UBC Theses and Dissertations

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UBC Theses and Dissertations

Flash photolysis of the oxides of chlorine Dogra, Sneh Kumar 1970

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FLASH PHOTOLYSIS OF THE OXIDES OF CHLORINE by B.Sc.(Hons M.Sc., SNEH KUMAR DOGRA , ) , Panjab U n i v e r s i t y , 1964 Panjab U n i v e r s i t y , 1965 THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department o f Chemistry We^accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1970 In presenting t h i s t h e s i s in p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree tha permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of Chemistry  The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada A p r i l 30, 1970 ABSTRACT The p r o d u c t i o n o f v i b r a t i o n a l l y e x c i t e d o x y g e n , O^, f o l l o w i n g t h e i s o t h e r m a l f l a s h p h o t o l y s i s o f C 1 0 2 , c^-2^ anc^ o f t h e CIO f r e e r a d i c a l has shown t o be due t o t h e r e a c t i o n s o f o x y g e n atom w i t h C 1 0 9 and CIO (1, 2) . I n b o t h r e a c t i o n s , t h e energy, d i s t r i b u t i o n i n t h e p r o d u c t s i s m a r k e d l y non-e q u i l i b r a t e d w i t h a l a r g e f r a c t i o n o f t h e e n e r g y l i b e r a t e d i n t h e f o r m o f v i b r a t i o n a l e x c i t a t i o n o f t h e o x y g e n m o l e c u l e . The h i g h e s t l e v e l o f O2 p r o d u c e d c o r r e s p o n d s t o t h e e x o t h e r - • m i c i t y o f t h e r e a c t i o n s . The r a t e c o n s t a n t s f o r t h e p r o d u c t i o n o f O2 xn l e v e l s v ' 1 = 6 v ' ' = 13 a r e a p p r o x i m a t e l y e q u a l . •k The r e l a x a t i o n o f 0^ by CIO..,, CIO and by C l and 0 atoms has b e e n s t u d i e d and t h e e x c e p t i o n a l e f f i c i e n c y o f t h e atoms d e m o n s t r a t e d . The r a t e c o n s t a n t f o r t h e b i m o l e c u l a r r e a c t i o n o f CIO r a d i c a l s (10) was m e a s u r e d u s i n g C 1 0 2 , C l 2 / 0 2 , C 1 2 0 , C l 2 0 / C l 2 as s o u r c e s o f t h e r a d i c a l s . The c o n s t a n c y o f t h e v a l u e o f ' 2,7 i 0.3 x 10 1 mole s e c o b t a i n e d f r o m a l l s y s t e m s c o n -7 -1 -] t r a s t s w i t h t h e l i t e r a t u r e v a l u e s o f 6.2 x 10 1 mole s e c 4.8 x io7 1 m o l e - 1 s e c - 1 and 2.4 x io7 1 m o l e - 1 s e c - 1 , o b t a i n e d f r o m t h e f i r s t t h r e e s y s t e m s . The c h l o r i n e and b r o m i n e p h o t o s e n s i t i s e d d e c o m p o s i t i o n o f C l 0 2 and C 1 2 0 have b e e n s t u d i e d and t h e e x t i n c t i o n c o e f f i -c i e n t o f CIO and BrO f r e e r a d i c a l s m e a s u r e d . Mechanisms have I l l b e e n p r o p o s e d f o r a l l s y s t e m s and a l l r e l e v a n t r a t e c o n s t a n t s have b e e n m e a s u r e d . The r e s u l t s a r e l i s t e d b e l o w . R e a c t i o n k ( l m o le 1. 0 + C 1 0 2 + CIO 4 0* ( v * : i i l 5 ) 3 .0 X 1 0 1 0 2. .0 + C10 + C l + 0*2 (v» ' <. 14) 7 .0 X 1 0 9 3. 0 4 C 1 2 0 + 2C10 5 .1 X 1 0 9 4. 0 + c . i 2 o c i 2 4 o 2 <<5 .2 X 1 0 9 5. C I + c i 2 o •* CIO 4 c i 2 4 .0 X 1 0 8 6 . CI + C 1 0 2 -* 2C10 5.1 X 9 10 7. C I + c i o 2 c i 2 4 o 2 <<5 .1 X i o 9 8. B r 4 C 1 0 2 BrO 4 C10 7 . 2 X i o 9 9. B r 4 C 1 2 0 -> B r C l 4 CIO 6 .1 X i o 8 10. CIO 4 c i 2 o •+ c i o 2 4 c i 2 2 .7 X i o 5 11. CIO 4 C 1 2 0 + C l 4 0 2 4 C l 2 6 .5 X i o 5 12. CIO 4 CIO + C l 2 4 0 2 2 ,7 X i o 7 13. CIO 4 BrO + B r C l 4 0 2 1 .5 X i o 9 14. BrO 4 BrO B r 2 4 0 2 1 .3 X 1 0 9 15. 0 + 0* ( v 1 '=12) + 0 4 0* (v' »<12) 2 X i o 1 0 16. 0 4 0 * ( v ' '=6) + 0 + 0 * ( v ' '<6) 9 X 1 0 9 17. CI 4 • 0* (v' '=12) + C l 4 0* (v' '<12) 7 X 1 0 9 18. C I 4 • 0* (v' '=6) •-»• C l 4 0* (v' *<6) 2 X i o 9 19. C 1 0 ( C 1 0 2 ) + 0* (v' '=12)-*- C 1 0 ( C 1 0 2 ) 4 0 2 ( v 1 ' < 1 2 ) 2 X i o 8 20. C 1 0 ( C 1 0 2 ) + 0*(v''=6) -* C 1 0 ( C 1 0 2 ) + + 0*(v 1'<6) 0 8 X 1 0 7 21. E x t i n c t i o n c o e f f i c i e n t . o f CIO(2772 A) 1 . 7 x 10 1 mole cm ° 3 - 1 22. E x t i n c t i o n c o e f f i c i e n t o f BrO(3208 A) 2.4 x 1 0 1 mole cm TABLE OF CONTENTS Page A b s t r a c t i i T a b l e o f C o n t e n t s i v L i s t o f T a b l e s v i L i s t o f F i g u r e s v i i i A c k nowledgement x i i CHAPTER I . INTRODUCTION A. F o r m a t i o n and Decay o f CIO 1 B. P h o t o l y s i s o f C 1 0 2 10 C. P h o t o l y s i s o f C l 2 0 16 D. F o r m a t i o n and Decay o f V i b r a t i o n a l l y E x c i t e d Oxygen . . 20 E . P r e s e n t I n v e s t i g a t i o n . . 21 I I . EXPERIMENTAL A. A p p a r a t u s 2 4 B. F i l t e r s 30 C. P h o t o g r a p h y and P h o t o m e t r y 32 D. D e t e r m i n a t i o n o f A b s o l u t e C o n c e n t r a t i o n s . 36 . E . M a t e r i a l s 40 F. P r e p a r a t i o n o f M i x t u r e s and P r o c e d u r e . . . 42 I I I . EXTINCTION COEFFICIENT AND DECAY OF CIO A. E x t i n c t i o n C o e f f i c i e n t o f CIO 44 B. Decay o f CIO R a d i c a l 57 V CHAPTER Page IV. REACTIONS OF OXYGEN ATOMS WITH CIO, C 1 0 2 AND C 1 2 0 A. R e a c t i o n o f Oxygen Atoms v / i t h CIO 74 B. R e a c t i o n s o f Oxygen Atoms w i t h C 1 0 2 and C 1 2 0 97 V. HALOGENS (CHLORINE AND BROMINE) PHOTOSENSITISED DECOMPOSITION OF C 1 2 0 AND C 1 0 2 A. P h o t o l y s i s o f C 1 2 0 117 B. R e a c t i o n o f B r o m i n e Atoms w i t h C 1 2 0 . . . . 135 C. R e a c t i o n o f C h l o r i n e Atoms w i t h C 1 0 2 . . . 141 D. B r o m i n e and Oxygen S y s t e m 153 E . R e a c t i o n o f B r o m i n e Atoms w i t h C 1 0 2 . . . . 159 V I . VIBRATIONALLY EXCITED OXYGEN A. P r o d u c t i o n o f V i b r a t i o n a l l y E x c i t e d Oxygen 187 B. V i b r a t i o n a l E n e r g y D i s t r i b u t i o n i n E x c i t e d Oxygen 206 C. R e l a x a t i o n o f E x c i t e d Oxygen 214 V I I . CONCLUSION 2 4 0 BIBLIOGRAPHY 245 APPENDICES 2 5 2 v i L I S T OF TABLES TABLE Page I Y o f t h e P h o t o g r a p h i c P l a t e a t D i f f e r e n t W a v e l e n g t h s ' . 35 I I E x t i n c t i o n C o e f f i c i e n t o f CIO f r o m t h e P h o t o l y s i s o f C 1 0 2 47 I I I E x t i n c t i o n C o e f f i c i e n t o f CIO f r o m t h e P h o t o l y s i s o f C 1 2 0 51 IV R a t e C o n s t a n t f o r t h e R e a c t i o n o f CIO w i t h CIO i n t h e C 1 0 2 S y s t e m 58 V R a t e C o n s t a n t f o r t h e R e a c t i o n o f CIO w i t h CIO i n C l 2 / 0 2 S y s t e m 64 VI R a t e C o n s t a n t f o r t h e R e a c t i o n o f CIO w i t h CIO i n t h e C 1 2 0 and i n t h e C l 2 0 / C l 2 Systems . . . . 67 V I I R a t e C o n s t a n t f o r t h e R e a c t i o n o f CIO w i t h CIO i n t h e C l o 0 / C 1 0 o S y s t e m 5 9 V I I I R a t e C o n s t a n t f o r t h e R e a c t i o n o f Oxygen Atoms w i t h CIO F o l l o w i n g t h e F l a s h P h o t o l y s i s o f C 1 0 2 83 IX R a t e C o n s t a n t f o r t h e R e a c t i o n o f Oxygen Atoms w i t h CIO F o l l o w i n g t h e F l a s h P h o t o l y s i s o f CIO 83 X R a t i o o f t h e R a t e C o n s t a n t s f o r t h e R e a c t i o n s o f Oxygen Atoms w i t h C 1 2 0 and CIO 105 XI R a t i o o f t h e R a t e C o n s t a n t s f o r t h e R e a c t i o n s o f Oxygen Atoms w i t h C 1 0 2 and C 1 2 0 112 X I I R a t i o s o f CIO f o r m e d i n t h e C l p O / C l O „ and i n t h e C 1 0 2 Systems . • 115 X I I I R a t e C o n s t a n t f o r t h e R e a c t i o n o f C h l o r i n e Atoms w i t h C 1 2 0 122 XIV R a t e C o n s t a n t s f o r t h e R e a c t i o n o f CIO R a d i c a l s w i t h C l 2 0 130 XV C a l c u l a t i o n o f t h e Quantum Y i e l d f o r t h e D e c o m p o s i t i o n o f C l 2 0 134 XVI R a t e C o n s t a n t f o r t h e R e a c t i o n o f B r o m i n e Atoms w i t h C l o 0 139 v i i TABLE Page X V I I C o m p a r i s o n o f t h e Amount o f CIO Formed i n C 1 0 2 and i n C 1 0 2 / C 1 2 Systems 1 4 8 X V I I I R a t e C o n s t a n t f o r t h e R e a c t i o n o f C h l o r i n e Atoms w i t h C 1 0 2 153 XIX R a t e C o n s t a n t f o r t h e R e a c t i o n o f BrO w i t h BrO i n t h e B r 2 / 0 2 S y s t e m . 158 XX C a l c u l a t i o n o f e(3208) o f BrO 167 XXI C a l c u l a t i o n o f k . n o f R e a c t i o n CIO + BrO B r C l ¥ 0 2 174 XX I I C a l c u l a t i o n o f k,, o f t h e R e a c t i o n B r + C 1 0 2 -»• BrO * CIO 182 X X I I I C a l c u l a t i o n o f R e l a t i v e P o p u l a t i o n s o f E x c i t e d Oxygen 213 * XXIV H a l f - l i v e s o f t h e 0 2 l e v e l s v''=6 and 12 f o l l o w i n g t h e F l a s h P h o t o l y s i s o f C 1 0 2 216 XXV C o n c e n t r a t i o n s o f C 1 0 2 , CIO and C l Atoms P r e s e n t a f t e r t h e P r i m a r y P h o t o l y s i s and C h e m i c a l R e a c t i o n s 225 XXVI C a l c u l a t e d H a l f L i v e s o f 0 2 a t V a r i o u s r V t S P e r c e n t a g e P r i m a r y P h o t o l y s i s o f CIO- U s i n g — (t) , (u) ", (v) t 227 XXVII R a t e C o n s t a n t s f o r Q u e n c h i n g o f 0* (0,12) by CIO, C 1 0 2 , C l and 0 Atoms 229 X X V I I I C o m p a r i s o n o f H a l f L i v e s o f O*(0,12) L e v e l W i t h and W i t h o u t ' C l 2 0 U s i n g 3400 A F i l t e r . . . 232 XXIX C o m p a r i s o n o f H a l f L i v e s o f O 2 ( 0 , 1 2 ) L e v e l W i t h and W i t h o u t C l 2 . . . • 235 XXX H a l f L i v e s o f O*(0,12) L e v e l F o l l o w i n g t h e F l a s h P h o t o l y s i s o f C l - 0 238 v i i i L I S T OF FIGURES FIGURE Page 1. S c h e m a t i c d i a g r a m o f a c o n v e n t i o n a l f l a s h p h o t o l y s i s a p p a r a t u s . . . 25 2. A u x i l i a r y lamp t r i g g e r i n g c i r c u i t 29 3. T r a n s m i s s i o n c h a r a c t e r i s t i c s o f t h e f i l t e r s . . 31 4. C h a r a c t e r i s t i c c u r v e s f o r HP3 P l a t e 34 5. H a l f p a t h p l o t f o r 12,0 band o f CIO 38 6. H a l f p a t h p l o t f o r 0,12 band o f O* 39 7. D e n s i t o m e t e r t r a c e o f 12,0 band o f CIO t a k e n i n s e c o n d o r d e r 55 8. A p l o t o f 1 / f C l O ] a g a i n s t t i m e f o l l o w i n g t h e f l a s h p h o t o l y s i s o f C l O ^ 59 9. R i s e and d e c a y o f CIO f o l l o w i n g t h e f l a s h p h o t o l y s i s o f C l 2 and 0 2 62 10. A p l o t o f 1/[C10] a g a i n s t t i m e f o l l o w i n g t h e f l a s h p h o t o l y s i s o f C l 2 0 66 10-a. B e h a v i o u r o f C 1 0 2 d u r i n g t h e d e c a y o f CIO. . . 72 11. C o m p a r i s o n o f t h e d e c a y o f CIO when C 1 0 2 i s f l a s h e d a t h i g h and low f l a s h e n e r g i e s . . . . 75 12. A p l o t o f 1/[C10] a g a i n s t t i m e a t s h o r t t i m e d e l a y s 78 13. A p l o t o f s l o p e s c a l c u l a t e d a t d i f f e r e n t t i m e s , f r o m f i g . 12, a g a i n s t t h e c o r r e s p o n d i n g v a l u e s o f [0] 80 [CIO] 14. A p l o t o f l o g ( [ C 1 0 J ) a g a i n s t t i m e 82 [ 0 ] 15. C o m p a r i s o n o f t h e d e c a y o f CIO w i t h and w i t h o u t f l a s h i n g t h e a u x i l i a r y lamp ( p h o t o -g r a p h i c p r i n t ) 86 XX FIGURE Page 1.6. C o m p a r i s o n o f t h e d e c a y o f CIO w i t h and w i t h o u t f l a s h i n g t h e a u x i l i a r y lamp ( p l o t o f [CIO] v s . t i m e ) 87 17. A p l o t o f l o g ( [ C I O ] ^ j a g a i n s t t i m e . 89 [CIO] t - [ C I O ] eo 18. C o m p a r i s o n o f t h e d e c a y o f CIO i n t h e p r e s e n c e and a b s e n c e o f 0 2 9 2 19. A p l o t o f 1/[C10] a g a i n s t t i m e f o l l o w i n g t h e f l a s h p h o t o l y s i s o f C 1 0 „ i n t h e p r e s e n c e o f C 1 2 0 7 . 100 20. A p l o t o f ([CIO] - [ C I O ] 1 ) a g a i n s t C 1 2 0 . . . 104 21. C o m p a r i s o n o f t h e d i s a p p e a r n c e o f C 1 0 2 i n t h e p r e s e n c e and a b s e n c e o f C 1 2 0 107 22. A p l o t o f [ C 1 0 2 ] a g a i n s t t i m e when C 1 0 2 i s f l a s h e d i n t h e a b s e n c e and p r e s e n c e o f C 1 2 0 . . 10 8 23. C o m p a r i s o n o f t h e [CIO] f o r m a t i o n when C 1 0 2 i s f l a s h e d i n t h e p r e s e n c e and a b s e n c e o f C 1 2 0 109 24. C o m p a r i s o n o f t h e f o r m a t i o n o f 0 2 when C 1 0 2 i s f l a s h e d i n t h e p r e s e n c e and a b s e n c e o f C 1 2 0 113 25. F l a s h p h o t o l y s i s o f C 1 2 0 119 26. F l a s h p h o t o l y s i s o f C l 2 0 / C l 2 121 27. A p l o t o f l o g ( [ C 1 2 0 ] o " [ C 1 0 ] t ) a g a i n s t t i m e . 123 [ C 1 0 ] o - [ c i o ] t 27-a. F o r m a t i o n o f C 1 0 2 i n t h e p h o t o l y s i s o f C l 2 0 . . 126 28. F l a s h p h o t o l y s i s o f b r o m i n e i n t h e p r e s e n c e o f CIO . . / 137 2 [ C 1 2 0 ] o " [ C l 0 ] t 29. A p l o t o f l o g ( ) a g a i n s t [C101 - [CIO], t i m e ? . . . 140 J X FIGURE ' Page 30. C o m p a r i s o n o f t h e d i s a p p e a r a n c e o f CIO2 when i t i s f l a s h e d i n t h e p r e s e n c e and a b s e n c e o f C l 2 . . . . 31. A p l o t o f [CIO2] a g a i n s t t i m e when i t i s f l a s h e d i n t h e p r e s e n c e and a b s e n c e o f C l 2 • • 143 32. R i s e and d e c a y o f CIO when CIO2 i s f l a s h e d i n t h e p r e s e n c e and a b s e n c e o f C l 2 1^5 33. A . p l o t o f [CIO] a g a i n s t t i m e when C10 2 i s f l a s h e d i n t h e p r e s e n c e and a b s e n c e o r C l 2 . . 1^6 34. R i s e and d e c a y o f BrO f o l l o w i n g t h e f l a s h p h o t o l y s i s o f B r 2 / 0 2 155 35. A p l o t o f l / [ B r O ] ' a g a i n s t t i m e 157 36. F l a s h p h o t o l y s i s o f b r o m i n e i n t h e p r e s e n c e o f C10 2 165 37. A p l o t o f 1/[CIO] a g a i n s t t i m e f o l l o w i n g t h e f l a s h p h o t o l y s i s o f b r o m i n e i n t h e p r e s e n c e o f C10 2 168 37-a. F i r s t o r d e r p l o t o f CIO f o l l o w i n g t h e f l a s h p h o t o l y s i s o f b r o m i n e i n t h e p r e s e n c e o f C10 2 170. 38. A p l o t o f l / [ B r O ] a g a i n s t t i m e f o l l o w i n g t h e f l a s h p h o t o l y s i s o f b r o m i n e i n t h e p r e s e n c e o f C 1 0 2 172 39. Decay o f CIO2 f o l l o w i n g t h e f l a s h p h o t o l y s i s o f b r o m i n e i n t h e p r e s e n c e o f C10~ 176 [ c i o 2 ] 40. A p l o t o f l o g ( ) a g a i n s t [ c i o 2 ] t - [ c i o 2 ] o o t i m e 178 [ C 1 0 2 ] -[CIO] 41. A p l o t o f l o g ( ) a g a i n s t [ C I O ] Q - [ C I O ] t t i m e 179 [ C l 0 2 ] o - [BrO] t 42. A p l o t o f l o g ( ) a g a i n s t t i m e . 180 [BrO] - [ B r O ] 1 x i FIGURE - Page * 43. Spectrum of 0 2 following the f l a s h photolysis of C10 2 189 * 44. A pl o t of [021 against percentage primary photolysis of C10 2 190 45. Spectrum of 0~ following the f l a s h photolysis of C1 20. . . 7 192 46. Rise and decay of C>2(0,12) l e v e l when CIO i s flashed 195 47. Population r a t i o of the (0,12) and (3,6) levels of 0* 207 48. F i r s t order p l o t of 0„ following the f l a s h photolysis of CIO, . . . 208 49. Rise and decay of lower leve l s and (0,12) l e v e l of O* following the f l a s h photolysis of CIO,-, . 219 50. Comparison of the decay of (0,12) l e v e l of 0* when C10 2 i s fl a s h photolysed at high (I060J) and low (260 J) energies . . . . . . . 220 51. A p l o t of t y ? of (0,12) l e v e l of 0 2 against the percentage photolysis of C10 2 221 52. Comparison of the decay of (0,12) l e v e l of 0 2 when C10„ i s flashed i n the presence and absence of C l 2 0 231 53. Comparison of the decay of (0,12) l e v e l of 0 2 when C10 2 i s f l a s h photolysed i n the presence and absence of C l 2 . 234 * 54. A f i r s t order p l o t of 0 2 when C1 20 i s f l a s h photolysed 237 ACKNOWLEDGEMENT I w i s h t o e x p r e s s s i n c e r e thanks t o Dr. N. Basco whose encouragement, generous a d v i c e , and c r i t i c i s m g u i d e d t h i s work a t many s t a g e s . I am i n d e b t e d t o Mr. Mark Vagg and Mr. E r i c F i s h e r f o r t h e i r i n v a l u a b l e a s s i s t a n c e i n c o n s t r u c t i n g t h e f l a s h a p p a r a t u s . I a l s o w i s h t o thank Mr. Don Morse f o r r e a d i n g t h e p r o o f o f the m a n u s c r i p t . F i n a l l y , I w i s h t o thank t h e many p e o p l e who, th r o u g h o u t t h e co u r s e o f t h i s i n v e s t i g a t i o n , have h e l p e d i n one way o r t h e o t h e r . I t i s w i t h the dee p e s t a p p r e c i a t i o n t h a t I acknowledge the f i n a n c i a l s u p p o r t o f t h e N a t i o n a l Research C o u n c i l o f Canada f o r t h e S t u d e n t s h i p c o v e r i n g t h e p e r i o d from May, 1967 t o A p r i l 1970. CHAPTER I INTRODUCTION F l a s h p h o t o l y s i s i s a t e c h n i q u e o f p h o t o c h e m i s t r y i n w h i c h f r e e r a d i c a l s o r o t h e r t r a n s i e n t s c a n be p r o d u c e d by a h i g h i n t e n s i t y l i g h t s o u r c e . The c o n c e n t r a t i o n s o f t h e s p e c i e s p r o d u c e d i s so h i g h t h a t t h e y c a n be d e t e c t e d i n t h e f a r i n f r a -r e d t o vacuum u l t r a v i o l e t by a b s o r p t i o n s p e c t r o s c o p y . A l s o a t t h e s e c o n c e n t r a t i o n s i n a d d i t i o n t o r e a c t i o n s o f atoms o r r a d i c a l s w i t h s t a b l e m o l e c u l e s , r a d i c a l - r a d i c a l r e a c t i o n s o r a t o m i c r e c o m b i n a t i o n r e a c t i o n s become more i m p o r t a n t . Thus i f s p e c t r o s c o p i c methods a r e u s e d t o s t u d y t h e i r k i n e t i c b e -h a v i o u r a f t e r f l a s h p h o t o l y s i s , t h e t e c h n i q u e i s known as k i n e t i c s p e c t r o s c o p y . D e t a i l e d r e v i e w s o f t h e t e c h n i q u e o f f l a s h p h o t o l y s i s and k i n e t i c s p e c t r o s c o p y and t h e i r a p p l i c a t i o n 1 2 3 t o t h e s t u d y o f t h e s e f a s t r e a c t i o n s a r e g i v e n by P o r t e r ' ' , 4 5 6 7 3 N o r r i s h ' ' , N o r r i s h and T h r u s h and T h r u s h . T h i s t e c h n i q u e has b e e n u s e d t o s t u d y e n e r g y t r a n s f e r r e a c t i o n s and t h i s 3 8 a s p e c t has b e e n r e v i e w e d by C a l l e a r . ' Above a l l , t h e e l e c -t r o n i c s p e c t r a o f many hew t r a n s i e n t s h a v e b e e n o b s e r v e d and i n f o r m a t i o n a b o u t t h e i r g e o m e t r i c s t r u c t u r e has b e e n o b t a i n e d f r o m t h e r o t a t i o n a l f i n e s t r u c t u r e s t u d i e d u n d e r h i g h r e s o l u t i o n , A. R e a c t i o n s o f CIO R a d i c a l S i n c e t h e l a t e t w e n t i e s , t h e CIO r a d i c a l has b e e n p r o -p o s e d as an i n t e r m e d i a t e i n r e a c t i o n s i n v o l v i n g c h l o r i n e and o x y g e n , e s p e c i a l l y i n c h l o r i n e . s e n s i t i s e d o x i d a t i o n and i n t h e 2 p h o t o l y s i s o f o x i d e s o f c h l o r i n e . Many p a p e r s and r e v i e w a r t i c l e s have b e e n p u b l i s h e d a b o u t t h e r e a c t i o n s o f CIO r a d i -c a l s s t u d i e d by f l a s h p h o t o l y s i s and o t h e r t e c h n i q u e s . The m a i n s o u r c e s o f p r o d u c t i o n o f CIO a r e : 1) C h l o r i n e s e n s i t i s e d o x i d a t i o n and f l a s h p h o t o l y s i s o f c h l o r i n e and o x y g e n . 2) C h l o r i n e s e n s i t i s e d d e c o m p o s i t i o n o f o z o n e . 3) P h o t o l y s i s and f l a s h p h o t o l y s i s o f c h l o r i n e d i o x i d e and c h l o r i n e m o n o x i d e . E a c h method w i l l be m e n t i o n e d b u t (3) w i l l . b e d i s c u s s e d i n d e t a i l b e c a u s e t h e s e two s y s t e m s have b e e n s t u d i e d e x t e n -s i v e l y , b o t h by t h e d i r e c t p h o t o l y s i s as w e l l as s e n s i t i s e d by h a l o g e n s ( i . e . c h l o r i n e and b r o m i n e ) . 1) I n 1929, B o d e n s t e i n , L e n h e r and Wagner s u g g e s t e d CIO as a c h a i n c a r r i e r i n t h e c h l o r i n e s e n s i t i s e d o x i d a t i o n o f c a r b o n m onoxide ( i . e . ) : C l + C0C1 + CIO + Though t h e r e a c t i o n has n o t b e e n s t u d i e d i n g r e a t d e t a i l i t becomes d i f f i c u l t t o e x p l a i n t h e o x i d a t i o n w i t h o u t h a v i n g CIO as an i n t e r m e d i a t e . S i m i l a r l y , t h e r e a r e o t h e r e x a m p l e s , e . g . c o n v e r s i o n o f c h l o r o f o r m t o c a r b o n y l c h l o r i d e , s t u d i e d by Schumacher and W o l f , and o t h e r e x a m ples w h i c h have b e e n r e v i e w e d by C l 2 + 2C1 CO -v C0C1 o 2 c o 2 + CIO CO -> C 0 „ + C l 9 Edgecombe, N o r r i s h and Thrush. 2) The r e a c t i o n of c h l o r i n e atoms wi t h ozone has been s t u d i e d t h e r m a l l y as w e l l as p h o t o c h e m i c a l l y . Bodenstein, P a d e l t and Schumacher 1^ s t u d i e d the thermal r e a c t i o n of c h l o r i n e atoms wi t h ozone and proposed the f o l l o w i n g mechanism. c i 2 + o 3 -> CIO + c i o 2 c i o 2 + o 3 ->- c i o 3 + o 2 C10 3 + 0 3 C10 2 + 20 2 C10 3 + C10 3 C l 2 + 30 2 CIO + c i o -> c i 2 + o 2 CIO + 0 3 -> C l + 20 2 They thought C10 3 was a c h a i n c a r r i e r and n e g l e c t e d the r e a c t i o n of ClO w i t h ozone, t h i n k i n g t h a t i t has a h i g h a c t i -v a t i o n energy. L a t e r on N o r r i s h and N e v i l l e 1 1 s t u d i e d the p h o t o s e n s i t i s e d decomposition of 0 3 by c h l o r i n e atoms and pos-t u l a t e d the f o l l o w i n g mechanism. C l 2 + hv . -»• 2C1 C l + o 3 •+ CIO + o 2 CIO + 0 3 -*• C l + 20 2 ClO + CIO C l ~ + 0 o -> 2 2 c i + o 3 + c i 2 c i o 3 + c i 2 C l + o 3 + o 2 c i o 3 + o 2 Though t h i s r e a c t i o n i s p h o t o s e n s i t i s e d , i t c a n be compared w i t h t h e above m e n t i o n e d scheme. The m a i n d i f f e r e n c e b e t w e e n t h e two i s t h a t B o d e n s t e i n e t a l . " ^ t h o u g h t C l O ^ was t h e c h a i n c a r r i e r w h e r e a s N o r r i s h e t a l . " ^ t h o u g h t i t t o be CIO. The l a t t e r scheme seems t o be more a p p r o p r i a t e and more i n k e e p i n g w i t h t h e r e a c t i v i t y o f CIO r a d i c a l o b s e r v e d l a t e r on i n t h e f l a s h work. T h i s r a d i c a l has b e e n f u r t h e r o b s e r v e d s p e c t r o s c o p i c a l l y 12 13 when M c G r a t h and N o r r i s h ' " f l a s h e d h a l o g e n s m t h e p r e s e n c e o f o z o n e . They o b s e r v e d b o t h CIO and BrO v i b r a t i o n a l l y e x c i t e d X 2 + hv •> 2X X + o 3 -* XO* + o 2 where X = c h l o r i n e and b r o m i n e and X0* i s t h e g r o u n d s t a t e v i b r a t i o n a l l y e x c i t e d . T h i s r e a c t i o n gave an a d d i t i o n a l w e i g h t t o t h e i r p o s t u l a t e t h a t most o f t h e e x o t h e r m i c i t y o f t h e r e -a c t i o n s o f t y p e A + BCD -> AB + CD g o e s t o t h e newly f o r m e d bond i n t h e f o r m o f v i b r a t i o n a l e n e r g y 3) C h l o r i n e and Oxygen S y s t e m Porter"'" was t h e f i r s t t o o b s e r v e t h e a b s o r p t i o n s p e c t r u m o f C10 when he f l a s h e d a m i x t u r e o f C l 2 / 0 2 / H 2 . . L a t e r he d i d t h e v i b r a t i o n a l a n a l y s i s . " ^ D u r i e and Ramsay d i d a more d e t a i l e d v i b r a t i o n a l a n a l y s i s and s t u d i e d t h e r o t a t i o n a l s t r u c -t u r e . P o r t e r and W r i g h t 1 " * s t u d i e d t h e k i n e t i c s o f t h e f o r m a -t i o n and d e c a y o f C l O by f l a s h i n g C I T / O , , and a l s o i n t h e p r e s e n c e o f e x c e s s o f i n e r t gas ( N 2 ) • They f o u n d t h a t t h e s y s t e m i s c o m p l e t e l y r e v e r s i b l e , i . e . C l 2 and 0 2 a r e t h e end p r o d u c t s , and t h e y c o u l d n o t d e t e c t any o t h e r o x i d e o f c h l o r i n e . I n o r d e r t o s t u d y t h e f o r m a t i o n o f CIO, t h e y v a r i e d t h e p r e s -s u r e o f o x y g e n and f o u n d t h a t c h l o r i n e atoms combine 46 t i m e s f a s t e r i n t h e p r e s e n c e o f o x y g e n t h a n i n t h e p r e s e n c e o f n i t r o g e n . They c o n c l u d e d t h a t CIO i s f o r m e d by t h e r e a c t i o n C l + 0 2 •+ C l - O - 0 (1) C l - O - 0 + C l -»• 2C10 (2) where C l - O - 0 was t h o u g h t t o be an u n s t a b l e t r a n s i t o r y i n t e r -m e d i a t e and i s q u i t e d i f f e r e n t f r o m t h e s t a b l e O - C l - 0 m o l e c u l e . CIO i s n o t f o r m e d d i r e c t l y i n t h e r e a c t i o n C l + 0 2 • CIO + 0 (3) as i t i s 55 K c a l e n d o t h e r m i c . L a t e r on t h i s r a d i c a l ( Cl-O-0) 17 was p r o p o s e d by B enson and A n d e r s o n m t h e i r s t u d y o f 18 t r a p p i n g o f c h l o r i n e o x i d e f r e e r a d i c a l s . M o r r i s and J o h n s t o n have f o u n d t h e a b s o r p t i o n s p e c t r u m o f C l O O i n f a r U.V. by t h e i r m o l e c u l a r m o d u l a t i o n t e c h n i q u e . 15 P o r t e r and W r i g h t c o u l d n o t d e t e r m i n e t h e r a t e c o n s t a n t o f r e a c t i o n (1) due t o t h e l i m i t a t i o n o f t h e l o n g l i f e t i m e o f 19 t h e i r f l a s h lamp b u t r e c e n t l y , N o r r i s h and N i c h o l a s h a v e f o u n d t h a t 6 C l + C>2 + M ->- C l - O - 0 + M - (1) C l - O - 0 + C l 2C10 (2) C l - O - 0 + C l -> C l 2 + 0 2 (4) = 6.2 - 1.1 x 1 0 8 m o l e 2 l 2 s e c ^ and k ^ / k 2 ^ 15 o r 7.7 d e p e n d i n g on t h e e x t i n c t i o n c o e f f i c i e n t o f CIO. The d e c a y o f CIO was f o u n d t o be u n a f f e c t e d b y t h e p r e -s e n c e o f c h l o r i n e , o x y g e n , n i t r o g e n o r c a r b o n d i o x i d e a nd t o be i n d e p e n d e n t o f t e m p e r a t u r e i n t h e r a n g e 293 t o 433°K. They c o u l d n o t m e a s u r e t h e a b s o l u t e r a t e c o n s t a n t o f t h e r e a c t i o n CIO + CIO -> C l 2 + 0 2 (5) s i n c e the e x t i n c t i o n c o e f f i c i e n t o f CIO was n o t known a n d they c o u l d n o t m e a s u r e i t f r o m t h e d e c r e a s e i n t h e c h l o r i n e c o n c e n -t r a t i o n . T hey f o u n d t h e r a t e c o n s t a n t i n t e r m s o f k^/e e q u a l 4 -1 t o 7.2 x 10 cm s e c (where e i s t h e e x t i n c t i o n c o e f f i c i e n t o 7 7 o f CIO a t 2577 A ) . Thus t h e v a l u e o f 4.8 x 10 o r 7.6 x 10 1 m o l e ^ s e c ^ c o u l d be a s s i g n e d t o d e p e n d i n g u p on t h e e x t i n c t i o n e f f i c i e n t . 4) CK>2 S y s t e m M ore i n f o r m a t i o n a b o u t t h e k i n e t i c s v w a s f o u n d b y 20 21 L i p s c o m b , N o r r i s h a n d T h r u s h ' i n t h e f l a s h p h o t o l y s i s o f C 1 0 2 . CIO was t h e m a j o r p r o d u c t o f t h e p h o t o l y s i s a t h i g h f l a s h e n e r g y . They f o u n d t h a t a s e c o n d o r d e r p l o t o f t h e d e c a y o f CIO g i v e s a s t r a i g h t l i n e a t a l l t h e f l a s h e n e r g i e s u s e d . The 7 s l o p e i n c r e a s e d w i t h i n c r e a s e of f l a s h energy but the i n t e r -cept was c o n s t a n t . The lower l i m i t c a l c u l a t e d f o r k r was 7 -1 -1 1.9 x 10 1 mole sec a t f l a s h energy 240 J and h i g h e s t • 7 - l - i 6.2 x 10 1 mole sec f o r f l a s h energy g r e a t e r than 1600 J . S i m i l a r behaviour was observed i n the d e t e r m i n a t i o n of the e x t i n c t i o n c o e f f i c i e n t . T h i s was o b t a i n e d by the assump-t i o n t h a t each molecule of C10 2 decomposed w i l l g i v e one mole-c u l e of CIO. The absorbance of CIO was c a l c u l a t e d by e x t r a -p o l a t i o n of the second order p l o t of CIO. At low f l a s h energy a c o r r e c t i o n was a p p l i e d t o the C10 2 decomposed due to the appearance of ClO^ spectrum. The e x t i n c t i o n c o e f f i c i e n t a t ° 3 - 1 - 1 2577 A v a r i e d from 1.4 t o 0.68 x 10 1 mole cm a t f l a s h energy 240 to 1600 J r e s p e c t i v e l y . In a l l t h e i r q u a n t i t a t i v e measurements a soda g l a s s f i l t e r was used i n order to a v o i d d i r e c t p h o t o l y s i s of ClO. 5) CI2O System 9 22 Edgecombe, N o r r i s h and Thrush ' c a r r i e d out the study of ClO f u r t h e r by f l a s h i n g Cl^O. Since the continuous spec-trum of C l 2 0 extends over the whole range of ClO bands, they o o c a r r i e d out measurements a t 2920 A (7,0) f o r CIO and 2912 A f o r C1 20 measurements. Though ClO does take p a r t i n the c h a i n p r o p a g a t i o n of C1 20 decomposition, these processes are slow as compared t o b i m o l e c u l a r decay of CIO. They o b t a i n e d a va l u e 7 — 1 — 1 of k 5 of (2.4 ± 0.4) x 10 1 mole" sec and were s a t i s f i e d on f i n d i n g t h a t t h e i r v a l u e l a y w i t h i n the range of t h a t found 21 by Lipscomb e t a l . Although t h e i r v a l u e s of e and k^/e are not g i v e n e x p l i c i t l y ^ from f i g . (2) of t h e i r paper, the 4 -1 s l o p e can be c a l c u l a t e d to be 4.9 x 10 cm sec and thus the e x t i n c t i o n c o e f f i c i e n t ^920 w a s c a ^ - c u x a ^ e < ^ a s 490 1 mole -1 23 cm . Clyne and Coxan have found k^/e from t h e i r p l o t as 4 -1 ° 3.1 x 10 cm sec a t 2577 A, by means of known r e l a t i v e o o o e x t i n c t i o n c o e f f i c i e n t s a t 2577 A, 2824 A and 2920 A and thus 760 1 mole ^ cm ^ c o u l d be assign e d to the e x t i n c t i o n c o e f f i -o c i e n t at 2577 A. However, no d i r e c t c o r r e l a t i o n was drawn because of the d i f f e r e n t systems and d i f f e r e n t wavelengths used 6) Flow System Inf o r m a t i o n r e g a r d i n g the r e a c t i o n s and k i n e t i c s of 23 24 25 CIO was g i v e n by Clyne and Coxan ' ' from t h e i r study i n a flow system. The CIO r a d i c a l s were generated by the r e a c t i o n of c h l o r i n e atoms wi t h c h l o r i n e d i o x i d e . They c a l c u l a t e d the e x t i n c t i o n c o e f f i c i e n t o f CIO e i t h e r by t i t r a t i o n w i t h oxygen atoms or with n i t r i c o x i d e . The va l u e of 1.9 - .06 x 10 1 mole cm was g i v e n to the e x t i n c t i o n c o e f f i c i e n t a t 2772 A, or 1.27 ± 0.04 x i o 3 1 m o l e - 1 c m - 1 a t 2577 A, and thus 2 agrees w i t h the upper l i m i t c a l c u l a t e d by Lipscomb and o t h e r s , The lower v a l u e s o b t a i n e d a t h i g h e n e r g i e s were e x p l a i n e d by 23 Clyne and Coxan t o be caused by the r e a c t i o n O + CIO •> 0 2 + C l (6) 23 k g has been c a l c u l a t e d by Clyne and Coxan and the lower l i m i t 9 - 1 - 1 found to be 6 x 10 1 mole sec , where k_/k, was found to / b be ^ 4. O + C 1 0 2 CIO + 0 2 • (7) The b i m o l e c u l a r d e c a y o f CIO was p l o t t e d by them and k r f o u n d , 7 i — i — i t o be 1.4 _ .1 x 10 1 mo lie s e c w h i c h a g r e e s w i t h t h e l o w e r 21 l i m i t o f L i p s c o m b e t a l . Two t y p e s o f mechanisms a r e p r o p o s e d f o r t h e C l O d e c a y : 15 21 22 1) P o r t e r and W r i g h t , L i p s c o m b e t a l . and Edgecombe e t a l . h ave p r o p o s e d a m o l e c u l a r mechanism f o r t h e o b s e r v e d s e c o n d o r d e r d e c a y o f CIO r a d i c a l s w h i c h c a n be summarized by t h e r e a c t i o n s . c i o + c i b ( + My-* C I 2 O 2 ( + M ) C L 2 0 2 ( + M). -» C l 2 + 0 2 ( + M) 2 6 2) B e n s o n and B u s s have a l t e r n a t i v e l y p r o p o s e d f o r t h i s r e -a c t i o n a f r e e r a d i c a l r e a c t i o n i n v o l v i n g a c h l o r i n e atom and t h e s h o r t l i v e d C100, p e r o x y r a d i c a l , i . e . , CIO + CIO •* C l - O - 0 + C l C l - O - 0 +1 C l + c i 2 + o 2 C l - O - 0 + M -»• C l + 0 2 + M They p o i n t e d o u t t h a t s l o w d e c a y o f CIO o b s e r v e d by L i p s c o m b 21 e t a l . a t low f l a s h e n e r g y c a n be e x p l a i n e d w i t h t h i s mechan-i s m by i n c l u d i n g r e a c t i o n (8) C l + C 1 0 2 -* 2C10 (8) T hey a l s o made i t c l e a r t h a t , s i n c e t h e r a p i d r e a c t i o n C l + C100 •> CIO + CIO i s e v i d e n t l y r e s p o n s i b l e f o r C l O f o r m a t i o n i n t h e f l a s h p h o t o -27 l y s i s o f c h l o r i n e / o x y g e n m i x t u r e (Burns and N o r r i s h , N o r r i s h 19 and N i c h o l a s ) t h e n e a r l y , t h e r m o n e u t r a l r e v e r s e r e a c t i o n m i g h t be p o s s i b l e t o be t h e r a t e i d e t e r m i n i n g s t e p f o r C l O r e m o v a l . They a l s o e x p l a i n e d t h e r e s u l t s o f t h e c h l o r i n e s e n s i t i s e d d e -2 8 c o m p o s i t i o n o f n i t r o u s o x i d e (Kaufman e t a l . ) by a s i m i l a r mechan sm. 10 25 C l y n e and Coxon " have c o n s i d e r e d b o t h mechanisms b u t 2 & p r e f e r r e d t h e s e c o n d , p r o p o s e d by B e n s o n and B u s s . They p u t f o r w a r d f o l l o w i n g e v i d e n c e : 1) They d e t e c t e d t h e r e d c h l o r i n e a f t e r g l o w s p e c t r u m ( B a d e r and O g r y z l o , 2 9 C l y n e and Coxon''^) t h o u g h i t i s r e l a t i v e l y r e d s h i f t e d . 