the a b i l i t y t o o b t a i n an enhancement at h i g h i o d i n e p r e s s u r e and the mechanism i n v o l v e d . The mechanism p r e s c r i b e d by Letokhov f o r the system i n v o l v e d h a v i n g t h e atoms produced by s e l e c t i v e p r e d i s s o c i a t i o n a d s o r b onto the w a l l s of t h e t e s t c e l l . Hence, t h i s mechanism e f f e c t i v e l y removes the atoms from the system t o some e x t e n t . The p r o c e s s does not pr o c e e d 100 p e r c e n t as the atoms, a r e a b l e t o come o f f the w a l l s a g a i n and scramble the i o d i n e m o l e c u l e s t h a t s t r i k e the adsorbed atoms a c c o r d i n g t o the a u t h o r s . 53 F o l l o w i n g L etokhov's d e s c r i p t i o n of the system one has t h e p a ramters : N, the o r t h o i o d i n e c o n c e n t r a t i o n , n, the p a r a - i o d i n e c o n c e n t r a t i o n , m, the vapour phase i o d i n e atom c o n c e n t r a t i o n , m', the c o n c e n t r a t i o n of a d s orbed i o d i n e atoms, a, the p r e d i s s o c i a t i o n r a t e c o n s t a n t , 0, the o r t h o - p a r a s c r a m b l i n g r a t e c o n s t a n t , 6, the i o d i n e atom r e c o m b i n a t i o n r a t e , K, the a d s o r p t i o n r a t e c o n s t a n t , and, K', the d e s o r p t i o n r a t e c o n s t a n t . W i t h th e s e so d e f i n e d the r e a c t i o n mechanism f o r the i o d i n e vapour system i s , N — - 2m m - » m' m' + N n + m' m — * - i N , ^ n The m a t h e m a t i c a l d e s c r i p t i o n p r e s e n t e d by the a u t h o r s r e q u i r e the assumptions t h a t (1) the f l u o r e s c e n c e s i g n a l decay i s due s o l e l y t o the d i s a p p e a r a n c e of i o d i n e m o l e c u l e s , (2) the s c r a m b l i n g of o r t h o - and p a r a - i o d i n e o c c u r s p r i m a r i l y on the w a l l s , (3) the number of i o d i n e atoms i s a l m o s t e q u a l t o the number adsorbed on t h e w a l l s , and (4) the i o d i n e atoms a r e e f f e c t i v e l y scavenged by the w a l l s . These assumptions p r o b a b l y h o l d v e r y w e l l a t low i n i t i a l p r e s s u r e s of i o d i n e vapour but may break down as the p r e s s u r e i s i n c r e a s e d . I f one a c c e p t s the above d e s c r i p t i o n then a g i v e n c e l l w i l l have N c a d s o r p t i o n s i t e s . At low i n i t i a l p r e s s u r e s , 5 54 mtorr f o r example, most of the i o d i n e atoms produced a r e a b l e t o s t i c k t o the w a l l s . Thus the s c r a m b l i n g of o r t h o - i o d i n e and p a r a - i o d i n e o c c u r s o n l y upon the c o l l i s i o n of a m o l e c u l e w i t h an adsorbed atom. From t h i s v i e w p o i n t the o r t h o - i o d i n e f l u o r e s c e n c e s h o u l d decay r a p i d l y a t f i r s t w h i l e the p a r a - i o d i n e c o n v e r s i o n s h o u l d be s l o w e r and a net enrichment i s e x p e c t e d . As t h e p r e s s u r e i s i n c r e a s e d , r o u g h l y the same pe r c e n t a g e of i o d i n e atoms a r e expected t o be produced. However, t h e a b s o l u t e number of a v a i l a b l e a d s o r p t i o n s i t e s remain c o n s t a n t . The " e x t r a " atoms formed remain i n the vapour phase and c o l l i s i o n a l l y r e l a x any enrichment t h a t had been o b t a i n e d r a t h e r q u i c k l y . U s i n g the e q u a t i o n s p r o v i d e d by Letokhov e t a l one may e s t i m a t e t h e s c r a m b l i n g c o e f f i c i e n t f o r the atoms on the m o l e c u l e s as f o l l o w s . At a 5 mt o r r i n i t i a l p r e s s u r e the c o n c e n t r a t i o n of adsorbed atoms i s , m'5 3.52*10" 1 0 moles/cm 3 w h i l e the o r i g i n a l c o n c e n t r a t i o n s of the o r t h o and pa r a m o l e c u l e s a r e , N 5 0 « 1.70«10" 1° moles/cm 3 , and, n 5 0 -1.22•10" 1°moles/cm 3. From the d a t a s u p p l i e d on f i g u r e 3.4 the f i n a l v a l u e s of t h e s e s p e c i e s a r e , N 5 = 0.36*10" 1 0 moles/cm 3 n 5 = 0.73*10" 1 0 moles/cm 3 and t h i s makes the f r e e i o d i n e atom c o n c e n t r a t i o n a t e q u i l i b r i u m , m5 * 3.52*10" 1 8 moles/cm 3. ( E f f e c t i v e l y z e r o ) At an 80 mtorr i n t i a l p r e s s u r e s i m i l a r c a l c u l a t i o n s show, assuming t h a t t h e number of adsorbed i o d i n e atoms remains c o n s t a n t a t the p r e v i o u s c o n c e n t r a t i o n , t h a t the 55 e q u i i b r i u m v a l u e s of the o r t h o - i o d i n e , p a r a - i o d i n e , and i o d i n e atoms a r e N 8 0 =* 2.18 -1 0 _ 9 moles/cm 3, n 8 0 1.56 • 1 0 _ 9 moles/cm 3 and, m 8 0 ^ 1.15-10 - 9 moles/cm 3. Thus the s c r a m b l i n g r a t e of the atoms may be e s t i m a t e d from t h e s e f a c t s and assuming t h a t the s c r a m b l i n g obeys a f o r m u l a of the t y p e , (n f c - n^) = ( n t - n.jEXP- (kmt) where k i s the s c r a m b l i n g r a t e , m i s the atom c o n c e n t r a t i o n i n the vapour phase, and t i s the t i m e . Hence, a h a l f l i f e -CL C= I n 2 k m may be d e f i n e d . In the case of the 80 mtorr c e l l the h a l f l i f e must be l e s s than 10 seconds as no o r t h o t o p a r a s h i f t was o b s e r v e d at a l l . Thus, from e q u a t i o n (3.20) k = 6•10 7cm 3/(mole s) which i s the same o r d e r of magnitude as the r a t e c o n s t a n t f o r the analogous r e a c t i o n i n hydrogen [1.5•107cm3/(mole s0-As e x p e c t e d , the a b i l i t y of i o d i n e atoms t o scramble any o r t h o - p a r a enhancement i s c o n s i d e r a b l e . Of c o u r s e the o b v i o u s way t o get around t h i s problem i s t o add a scavenger a l a Badger and Urmston. In the same paper, Letokhov e t a l r e p o r t e d on the p a r a - i o d i n e t o o r t h o - i o d i n e enhancement o b s e r v e d when 2-hexene was used as a scavenger f o r the s e l e c t i v e l y e x c i t e d o r t h o m o l e c u l e s . The a u t h o r s v a r i e d l a s e r power, c o n c e n t r a t i o n s of i o d i n e , and 5 6 2-hexene as summarized i n f i g u r e s 3.5 and 3.6.(taken from r e f e r e n c e 3) 57 fe i a ! oo 200 fj,(MT0J?») 300 F i g u r e 3.5 Enrichment c o e f f i c i e n t dependence upon the i n i t i a l i o d i n e p r e s s u r e i n systems of m i x t u r e s of i o d i n e and 2-hexene a f t e r 10 t o 12 minutes of i r r a d i a t i o n w i t h an argon i o n l a s e r . A g a i n , as the i o d i n e p r e s s u r e d e c r e a s e s , the enrichment i n c r e a s e s , (from r e f e r e n c e 3) F i g u r e 3.6 Dependence of the i n i t i a l r a t e of change of the o r t h o - i o d i n e c o n c e n t r a t i o n as a f u n c t i o n of l a s e r i n t e n s i t y , i n i t i a l i o d i n e c o n c e n t r a t i o n , and the square r o o t of the 2-hexene p r e s s u r e , (from r e f e r e n c e 3) 58 The u s u a l mechanism f o r such a r e a c t i o n i s v i a a r a d i c a l c h a i n as shown below. o - I 2 — - 2 1 Pc, I + X ==• XI . *-«• XI + I 2 -=r XI2 + I K», k-3 1 — * I 2 ^ 21 + M «- I 2 + M k & Under the ass u m p t i o n s t h a t ( 1 ) the l a s e r i n d uced f l u o r e s c e n c e i s c o m p l e t e l y i n d i c a t i v e of the amount of o r t h o or p a r a i o d i n e p r e s e n t ; once t h e l a s e r i s t u r n e d on ( 2 ) the i o d i n e atom c o n c e n t r a t i o n and the c o n c e n t r a t i o n of the s p e c i e s XI" a r e c o n s t a n t ( 3 ) the r e v e r s e r e a c t i o n 3 above i s n e g l i g i b l e , ( 4 ) c h a i n r u p t u r e o c c u r s v i a i o d i n e f b r m a t i o n a t the w a l l s , then one may e x p r e s s the r a t e e q u a t i o n s f o r each s p e c i e s as d ( I ) a ZPa. - ka(x)(x) + k-z (xi) + M X I K I J - k 4u) d t d(xi) = kx<x)(i.) - k - z ( X i ) - Mx i ) l i 2 ) (3.2x) c i t d j x i z ) * kaCxxKia.) ( 5 a 3 ) d t 59 S e t t i n g e q u a t i o n s 3.21 and 3.22 t o z e r o one has ( x i ) j - K t U H i j and, ( 1 ) 1 * I fk (3.7th) So t h a t , d ( X I * ) | =r 2 R , k . ( X ) ( l « ) d t i k 4 [ i 4 + k -z /k * , l which f i n a l l y y i e l d s T h i s i s i n disagreement w i t h Letokhov's statement t h a t d t J d t I I t i s i m p o r t a n t t o make t h i s d i s t i n c t i o n . Based upon the d a t a p r e s e n t e d i n f i g u r e s 3.6 and 3.7 Letokhov e t a l s t a t e t h a t the r a d i c a l c h a i n mechanism i s not the c o r r e c t i n t e r p r e t a t i o n as h i s da t a shows, d O - T . . ) ~ I L F C A / > ( 3 - 2 6 ) d t which i s not the same beh a v i o u r as e q u a t i o n ( 3 . 2 6 ) . The r e a c t i o n mechanism then remains u n c e r t a i n . I t i s n o t a b l e t h a t h i g h enrichment was observed e s p e c i a l l y as the i o d i n e p r e s s u r e was d e c r e a s e d . T h i s 60 i n c r e a s e i n enrichment w i t h d e c r e a s e i n p r e s s u r e may i n f a c t be due t o a r a d i c a l c h a i n mechanism. As one d e c r e a s e s the amount of i o d i n e p r e s e n t one a l s o d e c r e a s e s the c o n c e n t r a t i o n s of the s p e c i e s such as XI and I which w i l l s cramble the enhancement. T h e r e f o r e , i t i s p o s s i b l e t h a t two or more mechanisms a r e a t work s i m u l t a n e o u s l y . V. S. Letokhov e t a l have p r e s e n t e d some p r o m i s i n g , a l t h o u g h somewhat c o n t r a d i c t o r y , e v i d e n c e t h a t the r a t i o of p a r a - i o d i n e t o o r t h o - i o d i n e may be s h i f t e d away from the n a t u r a l v a l u e of 5:7 both i n a system of i o d i n e vapour a l o n e and, t o a l a r g e r e x t e n t , i n a system of i o d i n e and 2-hexene v i a s e l e c t i v e l a s e r e x c i t a t i o n . IV. EXPERIMENTAL RESULTS . I t i s s u r p r i s i n g t h a t a f t e r h a v i n g o b t a i n e d some enhancement i n the p a r a - i o d i n e t o o r t h o - i o d i n e r a t i o the r e v e r s e mechanism, i . e . the r e l a x a t i o n of t h i s q u a n t i t y t o the n a t u r a l v a l u e , was not s t u d i e d by Letokhov e t a l . The purpose of t h i s i n v e s t i g a t i o n was t o reproduce and perhaps improve upon the R u s s i a n s ' r e s u l t s and t o study the r e c o n v e r s i o n r a t e of o r t h o - and p a r a - i o d i n e by themselves and i n t h e presence of some paramagnetic s p e c i e s a l a F a r k a s . The t e c h n i q u e employed t o produce an o r t h o t o para r a t i o s h i f t was v e r y s i m i l a r t o t h a t of Letokhov w i t h the main d i f f e r e n c e b e i n g the method of m o n i t o r i n g the c o n c e n t r a t i o n s of t h e s e two s p e c i e s . A Coherent CR-15SG argon i o n l a s e r tuned t o the 5145 A l i n e was used t o s e l e c t i v e l y e x c i t e the o r t h o - i o d i n e m o l e c u l e s . ( B 3 n r , + X 1Z +  J 0*u g 43'- 0" P(13) and R ( l 5 ) t r a n s i t i o n s ) P e r i o d i c a l l y d u r i n g the p r o c e s s t h e argon i o n l a s e r beam was r e d i r e c t e d t o power a Coherent CR-699-21 s c a n n i n g dye l a s e r . The dye l a s e r beam was sent t h r o u g h the t e s t c e l l and the f r e q u e n c y scanned a c r o s s a g i v e n s p e c t r a l r e g i o n of i o d i n e . W i t h the a i d of an i o d i n e a t l a s 2 8 , the r o t a t i o n a l and v i b r a t i o n a l c o n s t a n t s of i o d i n e s u p p l i e d by H e r z b e r g " and some p r e d i s s o c i a t i o n d a t a o b t a i n e d by J . V i g u 6 9 , a r e g i o n which s a t i s f i e d the f o l l o w i n g c r i t e r i a c o u l d be found: the r e g i o n chosen s h o u l d (1) a f f o r d good dye l a s e r o utput power (100 mW or b e t t e r ) , (2) c o n t a i n a t l e a s t one o r t h o - i o d i n e and one p a r a - i o d i n e 61 62 s p e c t r a l l i n e b e l o n g i n g t o the same v i b r a t i o n a l t r a n s i t i o n , (3) the s p e c t r a l l i n e s i n the r e g i o n s h o u l d not be too c l u t t e r e d and b a d l y o v e r l a p p e d , and, (4) t h e t r a n s i t i o n s used t o m o n i t o r the o r t h o and para c o n c e n t r a t i o n s s h o u l d induce f a i r l y low p r e d i s s o c i a t i o n so as t o d i s t u r b the system as l i t t l e as p o s s i b l e . Once chosen, the f l u o r e s c e n c e of the i o d i n e due the argon i o n beam and the dye l a s e r beam was m o n i t o r e d w i t h an EMI 9558QB p h o t o m u l t i p l i e r and r e c o r d e d on a s t r i p c h a r t r e c o r d e r . The time dependence of t h e r a t i o of an o r t h o - i o d i n e t o p a r a - i o d i n e peak h e i g h t would ( t o f i r s t o r d e r ) i n d i c a t e whether or not any enhancement was o c c u r r i n g . With the above t e c h n i q u e i n mind, the remainder of t h i s c h a p t e r c o n t a i n s i n depth d e s c r i p t i o n s of each of the i n v e s t i g a t i o n s u n d e r t a k e n . A. IODINE VAPOUR EXPERIMENTS 1. SINGLE LASER BEAM IRRADIATION TECHNIQUE Based upon t h e r e s u l t s of Letokhov e t a l the g r e a t e s t enrichment f a c t o r s were o b t a i n e d w i t h low i n i t i a l p r e s s u r e s of i o d i n e v a p our. F u r t h e r , p r e v i o u s i n v e s t i g a t i o n s of F a r k a s i n t o m o l e c u l a r hydrogen i n d i c a t e d t h a t paramagnetic i m p u r i t i e s and e x c e s s i v e numbers of atoms c o u l d pose d i f f i c u l t i e s f o r t h i s e x p e r i m e n t . Hence, the f i r s t e x p e r i m e n t s i n v o l v i n g i o d i n e vapour a l o n e were c a r r i e d out 63 w i t h low p r e s s u r e s of i o d i n e vapour (about 3 mtorr) i n a s e a l e d c e l l . A c y l i n d r i c a l t e s t c e l l of l e n g t h 30 cm and volume 79 cm 3 was ev a c u a t e d on a g r e a s e l e s s gas m a n i f o l d equipped w i t h t e f l o n s t o p c o c k s t o a p r e s s u r e of r o u g h l y 2 x 1 0 - 5 t o r r . The c e l l was then f i l l e d w i t h the d e s i r e d p r e s s u r e of i o d i n e vapour by p l a c i n g a c o n s t a n t t e m p e r a t u r e bath around a c o n t a i n e r h o l d i n g i o d i n e c r y s t a l s (BDH a s s u r e d , a n a l y t i c a l r eagent grade, 99.99% pure) and a l l o w i n g the vapour t o d i f f u s e i n t o the t e s t c e l l . Having t h u s l y p r e p a r e d a t e s t sample, the c e l l was p i n c h e d o f f of the m a n i f o l d and set i n a c o n s t a n t temperature b a t h i n the a p p a r a t u s shown i n f i g u r e 4.1. A Coherent argon io r i l a s e r was used t o d r i v e a Coherent' CR-699-21 dye l a s e r which scanned a c r o s s the B 3n Q^g» * " X 1 I g 14'- 1" P(77) and R(82) s p e c t r a l l i n e s of m o l e c u l a r i o d i n e . The sweep speed was s e t t o 10 GHz i n 25 seconds and the l i n e s rescanned 5 t o 10 t i m e s b e f o r e i n i t i a t i n g the experiment t o g i v e an i n i t i a l r a t i o of the o r t h o - i o d i n e t o p a r a - i o d i n e c o n c e n t r a t i o n . Next, p r i s m P1 (see f i g u r e 4.1) was i n s e r t e d t o d e f l e c t the argon i o n l a s e r beam through the t e s t c e l l t o b e g i n the s e l e c t i v e o r t h o - i o d i n e p r e d i s s o c i a t i o n . N o t i c e t h a t b e f o r e e n t e r i n g the t e s t c e l l the argon i o n beam was expanded from about a 2 mm t o an 8 mm di a m e t e r so as t o i r r a d i a t e as much of the t e s t c e l l ' s volume as p o s s i b l e . The power of t h i s beam, a f t e r e x p a n s i o n , e n t e r i n g the c e l l was t y p i c a l l y between 3 and 4 w a t t s . |WV\AA ARGON ION LASER i PRISM, P2 PRISM PI v (REMOVABLE)\ BEAM EXPANDER CONSTANT TEMP. , WATER BATH EMI 9558 QB - i jfPHOTOMULTIPLIER CORNING CS2-62 FILTER DYE LASER TEST CELL TO STRIP CHART RECORDER F i g u r e 4.1. S i n g l e l a s e r beam i r r a d i a t i o n e x p e r i m e n t a l arrangement f o r the s e l e c t i v e e x i c i t a t i o n of o r t h o - i o d i n e . T e s t c e l l s c o n t a i n i n g i o d i n e vapour were p l a c e d i n a c o n s t a n t t e m p e r a t u r e water b a t h and, w i t h p r i s m PI i n p l a c e , i r r a d i a t e d w i t h the argon i o n l a s e r beam. By removing t h i s p r i s m , t h e dye l a s e r was powered and a g i v e n s p e c t r a l r e g i o n of m o l e c u l a r i o d i n e c o u l d be m o n i t o r e d . Note t h a t b oth f l u o r e s c e n c e s were r e c o r d e d u s i n g an EMI 9558QB p h o t o m u l t i p l i e r s h i e l d e d by a C o r n i n g CS2-62 r e d pass f i l t e r t o e l i m i n a t e t h e s i g n a l due t o s c a t t e r e d l a s e r l i g h t . 65 As the c h a r a c t e r i s t i c t i m e s of the r a t i o s h i f t were c l a i m e d t o be of the o r d e r of 150 t o 200 m i n u t e s by Letokhov et a l , the argon i o n beam was sent t o t h e dye l a s e r a t i n t e r v a l s of r o u g h l y one hour and the o r i g i n a l s p e c t r a l r e g i o n was r e r e c o r d e d . T h i s r e s c a n n i n g p e r i o d took between f i v e and t e n minutes a f t e r which time the arg o n i o n l a s e r beam was sent through the c e l l once more. Some t y p i c a l r e s u l t s o b t a i n e d w i t h a c e l l c o n t a i n i n g (3.75±0.15) mtorr of i o d i n e vapour i n i t i a l l y a r e summarized i n T a b l e s 4.1 t o 4.3 and i n f i g u r e s 4.2 and 4.3. TABLE 4.1 E x p e r i m e n t a l C o n d i t i o n s BACKGROUND PRESSURE INITIAL IODINE PRESSURE ARGON ION LASER POWER DYE LASER POWER WATER BATH TEMPERATURE DURATION OF EXPERIMENT DYE LASER MONITOR LINES 3 x l 0 " 5 t o r r (3.75±0.15)mtorr (3.9±0.1)W 80mW (27±1)°C (125±5)minutes B X (14'- 0") P(77),R(82) j>=*1 7034 cm" 166 a (x5) J t«0 t~60min U40) b t~l20min F i g u r e 4.2. T y p i c a l appearance of the 14'-1 B P(77) [peak ( a ) ] and the U ' - l " R(82) [ p e a k ( b ) ] s p e c t r a l l i n e s a f t e r h a v i n g i r r a d i a t e d the t e s t c e l l w i t h the 5145 A argon i o n l a s e r beam f o r the amount of time i n d i c a t e d below each spectrum, ( i . e . t=0 i m p l i e s b e f o r e any argon i o n i r r a d i a t i o n , t«*60 min. i s a f t e r an hour of such i r r a d i a t i o n , and, t»l20 min. i s t h e same r e g i o n a f t e r abour two hours of net argon i o n l a s e r i r r a d i a t i o n . ) 67 TABLE 4.2 Argon Ion and Dye L a s e r Induced F l u o r e s c e n c e v e r s u s Time TIME Ar* ( t ) [ 1 4 ' - I n P ( 7 7 ) ] [14'-1" R ( 8 2 ) ] (min.) A r " ( 0 ) [14!-1" P ( 7 7 ) ] [14'-1" R ( 8 2 ) ] 0 1.0 1.0 1.0 60±2 0.120±.008 0.136±0.003 0.13710.003 125±5 0.0321.007 0.038±0.002 0.039±0.002 TABLE 4.3 O r t h o and Para Peak H e i g h t s and R a t i o s v e r s u s Time TIME P(77) R(82) P(77) (min.) (arb.) ( a r b . ) R(82) 0 • 203.1±1.6 148.4±1.8 1.37±0.01 60±2 127.61.4 '"" 20.310.2 1 .3710.01 12515 7.810.3 5.810.2 1.3410.04 From t h e s e r e s u l t s one i m m e d i a t e l y o b s e r v e s t h a t a l t h o u g h t h e argon i o n in d u c e d f l u o r e s c e n c e d e c r e a s e s t o about 3 p e r c e n t of the i n i t i a l v a l u e as r e p o r t e d by Letokhov et a l , t h e 14'-1" P(77) and R(82) peak h e i g h t s both d e c r e a s e by t h i s amount a l s o over the c o u r s e of the e x p e r i m e n t . Hence, no enrichment of p a r a - i o d i n e has been o b s e r v e d . T h i s i s i n c o n t r a d i c t i o n w i t h L e t o k h o v ' s c l a i m t h a t an enrichment f a c t o r of r o u g h l y 2 i s t o be e x p e c t e d a t t h i s i n i t i a l p r e s s u r e . 68 F i g u r e 4.3. P l o t of t h e " f l u o r e s c e n c e i n d u c e d i n a (3.75±0.15)mtorr i o d i n e vapour c e l l (1.9 cm d i a m e t e r , 30 cm l e n g t h ) by the 5145 A l i n e of the argon i o n l a s e r beam, (O), and the r a t i o of t h e 14'-2" P(77) t o the 14'-1" R(82) peaks, ( O ) , over the c o u r s e of the ex p e r i m e n t . As the e x p e r i m e n t a l arrangement a l l o w e d o n l y one l a s e r beam e n t e r the c e l l a t a t i m e , the p e r i o d s when the i o d i n e was exposed t o the dye l a s e r beam but not the argon i o n beam a r e i n d i c a t e d by t h e shaded r e g i o n s . 69 The immediate q u e s t i o n a r i s e s , what a r e the d i f f e r e n c e s between t h e s e e x p e r i m e n t s and t h o s e performed p r e v i o u s l y ? Perhaps the d i f f e r e n c e was i n the argon i o n l a s e r used. I t i s p o s s i b l e t h a t the l a s e r employed here t o d r i v e the r e a c t i o n was s u f f i c i e n t l y broad t h a t a p p r e c i a b l e amounts of p a r a - i o d i n e were b e i n g e x c i t e d a l o n g w i t h t h e o r t h o - i o d i n e . To t e s t t h i s a f l u o r e s c e n c e c e l l was was p l a c e d i n f r o n t of a McPherson 0.75 meter g r a t i n g monochrometer w i t h a 30,000 l i n e p e r i n c h g r a t i n g used i n second o r d e r . The c e l l was i r r a d i a t e d w i t h the 5145 & l a s e r beam and, as the g r a t i n g was r o t a t e d , the s i g n a l coming out of the monochrometer was r e c o r d e d w i t h a p h o t o m u l t i p l i e r on a s t r i p c h a r t r e c o r d e r . The r e s u l t s of t h i s d i a g n o s t i c check a r e shown i n f i g u r e 4.4 which i n d i c a t e s t h a t p r i m a r i l y the o r t h o - m o l e c u l e s are b e i n g e x c i t e d by t h i s l a s e r r a d i a t i o n . Another d i f f e r e n c e between the two e x p e r i m e n t s l i e s i n the d i a m e t e r s of t h e t e s t c e l l s used. Letokhov et a l employed c e l l s of d i a m e t e r r o u g h l y 0.64 cm w h i l e i n the e x p e r i m e n t s performed by t h i s a u t h o r the c e l l ' s d i a m e t e r was 1.9 cm. From some s i m p l e gas k i n e t i c a n a l y s i s a t 3 mtorr i n i t i a l p r e s s u r e th e mean f r e e p a t h of an i o d i n e atom i s r o u g h l y , d = ( N a ) " 1 where N i s the number of p a r t i c l e s per cm 3, and a i s the c o l l i s i o n a l c r o s s - s e c t i o n between i o d i n e atoms and 70 CVJ ro O ro CVJ in CVJ ro cvj o CVJ CVJ cvj F i g u r e 4.4. The f l u o r e s c e n c e spectrum of an i o d i n e c e l l (about 150-200 m t o r r ) e x c i t e d by a Coherent CR-15SG argon i o n l a s e r tuned t o t h e 5145 A as r e c o r d e d w i t h a McPherson 0.75 m g r a t i n g monochromator equipped w i t h a 30,000 l i n e per i n c h g r a t i n g . The above t r a c e d i s p l a y s the 43'-7" band o b s e r v e d i n second o r d e r on the g r a t i n g . The lower numbers d i s p l a y s t h e w a v e l e n g t h s c a l e w h i l e above the peaks the l i n e assignment i s g i v e n . N o t i c e t h a t a l l t h e l i n e s c o r r e s p o n d t o odd r o t a t i o n a l t r a n s i t i o n s i m p l y i n g t h a t p r i m a r i l y t h e o r t h o - i o d i n e m o l e c u l e s a r e b e i n g e x c i t e d . o 71 m o l e c u l e s . As t h i s c r o s s - s e c t i o n i s unknown l e t i t be assumed t o be about the s i z e of t h e i o d i n e m o l e c u l e (about 2 A 2 ) so t h a t the mean f r e e p a t h becomes 24 cm. Hence i f the above a s s u m p t i o n i s t r u e then any i o d i n e atom produced w i l l s t r i k e the w a l l b e f o r e ever e n c o u n t e r i n g an i o d i n e m o l e c u l e i n e i t h e r c e l l . However, i f i t i s wrong and the mean f r e e p a t h i s o n l y of the o r d e r of 0.5 cm (or a = l 90A 2) then i t c o u l d be t h a t the i o d i n e atoms are e x t r e m e l y e f f i c i e n t a t s c r a m b l i n g the o r t h o and para s p e c i e s r e s u l t i n g i n no observed s h i f t i n t h e n a t u r a l r a t i o . I t i s a l s o p o s s i b l e t h a t t h e m a t e r i a l used t o c o n s t r u c t the t e s t c e l l s p l a y s a major r o l e i n t h e s e e x p e r i m e n t s . The c e l l s used by t h i s a u t h o r were pyrex w h i l e those of Letokhov et a l were made of molybdenum g l a s s . U n f o r t u n a t e l y , our g l a s s b l o w e r was unable t o o b t a i n any i n f o r m a t i o n about t h i s m a t e r i a l and as a r e s u l t t h e t e s t c e l l s had t o be made of p y r e x . I t i s c o n c e i v a b l e t h a t molybdenum g l a s s i s a much b e t t e r scavenger than pyrex f o r i o d i n e atoms and, hence, a net e f f e c t may not be p o s s i b l e w i t h p y r e x . The power d e n s i t i e s of t h e beam used i n each case were s l i g h t l y d i f f e r e n t a l s o . L e tokhov e t a l employed a 2.5 mm, 2.2 watt l a s e r beam w h i l e t h i s a u t h o r used an 8 mm, 4 watt beam. T h i s c o r r e s p o n d s t o t h e power d e n s i t y of Letokhov's l a s e r b e i n g about s i x t i m e s g r e a t e r than t h a t used i n the e x p e r i m e n t s p r e v i o u s l y r e p o r t e d h e r e . T h i s may have changed t h e c h a r a c t e r i s t i c time of t h e r e a c t i o n from 150 t o 900 m i n u t e s . Thus, a f t e r 120 m i n u t e s of i r r a d i a t i o n w i t h the 72 lower power d e n s i t y one might expect t o see the r e s u l t s o b t a i n e d by Letokhov a f t e r about 20 mi n u t e s . From r e f e r e n c e ( 3 ) , one has an enrichment f a c t o r of about 1.32 a f t e r 20 minutes of i r r a d i a t i o n or r o u g h l y a 25% s h i f t i n the o r t h o - i o d i n e t o p a r a - i o d i n e r a t i o . To w i t h i n a 5% u n c e r t a i n t y , no such s h i f t was ob s e r v e d . F i n a l l y , t he p r e s s u r e t o which the t e s t • c e l l was eva c u a t e d b e f o r e use was about 2x10' 5 t o r r here v e r s u s about 1 0 _ * t o r r f o r Le t o k h o v . I t would seem l o g i c a l t o assume t h a t the lower t h i s v a l u e the b e t t e r the r e s u l t s s h o u l d be as t h i s reduces the p o s s i b i l i t y of c o n t a m i n a t i o n of the c e l l by paramagnetic s p e c i e s . At w o r s t , Letokhov e t a l c o u l d have had about 2% c o n t a m i n a t i o n i n t h e i r c e l l s when the i n i t i a l i o d i n e p r e s s u r e was 5 mtorr w h i l e the c o n t a m i n a t i o n i n t h i s i n v e s t i g a t i o n would have been about 0.7%. ( T h i s , of c o u r s e , assumes t h a t no o t h e r d e g a s i n g of the c e l l t a k e s p l a c e d u r i n g an expe r i m e n t ) To summarize, t h e main d i f f e r e n c e s between Letokhov's work and t h i s f i r s t s e r i e s of e x p e r i m e n t s were (1) c e l l m a t e r i a l and d i m e n s i o n s , (2) t h e argon i o n l a s e r power d e n s i t y i n the c e l l , (3) the background p r e s s u r e i n the t e s t c e l l s b e f o r e f i l l i n g , and (4) t h e method of m o n i t o r i n g the o r t h o - and p a r a - i o d i n e c o n c e n t r a t i o n s . 2. DUAL BEAM IRRADIATION TECHNIQUE Another s e t of e x p e r i m e n t s w i t h i o d i n e vapour t e s t c e l l s was p r e p a r e d w i t h as many of the a f o r e m e n t i o n e d 73 parameters a d j u s t e d t o agree more c l o s e l y w i t h L e t o k h o v ' s . A new c e l l was c o n s t r u c t e d of pyrex i n a T shape h a v i n g d i m e n s i o n s of about 6 mm d i a m e t e r , 15 cm l e n g t h , and, a 6.0 cm 3 volume. T h i s was o u t f i t t e d w i t h a t e f l o n s t o p c o c k so as t o a v o i d any c o n t a m i n a t i o n of the c e l l due t o h e a t i n g up the pyrex t o remove i t from the gas m a n i f o l d a f t e r p r e p a r a t i o n . The reason f o r the T shape was t o a l l o w t h e experiment t o be performed w i t h two l a s e r beams g o i n g t h r o u g h the c e l l p e r p e n d i c u l a r t o one a n o t h e r s i m u l t a n e o u s l y . The new c e l l s were p l a c e d i n the a p p a r a t u s shown i n f i g u r e 4.5. In t h e s e e x p e r i m e n t s p r i s m P1 was r e p l a c e d w i t h a beam s p l i t t e r which sent p a r t of the argon i o n r a d i a t i o n t o the t e s t c e l l and the r e s t t o power t h e dye l a s e r . T h i s arrangement a l l o w e d roucfhly 2 w a t t s of argon i o n l a s e r beam and about 80 m i l l i w a t t s of dye l a s e r power t o e n t e r the t e s t c e l l s i m u l t a n e o u s l y . N o t i c e , t o o , t h a t i n t h i s arrangement no water b a t h was used t o c o n t r o l the c e l l t e m p e r a t u r e . A g a i n , b o t h s i g n a l s were r e c o r d e d by e i t h e r an EMI 9558B or 9558QB p h o t o m u l t i p l i e r whose s i g n a l s were sen t t o a PHILLIPS PM8252A s t r i p c h a r t r e c o r d e r . A t y p i c a l e x p e r i m e n t and the i n t e r e s t i n g r e s u l t s a r e d e s c r i b e d below. The p y r e x t e s t c e l l was p r e p a r e d f o r use by b a k i n g the c e l l ( t o about 200°C) w h i l e e v a c u a t i n g i t f o r a p e r i o d of r o u g h l y twenty f o u r h o u r s . A r e s i d u a l p r e s s u r e of 8 x 1 0 _ 6 t o r r was o b t a i n e d t h i s way. 74 ARGON ION LASER BEAM STOP 1 O EMI 9558 B PHOTOMULTIPLIER j " c S 2-62 FIL1 TEST CELL a! y CD-0 0 3 If) ^ hJQ. CO IO I CM in o ER BEAM-SPLITTER 7 ' STRIP CHART RECORDER DYE LASER F i g u r e 4.5. Dual l a s e r beam e x p e r i m e n t a l arrangement i s shown above. P r i s m P i was r e p l a c e d w i t h a beam s p l i t t e r w h ich sent p a r t of the argon i o n beam t o the t e s t c e l l w i t h the remainder powering the dye l a s e r . T h i s s e t up a l l o w e d both l a s e r beams t o e n t e r the T shaped t e s t c e l l (6 mm d i a m e t e r , 15 cm l e n g t h ) s i m u l t a n e o u s l y . The f l u o r e s c e n c e i n d u c e d by each beam was r e c o r d e d by EMI p h o t o m u l t i p l i e r s s h i e l d e d by C o r n i n g r e d pass f i l t e r s as b e f o r e . 75 The c e l l was then a l l o w e d t o c o o l t o room temperature and f i l l e d , as p r e v i o u s l y d e s c r i b e d , v i a temperature c o n t r o l l e d vapour p r e s s u r e t o (3.00±0.15) m t o r r . T h i s c e l l was then p l a c e d i n the e x p e r i m e n t a l a p p a r a t u s . The r e g i o n over which the dye l a s e r scanned was changed from the 14'-1" P(77) and R(82) l i n e s (v * 17034 cm" 1 t o the 15'-0" R(30) and P(25) s p e c t r a l l i n e s (v = 17408 cm" 1). T h i s r e g i o n , shown i n f i g u r e 4.6, produced s l i g h t l y more l a s e r power and had a comparable r e l a t i v e p r e d i s s o c i a t i o n r a t e t o the p r e v i o u s r a n g e . 