2) They f o u n d a d e c r e a s e i n t h e c h l o r i n e d i o x i d e c o n -c e n t r a t i o n d u r i n g t h e C l o d e c a y . The CIO c o n c e n t r a t i o n d o e s r e m a i n c o n s t a n t i n t h e p r e s e n c e o f c h l o r i n e d i o x i d e and f o l l o w s s e c o n d o r d e r d e c a y k i n e t i c s a f t e r t h e c h l o r i n e d i o x i d e i s u s e d up. 3) C o m p l e t e r e p l a c e m e n t o f t h e c h l o r i n e a f t e r g l o w s p e c t r u m by a d e e p e r r e d e m i s s i o n o c c u r r e d when b r o m i n e was added d u r i n g t h e CIO d e c a y . The l a t t e r s p e c t r u m i s due t o t h e e m i s s i o n o f B r C l (^TT+) . ^ 4) The r a t e o f d e c a y o f CIO i n c r e a s e d when H"2 was added t o t h e s y s t e m . A l l t h e s e r e s u l t s c o u l d o n l y be e x p l a i n e d i f c h l o r i n e atoms a r e p r e s e n t i n t h e s y s t e m d u r i n g t h e CIO d e c a y as ex-25 p l a i n e d by C l y n e and Coxon and a l s o t h e r e a c t i o n o f c h l o r i n e 25 8 —1 —1 atoms w i t h c h l o r i n e d i o x i d e (>5 x 10 1 mole s e c ) and 32 w i t h m o l e c u l a r b r o m i n e a r e v e r y f a s t . B. P h o t o l y s i s o f C h l o r i n e D i o x i d e 1) C h l o r i n e D i o x i d e S y s t e m The d e c o m p o s i t i o n o f c h l o r i n e d i o x i d e h as b e e n s t u d i e d b o t h t h e r m a l l y and p h o t o c h e m i c a l l y , w h i c h i n c l u d e s f l a s h and 11 33 s t e a d y s t a t e . Schumacher and S t e i g e r , ' f r o m t h e i r s t u d y o f t h e r m a l d e c o m p o s i t i o n s u g g e s t e d t h a t d e c o m p o s i t i o n i s a s m a l l c h a i n p r o c e s s and t h e p r i m a r y , s t e p i s cio 2 -> CIO + 0 The s t e a d y s t a t e p h o t o l y s i s has b e e n s t u d i e d by a number o f 33 34 35 w o r k e r s i n t h e gas p h a s e , ' and i n s o l u t i o n u s i n g C C l ^ as a s o l v e n t . A l l o f them have s t r e s s e d t h e f o r m a t i o n o f h i g h e r o x i d e s o f c h l o r i n e , e . g . c h l o r i n e , t r i o x i d e and c h l o r i n e h e x a -o x i d e , a l t h o u g h t h e p r i m a r y s t e p i s t h e same. The f i r s t s p e c -3 6 t r o s c o p i c e v i d e n c e was f o u n d by Goodeve and S t e i n i n t h e i r s t u d y o f t h e c h l o r i n e d i o x i d e s p e c t r u m . They o b s e r v e d p r e -d i s s o c i a t i o n i n t h e b a n d s p e c t r u m o f c h l o r i n e d i o x i d e , c o r r e s -p o n d i n g t o d i s s o c i a t i o n i n t o C 1 0 2 + hv + CIO + 0 t h u s g i v i n g t h e l o w e r l i m i t o f 45 K c a l p e r mole f o r t h e d i s s o c i a t i o n e n e r g y o f CIO, a l t h o u g h r e c e n t l y i t was f o u n d t o be 63 K c a l / m o l e , c a l c u l a t e d f r o m t h e a b s o r p t i o n s p e c t r u m o f 14,16 CIO. 37 I n 1931, F i n k e l b e r g and Schumacher, i n t h e i r i n v e s t i -g a t i o n o f s p e c t r u m and p h o t o c h e m i c a l d e c o m p o s i t i o n o f c h l o r i n e d i o x i d e o b s e r v e d t h a t t h e p r o c e s s C 1 0 2 + hv -»• CIO + 0 ( 3 P ) o o c c u r r e d by p r e d i s s o c i a t i o n a t w a v e l e n g t h s s h o r t e r t h a n 3753 A, They a l s o s u g g e s t e d t h a t t h e w a v e l e n g t h a p p r o a c h i n g t h e band 12 c o n v e r g e n c e (2560 A) w o u l d decompose t h e c h l o r i n e d i o x i d e m o l e c u l e by t h e r e a c t i o n C 1 0 2 + hv -* CIO + 0 ( 1 D ) 3 8 S p i n k s and P o r t e r c a r r i e d o u t t h e p h o t o l y s i s s t u d i e s f u r t h e r b o t h d r y and i n t h e p r e s e n c e o f w a t e r . The r e s u l t s o b t a i n e d by s t e a d y s t a t e p h o t o l y s i s c a n be summed up as C 1 0 2 + hv + CIO + 0 C 1 0 2 + 0 + M -+ C 1 0 3 + M CIO + C 1 0 2 + c l 2 ° 3 CIO + CIO -»• c i 2 + o 2 2C10- •+ Cl~Oc 3 2 b and H o 0 + Cl~Or •> HC10- + HC10. 2 2 b 3 4 2 C 1 2 0 3 + 2 H 2 ° ~y 2HC10 2 + HC10 + H C10 3 Though t h e CIO r a d i c a l has b e e n p r e d i c t e d and some o f t h e r e a c -t i o n s have b e e n d i s c u s s e d , a g r e a t e r r o l e has b e e n p l a y e d by t h e h i g h e r o x i d e s o f c h l o r i n e . The f l a s h p h o t o l y s i s o f c h l o r i n e d i o x i d e was c a r r i e d 21 o u t by L i p s c o m b e t a l . They f o u n d t h a t t h e m a j o r p r o d u c t o f t h e p h o t o l y s i s i s CIO. They a l s o o b s e r v e d t h e s p e c t r u m o f C 1 0 3 whose p r o d u c t i o n was f o u n d t o be i n d e p e n d e n t o f t o t a l p r e s s u r e f o r a f i x e d p r e s s u r e o f c h l o r i n e d i o x i d e . The i n t e n -s i t y o f C l O ^ s p e c t r u m i n c r e a s e d w i t h t h e d e c r e a s e o f f l a s h ' e n e r g y and a b s o r p t i o n was m e a s u r e d by u s i n g Goodeve and R i c h a r d s o n ' s d a t a . However, t h e maximum amount o f ClO-j p r o d u c e d i n t h e i r work was n o t more t h a n 10%, c a l c u l a t e d f r o m T a b l e 1 o f t h e i r p a p e r , i f 0.5 t o r r o f c h l o r i n e d i o x i d e i s a c c e p t e d as t h e i n i t i a l c o n c e n t r a t i o n . T h i s i s q u i t e d i f f e r -e n t f r o m t h e s t e a d y s t a t e work where C l O ^ i s t h e m a j o r p r o d u c t . They e x p l a i n e d t h i s d i f f e r e n c e i n terms, o f f o l l o w i n g c o m p e t i n g r e a c t i o n s 0 + C 1 0 2 -> C 1 0 3 (9) 0 + CIO + c i o 2 (10) 0 + 0 + M -> O, + M (11) S i n c e t h e r a t i o o f t h e c o n c e n t r a t i o n o f a t o m i c o x y g e n p l u s CIO t o t h e C 1 0 2 i s many t i m e s g r e a t e r i n t h e f l a s h e x p e r i m e n t s , t h e p r o b a b i l i t y o f r e a c t i o n (9) r e l a t i v e t o (10) and (11) w i l l c o r r e s p o n d i n g l y be r e d u c e d . Though t h i s e x p l a n a t i o n i s r e a -s o n a b l e , t h e r e a c t i o n s (6) and (7) seem more l i k e l y t h a n r e a c t i o n ( 9 ) . 21 L i p s c o m b e t a l . a l s o o b s e r v e d t h e r e a p p e a r a n c e o f C 1 0 2 s p e c t r u m w h i c h t h e y e x p l a i n e d by t h e f o l l o w i n g t y p e o f e q u i l i b r i u m C 1 0 2 + CIO + c l 2 ° 3 Thus as t h e C l O c o n c e n t r a t i o n d e c r e a s e s t h e e q u i l i b r i u m s h i f t s t o t h e l e f t and C 1 0 2 s p e c t r u m s t a r t s a p p e a r i n g . No s p e c t r o -s c o p i c e v i d e n c e f o r t h i s compound has been g i v e n b u t r e c e n t l y 40 41 M c h a l e and E l b e ' have e s t a b l i s h e d t h e e x i s t e n c e o f t h i s compound f r o m t h e i r s t u d y o f C l O , . 14 2) H a l o g e n P h o t o s e n s i t i s e d D e c o m p o s i t i o n ' o f CIO2 The c h l o r i n e p h o t o s e n s i t i s e d d e c o m p o s i t i o n o f C10 2 has n o t b e e n s t u d i e d i n a s t e a d y s t a t e s y s t e m . The m a i n t r o u b l e i n c h l o r i n e s e n s i t i s a t i o n i s t h a t t h e s p e c t r u m o f c h l o r i n e a l s o l i e s i n t h e same r e g i o n where t h e d i s s o c i a t i v e s p e c t r u m o f CIO2 e x i s t s . So i t becomes d i f f i c u l t t o decompose c h l o r i n e a l o n e w i t h o u t d e c o m p o s i n g C I C ^ . However i n a f l o w s y s t e m , r e a c t i o n o f c h l o r i n e atoms ( g e n e r a t e d by means o f r . f . d i s c h a r g e ) w i t h 25 CIO2 was u s e d as a s o u r c e o f C l O r a d i c a l s . C l y n e and Coxon have f o u n d t h e r a t e c o n s t a n t o f t h i s r e a c t i o n t o be g r e a t e r 8 -1 — 1 t h a n 5 x 10 1 mole s e c and t h e s t o i c h i o m e t r y o f t h e r e a c t i o n C l + C10„ + 2C10 (8) t o . be 1.9 - 0.1 as compared t o t h e r e a c t i o n C l + C10 2 C l 2 + 0 2 (12) w h i c h i s more e x o t h e r m i c t h a n t h e r e a c t i o n (8). The p h o t o s e n s i t i s e d d e c o m p o s i t i o n o f C10 2 by b r o m i n e i n p r i n c i p l e i s more e a s i l y s t u d i e d t h a n t h a t s e n s i t i s e d by c h l o r -i n e . The s p e c t r u m o f b r o m i n e l i e s much above t h e p r e d i s s o c i -a t i o n l i m i t o f CIO2 so t h a t b r o m i n e atoms c a n be g e n e r a t e d 42 w i t h o u t d e c o m p o s i n g C10 2. Schumacher f i r s t p r o p o s e d t h e mechanism o f i t s d e c o m p o s i t i o n as B r 2 + hv + 2Br (13) B r + C10 2 B r C l + 0 2 (14) 2BrCl -> B r 2 + C l 2 (15) 15 3 8 S p i n k s and P o r t e r s t u d i e d t h e same r e a c t i o n u s i n g o 5460 A r a d i a t i o n and s u g g e s t e d t h a t t h e e x c i t e d b r o m i n e m o l e -c u l e s a r e r e s p o n s i b l e f o r t h e a c t i v a t i o n o f C 1 0 2 m o l e c u l e s on 43 c o l l i s i o n and t h u s t h e r e a c t i o n i s s t a r t e d . B u t l a t e r t h e y a l s o f o u n d t h a t t h e quantum y i e l d and t h e c o u r s e o f t h e r e -o o a c t i o n i s s i m i l a r a t 3650 A and 5460 A , i n d i c a t i n g t h a t r e a c -ts t i o n a t 5460 A p r o b a b l y a l s o p r o c e e d s by means o f b r o m i n e atoms and n o t by means o f an e x c i t e d m o l e c u l e . F u r t h e r , t h e quantum y i e l d f o r t h e s e n s i t i s e d and u n s e n s i t i s e d r e a c t i o n i s e q u a l , i n d i c a t i n g t h a t , a p a r t f r o m t h e p r i m a r y a c t o f l i g h t a b s o r p t i o n , t h e mechanism o f t h e two r e a c t i o n s i s s i m i -42 • l a r . S i n c e Schumacher's mechanism c o u l d n o t e x p l a i n t h e f o r m a t i o n o f C l O ^ and h i g h e r o x i d e s o f c h l o r i n e , t h e y p o s t u -l a t e d a new mechanism: B r 2 + hv -*• 2 B r (13) B r + C 1 0 2 + CIO + BrO (16) BrO + C 1 0 2 + C 1 0 3 + B r (17) w h i c h w o u l d g i v e ' t h e same p r o d u c t s as t h e d i r e c t p h o t o c h e m i c a l r e a c t i o n . 44 45 C l y n e and Coxon ' a l s o s t u d i e d t h e r e a c t i o n o f b r o m i n e atoms v / i t h C 1 0 2 i n o r d e r t o c o r r e l a t e i t w i t h t h e r e a c t i o n o f c h l o r i n e atoms w i t h C 1 0 2 . They c o u l d n o t o b s e r v e t h e BrO and C l O s p e c t r u m , however, i n s t e a d o f t h e s e r a d i c a l s , t h e y o b s e r v e d 3 + 1 + t h e e m i s s i o n s p e c t r u m o f B r C l ( fr g) w h i c h was v e r y s i m i l a r t o t h a t o b t a i n e d by t h e d i r e c t r e c o m b i n a t i o n o f b r o m i n e and 16 c h l o r i n e atoms: B r + C l + M -> B r C l ( 3 T T + ) + M o They t h u s c o n c l u d e d t h a t r e a c t i o n (14) o c c u r r e d . R e a c t i o n (16) was r e j e c t e d on t h e g r o u n d s t h a t i t i s e n d o t h e r m i c by 3 K c a l and t h e y d i d n o t o b s e r v e CIO o r BrO. R e a c t i o n (14) i s s u f f i c i e n t l y e x o t h e r m i c t o a c c o u n t f o r t h e i r r e s u l t s . 25 R e c e n t l y , i n t h e i r r e i n v e s t i g a t i o n o f t h e above r e -a c t i o n , t h e y c o u l d i d e n t i f y CIO b u t c o u l d n o t d e t e c t BrO. S i n c e t h e y f o u n d t h a t r a t e o f d e c a y o f BrO i s n e a r l y 50 t i m e s f a s t e r t h a n CIO, t h i s may be t h e r e a s o n t h a t t h e y c o u l d n o t s e e t h e BrO s p e c t r u m , so t h e y a r e s t i l l d o u b t f u l a b o u t t h e o c c u r r e n c e o f r e a c t i o n (16). C. P h o t o c h e m i c a l D e c o m p o s i t i o n o f C1 20 L i k e CIO2, CI2O has a l s o b e e n s t u d i e d e x t e n s i v e l y by s t e a d y s t a t e and f l a s h p h o t o l y s i s . E a r l i e r w o r k e r s l i k e 46 47 Bowen a n d . B o d e n s t e i n and K i s t i a k o w s k y f o u n d t h a t decom-p o s i t i o n was p r o p o r t i o n a l t o t h e l i g h t a b s o r b e d and t h e o b s e r v e d quantum y i e l d was two. The r e a c t i o n was n o t a f f e c t e d by t h e a i r o r o x y g e n and c h l o r i n e d i o x i d e was shown s p e c t r o s c o p i c a l l y t o be p r e s e n t i n i n c r e a s i n g amounts as t h e r e a c t i o n p r o c e e d e d . 47 48 However, B o d e n s t e i n and K i s t i a k o w s k y and Goodeve and W a l l a c e showed t h a t t h e a b s o r p t i o n s p e c t r u m o f C l 2 0 was c o n t i n u o u s b e -o o tween 6200 A and 2300 A. Thus i n o r d e r t o e x p l a i n t h e y i e l d 17 49 o f two, Schumacher and Wagner s u g g e s t e d t h e f o l l o w i n g r e -a c t i o n mechanism. C 1 2 0 + hv -»• CIO + C l (18) C l + C 1 2 0 C l 2 + CIO (19) CIO + CIO -> C l 2 + 0 2 (20) I t was n o t u n t i l 1932 when F i n k e l b e r g , Schumacher and S t e i g e r , ^ i n t h e i r r e i n v e s t i g a t i o n o f g a s e o u s r e a c t i o n o f C l 2 0 , f o u n d t h a t t h e quantum y i e l d o f t h e p h o t o c h e m i c a l decom-p o s i t i o n i s 3.5 a t w a v e l e n g t h s 4360, 3650 and 3130 A. C 1 0 2 and h i g h e r unknown o x i d e s were shown t o be p r e s e n t . S u b s e -q u e n t l y Schumacher and T o w n e d ^ 1 s t u d i e d t h i s s y s t e m i n t h e r e g i o n where C 1 2 0 decomposes, i n t o atoms and f o u n d t h a t quantum o y i e l d a t 2500 A was 4.5. I n o r d e r t o e x p l a i n t h e h i g h e r q u a n -tum y i e l d and t h e f o r m a t i o n o f C 1 0 2 , t h e f o l l o w i n g r e a c t i o n s were s u g g e s t e d , a p a r t f r o m t h o s e m e n t i o n e d a b o v e : CIO + C 1 2 0 -> C 1 0 2 + C l 2 (20) CIO + C 1 2 0 -v C l + 0 2 + C l 2 (21) o w h e r e a s a t 2500 A t h e p r i m a r y p r o c e s s was C 1 2 0 + hv ->- 2C1 + . 0 (22) Schumacher and T o w n e d ^ 1 p r o p o s e d t h a t o x y g e n atoms do n o t r e -a c t w i t h C 1 2 0 i n o r d e r t o e x p l a i n t h e i r quantum y i e l d . The t h e r m a l d e c o m p o s i t i o n o f C 1 2 0 h a s b e e n s t u d i e d by 52 53 H x n s h e l w o o d and P r i c h a r d , H i n s h e l w o o d and Hughes. T h e y f o u n d t h a t a p a r t f r o m t h e p r i m a r y p r o c e s s , t h e mechanism o f 18 t h e p h o t o and t h e r m a l r e a c t i o n s a r e v e r y s i m i l a r . 22 Edgecombe, N o r r i s h and T h r u s h i n v e s t i g a t e d t h e p h o t o l y s i s o f C 1 2 0 by t h e f l a s h t e c h n i q u e . T h e y o b s e r v e d a s i m i l a r b e h a v i o u r t o t h a t o b s e r v e d by t h e e a r l i e r w o r k e r s i n t h e s t e a d y s t a t e . However, t h e y p o i n t e d o u t t h a t r e a c t i o n o f o x y g e n atoms w i t h C 1 2 0 i s q u i t e i m p o r t a n t , w h i c h i s a n a l o g o u s t o t h e r e a c t i o n ( 7 ) , i . e . 0 + C 1 0 2 -> CIO + 0 2 (7) 0 + C 1 2 0 -> 2C10 Th e y d i v i d e d t h e i r s t u d y i n t o t h r e e p a r t s . The f i r s t , l a s t i n g f o r 100 u s e e , c o r r e s p o n d s t o t h e d u r a t i o n o f p h o t o l y t i c f l a s h . D u r i n g t h i s p e r i o d , C l 2 0 c o n c e n t r a t i o n d r o p s s h a r p l y and CIO r e a c h e s i t s maximum c o n c e n t r a t i o n . I n t h e n e x t 10 msec, t h e s e c o n d s t a g e o f t h e r e a c t i o n , CIO d e c a y s v e r y s l o w l y . C 1 0 2 s t a r t s a p p e a r i n g a p p r o x i m a t e l y a t 1 msec, r e a c h e s i t s maximum v a l u e i n a b o u t 30 s e c , a f t e r t h a t d e c r e a s e s s l o w l y . T h i s i s t h e t h i r d s t a g e o f t h e r e a c t i o n . 2) H a l o g e n S e n s i t i s e d D e c o m p o s i t i o n o f C ^ O A l t h o u g h t h e c o n t i n u u m s p e c t r u m o f C l 2 0 e x t e n d s f r o m ° 0 48 2300 A -to 6200 A, t h e e x t i n c t i o n c o e f f i c i e n t f a l l s s o r a p i d l y o t h a t t h e d e c o m p o s i t i o n due t h e w a v e l e n g t h s a b o v e 3300 A i s n e g l i g i b l e . The c h l o r i n e s e n s i t i s e d r e a c t i o n c o u l d , t h e r e f o r e , be s t u d i e d . I n t h e s t e a d y s t a t e p h o t o l y s i s , t h e e a r l i e r w o r k e r f o u n d t h a t t h e quantum y i e l d o f C 1 2 0 d e c o m p o s i t i o n w i t h and w i t h o u t t h e c h l o r i n e were t h e same, i . e . two. L a t e r o n , 19 i n t h e f l a s h p h o t o l y s i s , t h e c o u r s e o f r e a c t i o n was f o u n d t o be s i m i l a r t o t h a t w i t h o u t t h e c h l o r i n e and a l s o s i m i l a r t o t h a t d i s c u s s e d by F i n k e l n b e r g e t a l . ~ ^ So t h e p r i m a r y s t e p can be C l 2 + hv 2C1 f o l l o w e d b y r e a c t i o n s ( 1 9 ) , ( 2 0 ) , (21) and ( 5 ) . Edgecombe e t 22 8 — 1 — 1 a l . a l s o f o u n d t h e l o w e r l i m i t o f 4"* 10 1 mole s e c f o r t h e r e a c t i o n ( 1 9 ) . 54 Brown and S p i n k s i n v e s t i g a t e d t h e b r o m i n e s e n s i t i s e d o d e c o m p o s i t i o n o f C 1 2 0 u s i n g 5460 A. T h e y f o u n d t h a t t h e c o u r s e o f t h e r e a c t i o n i s s i m i l a r t o t h a t s e n s i t i s e d and u n s e n s i t i s e d by c h l o r i n e . The f i n a l p r o d u c t s were s i m i l a r and t h e quantum y i e l d t h u s e v a l u a t e d was a l s o e q u a l t o t h a t f o u n d by F i n k e l n b e r g 50 e t a l . From t h e i r r e s u l t s t h u s t h e y g a v e t h e mechanism as B r 2 + hv + B r * (23) Br*, + M + 2Br + M (24) B r + C 1 2 0 ->• B r C l + CIO (25) o r B r * + C 1 2 0 '-»• B r 2 + C l + CIO (26) C l + c i 2 o -»• CIO + c i 2 A f t e r t h e s e p r i m a r y r e a c t i o n s , t h e o t h e r r e a c t i o n s a r e s i m i l a r t o t h o s e d i s c u s s e d i n t h e c a s e o f c h l o r i n e s e n s i t i s e d r e a c t i o n , i n o r d e r t o e x p l a i n t h e o v e r a l l quantum y i e l d and t h e r e s t o f t h e f i n a l p r o d u c t s . 20 D. Formation and Decay of V i b r a t i o n a l l y E x c i t e d Oxygen In the study of the f l a s h p h o t o l y s i s of C102 f Lipscomb 21 e t a l . a l s o observed v i b r a t i o n a l l y e x c i t e d oxygen w i t h up t o e i g h t quanta, c o r r e s p o n d i n g t o an energy of 34 K c a l , i n 9 the ground e l e c t r o n i c s t a t e . They found t h a t the amount of oxygen produced decreases i f the primary p h o t o l y s i s i s more than f i f t y p e r c e n t because t h e r e was not enough CIO2 l e f t t o r e a c t w i t h oxygen atoms a f t e r the primary p r o c e s s . The mechan-ism proposed f o r f o r m a t i o n of e x c i t e d oxygen was simple: C10 2 + hv -> CIO + 0 O + C10 2 -> CIO + 0 2* + *5§ K c a l O + c i o 2 + c i o 3 . C10 3 + hv -> CIO + 0 2* * where 0 2 i s v i b r a t i o n a l l y e x c i t e d oxygen. From t h e i r work on C10 2 and N0 2 as w e l l as from McKinley, 55 G a r v i n and Boudart, who observed i n f r a r e d e m i s s i o n from OH produced from the H/O^ r e a c t i o n , i t was c l e a r t h a t the energy d i s t r i b u t i o n i s not equilibrated'._/ ; : A l a r g e amount of energy i s found as v i b r a t i o n i n the newly formed bond. The study of CIO2 r e a c t i o n as w e l l as a number of s i m i l a r r e a c t i o n s studied 12 58 by f l a s h p h o t o l y s i s , l e d McGrath and N o r r i s h , ' to the f o l l o w i n g g e n e r a l i z a t i o n : "When an exothermic r e a c t i o n of the g e n e r a l form A +. BCD -*• AB + CD o c c u r s , the molecule AB w i t h newly formed bond takes a h i g h p r o p o r t i o n of exothermic energy o f the r e a c t i o n i n the form of xinequi l i b r a t e d v i b r a t i o n a l energy." 21 S i n c e the v i b r a t i o n a l t r a n s i t i o n s w i t h i n the ground s t a t e are o n l y allowed as magnetic d i p o l e or quadrupole r a d i -a t i o n , c o l l i s i o n a l d e a c t i v a t i o n i n the ground s t a t e must be c o n s i d e r e d . C o l l i s i o n a l d e a c t i v a t i o n of 0 2 was found t o be v e r y i n e f f e c t i v e f o r argon and n i t r o g e n (1 i n 10 7 e f f e c t i v e ) . 21 * They a l s o observed t h a t the h a l f l i f e of 0 2 (v"=6) v a r i e s between approximately 200 to 700 usee and was i n v e r s e l y p r o -p o r t i o n a l t o the i n i t i a l p r e s s u r e of CIC^. I t was thus con-c l u d e d t h a t C1C>2 and CIO are approximately e q u a l l y e f f e c t i v e i n d e a c t i v a t i o n 0 2 * (one i n 2000 e f f e c t i v e ) and t h a t a v a l u e 8 —1 —1 of k = 1.0 x 10 1 mole sec c o u l d be. a s s i g n e d to the r a t e c o n s t a n t f o r the p r o c e s s 0 2*(v"=6) + C l O ( o r C10 2) ->• 0 2*(v"=5) + C10(or C10 2) A g a i n due to the time r e s o l u t i o n of the apparatus and p r e s s u r e of C10 2 and argon used, they b e l i e v e d t h a t the energy d i s t r i b u t i o n they observed a t s h o r t e s t d e l a y s was v e r y c l o s e t o the i n i t i a l d i s t r i b u t i o n produced from the r e a c t i o n . They e s t i m a t e d t h a t a t the s h o r t e s t d e l a y the v"=5,6 and 7 l e v e l s of oxygen were e q u a l l y p o pulated w h i l e the v"=8 was p o p u l a t e d to a l e s s e r e x t e n t . E. P r e s e n t I n v e s t i g a t i o n Although e x t e n s i v e work has been done on the d i r e c t p h o t o l y s i s of the oxides of c h l o r i n e by v a r i o u s t e c h n i q u e s , i t can s t i l l be seen from the r e s u l t s t h a t t h e r e i s l a r g e v a r i -a t i o n i n the e x t i n c t i o n c o e f f i c i e n t and r a t e c o n s t a n t f o r the 22 25 decay of ClO. Clyne and Coxon's v a l u e s and those of L i p -21 22 15 scomb e t a l . , Edgecombe e t a l . and P o r t e r and Wright and the p r e s e n t f l a s h p h o t o l y s i s r e s u l t s themselves v a r y s i g n i f i -c a n t l y . One reason f o r the p r e s e n t work was, t h e r e f o r e , t o s o l v e the d i s c r e p a n c i e s which e x i s t between v a r i o u s workers s i n c e the CIO r a d i c a l has p l a y e d an important r o l e i n the ex-p l a n a t i o n of many r e a c t i o n mechanisms., The e a r l i e r f l a s h p h o t o l y s i s s t u d i e s of the decomposi-t i o n of CIO2 and Cl^O and of C ^ A ^ systems have been, t h e r e f o r e , repeated and the p r e v i o u s i n c o n s i s t e n c i e s r e s o l v e d . S e v e r a l new f e a t u r e s have been r e v e a l e d and i t has been p o s s i b l e t o measure the r a t e c o n s t a n t s f o r n e a r l y a l l the elementary r e -a c t i o n s i n v o l v e d i n the p h o t o l y s i s and c h l o r i n e p h o t o s e n s i t i s e d decomposition o f both CIO2 and C^O. R eactions of bromine atoms w i t h CIO2 and C^O have a l s o been s t u d i e d and the e x t i n c t i o n c o e f f i c i e n t f o r the BrO r a d i c a l measured. A second reason f o r t h i s work was t o study the energy d i s t r i b u t i o n i n the r e a c t i o n between oxygen and CIO2, as such i n f o r m a t i o n may p r o v i d e a deeper i n s i g h t i n t o the d e t a i l e d mechanism of a r e a c t i o n than can be o b t a i n e d f o r a knowledge o f r a t e c o n s t a n t a l o n e . The e x o t h e r m i c i t y of the r e a c t i o n (59 Kcal/mole) i s much more than the energy of the h i g h e s t 21 l e v e l observed by Lipscomb e t a l . In the r e a c t i o n between oxygen atoms and NO2 the same s i t u a t i o n e x i s t s ; the h i g h e s t 21 l e v e l observed by Lipscomb e t a l . . b e i n g a g a i n o n l y the 8tl whereas 47 K c a l are a v a i l a b l e . L a t e r s t u d i e s by Basco and 2 3 v 56 57 N o r r i s h and Basco and Morse have shown t h a t 0 2 w i t h v i b r a -t i o n a l energy up t o maximum e n e r g e t i c a l l y p o s s i b l e i s produced i n t h i s case. The p r e s e n t work has a l s o shown t h i s t o be t r u e f o r the C1C>2 r e a c t i o n . The r e l a t i v e r a t e s o f p r o d u c t i o n o f C»2 i n t o the l e v e l s observed have a l s o been measured and new i n f o r m a t i o n on the r e l a x a t i o n has been o b t a i n e d . Thus the p r e s e n t work has been d i v i d e d i n t o the f o l l o w -i n g s e c t i o n s . In Chapter t h r e e the e x t i n c t i o n c o e f f i c i e n t of CIO i s measured from C10 2 and supported from C l 2 0 and C ^ O / C l j systems. T h i s chapter a l s o i n c l u d e s the r a t e of recombination of CIO r a d i c a l s . The r e a c t i o n of oxygen atoms w i t h CIO, C ^ O and CIO2 have been r e p o r t e d i n chapter f o u r and the v a l u e s of the r a t e c o n s t a n t s are determined. Halogen (CI2 and B ^ ) p h o t o s e n s i t i s e d decomposition of CIO2 and CI2O and the d e t e r m i n a t i o n o f the r a t e c o n s t a n t s have been d i s c u s s e d i n chapter f i v e . In the l a s t c h apter (VI), the f o r m a t i o n , energy d i s t r i -* b u t i o n and r e l a x a t i o n of O2 from C10 2, C^O and CIO w i l l be d i s c u s s e d . CHAPTER I I EXPERIMENTAL A . Apparatus 1) The term f l a s h p h o t o l y s i s i s a p p l i e d t o h i g h i n t e n s i t y i r r a d i a t i o n f o r an extremely s h o r t time. The p r i n c i p l e s o f the method have been d e s c r i b e d by P o r t e r 1 i n d e t a i l . There are two t e c h n i q u e s , the photographic and the p h o t o e l e c t r i c , used t o d e t e c t the products and t r a n s i e n t i n t e r -mediates produced by f l a s h p h o t o l y s i s and the subsequent chemica r e a c t i o n s . a) In the photographic t e c h n i q u e , the a b s o r p t i o n spectrum o f the t r a n s i e n t s i s taken a t predetermined times a f t e r the p h o t o l y t i c f l a s h by means of a second f l a s h lamp and r e c o r d e d on photographic p l a t e s through a s p e c t r o g r a p h . The main advan-tage of t h i s technique i s t h a t a l a r g e wavelength r e g i o n can be s t u d i e d i n one experiment, the p r i c e b e i n g t h a t each d e l a y time r e q u i r e s a separate experiment. b) In the p h o t o e l e c t r i c . t e c h n i q u e the s i t u a t i o n i s r e -v e r s e d , the complete time behaviour of a narrow r e g i o n o f the spectrum i s f o l l o w e d by one experiment by means of a photo-m u l t i p l i e r and monochromator. The photographic technique was used throughout t h i s i n -v e s t i g a t i o n . The apparatus used i n t h i s i n v e s t i g a t i o n i s 59 e s s e n t i a l l y the same as t h a t used by Basco and N o r r i s h and Yee.^^ The main f e a t u r e s of the apparatus are shown i n f i g . ( 1 ) . 25 I LEGEND A - P h o t o l y s i s Lamp G - S p e c t r o g r a p h S l i t B - R e a c t i o n V e s s e l H - E l e c t r o n i c D e l a y U n i t C - B r a s s C o n t a i n e r I - H i g h V o l t a g e C h a r g i n g U n i t Di , „ , . „ J - I n d u c t i o n C o i l P i c k - u p _ x - 3 3 and 2yF D 2 K H y d r o g e n T h y r a t r o n E - T u n g s t e n E l e c t r o d e s • L - H i g h V o l t a g e S w i t c h F - S p e c t r o s c o p i c Lamp M - Gas I n l e t s f r o m Vaccum Systern F i g u r e 1. S c h e m a t i c D i a g r a m o f a C o n v e n t i o n a l F l a s h P h o t o l y s i s A p p a r a t u s . The o n l y d i f f e r e n c e i s t h e u s e o f manual f i r i n g i n s t e a d o f w i t h a T e s l a c o i l , i . e . , one e l e c t r o d e was c o n n e c t e d t o t h e c o n d e n s o r , t h e o t h e r t o one end o f t h e f i r i n g s w i t c h . The o t h e r end o f t h e s w i t c h was g r o u n d e d . The f i r i n g was done by c l o s i n g t h e s w i t c h . The a d v a n t a g e o f t h i s m a nual s w i t c h o v e r t r i g g e r i n g w i t h a p u l s e f r o m a T e s l a c o i l was t h a t spontaneous breakdown o f t h e lamps > was a v o i d e d . The p r e s s u r e o f a r g o n i n t h e lamp was o n l y 10 t o r r , a t w h i c h . p r e s s u r e t h e lamp a l w a y s f i r e d i m m e d i a t e l y on c l o s i n g t h e s w i t c h . The f l a s h e n e r g y u s e d i n t h e p r e s e n t work v a r i e s f r o m 150 t o 1300 J . I t was o b s e r v e d t h a t c h a r a c t e r i s t i c p r o f i l e o f t h e f l a s h lamp r e m a i n s a p p r o x i m a t e l y c o n s t a n t a t e a c h f l a s h e n e r g y u s e d . The h a l f l i f e t i m e o f t h e h i g h e n e r g y p u l s e , i . e . t h e t i m e t a k e n t o d e c a y h a l f o f i t s peak h e i g h t was f o u n d t o be 20 u s e e s . The f l a s h e n e r g y u s e d i n t h e s p e c t r o s c o p i c lamp was 'vlOO J . The h a l f l i f e o f s p e c t r o -s c o p i c lamp was f o u n d t o be 5 u s e e . The m i x t u r e s o f t h e g a s e s b e i n g s t u d i e d was s u b j e c t e d t o t h e h i g h e n e r g y , s h o r t d u r a t i o n p u l s e o f l i g h t w h i c h i s p r o d u c e d b y ' d i s c h a r g i n g t h e c o n d e n s o r s t h r o u g h t h e f l a s h lamp c o n t a i n i n g a r g o n . A p u l s e , p i c k e d by t h e c o i l w rapped a r o u n d t h e g r o u n d l e a d , i s f e d i n t o a d e l a y u n i t and, a f t e r a p r e -d e t e r m i n e d t i m e d e l a y , t h e s p e c t r o s c o p i c lamp f i r e s . Thus t h e l i g h t f r o m t h i s lamp, w h i c h m o n i t o r s t h e t r a n s i e n t s p e c i e s p r o d u c e d by m a i n f l a s h lamp, p a s s e s t h r o u g h t h e r e a c t i o n 27 v e s s e l and i s focused on the s p e c t r o g r a p h s l i t by q u a r t z l e n s e s . The a b s o r p t i o n spectrum of the s p e c i e s p r e s e n t i n the r e a c t i o n v e s s e l was photographed. 2) Double Lamp System S i m i l a r t o the c o n v e n t i o n a l f l a s h p h o t o l y s i s apparatus a double f l a s h system was developed which allowed the photo-l y s i s of t r a n s i e n t s p e c i e s . The arrangement f o r t h i s system over and above what i s r e q u i r e d f o r the c o n v e n t i o n a l apparatus i s shown i n f i g . ( 2 ) . A second p h o t o l y s i s lamp ( a u x i l i a r y ) , which can be e l e c t r o n i c a l l y delayed i n the same manner as the s p e c t r o s c o p i c lamp, i s p l a c e d p a r a l l e l t o the f i r s t p h o t o l y s i s lamp (main) on the o p p o s i t e s i d e of the r e a c t i o n v e s s e l . The s p e c t r o s c o p i c lamp i s then t r i g g e r e d from the a u x i l i a r y lamp. The t r i g g e r i n g c i r c u i t used to f i r e the a u x i l i a r y lamp i s s i m i l a r t o t h a t used to f i r e the s p e c t r o s c o p i c lamp i n the c o n v e n t i o n a l f l a s h p h o t o l y s i s apparatus, i . e . , a p u l s e p i c k e d up from an i n d u c t i o n c o i l (J) wound around the h i g h v o l t a g e l e a d i s f e d i n t o an e l e c t r o n i c d e l a y u n i t (H-i) c o n s i s t i n g of a c o l l e c t i o n o f c a l i b r a t e d R.C. c i r c u i t s . The d e l a y p u l s e i s strengthened by a s m a l l t h y r a t r o n - c a p a c i t o r c i r c u i t and i s d e l i v e r e d t o the g r i d of a l a r g e hydrogen t h y r a t r o n which i n t u r n d i s c h a r g e s the c a p a c i t o r through the a u x i l i a r y lamp. Sin c e a c c u r a t e time d e l a y s of the a u x i l i a r y lamp were r e q u i r e d , t r a c e s of the l i g h t output of the lamps were checked f o r each double lamp experiment on the s c r e e n of an o s c i l l o s c o p e . The d e l a y times quoted are measured from the 28 peak of the f i r s t f l a s h t o the peak of the second. 3) Vacuum System The u s u a l type of vacuum system was used, w i t h a two stage mercury d i f f u s i o n pump and a one stage r o t a r y o i l pump. The s t o p cocks were greased w i t h s i l i c o n grease s u p p l i e d by Dow C o r n i n g . T h i s grease was used i n o r d e r t o a v o i d the decomposition o f oxides o f c h l o r i n e because these gases were s e n s i t i v e t o o r g a n i c greases l i k e Apiezon. With the use of t h i s grease, i t was s t i l l p o s s i b l e t o pump the system t o l e s s than 10 t o r r . Most of the gases used were c o r r o s i v e t o mercury so the p r e s s u r e o f the gases was measured w i t h two s p i r a l gauges of r a t i o s 1:4 and 1 : 2 . These s p i r a l gauges were c a l i b r a t e d w i t h a mercury manometer. The t o t a l p r e s s u r e or c o o l i n g gas p r e s s u r e was measured w i t h a mercury manometer. 4) Spectrographs Most of the work d e s c r i b e d here was done on the J a r r e l l Ash s p e c t r o g r a p h but a few runs were done on the medium H i l g e r . The J a r r e l l Ash i s a 3 . 4 meter E b e r t mounting type s p e c t r o -graph w i t h the f o l l o w i n g c h a r a c t e r i s t i c s : 1) i n t e r c h a n g e a b l e plane g r a t i n g mounting; 2) two plane g r a t i n g s each w i t h 1 5 , 0 0 0 l i n e s per i n c h , o o one b l a z e d f o r 3300 A and the oth e r f o r 6000 A; o 3) r e c i p r o c a l l i n e a r d i s p e r s i o n o f 5 . 1 A per mm. i n the f i r s t o r d e r ; o 4) 2500 A range i n f i r s t o r d e r by two 10 x 4 i n c h photographic p l a t e s . ...... 29 F i g u r e 2. A u x i l i a r y Lamp T r i g g e r i n g C i r c u i t . Components w i t h S u b s c r i p t s are the d u p l i c a t e components of the conventional system^ otherwise the l a b e l i n g i s the same as i n f i g u r e 1. 30 The second s p e c t r o g r a p h was a medium H i l g e r p r i s m i n -strument u s i n g q u a r t z o p t i c s . I t was used o n l y f o r comparing the e x t i n c t i o n c o e f f i c i e n t and r a t e c o n s t a n t f o r the decay of 21 CIO w i t h those of Lipscomb e t a l . The spectrum o b t a i n e d o o from t h i s instrument extends from 2000 A to 10,000 A and i s 221 cm. l o n g , e n a b l i n g i t to be reco r d e d on a s i n g l e p l a t e . o The r e c i p r o c a l l i n e a r d i s p e r s i o n a t 2200 A i s approximately o o o 5 A per mm. whereas a t 6000 A i t i s about 150 A per mm. T h i s instrument i s most u s e f u l f o r e x p l o r a t o r y work or work i n v o l -v i n g continuous s p e c t r a because of the wide range covered a t the expense of d i s p e r s i o n and r e s o l u t i o n and a l s o because of i t s speed. B. F i l t e r s Almost a l l the work mentioned here r e q u i r e d the s e l e c -t i v e p h o t o l y s i s of e i t h e r CIO2, C^O or halogens. T h i s was achieved by p l a c i n g 3 mm t h i c k c o r n i n g g l a s s f i l t e r s between the p h o t o l y s i s lamp and the r e a c t i o n v e s s e l . The type of f i l t e r s used w i l l be mentioned i n the r e s p e c t i v e s e c t i o n . One of the f i l t e r s used was prepared from photographic p l a t e s a f t e r removing the g e l a t i n e completely; t h i s i s c a l l e d g l a s s f i l t e r A. The t r a n s m i s s i o n p r o p e r t i e s o f the f i l t e r s used are g i v e n i n f i g . ( 3 ) . The curves are reproduced from the Co r n i n g 61 G l a s s Works C i r c u l a r except f o r the f i l t e r which was p r e -pared from photographic p l a t e s and pyrex r e a c t i o n v e s s e l . F i g u r e 3 . Transmission C h a r a c t e r i s t i c s of the F i l t e r s . A - P y r e x r e a c t i o n vessel,-B =Glass f i l t e r A, C= 0 - 5 2 , D = 3 - 7 5 , E « 3 - 7 2 , F - 3 - 6 6 Corning F i l t e r s . 32 T h e i r t r a n s m i s s i o n was measured on Cary 14. The t r a n s m i s s i o n of the f i l t e r s was unchanged a f t e r many f l a s h e s . The nomi-n a l l y zero t r a n s m i s s i o n r e g i o n s o f the f i l t e r s had a measured o p t i c a l d e n s i t y o f g r e a t e r than f i v e . C. Photography and Photometery In the p r e s e n t work, o n l y I l f o r d p l a t e s were used t o photograph the s p e c t r a of d i f f e r e n t s p e c i e s . HP J were used o f o r wavelengths g r e a t e r than 2300 A and Q2 f o r wavelengths o below 2300 A. The s l i t width of the spe c t r o g r a p h was 30u i n a l l of the experiments except f o r Q2 where i t was lOOu • Sin c e Q2 p l a t e s were used o n l y t o monitor NOCl, which has a continuous spectrum, the s l i t width does not a f f e c t the appar-ent shape of the band. The photographic p l a t e s were developed i n Kodak D19 devel o p e r f o r 5 min. wit h continuous a g i t a t i o n . The tempera-t u r e of the developer was kept a t 20°C. The p l a t e s were dipped i n a stop bath (3% a c e t i c a c i d ) f o r 30 sec. and then f i x e d f o r two minutes i n the Kodak r a p i d f i x e r . The p l a t e s were washed i n running water f o r h a l f an hour, r i n s e d w i t h d i s t i l l e d water and d r i e d w i t h a stream" o f a i r a t room tempera-t u r e . The photographic p r i n t s shown i n the next chapters were made u s i n g a standard photographic e n l a r g e r and Agfa BNI p r i n t < i paper, the development of which i s the same as the p l a t e s except Agfa D e u t o l was used f o r d e v e l o p i n g the paper. 3 3 The d e n s i t y o f each p l a t e was measured on a Joyce-L o e b e l double beam r e c o r d i n g microdensitometer Model I I I , u s i n g an a c c u r a t e l y c a l i b r a t e d n e u t r a l d e n s i t y wedge. In most of the measurements, e s p e c i a l l y CIO, CIO2 and C^O, the 0 t o 2 wedge was used but f o r v i b r a t i o n a l l y e x c i t e d oxygen or othe r weak s p e c t r a , 0 t o 1 was used. The c a l i b r a t i o n of the wedge was s u p p l i e d by the makers of the instrument and i t was a l s o checked by u s i n g two n e u t r a l d e n s i t y f i l t e r s of o p t i c a l d e n s i t y 1.0 and 1.5. The v a l u e thus o b t a i n e d f o r c a l i b r a t i o n was w i t h i n e x p e r i m e n t a l e r r o r o f t h a t was s u p p l i e d . So the va l u e g i v e n by the makers was used, i . e . , 0.087 per cm (while our e x p e r i m e n t a l v a l u e was 0.09 ± 0.01). The q u a n t i t a t i v e measurements s t u d i e d w i t h the h e l p of photography does depend upon the p h y s i c a l p r o p e r t i e s o f emul-s i o n s ; e.g., c o n t r a s t , speed, l a t i t u d e or u s e f u l range of exposure, g r a i n i n e s s and s p e c t r a l s e n s i t i v i t y . Of these, the f i r s t t h r e e are the main ones which can be o b t a i n e d from the c h a r a c t e r i s t i c curve of emulsion, which shows the r e l a t i o n s h i p between the l i g h t exposure (on a l o g a r i t h m i c s c a l e ) r e c e i v e d by the emulsion and the r e s u l t a n t d e n s i t y o f the developed image.. The v a r i a t i o n i n the exposure f o r the p l o t s was ob-t a i n e d by v a r y i n g the s l i t width s i n c e the area i l l u m i n a t e d i s p r o p o r t i o n a l to the exposure i f e v e r y t h i n g e l s e i s kept con-s t a n t . Thus the c h a r a c t e r i s t i c curves were drawn f o r each wavelength wherever the measurements were done. I t can be seen from f i g . ( 4 ) t h a t the curves show a l i n e a r r e l a t i o n s h i p F i g u r e 4. C h a r a c t e r i s t i c Curves f o r tljie HP3 p l a t e f o r i i s e l e c t e d wave l e n g t h s . 1 Log.(Slit Width) 35 between the p l a t e d e n s i t y and exposure f o r the 0.2 to 0.8 p l a t e d e n s i t y r e g i o n . A l l i n t e n s i t y measurements were con-f i n e d t o t h i s r e g i o n . From these curves the s l o p e of the l i n e a r p o r t i o n , which i s known as the c o n t r a s t (y) , i . e . , the development f a c t o r o f the emulsion, was c a l c u l a t e d and i s l i s t e d i n Ta b l e I . I t was found t h a t Y changes w i t h time of development and the temperature of the d e v e l o p i n g bath, so f o r a l l the p l a t e s which were used f o r q u a n t i t a t i v e measurements, s i m i l a r c o n d i t i o n s were used w i t h f r e s h d e v e l o p i n g s o l u t i o n each time. Table I (A) 2577 2772 2920 3208 3383 3515 1.02 1.17 1.21 — — 1.30 ... y ; ; . . ' — 1.74 1.74 2.18 2.35 2.61 The Y.of the p l a t e s was checked from time t o time i n or d e r t o c o n f i r m t h a t i t does remain c o n s t a n t . In the p r e s e n t study two types of p l a t e s were used. The batch of p l a t e s which came i n the l a t e r p a r t o f the study was found t o have g r e a t e r c o n t r a s t than the p l a t e s used e a r l i e r . S i m i l a r c h a r a c t e r i s t i c curves were drawn f o r I l f o r d Q2 p l a t e s but here i t was found t h a t Y i s v e r y s e n s i t i v e to the p h y s i c a l c o n d i t i o n s than I l f o r d HP 3 p l a t e . So more care was taken here, although they were used v e r y r a r e l y . 36 D. Measurements of Ab s o l u t e C o n c e n t r a t i o n s In o r d e r t o c a l c u l a t e the a b s o l u t e c o n c e n t r a t i o n of any s p e c i e s , i t i s necessary t o know whether Beer-Lambert's law i s obeyed by i t . T h i s has been checked by the two path l e n g t h 6 3 method as d e s c r i b e d by N o r r i s h e t a l . The r e g u l a r mixture was f l a s h e d and the spectrum was taken. Then h a l f o f the r e a c t i o n v e s s e l was covered w i t h b l a c k paper and the f r e s h mixture was f l a s h e d w i t h the same energy and the spectrum was taken under the same c o n d i t i o n s as w i t h f u l l r e a c t i o n v e s s e l . A number of s t r i p s were taken u s i n g d i f f e r e n t d e l a y times, d i f f e r e n t p r e s s u r e s of C1Q 2 and C l 2 0 w i t h same t o t a l p r e s s u r e , d i f f e r e n t f l a s h e n e r g i e s and c o v e r i n g a d i f f e r e n t h a l f of the r e a c t i o n v e s s e l w i t h b l a c k paper, but care was taken t h a t the s t r i p s taken w i t h h a l f r e a c t i o n v e s s e l were under the same c o n d i t i o n s as f u l l r e a c t i o n v e s s e l . P l a t e s were photo-metered and the peak h e i g h t s h(with f u l l r e a c t i o n v e s s e l ) were p l o t t e d a g a i n s t the peak h e i g h t s h ^ (with h a l f the r e a c t i o n v e s s e l ) and the b e s t p o s s i b l e s t r a i g h t l i n e was drawn. I t i s an assumption i n the two path method t h a t the power t o which both the c o n c e n t r a t i o n and the l e n g t h must be r a i s e d i n des-c r i b i n g the o p t i c a l d e n s i t y , i . e . , D = e- ( c l ) n 2 so D/D( = h/h ( = 2 n I 3 7 t T h i s power 'n' can be determined from the r a t i o of h/h^ from f i g . (5) i n case of CIO and f i g . (6) i n case of (0-12) * band of 0 2 . For a curve, the power would vary over a range of o b s e r v a t i o n s , f o r a s t r a i g h t l i n e i t would be c o n s t a n t and f o r an a b s o r p t i o n obeying the Beer-Lambert law, the r a t i o h/hj^ would be equal to 2 and thus the r e s u l t i n g power ' n' would be one. I t i s thus p o s s i b l e t o c a l c u l a t e a n a l y t i c a l l y the de-pendence of h on c o n c e n t r a t i o n . 1) CIO In case of CIO t h i s power "n* i s found t o be 0.9 ! .02 f o r 12,0 band and thus the d e n s i t o m e t r i c h e i g h t should be r a i s e d t o the power 1.1 1.02 i n o r d e r to determine the a b s o l u t e c o n c e n t r a t i o n of CIO. Since the v a l u e of *n' o b t a i n e d i s i o , w i t h i n the e x p e r i m e n t a l e r r o r , i t was assumed t h a t CIO obeys the Beer-Lambert law. T h i s was a l s o checked i n another way. o The r a t i o of the a b s o r p t i o n a t 2772 A (12,0) t o the a b s o r p t i o n ° . a t 2577 A (continuum) was c a l c u l a t e d t o be 1.52 I .04, which 15 agrees w i t h the v a l u e o b t a i n e d by P o r t e r and Wright and a l s o 23 by Clyne and Coxon and remains c o n s t a n t over a l a r g e range of c o n c e n t r a t i o n and under o t h e r c o n d i t i o n s . S i n c e the Beer-Lambert law i s obeyed by the continuum, constancy of the r a t i o shows t h a t i t should be obeyed by 12,0 band as w e l l as by 7,0 o band ( d i f f u s e band, 2920 A ) . Thus the c o n c e n t r a t i o n of CIO was c a l c u l a t e d C = D/el where e = e x t i n c t i o n c o e f f i c i e n t of CIO; 1 = path l e n g t h . F i g u r e 5 . A p l o t of h(C10) against hJ(C10) . 4 u s o «-l|C\J 0 OJ 00 h (cm) 39 * C M O -p w C • H CO CO * OJ O o -p o rH ft vO cu O OJ o X3 40 2) V i b r a t i o n a l l y E x c i t e d Oxygen The same procedure as t h a t d e s c r i b e d f o r ClO measurements was used to measure e x c i t e d oxygen i n d i f f e r e n t bands. The d i f f i c u l t y w i t h e x c i t e d oxygen was t h a t n e a r l y a l l the bands were e i t h e r mixed up w i t h CK> 2 spectrum, ClO spectrum (banded and continuous) or the d i s p e r s i o n was not l a r g e enough to r e -s o l v e the lower bands of oxygen. The bands (0,12) and (3,6) were the b e s t out of the whole range of bands. I t was found t h a t the power 'n' f o r (0,12) i s not c o n s t a n t and i t does depend upon the amount of i n t e r f e r e n c e w i t h C10 2 (only the band head and a few r o t a t i o n a l l i n e s c o u l d be d e t e c t e d b e f o r e the band of C 1 0 2 ) . The average v a l u e found from f o u r d i f f e r e n t s e t s of experiments showed t h a t the power t o which h should be r a i s e d has a v a l u e 1.5 1 .25. As i t w i l l be seen l a t e r on, the r e l a t i v e p o p u l a t i o n of e x c i t e d oxygen was c a l c u l a t e d u s i n g 64 the power as 1.0 and 1.75. In the case of NO2, s i m i l a r behaviour i s observed i n our l a b o r a t o r y , i . e . , wherever the 0 2 i s not i n t e r f e r e d by NO or N0 2, i t obeys Beer-Lambert law, whereas otherwise departure from the Beer-Lambert law i s ob-served, s _ E. P r e p a r a t i o n and P u r i f i c a t i o n o f M a t e r i a l s C h l o r i n e d i o x i d e . C l ° 2 was prepared as d e s c r i b e d by Derby and H u t c h i n s o n . ^ 5 C h l o r i n e and a i r , a f t e r b u b b l i n g ~ through c o n c e n t r a t e d s u l p h u r i c a c i d , were mixed t o g e t h e r . The mixture was passed through a column l o o s e l y packed wi t h c r y s -t a l l i n e NaC102- The gases are then c o l l e c t e d i n a t r a p kept 41 a t acetone/dry i c e temperature ( i . e . - 7 8 ° C ) . The C10 2 i s then pumped v i g o r o u s l y a t l i q u i d n i t r o g e n temperature, d r i e d by p a s s i n g over P 2 ° 5 ^ n a t r a P a n < ^ f i n a l l y i t was d i s t i l l e d from acetone/dry i c e t o l i q u i d n i t r o g e n temperature. The sample a t l i q u i d n i t r o g e n was d i s p o s e d of and m a t e r i a l l e f t a t acetone/dry i c e temperature was pumped at t h i s temperature. 6 7 The p u r i t y o f the compound was t e s t e d by i t s vapour p r e s s u r e . The sample thus o b t a i n e d was always kept a t l i q u i d N2 temper-a t u r e . C h l o r i n e monoxide. CI2O was prepared by the method 66 suggested by Cady. A mixture of c h l o r i n e and a i r , a f t e r p a s s i n g through c o n c e n t r a t e d s u l p h u r i c a c i d , was passed through a column c o n t a i n i n g a mixture of m e r c u r i c oxide and g l a s s c h i p s . The g l a s s c h i p s were added t o a l l o w the passage of " g a s e s . The gases were c o l l e c t e d a t -78°C. P u r i f i c a t i o n was c a r r i e d out i n the same manner as above w i t h the d i f f e r e n c e t h a t m e t h a n o l / l i q u i d n i t r o g e n s l u s h was used f o r pumping the l a s t t r a c e s of c h l o r i n e p r e s e n t . The C^O was s t o r e d a t l i q u i d n i t r o g e n temperature whenever i t was not being used. N i t r o s y l c h l o r i d e . N0C1 was prepared by a l l o w i n g a mixture of pure dry c h l o r i n e and excess n i t r i c oxide to stand f o r t h r e e days. The products were condensed i n a l i q u i d n i t r o g e n t r a p w i t h the excess n i t r i c oxide and the l a t t e r was pumped o f f . A f t e r d i s t i l l a t i o n , the n i t r o s y l c h l o r i d e was s t o r e d a t -196°C u n t i l r e q u i r e d . 42 C h l o r i n e . C l 2 ( L i q u i d A i r L t d . ) , was degassed a t -196°C / d r i e d by p a s s i n g over P 2 0 5 a n c ^ f i n a l l y d i s t i l l e d from -78°C to -196°C. The gas was s t o r e d a t room temperature. Bromine. B r 2 ( A l l i e d Chems. ) was p u r i f i e d as c h l o r i n e except t h a t i t was d i s t i l l e d from an ice/common s a l t mixture and s t o r e d a t -78°C u n t i l r e q u i r e d . Oxygen, N i t r o g e n ( 9 8 % ) , Argon (99.999%) and Carbon  D i o x i d e were s u p p l i e d by L i q u i d A i r L t d . These gases were used as such without f u r t h e r p u r i f i c a t i o n except t h a t they were passed through a t r a p c o n t a i n i n g g l a s s wool and kept a t -78°C. The p u r i f i c a t i o n of a l l the gases except i n e r t gases was done under vacuum. F. P r e p a r a t i o n of a M i x t u r e and Procedure M i x t u r e of the r e a c t a n t s p e c i e s and argon or o t h e r i n e r t gas was done a t l e a s t t h r e e hours b e f o r e commencing the run t o ensure homogeneous mixin g . The mixture was made i n a l a r g e b ulb a t the same argon: r e a c t a n t r a t i o t o be used i n the experiments but i n s u f f i c i e n t h i g h e r t o t a l p r e s s u r e t o a l l o w the experiments t o be completed. _ S i n c e oxides of c h l o r i n e are s e n s i t i v e t o l i g h t , a l l the bulbs were blackened, a l s o the f e e d i n g l i n e s t o r e a c t i o n v e s s e l were covered w i t h black paper. A l l the experiments were done i n a dark room. Pr e s s u r e s l e s s than one mm were measured by expanding from a high p r e s s u r e i n a s m a l l volume to an evacuated b u l b . The 43 expansion r a t i o s of these l i n e s t o the bulbs were measured beforehand. There are some experiments where the mixing time i s l e s s than t h a t mentioned above ( i n order t o a v o i d dark r e a c t i o n ) but t h a t w i l l be mentioned a t the s p e c i f i e d p l a c e . There are twenty t h r e e s t r i p s a v a i l a b l e on each p l a t e which a l l o w s a t l e a s t 18 t o 19 of these to be time e x p e r i -ments. A t l e a s t two of the o t h e r s t r i p s were used f o r blanks to' c o n f i r m the constancy of the output of the s p e c t r o s c o p i c lamp throughout the experiment. The o t h e r exposures are the " b e f o r e , " which i s the a b s o r p t i o n spectrum of the r e a c t i o n mixture i n the c e l l b e f o r e the p h o t o l y s i s and " a f t e r " , an a b s o r p t i o n spectrum of products and r e a c t a n t s remaining a t r e l a t i v e l y long time (30 seconds) a f t e r the p h o t o l y s i s f l a s h . Each of these exposures i s necessary on every p l a t e t o a c t as a s t a n d a r d f o r the p a r t i c u l a r c o n d i t i o n s used. CHAPTER I I I EXTINCTION COEFFICIENT OF CIO AND DECAY OF CIO A. E x t i n c t i o n C o e f f i c i e n t of CIO 1) From C I O 2 P h o t o l y s i s The assumption i n v o l v e d i n the measurement of e x t i n c -t i o n c o e f f i c i e n t of CIO i n the f o l l o w i n g experiment i s t h a t one CIO r a d i c a l i s produced from each molecule of C I O 2 decom-posed and t h a t the t o t a l c o n c e n t r a t i o n of CIO produced i n i -t i a l l y may be measured by e x t r a p o l a t i o n back to zero time. C I O 2 was f l a s h e d u s i n g f o u r d i f f e r e n t f l a s h e n e r g i e s i : the presence of excess i n e r t gas. The r a t i o o f C I O 2 to i n e r t gas was v a r i e d from 1:750 t o 1:3300, a t which r a t i o s the p h o t o l y s i s i s i s o t h e r m a l . S i n c e the pyrex r e a c t i o n v e s s e l o (used i n our system) t r a n s m i t s some l i g h t below 3000 A, i n a l l the experiments 2 t o 3 mm of g l a s s f i l t e r A was used. o T h i s f i l t e r absorbs a l l the r a d i a t i o n below 3000 A ( f i g . 3 ) a n d thus the d i r e c t p h o t o l y s i s of CIO was avoided. Because of the s t r o n g a b s o r p t i o n of CIO and C I O 2 , the c o n c e n t r a t i o n of CK>2 used f o r the measurements of e x t i n c t i o n c o e f f i c i e n t of CIO v a r i e d between 0.06 t o 0.12 t o r r . Some experiments were done u s i n g 0.25 t o r r of C I O 2 but i t has been found t h a t the r e s u l t s were not v e r y a c c u r a t e due t o the s a t u r a t i o n of the photographic p l a t e . S i n c e t h e r e c o u l d be a s m a l l e r r o r i n making the C I O 2 mixture by the expansion method, though l e s s 45 than 10% even a t the lowest CIO2 p r e s s u r e , the CIO2 p r e s s u r e was checked by measuring i t s c o n c e n t r a t i o n from i t s a b s o r p t i o n spectrum. From a c c u r a t e l y measured p r e s s u r e s , the e x t i n c t i o n c o e f f i c i e n t a t 3515 A was measured and found to be i n e x c e l -25 -1 l e n t agreement w i t h the l i t e r a t u r e v a l u e of (3000 I mole cm" 1). A t low f l a s h energy where the primary p h o t o l y s i s i s l e s s than 25%, o n l y t h r e e processes need to be c o n s i d e r e d , e.g. C10 2 + hv •+ CIO + O (27) C10 2 + 0 + CIO + 0 2 (7) CIO + CIO -> C l 2 + 0 2 (5) C o n v i n c i n g reasons f o r b e l i e v i n g t h a t (27) i s the o n l y s i g n i f i c a n t primary process w i l l be g i v e n i n c o n n e c t i o n w i t h the mechanism of the p r o d u c t i o n of v i b r a t i o n a l l y e x c i t e d oxygen. The r a t e of decay of CIO i s v e r y slow compared to the r a t e of i t s p r o d u c t i o n and the second order p l o t f o r the decay i s s t r i c t l y l i n e a r over the range of d e l a y times used. The accuracy of the s h o r t l i n e a r e x t r a p o l a t i o n to zero time i s then l i m i t e d o n l y by a s m a l l u n c e r t a i n t y i n the d e f i n i t i o n of zero time due to f i n i t e l i f e t i m e of the p h o t o - f l a s h . T h i s uncer-t a i n t y of <_ 10 usee leads to a p o s s i b l e e r r o r of o n l y 1 5% i n the e x t i n c t i o n c o e f f i c i e n t . A t h i g h f l a s h e n e r g i e s , the second order p l o t f o r the CIO (as can be seen from f i g . 8, p. 59 ), becomes markedly non-46 l i n e a r a t d e l a y times below ^200 ysec, but, s i n c e the i n i t i a l r a p i d decay can be f o l l o w e d over most of the p e r i o d , the e x t r a -p o l a t i o n i s s t i l l v a l i d . Because of the c u r v a t u r e and the steepness a t h i g h f l a s h e n e r g i e s , the e x t r a p o l a t i o n i s more d i f f i c u l t and an e r r o r o f 10 ysec i n zero time would g i v e ^20% e r r o r i n the e x t i n c t i o n c o e f f i c i e n t . From the c l o s e n e s s of the agreement between the v a l u e s of e x t i n c t i o n c o e f f i c i e n t ob-t a i n e d a t h i g h and low f l a s h e n e r g i e s , i t would appear t h a t the zero time i s known w i t h i n 1 5 ysec. Taking i n t o c o n s i d e r a t i o n t h a t t h e r e i s no o t h e r r e -a c t i o n competing w i t h the above mentioned, then A[CIO,] = [CIO] = e e l = D o where A[C10 2] = decrease i n the [ClOj] D = o p t i c a l d e n s i t y e = e x t i n c t i o n c o e f f i c i e n t of CIO 1 = l e n g t h of the r e a c t i o n v e s s e l and the e x t i n c t i o n c o e f f i c i e n t of CIO can be c a l c u l a t e d from e = D/1[C10] Q = D/1A[C10 2] . The v a l u e s of e x t i n c t i o n c o e f f i c i e n t of CIO were c a l c u -l a t e d by measuring the c o n c e n t r a t i o n of CIO formed a t t h r e e d i f f e r e n t wavelengths and under the v a r i o u s c o n d i t i o n s . The d a t a thus o b t a i n e d are g i v e n i n T a b l e I I . Values w i t h * show the e f f e c t of working i n the n o n - l i n e a r r e g i o n of the p l a t e c h a r a c t e r i s t i c curve and are not i n c l u d e d i n the average. The o o o average v a l u e of e (CIO) a t 2577 A, 2772 A and 2920 A are 47 found to be 1.15 t O.0.6x 1 0 3 , 1.7 ± 0.07* 1 0 3 and 1.05 t 0 . 