1 TABLE 4.4 E x p e r i m e n t a l C o n d i t i o n s BACKGROUND .PRESSURE INITIAL IODINE PRESSURE ARGON ION LASER POWER DYE LASER POWER DURATION OF EXPERIMENT DYE LASER MONITOR LINES SCAN LENGTH SCAN TIME 8 x 1 0 " 6 t o r r (3.00±0.15)mtorr (1.8±0.1)W (80±10)mW 160 minutes B X (15'- 0") P(25),R(30) i>=<17034 cm" 1 10 GHz 25s 1From the d o c t o r a l t h e s i s of J . V i g u 6 9 the p r e d i s s o c i a t i o n quantum y i e l d of the 15' -0" P(25) and 14'—1." P(69) l i n e s may be deduced as (0.0576±0.0462) and (0.0526±0.0504) r e s p e c t i v e l y . 76 F i g u r e 4.6. The above f i g u r e shows the 17408 cm - 1 r e g i o n of the m o l e c u l a r i o d i n e spectrum e x c i t e d by the dye l a s e r . The t r a n s i t i o n s l a b e l l e d a, b, c, and d a r e the 15'-0" R ( 3 0 ) , 15'-0" P ( 2 5 ) , 18'-1" P ( 8 4 ) , and 19'-1" P(121) peaks r e s p e c t i v e l y . 77 The argon i o n l a s e r was p e r i o d i c a l l y sent through the t e s t c e l l w h i l e t h e dye l a s e r c o n t i n u o u s l y scanned a c r o s s the s e l e c t e d r e g i o n f o r a p e r i o d of about 40 minu t e s . A f t e r t h i s time the l a s e r s were c u t o f f and the system l e f t i n the dark f o r 120 m i n u t e s . The l a s e r s were t u r n e d on a g a i n and the r e s u l t s shown i n f i g u r e s 4.7 t o 4.9. 78 U I vJ (x2.5) b a I c t«0 t~!5min t~30min t~!30min F i g u r e 4.7. The 1 [peak b ] , and o b s e r v e d i n a ( 3 . a f t e r d i f f e r e n t l a s e r [ t r a c e s (1) dark f o r about r e l a t i v e s t r e n g t h o v e r time w i t h t r a n s i t i o n s . 5'-0" P(25) [peak a ] , l 5 ' - 0 " R(30) the 18'-1" P(84) [peak c ] t r a n s i t i o n s 00±0.15)mtorr i o d i n e vapour t e s t c e l l amounts of exposure t o the argon i o n t o ( 3 ) ] and a f t e r b e i n g l e f t i n the 100 m i n u t e s . [ t r a c e ( 4 ) J N o t i c e how the of the 18'-1" P(84) peak i n c r e a s e s r e s p e c t t o the !5'-0" P(25) and R(30) 79 TABLE 4.5 15'-0" P ( 2 5 ) , l 5 ' - 0 " R ( 3 0 ) , and 1 8' -1 " P(84) Peak H e i g h t R a t i o s v e r s u s Time. Time 15'-0" P(25) 15'-0" P(25) (min.±1 l 5 ' - 0 n R(30) 18'-1" P(84) min.) 0 1 .04310.036 1.25810.048 3.0 1.05110.043 1.27010.057 5.6 1 .04310.045 1.22710.058 10.3 1 .03810.049 1.26310.066 15.0 1.05410.060 1.23010.076 18.5 1.03410.056 1.21610.072 22.8 1 .03710.023 1.20610.029 25. 1 1.04410.026 1.20010.032 32.8 1.028+0.031 1.19410.039 37.9 1.035+0.029 1.163+0.035 160 1.07710.059 1.11310.058 80 TIME (min) F i g u r e 4.8 A p l o t of the 15'-0" P(25) : R(30) ( O ) , and, 15'~0" P(25) : 18'-1" P(121) ( A ) , peak h e i g h t s over produced i n a (3.00±0.15)mtorr i o d i n e vapour c e l l over th e c o u r s e of the experiment i s shown above. From t h i s g r aph the v a r i a t i o n of the 15'-0" P(25) : 18*-1 w P(84) peak h e i g h t s i s more c l e a r l y seen. A l a s e r i nduced enhancement of t h e p a r a - i o d i n e cannot be c o n c l u d e d from t h i s as t h e r a t i o c o n t i n u e s t o d e c r e a s e even i n the absence of t h e 5145 A i r r a d i a t i o n and because the 15'~0" P(25) : 15'~0" R(30) r a t i o remains c o n s t a n t t h r o u g h o u t . (As b e f o r e , t h e p e r i o d s when the argon i o n l a s e r i s p r e v e n t e d from e n t e r i n g the t e s t c e l l a r e marked by the shaded r e g i o n s on the graph w i t h the e x c e p t i o n of the r e g i o n d i s p l a y e d a f t e r the 34 minute mark which, a l t h o u g h not shaded, d i d not have t h e 5145 A beam e n t e r i n g the c e l l . ) 81 TABLE 4.6 Argon Ion Induced F l u o r e s c e n c e v e r s u s Time TIME FLUORESCENCE (min.±1.0 min.) (ar b . ) 0 1 .000 3.0 0.847±0.026 4.6 0.807±0.025 o f f • • • 9.1 0.829±0.026 10.3 0.742±0.024 15.0 0.644±0.023 15.2 0.641±0.023 o f f • • * 23.4 0.671±0.023 25.0 0.604±0.022 32.8 0.491±0.021 33.0 0.480±0.021 o f f • • • 82 w 0.800 < 5 0.900 1.000 u z UJ a 0.700 cr. 3 0.600 z 0.500 o g 0.400 1 I I I 1 0 4 8 12 16 20 24 28 32 36 40 F i g u r e 4.9 The decay of t h e argon i o n induced f l u o r e s c e n c e over t h e c o u r s e of the experiment i s shown above. The p e r i o d s when t h e 5145 A r a d i a t i o n was t u r n e d o f f a r e i n d i c a t e d by t h e dashed l i n e s between the e x p e r i m e n t a l p o i n t s . TIME ( m i n i I min) 83 A g a i n , based upon the r a t i o of the 15'-0" P(25) t o R(30) s p e c t r a l l i n e s t h e r i s no o r t h o t o para s h i f t t o w i t h i n 5% u n c e r t a i n t y . However, an e x t r e m e l y i n t e r e s t i n g r e s u l t i s o b t a i n e d w i t h the r a t i o of the 15'-0" P(25) t o 18'-1" P(84) l i n e s . T h i s r a t i o seems t o d e c r e a s e over time and would appear t o i n d i c a t e t h a t an o r t h o - t o p a r a - i o d i n e s h i f t has indeed been o b t a i n e d , w i t h an enrichment f a c t o r o f , r ( l. Zi>8 ± O.04-8 ) ( I. l b a ± o-o»5) I O B ± O . O T over a p e r i o d of t h i r t y m i n u t e s . ( L e t o k h o v r e p o r t s an enrichment of 1.3 a f t e r t h i r t y m inutes of i r r a d i a t i o n i n a 5 mtorr i o d i n e c e l l . ) T h i s enhancement i s not c l o s e t o the v a l u e r e p o r t e d by the R u s s i a n a u t h o r s but i t does show t h a t the r a t i o of two s p e c t r a l l i n e s b e l o n g i n g t o d i f f e r e n t v i b r a t i o n a l t r a n s i t i o n s and w i t h v e r y d i f f e r e n t r o t a t i o n a l quantum numbers ten d t o i n d i c a t e a v a r i a t i o n i n the o r t h o -t o p a r a - i o d i n e r a t i o over t i m e . Indeed a most f a s c i n a t i n g r e s u l t o c c u r s a f t e r h a v i n g l e f t the t e s t c e l l i n the dark f o r about two hours and r e - i r r a d i a t i n g the system — the l 5 ' - 0 " P(25) t o 18*-1" P(84) r a t i o has dropped even f u r t h e r w h i l e the 15*-0" P(25) t o R(30) r a t i o remained c o n s t a n t . A f i r s t c l u e as t o what may be happening may be o b t a i n e d by comparing t h i s r e g i o n of spectrum {v •* 17408 cm* 1) i n a low p r e s s u r e i o d i n e c e l l t o a h i g h p r e s s u r e i o d i n e c e l l as shown i n f i g u r e 4.10. One remarks immediately 84 c b Q F i g u r e 4.10. T y p i c a l i o d i n e s p e c t r a produced i n the 17408 c m - 1 r e g i o n v i a dye l a s e r i n d uced f l u o r e s c e n c e i n a low p r e s s u r e i o d i n e c e l l (about 3 m t o r r ) i n [1] and a h i g h p r e s s u r e i o d i n e c e l l (about 30 m t o r r ) i n [ 2 ] . The p e a k s , a, b, c, and, d, a r e i d e n t i f i e d as t h e l 5 ' - 0 " R ( 3 0 ) , 15'-0" P ( 2 5 ) , 18'-1" P ( 8 4 ) , and the 19'-1" P(121) t r a n s i t i o n s r e s p e c t i v e l y . In the the h i g h p r e s s u r e c a s e , [ 2 ] , t h e 18*-1" P(84) and 19'-1" P(121) peaks are more predominant than the c o r r e s p o n d i n g peaks i n the low p r e s s u r e spectrum owing p r o b a b l y t o g r e a t e r s e l f - q u e n c h i n g i n the h i g h p r e s s u r e c a s e . 85 t h a t the 18'-1" P(84) and t h e 19'-1" P(121) l i n e s ( l a b e l l e d " c " and "d" i n the f i g u r e r e s p e c t i v e l y ) a r e much more predominant a t h i g h p r e s s u r e s than a t low p r e s s u r e s . T h i s may be a t t r i b u t e d t o the f a c t t h a t the l i f e t i m e of a g i v e n s t a t e may be w r i t t e n a s , *c = WWi (4-. i) A + Bprcd + Q-where Bp r e ci * s a f a c t o r depending upon t h e p r e d i s s o c i a t i o n r a t e of a g i v e n s t a t e , and Q i s a f a c t o r d e p e nding upon the c o l l i s i o n a l quenching of a g i v e n s t a t e by e i t h e r a f o r e i g n gas or by s e l f - q u e n c h i n g . For a s t a t e t h a t i s h i g h l y p r e d i s s o c i a t e d , the l i f e t i m e i s not e x p e c t e d t o be s t r o n g l y m o d i f i e d by the a d d i t i o n of some f o r e i g n gas. A s t a t e t h a t i s not s t r o n g l y p r e d i s s o c i a t e d , on the o t h e r hand, may be g r e a t l y a f f e c t e d depending upon the r e l a t i v e s i z e of the p a r a m e t e r s A, B, and Q. In the case of the 15'-0" P(25) s t a t e t h e p r e d i s s o c i a t i o n quantum y i e l d i s r o u g h l y (0.0576±0.0462) w h i l e the 18'-1" P(84) and 19* — 1" P(121) t r a n s i t i o n s have quantum y i e l d s r o u g h l y (0 .287±0.191) and (0.539±0.394) r e s p e c t i v e l y . (These v a l u e s were e x t r a p o l a t e d from r e f e r e n c e ( 9 ) ) Hence, i f the c e l l i s l e a k i n g , o u t g a s i n g , or i f t h e i o d i n e atoms are d r i v i n g some i m p u r i t i e s o f f of the w a l l s o f the t e s t c e l l , t h e n , over time the i n t e r n a l p r e s s u r e i n t h e c e l l w i l l 86 i n c r e a s e and c o u l d cause some d i f f e r e n t i a l q uenching of the v a r i o u s t r a n s i t i o n s . In t h e s e e x p e r i m e n t s , the most p r o b a b l e cause of an i n c r e a s e i n i n t e r n a l p r e s s u r e would be due t o o u t g a s i n g . Leakage by the t e f l o n s t o p c o c k s may be r u l e d out as the s e produce a v e r y good s e a l e s p e c i a l l y over a two t o t h r e e hour p e r i o d . A l s o , i f i o d i n e atoms a r e d r i v i n g i m p u r i t i e s o f f of the w a l l s of the c o n t a i n e r t o a l a r g e e x t e n t , then one would e x p e c t t h a t the peak h e i g h t r a t i o s would remain more or l e s s c o n s t a n t a f t e r the argon i o n l a s e r beam was stopped from e n t e r i n g t h e c e l l . However, t h i s was not ob s e r v e d t o be so. To t e s t t h i s o u t g a s i n g h y p o t h e s i s one s h o u l d prepare s e v e r a l t e s t c e l l s which v a r y i n the degree t o which they were e v a c u a t e d . That i s , when one e v a c u a t e s a c e l l the p r e s s u r e time dependence goes a s , dP _ -yp + D (4.Z) dt where P i s the p r e s s u r e i n system, 7 i s a f a c t o r p r o p o r t i o n a l t o the pumping r a t e of the system, and D i s an o u t g a s i n g r a t e . C l e a r l y , as one reduces t h e u l t i m a t e p r e s s u r e o b t a i n e d then the d e g a s i n g f a c t o r a l s o d e c r e a s e s , i . e . a t e q u i l i b r i u m , D = 7P(=>) . The lower P(°°) i s the slower the d e g a s i n g r a t e of the c e l l when i t i s removed from the pump. By p r e p a r i n g c e l l s w i t h d i f f e r e n t background p r e s s u r e s and f i l l i n g them w i t h r o u g h l y the same amounts of i o d i n e 87 v a p o u r , one may t e s t t h e h y p o t h e s i s . A s e r i e s of such e x p e r i m e n t s was performed w i t h each of the c e l l s b e i n g t e s t e d as p r e v i o u s l y d e s c r i b e d u s i n g the s i m u l t a n e o u s i r r a d i a t i o n t e c h n i q u e . The r e s u l t s o b t a i n e d a r e r e p o r t e d below. (a) Run Number 1: P(°°) = 5 x l 0 ~ 6 t o r r A c e l l was p r e p a r e d by b a k i n g i t ( t o about 200°C) w h i l e e v a c u a t i n g i t f o r r o u g h l y 6 days p r i o r t o f i l l i n g . The c e l l was nex t f i l l e d w i t h (4.5±0.3)mtorr of i o d i n e vapour and i r r a d i a t e d w i t h both l a s e r beams s i m u l t a n e o u s l y . TABLE "4.7- E x p e r i m e n t a l C o n d i t i BACKGROUND PRESSURE INITIAL IODINE PRESSURE ARGON ION LASER POWER DYE LASER POWER DURATION OF EXPERIMENT DYE LASER MONITOR LINES SCAN LENGTH SCAN TIME 5 x 1 0 " 6 t o r r (4.5±0.3)mtorr (1 .85±0.10)W 80mW 110 minutes B X (15'- 0") P(25),R(30) 7408 cm- 1 30 GHz 75 s 88 (x2) a d vJy lMJL d J u u t=0 t~30min t~60min t~l6hrs F i g u r e 4.11. T y p i c a l i o d i n e s p e c t r a i n the 17408 cm" 1 r e g i o n i n d uced i n a (4.5±0.3)mtorr i o d i n e c e l l p r e p a r e d by e v a c u a t i n g the c e l l t o a r e s i d u a l p r e s s u r e of about 5x10' t o r r p r i o r t o use, as a f u n c t i o n of time d u r i n g the experiment a r e g i v e n above. In t h i s case the peaks i d e n t i f i e d as a - 15*-0" R ( 3 0 ) r b - 15'-0" P(25) c -18*-1" P ( 8 4 ) , and d - 19*-1" P ( 1 2 1 ) , m a i n t a i n e d c o n s t a n t r e l a t i v e h e i g h t s over the d u r a t i o n of the e x c i t a t i o n w i t h the argon i o n l a s e r but v a r i e d a f t e r a 16 hour p e r i o d of l e a v i n g the c e l l i n the dark. 89 TABLE 4.8 Time (min.±1 min. ) 0 4.6 7.1 . 9.6 12.1 14.6 27.6 32.6 36.4 43.4 "45.9 48.4 52.2 57. 1 16 h r s He i g h t R a t i o s 15'-0" P(25) 15'-0" R(30) (±0.055) 1 .054 1 .060 1 .061 1 .063 1.069 1 .063 1 .047 1 .057 1 .055 1 .061 1 .058 1 .048 1 .042 1 .049 1 .031 v e r s u s Time l 5 ' - 0 " P(25) 18'-1" P(84) (±0.080) 1 .438 1 .454 1.431 1 .444 1 .464 1 .436 1 .441 1 .438 1 .450 1 .451 1 .452 1 .454 1 .464 1 .448 1 .1 72 15'-0" P(25) 19*-1" P(121) (±0.125) 3.682 3.872 3.827 3.844 3.889 3.792 3.868 3.772 3.918 3.843 3.847 3.842 3.835 3.845 3.212 90 4 .0 - 4fi*1—J H — H H K c n o cc »-X o OL 3.0 20 l.of^ 0.60. -0—0—0—0—O- -O D--5-^-5—$—$ v V O- 0 -0—0—0-_L 10 20 30 TIME (min) 40 50 -16 hrs F i g u r e 4.12. The l 5 ' - 0 " P(25) : l 5 ' - 0 " R(30) (O), l 5 ' - 0 " P(25) : 18'-1" P(84) ( A ) , and l 5 ' - 0 " P(25) : 19'-1" P(121) ( D ) , peak h e i g h t r a t i o s over the c o u r s e of the exp e r i m e n t i n a t e s t c e l l e v a c u a t e d t o about 5 x l O ~ 8 t o r r p r i o r t o use and f i l l e d w i t h (4.5±0.3)mtorr of i o d i n e v a p o u r. A l l t h e r a t i o s appear t o remain c o n s t a n t d u r i n g the i r r a d i a t i o n of the c e l l w i t h the argon i o n l a s e r but v a r y a f t e r h a v i n g l e f t t h e c e l l i n the dark f o r a s i x t e e n hour p e r i o d . 9 1 TIME (min) F i g u r e 4.13 The decay of the argon i o n induced f l u o r e s c e n c e o b s e r v e d i n a t e s t c e l l f i l l e d w i t h (4.5±0.3)mtorr of i o d i n e vapour i n a c e l l e v a c u a t e d t o about 5 x l 0 " 6 t o r r b e f o r e use. A g a i n , t h e shaded r e g i o n s i n d i c a t e the p e r i o d s d u r i n g which the 5145 A r a d i a t i o n was p r e v e n t e d from e n t e r i n g the t e s t c e l l . 92 TABLE 4.9 Argon Ion L a s e r Induced F l u o r e s c e n c e v e r s u s Time TIME FLUORESCENCE (min.±1.0 min.) (ar b . ) (±0.020) 0 1 .000 2.0 0.888 6.9 0.857 11.9 0.821 14.9 0.800 18.0 * • • 19.5 0.792 20.9 0.808 25.9 0.784 30.9 0.762 34.2 0.748 38.0 0.735 38.0 • • • 42.5 on 43.2 0.731 48.2 0.705 57.2 0.680 58.0 o f f 61.5 on 62.2 0.693 65.2 0.674 ..cont.) 75.2 85.2 95.5 105.2 110.2 0.653 0.628 0.613 0.591 0.588 94 Hence one o b s e r v e s t h a t no o r t h o t o para r a t i o s h i f t has been o b t a i n e d but t h a t the v a r i a t i o n i n the r e l a t i v e peak h e i g h t s was l e s s than the p r e v i o u s t r i a l . T h i s would seem t o support the d e g a s i n g h y p o t h e s i s , however s t i l l more dat a was r e q u i r e d . 