0 4 1 -1 ~1 x 1 0 J mole cm respectively. The error l i m i t s are the standard deviations. Table II E x t i n c t i o n C o e f f i c i e n t of CIO from C 1 0 2 Photolysis P l a t e no. ,C 1 0 2 ( t o r r ) A r g o n ( t o r r ) E n e r g y J e x 10 2577 A 3 (1 m o l e " 2772 A 1 -1, cm ) 2920 A 104 0.25 200 1060 0.8* 0.86* 0.8* 105 0.06 200 1060 1.17 1.78 — 107 0.08 75 1060 — 1.66 1.10 114 0.12 200 1060 1.74 1.02 105 0.06 200 600 1.17 1.72 — 154 0 .1 200 600 — .1, 7 1.0 4 118 0.25 200 260 1.04 1.28* 1.0 120 0.1 75 260 1.14 1.76 1.02 156 0.1 200 260 1.16 1.71 1.04 164 0.25 200 260 1.25 1.65 1.10 118 0.25 200 160 1.16 1.55 1.05 156 0.1 200 160 1.12 1.71 1.10 A v e r a g e 1.15* Q.OG 1.70±0 .07 1.05 ± 0.04 T h e s e v a l u e s a r e n e g l e c t e d due t o s a t u r a t i o n o f p l a t e From t h e s e r e s u l t s t h e r a t i o o f e (2577 A) : e (2772 A) : o e(2920 A) i s f o u n d t o be 1.09 : 1.61' : 1.0. The r e l a t i v e v a l u e s o f e(CIO) f o r t h e s e w a v e l e n g t h s were a l s o c a l c u l a t e d f r o m 100 m e asurements o f C l O a b s o r p t i o n a t d i f f e r e n t t i m e d e l a y s . T h e s e 48 o o o gave an a v e r a g e v a l u e f o r e (2577 A) : e(2772 A) : e(2920 A) = 1.07 : 1.60 : 1.0, t h u s i n e x a c t a g r e e m e n t w i t h t h a t ob-o o t a i n e d by e x t r a p o l a t i o n . The r a t i o o f e(2577 A) : e(2772 A) =1.0 : 1.48, i s a l s o i n g o o d a g r e e m e n t w i t h t h e r a t i o c a l -15 c u l a t e d by P o r t e r and W r i g h t (1.0:1.5) and C l y n e and C o x o n 2 3 ( 1 . 0 : 1 . 5 4 ) . The v a l u e o f 1.15 x 1 0 3 1 m o l e - 1 c m - 1 f o r e(CIO) a t o 2577 A i s i n e x c e l l e n t a g r e e m e n t w i t h t h a t o b t a i n e d a t low 21 f l a s h e n e r g y v a l u e s by L i p s c o m b e t a l . b u t t h e v a l u e a t o 2920 A i s a p p r o x i m a t e l y t w i c e t h a t w h i c h c a n be o b t a i n e d f r o m 22 t h e work o f Edgecombe e t a l . t o be d i s c u s s e d m t h e n e x t s e c t i o n . 2) C a l c u l a t i o n o f E x t i n c t i o n C o e f f i c i e n t of CIO f r o m t h e  D i r e c t P h o t o l y s i s o f CI2O and C h l o r i n e P h o t o s e n s i t i s e d  D e c o m p o s i t i o n o f CI2O As i t c a n be s e e n f r o m t h e a b s o r p t i o n s p e c t r u m o f 48 0 C 1 2 0 , t h e l i g h t a b s o r b e d by C1 20 above 3000 A w i l l l e a d t o t h e f o l l o w i n g r e a c t i o n s : C 1 2 0 + hv •+ CIO + C l c i 2 o + C l CIO + c i 2 CIO + CIO -»• c i 2 + o 2 T h e r e a r e o t h e r r e a c t i o n s f o l l o w i n g t h e s e b u t t h e y a r e q u i t e s l o w and w i l l be c o n s i d e r e d i n C h a p t e r V o f t h i s work. C o n s i d e r i n g t h e s e t h r e e s i m p l e r e a c t i o n s as w i t h C10 2 p h o t o l y s i s , C1~0 was f l a s h e d a t t h r e e f l a s h e n e r g i e s and two 49 p r e s s u r e s , keeping a t o t a l i n e r t gas p r e s s u r e of 200 t o r r . The assumption t h a t one molecule of C1 20 decomposed w i l l g i v e r i s e t o one CIO r a d i c a l s t i l l h o lds i n the above r e a c t i o n scheme as w e l l as i n the c h l o r i n e s e n s i t i s e d p h o t o l y s i s of C1 20, i . e . C l 2 + hv -»• 2 C l C l + c i 2 o-> CIO + c i 2 T h e r e f o r e C1 20 was a l s o f l a s h e d i n the presence of c h l o r i n e . The r a t i o s v a r y approximately from 1:1 t o 1:4 a t t h r e e f l a s h e n e r g i e s . The c h l o r i n e atom c o n c e n t r a t i o n a t each f l a s h energy was c a l c u l a t e d from the t i t r a t i o n of c h l o r i n e atoms w i t h n i t r o -s y l c h l o r i d e , i . e . , by measuring the decrease i n the amount of n i t r o s y l c h l o r i d e by the r e a c t i o n C l + NOCl •> NO + C l 2 T h i s was achieved by f l a s h i n g N O C l / C l 2 / A r mixture under the same c o n d i t i o n s as C l 2 0 / C l 2 / A r . A d i f f i c u l t y n o t i c e d i n the measurement was t h a t the spectrum of C1 20 i s spread over the whole r e g i o n of t h a t of o ClO. Even a t 2350 A ClO has an e x t i n c t i o n c o e f f i c i e n t a p p r o x i -mately 50 t o 80 1 m o l e - 1 c m - 1 whereas C l j O has W100 1 m o l e - 1 cm ^. Although w i t h the eye the ClO spectrum looks d i s c r e t e , w i t h a densitometer t r a c e , . a l l the bands are mixed w i t h each o t h e r . T h i s w i l l be d i s c u s s e d l a t e r on. Thus t o f i n d the C1 20 decrease o r ClO produced, the r a t i o of the peak h e i g h t measured from the a c t u a l base l i n e t o the peak h e i g h t measured from the base l i n e (obtained by j o i n i n g the two s u c c e s s i v e 50 bands), was measured from the f l a s h p h o t o l y s i s of CIC^. I t was found t h a t a c t u a l peak h e i g h t of CIO band i s approximately 20% more than the apparent one. Thus, i n t h i s case D t = D (C1 20) + D (CIO) (a) = D (C1 00) + D (CIO) ^•+ 20% D (CIO) I app• aPP• T o t a l peak h e i g h t (D^ .) and apparent peak heightswere measured. The a c t u a l peak h e i g h t of CIO was thus c a l c u l a t e d by adding 20% of the apparent peak h e i g h t t o i t and hence the decrease i n the C1 20 was c a l c u l a t e d from the e x p r e s s i o n ( a ) . A s i m i l a r procedure was a p p l i e d t o the c h l o r i n e s e n s i t i s e d decomposition of C1 20. Thus the e x t i n c t i o n c o e f f i c i e n t S a t two wavelengths were c a l c u l a t e d w i t h the r e s u l t s l i s t e d i n T a b l e I I I . . The O O j^. averageSof e(2772 A) and e(2920 A) were found t o be 1.61 -0.05 x i o 3 and 0.99 *0.05 x i o 3 1 m o l e " 1 c m - 1 r e s p e c t i v e l y . Though t h i s procedure i s not v e r y a c c u r a t e due t o the i n t e r -f e r e n c e of two s p e c t r a , the v a l u e s o b t a i n e d are approximately of the same order as c a l c u l a t e d from C10 2 p h o t o l y s i s . D i s c u s s i o n I t can be seen from Table I I t h a t our v a l u e of e(ClO) o a t 2577 A agrees very w e l l w i t h those o b t a i n e d by Lipscomb 21 e t a l . , a t low f l a s h e n e r g i e s but a t h i g h f l a s h energy our 21 v a l u e i s twice t h a t o b t a i n e d by them. We a l s o found the same behaviour as n o t i c e d by them, i . e . , i f the e x t i n c t i o n 51 T a b l e I I I E x t i n c t i o n C o e f f i c i e n t of CIO from C l 2 0 and C l 2 0 / C l P h o t o l y s i s P l a t e C l 2 O x l O + 6 C l 2 x l 0 + 6 Ar Energy e x Io" 3 (1 mole ^cm ^ no. (mole/£) (mole/£) t o r r J 2772 A 2920 A. 167 34.1 137.5 200 260 1.69 — 169 63.0 68.0 200 600 1.58 — 170 35.6 137.5 200 600 1.57 0.97 171 63.3 137.5 200 600 -- 1.0 173 36.5 137.5 200 1060 0.90 184 57.8 — 200 830 1.60 1.10 191 52.3 — 200 1060 1.58 — 193* 33.0 — 200 1325 1.65 0.96 Average 1.61±0. 05 0.99±.0£ N.B. In a l l the experiments 2mm of g l a s s f i l t e r A was used except w i t h * . c o e f f i c i e n t i s c a l c u l a t e d from the l i n e a r e x t r a p o l a t i o n of second o r d e r p l o t a t a l l f l a s h e n e r g i e s , the v a l u e o b t a i n e d decreased w i t h i n c r e a s i n g f l a s h energy. From our measurements, ° ~ -1 -1 v a l u e s of e a t 2577 A as low as 400 1 mole cm would have been o b t a i n e d u s i n g t h i s procedure. It^therefore., seems v e r y probable t h a t the d i f f e r e n c e between our r e s u l t s and those 21 r e p o r t e d by Lipscomb e t a l . may be e x p l a i n e d by the f a c t t h a t they d i d not observe the i n i t i a l r a p i d decay of CIO. I t w i l l be shown l a t e r t h a t the reason f o r t h i s r a p i d decay i s the r e a c t i o n 0 + CIO -»• C l + 0 2 (6) In t h i s treatment, no account of the presence of ClO^ has been taken, the f o r m a t i o n of which i s the r e a c t i o n 0 + C10 2 -r C10 3 (9) 21 proposed by Lipscomb e t a l . and by the steady s t a t e workers. They found a weak continuous a b s o r p t i o n (mainly below o 3000 A), a p p a r e n t l y due to ClO^ which i s formed d u r i n g the f l a s h and which dis a p p e a r e d s l o w l y over a p e r i o d of one minute. o By measuring the c o n c e n t r a t i o n of ClO^ a t 2948 A (using efClO^) 39 found by Goodeve e t a l . ), a c o r r e c t i o n was a p p l i e d to the o a b s o r p t i o n a t 2577 A, the wavelength used as a measure of the CIO c o n c e n t r a t i o n . The c o r r e c t i o n of ClO^ produced was g r e a t -e s t a t low f l a s h e n e r g i e s between 240 and 400 J (at these f l a s h e n e r g i e s the decomposition of C10 2 was between 73% and 91%). A t e n e r g i e s of 1280 J and 1620 J where the C10 2 decom-p o s i t i o n was 98-99%, the p r o p o r t i o n of ClO^ produced was o n l y 3% and 2%. I f the e x i s t e n c e of ClO^ i s admitted, then D = D(CIO) + D(C10 3) and A[C10 2] = [ C 1 0 ] Q + [CIO3] where D and A[C10 2] have the same meaning as b e f o r e . The v a l u e of e(CIO) i n the p r e s e n t work denies the e x i s t e n c e of ClO^ so t h a t the e x t i n c t i o n c o e f f i c i e n t of CIO was measured simply from 5 3 D D  E = l [ C 1 0 ] o = 1A[C1 20] ° -1 -1 S i n c e our v a l u e of e(CIO) a t 2577 A (1150 1 mole cm ) i s 39 -1 -1 v e r y c l o s e t o e (ClO^) a t t h i s wavelength ('vllOO 1 mole cm ) , o the c o r r e c t i o n i s n e g l i g i b l e ( l e s s than 1%) and even a t 2772 A the maximum e r r o r would have been o n l y ^3%. I f the v a l u e of - 1 - 1 21 . 690 1 mole cm g i v e n by Lipscomb e t a l . i s used, the maxi-mum e r r o r would be o n l y 15% but the evidence p r e s e n t e d here 21 and the r e s u l t s of Clyne and Coxon make the h i g h e r v a l u e more pr o b a b l e . Although the presence of C10-j i s i r r e l e v a n t t o the d i s -c u s s i o n on £(CIO), i t i s convenient t o c o n s i d e r the . evidence f o r i t s f o r m a t i o n here. To d e t e c t the u n d e r l y i n g continuous a b s o r p t i o n i n the presence of d i s c r e t e bands, i t i s necessary e i t h e r t h a t the l a t t e r i s completely r e s o l v e d or t h a t s u c c e s s i v e bands of the d i s c r e t e spectrum s h o u l d be f r e e from o v e r l a p . Although, v i s u a l l y , t h i s l a t t e r c o n d i t i o n appears t o be f u l -f i l l e d f o r the ClO spectrum recorded on medium q u a r t z s p e c t r o -21 graph (e.g. f i g . ( 2 ) of L.N.T. ), a microdensitometer t r a c i n g shows t h a t t h i s i m p r e s s i o n i s f a l s e and o v e r l a p i s s t i l l appar-ent on our s p e c t r a o b t a i n e d a t two t o t h r e e times g r e a t e r d i s -p e r s i o n where the r o t a t i o n a l s t r u c t u r e appears, v i r t u a l l y , t o be l a r g e l y r e s o l v e d . In s p e c t r a taken i n the f o u r t h o r d e r of a 21 f t . g r a t i n g s p e c t r o g r a p h 1 ^ the r o t a t i o n a l s t r u c t u r e i s d e s c r i b e d as sharp but complex owing t o the o v e r l a p p i n g of s u c c e s s i v e v i b r a t i o n a l bands. A t r a c i n g of a spectrum taken i n s e c o n d o r d e r o f 3.4 m e t e r g r a t i n g s p e c t r o g r a p h u n d e r c o n -d i t i o n s r e p o r t e d f a v o u r a b l e t o t h e f o r m a t i o n o f C10 3, no i n d i c a t i o n o f an u n d e r l y i n g c o n t i n u u m s p e c t r a i s a p p a r e n t . S p e c t r a t a k e n a t h i g h f l a s h e n e r g y a r e s i m i l a r . A t l o n g d e l a y s ( f r o m 5 s e c o n d s t o 5 m i n u t e s ) where t h e CIO c o n c e n -t r a t i o n i s n e g l i g i b l e , t h e s p e c t r a show a g a i n no i n d i c a t i o n 21 o f c o n t i n u o u s a b s o r p t i o n , a l t h o u g h a c c o r d i n g t o L.N.T. t h e C l O ^ s p e c t r u m s h o u l d s t i l l be p r e s e n t a t t h e s e t i m e s ( f i g . 7 ) . F u r t h e r r e a s o n s f o r d e n y i n g t h e p r e s e n c e o f d e t e c t a b l e c o n c e n t r a t i o n o f C l O ^ i n t h e p r e s e n t work a r e t h a t t h e CIO s p e c t r u m i s u n c h a n g e d when i t i s p r o d u c e d f r o m o t h e r s o u r c e s ( C l 2 / 0 2 system) where t h e p r o d u c t i o n o f C l O ^ i s l e s s l i k e l y and t h e r a t e c o n s t a n t s f o r t h e d e c a y o f CIO p r o d u c e d i n t h e v a r i o u s ways (CI2O, C1 20/C1 2 and CI2/O2 s y s t e m s ) a r e i n g o od a g r e e m e n t . We c o n c l u d e t h a t t h e r e i s no e v i d e n c e o f C l O ^ o r t h a t i f i t i s p r o d u c e d , i t i s v e r y l i k e l y r a p i d l y removed by t h e f o l l o w i n g r e a c t i o n s : 0 + C10 3 ->• C10 2 + 0 2 (28) CIO + C10 3 -> 2C10 2 (29) C l + C10 3 + CIO + C10 2 (30) * The p o s s i b i l i t y o f p r o d u c t i o n o f O2 i n t h e r e a c t i o n (28) o r 21 as p r o p o s e d b y L.N.T. i n t h e p h o t o l y s i s o f C10 3 C10 3 + hv •+ CIO + 0* (31) w i l l be c o n s i d e r e d i n a l a t e r s e c t i o n . Fifur.e 7 . Densitometer t r a c i n g of CIO bands taken i n second order of 3 - 4 meter g r a t i n g spectrograph under c o n d i t i o n s reported favourable of ClOgformation. CIO2 - 0.2 t o r r , A r g o n r 200 t o r r , E r 2 6 0 J . 2 0 0 yx.sec Blank 1 3 , 0 1 2 . 0 . 1 , 0 1 0 , 0 3 0 s e c , i ' w i Blank ^ min Blank 56 o Although our values of r a t i o of £(ClO) at 2772 A to o e(CIO) at 2577 A agree w e l l w i t h t h a t found by Clyne and 25 Coxon, our values at these wavelengths are ^10% l e s s than 25 those of Clyne and Coxon. Although t h i s d i f f e r e n c e i s w i t h i n experimental e r r o r , the p o s s i b i l i t y t h a t i t arose from i n s t r u m e n t a l , thermal or n o n e q u i l i b r i u m e f f e c t s were considered. The p o s s i b i l i t y of i n s t r u m e n t a l e f f e c t s was t e s t e d by measuring the e x t i n c t i o n c o e f f i c i e n t u sing a medium quartz spectrograph as w e l l as the 3.4 meter g r a t i n g spectrograph w i t h v a r i o u s s l i t widths but no d i f f e r e n c e was noted. Since o 2920 A r e f e r s to 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 of CIO 2 ( ^2/2' v " = ^ a t t h e t e m P e r a t u r e o f t n e system, only a s m a l l temperature change i s needed to account f o r the discrepancy. However, s i n c e the v a l u e s obtained were the same w i t h a 800 f o l d d i l u t i o n of C10 2 w i t h i n e r t gas as w i t h a 3300 f o l d d i l u -t i o n , i t would appear t h a t i n both cases the system was s t r i c t l y i s o t h e r m a l or t h a t the e f f e c t i v e temperature c o e f f i c i e n t of e i s s m a l l . I t remains p o s s i b l e t h a t the p o p u l a t i o n of CIO 2 ( 7 r3/2 / v " = ^) produced e i t h e r by p h o t o l y s i s or by chemical r e a c t i o n was not s t r i c t l y e q u i l i b r a t e d , but as there i s no i n -dependent evidence f o r t h i s , we conclude t h a t the s m a l l d i s -25 crepancy between our r e s u l t s and those of Clyne and Coxon i s caused by experimental e r r o r . 57 B. - Bi m o l e c u l a r Decay of the CIO'Radical CIO r a d i c a l , as already mentioned i n the i n t r o d u c t i o n , can be generated by d i f f e r e n t methods and thus i t s b i m o l e c u l a r decay can be s t u d i e d by d i f f e r e n t methods, so long as other r e a c t i o n s of CIO do not i n t e r f e r e during i t s decay. We have generated CIO r a d i c a l .by f i v e d i f f e r e n t methods and the r e s u l t s obtained are dis c u s s e d below. 1) . From CK>2 P h o t o l y s i s As mentioned i n the l a s t s e c t i o n on the determination of e x t i n c t i o n c o e f f i c i e n t of CIO r a d i c a l , C10 2 was f l a s h e d using four f l a s h energies (mentioned i n Table I V ) . C10 2 pressure used was v a r y i n g from 0.06 t o 0.25 t o r r and two argon pressures were used, 75 t o r r and 200 t o r r . Thus the r a t i o of CIO2 t o i n e r t gas (argon) was between 300 t o 3300. In a l l the experiments used t o determine the r a t e constant of b i m o l e c u l a r decay, 2mm g l a s s f i l t e r A was placed between the r e a c t i o n v e s s e l and f l a s h lamp i n order t o avoid the d i r e c t p h o t o l y s i s of CIO. I t was found t h a t at a l l f l a s h e n e r g i e s , the decay of CIO produced from the above sets of experiments was s t r i c t l y second order f o r the delay times above 100 t o 250 ysec ( f i g . 8 ) . The r e a c t i o n i n v o l v e d i s taken t o be 2C10 -> C l 2 + 0 2 (5) The r a t e constant, I C 5 , d e f i n e d by the equation 58 Table IV P l a t e [cio 2] Argon Energy k 5 x 10 7 ( 1 mole 1 -1 sec no. ( t o r r ) ( t o r r ) 2577 0 A 2772 A. 2920 . 104 0.25 200 1060 2.97 2.96 2.60 105 0.06 200 1060 2.90 2.20 106 0.25 75 1060 2.97 2.65 3.25 107 0.08 75 1060 — 2.3 2.98 114 0.12 200 1060 2.99 2.96 2.45 115 0.25 200 1060 — 2.69 2.28 152 0.25 200 1060 2.62 2.10 — 104 0.25 200 600 2.77 2.49 2.43 105 0.06 200 600 2.69 2.49 106 0.25 75 600 3.36 2.41 2.43 107 0.08 75 600 2.62 — — 119 0.25 75 600 2.03 2.20 — 120 0.1 75 600 2.93 3.04 154 0.1 200 600 — 2.20 2.53 118 0.25 200 260 2.60 2.52 2.4 119 0.25 75 260 2.13 2.52 — 120 0.1 75 260 3.07 2.70 2.98 121 0.25 200 260 2.8 2.3 2.95 156 0.1 200 260 2.75 2.52 3.10 164 0.25 200 260 2.25 2.72 2.80 110 0.25 200 160 — 2.65 2.67 156 0.1 200 160 2.52 2.6 2.73 121 0.25 200 160 2.62 2.77 2.64 Average k5 =2 .65 ± o.: 29 7 x 10 I mole -1 sec 1 F i g u r e 8 . A p l o t of 1/ C C 1 01 against time f o l l o w i n g the f l a s h p h o t o l y s i s of C10 2. C10 2= 0 .25 t o r r , Argon-200 t o r r . W TO • P X C -•*—« " *— E 1060 J * * * E 260 J >> -p t-i w cu •o 8 rH CO O • H - P ft O t(m sec) VD c a n be i n t e g r a t e d t o g i v e t h e f o r m : 1 = 1 + k t [CIO] [ c i o ] 0 5 T h i s c a n be t r a n s f o r m e d i n t o 1 1 + k 5 t D D 0 £ i where D = o p t i c a l d e n s i t y a t any t i m e t = e [ C 1 0 ] t l D 0= e [ C l O ] Q = o p t i c a l d e n s i t y a t t i m e t = 0. 1/D was p l o t t e d a g a i n s t t i m e t and t h e d a t a were f o u n d t o f o l l o w s t r a i g h t l i n e s a f t e r 200 u s e e a t a l l f l a s h e n e r g i e s . From t h e s l o p e s o f t h e p l o t and t h e e x t i n c t i o n c o e f f i c i e n t d e t e r m i n e d as d e s c r i b e d i n t h e l a s t s e c t i o n , k,- was c a l c u l a t e d . The CIO c o n c e n t r a t i o n was m e a s u r e d a t t h r e e w a v e l e n g t h s and t h e d e c a y f o l l o w e d a t a l l f l a s h e n e r g i e s , CIO2 p r e s s u r e % " and t o t a l p r e s s u r e s m e n t i o n e d a b o v e . The r e s u l t s a r e l i s t e d i n T a b l e IV. From T a b l e IV, i t i s c l e a r t h a t k^ i s i n d e p e n d e n t o f f l a s h e n e r g y , t o t a l p r e s s u r e and CIO 2 p r e s s u r e . The a g r e e m e n t b e t w e e n t h e r e s u l t s o b t a i n e d a t t h r e e w a v e l e n g t h s i s g o o d . The a v e r a g e v a l u e o f a l l f i f t y s e v e n d e t e r m i n a t i o n s o f k,- i s (2.65 t 0.29) x 10 1 m o l e " s e c . The a v e r a g e o f f i v e d e t e r -m i n a t i o n s ' u s i n g medium q u a r t z s p e c t r o g r a p h was i d e n t i c a l . The v a l u e o f kg o b t a i n e d i s i n s a t i s f a c t o r y a g r e e m e n t w i t h t h a t o f (2.4 + 0.4) x i o 7 1 m o l e - 1 s e c - 1 , o b t a i n e d b y Edgecombe e t a l . 2 2 f r o m t h e f l a s h p h o t o l y s i s o f C1 20 and i t i s s i m i l a r t o t h e v a l u e s 21 o b t a i n e d by L.N.T. a t the two lowest f l a s h e n e r g i e s used. The r e s u l t s g i v e n here, however, d i f f e r s i g n i f i c a n t l y from those 15 25 r e p o r t e d by P o r t e r and Wright and Clyne and Coxon. From 15 the f l a s h p h o t o l y s i s of C l 2 / 0 2 m i x t u r e s , P o r t e r and Wright 4 - 1 ° found = 7.2 x 10 cm sec a t 2577 A, which, combined w i t h ° 7 - 1 - 1 our v a l u e of e a t 2577 A, g i v e s k j = 8.3 x 10 1 mole sec 25 Clyne and Coxon from t h e i r flow system i n which CIO r a d i c a l s were generated by the r e a c t i o n of c h l o r i n e atoms w i t h CIO2, + 7 -1 -1 got average v a l u e s of kt- as (1.4 _ 0.1) x 10 1 mole sec 2) P r o d u c t i o n of CIO R a d i c a l s by F l a s h i n g a M i x t u r e of - C h l o r i n e and Oxygen C o n s i d e r i n g the g r e a t d i f f e r e n c e between the v a l u e of kj-determined by us from C10 2 p h o t o l y s i s and t h a t of kg found by 15 . P o r t e r and Wright from the. c h l o r i n e and oxygen system, we t h e r e f o r e r e i n v e s t i g a t e d t h i s system. C l 2 w i t h p r e s s u r e 5 t o r r and 7.5 t o r r , and oxygen 50 or 66 t o r r was f l a s h e d w i t h a t o t a l p r e s s u r e of 200 t o r r u s i n g i n e r t gas ( n i t r o g e n or argon). The above mixtures were f l a s h e d w i t h o n l y one f l a s h energy (1325J^ u s i n g both pyrex and q u a r t z r e a c t i o n v e s s e l s . In two of the runs, the same mixture was f l a s h e d a g a i n and a g a i n to see i f t h e r e was any v a r i a t i o n i n the r e s u l t s o b t a i n e d by r e f l a s h i n g the mixture or t a k i n g f r e s h one each time. The spectrum thus 14 o b t a i n e d i s s i m i l a r t o t h a t o b t a i n e d by P o r t e r , i s shown i n f i g . 9 ( p l a t e 208). The ClO c o n c e n t r a t i o n was measured a t two 0 0 wavelengths, i . e . , 2772 A and 2577 A t o f o l l o w i t s decay. 62 2 300 2^00 A | B l a n k B e f o r e 10 ytcsec 20 40 71 120 201 431 630 1.02 msec 1.97 2.73. 3.67 4.32 5.2 6.1 A f t e r F i g u r e 9« R i s e . a n d Decay o f CIO. Cl2-5-0 t o r r , 02=50 t o r r , Argon=l45 t o r r , E-1325J 63 o The measurements a t 2920 A were n e g l e c t e d due t o the i n t e r f e r -ence of continuum of c h l o r i n e a b s o r p t i o n . The r e s u l t s were t r e a t e d i n the same manner as i n C10 2 p h o t o l y s i s and the r e -s u l t s are l i s t e d i n T a b l e V. The average of f o u r t e e n d e t e r -m i n a t i o n s of k^ was 2.77 1 0.26 1 0 7 1 mole ^sec ^ and thus i n e x c e l l e n t agreement w i t h the v a l u e o b t a i n e d i n the p r e v i o u s system. The same r e s u l t s were o b t a i n e d w i t h n i t r o g e n and. argon a s , i n e r t gas and w i t h q u a r t z and pyrex r e a c t i o n v e s s e l s . I t can a l s o be seen from Table V, r e s u l t s were the same i n the experiments where the same mixture of C l 2 / 0 2 / A r was used f o r a l l d e l a y s and those where a f r e s h mixture was used f o r each d e l a y time. The v a l u e of k,- o b t a i n e d i s much lower than t h a t o b t a i n e d by P o r t e r and Wright"^ but no e x p l a n a t i o n i s o f f e r e d f o r the d i f f e r e n c e between the two r e s u l t s . 3) P r o d u c t i o n of CIO R a d i c a l s from C1?0 and C h l o r i n e  S e n s i t i s e d P h o t o l y s i s of C 1 ? 0 ; In view of the p o s s i b l e complexity of the CIO2 system ( i n p a r t i c u l a r the proposed f o r m a t i o n of ClO^ and CI2O3) and a l s o as t h e r e was no obvious reason f o r the h i g h e r v a l u e s o b t a i n e d by P o r t e r and Wright,"^ we s t u d i e d the decay of CIO r a d i c a l s produced i n the p h o t o l y s i s of CI2O and i n the c h l o r i n e s e n s i t i s e d decomposition of C l 2 0 . The CI2O a t two p r e s s u r e s but w i t h the same t o t a l p r e s s u r e of argon was f l a s h e d a t t h r e e f l a s h e n e r g i e s . In two of the 64 Table V P l a t e no. [ c i 2 ] ( t o r r ) [o 2] ( t o r r ) R.V. k 5 x 1 0 ~7 ( 1 mole 2577 A -1 -1, sec ) 2772 A 201 5.0 56 P 2.65 3.05 202 5.0 56 Q 2.95 2.94 203 5.0 50 Q 2.70 — 204 7.5 50 Q 2.99 (1.90J 205* 7.5 50 Q 3.10 2.70 206* 7.5 50 P 3.0 2.60 207 7.5 50 P — 2.53 208 7.5 50 P 2.62 2.15 Average 2.86±0 .18 2.66*0.26 Mean = (2.77+0.26) x 1 0 7 1 mole sec * N i t r o g e n was used as i n e r t gas P = pyrex Q = q u a r t z experiments two mm g l a s s f i l t e r A was p l a c e d between the photo-l y s i s lamp and the r e a c t i o n v e s s e l t o see i f t h e r e i s any d i f f e r e n c e i n the r e s u l t s . The measurements of CIO were o — o c a r r i e d out a t the 12,0 (2772 A) and 7,0 (2920 A) bands i n the same manner as has been d e s c r i b e d i n the c a l c u l a t i o n of e x t i n c -t i o n c o e f f i c i e n t of CIO. I t was found t h a t the c o n c e n t r a t i o n of CIO produced or CI2O decomposed was g r e a t e r when no f i l t e r was used. T h i s was q u i t e expected, f i r s t l y due t o the i n c r e a s e i n the e x t i n c t i o n c o e f f i c i e n t of C l 2 0 towards the lower wavelengths and secondly due t o decomposition of C l 2 0 i n t o atoms (which w i l l be d i s -cussed i n Chapter V ) , f o l l o w e d by t h e i r r e a c t i o n s . Except f o r t h i s no d i f f e r e n c e i n the decay of ClO was noted. In a s i m i l a r way as w i t h the C10 2 p h o t o l y s i s , 1/D was p l o t t e d a g a i n s t time and a reasonably good s t r a i g h t l i n e was observed as shown i n f i g . ( 1 0 ) , i n d i c a t i n g t h a t o t h e r p r o c e s s e s d u r i n g the ClO decay are slow. These w i l l be d i s c u s s e d i n more d e t a i l i n Chapter V. Combining the s l o p e S o f the s t r a i g h t l i n e s w i t h the e x t i n c t i o n c o e f f i c i e n t was c a l c u l a t e d and v a l u e s are l i s t e d i n Table VI. -The average of seven runs was found to be 2.77 t 0.22 x 10 7 1 m o l e ' ^ e c " 1 . The CIO r a d i c a l was a l s o generated by f l a s h i n g C l 2 i n the presence of C1 20 i n the pyrex r e a c t i o n v e s s e l and two mm of g l a s s f i l t e r A was p l a c e d between the f l a s h lamp and r e a c t i o n v e s s e l . Two f l a s h e n e r g i e s v i z . 600 and 1060 J , two C1 20 -5 -5 c o n c e n t r a t i o n s (3.65 x io and 5 .8 x 10 mole/1) and one c h l o r i n e p r e s s u r e was used. D e t a i l s of the r e a c t i o n of the c h l o r i n e atom w i t h C l 2 0 and o t h e r secondary p r o c e s s e s w i l l be d i s c u s s e d i n Chapter V. A s i m i l a r procedure was used t o c a l -c u l a t e , the r a t e c o n s t a n t f o r ClO decay and the r e s u l t s are l i s t e d i n T a b l e VI. The average of f o u r r e s u l t s was found to be (2.88 t 0.3) x i o 7 1 mole" 1 s e c " 1 . I t can be seen from the r e s u l t s of Table VI t h a t t h e r e i s a s a t i s f a c t o r y agreement between our v a l u e and t h a t of 22 Edgecombe e t a l . The agreement may, however, be c o i n c i d e n t a l F i g u r e 10. A p l o t of 1/ CCldJ against time f o l l o w i n g the f l a s h p h o t o l y s i s o f C1 2 0 . ClgO= 0 . 6 T ; t o r r , Argon- .200 t o r r , E = 1325 J. 67 i n t h a t t h e v a l u e o f z (CIO) m e a s u r e d by them a t 2920 A ( t h e w a v e l e n g t h u s e d t o f o l l o w t h e CIO d e c a y ) i s n o t e x p l i c i t l y 25 s t a t e d . C l y n e and Coxon h a v e c a l c u l a t e d f r o m t h e r e l a t i v e e x t i n c t i o n c o e f f i c i e n t s o f CIO a t 2577 A and 2824 A (10,0) g i v e n by P o r t e r and W r i g h t 1 " * and f r o m t h e r e l a t i v e i n t e n s i t i e s 14 o f 10,0 and 7,0 bands g i v e n by P o r t e r , t h a t Edgecombe, 22 N o r r i s h and T h r u s h , w o u l d h a v e o b t a i n e d a v a l u e o f e a t 2577 A o f 760 1 m o l e - 1 c m - 1 w i t h k 5/ £ = 3.1 x 1 0 4 cm s e c - 1 a t 22 t h i s w a v e l e n g t h . From t h e d a t a g i v e n b y E.N.T., we c a l c u -l a t e d k 5/ £ ^ 4.8 x 1 0 4 cm s e c " 1 and e = 500 1 m o l e - 1 c m - 1 a t o 2920 A. I n b o t h c a s e s a s i g n i f i c a n t d i s c r e p a n c y i s a p p a r e n t . T a b l e V I P l a t e no. [ci 2o] (mole / 1 t c i 2 ] x 10-6) F i l t e r E n e r g y cr k 5 x i o - 7 (1 2772 A -1 -1\ m o l e s e c ) 2920 A 170 35.6 137.5 A 600 2.82 — 171 63.3 137.5 A 600 2.92 2.59 173 36.5 137.5 A 1060 2.88 t 3.18 0.3 184 57.8 — A 830 2.8 2.45 191 52.3 — A 1060 3.1 192 55.0 — - . 1325 2.62 2.8 193 33.0 - 1325 3.1 2.55 A v e r a g e 2.77 t 0.22 68 4) Flash' P h o t o l y s i s of a M i x t u r e of ClO^ tend Cl^O Though main reasons f o r f l a s h i n g mixtures of C^O and C l O j a t f l a s h e n e r g i e s 1060 and 1325 J were to compare the r a t e c o n s t a n t s f o r the r e a c t i o n of oxygen atoms wi t h CIO r a d i -c a l s and C I 2 O and C I O 2 as w i l l be d i s c u s s e d i n the next c h a p t e r , a study of the b i m o l e c u l a r decay of CIO was a l s o c a r r i e d out a f t e r the p r e l i m i n a r y r e a c t i o n s of atoms are over (approx. 200 to'300 u s e e ) . The d i f f e r e n t mixtures (as mentioned i n T a b l e o VII) of C l 2 0 and C 1 0 2 were f l a s h e d u s i n g 3400 A f i l t e r . T h i s f i l t e r was used to prevent the decomposition of C^O. I t was found t h a t r a p i d decay of ClO i n the e a r l y p a r t w i t h h i g h f l a s h energy was reduced and r e p l a c e d by a steady i n c r e a s e i n i t s c o n c e n t r a t i o n depending upon the r a t i o of C I O 2 t o C ^ O as can be seen from f i g . 19 (page 100). A f t e r the e a r l y r e a c t i o n s , o the ClO c o n c e n t r a t i o n was monitored a t 2772 A and the 1/D was p l o t t e d a g a i n s t time which was found to be a s t r a i g h t l i n e . The v a l u e s of c a l c u l a t e d from the s l o p e s of the s t r a i g h t l i n e s , combined w i t h e x t i n c t i o n c o e f f i c i e n t s are l i s t e d i n T a b l e V I I . The average of two (without C^O) and s i x runs + 7 -1 -1 (with C I 2 O ) was found t o be (2.4 _ 0.2)x 10 1 mole sec which i s i n good agreement w i t h the v a l u e of k^ o b t a i n e d by p r e v i o u s methods. Mechanism of the Decay of ClO In view of the c o n s i s t e n c y of the r e s u l t s of k5 o b t a i n e d by f i v e d i f f e r e n t f l a s h p h o t o l y s i s methods, i t appears p r o b a b l e 69 T a b l e V I I P l a t e no. [cio 2 ] ( I O - 6 mole/1) [ci 2o] ( 1 0 - 6 m o l e / l ) E n e r g y J. ka x 1 0 " 7 -1 -1 (1 m o l e s e c ) 186 4.4 — 1060 2.6 186 4.4 6.9 1060 2.4 187 4.4 2.75 1060 2.5 187 4.4 5.5 1060 2.4 188 4.4 — 1325 2.4 188 4.4 9.6 1325 2.1 189 4.4 4.1 1325 2.4 189 4.4 6.9 1325 2.2 A v e r a g e = (2.4 t 0.2) x 1 0 7 1 m o l e - 1 -1 s e c 25 + t h a t t h e d i f f e r e n c e b e t w e e n C l y n e and Coxon's v a l u e (1.4-0.01) 7 - 1 - 1 x 10 1 mole s e c and o u r s a r i s e s f r o m b e c a u s e t h e mechanism o f t h e r e a c t i o n c h a n g e s a t low p r e s s u r e (<^lmm) u s e d i n t h e d i s -25 c h a r g e f l o w e x p e r i m e n t s o f C l y n e and Coxon. Thus t h e y a d o p t e d — • 26 t h e f r e e r a d i c a l m echanism p r o p o s e d by B e n s o n and B u s s and t h e i r k$ i s i d e n t i f i e d w i t h k 2 Q , k 2C10 - V 2 0 C l - O - 0 + C l k 1 -20 70 T h i s i n t e r p r e t a t i o n was supported by the e f f e c t s produced when c h l o r i n e atom scavengers (C10 2, O3, B r 2 ^ were added to the ClO system. A r a p i d removal of both C10 2 and O^ was observed i n c h a i n r e a c t i o n s whose r a t e s were c l o s e l y r e l a t e d t o t h a t of the CIO recombination r a t e . B r 2 i s e f f e c t i v e i n s u p p r e s s i n g the c h a i n r e a c t i o n of H 2 w i t h ClO. The mechanism of P o r t e r and W r i g h t 1 5 i n v o l v e s the i n t e r -mediate C 1 2 0 2 and perhaps the s i m p l e s t s o l u t i o n i s t o combine the two mechanisms u s i n g C l 2 0 2 as the common i n t e r m e d i a t e . 2C10 + c l 2 ° 2 c i 2 o 2 + Cl-O-0 + C l C l 2 ° 2 + M • "* C 1 2 + °2 + M C l + Cl-O-0 -+ c i 2 + o 2 T h i s leads to the same l i m i t i n g h i g h and low p r e s s u r e behaviour 25 as the s i m i l a r mechanism proposed by Clyne and Coxon, i . e . , t h e i r mechanism: k 90 2C10 • + M c l9 0-? + M -90 k c i 2 o 2 J - 0 0 c i 2 + ~ o 2 . • i d e n t i f i e s k^ w i t h kgg x k^OO ^ ^-90 ^ w h e r e ^-QQ a n ^ ^100 c o r r e s " 25 pond to kg and k^^ of Clyne and Coxon's r e s p e c t i v e l y ) . 26 Evidence t h a t the Benson and Buss mechanism i s not important a t h i g h p r e s s u r e i s p r o v i d e d by the f o l l o w i n g obser-v a t i o n s . T h e d e c a y o f C I O p r o d u c e d f r o m C l O ^ a t h i g h f l a s h e n e r g i e s w h e r e a n a p p r e c i a b l e c o n c e n t r a t i o n o f c h l o r i n e a t o m s i s p r o d u c e d i n r e a c t i o n (6) i s t h e same a s a t l o w f l a s h e n e r -g i e s . A t l o w f l a s h e n e r g i e s , w h e r e a n a p p r e c i a b l e c o n c e n -t r a t i o n o f CIO2 i s p r e s e n t d u r i n g t h e d e c a y o f C I O , n o d e -c r e a s e o f CIO2 c o n c e n t r a t i o n was o b s e r v e d a s c a n b e s e e n f r o m f i g . ( 1 0 - a ) , u n l i k e r e s u l t s a t l o w p r e s s u r e o f C l y n e a n d 25 C o x o n , a l t h o u g h t h e r e a c t i o n o f c h l o r i n e a t o m s w i t h CIO2 i s 8 - 1 - T 2 5 9 v e r y f a s t ( k Q > 5 x 10 1 m o l e s e c o r k g = 5 x 10 - 1 - 1 31 1 m o l e s e c ) . T h i s o b s e r v a t i o n was c o n f i r m e d d u r i n g t h e p h o t o s e n s i t i s e d d e c o m p o s i t i o n o f CIO2 b y c h l o r i n e a t o m s , i n w h i c h t h e CIO2 c o n c e n t r a t i o n d e c r e a s e s i n t h e b e g i n n i n g (due t o t h e r e a c t i o n o f c h l o r i n e a n d o x y g e n a t o m s w i t h CIO2) a n d t h e C I O c o n c e n t r a t i o n i n c r e a s e s . A f t e r t h i s CIO2 r e m a i n s c o n s t a n t a n d t h e r a t e o f C I O d e c a y i s l i k e w i s e n o t d e c r e a s e d i n t h e p r e s e n c e o f CIO2. F i n a l l y , w h e n C I O i s p r o d u c e d f r o m C ^ O o r f r o m CI2O/CI2/ t h e r e i s a l a r g e e x c e s s o f C l a t o m s . S u b s e q u e n t l y , t h e CI2O b e h a v e s a s a n e f f i c i e n t c h l o r i n e a t o m s c a v e n g e r (k 1 9> 4 x 10 8 1 m o l e _ 1 s e c _ 1 b y E . N . T . 2 2 a n d k i g = 4 x 1 0 8 1 m o l e ^ s e c 1 i n t h e p r e s e n t w o r k ) y e t t h e same v a l u e f o r t h e r a t e c o n s t a n t k^ was o b s e r v e d i n a l l t h e s y s t e m s s t u d i e d . 72 —1 I t t I E 3 Blank Before 5 /isec 10.6 40 80 195 405 620 940 1 . 6 3.2 5.9 7.1 lO.o 18.5 30.5 41 . 5 50.0 6o.o 91.0 msec CIO, 3434 3515 ft F i g u r e 10-a. Behaviour of C102 during the decay of CIO, C 1 0 2 - 0.1 t o r r , Argons 200 t o r r , E=l6o J 72-a 99 A f t e r the thesis was written, Johnston, Morris and Vam den Bogaerde reported t h e i r r e s u l t s of a d e t a i l e d k i n e t i c study of photolysis of chlorine i n the presence of oxygen with square wave excited UV lamps at frequencies of 0.25 - 32 Hz. Using a new molecular modulation technique, the UV and IR spectra a t t r i b u t e d to the C£00 r a d i c a l were found i n the regions 2300 A - 2600 A and 1430 - 1460 cm - 1 and the UV spectrum of CZO was also observed. A most s i g n i f i c a n t feature of t h e i r r e s u l t s was an almost l i n e a r dependence of the rate constant f o r the decay of C£0 on t o t a l pressure in the range studied (50 - 760 mm). This complete mechanism, preserving t h e i r nomenclature, i s C£ 2 + hv £ 2 C t ro o, -•- M k ro,oo j- M c CZ + CSLOO J 2 C£0 e C£ + C£00 t Ql2 + 0 2 2 C£0 + M | C £ 2 0 2 + M h CJt 20 2 + M * C£ 2 + 0 2 . + M 2 C i + M ^ C£„ -»• 2. and they f i n d e = 3.8 x 10^ l i t r e mole * sec ^ 10 2 "2 _ i i g = 1.2 x 10 l i t r e mole sec (Argon) ~h 72-b Fo r t h e t i m e dependence o f C£0, t h e y d e r i v e t h e e q u a t i o n d[C£0] = 2cd[C£00r - 2(e + ) [C£0] 2 2 ~dt~ "Flop h so t h a t A t p r e s s u r e s o f ^ 2 mm, t h e f i r s t t erm i s more i m p o r t a n t and t h e v a l u e o f K2 o b t a i n e d from t h i s e q u a t i o n i s i n good agreement w i t h t h a t f o u n d 25 e x p e r i m e n t a l l y by C l y n e and Coxon. Above 60 mm i g [ M ] ) 10 e and ~ h t h e r e a c t i o n i s e s s e n t i a l l y t h i r d o r d - v r 2 CZO + A r -> Cl2 + 0 2 + A r 8 - 1 -1 w i t h a c a l c u l a t e d r a t e c o n s t a n t K 2 a t 200 mm o f 2.6 x 10 l i t r e mole s e c Th e r e p o r t e d p r e s s u r e dependence o f i s i n c o m p l e t e d i s a g r e e m e n t w i t h our r e s u l t s and w i t h t h e work o f P o r t e r and W r i g h t ^ , who showed t h a t t h e r a t e was i n d e p e n d e n t o f t o t a l p r e s s u r e o v e r t h e r a n g e 55-610 mm. 8 M o r e o v e r , t h e c a l c u l a t e d v a l u e o f t h e r a t e c o n s t a n t K 2 o f 2.6 x 10 l i t r e mole 1 s e c 1 a t 200 mm t o t a l p r e s s u r e i s h i g h e r t h a n o u r v a l u e by a f a c t o r 99 o f ^ 10. J o h n s t o n , M o r r i s and Van den Bogaerde draw a t t e n t i o n t o t h e -1 25 a c t i v a t i o n energy o f 2.5 k c a l mole f o u n d by C l y n e and CoKon f o r t h e r e a c t i o n and t o t h e i r own o b s e r v a t i o n o f a p r e s s u r e dependence. They s t a t e t h e r e was a l a r g e a d i a b a t i c t e m p e r a t u r e r i s e i n P o r t e r ' s s y s t e m so t h a t a r e d u c t i o n i n t o t a l p r e s s u r e w o u l d t e n d b o t h t o i n c r e a s e t h e r a t e because o f t h e r e s u l t i n g i n c r e a s e i n t e m p e r a t u r e and t o l o w e r i t because o f t h e p r e s s u r e e f f e c t . "Thus t h e n e a r c a n c e l l a t i o n o f t h e s e two e f f e c t s may have ca u s e d P o r t e r t o m i s s b o t h o f them." - T h i s e x p l a n a t i o n o f 72-c the d i f f e r e n c e between t h e i r r e s u l t s and those of Porter i s however 25 unconvincing. F i r s t , the a c t i v a t i o n energy found by Clyne and Co)lon surely applies to rea c t i o n e which i s of n e g l i g i b l e importance at the pressures used except at impossibly high temperatures. For example at 300 mm t o t a l pressure, e = A j a l ^ J only at ^  3700° h. Accidental c a n c e l l a t i o n of the two e f f e c t s of pressure would not be possible over the range of pressures studied by Porter. Secondly, i f any s i g n i f i c a n t temperature r i s e did occur i n Porter's system and i f the reaction does have a p o s i t i v e temperature c o e f f i c i e n t , his value f o r K2 would have been 99 higher than those of Johnston et a l , whereas the reverse i s the case. Our r e s u l t s show that K 2 i s independent of pressure and that the small temperature r i s e s which may occur i n the systems studied have no measureable e f f e c t on the rate constant. In the system, f o r example, the two factors which l a r g e l y determine the temperature r i s e v i z . f l a s h energy and the r a t i o [CJIC^] [Ar] have both been var i e d at the same t o t a l pressure without changing the value of K2 obtained (table IV). 99 15 The c o n f l i c t s between the r e s u l t s of Johnston et a l , Porter and those presented i n t h i s thesisemain to encourage further studies of th i s r e a c t i o n . CHAPTER IV REACTIONS OF OXYGEN ATOMS WITH ClO, C l 2 0 AND C10 2 In t h i s c h apter the r e a c t i o n s of oxygen atoms w i t h CIO, CK> 2 and C1 20 w i l l be d i s c u s s e d . The r e a c t i o n of oxygen atoms w i t h CIO and C10 2 i n the C10 2 system and w i t h C l 2 0 and CIO i n the C l 2 0 system take p l a c e s i m u l t a n e o u s l y , depending upon the e x t e n t of decomposition of the parent compound i n the primary p r o c e s s . So i t i s q u i t e c o m p l i c a t e d to f i n d the r a t e c o n s t a n t of the i n d i v i d u a l r e a c t i o n s . But C10 2 has two advantages over the C1 20 system. .1) Using g l a s s f i l t e r A, t h e r e i s o n l y one primary p r o c e s s , i . e . C10 2 + hv -»• CIO + 0 (27) f o l l o w e d by C10 2 + 0 + CIO + 0 2 (7) 2) CK> 2 absorbs v e r y s t r o n g l y as compared t o C1 20 and hence the complete removal of C10 2 can be achieved w i t h com-p a r a t i v e l y low f l a s h energy. Thus the C10 2 was used to f i n d the r a t e c o n s t a n t s of the r e a c t i o n s of oxygen atoms w i t h CIO and the mixture of C l 2 0 arid CIO 2 was used t o compare the r a t e c o n s t a n t s of oxygen atoms wi t h CIO to C10 2 or t o C l 2 0 depending upon the f l a s h energy and the r a t i o of C10 2 to C1 20. In the f i r s t s e c t i o n , the r e a c t i o n of oxygen atoms w i t h ClO s t u d i e d i n two ways w i l l be d i s c u s s e d and c o n f i r m a t o r y evidence o b t a i n e d by one method w i l l be p r e s e n t e d . T h i s w i l l be f o l l o w e d by an account of the r e a c t i o n of oxygen atoms w i t h C l j O and C1C>2, r e s p e c t i v e l y . A. R e a c t i o n of Oxygen Atoms w i t h CIO 1) Using CIO? as the Parent Compound As mentioned b e f o r e , CIO2 absorbs v e r y s t r o n g l y and v i r -t u a l l y complete decomposition of p r e s s u r e s i n the range 0.05 t o 0.25 t o r r can be achieved by r e l a t i v e l y low f l a s h e n e r g i e s (400 t o 800 J) depending upon the type of the r e a c t i o n v e s s e l and f l a s h lamp e n c l o s u r e used. A t any h i g h f l a s h energy, i f more than h a l f of the CIO2 i s p h o t o l y s e d and t h e r e must be an excess of oxygen atoms over t h a t necessary to complete the removal of CIO2 i n the r e a c t i o n O + C10 2 + CIO + 0 2 (7) The l i k e l y f a t e of the excess oxygen atoms i s the r e a c t i o n 0 + CIO •* GI + 0 2 (6) and we propose t h a t t h i s r e a c t i o n i s f a s t and t h a t i t i s r e s -p o n s i b l e f o r the i n i t i a l f a s t decay of CIO observed a t h i g h f l a s h e n e r g i e s . T h i s can be seen from the second order p l o t o f CIO vs time ( f i g . 8, page 59) . A t low f l a s h e n e r g i e s the p l o t i s l i n e a r from the peak of CIO whereas a t h i g h f l a s h e n e r g i e s the p l o t d eparts from l i n e a r i t y though the peak can be reached w i t h i n 10 y s e c . The decay of CIO a t f l a s h e n e r g i e s 10600"and 160Jare compared i n f i g . (11) ( p l a t e s 106 and 110) . F i g u r e 11. CIO r Comparison of the decay o f CIO f o l l o w i n g the f l a s h p h o t o l y s i s o f C10 2 at high(1060 J) and low(l6o J) energies. 0.25 t o r r , Argon =200 t o r r . 2750 X 2850 A E =1060 J E = 160 J 76 The importance of r e a c t i o n (6) a t low e n e r g i e s depends 23 upon the r e l a t i v e v a l u e s of kg and k^. Clyne and Coxon found ky/kg t o be approximately 4, so the r e a c t i o n (6) must be taken i n t o account where the primary process exceeds^30%. The o v e r a l l p r o d u c t i o n of CIO i s , however, not a f f e c t e d u n l e s s the primary p h o t o l y s i s exceeds 50% and t o t a l decomposition of CIO2 i s a c h i e v e d , s i n c e the c h l o r i n e atoms produced i n r e a c t i o n (6) w i l l r e a c t r a p i d l y w i t h any CIO2 l e f t i n the r e -a c t i o n C l + C10 2 •* 2C10 (8) The r a t e c o n s t a n t f o r t h i s r e a c t i o n has been g i v e n by Clyne 25 8 —1 —1 and Coxon as >5 x 10 1 mole sec and we have found 9 - 1 - 1 kg = 5 x 10 1 mole sec Evidence t h a t r e a c t i o n (6) i s o c c u r r i n g t o a s i g n i f i -2 c a nt e x t e n t i n the experiments of Lipscomb, N o r r i s h and Thrush has a l r e a d y been pr e s e n t e d i n the d i s c u s s i o n on the e x t i n c t i o n c o e f f i c i e n t of ClO. Support f o r t h i s view i s p r o v i d e d by T a b l e I of t h e i r paper i n which i t i s seen t h a t the percentage decomposition of CIO2 achieved i s remarkably i n s e n s i t i v e t o the f l a s h energy. For example, i n c r e a s i n g the energy from 400 t o 1620 J , r a i s e s the decomposition from 91% t o 99% w h i l e i t o n l y f a l l s t o ^73% a t 240 J . The same ex p e r i m e n t a l procedure was used i n s t u d y i n g r e a c t i o n (6) as i n the d e t e r m i n a t i o n of b i m o l e c u l a r decay of ClO except t h a t o n l y the e a r l y p a r t o f the second o r d e r p l o t 77 was used f o r one energy, 1060 J . T h i s b e g i n n i n g p o r t i o n of the p l o t of 1/D ( r e p r e s e n t a t i o n of CIO c o n c e n t r a t i o n ) has been r e p l o t t e d i n f i g . (12) on a l a r g e r s c a l e . I t can be seen v e r y c l e a r l y t h a t t h i s p o r t i o n i s markedly n o n - l i n e a r . A t h i g h f l a s h e n e r g i e s then, the decay of CIO i s r e p r e -sented by the e q u a t i o n " d [ C 1 0 ] = k . t C I O ] 2 + k c [ C l O ] [ 0 ] (b) d t 5 6 w i t h the reasonable assumption t h a t the e f f e c t o f c h l o r i n e atoms on the decay of CIO may be n e g l e c t e d . The r a t e c o n s t a n t kg was measured as f o l l o w s from f i g . ( 1 2 ) . The CIO c o n c e n t r a t i o n , measured from the curve p a s s i n g through the e x p e r i m e n t a l p o i n t s a t v e r y s h o r t d e l a y s (^ 5 t o 8 y s e c ) , was e x t r a p o l a t e d to zero time. The v a l u e a t zero time [C10] q i s the t o t a l [CIO] produced and s i n c e 100% decom-p o s i t i o n of CIO2 was a c h i e v e d , a check on the accuracy of the e x t r a p o l a t i o n i s p r o v i d e d by the c o n d i t i o n [ C 1 0 ] Q = [C102] Q. The l i n e a r p a r t of the second order p l o t i s e x t r a p o l a t e d t o zero time, t h i s l i n e r e p r e s e n t i n g the b i m o l e c u l a r decay of CIO f o r an i n i t i a l c o n c e n t r a t i o n [CIO] . The c o n c e n t r a t i o n of CIO removed by r e a c t i o n (6) and hence the i n i t i a l oxygen atom c o n c e n t r a t i o n which i s l e f t a f t e r r e a c t i n g w i t h CIO2 i s g i v e n by 1 [ 0 ] Q = [ C 1 0 ] Q - [ c i o r o A t any time the oxygen atom c o n c e n t r a t i o n i s o b t a i n e d from the e q u a t i o n j [0] = [CIO] - [CIO] (b») Figure 12. A p l o t of 1/£C1Q3 against time at short time, delays. C10 2=0 . 2 5 t o r r , Argon•= 2 0 0 t o r r , E ~ 1 0 6 o J. 1 5 0 r 125 100 75 L o 2 0 40 time (yusec) 6 0 8 0 100 120 00 79 where [CIO] i s the measured CIO c o n c e n t r a t i o n and [CIO]' i s the v a l u e o b t a i n e d from the l i n e a r e x t r a p o l a t i o n . The r e s u l t s were t r e a t e d i n two ways: i ) E q u a t i o n (b) can be converted i n t o from f i g . ( 1 2 ) s l o p e s of the second o r d e r p l o t were measured and at the same time the r a t i o o f oxygen atoms to CIO r a d i c a l s were c a l c u l a t e d . Thus the s l o p e s c a l c u l a t e d i n t h i s way were p l o t t e d a g a i n s t the r a t i o of oxygen to CIO r a d i c a l s . As expected from the above e q u a t i o n t h i s p l o t was l i n e a r ( f i g . 13) and the v a l u e s of k, thus o b t a i n e d are l i s t e d i n Table V I I I . The 6 + 9 - 1 average of f i v e experiments i s (7.05 _ 0.42) x 10 1 mole sec 1 . From e q u a t i o n (c) i t i s a l s o c l e a r t h a t the i n t e r c e p t of t h i s p l o t should g i v e the r a t e c o n s t a n t f o r the b i m o l e c u l a r decay of CIO but i t was found t h a t the i n t e r c e p t i s too s m a l l t o be measured a c c u r a t e l y . S t i l l i t i s of the o r d e r o f the v a l u e of p r e v i o u s l y determined. i i ) As has been seen from procedure (i) the i n t e r c e p t i s too s m a l l t o be determined a c c u r a t e l y and a l s o i t can be seen from the l i n e a r e x t r a p o l a t i o n t h a t [CIO]' i s n e a r l y con-s t a n t over the p e r i o d 200usec, t o the f i r s t approximation the f i r s t term i n the e q u a t i o n (b) can be n e g l e c t e d and thus e q u a t i o n (b) reduces t o -1 d[ClO] [ C I O ] 2 d t = k 5 + k 6 [CIO] [O] (c) F i g u r e 13. A p l o t of slopes c a l c u l a t e d at d i f f e r e n t times from f i g . 12 a g a i n s t [ o ] / [ C I O ] at the same t i m e . 3 . 0 L i 1 1 1 . 1 I I I I • 1 0 0 . 8 1 . 6 2.4 3 . 2 ' 4.0 [ 0 ] / [Cl6] X l O 81 - d [ c l 0 ] = k c [ 0 ] [CIO] d t 6 ,v = kg [CIO] ([C I O ] - [CIO] J a f t e r s u b s t i t u t i n g f o r o x y g e n atoms a t any t i m e . CIO, t h e above e q u a t i o n becomes - ^ = k.x (x - a) o r I n x _ a = a k 6 t + c W r i t i n g x f o r where c i s c o n s t a n t o f i n t e g r a t i o n . When t = 0, x = x Q s o , c = I n x. x Q - a x i . e . I n = a k,t + I n x - a 6 x ^ - a o r l o g [CIO] a k. [ C I O ] - [ C I O ] 2.303 + l o g [CIO] [ C I O ] Q - [ C I O ] (d) l o g was p l o t t e d a g a i n s t t i m e ( f i g . 1 4 ) and f o u n d t o be x a l i n e a r as e x p e c t e d f r o m e q u a t i o n (d) . The r e s u l t s a r e shown i n T a b l e IX and t h e a v e r a g e o f f i v e f o u n d t o be 6.9 - 0.44 x 9 - 1 - 1 •+-10 1 m o l e s e c . The o v e r a l l a v e r a g e v a l u e 7.0 _ 0.44 x 9 —1 -1 9 10 1 mole s e c i s c o n s i s t a n t w i t h t h e l o w e r l i m i t of 6 x 10 -1 -1 23 m o l e s e c g i v e n by C l y n e and Coxon f o r t h i s r a t e c o n s t a n t . 83 T a b l e V I I I j C a l c u l a t i o n o f t h e R a t e C o n s t a n t o f R e a c t i o n 0 + CIO P l a t e [ C 1 0 2 ] A r g o n E n e r g y k f i x 10s (1 m o l e " s e c " ) ( t o r r ) ( t o r r ) J Method I Method I I 104 0.25 200 1060 6.7 6.4 152 0.25 200 1060 7.2 6.8 107 0.08 200 1060 7.2 7.3 105 0.06 200 1060 7.7 7.6 106 0.25 75 1060 6.4 6.4 A v e r a g e 7.04±0.42 6.910.44 (7.00+0.44) x 1 0 9 1 m o l e " s e c -T a b l e IX P l a t e [CIO,] A r g o n D e l a y t i m e CIO x 1 0 + 6 k c x 10 ( t o r r ) ( t o r r ) y s e c (mole/1) (1 m o l e - s e c 214 2.0 360 200 5.3 8.1 215 2.0 360 100 7.2 8.3 219* 2.0 188 100 8.75 6.8 219* 2.0 188 166 - ~7.3 6.7 A v e r a g e 7;5 +0.8 T h e s e two e x p e r i m e n t s c o n t a i n 10 t o r r o f CO i 84 i . 1 2) F l a s h P h o t o l y s i s of ClO R a d i c a l s j F l a s h p h o t o l y s i s has been used f o r producing the t r a n s i e n t s p e c i e s so f a r but i n t h i s work we have a l s o used a f l a s h p h o t o l y s i s technique to p h o t o l y s e the 1 t r a n s i e n t s . T h i s has been achieved by f i r i n g one f l a s h lamp (known as the main lamp) t o generate the t r a n s i e n t s , f o l l o w e d by a f i r i n g o f second lamp ( a u x i l i a r y lamp) a t a known d e l a y (0 to 214 usee) a f t e r the main lamp. The o t h e r d e t a i l s of t h i s c i r c u i t have been mentioned i n the experimental s e c t i o n . T h i s technique has been used here t o f l a s h the t r a n s i e n t , ClO, i n order to f i n d the r a t e c o n s t a n t of the r e a c t i o n of oxygen atoms w i t h CIO and t o see i f we can g e t a v i b r a t i o n a l l y e x c i t e d oxygen i n i t s ground e l e c t r o n i c s t a t e . The former w i l l be d i s c u s s e d here whereas the l a t t e r w i l l be i n Chapter V I . As u s u a l , ClO r a d i c a l s were generated by f l a s h i n g CIO2 p r e s s u r e v a r y i n g from 0.5 to 2.0 t o r r w i t h 360 t o r r of argon or n i t r o g e n i n q u a r t z or pyrex r e a c t i o n v e s s e l . The f l a s h energy used i n the main lamp was 1060 J . The behaviour of CIO2 and ClO i n the q u a r t z r e a c t i o n v e s s e l was s i m i l a r t o t h a t w i t h pyrex having two mm of e x t r a g l a s s f i l t e r , i . e . v e r y f a s t decay of CIO r a d i c a l s i n the b e g i n n i n g f o l l o w e d by the u s u a l b i m o l e c u l a r decay. Then the mixture was f l a s h e d w i t h the a u x i l i a r y lamp a t v a r i o u s f l a s h e n e r g i e s and a t d i f f e r e n t time d e l a y s a f t e r a l l the CIO2 had decomposed. At these times, the mixture con-t a i n s CIO, C l - and 0„ (from the secondary r e a c t i o n s ) and some 85 c h l o r i n e atoms from r e a c t i o n (6). The s p e c t r a were reco r d e d a t d i f f e r e n t d e l a y times a f t e r the f i r i n g o f a u x i l i a r y f l a s h . The t r a c e o f the main, a u x i l i a r y and s p e c t r o s c o p i c lamps were observed on the s c r e e n of o s c i l l o s c o p e every time i n order t o assure t h a t a u x i l i a r y lamp was f i r i n g a t the proper d e l a y . The decay of CIO, as can be seen from f i g . ( 1 5 ) ( p l a t e 214) and (16), i s f a s t e r a f t e r the a u x i l i a r y f l a s h lamp than t h a t observed w i t h o u t the a u x i l i a r y lamp. A f t e r the i n i t i a l f a s t decay, the CIO decays s l o w l y as has been observed i n the CIO2 when f l a s h e d w i t h h i g h f l a s h energy. T h i s f a s t decay can be e x p l a i n e d by the primary process (6-1) or (6-2) f o l l o w e d by (6-5) because the p r e s s u r e of argon or n i t r o g e n used was enough t o have (6-4) f a s t e r than (6-3), i . e . , CIO + hv ( p r e d i s s o c i a t i o n ) C l + 0 ( 3 P ) (6-1) CIO + hv (< 2 8 0 0 A) -> C l + 0 (^D) (6-2) f o l l o w e d by 0( 1D) + C 1 0 ( 2 7 T ) + C1( 2P) + 0 2 ( 3 E g ) (6-3) 0 (^D) + M -»• 0 ( 3P) + M (6-4) 0( 3 P ) + CIO ( 2 7 T ) -> C1( 2P) +_0 2( 3Sg) (6-5) ' The d e t a i l s of the p r e f e r e n c e of (6-5) over (6-3) w i l l be d i s c u s s e d i n Chapter VI. At the moment, we w i l l c o n s i d e r only (6-5). The slow decay i n the l a t t e r p a r t i s e x p l a i n e d by r e a c t i o n (5). CIO + CIO •> C l 2 + 0 2 (5) F i g u r e 15. Decay of CIO w i t h and without f l a s h i n g w i t h a u x i l i a r y lamp. ClO^- 2.0 t o r r , Argon= 360 t o r r , R e a c t i o n V e s s e l s Quartz, E M=1060 J E a u x . = 1 3 2 5 J ; Without Aux. Lamp With Aux, Lamp 2750 A 27 ,50 A 12,0 1Q0 12,0 10,0 Blank H.D. 5.2 ^ s e c 11 21 42.5 82 164 Blank 00 o rH o F i g u r e l 6 . Comparison of decay of CIO w i t h and without f l a s h i n g the a u x i l i a r y lamp. C10 2=2.0 t o r r , Argon = 360 t o r r , E m=1060 J , E>iux=1325 J , Delay time =20ojnSec. tt n n I n d i c a t e s CIO without f l a s h i n g aux. lamp. I n d i c a t e s decay of CIO a f t e r f l a s h i n g the aux. lamp. 0 25 50 75 100 125 time (/Asec) 150 175 00 8 8 The r a t e c o n s t a n t of r e a c t i o n (6-5) was c a l c u l a t e d as f o l l o w s . The oxygen atom c o n c e n t r a t i o n i s g i v e n by -^r (a-b) where a = [CIO] b e f o r e the a u x i l i a r y f l a s h ; b = (CIO] where a l l o the oxygen atoms have r e a c t e d w i t h CIO, found by e x t r a p o l a t i n g the slow decay p a r t of CIO c u r v e . I t has been assumed t h a t r e a c t i o n (5) can be n e g l e c t e d as slow compared to (6-5). The [0] t a t any time i s g i v e n by [ 0 ] t = x-b where x i s the concen-t r a t i o n of CIO a t any time t . Thus the decay of CIO can be r e p r e s e n t e d by - d-£1°! V k [oncio] d t 6 or - d x = k c x (x-b) d t 6 or l o g — - — = ^ t + l o g — - — (e) x-b 2.303 B - b where B = [CIO] j u s t a f t e r the a u x i l i a r y f l a s h = a - (a-b) = i x -l-(a+b) . Thus l o g — r - , p l o t t e d agaxnst time, was l i n e a r as £ X — D shown i n f i g . ( 1 7 ) . The v a l u e s of kg are l i s t e d i n Table IX. , +9 - i The average of f o u r are found to be 7.510.8 x 10 1 mole sec 1 , which i s i n s a t i s f a c t o r y agreement w i t h t h a t o b t a i n e d p r e v i o u s l y . T h i s method has c e r t a i n l i m i t a t i o n s . F i r s t l y , as i t can be seen from e q u a t i o n (e) t h a t v a l u e of kg o b t a i n e d i s dependent on the s e l e c t i o n of b, the amount of [ C l O ] ^ . The v a r i o u s v a l u e s of b were s e l e c t e d and i t was found t h a t any v a l u e of b taken b e f o r e 150 ysec does not g i v e a l i n e a r p l o t i n the whole r e g i o n and the p l o t behaves as shown i n f i g . ( 1 7 ) . F i g u r e 1 7 . A p l o t of. log( *-/x-b) against time, b = [CloJ , x = [ c i 6 ] t . C 1 0 2 = 2 . 0 t o r r , Argons 3 6 0 t o r r , E m = 1 0 6 0 J , E a u x= 1 3 2 5 J , Delay time- 200 jmsec. time ( usee) 00 VO 90 The v a l u e s of kg c a l c u l a t e d i n t h i s manner were found t o v a r y by a f a c t o r of 2. T h i s i n d i c a t e s t h a t r e a c t i o n (6-5) i s not complete w i t h i n t h i s p e r i o d . Whereas between 150 and 200 usee, any v a l u e of b s e l e c t e d would change the v a l u e of kg by o n l y 10% which i s w i t h i n the e x p e r i m e n t a l e r r o r . But a f t e r 200 usee kg decreases r a p i d l y ^ a s w e l l , a s the p l o t departs from l i n e a r i t y , thus i n d i c a t i n g t h a t i n t h i s r e g i o n r e a c t i o n (5) i s more s i g -n i f i c a n t than ( 6 - 5 ) . The o t h e r e r r o r can be due to the f i n i t e l i f e time of the f l a s h , i . e . , oxygen atoms are consumed by the r e a c t i o n (6-5) as w e l l as produced by (6-1) or (6-2) . The curve was drawn f i r s t w i t h the assumption t h a t o n l y the p h o t o l y s i s i s t a k i n g p l a c e d u r i n g the f l a s h but i t was m o d i f i e d by t a k i n g i n t o account the amount of CIO a c t u a l l y p r e s e n t . I t was found t h a t l e s s than 5% of the t o t a l p h o t o l y s i s takes p l a c e a f t e r the times t h a t were used to f i n d k,. These curves were drawn 6 w i t h the assumption t h a t decomposition i s l i n e a r l y p r o p o r t i o n a l t o the f l a s h energy. Thus t a k i n g i n t o c o n s i d e r a t i o n these l i m i t a t i o n s , the v a r i a t i o n s i n the i n d i v i d u a l v a l u e s as mentioned i n Table IX are not' bad and g i v e a s a t i s f a c t o r y agreement w i t h t h a t c a l c u -l a t e d p r e v i o u s l y . 3) P h o t o l y s i s of C10 ? i n Presence of Oxygen The CIO2 was f l a s h e d i n the. presence of oxygen w i t h r a t i o s v a r y i n g from 1:5 to 1:3000. The experiments i n which t h e t o t a l p r e s s u r e was l e s s t h a n 200 t o r r , was made 200 t o r r by a d d i n g a r g o n . The f l a s h e n e r g i e s u s e d were 1060 and 1325 J and t h e r e a c t i o n v e s s e l u s e d was p y r e x w i t h and w i t h o u t t h e p r e s e n c e o f 2 mm o f g l a s s f i l t e r A. The r e s u l t s o b t a i n e d a r e m o s t l y q u a l i t a t i v e a l t h o u g h some a p p r o x i m a t e q u a n t i t a t i v e r e s u l t s a r e o b t a i n e d . When t h e C l C ^ t o r a t i o i s low (1:5) , t h e b e h a v i o u r o f e a c h s p e c i e s i s s i m i l a r t o t h a t when no o x y g e n was a d d e d t o t h e s y s t e m , i . e . , CIO and e x c i t e d o x y g e n ( i n c h a p t e r VI) d e c a y v e r y f a s t as u s u a l . As t h e r a t i o i s i n c r e a s e d (^500), t h e r a t e o f d e c a y o f CIO i s r e d u c e d and a t t h e same t i m e a c o n t i n u o u s o s p e c t r u m h a v i n g a maxima a r o u n d 2500 A a p p e a r s . The i n t e n s i t y o f t h e c o n t i n u o u s s p e c t r u m depends upon t h e p r e s s u r e o f oxygen u s e d . A t t h e h i g h e s t r a t i o u s e d ( 1 : 2 8 0 0 ) , t h e CIO2 s p e c t r u m does n o t d i s a p p e a r c o m p l e t e l y and e v e n seems t o h a ve i n c r e a s e d a t l o n g t i m e s . S i m i l a r r e s u l t s i n t h e d e c a y o f C l O were o b s e r v e d when two mm o f g l a s s f i l t e r A was u s e d . I n t h i s c a s e t h e o x y g e n p r e s s u r e was so a d j u s t e d t h a t t h e CIO2 was c o m p l e t e l y r e a c t e d s i n c e t h e s e r e s u l t s were u s e d q u a n t i t a t i v e l y . T h i s has b e e n shown i n f i g . ( 1 8 ) ( p l a t e 2 9 3 ) . The p o s s i b l e r e a c t i o n s a r e C10 2 + hv + CIO + O (80%) (27) C10 2 + 0 CIO + 0 2 ( c o m p l e t e ) (7) 9 2 F i g u r e 18. Decay of CIO i n the presence and absence of 0 2 , C10 2= 0.05 t o r r A r g o n r 200 t o r r C 1 0 2 r 0.05 t o r r 0 2 = 50 t o r r Argon ~ 150 t o r r 27BP A 2§50 A 2j*j0 A Blank 5 /usee 10 2850 A 20 40 71 120 201 430 630 1.02msec 1.97 2.73 4 .32 5.1 6.1 Blank Before Blank Before 200 yusec 10 40 7 1 120 5 430 620 1.02 msec 1.97 2.73 4.32 5.1 6.1 A f t e r Before 12,0 10,0 12,0 10,0 0 + CIO -r C l + 0 2 (6) 0 + 0 2 + 0 2 -> 0 3 + 0 2 (32) C l + 0 3 •> CIO + 0 2 (33) C l + o 2 + o 2 •> C l - O-0 + o 2 (1) C l + C l - O-0 ->• c i 2 + o 2 (4) C l + C l - O-0 2 CIO (2) O + O - •* 0 o + 0„ (34) There can be two e x p l a n a t i o n s f o r t h e slow decay o f CIO i n t h e p r e s e n c e o f oxygen. 