95 (b) Run Number 2: P(») = 1.6X10" 5 t o r r T h i s sample c e l l was p r e p a r e d as b e f o r e by b a k i n g i t out over n i g h t w h i l e e v a c u a t i n g i t but w i t h o u t u s i n g a l i q u i d n i t r o g e n c o l d t r a p on the gas m a n i f o l d . The r e s u l t i n g r e s i d u a l p r e s s u r e was 1.6X10' 5 t o r r . TABLE 4.10 E x p e r i m e n t a l C o n d i t i o n s BACKGROUND PRESSURE INITIAL IODINE PRESSURE ARGON ION LASER POWER DYE LASER POWER DURATION OF EXPERIMENT DYE LASER MONITOR LINES SCAN LENGTH SCAN TIME 1 . 6 x 1 0 - 5 t o r r (3.3±0.3)mtorr (1.9±0.1)W 50mW 60 minutes B X (15'- 0") P(25),R(30) v=*\7408 cm- 130 GHz 75 s F i g u r e 4.14. T y p i c a l i o d i n e s p e c t r a i n t h e 17408 cm - 1 r e g i o n induced i n a (3.3±0.3)mtorr i o d i n e c e l l p r e p a r e d by e v a c u a t i n g t h e c e l l t o a r e s i d u a l p r e s s u r e of about 1 . 6 x l 0 " 5 t o r r p r i o r t o u s e , as a f u n c t i o n of time d u r i n g t h e experiment a r e g i v e n above. In t h i s c a s e the peaks, i d e n t i f i e d as a - l 5 ' - 0 " R ( 3 0 ) , b - 15'-0" P ( 2 5 ) , c -18'-1" P ( 8 4 ) , and d - 19'-1" P ( 1 2 1 ) , v a r i e d i n r e l a t i v e h e i g h t s over the d u r a t i o n of the e x c i t a t i o n w i t h the argon i o n l a s e r and t o a g r e a t e r e x t e n t a f t e r l e a v i n g the c e l l i n the dark f o r about 48 h o u r s . 97 TABLE 4.11 Peak H e i g h t R a t i o s v e r s u s Time Time (min.±1 min.) 15'-0" P(25) 15'-0" R(30) (±0.055) 15'-0" P(25) 18 * - 1 " P(84) (±0.080) 15'-0" P(25) 1 9 ' - 1 " P(121) (±0.125) 0.0 3.4 8.0 11.1 16.1 21.3 55.0 60.0 48hrs 1.072±0.030 1.060±0.026 1.055±0.028 1.047±0.030 1.042±0.031 1.041±0.036 1.043±0.028 1.033±0.029 1 .042±0.027 1.280±0, 1.288±0, 1.305±0, 1.256±0, 1.258±0, 1.234±0, 1.126±0, 1.115±0. 0.672±0, 030 035 038 039 042 046 028 032 014 3.520±0.120 3.473±0.114 3.493±0.122 3.400±0.127 3.381±0.133 3.389±0.153 2.992±0.159 3.092±0.108 1.678±0.040 98 4.00 r-60 48hrs TIME (min) F i g u r e 4.15. The 1 5 ' - D " P(25) : 15'-0" R(30) (O), l 5'-0" P(25) : 18'-1" P ( 8 4 ) ( A ) , and l 5'-0" P(25) : 19'-1" P(121) (Q),peak h e i g h t r a t i o s over t h e c o u r s e of the exp e r i m e n t i n a t e s t c e l l e v a c u a t e d t o about 1 . 6 x i 0 - 5 t o r r p r i o r t o use and f i l l e d w i t h (3.3±0.3)mtorr of i o d i n e vapour. As i n the c a s e s w i t h the t e s t c e l l e v a c u a t e d t o 8 x 1 0 " 6 t o r r o r i g i n a l l y , (see page ), d u r i n g t h e i r r a d i a t i o n o f the c e l l w i t h the argon i o n l a s e r , t h e r a t i o s of the l 5'-0" P(25) : 18'-1" P(84) and the l 5'-0" P(25) : 19'-1" P(121) r a t i o s d e c r e a s e d and c o n t i n u e d t o do so even when the t e s t c e l l was kept i n the dark f o r about 48 h o u r s . 99 TABLE 4.12 Argon Ion Laser Induced F l u o r e s c e n c e v e r s u s Time. TIME FLUORESCENCE (min.±1.0 min.) (arb.±0.030) 0 2.2 7.2 12.2 17.2 22.2 36.2 50.4 55.4 60.4 62.0 1 .000 0.944 0.896 0.848 0.814 0.785 0.723 0.679 0.670 0.662 0.642 100 I I I I 1 I I I I I I I l_ O 10 20 30 40 50 60 TIME (min) F i g u r e 4.16 The decay of the argon i o n i n d u c e d f l u o r e s c e n c e o b s e r v e d i n a t e s t c e l l f i l l e d w i t h (3.3±0.3)mtorr of i o d i n e vapour i n a c e l l e v a c u a t e d t o about 1 . 6 x l 0 " 5 t o r r b e f o r e use. The b e h a v i o r of the decay i s s i m i l a r t o t h a t p r e v o u s l y observed i n run #1. 101 (c) Run Number 3: P(») = 9.5x10" 5 A g a i n the t e s t c e l l was baked out o v e r n i g h t and the l i q u i d n i t r o g e n c o l d t r a p removed from the system. In o r d e r t o d e c r e a s e the vacuum f u r t h e r from the r e a d i l y o b t a i n a b l e 2X10' 5 t o r r l e v e l , one m a n i f o l d of the gas f i l l i n g system which had been pumped down t o o n l y about 10" 3 t o r r was opened t o the m a n i f o l d c o n t a i n i n g the c e l l a l o n e b r i e f l y . T h i s r e s u l t e d i n an i n c r e a s e i n p r e s s u r e t o 9.5x10" 5 t o r r . At t h i s p o i n t the pumps were c l o s e d o f f from the main m a n i f o l d and the c e l l f i l l e d w i t h r o u g h l y (3.3±0.3) mtorr of i o d i n e . U n f o r t u n a t e l y , as t h i s vacuum p r e s s u r e was not e a s i l y o b t a i n a b l e i t i s exp e c t e d t h a t the r e s u l t s of t h i s experiment would be the most q u e s t i o n a b l e . TABLE 4.13 E x p e r i m e n t a l C o n d i t i o n s BACKGROUND PRESSURE INITIAL IODINE PRESSURE ARGON ION LASER POWER DYE LASER POWER DURATION OF EXPERIMENT DYE LASER MONITOR LINES SCAN LENGTH SCAN TIME 9 . 5 x l 0 " s t o r r (3.3±0.3)mtorr (1.7±0.1)W 1 OOmW 60 m i n u t e s B X (15'-0" ) P(25),R(30) »>=*1 7408 cm' 1 30 GHz 75 s 102 b a t~0 t~40min t~60min t~48hrs F i g u r e 4.17. T y p i c a l i o d i n e s p e c t r a i n the 17408 cm" 1 r e g i o n i n d u c e d i n a (3.3±0.3)mtorr i o d i n e c e l l p r e p a r e d by e v a c u a t i n g the c e l l t o a r e s i d u a l p r e s s u r e of about 9 . 5 x l 0 " 5 t o r r p r i o r t o use, as a f u n c t i o n of time d u r i n g the e x p eriment a r e g i v e n above. In t h i s case t h e peaks, i d e n t i f i e d as a - 15'-0" R ( 3 0 ) , b - 15'-0" P ( 2 5 ) , c -18'-1" P ( 8 4 ) , and d - 19'-1" P ( 1 2 1 ) , v a r i e d i n r e l a t i v e h e i g h t s over the d u r a t i o n of the e x c i t a t i o n w i t h the argon i o n l a s e r and a f t e r l e a v i n g the. c e l l i n the dark f o r about 48 hours i n a manner comparable t o t h a t of the p r e v i o u s c e l l . 103 TABLE 4.14 Peak H e i g h t R a t i o s v e r s u s Time Time 15'-0" P(25) 15'-0" P(25) . 15'-0" P(25) (min.±1 15'-0" R(30) 18'-1" P(84) 19'-1" P(121) min.) 0 1 .031±0. 035 1 .31010. 051 3. 48710. 160 2.9 1 .037±0. 039 1 .32310. 057 3. 59310. 187 4.8 1 .033±0. 022 1 .33410. 032 3. 64410. 174 8.5 1 .034±0. 025 1 .32810. 036 3. 52610. 188 11.0 1 .03210. 027 1 .30010. 038 3. 53110. 204 13.4 1 .034±0. 029 1 .29410. 040 3. 60710. 227 15.6 1 .039±0. 030 1 .31010. 044 3. 52110. 229 17.9 1 .028±0. 032 1 .28510. 045 . 3. 47610. 146 20.2 •1 .026±0. 034 1 .29310. 048 3. 45410. 1 52 24.7 1 .034±0. 037 1 .29210. 052 3. 48610. 1 70 29.2 1 .03010. 020 1 .23510. 027 3. 33310. 1 40 54.7 1 .04410. 032 1 . 19110. 039 3. 23210. 203 57.0 1 .04110. 016 1 .17710. 020 3. 18010. 102 61 .4 1 .02410. 017 1 .19710. 021 3. 25810. 1 12 48hrs 1 .04410. 080 0 .70010. 025 1 . 76810. 130 1 04 F i g u r e 4.18. The 15'-0 B P(25) : 15'~0" R(30) (O), 15'~0" P(25) : 18'-1" P(84) ( & ) , and 15'-0" P(25) : 19'-1" P( 1 21 )(P)peak h e i g h t r a t i o s over the c o u r s e of the experiment i n a t e s t c e l l e v a c u a t e d t o about 9 . 5 x l 0 " 5 t o r r p r i o r t o use and f i l l e d w i t h (3.3±0.3)mtorr of i o d i n e vapour. As i n run #2, d u r i n g the i r r a d i a t i o n of the c e l l w i t h the argon i o n l a s e r , the r a t i o s of the 15'-0" P(25) : 18'-1" P(84) and the 15'-0" P(25) : 19'-1" P(121) r a t i o s d e c r e a s e d and c o n t i n u e d t o do so even when t h e t e s t c e l l was kept i n the dark f o r about 48 h o u r s . 105 TABLE 4.15 Argon Ion L a s e r Induced F l u o r e s c e n c e v e r s u s Time, TIME (min.±1.0 min.) FLUORESCENCE (arb.±0.030) 5.2 10.2 15.2 20.2 25.2 30.2 35. 1 40. 1 45. 1 . 50. 1 53.0 54.4 57.7 61.7 0.872±0.037 0.802±0.042 0.742±0.040 0.694±0.039 0.669±0.030 0.640±0.038 0.610±0.037 0.592±0.027 0.574±0.028 0.559±0.025 0.551±0.026 0.543±0.025 0.548±0.048 0.545±0.025 1 06 TIME (min) F i g u r e 4.19 The decay of the argon i o n induced f l u o r e s c e n c e o b s e r v e d i n a t e s t c e l l f i l l e d w i t h (3.3±0.3)mtorr of i o d i n e vapour "in a c e l l e v a c u a t e d t o about 1 . 6 x l 0 ' 5 t o r r b e f o r e use. The b e h a v i o r of the decay i s s i m i l a r t o t h a t p r e v o u s l y observed i n run #1. 107 From these r e s u l t s one ob s e r v e s t h a t i n d e e d no o r t h o t o p a r a enhancement i s produced d u r i n g the s e l e c t i v e i r r a d i a t i o n of the i o d i n e but t h a t the r a t i o s of the h e i g h t s o f s p e c t r a l l i n e s b e l o n g i n g t o d i f f e r e n t v i b r a t i o n a l and r o t a t i o n a l s t a t e s do v a r y over the c o u r s e of the e x p e r i m e n t . F u r t h e r , t h i s v a r i a t i o n appears t o depend h e a v i l y upon how c l e a n the t e s t c e l l was made p r i o r t o use s u p p o r t i n g the h y p o t h e s i s t h a t the above v a r i a t i o n i s caused by d i f f e r e n t i a l q u e n c h i n g of the v i b r a t i o n a l s t a t e s by i m p u r i t i e s outgased by t h e c e l l w a l l s d u r i n g the e x p e r i m e n t . Hence, no o r t h o - i o d i n e t o p a r a - i o d i n e s h i f t i s p o s s i b l e by t h i s method. B. IODINE AND SCAVENGERS The major problem, i t would be f a i r t o assume, was t h a t i o d i n e atoms can v e r y e f f i c i e n t l y scramble the o r t h o - and p a r a - i o d i n e m o l e c u l e s v i a , I + o - 1 2 p-I 2 + I t o q u i c k l y r e t u r n t h e r a t i o s of t h e s e two s p e c i e s t o the o r i g i n a l v a l u e . The l o g i c a l s t e p , t h e n , i s t o add a s u i t a b l e s c a v e n g e r t o the system which w i l l e f f e c t i v e l y mop up e i t h e r t h e atoms or the e x c i t e d i o d i n e m o l e c u l e s i m m e d i a t e l y a f t e r t h e y have been formed. To t h i s end s e v e r a l d i f f e r e n t o r g a n i c m o l e c u l e s and some i n o r g a n i c ones were used and the r e s u l t s d e s c r i b e d below. 108 1. IODINE AND 2-HEXENE The f i r s t c h o i c e f o r a scavenger was 2-hexene as t h i s was r e p o r t e d t o r e a c t r e a d i l y w i t h e i t h e r the e x c i t e d i o d i n e m o l e c u l e s o r i o d i n e atoms by b o t h Badger and U r m s t o n 2 7 and Letokhov e t a l 2 3 . A m i x t u r e of c i s - and trans-2-hexene was o b t a i n e d from t h e A l d e r i c h Chemical Company (GOLD, 99+%). T h i s was p l a c e d i n a pyrex v e s s e l and a t t a c h e d t o the gas f i l l i n g m a n i f o l d on the vacuum l i n e then d i s t i l l e d between two t r a p s . The subsequent 2-hexene was s t o r e d i n vacuo and c o v e r e d by a dark c l o t h on the gas l i n e and used as n e c e s s a r y . The f i r s t experiment was performed based upon i n f o r m a t i o n s u p p l i e d by Letokhov e t a l . As t h e enrichment f a c t o r i n c r e a s e d w i t h d e c r e a s i n g i n i t i a l i o d i n e p r e s s u r e i t was d e c i d e d t o use a v e r y low p r e s s u r e i o d i n e c e l l t o o b t a i n the maximum e f f e c t . To t h i s end a c e l l c o n t a i n i n g about 2 mtorr of i o d i n e and 1.0 t o r r of 2-hexene was p r e p a r e d and i r r a d i a t e d w i t h the dye l a s e r i n s t e a d of the argon i o n l a s e r . The r e g i o n used i s shown i n f i g u r e 4.20 and the m o n i t o r l i n e s were the 18* — 1" R(42) and P(37) t r a n s i t i o n s and the 19'-1" P(95) l i n e was used t o d r i v e t h e r e a c t i o n . The 19'-1" P(95) t r a n s i t i o n i s more h i g h l y p r e d i s s o c i a t e d than the 43'-0" P(13) or R(15) l i n e s e x c i t e d by the argon i o n l a s e r . However, t h e argon i o n l a s e r produces more power and w i l l e x c i t e two t r a n s i t i o n s from the v"=0 l e v e l which, at room t e m p e r a t u r e , i s more h i g h l y p o p u l a t e d than the v"=1. Thus, i t i s not c l e a r whether or not the a c t i o n of t h i s 109 a d b UL F i g u r e 4.20. The 17476 cm - 1 s p e c t r a l r e g i o n of m o l e c u l a r i o d i n e i s shown above. The peaks, a, b, c, d, and, e, a r e i d e n i f i e d as the 18'-1" P ( 3 7 ) , l 6 ' - 0 " R ( 6 1 ) , 17'-0" R ( 1 1 1 ) , 19'-1" P ( 9 5 ) , and 18'-1" R(42) r e s p e c t i v e l y . 110 t r a n s i t i o n w i l l be s i m i l a r t o t h a t of the argon i o n l a s e r or not . Indeed, i f the dye l a s e r produced more i o d i n e atoms than the argon i o n beam then t h i s i s bad from the p o i n t of view t h a t Letokhov e t a l c l a i m e d t h a t no r e a c t i o n o c c u r s between the 2-hexene and i o d i n e atoms. The main reason f o r c h o o s i n g t h i s t r a n s i t i o n was the f a c t t h a t u s i n g the dye l a s e r a l o n e made i t v e r y s i m p l e t o mo n i t o r the o r t h o and para l i n e s a t s p e c i f i c time i n t e r v a l s and c u t down on the n o i s e i n the s i g n a l . The o n l y way t o f i n d out i f t h i s had been a bad c h o i c e was t o p e r f o r m the ex p e r i m e n t . To o b t a i n a v e r y low p r e s s u r e of i o d i n e vapour i n the t e s t c e l l a known volume volume, V f i =* 6.0 cm 3, equipped w i t h a break s e a l was a t t a c h e d t o the t e s t c e l l (1.9 cm d i a m e t e r , 30 cm l e n g t h , and 79 cm 3 volume). T h i s s e c t i o n , V f i, was evacuated t o about 8X 1 0 " 6 t o r r , f i l l e d w i t h (30.0±0.3) mtorr of i o d i n e v a pour, and s e a l e d . The t e s t c e l l i t s e l f was then e v a c u a t e d and f i l l e d w i t h (1.0±0.3) t o r r of 2-hexene vapour. At t h i s p o i n t the 2-hexene was condensed i n a s i d e arm of the t e s t c e l l by immersing t h e arm i n l i q u i d n i t r o g e n and the s e a l between the c e l l and volume V f i broken, a l l o w i n g the i o d i n e vapour i n t o the t e s t c e l l and s u b l i m i n g i t i n the s i d e arm as w e l l . A f t e r p i n c h i n g o f f the volume, V f i, the p r e s s u r e of t h e i o d i n e c o u l d be c a l c u l a t e d as (2.2410.20) m t o r r . W i t h b o t h c o n s t i t u e n t s s t i l l condensed, the t e s t c e l l was r e e v a c u a t e d f o r ten minutes t o remove any a i r t h a t may have outg a s e d d u r i n g the f i l l i n g p r o c e d u r e . F i n a l l y , the 111 DYE LASER TO STRIP CHART RECORDER MIRROR M2 ^EMI 9558 QB PHOTOMULTIPLIER CS-2-62 FILTER TEST CELL -MIRROR Ml F i g u r e 4.21 The e x p e r i m e n t a l arrangement used f o r t e s t c e l l s (1.9 cm d i a m e t e r , 30 cm l e n g t h ) c o n t a i n i n g about 2.24 m t o r r of i o d i n e vapour and 1 t o r r of 2-hexene gas. The dye l a s e r was used b o t h t o d r i v e t h e r e a c t i o n u s i n g t h e 19'-1" P(95) s p e c t r a l l i n e and m o n i t o r t h e o r t h o - t o p a r a - i o d i n e r a t i o w i t h t h e 18'-1" P(37) and R(42) peaks. Note t h a t the l a s e r beam was sent t h r o u g h the t e s t c e l l t h r e e t i m e s w i t h m i r r o r s M1 and M2. 1 12 c e l l was removed from the vacuum l i n e and a r r a n g e d as shown i n f i g u r e 4.21.(Note t h a t the dye l a s e r beam t r a n s i t t e d the c e l l volume t h r e e t i m e s w i t h the a i d of m i r r o r s M1 and M2.) The r e s u l t s of such an experiment a r e d e s c r i b e d below. TABLE 4.16 E x p e r i m e n t a l C o n d i t i o n s BACKGROUND PRESSURE INITIAL IODINE PRESSURE INITIAL 2-HEXENE PRESSURE DYE LASER POWER EXCITATION LINE DYE LASER MONITOR LINES SCAN LENGTH SCAN TIME 2 x 1 0 - 5 t o r r (2.24±0.20)mtorr (1.0±0.2)torr (310±10)mW B X (19'-1") P(95) B X (18'-1") P(37),R(42) v^l 7476 cm" 1 30 GHz 75 s 1 1 3 t =0 t~l5min t ~ 40min t~70min F i g u r e 4.22 The time dependent b e h a v i o u r of t h e 17476 c m - 1 r e g i o n of the i o d i n e spectrum i n a t e s t c e l l c o n t a i n i n g about 2.24 m t o r r of i o d i n e and 1 t o r r of 2-hexene. The l i n e b e i n g used t o d r i v e the r e a c t i o n between the two s p e c i e s , t h e 19'-1" P(95) t r a n s i t i o n , i s l a b e l l e d above as E and the peaks used t o m o n i t o r the o r t h o t o para r a t i o of the sample a r e t h e 18'-1" P ( 3 7 ) , [ a ] , and the 18'-1" R ( 4 2 ) , [ b ] , l i n e s . No o r t h o - t o p a r a - i o d i n e r a t i o s h i f t i s a p p a r e n t . 1 14 F i g u r e 4.23. A p l o t of the decay of the 19'-1" P(95) i n d u c e d f l u o r e s c e n c e (O), and t h e 18'-1" P(37) : R(42) peak h e i g h t r a t i o , ( O), i n a c e l l c o n t a i n i n g about 2.24 m t o r r of i o d i n e vapour and 1 t o r r of 2-hexene gas over th e c o u r s e of the ex p e r i m e n t i s shown above. As b e f o r e , t h e dashed l i n e s i n d i c a t e the p e r i o d f o r which the r e a c t i o n was not b e i n g d r i v e n - w i t h the 19'-1" P(95) t r a n s i t i o n . TABLE 4.17 19'-1" P(95) F l u o r e s c e n c e S i g n a l v e r s u s Time TIME FLUORESCENCE (min.±1.0 min.) (arb.±0.010) 0 1 .000 2.5 0.915 5.0 0.843 7.5 0.770 10.0 0.759 16.3 0.769 18.8 0.740 21 .3 0.714 23.8 0.682 26.3 0.664 28.8 0.642 31 .3 0.623 33.8 0.602 36.3 0.589 45.6 0.544 48. 1 0.503 50.6 0.483 53. 1 0.462 55.6 0.448 58. 1 0.432 60.6 0.423 63.1 0.412 1 16 (...cont.) 65.6 0.397 67.1 0.391 TABLE 4.18 18' — 1" P(37):R(42) Peak H e i g h t R a t i o s v e r s u s Time TIME 18 1 - 1" P(37) (min.±1.0 min.) 18 * - 1 " R(42) 0.0 1.164±0.049 13.0 1.16B±0.020 41.0 1.174±0.025 70.0 1.192±0.026 No o r t h o t o p a r a r a t i o s h i f t was observed. The reason f o r t h i s i s ambiguous, however, as i t may s i m p l y be caused by the f a c t t h a t t o o many i o d i n e atoms were produced and were not scavenged by the 2-hexene. To s e t t l e t h i s m a tter an experiment was performed c l o s e l y r e p r o d u c i n g t h e c o n d i t i o n s r e p o r t e d by Letokhov e t a l . A m i x t u r e of r o u g h l y 30 m t o r r of i o d i n e and 2 t o r r of 2-hexene vapour c o n t a i n e d i n a T-shaped t e s t c e l l (6 mm d i a m e t e r , 15 cm l e n g t h and about 18 . 0 cm 3 volume) was i r r a d i a t e d w i t h t h e 5145 A argon i o n l i n e and the dye l a s e r tuned t o the 17408 cm' 1 r e g i o n s i m u l t a n e o u s l y . The r e s u l t i n g f l u o r e s c e n c e was r e c o r d e d on a s t r i p c h a r t r e c o r d e r . 1 17 The t e s t c e l l was p r e p a r e d f o r use by b a k i n g i t out under a vacuum f o r 24 hours p r i o r t o use. I o d i n e vapour was added t o the c e l l by d i f f u s i o n as p r e v i o u s l y d e s c r i b e d to g i v e (30±3) mt o r r of p r e s s u r e . The 2-hexene was added t o the c e l l by p l a c i n g a (-20.0±0.5)°C bath around the 2-hexene v e s s e l . The v e s s e l was opened t o p a r t of the m a n i f o l d (volume V ^ l 35.05cm 3) and the p r e s s u r e . i n t h i s s e c t i o n a l l o w e d t o r e a c h the e q u i l i b r i u m v a l u e f o r t h i s t e m p e r a t u r e , 16.5 t o r r . The 2-hexene c o n t a i n e r was s e a l e d o f f and the gas i n volume V, a l l o w e d t o expand i n t o a second p a r t of the m a n i f o l d (volume V 2-821.39 cm 3) and the t e s t c e l l i n the dark so as t o a v o i d i n i t i a t i n g a r e a c t i o n between the i o d i n e and 2-hexene p r e m a t u r e l y . The net r e s u l t of t h i s was t o produce a t e s t sample c o n t a i n i n g (2.3±0.5) t o r r of 2-hexene i n a d d i t i o n t o the i o d i n e . T h i s c e l l , kept i n the d a r k , was p l a c e d i n the e x p e i m e n t a l s e t u p shown i n f i g u r e 4.5 and the experiment i n i t i a t e d . 118 TABLE 4 . 1 9 E x p e r i m e n t a l C o n d i t i o n s BACKGROUND PRESSURE INITIAL IODINE PRESSURE INITIAL 2-HEXENE PRESSURE ARGON ION LASER POWER DYE LASER POWER DURATION OF EXPERIMENT DYE LASER MONITOR LINES SCAN LENGTH SCAN TIME 6 x 1 0 " 6 t o r r (30±3)mtorr (2.3±0.5)torr ' "" - (1 .60±0. 1 0)W 80mW 20 minutes B X (15'-0") P(25),R(30) 7408 cm" 1 30 GHz 75 s 1 19 G t = 0 t~7 min t~l8min F i g u r e 4.24. T y p i c a l f l u o r e s c e n c e s p e c t r a of t h e 17408 c m - 1 r e g i o n e x c i t e d i n a t e s t c e l l (6 mm d i a m e t e r , 1 5 cm l e n g t h ) c o n t a i n i n g (30±3)mtorr of i o d i n e vapour and ( 2 . 3 + 0 . 5 ) t o r r of 2-hexene. In t h i s c a s e , the d u a l beam i r r a d i a t i o n t e c h n i q u e was used once a g a i n so t h a t the r e a c t i o n was i n i t i a t e d w i t h t h e 5145 A argon i o n l a s e r l i n e and the o r t h o - and p a r a - i o d i n e r a t i o s were m o n i t o r e d w i t h the 15'-0" P(25) and R(30) t r a n s i t i o n s . (The peaks l a b e l l e d a, b, and c a r e t h e l 5 ' - 0 " P ( 3 0 ) , 15'-0" P ( 2 5 ) , and t h e 18'-1" P(84) l i n e s r e s p e c t i v e l y . ) 120 TABLE 4.20 15'-0" P(25):R{30) Peak H e i g h t R a t i o v e r s u s Time TIME l 5 ' - 0 " P(25) ±1.0 min.) 15'-0 B R(30) 0 1.06310.033 3.4 1.04410.051 4.6 1 .07410.061 6.2 1.07210.065 7.4 1.07810.066 12.4 1.02810.175 17.6 1.07610.104 20.0 1.07610.105 121 TIME (min) F i g u r e 4.25. A p l o t of the decay of the 5145 A induced f l u o r e s c e n c e ( • ), and the 1 5 * - 0 " P(25) : R ( 3 0 ) peak h e i g h t r a t i o , ( O ) , i n a c e l l c o n t a i n i n g about 3 0 mtorr of i o d i n e vapour and 2 . 3 t o r r of 2-hexene gas over the c o u r s e of t h e experiment i s shown above. 122 TABLE 4.21 Argon Ion Induced F l u o r e s c e n c e v e r s u s Time TIME FLUORESCENCE (min.±1.0 min.) ( a r b . ) 0 1.000 1.0 0.876±0.024 2.0 0.837±0.024 3.0 0.825±0.023 4.0 0.819±0.023 5.0 0.809±0.023 8.0 0.831±0.024 9.0 0.787±0.023 10.0 0.77610.023 "11.0 1 0.776±0.023 12.0 0.770±0.023 13.0 0.766±0.022 14.0 0.768±0.022 15.0 0.762±0.022 16.0 0.756±0.022 Based upon t h e s e r e s u l t s , a f t e r r o u g h l y 12 minutes of i r r a d i a t i o n w i t h the argon i o n l a s e r beam, no o r t h o t o para e n r i c h m e n t has o c c u r r e d t o w i t h i n a 10 p e r c e n t u n c e r t a i n t y . A g a i n t h i s c o n t r a d i c t s the f i n d i n g s of Letokhov e t a l who c l a i m e d t h a t w i t h t h e s e i n i t i a l p r e s s u r e s an enrichment f a c t o r i n e x c e s s of 4 was t o be e x p e c t e d — a 75% decrease i n the o r t h o - t o p a r a - i o d i n e r a t i o - over t h i s t i m e p e r i o d . 1 2 3 P o s s i b l e c auses f o r t h i s c o u l d be t h a t e i t h e r the 2-hexene i s not r e a c t i n g w i t h the i o d i n e m o l e c u l e s or atoms or t h a t t h e r e a c t i o n does proceed v i a a r a d i c a l c h a i n , i . e . l e t X r e p r e s e n t a 2-hexene m o l e c u l e , o - I 2 — - 2 1 I + X — XI XI + 1 2 — I + XI2 XI + I — - X I 2 As the c o n c e n t r a t i o n of i o d i n e m o l e c u l e s i s l a r g e r than t h a t of the atoms, a t l e a s t i n i t i a l l y , then i f such a mechanism i s o p e r a t i n g the most p r o b a b l e r e s u l t of i t would be t h a t ' the XI r a d i c a l s formed would a t t a c k the m o l e c u l a r i o d i n e and l i b e r a t e i o d i n e atoms. Both the XI and I s p e c i e s c o u l d c o n c e i v a b l y scramble any enhancement produced. However, Letokhov e t a l r u l e d out t h i s p o s s i b i l i t y based on the argument t h a t t h e r e a c t i o n r a t e of the i o d i n e m o l e c u l e s w i t h 2-hexene does not f o l l o w the e x p e c t e d form f o r such a mechanism (see c h a p t e r 3 ) and t h a t when a m i x t u r e of 2-hexene and i o d i n e was i r r a d i a t e d a t 4880 A.which l i e s above the d i s s o c i a t i o n l i m i t of the B 3 I I A + s t a t e of 0*u m o l e c u l a r i o d i n e , no change i n the m o l e c u l a r c o n c e n t r a t i o n was o b s e r v e d . U n f o r t u n a t e l y , no d a t a was p r o v i d e d t o support the l a t t e r c l a i m . As i t was not the purpose of t h i s i n v e s t i g a t i o n t o study the k i n e t i c s and r e a c t i o n mechanism of the above 124 s p e c i e s , t h i s a r e a of r e s e a r c h was abandonned i n s e a r c h of a more f a v o r a b l e s c a v e n g e r . 2. IODINE AND ACETYLENE A s e r i e s of t h r e e papers were produced by V. S. K u s h a w a h a 2 9 3 0 3 1 i n which l a s e r i n d u c e d p h o t o c h e m i c a l r e a c t i o n s between m o l e c u l a r i o d i n e and a c e t y l e n e , C 2 H 2 , were p e r f o r m e d . The paper of t h e most r e l e v a n c e t o t h i s w o r k 3 0 d e s c r i b e d a l a s e r i n d u c e d i s o t o p e s e p a r a t i o n of I 2 1 2 9 from I 2 1 2 7 u s i n g a c e t y l e n e as a scavenge r . The d e t a i l s of the experiment a r e l i s t e d i n t a b l e 4.21. . . 129 TABLE. 4.22 E x p e r i m e n t a l C o n d i t i o n s f o r I 2 I s o t o p e s e p a r a t i o n . BACKGROUND PRESSURE INITIAL AMOUNT OF IODINE 127 INITIAL AMOUNT OF IODINE 129 INITIAL AMOUNT OF ACETYLENE TEST CELL DIMENSIONS DYE LASER IRRADIATION REGION DYE LASER POWER 1 0 " 5 t o r r 1.0x10-*g 1.0xl0-'g (?) 30 t o r r (PYREX) 2.5 cm di a m e t e r 5.0 cm l e n g t h (6040±2)A' ( i ) lOOmW b e f o r e r e a c t i o n ( i i ) l5mW d u r i n g r e a c t i o n 125 To e f f e c t an i s o t o p e s e p a r a t i o n , Kushawaha p r e p a r e d two c e l l s , one c o n t a i n i n g i o d i n e - 1 2 7 a l o n e — the i n t r a c a v i t y c e l l — and one c o n t a i n i n g an e q u a l m i x t u r e of of the two i s o t o p e s and some a c e t y l e n e — t h e t e s t c e l l . The t e s t c e l l was i r r a d i a t e d w i t h a dye l a s e r beam, (6040±2)A, a t 100 mW of power f o r r o u g h l y 10 m i n u t e s and the p r o d u c t s sent through a mass s p e c t r o m e t e r f o r a n a l y s i s . A second s i m i l a r l y p r e p a r e d c e l l was i r r a d i a t e d w i t h t h dye l a s e r beam but w i t h the i o d i n e - 1 2 7 s p e c t r a l l i n e s removed by p l a c i n g the i n t r a c a v i t y c e l l i n s i d e the l a s e r c a v i t y . I n t h i s c a s e about 15 mW of power e x i t t e d t h e dye l a s e r and the c e l l i r r a d i a t e d f o r 60 minutes a f t e r which t i m e t h e p r o d u c t s were a g a i n a n a l y z e d by a mass s p e c t r o m e t e r . Kushawaha's r e s u l t s are shown i n f i g u r e 4.26 ( t a k e n from r e f e r e n c e 30) 126 F i g u r e 4.26. R e s u l t s o b t a i n e d by V. S. K u s h a w a h a 3 0 u s i n g a c e t y l e n e i n an i s o t o p i c s e p a r a t i o n of i o d i n e - 1 2 9 . The above two mass s p e c t o g r a p h s show the I 2 n and the C 2 H 2 I 2 n peaks a f t e r n o n - s e l e c t i v e i r r a d i a t i o n of a t e s t c e l l c o n t a i n i n g i o d i n e - 1 2 7 , i o d i n e - 1 2 9 , and a c e t y l e n e , A, and a f t e r s e l e c t i v e i r r a d i a t i o n of the I 2 1 ^ » 127 From the above s p e c t o g r a p h s i t appears t h a t , i n d e e d , an i s o t o p e s e p a r a t i o n has been produced. However, s e v e r a l unaddressed problems need t o be p o i n t e d o u t . F i r s t , i o d i h e - 1 2 9 i s a r a d i o a c t i v e s u b s t a n c e and, as s u c h , h a r d t o h a n d l e . As a r e s u l t of t h i s Kushawaha p l a c e d a c r y s t a l of the s u b s t a n c e i n each t e s t c e l l w i t h o u t w e i g h i n g i t and judged the weight by comparing i t s s i z e t o the s i z e of an i o d i n e - 1 2 7 c r y s t a l w e i g h i n g about 1.0x10"* gram a l s o p l a c e d i n the c e l l . The a c t u a l amount of the i o d i n e - 1 2 9 o r i g i n a l l y i n any g i v e n t e s t c e l l was unknown and v e r y l i k e l y v a r i e d c o n s i d e r a b l y from p r e p a r a t i o n t o p r e p a r a t i o n . Thus one can not r i g o r o u s l y compare r e s u l t s o b t a i n e d from d i f f e r e n t t e s t c e l l s . A c u r i o u s f e a t u r e of t h e r e s u l t o b t a i n e d when the sample was i r r a d i a t e d on a l l the s p e c t r a l l i n e s i s the f a i r l y l a r g e C 2 H 2 I 1 2 7 I 1 2 9 peak. T h i s would seem t o i n d i c a t e t h e e x i s t e n c e of some s c r a m b l i n g mechanism t a k i n g p l a c e d u r i n g the r e a c t i o n as t h e r e was v e r y l i t t l e of t h i s mixed s p e c i e s o r i g i n a l l y p r e s e n t i n t h e t e s t c e l l s . That i s , i f the r e a c t i o n proceeded s o l e l y v i a the r e a c t i o n , I 2° * + C 2 H 2 — C 2 H 2 I 2 n then one would expect the f i n a l p r o d u c t s t o r e f l e c t the o r i g i n a l r a t i o s of the I 2 1 2 7 : I 2 1 2 9 : i 1 2 7 j i 2 s S p e c i e s . T h i s dilemma i s f u r t h e r compounded by the f a c t t h a t no mixed s p e c i e s peak was o b s e r v e d a f t e r the s e l e c t i v e i r r a d i a t i o n of 128 the i o d i n e - 1 2 9 . With t h e s e c o n c e r n s a s i d e , the d a t a p r o v i d e d appears q u i t e p r o m i s i n g . Indeed i f a c e t y l e n e gas does r e a c t p r e f e r e n t i a l l y w i t h e x c i t e d i o d i n e m o l e c u l e s then i t would proove t o be an e x c e l l e n t scavenger w i t h which t o perfo r m an o r t h o - p a r a enhancement. A c e t y l e n e gas, 99.6% p u r e , was o b t a i n e d from Matheson Co. and i n t r o d u c e d t o s t o r a g e b u l b a t t a c h e d t o the vacuum l i n e . The gas was d i s t i l l e d from t r a p t o t r a p s e v e r a l times t o ensure i t s p u r i t y and remove any a i r t h a t c o u l d have c o n t a m i n a t e d the sample w h i l e f i l l i n g the b u l b . O b v i o u s l y one wis h e s t o a v o i d the f o r m a t i o n of a r a d i c a l c h a i n d u r i n g t h e s e e x periments as t h e s e w i l l l e a d t o the f o r m a t i o n of s p e c i e s t h a t may be a b l e t o scramble the o r t h o - and p a r a - i o d i n e . To a v o i d t h i s d i f f i c u l t y t he 15'-0" P(25) l i n e , which has low p r e d i s s o c i a t i o n , was chosen t o d r i v e the r e a c t i o n and, a l o n g w i t h the 15* — 0" R ( 3 0 ) , used t o m o n i t o r the o r t h o t o p a r a r a t i o . The f i r s t e x p e r i m e n t s performed w i t h a c e t y l e n e and i o d i n e was c a r r i e d out i n a pyrex c e l l w i t h a 1.9 cm di a m e t e r and 30 cm l e n g t h evacuated and f i l l e d w i t h the d e s i r e d amounts of i o d i n e and a c e t y l e n e . The c e l l was then removed from the vacuum l i n e and a r r a n g e d i n an appara t u s shown i n f i g u r e 4.27. The dye l a s e r beam was expanded t o r o u g h l y a 1.5 cm di a m e t e r b e f o r e e n t e r i n g t h e c e l l and the f l u o r e s c e n c e r e c o r d e d , as b e f o r e , on a s t r i p c h a r t r e c o r d e r . Note t h a t when the dye l a s e r was 129 DYE LASER TO STRIP CHART RECORDER BEAM ST0P^7 1 CJ77777, EMI 9558 OB • PHOTOMULTIPLIER + CS2-62 FILTER TEST CELL J 1— BEAM EXPANDER F i g u r e 4.27 The e x p e r i m e n t a l arrangement used f o r the s e l e c t i v e i r r a d i a t i o n of o r t h o - i o d i n e i n the presence of a c e t y l e n e . A t e s t c e l l , (1.9 cm d i a m e t e r , 30 cm l e n g t h ) , c o n t a i n i n g about (27±3)mtorr of i o d i n e vapour and (30±2) t o r r of a c e t y l e n e gas was i r r a d i a t e d w i t h t h e dye l a s e r b o t h t o d r i v e t h e r e a c t i o n u s i n g the 15'-0" P(25) s p e c t r a l l i n e and mo n i t o r the o r t h o - t o p a r a - i o d i n e r a t i o w i t h the 15'-0 B P(25) and R(30) peaks. Note t h a t the l a s e r beam was expanded b e f o r e n t e r i n g the c e l l . 