1) C h l o r i n e atoms produced i n t h e r e a c t i o n (6) r e a c t w i t h oxygen v i a r e a c t i o n (1) t o form the i n t e r m e d i a t e Cl-O-0 (peroxy r a d i c a l ) , f o l l o w e d by r e a c t i o n (4) o r ( 2 ) . N i c h o l a s and i 9 N o r r i s h have c a l c u l a t e d t h e r a t e c o n s t a n t o f r e a c t i o n (1) t o 3 2 —• 2 1 b e 6 . 2 _ l . l x l 0 1 mole sec and a l s o found t h a t r e a c t i o n (4) i s 14 ti m e s f a s t e r t h a n r e a c t i o n ( 2 ) . Hence r e a c t i o n (1) w i l l be f o l l o w e d by s t e p (4) p r e f e r e n t i a l l y and thus cannot a c c o u n t v e r y much f o r t h e slow decay of CIO. Though t h e r e can be some doubt about t h e continuum o around 2500 A s i n c e t h e CIO continuum a l s o e x t e n d s o v e r t h i s r e g i o n , the i n t e n s i t y o f t h i s continuum was g r e a t e r t h a n t h e i n t e n s i t y due t o CIO continuum c a l c u l a t e d by comparing i t w i t h o t h a t a t 2772 A. T h i s s u g g e s t s t h a t t h e continuum i s due t o o ozone w h i c h has i t s maximum a b s o r p t i o n around 2500 A. Thus r e a c t i o n s (1) , (4) and (2) cannot e x p l a i n t h e s l o w decay o f ClO c o m p l e t e l y though t h e y may be t a k i n g p a r t i n t h e r e a c t i o n scheme. 94 2) The m o s t l i k e l y e x p l a n a t i o n seems t o be c o m p e t i t i o n b e t w e e n r e a c t i o n s (6) and ( 3 2 ) . As h a s b e e n s e e n , t h e s l o w d e c a y o f CIO and t h e i n c r e a s e i n t h e i n t e n s i t y o f t h e c o n t i n u u m w i t h i n c r e a s e o f o x y g e n p r e s s u r e f a v o u r s r e a c t i o n (32) o v e r ( 6 ) . A l t h o u g h t h e d e c a y o f o z o n e i n t h i s e x p e r i m e n t was n o t m e a s u r e d , i t d o e s seem t o be s l o w . The r e a c t i o n (34) c a n be n e g l e c t e d b e c a u s e i t i s v e r y s l o w ( k ^ = 1 . 5 x 10^ 1 m o l e - " s e c - 1 , o r 4.0 x 1 0 6 1 m o l e - 1 s e c - 1 ) 6 9 ^ 7 0 ' 7 1 c o m p a r e d t o t h a t o f o x y g e n atoms w i t h CIO. A l s o , t h e c o n c e n t r a t i o n o f o z o n e p r e -s e n t w i l l be l e s s t h a n t h e c o n c e n t r a t i o n o f CIO. Thus i t seems t h a t o z o n e e i t h e r r e a c t s w i t h c h l o r i n e atoms whose h a l f l i f e i s f o u n d t o be a t l e a s t 400 / j s e c , t a k i n g t h e minimum v a l u e o f k 3 3 8 - 1 -1 °^ as 4 x 10 1 m o l e '"" s e c f o u n d by C l y n e and C o x o n , " " o r by 25 s l o w r e a c t i o n w i t h CIO as a l s o s u g g e s t e d b y C l y n e and C o x o n . I f we n e g l e c t t h e r e a c t i o n s c o n s i d e r e d a b o v e and a l s o t h e r e a c t i o n o f c h l o r i n e atoms w i t h o z o n e , t h e d e c a y o f CIO a n d o x y g e n atoms c a n be r e p r e s e n t e d as _ d J C l O i = k [ 0 ] [ C 1 Q ] d t b w h e r e t h e r e i s no o x y g e n , and " ^ n r = k 6to] [cio] +'k 3 2[o] [ o 2 ] 2 when t h e r e i s o x y g e n p r e s e n t i n t h e s y s t e m . D i v i d i n g t h e two e q u a t i o n s d[o] k 6 [o] [cio] + k 3 2 [ o ] [ o 2 ] 2 k.32 [o 2.] 2 = 1 + d[C10] kg [0] [CIO] kg [CIO] 1 + a [CIO] k k. ^  where a = — [ 0 o ] 2 or / [0,] [M] where M= T o t a l Pressure 2 k6 as the amount of oxygen i s so l a r g e t h a t i t can be c o n s i d e r e d as c o n s t a n t . I n t e g r a t i n g the above equation [O] = [CIO] + a ln[C10] + c when t = 0. [0] = [O] and [CIO] = [CIO] c = [O] - [CIO] - a In[CIO] o o o s u b s t i t u t i n g the v a l u e of c o n s t a n t c i n the above e q u a t i o n we have [O] - [O] = [CIO] - [CIO] + a l n 1°-^°-! (e' ) ' ° ° [ c i o ] o when t = 0 0 , i . e . , a l l the oxygen atoms have r e a c t e d w i t h CIO or oxygen [ Q ] = Q e q u a t i o n (e' ) reduces t o [CIO] [0] = [CIO] - [CIO] + a l n [0] = A [CIO] .+ a l n [CIO] [C101 ° [ C I O ] [ 0 ] Q can be measured as d e s c r i b e d i n the s e c t i o n ( 1 ) , i . e . i [ C 1 0 ] Q - [ C 1 0 ] O and A [ C I O ] r e p r e s e n t s the amount of oxygen atoms that'ihave r e a c t e d w i t h C I O i n presence of oxygen. The r e s u l t s of t h r e e experiments gave the r a t i o of kg/k^,, to be 7 5 , 60 and 36 when the oxygen p r e s s u r e used was 7 5 t o r r , 1 0 0 t o r r and 2 0 0 t o r r , r e s p e c t i v e l y . Thus depending upon the v a l u e 7 9 7 2 — 2 — 1 of k ^ 2 s e l e c t e d (as k^ 2 v a r i e s from 7 x 1 0 1 mole sec 8 2 — 2 — 1 t o 2 x 1 0 1 mole sec ), the v a l u e of kg v a r i e s from2.6 t o 1.5 X 10"*"^  1 mole 1 s e c 1 . I t was f u r t h e r f o u n d t h a t e v e n an e r r o r o f 5% i n t h e c a l c u l a t i o n o f [ClO] c a n c a u s e an e r r o r o f 25 t o 50% i n t h e f i n a l v a l u e o f kg. Though t h e r e i s t h u s a l a r g e d e g r e e o f u n c e r t a i n t y i n t h e v a l u e o f kg o b t a i n e d i n t h i s way, b u t t h e a g r e e m e n t i s s u f f i c i e n t l y good t o s u g g e s t t h a t t h e e a r l y f a s t d e c a y o f CIO i s due t o i t s r e a c t i o n w i t h o x y g e n atoms and t h a t t h i s i s r e d u c e d b e c a u s e o f t h e c o m p e t i t i o n f o r o x y g e n atoms i n r e a c t i o n (6) and ( 3 2 ) . I f we i n c l u d e r e a c t i o n (33) i n t h e r e a c t i o n scheme t h e d e c a y o f e a c h s p e c i e s c a n be r e p r e s e n t e d as d [ C l O ] = kg [O] [ C I O ] - k B [ c i ] [03] = kg [o] [ C I O ] + k 3 2 [ o ] [ o 2 ] 2 d t * [91 d t ^ L i , k [ 0 ] [ClO] - U C 1 ] [0,] = -d t b M i d t Though t h e r e a c t i o n o f c h l o r i n e atoms w i t h o z o n e i s 8 — 1 — 1 2 5 q u i t e f a s t (>4 x 10 1 mole s e c ) , i t i s s l o w as compared t o r e a c t i o n (6) and ( 3 2 ) . S o l v i n g t h e s e e q u a t i o n s w i l l l e a d us t o t h e f o l l o w i n g r e s u l t s w i t h t h e a s s u m p t i o n t h a t ^10= c o n s t a n t A [CIO]' _ [CIO] + a A[CIO] [CIO] -a where A [ C I O ] 1 = o x y g e n atoms r e a c t e d w i t h C l O i n t h e a b s e n c e o f o x y g e n ; A [ C l O ] = o x y g e n atoms r e a c t e d w i t h C l O i n p r e s e n c e o f o x y g e n ; 97 [CIO] = c o n c e n t r a t i o n o f CIO o b t a i n e d by e x t r a p o l a t i n g t h e l i n e a r p a r t o f t h e p l o t k 2 k . a = _32 [ 0 2 ] W J 2 r 0 -| r M - j v h e r e M = T o t a l ± - r e s s u r e k c K6 d' D S u b s t i t u t i n g t h e e x p e r i m e n t a l d a t a o f t h e t h r e e e x p e r i -m ents, t h e r a t i o o f k g / k 3 2 w a s f°und t o be 2 5 2 , 2 2 4 and 173 m o l e / l o x y g e n p r e s s u r e was 7 5 t o r r , 100 t o r r and 200 t o r r , r e s p e c t i v e l y . I t seems f r o m t h e s e t h r e e , r e s u l t s t h a t e i t h e r r e a c t i o n o f c h l o r i n e w i t h o z o n e i s t o o s l o w t o o b s e r v e any change i n CIO c o n c e n t r a t i o n o r t h a t t h e r e i s no r e a c t i o n w i t h o z o n e . The f i r s t a rgument seems t o be more r e a s o n a b l e t h o u g h o u r r e s u l t s a r e n o t a c c u r a t e enough t o p r o v e t h i s p o i n t . B. R e a c t i o n of Oxygen Atoms w i t h CIO,, and Cl^O As a l r e a d y m e n t i o n e d i n t h e b e g i n n i n g o f t h i s c h a p t e r , t h e r a t e c o n s t a n t o f o x y g e n atoms w i t h C 1 2 0 and C 1 0 2 c a n o n l y be compared w i t h t h a t o f o x y g e n atoms w i t h CIO, when a m i x t u r e o f C 1 0 2 and C l 2 0 i s f l a s h e d . I n t h i s s e c t i o n g e n e r a l d e t a i l s o f t h e e x p e r i m e n t a l p r o c e d u r e w i l l be d i s c u s s e d . The c a l c u l a -t i o n o f r a t e c o n s t a n t s o f r e a c t i o n s o f o x y g e n atoms w i t h C 1 2 0 and C 1 0 2 w i l l be c o v e r e d s e p a r a t e l y . C 1 0 2 was f l a s h p h o t o l y s e d i n t h e r a n g e o f 260 t o 1300 J . — 6 The c o n c e n t r a t i o n o f C 1 0 2 was v a r i e d f r o m 2.4 t o 4.75 x 10 -7 -5 m o l e / l i t e r and t h a t o f C 1 2 0 f r o m 5.5 x 1 0 t o 6 . 1 x 10 m o l e / l i t e r . I n t h e s e e x p e r i m e n t s , p h o t o l y s i s o f CT^O was a v o l d e d 98 by means o f a C o r n i n g 0-52 f i l t e r . The p r e s s u r e o f C1C>2 and CI2O were so a d j u s t e d t h a t t h e p l a t e s a t u r a t i o n was a v o i d e d . The r e s u l t s o f t h e s e e x p e r i m e n t s a r e g i v e n i n f i g s . ( 1 9 ) , (20), (21), (22) and (23) ( p l a t e 177) and T a b l e s X, XI and X I I . I t has b e e n f o u n d t h a t a t low f l a s h e n e r g i e s , t h e d e c o m p o s i t i o n o f C1C>2 was d e c r e a s e d i n t h e p r e s e n c e o f C l 2 0 ( f i g . 21 and 20) and a t t h e same t i m e , t h e c o n c e n t r a t i o n o f CIO, was i n c r e a s e d ( f i g . 23) . T h e r e i s a d e c r e a s e i n t h e amount o f e x c i t e d o x y g e n p r o d u c e d and u s i n g a r a t i o o f CIO2 t o C ^ O = 1:20, t h e v i b r a t i o n a l l y e x c i t e d o x y g e n i s h a r d l y v i s i b l e * ( f i g . 24, page 1 1 3 ) . The r a t e o f d e c a y o f O2 i s , however, s i g n i f i c a n t l y r e d u c e d . T h e s e e f f e c t s i n c r e a s e as t h e r a t i o o f C ^ O t o CIO2 i s i n c r e a s e d . A t h i g h f l a s h e n e r g i e s , t h e e f f e c t o f C ^ O i s much more ma r k e d . The i n i t i a l r a p i d d e c a y o f CIO w h i c h i s a f e a t u r e o f t h e h i g h f l a s h e n e r g y p h o t o l y s i s o f CIO2' i s r e d u c e d ( f i g . 19) * and t h e h a l f l i f e o f O2 w h i c h i s v e r y s h o r t a t t h e s e f l a s h e n e r g i e s i s d r a m a t i c a l l y i n c r e a s e d . A c o n v e n i e n t m e a s u r e o f t h e e f f e c t o f C ^ O on CIO d e c a y i s p r o v i d e d by e x t r a p o l a t i n g t h e l i n e a r p a r t o f t h e p o r t i o n o f t h e s e c o n d o r d e r p l o t o f ClO -1 \ ' [CIO] . v s . t i m e j t o z e r o t i m e . The v a l u e s o f [CIO] t h u s o b t a i n e d w i t h and w i t h o u t C ^ O and t h e i r r a t i o s a r e l i s t e d i n T a b l e X I I (page 115). t T h e s e a s s u m p t i o n s may be e x p l a i n e d by t h e r e a c t i o n s O + C10 2 -*• CIO + 0* (7) 0 + CIO + C l + O* (6) O + C1 20 ->- 2C10 ! (35) A t low f l a s h e n e r g i e s , C10 2 and Cl.,0 compete f o r oxygen atoms and t h i s accounts f o r the r e d u c t i o n i n the amount of C10 2 decomposed and 0 2 produced and f o r the i n c r e a s e i n the CIO c o n c e n t r a t i o n produced. A t h i g h f l a s h e n e r g i e s , CIO and C1 20 compete f o r the excess oxygen atoms and the e x t e n t of the i n i t i a l r a p i d decay of ClO caused by the r e a c t i o n (6), i s reduced by the r e a c t i o n (35). At s u f f i c i e n t l y high r a t i o s of C l j O t o C10 2, r e a c t i o n (35) w i l l cause the ClO c o n c e n t r a t i o n t o i n c r e a s e i n i t i a l l y and t h i s e f f e c t has been observed ( f i g . 1 9 ) . * A d e t a i l e d d i s c u s s i o n of the behaviour of 0 2 w i l l be p r o v i d e d i n Chapter VI. The c o n c l u s i o n reached t h e r e i s t h a t * the r a p i d decay of 0 2 a t h i g h f l a s h e n e r g i e s i s due t o the e x c e p t i o n a l l y h i g h e f f i c i e n c y of c h l o r i n e and oxygen atoms i n the v i b r a t i o n a l r e l a x a t i o n . On t h i s b a s i s , the e f f e c t o f C1 20 * on the decay of 0 2 i s r e a d i l y understood i n terms of the reduc-t i o n i n the oxygen atom c o n c e n t r a t i o n by r e a c t i o n (35) and i n the p r o d u c t i o n of c h l o r i n e atoms by r e a c t i o n (6). A secondary e f f e c t of C1 20 i s t o i n c r e a s e the r a t e o f decay of those c h l o r i n e atoms which are formed i n r e a c t i o n (6). 8 —1 —1 With a r a t e c o n s t a n t k^g of 4 x 10 1 mole sec f o r the r e a c t i o n C l + C1 20 > C l 2 + CIO (19) the h a l f l i f e i s reduced to 64 usee f o r a C1 20 p r e s s u r e of 0.5 t o r r . In most of the experiments, t h i s p r e s s u r e i s s u f f i c i e n t l y F i g u r e 1 9 . A p l o t o f l/[C10] against time In the'presence of C1 ?0. C 1 0 2 = 2 . 4 4 x 10 M, Argon-= 200 t o r r , E - 1 0 6 0 J . 0 ' 100' 200 time ( /xsec) 101 l a r g e t o s u p p r e s s t h e f o r m a t i o n o f c h l o r i n e atoms and t h i s s e c o n d a r y e f f e c t i s u s u a l l y much l e s s i m p o r t a n t t h a n r e a c t i o n (35) . Thus t h e q u a n t i t a t i v e e f f e c t o f C l 2 0 on t h e f o r m a t i o n and d e c a y o f CIO a l l o w s t h e r e l a t i v e v a l u e s o f k ^ r k ^ r k ^ ^ t o be m e a s u r e d . F i r s t k g : k 3 5 w i l l be d i s c u s s e d and t h e n k ^ r k ^ . 1) The R e a c t i o n o f Oxygen Atoms w i t h C l ^ O A t h i g h f l a s h e n e r g i e s (1060 and 1325 J ) , where more t h a n 50% p r i m a r y p h o t o l y s i s o f C 1 0 2 i s a c h i e v e d , we may c o n -s i d e r t h a t t h e p h o t o l y s i s i s e s s e n t i a l l y i n s t a n t a n e o u s and t h a t t h e s m a l l amount o f C 1 0 2 r e m a i n i n g i s v e r y r e a d i l y removed by t h e r e a c t i o n w i t h o x y g e n atoms.. The e x c e s s oxygen atoms t h e n r e a c t w i t h CIO and C l 2 0 and t h e d e c a y i s g i v e n by t h e e q u a t i o n _ g[C10] = k 5 t C l O ] 2 + k g [ 0 ] [ C I O ] - 2 k 3 5[o] [ C 1 2 0 ] ( f ) d t The r e a c t i o n s CIO + C 1 2 0 -»• C 1 0 2 + C l 2 (20) CIO + C 1 2 0 C l + C . l 2 + 0 2 (21) may be o m i t t e d s i n c e (k„_ + k~,,' i n 6 , " , - 1 - 1 ^ 4.u = 4. J 20 21)< 10 1 mole s e c s o t h a t ( k 2 Q + k 2 1 ) [CIO] << kg [CIO] << k 3 5 [ C 1 2 0 ] e x c e p t a t v e r y l o n g t i m e s . T h e s e r e a c t i o n s w i l l be c o n s i d e r e d i n d e t a i l i n C h a p t e r V. 102 S i n c e r e a c t i o n (6) i s t h e o n l y s o u r c e o f c h l o r i n e atoms and s i n c e k^g << k^,-, t h e r e a c t i o n s (8) and ( 1 9 ) , i . e . C l + C 1 0 2 -v 2 CIO (8) C l + C 1 2 0 ->• C l 2 + CIO (19) c a n l i k e w i s e be o m i t t e d . I f , t h e r e f o r e , t h e r e l a t i o n s h i p k g [CIO] = 2 K 3 5 [ C 1 2 0 ] c a n be s a t i s f i e d , t h e e q u a t i o n ( f ) r e d u c e s t o _ a r c i o ] = k [ c i o ] 2 d t and t h e i n i t i a l r a p i d d e c a y o f CIO c a n be e l i m i n a t e d . The s e c o n d o r d e r p l o t t h e n r e m a i n s l i n e a r e v e n a t s h o r t d e l a y s and e x t r a p o l a t e s b a c k t o g i v e [CIO] = [C10~] . V a r i o u s r a t i o s o f ^ o 2 o C 1 0 2 t o C 1 2 0 were t r i e d i n o r d e r t o a c h i e v e t h i s c o n d i t i o n as c l o s e l y as p o s s i b l e . I n p r a c t i c e t h e r e was a l w a y s a s m a l l i n i t i a l i n c r e a s e o r d e c r e a s e i n t h e r a t e o f d e c a y o f CIO ( f i g . 19) and e x t r a p o l a t i n g t h e s u b s e q u e n t s e c o n d o r d e r p l o t t l i n e a r l y t o z e r o t i m e g a v e [CIO] 4 [CIO] . The d i f f e r e n c e . o o [ C 1 0 ] q - [ C 1 0 ] q was c a l c u l a t e d . A s m a l l c o r r e c t i o n i n t h i s c a l -c u l a t i o n was a p p l i e d by t a k i n g i n t o c o n s i d e r a t i o n t h a t [ C 1 0 2 ] q i s t h e c o n c e n t r a t i o n o f C 1 0 2 t h a t has b e e n decomposed i n t h e p r e s e n c e o f C 1 2 0 r a t h e r t h a n t h e i n i t i a l c o n c e n t r a t i o n o f C 1 0 2 u s e d . E x p e r i m e n t a l l y o n l y 4 t o 10% C 1 0 2 was l e f t d e p e n d i n g upon t h e p r e s s u r e o f C 1 2 0 and f l a s h e n e r g y . Thus t h e d i f f e r e n c e [C10]' o - [CIO] was p l o t t e d a g a i n s t [ C 1 2 0 ] as 103 shown i n f i g . ( 2 0 ) and t h e c o n c e n t r a t i o n o f C 1 2 0 was f o u n d w h i c h w i l l g i v e z e r o d i f f e r e n c e . The a v e r a g e o f f o u r s e t s o f e x p e r i -ments gave k-^/kg = 0.75 _ 0.05 ( T a b l e X ) . E a c h s e t o f e x p e r i -ments c o n s i s t s o f one e x p e r i m e n t w i t h o u t C ^ O and t h r e e t o f i v e w i t h C ^ O o f d i f f e r e n t c o n c e n t r a t i o n . The c o n c e n t r a t i o n o f C ^ O was s e l e c t e d t o have minimum d i f f e r e n c e so t h a t t h e f u r t h e r e r r o r i n p l o t t i n g c o u l d be r e d u c e d . U s i n g t h e v a l u e o f k g , d e t e r m i n e d p r e v i o u s l y , we f i n d 9 - 1 - 1 t h a t k _ r = 5.3 x 10 1 mole s e c 3D The v a l i d i t y o f t h e a s s u m p t i o n s made i n t h i s d e t e r m i n -a t i o n o f k ^ has b e e n c h e c k e d by d e t a i l e d c a l c u l a t i o n o f t h e i r e f f e c t on t h e p r o d u c t i o n o f CIO. V a r i o u s v a l u e s f o r t h e r a t i o s o f t h e c o n s t a n t s , k _ : k ^ : k 0 r : k , _ were u s e d f o r v a r i o u s p e r c e n -t a g e s o f t h e p r i m a r y d e c o m p o s i t i o n and t h e d e p l e t i o n o f C ^ O was t a k e n i n t o a c c o u n t . The a s s u m p t i o n t h a t t h e p h o t o l y s i s was i n s t a n t a n e o u s ( i . e . p h o t o l y s i s was much f a s t e r t h a n t h e s u b s e -q u e n t c h e m i c a l r e a c t i o n s ) i s known t o be r e a s o n a b l e f r o m t h e f l a s h p r o f i l e and a l s o c a n be s e e n f r o m t h e p r i m a r y p h o t o l y s i s o f C I O 2 . The d e t a i l e d c a l c u l a t i o n s were a l s o done by a s s u m i n g t h a t p h o t o l y s i s and c h e m i c a l r e a c t i o n s a r e o c c u r r i n g s i m u l t a n -e o u s l y . I t was f o u n d t h a t o n l y ^ 5% more CIO w o u l d have b e e n p r o d u c e d a c c o r d i n g t o l a t e r c a l c u l a t i o n s . The a s s u m p t i o n t h a t a l l C I O 2 was decomposed b e f o r e t h e e x c e s s o x y g e n atoms s t a r t r e a c t i n g w i t h CIO and C ^ O was t e s t e d by c a l c u l a t i n g t h e e f f e c t o f a l l o w i n g a l l t h r e e r e a c t i o n s o f o x y g e n atoms t o o c c u r 104 • j 10 5 simultaneously as well as the reactions o f , c h l o r i n e atoms- with C1C>2. These c a l c u l a t i o n s showed that the amount of ClO pro-duced would be reduced by ^5% and that a small amount (^ 2 to 8%) of CIO2 would remain undecomposcd even f o r 75% primary process. Experimentally, t h i s c a l c u l a t i o n was con-firmed by the observation that between 4 to 10% of the G10 2 d i d i i i f a c t remain unreacted. We thus conclude that no s i g n i f i c a n t e r r o r a r i s e s from the assumption used. The value of k^ ,. obtained, i s i n f a i r • . • 9 - 1 - 1 agreement with the value of 8.3 x 10 1 mole sec given by 73 P h i l l i p s from the f a s t flow mass spectrometer study. ••' • • Table X 6 [CIO2] * 10 Argon . Energy p c ^ Q required k ^ / k g "2 (mole/1) '.  . (torr) J . (torr) 4.4 • - 200 10G0 0 .057 0.67 4.4 200 1325 0.049 0.78 2.44 r 200 ' 1060 0.032 ! ; 0.72 2.44 200 " 1325 0.028 • 0.80 ;. Average k 3 5 / k g = .0.74 _ 0.06 k35 = 0 , 7 i f X 7 , 0 x l ° 9 = x 1 q 9 1 mol©" 1 s e c " 1 106 2) R e a c t i o n o f Oxygen Atoms w i t h C1C>2 A t low f l a s h e n e r g i e s , o n l y a s m a l l f r a c t i o n o f t h e o x y g e n atoms p r o d u c e d i n t h e p h o t o l y s i s o f CIO2 r e a c t w i t h CIO. I n t h e p r e s e n c e o f a Cl.,0 c o n c e n t r a t i o n s u f f i c i e n t t o compete w i t h C10 2 f o r o x y g e n atoms, r e a c t i o n (6) may be n e g -l e c t e d . T h r e e e f f e c t s have b e e n n o t e d when C10 2 i s f l a s h e d i n e x c e s s o f C1 ?0. F i r s t , i t has b e e n f o u n d t h a t t h e r e i s a r e d u c t i o n i n t h e d e c o m p o s i t i o n o f C10 2 f i g s . (21) and (22). S e c o n d l y , r e d u c t i o n i n t h e amount o f 0 2 p r o d u c e d , f i g . 2 4 (page 113) and t h i r d l y , t h e i n c r e a s e i n t h e CIO c o n c e n t r a t i o n s h a v e b e e n n o t i c e d as shown i n f i g s . (23) and ( 1 9 ) . The f i r s t two o f t h e t h r e e have b e e n u s e d t o d e t e r m i n e t h e v a l u e s o f t h e r a t i o s o f t h e r a t e c o n s t a n t s k_,:k_ r. The t h i r d one has b e e n 7 3 5 u s e d t o p r e d i c t t h e amount o f CIO p r o d u c e d by u s i n g t h i s r a t i o o f k^/k^,-. The v a l u e s o b t a i n e d a r e l i s t e d i n T a b l e X I I (page 115) and a r e c o m p a r e d w i t h those o b t a i n e d e x p e r i m e n t a l l y . M e thod 1 I n t h e a b s e n c e o f C1 20 and f o r l e s s t h a n 50% p h o t o l y s i s , t h e o v e r a l l d e c o m p o s i t i o n o f C10 2, ( m o l e / l i t e r ) , i s s i m p l y t w i c e t h e c o n c e n t r a t i o n p h o t o l y s e d , i . e . , A x = 2 a where a = t h e amount o f C10 2 p h o t o l y s e d and b e c a u s e o f r e a c -t i o n (8) t h i s i s i n d e p e n d e n t o f r e a c t i o n ( 6 ) . I f , t h e n , i n t h e p r e s e n c e o f C l 2 0 , a f r a c t i o n $ o f t h e o x y g e n atoms have r e a c t e d w i t h C10 2, t h e o v e r a l l decompo-s i t i o n o f C10 9 ( A 9 ) i s g i v e n by F i g u r e 21. Disappearance o f C10 2 when i t i s f l a s h e d w i t h and without C1 P0. C10 2= 4 .Ox 10 M, Argon =200 t o r r , E = 6 0 0 J F i g u r e 22. A p l o t of ClOg against time i n the presence and absence of ClgO. C1G - 4 . 7 5 x 10 ° M, C l 0 = 36.2 x 10 M, Argon =200 t o r r , Er260.":J. F i g u r e 2 3 . Comparison o f CIO when CIO2 i s f l a s h e d w i t h and without ClgO. ClOo- 4 . 0 x 10 Iff, Argon = 200 t o r r , E= 6 0 0 J 110 . A 1 ( l + 6) A 2 = a ( l +6) = 2 I (g) i f r e a c t i o n (6) i s o m i t t e d . The v a l u e s o f 3 a r e o b t a i n e d f r o m t h e m e a s u r e d v a l u e s o f A ^ and A 2 f o r v a r i o u s r a t i o s o f [C1 20] : [CIO,,] w i t h low e n e r g i e s . A t t h e f l a s h e n e r g i e s and t h e C1 20 p r e s s u r e u s e d f o r t h e d e t e r m i n a t i o n o f k ^ r k ^ g , t h e o v e r a l l d e c o m p o s i t i o n o f C1C>2 i s s u f f i c i e n t l y s m a l l f o r t h e C10 2 c o n c e n t r a t i o n t o be t a k e n as c o n s t a n t a t i t s a v e r a g e v a l u e [ C I C ^ ] ^ . The c o n c e n t r a t i o n o f CI2O, t o an e v e n b e t t e r a p p r o x i m a t i o n , i s l i k e w i s e t a k e n as c o n s t a n t ( w i t h i n 10%). Then k 7 [ C 1 CV J av 3 = V C 1 0 2 W k 3 5 [ C l 2 0 ] a v From e q u a t i o n ( g ) , t h e v a l u e o f g i s 2 A 2 - A 3 = i -A l T h u s k 7 [ C 1 Q 2 J a v = 2 A2 - A l k 7 [ C 1 0 2 ] a v + k 3 5 [ C l 2 0 ] a v A x o r t h i s c a n be r e d u c e d t o :7 k„ 2 A z - A ^ [ C 1 2 0 ] ^ k 3 5 2 ( A 1 - A 2 ) [ C l 0 2 ] a v Thus f r o m t h e m e a s u r e d v a l u e s o f and A 2 a t d i f f e r e n t f l a s h e n e r g i e s and v a r i o u s r a t i o s o f [C1 20] : [ C I C ^ ] , t h e a v e r a g e v a l u e o f k ^ / k ^ e q u a l t o 5.8 1 0.1 was o b t a i n e d by u s i n g t h e above e q u a t i o n . The o t h e r v a l u e s a r e l i s t e d i n T a b l e X I . I l l The v a l i d i t y of t h i s formula was t e s t e d by c a l c u l a t i n g — — as a f u n c t i o n of k ^ / k ^ f o r v a r i o u s v a l u e s of k^/kg, i n i t i a l [ C l 2 0 ] : [ClOj] r a t i o s , and percentage primary photo-l y s i s . Allowance was made f o r the change i n C1C>2 and C1 20 c o n c e n t r a t i o n s as both p h o t o l y s i s and chemical r e a c t i o n s p r o -ceeded and r e a c t i o n of c h l o r i n e atoms w i t h C1C>2 and C1 20 was a l s o taken i n t o account. I t was found t h a t f o r our experiments, the e r r o r i n the equ a t i o n A,(1 + 8) A2 = — . 2 which a r i s e s from the n e g l e c t of r e a c t i o n ( 6 ) was s m a l l and almost equal t o t h a t a r i s i n g from the n e g l e c t of the use of [C10_] i n the c a l c u l a t i o n o f 8. F o r t u n a t e l y , these e r r o r s — av were o f o p p o s i t e s i g n and we conclude t h a t n e g l i g i b l e e r r o r i s i n t r o d u c e d by the use of the simple formula g i v e n f o r k ^ / k ^ . Method 2. * The c o n c e n t r a t i o n o f 0 2 produced i s reduced by a f a c t o r 8 by C1 20. Measurements u s i n g the e x t r a p o l a t e d v a l u e s * of 0 2 a t zero time y i e l d e d the average v a l u e of k ^ / k ^ = 5.8 ± 0.4. The o t h e r v a l u e s are l i s t e d i n Tab l e XI. The approximations i n t r o d u c e d by n e g l e c t i n g r e a c t i o n ( 6 ) and u s i n g C cl°2^av t o c a l c u l a ' ' - e B are r a t h e r l e s s s a t i s f a c t o r y i n t h i s case s i n c e the e r r o r c a n c e l l a t i o n i s l e s s complete. 31 Evidence has been o b t a i n e d t h a t the oxygen molecules p r o -112 i duced i n r e a c t i o n (6) are v i b r a t i o n a l l y e x c i t e d as i n r e a c t i o n • I (7), but the r e l a t i v e p o p u l a t i o n s o f the e x c i t e d l e v e l s a re not known t o be the same as i n the case of C I C ^ . Even so, the e r r o r o f l e s s than 10% i s p r o b a b l y exceeded by'the e x p e r i m e n t a l e r r o r i n measuring C > 2 c o n c e n t r a t i o n s . The a g r e e -ment between the methods of c a l c u l a t i n g k^/k^^ i s b e t t e r than might have been e x p e c t e d . .From the p r e v i o u s l y d e t e r m i n e d v a l u e of k^ ,- w e thus obtain- k.^  = 3.0. * 1 0 1 0 1 m o l e " 1 s e c " 1 and ky/kg = 4 . 3 . These r e s u l t s a r e i n s a t i s f a c t o r y agreement w i t h those o f C l y n e and 23 nn - i - i Coxon, i . e . , k- > 2 .4 x 10 1 mole sec and k~/k r "^4. T a b l e XT P l a t e no. ' [cio 2] x i o + 6 (mole / 1 ) [C1 ? 0] x (mole / 1 ) 1 0 + ^ Argon ( t o r r ) Energy J c i o 2 k ? / k 3 5 from °2 176 4.06 57 .0 200 260 i 5.8 --177 4 .0 44 .0 200 600 5.9 • 5.6 179 4 .75 ' 36 .2 200 260 5.7 6 . 3 180 4 .70 36 .2 200 600 5.9 5.6 Average 5.8 5.8 Mean k ^ / k ^ c , = 5 . 8 k_ = 3.Q x 1 0 1 0 1 m o l e " 1 sec F i g u r e 24. Formation o f 0 2 when C10 2 i s fl a s h e d w i t h and without C1 2 0 , C10 2= 0.05 t o r r , Argon = 200 t o r r , E - 260 J No C I 2 0 ClgO = 1.0 t o r r 5.2 /Usee 10.3 16.7 40 71 138 201 Blank Before o 2 —11 C10 2 0, O.63 msec 1.18 C10 2 114 Method 3 A t low f l a s h e n e r g i e s t h e t o t a l c o n c e n t r a t i o n o f CIO p r o d u c e d i n t h e p r e s e n c e o f C 1 2 0 i s g i v e n by t A X [CIO] = (3 -6) -° 2 w h i l e i n i t s a b s e n c e [ C 1 0 ] o - A x i f r e a c t i o n (6) i s n e g l e c t e d i n b o t h t h e c a s e s . The v a l u e s o f i - — ^ — — a r e compared w i t h t h e e x p e r i m e n t a l v a l u e s o f ^ l O ] Q # [ C 1 0 ] Q The a g r e e m e n t i s n o t b a d , b u t i t s h o u l d be n o t e d t h a t , s i n c e 3 -• 8 a l l t h e v a l u e s o f must l i e i n t h e r a n g e o f 1.25 1 0.25, 2 t h i s i s n o t t h e s t r o n g e s t t e s t o f t h e a c c u r a c y o f t h e r a t i o k^:k^^. The more d e t a i l e d c a l c u l a t i o n s u s e d t o t e s t t h e v a l i -d i t y o f t h e a p p r o x i m a t i o n s made i n t h e d e t e r m i n a t i o n o f t h e t r a t i o k ^ r k ^ j - , y i e l d e d t h e t h e o r e t i c a l v a l u e s o f [CIO] / [ C l O ] o i n b e t t e r a g r e e m e n t w i t h e x p e r i m e n t - ( T a b l e X I I ) . The most s i g n i f i c a n t f a c t i s t h a t t h e r a t i o ^ C 1 0 ^ ° [ C l 0 ] o i s g r e a t e r t h a n one and a p p r o a c h e s 1.5. T h i s p r o v i d e s s t r o n g e v i d e n c e t h a t t h e i n h e r e n t l y u n l i k e l y r e a c t i o n 0 + C 1 2 0 + C l 2 + 0 2 (36) w h i c h was i g n o r e d i n t h e d e t e r m i n a t i o n o f r a t i o kg t o k^^ i s n e g l i g i b l e compared t o r e a c t i o n ( 3 5 ) : 0 + C 1 2 0 -> 2C10 (35) F u r t h e r e v i d e n c e f o r t h i s c o n c l u s i o n has b e e n o b t a i n e d f r o m t h e measurement o f t h e v i b r a t i o n a l l y e x c i t e d o x y g e n d i s c u s s e d i n C h a p t e r V I . Table XII Comparison of the Ratios of ClO formed i n the Cl 20/C10 2 and i n the C10 2 Systems. Plate Energy [C10 1 [C1_0] % Photo. Experimental Ratio Calculated Ratio no. J z (Prim.) A B C 187 1060 4.4 2.75 70 2.89 4. 39 1.52 4.18 1.45 187 1060 4.4 5.5 70 2.89 4.65 1.61 4.98 1.72 186 1060 4.4 6.9 70 2.89 5.54 1.92 5.28 1.83 . 189 1325 4.4 4.1 . 83 1.52 4.56 3.00 4.75 3.1 189 1325 4.4 6.9 83 1.52 5.10 3.36 5.72 3.76 188 1325 4.4 9.6 83 1.52 6.4 4.04 6.38 4.2 291 1060 2.44 0.55 75 1.22 1.66 1.52 1.74 1.43 290 1060 2.44 1.1 75 1.22 2.2 1.80 2.09 1.71 287 1060 .2.44 2.2 75 1.22 2.9 2.37 2.58 2.11 288 1060 2.44 3.3 75 1.22 3.45 2.83 3.48 2.85 289 1060 2.44 4.4 75 1.22 4.11 3.36 3.76 3.0 291 1325 2.44 0.55 85 0.73 1.56 2.13 1.49 2.04 290 1325 2.44 1.1 85 0.73 1.93 2.64 2.00 2.74 287 1325 2.44 2.2 85 0.73 2.64 3.61 2.58 3.53 288 1325 2.44 3.3 85 0.73 3.27 4.48 3.06 , 4.19 289 1325 2.44 4.4 85 0.73 4.11 5.63 4.02 5.5 N.B. A=[C10] without C1 20, B = [ClO] with C1 20 [ C I O ] w i t h C 1 2 0 . A l l the concentrations have the units 10 m o l e / l i t r e . 116 A t h i g h f l a s h e n e r g i e s , t h e e f f e c t o f Cl^O c a n be e x p r e s s e d as t h e r a t i o o f CIO c o n c e n t r a t i o n a t z e r o t i m e ob-t a i n e d by l i n e a r e x t r a p o l a t i o n o f t h e s e c o n d o r d e r d e c a y p l o t i n t h e p r e s e n c e o f C 1 2 0 t o t h a t i n i t s a b s e n c e . I n t h i s c a s e , [ C 1 0 J o - A 1 and h e n c e t h e r a t i o o f ^ C l ° ^ o w o u l d be g r e a t e r t h a n 1.5, [ C 1 0 ] 0 d e p e n d i n g upon t h e r a t i o o f C l 2 0 t o C1C>2 and i n c r e a s e s w i t h t h e i n c r e a s e o f f l a s h e n e r g y and h e n c e t h e p r i m a r y p h o t o l y s i s o f c i o 2 . Table X l l - a Comparison of the ^ a+ios of CIO formed in the Cl^O/ClO^ and in the c 1 0 2 systems. Plate 1c [C10 2 J L C l o 0 J Experimental Watio Calculated no. (J) ^ ^ A B C D C / A D/A 179 260 4.75 36.2 2.9 3.5 1. 21 3. 88 3.68 1. 34 1.29 176 260 4.06 57.0 1.79 2.39 1. 34 2. 49 ,2.43 1. 39 1.36 178 260 4.06 61.0 1.92 2.5 1. 30 2. 51 2.67 1. 3 1.36 180 600 4.7 36.2 3.74 5.1 1. 37 5. 2 4.88 1. 4 1.3 177 600 .4.0 44.0 3.22 4.4 1. 37 4. 6 4.3 1. 