1 30 se t t o scan a c r o s s the s p e c t r a l r e g i o n of i n t e r e s t t o mo n i t o r the o r t h o - and p a r a - i o d i n e peaks the i n t e n s i t y of the beam was de c r e a s e d by a f a c t o r of 100 w i t h the use of a 2.0 n e u t r a l d e n s i t y f i l t e r . TABLE 4.23 E x p e r i m e n t a l C o n d i t i o n s BACKGROUND PRESSURE 3 x 1 0 ' 5 t o r r INITIAL IODINE PRESSURE (27±3)mtorr INITIAL ACETYLENE (30±2)torr PRESSURE TEST CELL DIMENSIONS (PYREX) 1.9 cm diameter 30 cm l e n g t h DYE LASER POWER (150±10)mW EXCITATION LINE 15 *-0" P(25) DYE LASER MONITOR LINES B X (15 *-0") P ( 2 5 ) f R ( 3 0 ) »>=17408 cm - i SCAN LENGTH 30 GHz SCAN TIME 75 S 131 1.25 r co o || 1.001-l -I g UJ i U J 0 . T i 0.50' J L _ 10 TIME (min) 20 3 0 3 3 F i g u r e 4.28 The 15'-0 B P(25) : R(30) ( O ) , and 15'-0" P(25) : 18• — 1" P(84) ( ^ ) , peak h e i g h t r a t i o s as a f u n c t i o n of time over the c o u r s e of the experiment o b s e r v e d i n a c e l l c o n t a i n i n g about 27 mtorr of i o d i n e and 30 t o r r of a c e t y l e n e . 132 TABLE 4.24 Peak H e i g h t R a t i o s v e r s u s Time Time 15'-0" P(25) l 5 ' - 0 n P(25) (min.±1 15'-0" R(30) 18'-1" P(84) min.) 0 1.143±0.032 0.645±0.024 15 1.12610.031 0.643±0.020 33 1.165±0.045 0.636±0.019 82 1.158±0.048 0.638±0.024 In t h i s experiment the dye l a s e r scanned a c r o s s the 30 GHz s e c t i o n of spectrum s e v e r a l t i m e s and then tuned t o the 15'-0" P(25) l i n e . At t h i s f r e q u e n c y t h e c e l l was i r r a d i a t e d f o r about f i f t e e n , minutes and then the spectrum rescanned s e v e r a l t i m e s . T h i s p r o c e d u r e was r e p e a t e d t w i c e more i r r a d i a t i n g the c e l l w i t h the P(25) peak f o r 13 and 45 minutes r e s p e c t i v e l y . As i s r e a d i l y o bserved from the r e s u l t s shown on f i g u r e 4.28 and t a b l e 4.24, no e v i d e n c e f o r any s h i f t i n the r a t i o of the o r t h o - i o d i n e t o p a r a - i o d i n e was o b s e r v e d t o w i t h i n 5%. A f i n a l attempt t o produce an enhancement u s i n g a c e t y l e n e as a scavenger was made. T h i s time the l a s e r power was d e c r e a s e d t o about 150 mW w i t h o u t expanding the beam, the i n i t i a l p r e s s u r e s of i o d i n e and a c e t y l e n e were d e c r e a s e d , and the s i z e of the c e l l used was d e c r e a s e d . As b e f o r e the t e s t c e l l was p r e p a r e d by b a k i n g i t out w h i l e e v a c u a t i n g i t f o r r o u g h l y 21 hours p r i o r t o use. 1 33 I o d i n e vapour and a c e t y l e n e gas were a l l o w e d t o d i f f u s e i n t o the c e l l c o n t r o l l e d by c o n s t a n t t e m p e r a t u r e b a t h s . TABLE 4.25 E x p e r i m e n t a l C o n d i t i o n s BACKGROUND PRESSURE INITIAL IODINE PRESSURE INITIAL ACETYLENE PRESSURE TEST CELL DIMENSIONS DYE LASER POWER EXCITATION LINE DYE LASER MONITOR LINES SCAN LENGTH SCAN TIME 3 . 6 x l 0 ' 6 t o r r (17±1)mtorr (7.0±0.4)torr (PYREX) 0.6 cm dia m e t e r 15 cm l e n g t h (l70±10)mW 15'-0" P(25) B X t15'-0") P ( 2 5 ) r R ( 3 0 ) 7408 cm" 1 30 GHz 75 s TABLE 4.26 Peak H e i g h t R a t i o s v e r s u s Time Time 15'-0" P(25) 15'-0" P(25) (min.±1 15'.-0" R(30) 18'-1" P(84) min.) 0 1.08310.028 0.63410.013 31 1.11110.032 0.63910.014 44 1.083+0.033 0.63110.015 134 A g a i n , no o r t h o t o para r a t i o s h i f t was o b s e r v e d . 3. IODINE AND NITRIC OXIDE A f t e r these s e t b a c k s an attempt t o f i n d a s i m p l e r scavenger was i n i t i a t e d . A paper w r i t t e n by N. Basco and J . E. H u n t 5 6 r e p o r t e d on the r e c o m b i n a t i o n of i o d i n e atoms i n the presence of n i t r i c o x i d e and a r g o n . The proposed mechanism was, I + NO + Ar — I N O + Ar I + INO — - I 2 + NO 2INO — - I 2 + NO where k, = 3 . 5x 1 0 9 l 2 / m o l e 2 s, k 2 ~ 1 x l 0 1 1 l / m o l e s, and k 3 2 . 1 x l 0 7 l / m o l e s. N i t r i c o x i d e may make a good scavenger f o r the i o d i n e m o l e c u l e s or perhaps s i m p l y h e l p them t o recombine. U n f o r t u n a t e l y , n i t r i c o x i d e i s a paramagnetic s p e c i e s and, as such, c a p a b l e of c o n v e r t i n g o r t h o - i o d i n e t o p a r a - i o d i n e and v i c e - v e r s a . I f the r a t e a t which t h i s c o n v e r s i o n o c c u r s i s l a r g e compared t o k, and k 3 then no s h i f t w i l l be o b s e r v e d . On the o t h e r hand, f o r the case where th e r a t e c o n s t a n t i s of the same o r d e r of magnitude as t h a t f o r the c o n v e r s i o n i n hydrogen then the h a l f - l i f e f o r t h e c o n v e r s i o n w i l l be about 17 minutes f o r a c e l l c o n t a i n i n g 50 t o r r of n i t r i c o x i d e and 330 minutes f o r one w i t h 3 t o r r . 1 3 5 Q I J UL 1/ t~30min t ~60 min F i g u r e 4.29. The above a r e t y p i c a l s p e c t r a of the 14'-1" P(77) [a] and 14'-1" R(82) [b] peaks o b s e r v e d i n a c e l l c o n t a i n i n g about 3 m t o r r of i o d i n e and 3 t o r r of n i t r i c o x i d e a f t e r d i f f e r e n t amounts of argon i o n l a s e r i r r a d i a t i o n . N o t i c e t h a t the s p e c t r a do not v a r y a p p r e c i a b l y , not even i n h e i g h t . 1 36 To f i n d out i f t h i s would work as a s c a v e n g e r , s e v e r a l e x p e r i m e n t s were performed u s i n g the method d i s c r i b e d on page on c e l l s c o n t a i n i n g r o u g h l y 3 m t o r r of i o d i n e , 20 t o r r of h e l i u m , ( h e l i u m was used i n s t e a d of argon because i t quenches th e f l u o r e s c e n c e of i o d i n e t o a l e s s e r e x t e n t ) and between 3 and 58 t o r r of n i t r i c o x i d e . As summarized by f i g u r e 4.29 w h i c h d i s p l a y s a t y p i c a l spectrum, no e v i d e n c e f o r any enhancement was o b s e r v e d . T h i s was the c a s e when the e x c i t a t i o n l i n e was the argon i o n beam or the dye l a s e r beam tuned t o t h e 1 6* — 1 ** P ( 8 3 ) , a more h i g h l y p r e d i s s o c i a t i n g t r a n s i t i o n . Thus one must c o n c l u d e t h a t the n i t r i c o x i d e r e a d i l y s c r a m b l e d the two s p e c i e s . 4 . IODINE AND NITROSYL CHLORIDE To t r y and a v o i d t h e problems encountered w i t h h a v i n g a f r e e r a d i c a l p r e s e n t , the n i t r i c o x i d e was r e p l a c e d w i t h n i t r o s y l c h l o r i d e , NOC1. I t was hoped t h a t t h i s compound would p r o v i d e e f f i c i e n t s c a v e n g i n g of the e x c i t e d i o d i n e m o l e c u l e s v i a , I 2 * + NOC1 INO +IC1 The same pr o c e d u r e as p r e v i o u s l y d e s c r i b e d u s i n g the dye l a s e r b o t h t o d r i v e the r e a c t i o n and m o n i t o r the o r t h o -and p a r a - i o d i n e c o n c e n t r a t i o n s was employed on t e s t c e l l s f i l l e d w i t h r o u g h l y 3 t o r r of n i t r o s y l c h l o r i d e and 150 137 mtorr of i o d i n e vapour. T y p i c a l e x p e r i m e n t a l c o n d i t i o n s a r e l i s t e d i n t a b l e 4.27. TABLE 4.27 E x p e r i m e n t a l C o n d i t i o n s BACKGROUND PRESSURE INITIAL IODINE PRESSURE INITIAL N0C1 PRESSURE TEST CELL DIMENSIONS DYE LASER POWER EXCITATION LINE DYE LASER MONITOR LINES SCAN LENGTH SCAN TIME 3 x 1 0 " 5 t o r r (150±10)mtorr (3±1)torr (PYREX) 1.9 cm diameter 30 cm l e n g t h (300±lO)mW 1 9 ' - I n P(95) B X (18'-1") P(37),R(42) j>*O7408 cm" 1 30 GHz 75 s The r e s u l t s of the s e e x p e r i m e n t s were q u i t e unexpected. Upon f i r s t i r r a d i a t i n g the t e s t c e l l w h i l e s c a n n i n g the 30 GHz r e g i o n of i n t e r e s t no m o l e c u l a r i o d i n e spectrum was e v i d e n t . W i t h the a i d of a c e l l c o n t a i n i n g i o d i n e vapour by i t s e l f , t he dye l a s e r was tuned t o the 19* — 1" P(95) peak and the t e s t c e l l was i r r a d i a t e d a t t h i s wavelength f o r about f i f t e e n m i n u t e s . D u r i n g t h i s time the f l u o r e s c e n c e caused by the dye l a s e r beam a t t h i s f r e q u e n c y i n c r e a s e d by a f a c t o r of two. Upon r e s c a n n i n g the dye l a s e r , m o l e c u l a r i o d i n e peaks were c l e a r l y v i s i b l e . T h i s p r o c e d u r e was 138 F i g u r e 4.30. The above two s p e c t r a were r e c o r d e d w i t h the dye l a s e r s c a n n i n g a c r o s s the 17408 cm -' r e g i o n i r r a d i a t i n g a t e s t c e l l c o n t a i n i n g about 150 m torr of i o d i n e and 3 t o r r of n i t r o s y l c h l o r i d e . The f i r s t t r a c e , ( a ) , was t a k e n b e f o r e any s t r o n g i r r a d i a t i o n of the c e l l had t a k e n p l a c e . The second t r a c e , ( b ) , was r e c o r d e d a f t e r i r r a d i a t i n g the m i x t u r e f o r about 15 m i n u t es w i t h the the 19'-1" P(95) t r a n s i t i o n . The i o d i n e spectrum i s c l e a r l y v i s i b l e a f t e r the i r r a d i a t i o n of the t e s t c e l l (as i n d i c a t e d by the arrows above) but not d i s c e r n a b l e o r i g i n a l l y . 139 r e p e a t e d once more and the i o d i n e l i n e s became more pronounced. From th e s e r e s u l t s i t would appear t h a t n i t r o s y l c h l o r i d e r e a c t e d w i t h t h e ground s t a t e m o l e c u l e s t o form perhaps IC1. Upon i r r a d i a t i o n , the I C l d i s s o c i a t e d t o produce I 2 and C l 2 . i . e . 1 2 + N0C1 —•» I + I C l + NO I + NOC1 — * - I C l + NO 2IC1 .+ hv —*- I 2 + C l 2 T h i s i s not n e c e s s a r i l y what happens, but i s a p l a u s i b l e e x p l a n a t i o n . In any c a s e , n i t r o s y l c h l o r i d e i s o b v i o u s l y a poor c h o i c e f o r a scavenger of e x c i t e d i o d i n e m o l e c u l e s a l o n e . 5. IODINE AND ETHYL IODIDE A l l of the p r e v i o u s r e s u l t s i n d i c a t e t h a t the i o d i n e m o l e c u l e s are v e r y s u s c e p t i b l e t o any f r e e r a d i c a l c o n t a m i n a t i o n , such as f r e e i o d i n e atoms, n i t r i c o x i d e , or a complex r a d i c a l formed by the a d d i t i o n of an i o d i n e atom t o an o r g a n i c compound. A f i n a l i d e a t h a t was t r i e d was t o exchange an i o d i n e atom from the s e l e c t i v e l y e x c i t e d i o d i n e m o l e c u l e w i t h one a t t a c h e d t o an i o d i n a t e d o r g a n i c compound. T h i s i s s i m i l a r t o what R. N. Zare e t a l 3 3 d i d t o produce a p h o t o c h e m i c a l 1 40 i s o t o p i c s e p a r a t i o n of 3 5C1 2 and 3 7C1 2. I t i n v o l v e d t a k i n g a n o n r a d i c a l compound c o n t a i n i n g a t l e a s t one i o d i n e atom a t t a c h e d t o i t , X I . T h e o r e t i c a l l y , as the e x c i t e d i o d i n e m o l e c u l e came i n t o c o n t a c t w i t h XI i t would exchange one of i t s i o d i n e atoms w i t h one a l r e a d y a t t a c h e d t o the scavenger. In the b e s t p o s s i b l e s c e n a r i o t h e s p e c i e s , X I , would r e a c t s o l e l y w i t h the e x c i t e d i o d i n e m o l e c u l e s and be i n e r t as re g a r d s the ground s t a t e m o l e c u l e s . A f t e r each exchange the r e s u l t i n g i o d i n e m o l e c u l e would have a 7/12 chance of bein g an o r t h o - i o d i n e and a 5/12 chance of b e i n g a p a r a - i o d i n e . Hence, i f one were s e l e c t i v e l y e x c i t i n g the o r t h o s p e c i e s , one would o b t a i n a net de c r e a s e i n the number of o r t h o m o l e c u l e s i n the system. E t h y l - i o d i d e , C 2 H 5 I , ( a b b r e v i a t e d EI here) was chosen as a t r i a l exchange p a r t n e r m a i n l y due t o time c o n s t r a i n t s , t he f a c t t h a t i t was r e a d i l y a v a i l a b l e , and because some data, on m e t h y l - i o d i d e , C H 3 I 3 * , a s i m i l a r m o l e c u l e , s uggested t h a t m e t h y l - i o d i d e exchanges atoms w i t h m o l e c u l a r i o d i n e i n the gas phase. F i s c h e r c e r t i f i e d e t h y l - i o d i d e was p l a c e d on the vacuum l i n e and p r e p a r e d f o r use by t r a p t o t r a p d i s t i l l a t i o n and then s t o r e d i n a pyrex c o n t a i n e r a t t a c h e d t o the m a n i f o l d . A t e s t c e l l was p r e p a r e d by e v a c u a t i n g i t f o r about 24 hours and then f i l l i n g i t w i t h t h e d e s i r e d p r e s s u r e s of each s u b s t a n c e . The c e l l was then p i n c h e d o f f of the system. To d r i v e the r e a c t i o n t h e 15'~0" P(25) s p e c t r a l l i n e was chosen t o have as few i o d i n e atoms p r e s e n t as p o s s i b l e d u r i n g t he ex p e r i m e n t . As u s u a l , t he dye l a s e r was scanned 141 a c r o s s a p r e s e l e c t e d r e g i o n of i o d i n e spectrum s e v e r a l times b e f o r e i n i t i a t i n g the experiment and the f l u o r e s c e n c e r e c o r d e d t o a s c e r t a i n the time z e r o peak h e i g h t r a t i o . T h i s a c c o m p l i s h e d , the dye l a s e r was tuned t o the P(25) t r a n s i t i o n and the c e l l i r r a d i a t e d f o r a s p e c i f i c amount of time and then the dye l a s e r scanned a g a i n . The r e s u l t s of t h i s p r o c e d u r e are g i v e n below. TABLE 4.28 E x p e r i m e n t a l C o n d i t i o n s BACKGROUND PRESSURE 3x10" 51 o r r INITIAL IODINE PRESSURE (30±2)mtorr INITIAL ETHYL IODIDE (l3±2)torr PRESSURE TEST CELL DIMENSIONS (PYREX) 1.9 cm di a m e t e r 30 cm l e n g t h DYE LASER POWER (190±10)mW EXCITATION LINE l 5 ' - 0 n P(25) DYE LASER MONITOR LINES B X (15'-0") P(25),R(30) •^17408 cm - 1 SCAN LENGTH 30 GHz SCAN TIME 75 S 142 1.30 120 -V) 1.10 -g J £ 1.00 -1 0.90 * 0.80 2 0.70 D_ o.eoh 0.50 _L 20 40 60 TIME (min) f i 80 100 -fh 160 F i g u r e 4.31. A p l o t of l 5 ' - 0 " P(25) : 15'-0" R ( 3 0 ) , (O), 15'-0" P(25) : 1B:-1 - P ( 8 4 ) f ( A ) , and l 5 ' - 0 " P(25) : 19'-1" P ( 1 2 1 ) , (•), peak h e i g h t r a t i o s v e r s u s time i n a c e l l c o n t a i n i n g about 30 mtorr of i o d i n e and 13 t o r r of e t h y l e n e i o d i d e . No e v i d e n c e f o r an o r t h o - t o p a r a - i o d i n e r a t i o s h i f t was o b s e r v e d . 143 TABLE 4.2? Peak H e i g h t R a t i o s v e r s u s Time Time 15'-0" P(25) 15'-0" P(25) 15'-0" P(25) (min.±1 15'-0" R(30) 18'-1" P(84) 19'-1" P(121) min.) 0 1„056±0.011 0.631±0.005 1.253±0.010 62.3 1.060+0.013 O.633±0.006 1.248±0.017 97.0 1.06210.013 0.629±0.012 1.25110.017 161.0 1.05510.014 0.62910.008 1.25310.018 As i n a l l of the p r e v i o u s e x p e r i m e n t s , an i n d i c a t i o n t h a t an o r t h o t o para r a t i o s h i f t c o u l d have o c c u r r e d would have been a de c r e a s e i n the 15'~0" P(25) induced f l u o r e s c e n c e d u r i n g the i r r a ' d i a t i o n p e r i o d . In t h i s case almost no change was observed a t a l l and, i n d e e d , the r a t i o s of the o r t h o - i o d i n e peak h e i g h t t o the p a r a - i o d i n e peak h e i g h t s d i s p l a y e d no t r a c e of a s h i f t . V. CONCLUSIONS AND DISCUSSION A. COMMENTS ON THIS RESEARCH The main impetus f o r t h i s r e s e a r c h , as has been s t a t e d p r e v i o u s l y , began w i t h the p r o m i s i n g r e s u l t s r e p o r t e d by Letokhov e t a l and, i n d e e d , t h i s a u t h o r a t t e m p t e d t o reproduce t h e i r e x p e r i m e n t a l c o n d i t i o n s as c l o s e l y as p o s s i b l e . U n f o r t u n a t e l y , the f i n d i n g s of t h i s work show no e v i d e n c e t o support the c l a i m t h a t s e l e c t i v e p r e d i s s o c i a t i o n of the o r t h o - i o d i n e m o l e c u l e s w i t h the 5145 A l i n e of an argon i o n l a s e r l e a d s t o any r e l a t i v e enhancement of the p a r a - i o d i n e p o p u l a t i o n e i t h e r i n a system of pure i o d i n e vapour or i n a m i x t u r e of i o d i n e and any of the sc a v e n g e r s 2-hexene, a c e t y l e n e , n i t r i c o x i d e , n i t r o s y l c h l o r i d e , or e t h y l - i o d i d e . The q u e s t i o n i m m e d i a t e l y a r i s e s as how t o r e c o n c i l e the two c l a i m s . To a d d r e s s t h i s q u e s t i o n the e x p e r i m e n t s i n v o l v i n g i o d i n e vapour a l o n e and those w i t h a m i x t u r e of i o d i n e p l u s some o t h e r c o n s t i t u e n t w i l l be examined s e p a r a t e l y . In t h e former c a s e , the system i s much s i m p l e r than the l a t t e r as i t i n v o l v e s p r i m a r i l y o n l y i o d i n e m o l e c u l e s , atoms, and, perhap s , some w a l l e f f e c t s . I n both the i n v e s t i g a t i o n s of Letokhov e t a l and t h i s a u t h o r f l u o r e s c e n c e measurements were used t o determine the r e l a t i v e amounts of o r t h o - and p a r a - i o d i n e . However, the t e c h n i q u e employed h e r e , namely u s i n g a dye l a s e r t o scan a c r o s s a s p e c i f i c r e g i o n of the m o l e c u l a r i o d i n e spectrum 144 i 145 c o n t a i n i n g a t l e a s t one o r t h o and one para peak b e l o n g i n g to the same v i b r a t i o n a l t r a n s i t i o n and h a v i n g s i m i l a r r o t a t i o n a l quantum numbers, was s u p e r i o r t o t h a t of Letokhov e t a l e s p e c i a l l y when the dye l a s e r and the argon i o n l a s e r were used s i m u l t a n e o u s l y . F i r s t of a l l t h i s t e c h n i q u e reduced t h e p o s s i b i l i t y of r e l a t i v e peak h e i g h t changes i n o r t h o and para s p e c i e s due t o h e a t i n g of t h e t e s t c e l l by the l a s e r and t o d i f f e r e n c e s of c o l l i s i o n a l quenching of d i f f e r e n t v i b r a t i o n a l t r a n s i t i o n s . The former may not be v e r y i m p o r t a n t but the l a t t e r , as was i n d i c a t e d by the s e r i e s of e x p e r i m e n t s i n w h i c h the r e s i d u a l p r e s s u r e t o which the t e s t c e l l s were e v a c u a t e d , may p l a y a l a r g e r o l e . I t seems t h a t the v i b r a t i o n a l s t a t e s a r e d i f f e r e n t i a l l y quenched by f o r e i g n gas. Thus i f the c e l l degases over the c o u r s e of the experiment ("1 t o 2 hours) then as the p r e s s u r e i n s i d e the c e l l i n c r e a s e s r e l a t i v e l i n e s t r e n g t h s v a r y . T h i s v a r i a t i o n would be the most a p p a r a n t at low i n i t i a l vapour p r e s s u r e s of i o d i n e as i n t h e s e c a s e s . t h e r e i s l i t t l e s e l f q u e n c h i n g . S e c o n d l y , t h i s t e c h n i q u e a l l o w s t h e e x p e r i m e n t e r t o more r e a d i l y o b s e r ve any change o c c u r r i n g i n the c o n c e n t r a t i o n s of each i o d i n e m o d i f i c a t i o n w i t h o u t h a v i n g t o s t o p the p r i m a r y i r r a d i a t i o n p r o c e s s and p r e v e n t i n g any e x t r a r e v e r s e reaction from o c c u r r i n g . The w i l d c a r d i n t h i s whole a f f a i r was the m a t e r i a l used f o r c o n s t r u c t i n g t h e t e s t c e l l s . L etokhov e t a l made t h e i r c e l l s of molybdenum g l a s s w h i l e the ones used f o r t h i s r e s e a r c h were p y r e x . As p r e v i o u s l y s t a t e d , our g l a s s b l o w e r 146 was unable t o o b t a i n any i n f o r m a t i o n on t h i s type of g l a s s and, hence, the c e l l m a t e r i a l was not m o d i f i e d d u r i n g the e x p e r i m e n t s d e s c r i b e d i n c h a p t e r 4. In a r e c e n t p e r s o n a l communication w i t h Dr. W. M a j e w s k i , who v i s i t e d the U n i v e r s i t y of B r i t i s h Columbia t o g i v e a seminar, informed me t h a t molybdenum g l a s s i s a type of g l a s s made t o have the same c o e f f i c i e n t of e x p a n s i o n as molybdenum so t h a t molybdenum e l e c t r o d e s may be p l a c e d i n s i d e g l a s s c e l l s . G e n e r a l l y i t i s s l i g h t l y more porous than p y r e x . I t may be t h a t the i o d i n e atoms a r e more r e a d i l y adsorbed and h e l d by t h i s g l a s s than by pyrex and t h a t t h i s s u r f a c e i s a b e t t e r one t o use t o o b t a i n the d e s i r e d e f f e c t . In the c a s e s where scavenger f o r the i o d i n e atoms and/or m o l e c u l e s were added t o the system t h e i n t e r p r e t a t i o n of the r e s u l t s i s more c o m p l i c a t e d and more s p e c u l a t i v e . For example, i n the case of 2-hexene, the r e s u l t s once a g a i n c o n t r a d i c t e d the f i n d i n g of Letokhov e t a l . As mentioned b e f o r e , Letokhov c l a i m e d an enrichment f a c t o r of up t o 4 w i t h m i x t u r e s of 40 m t o r r of i o d i n e vapour and about 2 t o r r of 2-hexene, w i t h the l o w e s t v a l u e of 2 a t somewhat h i g h e r p r e s s u r e s of i o d i n e vapour. The e x p e r i m e n t s r e p o r t e d here showed no enrichment whatsoever when s i m i l a r m i x t u r e s were s t u d i e d . From the r e s u l t s g i v e n i n c h a p t e r 4 i t would be p o s t u l a t e d t h a t the r e a c t i o n mechanism of t h e two m o l e c u l e s i n v o l v e d a r a d i c a l c h a i n i n which the 2-hexene p i c k e d up an i o d i n e atom from the e x c i t e d m o l e c u l e and p r o d u c i n g a f r e e atom p l u s the r a d i c a l X I . Both of t h e s e s p e c i e s may be a b l e 147 t o a t t a c k t h e ground s t a t e m o l e c u l e s by e i t h e r b r e a k i n g them up t h e r e b y p e r p e t u a t i n g the c h a i n or c a u s i n g o r t h o - t o p a r a - i o d i n e c o n v e r s i o n s and v i c e - v e r s a . L e t o k h o v , however, s t a t e s t h a t the i n i t i a l r a t e of change i n t h e o r t h o - i o d i n e c o n c e n t r a t i o n goes as the square r o o t of t h e 2-hexene c o n c e n t r a t i o n which does not s u p p o r t such a r a d i c a l c h a i n mechanism nor does i t f o l l o w from the d i r e c t a d d i t i o n of an e x c i t e d i o d i n e m o l e c u l e t o the 2-hexene. A g a i n , i t c o u l d be t h a t the d i r e c t a d d i t i o n of t h e e x c i t e d i o d i n e m o l e c u l e s t o the 2-hexene i s c a t a l y s e d somehow by the molybdenum g l a s s but not by p y r e x . A c e t y l e n e ; t o o , I b e l i e v e , r e a c t s w i t h t h e e x c i t e d i o d i n e v i a a r a d i c a l c h a i n as d e s c r i b e d above. T h i s i s c o n t r a r y t o the view of Kushawaha. However, the b a s i s f o r h i s c l a i m i s q u e s t i o n a b l e . As p r e v i o u s l y p o i n t e d o u t , problems w i t h Kushawaha's i n t e r p r e t a t i o n of t h e d a t a stem from i n h e r e n t u n c e r t a i n t y i n the method of f i l l i n g each t e s t c e l l w i t h t h e i o d i n e - 1 2 9 which makes r e p r o d u c i b i l i t y and comparison of r e s u l t s d u b i o u s . Another d i f f i c u l t y l a y i n t h e f a c t t h a t t h e two mass s p e c t o g r a p h s p u b l i s h e d i n r e f e r e n c e (30) c o n c e r n i n g the i s o t o p i c s e p a r a t i o n of i o d i n e do not f o l l o w one's i n t u i t i o n as t o what s h o u l d happen f o r a d i r e c t a d d i t i o n r e a c t i o n ( s e e pJ27) but p o i n t s t o a r a d i c a l c h a i n , a t l e a s t i n t h e f i r s t c a s e of the n o n s e l e c t i v e i r r a d i a t i o n of the sample. Indeed i f a s e l e c t i v e i r r a d i a t i o n of t h s i o d i n e - 1 2 9 r e s u l t e d i n an i s o t o p i c s e p a r a t i o n t h e n an o r t h o -t o p a r a - i o d i n e s h i f t s h o u l d a l s o have been o b s e r v e d . 148 U n f o r t u n a t e l y , the r e s u l t s of the i n v e s t i g a t i o n s r e p o r t e d here show no e v i d e n c e f o r such a s h i f t . U l t i m a t e l y n i t r i c o x i d e , n i t r o s y l c h l o r i d e , and e t h y l i o d i d e prooved i n e f f e c t i v e a l s o i n the e f f o r t s t o s h i f t the r a t i o . The e x p l a n a t i o n s f o r each of t h e s e i s somewhat s i m p l e r . N i t r i c o x i d e i t s e l f i s paramagnetic and, hence, undoubtedly causes o r t h o - p a r a c o n v e r s i o n f a i r l y r a p i d l y i n m o l e c u l a r i o d i n e . As d i s c u s s e d i n c h a p t e r 2, the r a t e of c o n v e r s i o n of o r t h o - i o d i n e t o p a r a - i o d i n e may be from 25 to 10 5 t i m e s l a r g e r than the c o r r e s p o n d i n g r a t e i n m o l e c u l a r hydrogen. Hence a t 3 t o r r of n i t r i c o x i d e a 330 minute h a l f - l i f e f o r the s h i f t i s e x p e c t e d f o r hydrogen which may drop down t o o n l y a m a t t e r of seconds i n the case of i o d i n e . N i t r o s y l c h l o r i d e i s not u s e f u l as a scavenger s i m p l y because i t appears t o r e a c t w i t h t h e ground s t a t e i o d i n e m o l e c u l e s s p o n t a n e o u s l y . I t i s i n t e r e s t i n g , though, t h a t upon i r r a d i a t i o n w i t h a l a s e r beam the m o l e c u l a r i o d i n e s p e c i e s r e a p p e a r s and does not seem t o d i s a p p e a r a g a i n , at l e a s t over the c o u r s e of about h a l f an hour. (By comparison the n i t r o s y l c h l o r i d e o r i g i n a l l y r e a c t e d w i t h the i o d i n e w i t h i n a p e r i o d of t e n minutes w h i l e the t e s t c e l l was kept i n the d a r k . ) As p r e v i o u s l y p o s t u l a t e d , t h i s may be due t o t h e spontaneous f o r m a t i o n of IC1 and i t s d e c o m p o s i t i o n i n t o I 2 and C l 2 under i r r a d i a t i o n . E t h y l i o d i d e , which had appeared t o be the most p r o m i s i n g method t o o b t a i n an enhancement, d i d not exchange i o d i n e atoms t o any o b s e r v a b l e e x t e n t over the c o u r s e of the 149 • i r r a d i a t i o n p e r i o d . I t i s p o s s i b l e t h a t much more energy i s r e q u i r e d t o o b t a i n such an exchange than was p r o v i d e d by the f r e q u e n c y t o which the dye l a s e r was t u n e d . Based upon these f i n d i n g s , no o r t h o - t o p a r a - i o d i n e r a t i o s h i f t has been d e t e c t e d t o w i t h i n an u n c e r t a i n t y of f i v e p e r c e n t v i a the s e l e c t i v e p r e d i s s o c i a t i o n of o r t h o - i o d i n e m o l e c u l e s u s i n g l a s e r i r r a d i a t i o n or a l a s e r i n d u c e d p h o t o c h e m i c a l r e a c t i o n of o r t h o - i o d i n e m o l e c u l e s w i t h the s c a v e n g e r s 2-hexene, a c e t y l e n e , n i t r i c o x i d e , n i t r o s y l c h l o r i d e , or e t h y l i o d i d e . The o b s e r v a t i o n s and r e s u l t s o b t a i n e d seem t o i n d i c a t e the f o r m a t i o n of f r e e r a d i c a l s d u r i n g the a f o r e m e n t i o n e d p r o c e s s e s and i t appears t h a t m o l e c u l a r i o d i n e i s q u i t e s u s c e p t i b l e t o o r t h o - p a r a c o n v e r s i o n c a t a l y s e d by t h e s e r a d i c a l s . B. SUGGESTIONS FOR FUTURE WORK T h i s r e s e a r c h l e f t the q u e s t i o n as t o how i m p o r t a n t the m a t e r i a l used f o r the c o n s t r u c t i o n of the t e s t c e l l i s t o the outcome of t h e e x p e r i m e n t . S p e c i f i c a l l y , do molybdenum g l a s s c e l l s behave v e r y d i f f e r e n t l y from pyrex ones? I f one were t o c o n t i n u e t h i s work, t h e n , i t would be d e s i r a b l e t o r e p e a t t h e i o d i n e vapour and i o d i n e p l u s 2-hexene e x p e r i m e n t s u s i n g the two l a s e r s i m u l t a n e o u s i r r a d i a t i o n t e c h n i q u e w i t h the samples c o n t a i n e d i n molybdenum g l a s s c e l l s . ( I ndeed, Dr.W. Majewski b e l i e v e s t h a t C o r n i n g produces a t ype of g l a s s s i m i l a r t o t h e type used by 150 Letokhov e t a l ) Another p r o m i s i n g t a c t i c would be t o s e a r c h more c a r e f u l l y f o r a s p e c i e s l i k e e t h y l i o d i d e t o produce an enhancement. I d e a l l y t h i s t e c h n i q u e has the major advantage of c a t a l y s i n g a s h i f t i n the o r t h o t o p a r a r a t i o w i t h o u t p r o d u c i n g any i o d i n e atoms. C o n c e i v a b l y t h i s would e l i m i n a t e e v e r y p o s s i b i l i t y of paramagnetic c o n v e r s i o n of the p a r a - i o d i n e m o l e c u l e s (assuming t h a t one i s s e l e c t i v e l y e x c i t i n g the o r t h o m o d i f i c a t i o n ) and one may be a b l e t o o b t a i n s u b s t a n t i a l enhancement. For an exchange p a r t n e r I would suggest a s t a b l e o r g a n i c m o l e c u l e h a v i n g a good vapour p r e s s u r e a t room t e m p e r a t u r e , and c o n t a i n i n g s e v e r a l i o d i n e atoms a t t a c h e d t o i t . T h i s would h e l p p r o v i d e more exchange s i t e s on each m o l e c u l e and reduce t h e bond energy between the i o d i n e atoms and the main c o r e of the compound making atomic i o d i n e exchange s i m p l e r . I f one were t o b e g i n t h i s r e s e a r c h anew, i t would be a d v i s a b l e t o attempt an o r t h o t o p a r a enhancement i n bromine or c h l o r i n e f i r s t r a t h e r than i o d i n e . The reason f o r t h i s i s t h a t both of t h e s e s p e c i e s occur n a t u r a l l y i n two i s o t o p e s , 3 5C1, 3 7C1, and 7 9 B r , 8 1 B r and t h e mixed s p e c i e s 3 5C1 3 7C1 and 7 9 B r 8 1 B r . By s e l e c t i v e l y e x c i t i n g an o r t h o or para m o d i f i c a t i o n of one of the i s o t o p e s i n the presence of a scavenger one c o u l d have o b s e r v e d whether any i s o t o p i c and/or o r t h o - p a r a s e p a r a t i o n had o c c u r r e d . I f the i s o t o p i c s e p a r a t i o n o c c u r r e d but no o r t h o - p a r a enhancement then one would have some i n f o r m a t i o n about the r e l a x a t i o n of the 151 o r t h o and p a r a s p e c i e s . I f n e i t h e r o c c u r s but r a t h e r a c o m p l e t e l y n o n s e l e c t i v e r e a c t i o n t a k e s p l a c e then one has more e v i d e n c e t h a t r a d i c a l c h a i n s a r e i n v o l v e d . Hence t h e presence of two i s o t o p e s , a l b e i t more c o m p l i c a t e d from a s p e c t r o s c o p i c v i e w p o i n t , p r o v i d e a much b e t t e r d i a g n o s t i c t o o l . Indeed t h i s p r o j e c t has proved t o be a v e r y c h a l l e n g i n g one c o m b i n i n g s e l e c t i v e p h o t o c h e m i s t r y , s p e c t r o s c o p y , and many hours of t h o u g h t . I am c e r t a i n t h a t t h i s s u b j e c t i s not ye t c l o s e d but t h a t some new r e s e a r c h i n t o t h i s t o p i c w i l l i n e v i t a b l y be u n d e r t a k e n . BIBLIOGRAPHY 1. B a z h u t i n , S . A., Letokhov,V. S., Makarov,A. A., and Semchishen,V. A., Sov. Phys. JETP L e t t e r s , 18, 303 (1973). 