37 1.34 N.B. A l l the concentrations have the. units..r 10 i mole/liter. A= [C10] without C1 20 B-[ C10] with C]_20 C= [C10] calculated with approx. Calculation. w i t h Cl^O. D= [CIO] calculated with detailed calculations i n the presence of c i 2 o CHAPTER V REACTIONS OF HALOGEN ATOMS ( C l AND Br) I WITH C 1 2 0 AND C I O 2 i i A. P h o t o l y s i s o f C I 2 0 The p h o t o l y s i s o f C l 2 0 was r e i n v e s t i g a t e d i n o r d e r t o c o r r e l a t e t h e r e s u l t s t h a t were o b t a i n e d f r o m C 1 0 2 p h o t o l y s i s , d i s c u s s e d i n C h a p t e r I I I , IV, and V I . Some new r e s u l t s were o b t a i n e d w h i c h a r e r e l e v a n t t o t h e mechanism o f t h e p h o t o l y s i s and o f t h e C l 2 p h o t o - s e n s i t i s e d d e c o m p o s i t i o n o f C 1 2 0 . C l 2 0 was f l a s h e d a t t h r e e f l a s h e n e r g i e s w i t h b o t h p y r e x and q u a r t z r e a c t i o n v e s s e l s , as w e l l as w i t h a g l a s s f i l t e r A, b u t a t o n l y two p r e s s u r e s o f C i 2 0 as m e n t i o n e d i n T a b l e XV and one t o t a l p r e s s u r e (200 t o r r ) o f a r g o n . The r e s u l t s o b t a i n e d (as c a n be s e e n f r o m f i g . 25) when a two mm f i l t e r A was u s e d a r e s i m i l a r t o t h o s e o b t a i n e d by 2 2 E.N.T. and f i t t h e i r r e a c t i o n scheme v e r y w e l l w i t h few m o d i -f i c a t i o n s . The t h r e e s t a g e s n o t e d by them and by us a r e : 1) P r i m a r y p h o t o l y s i s o f C ^ O ( d e p e n d i n g upon t h e w a v e l e n g t h o f e x c i t a t i o n ) f o l l o w e d by r a p i d d e c r e a s e o f t h e C 1 2 0 c o n c e n t r a t i o n and t h e n by an i n c r e a s e i n t h e CIO c o n c e n -t r a t i o n : C 1 2 0 + hV (53000 A) CIO + C l (18) C 1 2 0 + h»U3000 A) -> 2C1 + 0 (22) C 1 2 0 + C l CIO + C l 2 (19) 118 C 1 2 0 + 0 •> 2C10 ' (35) CIO + 0 + 0* + C l ^ (6) 2) B i m o l e c u l a r d e c a y o f CIO and d e c r e a s e i n t h e C1~0 ! c o n c e n t r a t i o n v e r y s l o w l y CIO + CIO -> C l 2 + 0 2 (5) 3) Slow a p p e a r a n c e o f C 1 0 2 (which s t a r t s i n t h e end o f t h e s e c o n d s t a g e ) , a t t a i n i n g maximum c o n c e n t r a t i o n a t n e a r l y 30 s e c o n d s and r e m a i n i n g c o n s t a n t f o r a t l e a s t 1 m i n u t e . The r e a c t i o n s i n v o l v e d a r e C 1 2 0 + CIO + C 1 0 2 + C l 2 (20) C 1 2 0 + CIO -*• C l + C l 2 + 0 2 (21) The s e c o n d s t a g e has b e e n d i s c u s s e d i n C h a p t e r I I I - B . The r e a c t i o n s o f o x y g e n atoms w i t h C l 2 0 has b e e n m e n t i o n e d i n C h a p t e r IV-B and p r o d u c t i o n o f v i b r a t i o n a l l y e x c i t e d o x y g e n w i l l be d i s c u s s e d i n C h a p t e r V I . O n l y r e a c t i o n o f c h l o r i n e atoms and t h e t h i r d s t a g e w i l l be d e s c r i b e d h e r e . 1) C h l o r i n e S e n s i t i s e d P h o t o - d e c o m p o s i t i o n o f C l 2 O o A f t e r t h e p r i m a r y p r o c e s s w i t h l i g h t a bove 3000 A, t h e i m m e d i a t e r e a c t i o n i s t h a t o f c h l o r i n e atoms w i t h C 1 2 0 t o g i v e CIO ( r e a c t i o n 1 9 ) . I n o r d e r t o c a l c u l a t e t h e r a t e c o n s t a n t k ^ g , a m i x t u r e o f C 1 2 0 and C l 2 i n t h e r a t i o o f 1:1 t o 1:5 was f l a s h o p h o t o l y s e d a t t h r e e f l a s h e n e r g i e s u s i n g l i g h t above 3000 A. Though t h e C 1 2 0 has an a b s o r p t i o n i n t h i s r e g i o n , t h e e x t i n c t i o n CIO, < Figure 25. F l a s h Photolysis of C l o 0 . C1 P 0 = 0.6 t o r r , A r g 0 n = 2 0 0 t o r r , E = 1325 J. £ d VO 120 c o e f f i c i e n t i s q u i t e s m a l l . A s m a l l c o r r e c t i o n was made t o c a l c u l a t e t h e d e c r e a s e i n t h e C 1 2 0 o r f o r m a t i o n o f CIO by c o m p a r i n g i t w i t h t h a t when Cl.,0 was f l a s h e d a l o n e a t t h e same c o n d i t i o n s (see f i g . 2 6 ) . The measurement o f CIO was c a r r i e d o u t i n t h e same manner as d e s c r i b e d i n C h a p t e r I I I , s e c t i o n 2. S i n c e t h e r a t e o f r e -c o m b i n a t i o n o f c h l o r i n e atoms i s q u i t e s m a l l d u r i n g t h i s p e r i o d , a l l o f t h e c h l o r i n e atoms p r o d u c e d w i l l r e a c t w i t h C 1 2 0 by t h e r e a c t i o n (19) and t h e r a t e c o n s t a n t c a n be o b t a i n e d f r o m t h e e q u a t i o n d f c i p i = _ d [ C . l 2 Q ] = k [ c l ] [ c l 0 ] ( j ) d t d t I t was d i f f i c u l t t o c a l c u l a t e k i g d i r e c t l y f r o m t h e C l 9 0 m easurements s i n c e l e s s t h a n 20% o f t h e C 1 2 0 i s decomposed and measurements o f t h e CIO c o n c e n t r a t i o n a r e much more a c c u r a t e t h a n t h a t o f C 1 2 0 . Thus k-^ was c a l c u l a t e d as f o l l o w s . [ C l ] 0 = [ C 1 0 ] q = a [ C l ] t = [CIO] Q - [ C 1 0 ] t = a-x [ c i 2 o ] t = [ c i 2 o ] o - [ C l O ] t = b-x where x i s t h e amount o f [C10J p r e s e n t a t t i m e t and b t h e amount o f C 1 2 0 t o s t a r t w i t h . E q u a t i o n ( j ) c a n be r e d u c e d t o dx = k, Q-(a-x) (b-x) d t i y l o g = (b-a) k 1 9 t + l o g b / a (k) a X 2.303 2600 A 2800 A 3000 A F i g u r e 26. F l a s h P h o t o l y s i s o f C l g O / C l g . a) C l 2 0 = 6 . 3 x 10 M, Argonr200 t o r r , • E r 600 J . b) C1 20= 6 3 x 1 0 ~ 6 M, C l 2 = 137.5 x 10 ^ M, Argon=200 t o r r , E= 600 J . H 10 j 122 b-x l o g was p l o t t e d a g a i n s t t i m e as shown i n f i g . (27) and a a x s t r a i g h t l i n e was o b t a i n e d as e x p e c t e d from 1 e q u a t i o n (k) . • From t h e s l o p e o f t h e l i n e a r p l o t , was c a l c u l a t e d and i s l i s t e d i n t h e T a b l e X I I I . The C l O c o n c e n t r a t i o n , was m e a s u r e d a t two w a v e l e n g t h s . B l a n k s a r e l e f t i n T a b l e X I I I f o r d a t a i n t h e n o n - l i n e a r p a r t o f t h e p h o t o g r a p h i c p l a t e o r when t h e CIO f o r m e d i s t o o s m a l l t o be a c c u r a t e l y m e a s u r e d . The a v e r a g e v a l u e o f s i x m easurements was f o u n d t o be 4.1 t 0.4 x 10^ 1 mole 1 s e c 1 8 — 1 — 1 w h i c h a g r e e s w i t h t h e l o w e r l i m i t o f 4 x 10 1 m o l e s e c 22 c a l c u l a t e d by E.N.T. f r o m t h e f a c t t h a t t h e d e c o m p o s i t i o n o f CI2O was v i r t u a l l y c o m p l e t e i n a p p r o x i m a t e l y , 2 0 0 u s e e . x a J i G A X — — C a l c u l a t i o n o f k ^ o f R e a c t i o n o f C h l o r i n e Atoms w i t h c i 2 o P l a t e no. [C1 20] (10-6 M ) [ c i 2 l ( I O - 6 M) Energy J k19 V ° 2772 A n -1 -1* (1 mole sec ) 2920 A 167 34.1 137.5 260 5.0 — 169 6 3.0 6 8.0. 600 4.0 — 170 35.6 137.5 600 3.9 3.7 171 63.3 137.5 600 — 4.3 173 36 .5 137.5 1060 — 3.8 Average k 1 9 = 4.1 - 0.4 x 10^ 1 mole 1 sec 1 123 124 2) Stage 3 j ' T h i s s e c t i o n w i l l be d i v i d e d i n t o two p a r t s . The f i r s t w i l l d e a l w i t h the f o r m a t i o n of C10 2 and the second w i l l be about the c a l c u l a t i o n of the quantum y i e l d . '; a) Formation of CIO2 The f o r m a t i o n of C10 2 was observed i n agreement w i t h the 49 50 r e s u l t s o b t a i n e d i n the steady s t a t e ' as w e l l i n the f l a s h 22 p h o t o l y s i s of C l 2 0 . The C1C>2 spectrum s t a r t s appearing a t r o u g h l y 1 msec, a t t a i n s a maximum i n t e n s i t y around one second, and remains v i r t u a l l y c o n s t a n t f o r a t l e a s t one minute. The amount of C10 2 formed depends upon the primary p h o t o l y s i s of C1 20. For example u s i n g a q u a r t z r e a c t i o n v e s s e l and h i g h f l a s h energy (1060 J ) , when n e a r l y a l l the C1 20 has been decom-posed w i t h i n 200 t o 300 usee, no C1C>2 was d e t e c t e d . A l s o w i t h o a 3000 A c u t o f f f i l t e r a t low f l a s h energy, when o n l y a v e r y s m a l l amount of decomposition o c c u r s , the c o n c e n t r a t i o n of C10 2 i s a l s o l e s s . T h i s suggests t h a t i t must be a r e a c t i o n between C1 20 and ClO which i s r e s p o n s i b l e f o r the C10 2 p r o d u c t i o n . The 22 r e a c t i o n proposed by E.N.T. accounts s a t i s f a c t o r i l y f o r the f o r m a t i o n of C10 2. C1 20 + CIO -»• C10 2 + C l 2 (20) T h i s i s not the o n l y r e a c t i o n of CIO r a d i c a l s w i t h C l 2 0 because i t has been observed t h a t the r a t e of f o r m a t i o n of C10 2 i s 125 slower than the decay of CIO and Cl„0. Thus the C l o 0 must be -• 2 | 2 reacting with CIO by some reaction other than reaction (20) and 22 we agree with the reaction suggested by E.N.T.: I C1 20 + CIO -y C l + C l 2 + 0 2 (21) followed by: C l + c i 2 o + CIO + c i 2 The re s u l t s obtained thus agree q u a l i t a t i v e l y with those of 22 E.N.T. They mentioned the p o s s i b i l i t y that the late appearance of C10 2 might be due to the following equilibrium: c i o + c i o 2 c i 2 o 3 21 This equilibrium was also suggested by L.N.T. i n order to explain the reappearance of C10 2 i n t h e i r photolysis of CIO.,. 40 41 Recently Mchale and Elbe ' have also found the new oxide of chlorine with the formula C1 20 3 i n t h e i r study of C10 2 explosions but we have not observed t h i s i n our system. They also rejected the p o s s i b i l i t y that the late appearance of C10 2 was due to i t s removal at short times by oxygen atoms produced i n the second primary process. Our measurement of the rate constant confirms that oxygen atoms can be neglected at short times. We did not try to look for the ClO^ spectrum because the continuum spectrum of C l 2 0 i s i n the same region as ClO^ and because of the overlap of the successive CIO bands. The r i s e of C10 2 concentration i s shown i n fig.(25-a) and the rate constant for i t s formation has been calculated with j 127 22 t h e e q u a t i o n d e r i v e d by E.N.T. and a l s o by f i n d i n g s l o p e s a t v a r i o u s p o i n t s . D e t a i l s o f t h i s c a l c u l a t i o n w i l l be g i v e n l a t e r . The a v e r a g e v a l u e o f k 2 Q w a s f o u n d t o be 2 . 8 ± 0 . 6 x 1 0 5 - 1 - 1 1 1 m o l e s e c , n e a r l y t h r e e t i m e s l a r g e r t h a n t h a t c a l c u l a t e d by E . N . T . 2 2 22 The s l o w d e c a y o f 0 1 0 2 o b s e r v e d by E.N.T. was e x p l a i n e d b y t h e r e a c t i o n j C 1 0 2 + C 1 2 0 C l 2 + 0 2 + C l O (20-a) S i n c e i n o u r s y s t e m t h e C 1 0 2 c o n c e n t r a t i o n d o e s n o t d e c r e a s e , a t l e a s t f o r one m i n u t e , we n e g l e c t t h i s r e a c t i o n . I t c a n a l s o be r e j e c t e d f r o m t h e f o l l o w i n g e x p e r i m e n t s . M i x t u r e s o f C 1 2 0 and C 1 0 2 were p r e p a r e d i n t h e r a t i o 1:1, 1:1.5 and 1:2 w i t h a t o t a l p r e s s u r e o f 200 mm o f a r g o n i n t h e b l a c k b u l b s . The s p e c t r u m o f e a c h m i x t u r e was t a k e n a f t e r 2, 8, 9.5, 15 and 33 h o u r s and were compared w i t h t h a t t a k e n o f C 1 0 2 . I t was f o u n d t h a t t h e r e i s no d e c r e a s e i n t h e C 1 0 2 and C l 2 0 c o n c e n t r a t i o n . M i x t u r e s w i t h l a r g e r r a t i o s ( i . e . up t o 1:20) were a l s o f l a s h e d and t h e c o n c e n t r a t i o n s o f C 1 0 2 and C 1 2 0 were m e a s u r e d b e t w e e n t h r e e t o f i v e h o u r s l a t e r and s i m i l a r r e s u l t s were o b s e r v e d . Thus i t a p p e a r s t h a t i f t h e r e i s a d e c a y o f C 1 0 2 , i t w i l l be by t h e r e a c t i o n o f c h l o r i n e atoms w i t h C 1 0 2 and n o t b y r e a c t i o n ( 2 0 - a ) . The above q u a l i t a t i v e r e s u l t s c a n be p u t i n t o d i f f e r e n t i a l 22 e q u a t i o n s w i t h t h e same a s s u m p t i o n s as made by E.N.T. b e c a u s e t h e y a l s o h o l d h e r e t o o . They a r e : 128 1) k ^ g i s g r e a t e r than k ^ and much gre a t e r than k^Q and k 2^. Under these c o n d i t i o n s , the c h l o r i n e atom c o n c e n t r a t i o n can be neglected and equation (19) can be added t o equation (21). 2) The r e a c t i o n of c h l o r i n e atoms w i t h C10 2 can be neglected. 3) The percentage change i n the C1 20 c o n c e n t r a t i o n during t h i s time i s s m a l l . Thus from the decay of CIO and C l 2 0 , the formation of C10 2 can be represented as f o l l o w s : ^ _ d[ClO]_ = k [ c l 0 ] 2 + k rcio] [C1,0] (1) dt D U - d [ C l 2 ° ] = k 2 Q [ C l O ] [C1 20] + 2 k 2 1 [ C l O ] [C1 20] (m) dt d [ C l 0 2 ] = k [CIO] [Cl-O] (n) dt Z U From equation (1) the c o n c e n t r a t i o n of CIO at any time can be given by e - k 2 ( ) [ C l 2 0 ] t [ c l 0 ] = k~ - r c i o i k t 1/[C10] Q + 7 (1 - e l c ± 2 U J K 2 0 t : ) k g [ C l 2 0 ] s u b s t i t u t i n g the c o n c e n t r a t i o n of CIO i n t o equation (n), the equation can be i n t e g r a t e d i n t o the f o l l o w i n g form: k20 k 5 [ C 1 0 l - k 9 n [ c i , 0 ] t , [CIO,] = ~7 [C1 90] l n [1 + — (1-e Z U Z ) J ( A ) k 5 k 2 0 [ C l 2 O ] where [C10] q i s the amount of CIO present immediately a f t e r the f l a s h . k 2 Q was c a l c u l a t e d by s u b s t i t u t i n g the amount of C10 2 formed at 1 sec and making successive approximations of k 2Q. ! 129 The average v a l u e of k^Q from t h r e e experiments found t o be 2.8 t 0 . 7 x i o 5 1 m o l e - 1 s e c - 1 as l i s t e d i n T a b l e XIV. The e q u a t i o n (n) was a l s o s o l v e d by f i n d i n g the s l o p e a t d i f f e r e n t times ( f i g . 25-a) and s u b s t i t u t i n g , the amount of CIO and CI2O p r e s e n t a t the same time. The v a l u e s are l i s t e d i n T a b l e XIV and average of f i v e found to be 2.64 _ 0.3 x 1 0 5 1 mole 1 sec 1 , which i s i n e x c e l l e n t agreement w i t h the v a l u e found from the i n t e g r a t e d e x p r e s s i o n . b) Decay of C^O The r a t e c o n s t a n t of the second r e a c t i o n of C l 2 0 w i t h CIO can be c a l c u l a t e d from e q u a t i o n (m) by two methods. 1) The s l o p e s of decay of C1 20 (S 2) and r i s e of C10 2 (S^) were measured a t the same time and s u b t r a c t i n g e q u a t i o n (n) from (m) S 2 " S l = 2 k 2 i [ c l ° 3 [C1 20] K21 was c a l c u l a t e d by s u b s t i t u t i n g the amount of CIO and C l 2 0 p r e s e n t a t t h a t time. The average v a l u e o b t a i n e d f o r k 2 ^ was found t o be 6 .2 - 1.5 x i o 5 l m o l e - 1 s e c - 1 (Table XIV). 2) The two equations were d i v i d e d t o g e t S 2 / S l = 1 + 2 k 2 1 / k 2 0 thus k 2 1 = 1 / 2 ( 8 2 ^ - 1) k 2 Q . The v a l u e s of the s l o p e s S 2 and ( c a l c u l a t e d i n method 1) were s u b s t i t u t e d and the v a l u e s of k ^ thus o b t a i n e d are l i s t e d i n Table XIV. The average v a l u e of k 0 1 thus o b t a i n e d was • 130 i I 5 _ i _ i found t o be 6.8 t 0.9 10 1 mole sec which agrees w i t h the above v a l u e w i t h i n e x p e r i m e n t a l e r r o r . T h e ' o v e r a l l v a l u e of 5 -1 -1 1 k 2 1 = 6 - 5 "- 1.3 10° 1 mole ^ sec -\ The v a l u e o f k ^ found by us i s 10 t o 12 times g r e a t e r 22 ' than t h a t c a l c u l a t e d by E.N.T. . From f i g . (3) of t h e i r paper, the s l o p e s of decay of C 1 2 0 and r i s e o f CIO., were measured. The r a t i o o f the two s l o p e s thus found was s u b s t i t u t e d i n t h e i r equations and the v a l u e of k 2 ^ found t o be 7 to 10 times g r e a t e r than k 2 Q but they found k 2 ^ to be h a l f of ^ Q . In c a l c u l a t i n g t h i s r a t i o from t h e i r paper, i t was assumed by us t h a t f i g . (3) r e p r e s e n t s the a b s o l u t e c o n c e n t r a t i o n . S i m i l a r r e s u l t s w i l l be d e r i v e d from the c a l c u l a t i o n o f quantum y i e l d mentioned i n the next s e c t i o n . T a b l e XIV P l a t e [ C 1 2 0 ] x 1 0 + 6 Energy k 2 Q x i O 5 k 2 1 x_o 5 n ° * (mole/1) J (1 m o l e " 1 s e c " 1 ) ( 1 m o l e - 1 s e c " 1 by Eq.A. Method I Method I I 192 55.0 1325 3.0 2.9 8.0 8.0 • 2.3 5.0 6.2 193 33.0 1325 3.5 3~0 8.2 7.8 2.3 4.9 5.8 2.7 5.0 6.0 191* 52.3 1325 2.0 — — — Average 2.64 6.2 6.8 + 0.3 +1.5 +0.9 - 6.5+1. 3 * Two mm of g l a s s f i l t e r A was p l a c e d between the r e a c t i o n v e s s e l and f l a s h lamp. I 131 '.'' • i . . • . • i 3) Quantum Y i e l d g i n Both S e n s i t i s e d and N p n s e n s i t i s e d Decomposition of Cl^O ' ' • I t i s q u i t e d i f f i c u l t i n our system t o f i n d quantum y i e l d a t d i f f e r e n t wavelengths as i t i s d i f f i c u l t 1 t o f i l t e r the r a d i a -t i o n . S i n c e the e x t i n c t i o n c o e f f i c i e n t of C^O decreases o s h a r p l y above 3400 A and u s i n g two mm of g l a s s f i l t e r A, the r a d i a t i o n which can cause decomposition i n our system i s mostly o o ' • • w i t h i n 300 A of 3000 A. The o v e r a l l quantum y i e l d was c a l c u l a t e d f o r these wavelengths and found t o be 3.6 + 0.2 which agrees 50 w i t h t h a t found by F i n k e l n b e r g e t a l . , i . e . , 3.5 a t 10°C. I f we apply the c o r r e c t i o n f o r the temperature c o e f f i c i e n t , t h e i r v a l u e w i l l become M.O a t room temperature, even then the agreement i s s a t i s f a c t o r y . I t can be seen from the r e a c t i o n scheme t h a t i f r e a c t i o n (18) i s the primary step f o l l o w e d r a p i d l y by r e a c t i o n (19), the change i n the C^O c o n c e n t r a t i o n produced i s the same as i n the case of c h l o r i n e p h o t o s e n s i t i s e d decomposition of C ^O having (18-a) as the primary step and both the c h l o r i n e atoms reacting w i t h C^O. Thus the quantum y i e l d i n both systems should be the same and i t can be seen from T a b l e XV t h a t average v a l u e of the quantum y i e l d o b t a i n e d by c h l o r i n e p h o t o s e n s i t i s a t i o n i s 3.5 + 0.2 which agrees w i t h the above r e s u l t s . The o v e r a l l quantum y i e l d f o r the wavelengths between o 51 2300 to 2750 A was c a l c u l a t e d by Schumacher e t a l . t o be 4.5. 132 They e x p l a i n e d i t by assuming t h a t oxygen atoms do not r e a c t 31 73 w i t h CI2O. But i t was found by us and by P h i l l i p s e t a l . 9 t h a t oxygen atoms r e a c t v i a r e a c t i o n (35) w i t h k 3 r. = 5.3 x 10 1 mole 1 sec 1 or 8.3 x 10^ 1 mole 1 sec ^, r e s p e c t i v e l y . I f r e a c t i o n (22) i s the primary step f o l l o w e d by (19), (35), or (36) r a p i d l y and f o l l o w e d by r e a c t i o n s (20) and (21), the quantum y i e l d s hould have been 5.5. In our' system i t i s q u i t e d i f f i c u l t t o se p a r a t e the r e a c t i o n s (18) and (22), f o l l o w e d by the - c o m p e t i t i o n between (35) and (6), although the o v e r a l l change would have been the same even i f r e a c t i o n (6) i s t a k i n g p l a c e . Thus i t seems more reasonable t o e x p l a i n the s m a l l i n c r e a s e i n quantum y i e l d towards s h o r t e r wavelengths, found by Schumacher e t a l . , ^ 1 t o be due t o a l i m i t e d o c c urrence of r e a c t i o n (22) f o l l o w e d by (19), (35) or ( 6 ) . I t can be s e e n , a l s o , t h a t a f t e r the f i r s t stage of r e -a c t i o n s are over, the quantum y i e l d of the decomposition of CI2O should be the same i n both c h l o r i n e s e n s i t i s e d and d i r e c t p h o t o l y s i s . The formula can be d e r i v e d as f o l l o w s ( d e t a i l s can be seen i n Appendix I ) : c i 2 o = [C l 2 0 ] o [C101 - ( 1 + ^ [C.10] rt c o ~b f c i 2 o i -1/c where b = '20 '20 + 2 k 21 c = k 2 0 + 2 k 2 1 - 1 133 c _ k 5 ~ 2 k 2 1 | K20 ! I [ C l 2 0 ] o = [C1 20] l e f t a f t e r f l a s h and C l + C l 2 0 r e a c t i o n i s f i n i s h e d . [ C I O ] =[C10] a f t e r the f l a s h and C l + C l ~ 0 r e a c t i o n o • 2 i s f i n i s h e d . The quantum y i e l d (cj>) of C l j O decomposition was c a l c u -l a t e d by s u b s t i t u t i n g k<., k 2Q and k 2 ^ determined by us, ^ u s i n g our v a l u e of k,. and k 2 ^ but E.N.T.'s v a l u e of k 2 ^ , cf) 1 1 u s i n g our v a l u e of k,. but E.N.T.'s v a l u e of k 2 Q and and the v a l u e s o b t a i n e d are l i s t e d i n T a b l e XV and a l s o the <f>e (experimental) are i n c l u d e d i n T a b l e XV.. I t can be seen from T a b l e XV t h a t by u s i n g the v a l u e s of k 2Q and k 2 ^ , determined 22 ' by E.N.T., very low v a l u e s of c)> are o b t a i n e d and hence the o v e r a l l quantym y i e l d would be 2.3 + 0.1, but i f we use our v a l u e s , the o v e r a l l quantum y i e l d i s found to be 3.24 + 0.22, which i s low but i n s a t i s f a c t o r y agreement w i t h the experiment T h i s i s f u r t h e r evidence t h a t the v a l u e of k 2 g and k 2 ^ c a l c u -l a t e d by us i s more reasonable than those c a l c u l a t e d by E.N.T. 134 Tabl e XV j C a l c u l a t i o n of Quantum Y i e l d of C1 20 i P h o t o l y s i s P l a t e no. [ C l 2 O ] x l 0 + 6 (mole/1) [ C l 2 ] x l 0 + 6 (mole/1) Energy J F i l t e r 4» + 1 i l l <P 170 35.6 137.5 600 A 3.3 3.6 2.3 2.2 171 63.3 137.5 600 A ;3.5 3.5 2.4 2.3 172 63.3 137.5 1060 A 3.1 3.8 2.2 2.1 173 36.5 137.5 1060 A 2.9 3.3 2.2 2.1 184 57.8 — 830 A 3.2 3.5 2.3 2.2 191 52.3 — 1060 A 3.2 3.5 2.3 2.4 192 55.0 — 1325 - 3.0 3.7 2.7 2.2 193 33.0 - 1325 - 3.6 3.6 2.3 2.2 3.24 3.6 2.3 2.2 Average +0.22 +0.2 +0.1 + 0.15 A = g l a s s f i l t e r A <{> = c a l c u l a t e d by u s i n g our v a l u e o f r a t e c o n s t a n t s cj> = c a l c u l a t e d e x p e r i m e n t a l l y (f)1 = c a l c u l a t e d by u s i n g our v a l u e of k,- and k,^ but E.N.T.'s va l u e o f k 2 ^ 11 <j> = c a l c u l a t e d by u s i n g our v a l u e of k_ but E.N.T.'s v a l u e of k 2 g and k 2^. 135 B. Bromine P h o t o s e n s i t i s e d Decomposition of ClpO 54 Brown and Spinks i n t h e i r study of bromine photosen-s i t i s e d decomposition of C^O found t h a t t h i s system behaves i n the same manner as c h l o r i n e p h o t o s e n s i t i s e d or d i r e c t p h o t o l y s i s . They found t h a t the quantum y i e l d i s 4.3 and t h a t C1C>2 i s formed. The r e a c t i o n s suggested by them are simply: Br + C1 20 -> CIO + B r C l ^ (25) w i t h subsequent r e a c t i o n s of CIO and C1 20 i d e n t i c a l t o those o c c u r r i n g i n the C l 2 / C l 2 0 sys1?em. Thus the r a t e c o n s t a n t k 2 ^ can be measured i n a s i m i l a r way t o t h a t used f o r the C l + C l 2 0 r e a c t i o n . A f u r t h e r important a p p l i c a t i o n of t h i s r e a c t i o n i s i t s use as a method f o r e s t i m a t i n g bromine atom c o n c e n t r a t i o n which i s more a c c u r a t e than measurements of the decrease of B r 2 c o n c e n t r a t i o n . B r 2 and C^O mixtures i n the r a t i o of n e a r l y 1:1 and 1:4 were f l a s h e d a t two f l a s h e n e r g i e s . Two l i g h t f i l t e r s o o 4400 A and 5600 A were used and thus o n l y Br 2 was p h o t o l y s e d . The t o t a l p r e s s u r e used was 200 mm of argon i n a l l the c a s e s . The r e s u l t s o b t a i n e d were s i m i l a r t o those o b t a i n e d i n the C l 2 p h o t o s e n s i t i s e d and i n the n o n s e n s i t i s e d p h o t o l y s i s of C^O. A t y p i c a l run i s shown i n the f i g . (28). The measurement of CIO was c a r r i e d out i n the same manner as d e s c r i b e d i n C h a p t e r l l l , s e c t i o n A-2. I t i s found t h a t CIO c o n c e n t r a t i o n a t t a i n s i t s maximum v a l u e w i t h i n 200 usee and then s t a r t s decaying s l o w l y . No o t h e r s p e c t r a i n the 136 o o range of 2300 A t o 4500 A were observed d u r i n g the f i r s t 500 ysec, i n p a r t i c u l a r no BrO spectrum. The C10 2 spectrum s t a r t s appearing a t approximately 1 msec, a t t a i n s i t s maximum v a l u e a t 1 sec and then decreases s l o w l y . I t appears from the r e s u l t s t h a t a r e a c t i o n scheme s i m i l a r t o C1 2/C1 20 can be w r i t t e n i n t h i s case t o o . B r 2 + hv -»- 2 Br (23-a) Br + C1 20 -> CIO + B r C l (25) The subsequent r e a c t i o n s of CIO and C1 20 are the same as d i s c u s s e d b e f o r e i n the d i r e c t p h o t o l y s i s . The bromine atoms i n v o l v e d i n t h i s r e a c t i o n are presum-a b l y ground s t a t e because the p r e s s u r e of argon and bromine used are enough f o r the quenching of e x c i t e d atoms. Quenching r a t e s of e x c i t e d bromine atoms, c a l c u l a t e d by Donovan and 8 0 Hussain are used. The subsequent r e a c t i o n s of CIO and C1 20 are the same as d i s c u s s e d b e f o r e i n the d i r e c t p h o t o l y s i s . T h i s i s confirmed from the f o l l o w i n g r e s u l t s . 1) The behaviour of C10 2 i s s i m i l a r t o t h a t observed i n the case of n o n s e n s i t i s e d experiments though a d e t a i l e d study was not c a r r i e d out. 1 2) [CIti] was p l o t t e d a g a i n s t time and the s l o p e s were c a l c u l a t e d . Combining them w i t h the e x t i n c t i o n c o e f f i c i e n t ( c a l c u l a t e d p r e v i o u s l y ) , k,- was c a l c u l a t e d and the average found t o be 3.0 * 0.5 x 10^ 1 m o l e - 1 s e c - 1 . Although t h i s v a l u e i s h i g h e r than t h a t found i n case of CIO- p h o t o l y s i s • ' '. ~, F i g u r e 2 8 . p i a s h P h o t o l y s i s o f bromine i n tEte presence of C1 20. C l 2 0 = 2 4 . 4 x 10 6 , Argon = 2 0 0 t o r r , Br2~ 2 7 . 5 x 10 6M, E = 1 0 6 0 J, F i l t e r 4400 2 6 0 0 A 2 8 0 0 ! 3 0 0 0 1 1 1 i i M i l l • U M T . 1 1 H i l l 1 I I 1 m id J m i l A Blank Blnak Before 10 A s e c 2 0 4 o 71 1 2 0 1 6 6 24l 4 3 0 6 3 0 Blank 1 . 0 2 msec 2 . 7 3 4 . 3 2 6 . 1 10 2 2 55 A f t e r Blank ci 2o 1 5 , 0 10,0 7 , 0 C10 138 (2.65 0.29 X 10 i l mole sec ), the d i f f e r e n c e i s not enough . t o r e q u i r e the i n c l u s i o n of the f o l l o w i n g r e a c t i o n B r C l + CIO -> BrO + C l 2 w h i c h i s o n l y one K c a l e n d o t h e r r a i c . 3) S i m i l a r t o t h e c h l o r i n e p h o t o s e n s i t i s e d p h o t o l y s i s o f C 1 2 0 , t h e o v e r a l l quantum y i e l d s f o r t h e d e c o m p o s i t i o n o f C1 20 by bromine atoms were c a l c u l a t e d . The average o f s i x measurements was 3.7 + 0.3, r e g a r d l e s s o f t h e r a d i a t i o n used. T h i s v a l u e i s i n s a t i s f a c t o r y agreement w i t h t h e v a l u e found by 54 Brown and S p m k s , and a l s o t h a t c a l c u l a t e d from the. C l 2 s e n s i t i s e d and t h e n o n - s e n s i t i s e d p h o t o l y s i s o f C l - 0 . s i m i l a r t o t h a t s e n s i t i s e d by c h l o r i n e and t h e o n l y e x t r a s p e c i e s , B r C l , p r e s e n t i n t h i s system does not i n t e r f e r e w i t h t h e r e a c t i o n scheme. C a l c u l a t i o n o f Rate C o n s t a n t of Bromine Atoms w i t h C1„0 The r a t e c o n s t a n t of bromine atoms w i t h C l ~ 0 was measured i n a s i m i l a r way as- t h a t f o r t h e r e a c t i o n of c h l o r i n e atoms w i t h C 1 2 0 , i . e . , from th e r a t e of f o r m a t i o n o f CIO r a d i c a l s . The r a t e e q u a t i o n can be w r i t t e n as The above r e s u l t s show t h a t t h e r e a c t i o n mechanism i s •2-2 d [ C l 2 0 ] d[CIO] = k 2 5 [ B r ] [C1 20] d t d t I f [ C 1 0 ] Q = a [CIO] - [ C 1 0 ] t = a - x 139 [ c i 2 o ] t = [ C 1 2 0 ] Q - [ C 1 0 ] t = b - x t h u s •.=i£=2L. = J * _ = k ( a - X ) ( b - x) d t d t " , b - x (b - a ) k 2 5 . . T b or l o g = t + l o g — a - x 2.303 a b ~" X i l o g was p l o t t e d a g a i n s t t i m e as shown i n f i g . ( 2 9 ) and a - x j f r o m t h e s l o p e o f t h e s t r a i g h t l i n e as e x p e c t e d f r o m t h e e q u a -t i o n , t h e v a l u e o f k 2 ^ was c a l c u l a t e d and i s l i s t e d i n T a b l e X V I . The CIO c o n c e n t r a t i o n was m e a s u r e d a t two w a v e l e n g t h s o o (2772 A and 2920 A ) . The a v e r a g e o f e l e v e n m easurements was + 8 —1 —1 f o u n d t o be 6.1 1 0.6 x 10 1 mole s e c T a b l e XVI P l a t e E n e r g y [ C 1 2 0 ] x i o [ B r 2 ] x l O F i l t e r k * 1 0 _ r no: J (mole/1) (mole/1) A ( I m o l e ~ x s e c 0 2772 A 2920 A 257 1060 25.7 20.6 4400 6.7 6.1 258 1060 24.4 27.5 4400 6.5 6.2 259 1060 39.6 41.3 4400 — 6.2 260 600 36.3 41.3 - 4400 6.3 5.5 261 600 23.4 20.6 4400 6.6 6.4 296 1060 39.2 137.5 5600 5.4 5.0 A v e r a g e k^g = 6 . l l o . 6 x 1 0 8 1 , -1 mole s e c ~ x F i g u r e 29. A p l o t of log(a-x/b-x) against time. b = [ci0] a = [ci 0] C l 0=24.4 x 10 ° M, Br^ r27.5 x 10 , Argon =200 t o r r . > Xr[ci0]. O 1 E --IO60 J , F i l t e r 4400 A. 2.0 1 1 1 | 1.5 — — 1.0 -0.5 — 0 1 1 1 1 0 20 40 60 80 100 time ( /Asec) C. Chlorine' P h o t o s e n s i t i s e d Decomposition of CIO? Mixtures of CIO2 and C l 2 i n r a t i o s v a r y i n g from 1:15 to 1:35 were f l a s h photolysed using energies 160 and 260 J . The argon pressure was 200 t o r r i n a l l the; experiments and 2 mm of g l a s s f i l t e r A was used t o prevent the p h o t o l y s i s of CIO. The purpose of using low f l a s h energies was t o reduce the primary ' p h o t o l y s i s of CIO2 and thus to avoid the r e a c t i o n 1 O + CIO -»• C l + 0 2 (6) The r a t i o of CI2 to CIO2 was kept high i n order to have enough c h l o r i n e atoms to produce a reasonable decrease i n CIO2 con-c e n t r a t i o n by the r e a c t i o n C l + C10 2 -v 2 CIO (8) because the e x t i n c t i o n c o e f f i c i e n t of c h l o r i n e i s much sm a l l e r than C10 2. The main d i f f i c u l t y i n t h i s system i s t h a t both CK>2 and CI2 absorb i n the same re g i o n and hence both w i l l be photolysed. Thus i n order t o f i n d the amount of C10 2 decom-posed by c h l o r i n e atoms, the r e s u l t s of the p h o t o l y s i s of CIO2 w i t h and without c h l o r i n e were compared. The decay of C10 2 w i t h and without c h l o r i n e i s shown i n f i g . ( 3 0 ) ( p l a t e 196) and f i g . (31) . I t can be seen from f i g . (31) t h a t a f t e r approximately 200 t o 300 usee, the C10 2 remains constant over the p e r i o d of our r e a c t i o n time (20 msec), i n d i -c a t i n g t h a t a l l the r e a c t i o n s i n v o l v i n g C10 2 are over w i t h i n t h i s p e r i o d . The decrease i n the CIO- c o n c e n t r a t i o n or the -Comparison of the behaviour of ClO^ i n the absence and presence of C l ^ . ~6 a) C10 2=4.0 x 10 M, Argon = 200 t o r r , E=260 — ft ~f\ b) C 1 0 2 = 4 . 