2. L e t o k h o v , V.S. and Semchishen, V.A., Sov. Phys. D o k l . , 20, 423 (1974). 3. B a l y k i n , V. I . , L e t o k h o v , V. S., Mi s h e n , V. I . , and, Semchishen, V. A., Chem. P h y s . ( N e t h s . ) , 17, 111 (1976). 4. H e r z b e r g , G., M o l e c u l a r S p e c t r a and M o l e c u l a r S t r u c t u r e I . S p e c t r a of D i a t o m i c M o l e c u l e s , 2nd Ed.(D.Van N o s t r a n d Company, I n c . , New York, 1965) 5. P a u l i n g , L. and W i l s o n , E. B., I n t r o d u c t i o n t o Quantum Mec h a n i c s , ( M c G r a w - H i l l , New York,1935) 6. G o l d s t e i n , H., C l a s s i c a l M e c h a n i c s , ( A d d i s o n - W e s l e y , New York, I 9 6 4 ) p . l 0 7 f f . 7. K r o n i g , R.de L. and R a b i , I . I . , Phys. Rev., 20, 262, (1927) 8. B r o y e r , M., These de Doctorat. d ' E t a t , U n i v e r s i t e P i e r r e e t M a r i e C u r i e , P a r i s V I , 1977. 9. V i g u e , J . , These de D o c t o r a t d ' E t a t , U n i v e r s i t y P i e r r e e t M a r i e C u r i e , 1978. 10. B r o y e r , M., V i g u e , J . , and Lehmann,J. C , J o u r n a l de P h y s i q u e , 39, 591 (1978). 11. Ramsey, N. F., N u c l e a r Moments, (John W i l e y and Sons, New York, 1953) 12. P i q u e , J . P., Hartmann, F., B a c i s , R., Churas s y , S., and K o f f e n d , J . B., Phys. Rev. L e t t . , 57, 267 (1984). 13. R a i c h , J . C. and Good, R. H., J r . , A s t r o p h y s . J . (U.S.A.), 139, 1004 (1964) 14. F a r k a s , A., Orthohydrogen, Parahydrogen, and Heavy Hydrogen, (Cambridge U n i v e r s i t y P r e s s , London, 1935) 15. Wigner, E., Z. f . p h y s i k a l Chemie, B4, 126 (1929) 16. K a l k a r , F. and T e l l e r , E., P r o c . Roy. Soc. (London), A150, 528 (1935) 17. Edmonds, A. R., A n g u l a r Momentum i n Quantum Mechanics, ( P r i n c e t o n 528 (1935) U n i v e r s i t y P r e s s , New J e r s e y , 1974) 152 153 18. H e i s e n b e r g , W., Z. f . P h y s i k , 38, 411 (1926) 19. Hund, F., Z. f . P h y s i k , 42, 93 (1927) 20. B o n h o e f f e r , K. F. and H a r t e c k , P., N a t u r w i s s . , 1 7 , 182 (1929) 21. B o n h o e f f e r , K. F. and H a r t e c k , P., S i t z b e r . P r e u s s . Akad. Wiss.,1929, 103 22. B o n h o e f f e r , K. F. and H a r t e c k , P., Z. f . p h y s i k a l Chemie, B4, 113 (1929) 23. B o n h o e f f e r , K. F. and H a r t e c k , P., Z. f . E l e c t r o c h e m i e . , 35, 621 (1929) 24. F a r k a s , A.and B o n h o e f f e r , K. F., Z. f . p h y s i k a l Chemie, B, Bondensteinband, 638 (1931) 25. G e i b , K. H., and H a r t e c k , P., Z. f. p h y s i k a l Chemie, BIO, 419 (1930) 26. F a r k a s , L. and Sac h s s e , H., S i t z . P r e u s s . Akad. Wiss., 1933, 268 27. Badger, R. M. and Urmston, J . W., P r o c . Nat. Acad. S c i . , 1 6 , 808 (1930) 28. G e r s t e n k o r n , S. and Luc,. P., A t l a s du S p e c t r e d ' A b s o r p t i o n de l a M o l e c u l e d ' l o d e , L a b o r a t i o r e A i m e-Cotton, C.N.R.S. I I , 91405, Orsay, France (1977) 29. Kushawaha, V. S. , J . Amer. Chem S o c , 102, 256 ( 1980) 30. Kushawaha, V. S., Opt. and Quant. E l e c t r . , 12, 269 (1980) 31. Kushawaha, V. S., Chem. Phys. L e t t . , 72, 451 (1980) 32. Basco, N. and Hunt, J . E., I n t . J . Chem. K i n e t . , 10, 733 (1978) 33. B r e n n e r , D. M., D a t t a , S. and Zar e , R. N., J . Amer. Chem. S o c , 97, 2557 (1975) 34. Schmied, H. and F i n k , R. W., J . Chem. Phys., 27, 1034 (1957) 35. V a n d e r l i n d e , J . , Lev y , C. D. P., B i c c h i , P. and Dalby , F. W., Phys. Rev., A, 30, 1325 (1984) APPENDIX A : ORDER OF MAGNITUDE ESTIMATE OF THE HYPERFINE ORTHO-PARA COUPLING As has been s t a t e d , the h y p e r f i n e e f f e c t s i n m o l e c u l a r i o d i n e may c o u p l e o r t h o and para l e v e l s from u and g e l e c t r o n i c s t a t e s t h e r e b y d e s t r o y i n g t h i s symmetry. The purpose of t h i s appendix i s t o e s t i m a t e t h e or d e r of magnitude of t h i s m i x i n g . The magnetic d i p o l e and e l e c t r i c q u a d r u p o l e terms, b e i n g the dominant i n t e r a c t i o n s , a l o n e w i l l be c o n s i d e r e d . (a) Magnetic D i p o l e Terms The magnetic, d i p o l e i n t e r a c t i o n s , where each term has been d e f i n e d p r e v i o u s l y on page i s c a p a b l e of c o u p l i n g o r t h o and para s t a t e s t h r o u g h H ^ O ) and ( A.I) C A . Z ; 154 155 E x p l i c i t l y , C o n s i d e r f i r s t t he H L I terms and expand the q u a n t i t i e s r i e ~ 3 a c c o r d i n g t o f i g u r e A.1. F i g u r e A.1 The c o o r d i n a t e system used t o d e s c r i b e the p o s i t i o n s of the e l e c t r o n s - and n u c l e i i n a d i a t o m i c m o l e c u l e i n t h i s appendix i s shown above. 1 = * t - 1 f 1 - 3_B cos© +--| (^40 c 1 f I + «3R cose+•• • ! CA4-b) r* I re J J . . i - a 1^ [ r e V R ^ i P - ^ c o s e J * 156 Fo r the purposes of t h i s e s t i m a t i o n i t w i l l s u f f i c e t o :ate t h e s e r i e s a f t e r the : (A3a) and (A3b) become s i m p l y , t r u n c a t R/r" e terms so t h a t e q u a t i o n s ~ ± ) I - 3 R c o s 6 I ( A A O |* | - 3 c s 6 "j. ~ i f / + 3RcosO 7 r 3 r a r 3 Thus u s i n g (A4c) and (A4d), H L I(D + MLlu> » - £ Z H B M M 3 I , f t ? - + k i ] (A5) L r*s r 1 J •ie, 'ifc ' becomes, H t l ( i ) + H L X U J cr - E ^HaM-g i . Jl' +i3cosej(i/c) -23cose(i,-i£) £u) To c o u p l e o r t h o and p a r a s t a t e s one must mix u and g e l e c t r o n i c s t a t e s by t h e symmetry arguments mentioned i n c h a p t e r 2. T h i s i s o n l y p o s s i b l e by h a v i n g terms l i k e cost?, c o s 3 t ? , e t c . i n the o p e r a t o r . F u r t h e r , the miximg H a m i l t o n i a n must o p e r a t e on e i t h e r I , or I 2 a l o n e and not on I as i n the former case s t a t e s w i t h AI=1 i s p o s s i b l e w h i l e i n t h e l a t t e r AI=0 h o l d s 1 7 . C o n s e q u e n t l y o n l y the f i n a l term i n e q u a t i o n ( A 6 ) s a t i f i e s t h e s e c r i t e r i a . (A.l) 157 where z = r cost), e e The m a t r i x element shown i n e q a t i o n (A8) w i l l y i e l d the s t r e n g t h of - the c o u p l i n g . - < s X X n ; v ; J ; i ; F ; M F , ] H ^ I s , * , / * * < X B ) which u s i n g Hund's c o u p l i n g scheme (a) becomes, M t x * 4p«MB9i, £ < J'v'-a'J R z e I J v J l > * 5 (A -3 ) * < J'I'F'MF' I I.- i e I J I P M F ) To e s t i m a t e t h i s q u a n t i t y c o n s i d e r each term s e p a r a t e l y b e g i n n i n g w i t h the second, T 2 T a . = < J ' l ' F ' M p ' l I,-ie I J . I F M F ) lA.|0> But, I.-ft « + I.-iet- + I l t i e t * * X . * l e * ( A l l ) 2. C o n s i d e r the f i r s t term, 158 which may be evaluated with the help of Clebsche-Gordon c o e f f i c i e n t s . For an angular momentum j3 C D2 + 3 a » a given state |j3m3> may be decomposed as, 4 'n.lml where m3 = m,+m2 and <j ,m, j 2 m 2 1 3 1 0 * j s i n 3 > i s the Clebshe-Gordan coupling c o e f f i c i e n t . In equation (A12) P=T+3 and T = T , + T 2 so that, | I J F M F > » E H . M 1 l l m T > < I l m 1 I » r n L | I , I » I M x > Hence equation (A12) becomes, A. 14 A.15 * < 3.1* l'M x*f J I, mi+i I* m j ) « < X , m,*| I f t M j - m . l I M x JM r-M x> Using the r e l a t i o n s , , - m . ) ( i , + m,+ i ) A.|fo and simplifying the sums re l y i n g upon the delta functions above and M F=MJ+MJ, MI=m,+m2, T 2 A reduces to, T x » 22 J ' l 'F' M F ' 11 'Mi J ' M j ' X 1,1,1'Mx' I I, m,' I, r^' > m,,m» (A.<8> M * . M J * < J' M j ' I l e - I T M,> 159 To e v a l u a t e (A18) c o n s i d e r the maximum p o s s i b l e v a l u e s of each of the q u a n t i t i e s i n the square r o o t s . For the n u c l e a r s p i n term, JL [(I.-m.KI,* m,+ i)] = - (i.+ m . - n ) + * O ( A , I 9 ) 3 m, the maximum o c c u r s f o r m,=0.5 o r , (>/(^ r»1)(ll+rn^ rT)MAK * 3 (A-tt>) The e l e c t r o n i c a n g u l a r momentum obeys a s i m i l a r r e l a t i o n s h i p , however A i s a whole number so t h a t , J{L+ A ) ( L - A * l) - /L ( L . + l / (A.20-For s t a t e s c o u p l e d as 3=8+(E+§) w i t h 0=0, and S=1, L = 2J+1 so t h a t max 7(L+A)(L-A+0 £ (2.J+0 Q A. 2 Z; Us i n g t h e s e r e l a t i o n s T 2 a becomes V £ £ 3 ( w + i K J , i > ' M ' | l ' M l * , i r M ' " M x ) x< I , T . m J X M x J O .Z3 ; 160 To e v a l u a t e the Clebsche-Gordan c o e f f i c i e n t s c n s i d e r t h e i r p r o p e r t i e s ; f o r an a n g u l a r momentum s t a t e | J 3 m 3 > composed of 3 i + D 2 w i t h | j i | ^ 1321 t h e r e w i l l be, a t most, 2j,+1 terms | j i m 1 j 2 m 2 > making up t h i s s t a t e . That i s , I j » m 3> » a± | j , j t j j i ,m 5 - j l > + Q z I j t j r l ; r n , - j l + i > J (A. 14) The sum of thes e Clebsche-Gordan c o e f f i c i e n t s , a., has ' 1 the p r o p e r t y t h a t , i T h i s may be shown as f o l l o w s ; c o n s i d e r t h a t u s i n g the n o r m a l i z a t i o n c o n d i t i o n , £ I I* * " 1 (A.2fc) i t h e sum, S, may be w r i t t e n as ( l e t t i n g n=2j,+1) S = ai + a t + a 3 + • • • + (A -71) 161 To f i n d l o c a l minima or maxima one needs t o take the p a r t i a l d e r i v a t i v e s of S w i t h r e s p e c t t o each of the a^ and s e t them e q u a l t o z e r o . To b e g i n , or, ax - I 1 - Os - *Z - -» » - Qn (A.28b) which, when r e p l a c e d i n e q u a t i o n (A25) g i v e s , 1 + Z A - a 3 * - a + l a £ 4 -lra"x Cft-W)-By c o n t i n u i n g t h i s p r o c e s s f o r each c o e f f i c i e n t one e v e n t u a l l y o b t a i n s , n-l ) / t ~ On* and 35 = - On + i 3an o r , 162 and Op-'n-z ( n - z ) Jnin-z) »n U l t i m a t e l y one o b t a i n s a 1 = a 2 = . . .=aN=1/>/r? so t h a t , 2J.*I To show t h a t the above i s i n d e e d a maximum c o n s i d e r U.32.) 3*S < O 5a; daj f o r a maximum. Case (1) i # j L e t S be e x p r e s s e d a s , "5 = / i - ai -aft* an* + Q* so t h a t , 3*S •4- • • • + Q, daiSaj ( l - v Q n * J * (A. 3 3 ) 163 T h i s q u a n t i t y i s l e s s than z e r o f o r the c h o i c e of t h e a.= 1//TT . Case (2) i = j A g a i n , da-Lx -a,1 - Qj1 t h i s i s l e s s than z e r o f o r the above c h o i c e of the c o e f f i c i e n t s . Thus the c l a i m t h a t t h e s e v a l u e s l e a d t o a maximum i n S i s c o r r e c t . U s i n g r e l a t i o n (A30) i n e q u a t i o n (A23) one o b t a i n s , Tz0" £ H 3QJ+1). _ L _ . . _1_ 1 T x a £ 3 ( 1 3 + 1 ) The next q u a n t i t y t o e s t i m a t e i s T 2 . Ta. b = < J ' I ' F ' M F ' I I . Z . Xez I J I F ^ F > 164 T h i s term w i l l behave e x a c t l y as the p r e v i o u s one as r e g a r d s Clebsche-Gordan c o e f f i c i e n t s and the o n l y d i f f e r e n c e w i l l be t h a t i n s t e a d of t a k i n g the maximum v a l u e s of I 1 + and 1 , one need o n l y c o n s i d e r m, m a x and I I m a x i . e . e - J 1 ez F i n a l l y one must e s t i m a t e , In the B 3 n n + u and X 1!^ s t a t e s of I 2 the i n t e r n u c l e a r d i s t a n c e , 2R, i s between one and two angstroms. C o n s e q u e n t l y , one would expect t h a t * fc> ^ <{?> * ( A M > Hence, T T £ I O l * c m * < V I V > CA-40) Combining t h e s e r e s u l t s , For a s t a t e w i t h A=0, M ~ ( o . o t i ) <v'|v> ( 2 J + 1 ) cm* 1 (A.4-2.) 165 Assuming s i m i l a r c o n t r i b u t i o n s from t he H g I and H f I terms, M ~ (o.034>) <v'lv> UJ+ i ) cm' (A43) f o r a A=0 s t a t e , and M ( o . O H ) <v'|v> ( 2 J + ^ ) crn 1 (A.44-) f o r a A=1 s t a t e . To o b t a i n an o r d e r of magnitude e s t i m a t e assume t h a t <v'|v> N y~ ' where N y i s the number of v i b r a t i o n a l s t a t e s i n a g i v e n e l e c t r o n i c l e v e l . Here N y 80, and, f o r J*10 one o b t a i n s , (b) E L e c t r i c Quadrupole Terms w> - ^ — 1 * * (A. 44,) e r W E Q l t ) « + f s f l W ^ + l ^ - l / W (6,47) 166 Expanding the second d e r i v a t i v e s of the p o t e n t i a l , V, f o r H E Q ( 1 ) and H E Q ( 2 ) and a d d i n g the two t o g e t h e r , keeping o n l y those terms which a r e a b l e t o c o u p l e u and g e l e c t r o n i c s t a t e s and o r t h o and para s t a t e s i n a n a l o g y w i t h the magnetic d i p o l e terms one o b t a i n s , L e t K, be so t h a t x < I ' J ' P ' M F ' I i . - J I U P N I F / where F*<F+1, J'=J±1, and I'=1±1, Thus, Kx £ 2(X+D(J + i)<x'j'F'rA F' | I ,• J | X JFNAr> ^ 167 The above e x p r e s s i o n has a l r e a d y been e v a l u a t e d i n the f i r s t s e c t i o n of t h i s appendix and as t h e v a l u e of (eQq) i s known f o r both the X 1 ! * and the B 3 n f t t e l e c t r o n i c s t a t e s of g 0 u i o d i n e and may be ta k e n from r e f e r e n c e ( 3 5 ) , one may r e a d i l y e s t i m a t e the quad r u p o l e m i x i n g a s , CA52) F u r t h e r , as I,=5/2 and 1^5, 5 J U T - I ) which becomes < ^',1 ± I | H V Q I ^ X > ~ O.OOT c m 1 ( A - 6 4 ) i n t h e X 1 E * and <u"L± 1 | W F EQ I ^ I > " - c o o l cm*1 (A.~5^>> i n t h e B 3 I1Q + u s t a t e . Thus the net c o n t r i b u t i o n of the h y p e r f i n e magnetic and q u a d r u p o l e c o u p l i n g of t h e o r t h o and p a r a l e v e l s between a u and g e l e c t r o n i c s t a t e i s 168 

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