0 x 10 M, Cl2=66.0 x 10 M, Argon =200 t o r r , E — 260 J. i Blank Before 5.0 /isec 10.6 20 29 40 71 120 201 431 Blank 630 1.02 msec 2-73 4.32 6.1 10 20 3515 A 3434" J ^ l 0 1 0 2 F i g u r e 31. Decay of C10 2 i n the absence and presence of C l p . -6 _ f-C10 2= 4.0 x 10 M, C l 2 = 6 6 . 0 x 10 M, Argon = 200 t o r r , E - 2 6 0 J . i — i — i " r ^0 0.1 0.2 0.3 0.4 . 0.5 time ( msec) 144 c o n c e n t r a t i o n of c h l o r i n e atoms formed, was c a l c u l a t e d from f i g . ( 3 1 ) and compared w i t h the c o n c e n t r a t i o n of c h l o r i n e atoms formed c a l c u l a t e d by the t i t r a t i o n of c h l o r i n e atoms w i t h N0C1. The agreement between the two values was s a t i s f a c t o r y (^10 t o 15%). The decomposition of C10 2 or amount of c h l o r i n e formed was a l s o checked by measuring the c o n c e n t r a t i o n of CIO, w i t h and without C l 2 present. These values were compared w i t h the values obtained from the decrease i n the C10 2 c o n c e n t r a t i o n . The values are l i s t e d i n Table XVII and the agreement i s q u i t e good. A comparison of CIO formed w i t h and without C l 2 i s g i v e n i n f i g s . (32) and (33). 1) •Stoichiometry of the Reaction The r e s u l t s of the above experiments can be e x p l a i n e d i n terms of the f o l l o w i n g r e a c t i o n s . In the absence of c h l o r i n e C10 2 + hv -*• CIO + 0 (27) O + C10 2 -*• CIO + 0 2 (7) CIO + CIO C l 2 + 0 2 (5) The r e a c t i o n s 0 + CIO C l + 0 2 (6) C l + C10 2 -*• 2C10 (8) 145 Figure 32. R i s e and Decay of CIO when C10 2 i s flashed w i t h and without Clp. a) C10 2= 4.0 x 10~ 6 M, Argon= 200 t o r r , E r 260 J . b) C10 2r 4.0 x i o " 6 M, C l 2 - 66.0 x 10 6 M, Argonr200 t o r r , E- 260 J . 2750 2850 A Blank 2750 I 2850 A [T2 f ,0 1 „ 1 ,0 I Before I 5 M-sec 1 10.6 20 29 40 71 120 201 431 Blank 630 1.02 msec 2.73 4.32 6.1 10 20 T270 147 c o n t r i b u t e l e s s than 5% t o the o v e r a l l decay of oxygen atoms. In the presence of c h l o r i n e , the f o l l o w i n g r e a c t i o n s may occur: C l 2 + hv + 2 C l C l + C10 2 + 2 CIO (8) C l + CK> 2 C l 2 + 0 2 (8-a) 0 + C l 2 -* CIO + C l (38) The recombination of c h l o r i n e atoms can be neglected because 29 30 i t i s too slow i f the r a t e constant given by Bader e t a l . ' 79 • and L i n n e t et a l . , AS used. I f c h l o r i n e atoms r e a c t w i t h C10 2 v i a r e a c t i o n (8-a), there would not be any i n c r e a s e i n the ClO c o n c e n t r a t i o n when i t i s f l a s h e d i n the presence of c h l o r i n e . I f r e a c t i o n (8) i s predominant, there w i l l be 2 CIO formed f o r each molecule of C10 2 decomposed. Otherwise, the amount of ClO formed would vary between 0 to 2 times the change i n the C10 2 c o n c e n t r a t i o n , depending upon the r e a c t i o n which i s dominant. I t can be seen from Table XVII t h a t the amount of CIO formed due to the reac-t i o n of c h l o r i n e atoms i s approximately 10% more than the amount t h a t would have been obtained by r e a c t i o n (8). Taking t h i s as an experimental e r r o r , i t i s c l e a r t h a t the s t o i c h i o -25 metry of the r e a c t i o n i s two. Clyne and Coxon have found the s t o i c h i o m e t r y t o be 1.9 1. 0.1 and the agreement i s s a t i s -f a c t o r y between the two v a l u e s . On these grounds, r e a c t i o n (8-a) can be neglected, the c o n t r i b u t i o n probably being l e s s I ' 148 i t h a n 5% o f r e a c t i o n (8) . j A l t h o u g h a 10% d i f f e r e n c e i n t h e c o n c e n t r a t i o n o f C l O c a n be r e g a r d e d as w i t h i n e x p e r i m e n t a l e r r o r much o f i t c a n be e x p l a i n e d by t a k i n g i n t o c o n s i d e r a t i o n t h e ' r e a c t i o n ( 3 8 ) , i . e . , e a c h o x y g e n atom r e a c t i n g w i t h a Cl^ m o l e c u l e w i l l g i v e two 7 - 1 - 1 e x t r a CIO r a d i c a l s . T a k i n g k^g = 5.0 x 10 1 mo l e s e c , c a l c u l a t e d by C l y n e and C o x o n ^ and b y N i k i and W e i n s t o c k , 8 ^ i t h e d i f f e r e n c e was r e d u c e d t o 5%. I t seems l i k e l y t h a t r e a c t i o n (38) d o e s o c c u r t o some e x t e n t t h o u g h i t i s n o t v e r y i m p o r t a n t i n o u r s y s t e m . T a b l e X V I I Amount o f C l O Formed w i t h and W i t h o u t C h l o r i n e P l a t e no. E n e r g y J [ c i o 2 ] [ c i 2 ] [CIO] A w i t h o u t [C1-] B ^ [ C 1 0 ] w i t h C l ~ C D 196 260 4.0 66.0 2.48 2.62 4.46 4.65 197 260 2.65 41.3 1.58 1.66 2.82 3.07 198 160 4.0 137.5 1.68 1.75 3.96 4.47 198' 160 4.0 137.5 1.68 1.75 3.96 4.3 200 160 4.0 87.5 1.68 1.75 3.02 3.24 N o t e : A l l c o n c e n t r a t i o n s a r e m e a s u r e d i n 10 m o l e / 1 CIO c o n c e n t r a t i o n i n columns A and C a r e c a l c u l a t e d f r o m t h e d e c r e a s e i n t h e C 1 0 2 and i n co l u m n s B and D f r o m t h e d i r e c t measurement o f CIO. 1 149 I 2) C a l c u l a t i o n o f k g j T h e r a t e c o n s t a n t k g c a n b e c a l c u l a t e d e i t h e r b y f o l l o w i n g t h e d e c a y o f C K > 2 o r b y t h e i n c r e a s e i n t h e C I O l c o n c e n t r a t i o n . I t w i l l b e s e e n t h a t i n b o t h t h e m e t h o d s c e r -t a i n a p p r o x i m a t i o n s h a v e t o b e u s e d b e c a u s e t h e d i f f e r e n t i a l e q u a t i o n s o b t a i n e d a r e d i f f i c u l t t o s o l v e . F r o m t h e C I O , , M e a s u r e m e n t j T h e r a t e o f t h e r e a c t i o n s a f t e r t h e p h o t o l y s i s c a n b e w r i t t e n a s - d [ C l ° 2 ] = k 7 [ 0 ] [ C 1 0 2 ] d t w i t h o u t c h l o r i n e a n d w i t h c h l o r i n e d [ C 1 0 2 ] = k ? [ 0 ] [ C 1 0 2 ] + k 8 [ C l ] [ C 1 0 2 ] ne d t g l e c t i n g r e a c t i o n ( 6 ) i n b o t h c a s e s . S i n c e 98 % o f o x y g e n a t a t o m s h a v e b e e n r e a c t e d v i t h C 1 0 2 w i t h i n 60 ju.sec, when C 1 0 2 i s f l a s h e d a l o n e , t h e d i f f e r e n c e i n t h e k 7 [ 0 ] [ C 1 0 2 ] t e r m o f t h e two e q u a t i o n s b e c o m e s s m a l l a f t e r t h a t t i m e . T h u s t o t h e f i r s t a p p r o x i m a t i o n b y s u b r a c t i n g f i r s t e q u a t i o n f r o m t h e s e c o n d d [ A C I O . ] -. £_ = k 8 [ c i ] [ c i o 2 ] (o) d t w h e r e A [ C 1 0 2 ] i s t h e d i f f e r e n c e o f C 1 0 2 c o n c e n t r a t i o n d e c o m p o s e d w i t h a n d w i t h o u t c h l o r i n e a t o m s . T h e c h l o r i n e a t o m c o n c e n t r a t i o n c a n b e c a l c u l a t e d a s f o l l o w s 150 [ C l ] o = ( t C 1 0 2 ] a - [ C 1 0 2 ] J „, = A [ C 1 0 2 ] ro where [ C l 0 2 ] a = amount of C10 2 present at °° without C l 2 [ C l 0 2 ] k = amount of C10 2 present at 0 0 w i t h C l 2 and [ C l ] t = . A[C10 2] o o - A [ C i a 2 J t Thus equation (o) can be w r i t t e n as d [ A C l 0 2 ] = kg (A[C10 2] o o - A [ C 1 0 2 ] t ) [C10 2] (p) dt This equation was solved as f o l l o w s . The decay curve of C10 2 was d i v i d e d i n t o s m a l l segments of 10 usee i n t e r v a l s . Taking the average value of C10 2 between the two successive i n t e r v a l s and assuming t h i s t o be a constant, the equation (p) was i n t e g r a t e d . - l n ( A [ C 1 0 2 ] o o - A [ C l 0 2 ] t ) = k g [ C 1 0 2 ] a v t - In ( A [ C 1 0 2 ] J (q) In t h i s way kg was c a l c u l a t e d by computing the value of each species a f t e r time i n t e r v a l of 10 usee a f t e r the i n i t i a l 60 usee when the r e a c t i o n of oxygen atoms w i t h C10 2 was almost complete. The values thus obtained are l i s t e d i n Table XVIII + 9 - 1 - 1 and average of kg = 5.1 _ 0.81 x 10 1 mole sec was obtained. Although the use of t h i s equation r e s t r i c t s the p e r i o d over which the r e a c t i o n i s f o l l o w e d to one where measure-ments are d i f f i c u l t , the standard d e v i a t i o n of 20 determinations i s q u i t e s m a l l . 1 151 k Q was a l s o c a l c u l a t e d by using the simple procedure 8 i of measuring the h a l f l i f e of r e a c t i o n (8). A f C l C ^ ] ^ was measured and the time t o reach ^ - A t C l C ^ J ^ was found from f i g . (31), Using the simple r e l a t i o n j kg = ' 6 9 (r) tt/l [C10 2] av k_ was c a l c u l a t e d . [C10 o] was the c o n c e n t r a t i o n of C10. o z. av z. between the t = 0 and t = ti / ^ . The values of i k g obtained i n t h i s way are l i s t e d i n Table XVIII and the average found to be + 9 - 1 - 1 5.1 _ 0.6 x 10 1 mole sec , i s i n agreement w i t h t h a t obtained above. From the Measurement of CIO A s i m i l a r procedure was used f o r c a l c u l a t i n g kg by measuring the CIO c o n c e n t r a t i o n . 1) By Measuring Half L i f e . A s i m i l a r method was used f o r c a l c u l a t i n g the time taken t o reach h a l f of the c o n c e n t r a t i o n of CIO produced by r e a c t i o n (8). Using equation (r) , kg was c a l c u l a t e d and the average of kg was found t o be 4.5 j l 0.4 x 10^ 1 mole 1 sec 1. The other values are l i s t e d i n Table XVIII and the value obtained agrees w i t h t h a t c a l c u l a t e d p r e v i o u s l y . 2) -The r a t e of the equation (8) can be w r i t t e n i n terms of CIO as f o l l o w s . d [ A C l ° 3 = 2 k q [ C 1 0 , ] [ C l ] dt y * where A [CIO] i s c o n c e n t r a t i o n produced by c h l o r i n e atoms. The c h l o r i n e atom c o n c e n t r a t i o n can be c a l c u l a t e d as I 152 [C 1 ] G = i <[ClO] b - [ C 1 0 ] a ) o = i A , [ C l O ] o [ClO]^ = t o t a l amount of CIO produced in,presence of C I 2 . [C10] a = t o t a l amount of ClO produced without C l 2 and ! [ C l ] t = - j - (A [CIO] Q - A [ C 1 0 ] T ) thus the above equation can be w r i t t e n as d [ A C 1 0 ] = k p (A[C101 - A [CIO].) ( c l p j dt 8 ° * 2 This equation was t r e a t e d i n the s i m i l a r manner as equa-t i o n (p) , i . e . , by t a k i n g s m a l l i n t e r v a l s o f time and a l s o using the average value [ C 1 0 2 ] at the beginning and end of the i n t e r v a l . Thus i t can be i n t e g r a t e d to -In ( A [ C 1 0 ] Q - A [ C 1 0 ] T ) = k G [ C 1 0 2 ] a v t - l n ( A [ C 1 0 ] Q ) (s) Thus kg was c a l c u l a t e d from the ClO c o n c e n t r a t i o n 4- 9 measurements and the average value found t o be 5.2 I 0.7 x 10 1 mole 1 sec 1 , 'agrees w i t h the values obtained p r e v i o u s l y . 9 The average of a l l the values was found to be 5.0±0.7 x 10 1 mole x sec x . This value i s n e a r l y 10 times g r e a t e r than the 25 lower l i m i t found by Clyne and Coxon. 153 Table XVIII C a l c u l a t i o n o f k 8 of the R e a c t i o n C l + c i o 2 = 2 CIO P l a t e no. Energy J [cio 2] (10" 6M) t c i 2 ] (10~6M) kg x 10 (1 mole" sec ) From CIO F rom C10 2 = n ( r ) = n ( s ) = n ( r ) = n ( q ) 196 260 4.0 66.0 4.0 ! £.9 (4) I 4.7 5.0(4) 197 260 2.65 41.3 4.6 5.3(3) 5.0 4.8(4) 198 160 4.0 137.5 4.5 5.1(3) 4.5 5.4(4) 198' 160 4.0 137.5 — 4.9(5) 5.5 5.1(5) 199 160 2.65 66.0 — —• • 5.8 4.7(5) 200 160 4.0 87.5 4.9 5.7(5) 5.5 5.6(5) Average 5. IS t.33 5._.G> Mean kg = 5.a _ 0.7 x 10 9 1 mole •1 -1 sec Note: Numbers i n parentheses show the nos. of measurements used i n o b t a i n i n g the average of each experiment. D. Bromine and Oxygen System I t has been observed t h a t BrO r a d i c a l s a r e produced when 0 2 / B r 2 i s f l a s h photolysed, 1** but no measurements have been 12 r e p o r t e d on i t s decay. McGrath and N o r r i s h showed t h a t BrO i s a l s o produced when B r 2 i s f l a s h e d i n the presence of ozone: Br + 0 3 •> BrO + 0 2 j : 154 i i 25 T h i s r e a c t i o n has a l s o been s t u d i e d by Clyne and Coxon i n the flow system. They generated bromine atoms by the r e a c t i o n of c h l o r i n e atoms w i t h B ^ . They a l s o found t h a t a second o r d e r ° 1 - + 5 decay w i t h a v a l u e f o r k 3 g / e a t 3383 A of 4.5 _ 0.5 x 10 cm sec x . T h e r e f o r e bromine and oxygen mixtures i n the r a t i o v a r y i n g from 1:80 t o 1:180 were f l a s h e d w i t h f l a s h energy 1325 J , u s i n g a q u a r t z r e a c t i o n v e s s e l , as t h i s system looks s i m p l e r than the one s t u d i e d above. The oxygen p r e s s u r e was v a r i e d from 400 t o 780 t o r r , so no o t h e r i n e r t gas was added as a d i l u t e n t . Experiments were done u s i n g the f r e s h mixture each time and a l s o f l a s h i n g the same mixture r e p e a t e d l y . The spectrum observed "is the same as observed by D u r i e and Ramsay, 1^ as shown i n f i g . (34) and no o t h e r spectrum was o o seen i n the r e g i o n 3000 A to 4000 A. The spectrum of BrO was observed a t the s h o r t e s t d e l a y s used, reaches a maximum at approximately 40 t o 50 ysec and decays f a s t e r than CIO, as 25 has been seen by Clyne and Coxon. The BrO c o n c e n t r a t i o n o was measured at two wavelengths, i . e . , a t 3208 A and a t o 3383 A. The c o n c e n t r a t i o n i n terms of o p t i c a l d e n s i t y i s g i v e n i n Table XIX. I t can be seen t h a t the amount of BrO formed i n c r e a s e s w i t h the i n c r e a s e of oxygen p r e s s u r e , whereas i t remains c o n s t a n t i f the c o n c e n t r a t i o n of bromine i s changed. T h i s i n d i c a t e s t h a t i t i s the oxygen which i s more important i n the f o r m a t i o n of BrO and not bromine. F i g u r e 3 4 . R i s e and Decay o f BrO. B r 2 = 5 . 0 t o r r , 0 2 = 4 0 0 t o r r , E = 1 3 2 5 J . 3 iqo 3200 3 3 9 0 34,00 3 ^ 0 0 A Blank Before 5 JUL sec 10 2 0 40 7 1 1 2 0 1 6 6 241 3 1 2 4 3 1 530 630 1 , 0 2 0 Blank 10,0 5 , 0 1 , 0 i 156 l/[BrO] i n terms o f o p t i c a l density^was p l o t t e d a g a i n s t time as shown i n f i g . (35) and p l o t s were found t o be good i s t r a i g h t l i n e s . The s l o p e was measured and the k^g/e thus c a l c u l a t e d are l i s t e d i n Tabl e XIX. The average v a l u e s o f ° , 5 nine measurements a t 3208 A was found t o be (5.4 I 0.9)x 10 -1 ° cm sec and the average v a l u e of e i g h t a t 3383 A was 5 —1 ° (4.3 1 0.2) x 10 cm sec . The v a l u e o b t a i n e d a t 3383 A i s 25 m reasonable agreement w i t h t h a t found by Clyne and Coxon (4.5 + 0.5 x 10^ cm sec x ) . Combining t h i s v a l u e w i t h the e x t i n c t i o n c o e f f i c i e n t of BrO c a l c u l a t e d from the r e a c t i o n of bromine atoms wi t h CIO2 ( d i s c u s s e d i n the next s e c t i o n ) , k^g 9 -1 -1 was found t o be 1.25 x 10 1 mole sec , which i s n e a r l y 25 t h r e e times g r e a t e r than t h a t o b t a i n e d by Clyne and Coxon. They c a l c u l a t e d the e x t i n c t i o n c o e f f i c i e n t o f BrO from the measured a b s o r p t i o n due to BrO by s a y i n g the f o l l o w i n g : " I t appeared t h a t the e x t i n c t i o n c o e f f i c i e n t s of BrO a t the intense. band heads a t 3289 and 3383 A are s i m i l a r t o £ ( C l O ) a t 2772 A," 8 _ 1 _ i and o b t a i n e d k^g t o be 4 x 10 1 mole sec From the r e s u l t s we have, i t appears t h a t k^g does not depend on bromine, oxygen o r t o t a l p r e s s u r e . I t i s a l s o independent of whether the f r e s h mixture i s used o r the same mixture i s f l a s h e d r e p e a t e d l y . Thus the decay of BrO resembles the decay of CIO and hence a s i m i l a r mechanism f o r i t s decay may be proposed. BrO. + BrO -> (BrO) _ (BrO) 2 -*• B r 2 + 0 2 F i g u r e 35. A p l o t of 1/QBrO] against time. Br 2=5.0 t o r r , 02~ 400 t o r r , E =1325 J . T a b l e XIX 158 P l a t e n o . [ B r 2 ] ( t o r r ) [o 2 ] ( t o r r ) O.D 4,0 • 8 ,0 D 4 , 0 / D A '8,0 B k-jg/edO^cm s e c 4,0 8,0 241 5.0 400 0.117 0 .103 1.2 1.13 4.1 5.0 243 5.0 500 0.107 0 .1 1.05 1.07 l ] 4.5 7.5 n e g l e c t e d 244 5.0 500 0.14 0 .11 1.3 1.27 4.5 6.3 246 5.0 600 0.14 0 .127 1.13 1.10 4.3 5.5 245 5.0 690 0 .137 0 .136 1.06 1.0 4.5 4.8 245 5.0 780 0.185 0 .15 1.28 1.22 3.4 5.0 242 5.0 400 0.132 0 .11 — 1.2 4.4 6.2 242 4.2 500 — 0 .12 — — 4.6 244 2.8 500 0.115 0 .095 1.26 1.21 4.2 6.0 A v e r a g e 1.18 + .09 1.22 ±.07 4.4 ± 0 . 2 5.4 ± 0 . 9 N o t e : I n c o l u m n A r a t i o s o f D^ Q/^Q Q a r e c a l c u l a t e d by method (1) and i n c o l u m n B by method ( 2 ) . R a t i o s o f O p t i c a l D e n s i t i e s a t 3383 and 3208 A The r a t i o o f t h e o p t i c a l d e n s i t i e s a t two w a v e l e n g t h s was c a l c u l a t e d by two m e t h o d s . 1) The o p t i c a l d e n s i t y was c a l c u l a t e d f r o m t h e l i n e a r e x t r a p o l a t i o n o f t h e s e c o n d o r d e r p l o t a t t=0 and t h u s t h e r a t i o s o b t a i n e d a r e l i s t e d i n T a b l e XIX and t h e a v e r a g e o f e i g h t f o u n d t o be 1.18 ± 0.09. I 159 I 2) The o p t i c a l d e n s i t i e s were measured at each delay and the r a t i o s were c a l c u l a t e d . The average of each e x p e r i -ment was c a l c u l a t e d and l i s t e d i n Table XIX. In t h i s way the average of 70 measurements, made at d i f f e r e n t delays and d i f f e r e n t c o n c e n t r a t i o n s o f BrO gave the value to be 1.22 + 0.07 which i s i n good agreement w i t h t h a t c a l c u l a t e d by procedure (1). The average of these two methods was compared w i t h r a t i o s of k^g/e which was found t o be 1.26 • The agreement i s s a t i s -f a c t o r y and £3333/^3207 = 1 * 2 w a s adopted. E. The Bromine P h o t o - s e n s i t i s e d Decomposition of ClO^ 3 g Spinks and P o r t e r found Cl 2Og as the main product of the bromine p h o t o - s e n s i t i s e d decomposition of C10 2 at 15°C, c h l o r i n e and oxygen being formed a t 30°C. For t h i s reason, Spinks e t a l . " ^ r e j e c t e d the mechanism proposed by Schumacher.^ 2 Br + C10 2 ->• B r C l + 0 2 2 B r C l + B r 2 + C l 2 and suggested t h a t the primary r e a c t i o n i s Br +• C10 2 -> BrO + CIO fo l l o w e d by BrO + C10 2 •+ Br + CK> 3 In e i t h e r case, bromine atoms r a t h e r than e x c i t e d bromine 43 0 molecules are i n v o l v e d s i n c e the quantum y i e l d s a t 3650 A o and 5460 A were found t o be equal. ! 160 44 45 Clyne and Coxon ' s t u d i e d the r e a c t i o n s of bromine i atoms w i t h C10 2 i n a flow system at low (1 ,to 3 t o r r ) t o t a l p r essures. They found a r a p i d r e a c t i o n accompanied by emission of a deep red chemiluminescence from the r e a c t i o n zone which was i d e n t i f i e d as a r i s i n g from the t r a n s i t i o n B r C l ( 3 T T O + ) B r C l ( ' z + ) The same spectrum was observed i n the r a d i a t i v e combination of bromine and c h l o r i n e atoms, accompanied by some weak bands of the c h l o r i n e a f t e r f l o w spectrum. The ab s o r p t i o n spectrum of CIO and BrO could not be detected over a wide range of re a c t a n t p a r t i a l pressures. The p o s s i b i l i t y t h a t the reason f o r absence of BrO and CIO was due t o a r a p i d r e a c t i o n between them was considered but thought u n l i k e l y because of the corresponding s i r e a c t i o n of CIO + CIO, Qn these grounds and because of i t s endothermicity, the r e a c t i o n Br + C10 2 BrO + CIO - 3 Kcals was r e j e c t e d . 3 + The formation of B r C l ( IT ) was, t h e r e f o r e , e x p l a i n e d by the r e a c t i o n Br + C10 2 -> B r C l + 0 2 w i t h the t r a n s i t i o n s t a t e being Br.C10 2 r a t h e r than OClOBr. 3 + The p o s s i b i l i t y t h a t B r C l ( T\Q ) was formed by r a d i a t i v e combin-a t i o n of c h l o r i n e and bromine atoms which might be formed as intermediates i n the r e a c t i o n between bromine atoms and C10 2 was r e j e c t e d because of the absence of the c h l o r i n e a f t e r g l o w . 161 The o v e r a l l r a t e constant at 300°K f o r the r e a c t i o n Br + C10 2 + B r C l + ° 2 ! 3 1 + which i n c l u d e s the formation of both B r C l ( IT + ) and B r C l ( £ ), i o o was measured by f o l l o w i n g the decay of C10 2 at 3515 A under c o n d i t i o n s of equal C10 2 and bromine atom c o n c e n t r a t i o n s . Although the second order p l o t deviated from l i n e a r i t y , the l i m i t i n g slope at zero time gave the value k - 3.1 ± 0.3 x 10 i i -1 ' " I 1 mole sec 45 -Clyne and Coxon a l s o found t h a t the r e a c t i o n between bromine atoms and C10 2, was s e l f propagating a f t e r i n i t i a t i o n 3 and considered a r a o i d r e a c t i o n between B r C l ( IT +) and e i t h e r o C10 2 and B r 2 . B r C l ( 3 i T +) + CIO- B r C l ( ' z + ) + C l + 0 o B r C l ( 3 7 T +) + B r , -» B r C l ( ' E + ) + 2Br In the former case bromine atoms would be regenerated i n the subsequent r e a c t i o n s C l + C10 2 -> 2 CIO C l + B r 2 -»• B r C l + Br C l + B r C l C l 2 + Br A c o n s i d e r a b l e s t a t i o n a r y s t a t e c o n c e n t r a t i o n of c h l o r i n e atoms would be expected. Since no c h l o r i n e a f t e r g l o w was observed, the r e a c t i o n w i t h bromine was p r e f e r r e d . 162 25 In a l a t e r paper, Clyne and Coxon jreported an e x p e r i -ment i n which bromine atoms produced by the r e a c t i o n C l + B r 2 -> B r C l + Br I were reacted w i t h C10 2. In t h i s case the ClO r a d i c a l was i d e n t i f i e d as a product, but again no BrO was observed. I t was a l s o found t h a t BrO r a d i c a l s produced i n the r e a c t i o n of bromine atoms wi t h ozone decayed much f a s t e r than ClO. They concluded t h a t t h e i r e a r l i e r i n t e r p r e t a t i o n of r e a c t i o n between bromine atoms and C10 2 was probably erroneous. Chemiluminescence observed during the decay of CIO i n the absence and i n the presence of bromine was then explained by the r e a c t i o n s 2 CIO -»• C100 + C l C l + B r 2 -»• B r C l + Br C l + C100 C1 0( 3TT +) + 0 o + 46 Kcals 2 O 2. Br + C100 -»• BrCl ( 3 T r +) + 0 9 + 45 Kcals I t should be noted, however, t h a t the r e a c t i o n s of bromine atoms w i t h both C10 2 and C100 are i n s u f f i c i e n t l y exo-thermic to account f o r the degree of e x c i t a t i o n observed 25 -(57 K c a l s , v' = 8 ) . Clyne and Coxon noted t h i s f o r the r e -a c t i o n of bromine atoms w i t h C10 2, but the r a t e constant obtained allowed an a c t i v a t i o n energy of up t o 5.5 K c a l s , whereas only 4 Kcals were needed to account f o r the energy of 3_ 25 the h i g h e s t l e v e l of B r C l ( TT 0 +) observed. I f Clyne and Coxon's value f o r D Q(C100) of 7 Kcals i s accepted, the discrepancy of 7 Kcals f o r the r e a c t i o n (14) i s r a t h e r harder to b e l i e v e . In view of the r a t h e r u n c e r t a i n s i t u a t i o n which seems t o e x i s t w i t h respect to the r e a c t i o n of bromine atoms and CIO2/ a p r e l i m i n a r y r e i n v e s t i g a t i o n of t h i s r e a c t i o n was undertaken. S Mixtu r e s o f CIO,, (^0.6 t o r r ) and B r 2 i n the r a t i o s of 1:16 to 1:50 w i t h excess argon or n i t r o g e n were f l a s h photo-l y s e d w i t h an energy of 1060 J . To prevent the d i r e c t p h o t o l y s i s of C10 2, Corning 3-72 and 3-66 f i l t e r s were used w i t h c u t - o f f o o wavelengths of 4400 A and 5600 A. Separate experiments without bromine showed t h a t no d e t e c t a b l e p h o t o l y s i s of CIO,, d i d i n f a c t occur under these c o n d i t i o n s . Since mixtures of CIO,, and B r 2 were found to r e a c t on standing, a l l p a r t s of the apparatus were pa i n t e d black or covered w i t h black paper and the e x p e r i -ments were performed under subdued l i g h t i n g . The time allowed f o r mixing the bromine w i t h i n e r t g a s / C l 0 2 mixtures was reduced t o 20 t o 30 minutes from the usual 2 to 3 hours normally allowed i n other experiments. Nevertheless the method used ensured t h a t the r e s u l t i n g mixtures were homogeneous. This was t e s t e d by repe a t i n g c e r t a i n time delays a t the end of the run w i t h i d e n t i c a l r e s u l t s . Under these c o n d i t i o n s no d e t e c t a b l e decomposition of C10 2 occurred on standing over the p e r i o d from the o r i g i n a l mixing to the end of the run. On f l a s h p h o t o l y s i s of the B r 2 / C l 0 2 , there was an i n i t i a l r a p i d removal of C10 2 over a p e r i o d of 200 t o 300 usee. This was f o l l o w e d by a f u r t h e r slow decrease i n the C10 2 concentra-t i o n extending over t>l msec and even up to 45 sec. Complete 164 removal of the C10 2 was achieved only at the higher bromine o pressures and w i t h the 4400 A f i l t e r . Under a l l c o n d i t i o n s the s p e c t r a of CIO and BrO were observed i n a b s o r p t i o n . Both species are produced r a p i d l y , a t t a i n maximum concentrations at 'vlOO usee and then decay q u i t e r a p i d l y over a p e r i o d of ^1 o msec. The c o n c e n t r a t i o n of CIO was measured at 2772 A (12,0), o \ t h a t of BrO at 3208 A (8,0 band). Both are found f r e e from i n t e r f e r e n c e by C10 2 whose c o n c e n t r a t i o n was measured at 3515 o A. The behaviour of a l l three species i s shown i n f i g . (36). 1) The Stoichiometry and Mechanism of the Fast Reaction The decrease i n C10 2 c o n c e n t r a t i o n i n the i n i t i a l r e -a c t i o n was measured by e x t r a p o l a t i o n of the slow decay r e g i o n to zero time. The t o t a l c o n c e n t r a t i o n of CIO and BrO produced ^[C10] q and [Bro] Qjwas measured by e x t r a p o l a t i o n of t h e i r decay p l o t s t o zero time. For these the p l o t s of r e c i p r o c a l o p t i c a l d e n s i t y a g a i n s t time were most s u i t a b l e , being l i n e a r over a wide range ( f i g s . 37 and 38). Under a l l c o n d i t i o n s i t was found t h a t [C10] Q = A[C10 2] I t can, t h e r e f o r e , reasonably be assumed t h a t the o v e r a l l r e -a c t i o n i s represented by the equation Br + C10 2 + BrO + CIO o The o p t i c a l d e n s i t y at 3208 A corresponding to [BrO] , D Q, i s r e l a t e d to the e x t i n c t i o n c o e f f i c i e n t at t h i s wavelength by the 2772 A 3200 3300 3400 • 1 — — 1 — — 12. ^ , 0 Blank Before l % L s e c 20 40 71 120 201 430 630 530 312 166 5.9 1,020 A f t e r CIO 3500 A r 10,0 8,0 7,0 5,0 4 , 0 C1Q, BrO F i g u r e 36. F l a s h P h o t o l y s i s o f bromine i n the presence of ClOg. C 1 0 2 n 3 . l 6 x 10 6 M, B r 2 = 137 .5 x IO* 6, Argon = 200 t o r r , E =1060 J , F i l t e r 4400 A. equation D D. D o e3208 A o o o [BrO] 1 [ C 1 0 ] n l A[C10 2]1 The values obtained are l i s t e d i n Table XX1, the average of a l l 3 — 1 — 1 measurements from C10 2 was 2.26 1 0.2 x 10 1 mole cm and 3 -1 from the same number using CIO was 2.38 i 0.1 x 10 1 mole cm ^, i s very much higher than the value of 900 1 mole 1 cm 25 used by Clyne and Coxon. I f our value i s i n e r r o r then because of the e q u a l i t y [C10] Q = A f C K ^ ] , the tru e value must be even higher. The value obtained here has been used t o determine the r a t e constants f o r the r e a c t i o n s 2 BrO -*• B r 2 + 0 2 and CIO + BrO -> B r C l + 0 2 2) Decay of CIO and BrO Both CIO and BrO r a d i c a l s were found t o decay more r a p i d l y than would be expected from the known r a t e s of the r e a c t i o n s 2 CIO -> C l 2 + 0 2 (5) 2 BrO •+ B r 2 + 0 2 (39) The d i f f e r e n c e i n decay r a t e i s p a r t i c u l a r l y marked f o r CIO and over the f i r s t 400 ysec, when the BrO and CIO concentrations are comparable. This behaviour can only be explained by the Table XX C a l c u l a t i o n of e(3208) of BrO P l a t e no. [ c i o 2 ] ( 1 0 ~ 6 M L . [ B r 2 ] (10 6M) P T t o r r F i l t e r o A E J A[C10 2] (10 _ 6M) [CIO]=[BrO] (10-6M) e x 10 (1 mole 1 A 3 cm 1) B 248 3.21 82.5 200 (N 2) 4400 1060 1.51 I 1.42 2.48 2.35 249a 2.62 137.5 200(N 2) 4400 1060 — 1.44 2.33 — 250 3.16 137.5 200 4400 1060 1.96 1.72 2.42 2.12 251 2.54 41.3 200 4400 1060 0.99 1.06 2.49 2.67 252a 3.00 137.5 100 4400 1060 2.1 1.74 2.34 2.00 252b 2.96 137.5 500 4400 1060 1.4 1.28 2.50 2.29 253 3.7 82.5 200 4400 1060 1.7 1.60 2.45 2.31 254 3.49 137.5 200 5600 1060 1.0 0.92 2.35 2.16 255 3.95 220 200 5600 1060 1.25 1.18 2.30 2.18 256a 3.30 137.5 500 5600 1060 1.0 0.92 2.26 2.10 256b 3.1 137.5 100 5600 1060 1.0 1.04 2.22 2.40 Mean e3208 = 2.32+.0 .15 x 103 1 mole" 1 - 1 xcm Average 2.38+0.09 2.26+0.2 Note: a = from [CIO] measurements; b = from [C10 2] measurements. F i g u r e 37 . A p l o t of 1/jjDlcQ against time f o l l o w i n g the f l a s h p h o t o l y s i s of bromine i n the presence of CIO . C10 2 = 3 . l 6 x 10 ° M, Br =-137.5 x 10 M, Argon =200 t o r r , E =106o J , F i l t e r 4400 A. 1.0 w •P •H c as •P •H rO U as 0.8 0.6 o i—i 0.4 0.2 _ J I I I I 0 . 2 0 .4 0 .6 time ( msec ) 169 r e a c t i o n CIO +. BrO •* B r C l + 0 2 + 52 K c a l (40) For CIO the other p o s s i b l e r e a c t i o n s are CIO + B r 0 -> BrO B r C l - 1 K c a l (42) CIO + Br -»• BrO +. C l K c a l (41) CIO + Br -> B r C l + 0 - 1 1 K c a l (41-a) Since the l a s t two, r e a c t i o n s of CIO w i t h bromine atoms are too endothermic, they are not considered. Reaction (42) can a l s o be r e j e c t e d because the r a t e of ClO decay produced i n the bromine p h o t o s e n s i t i s e d decomposition of C1 20 i s normal, and no BrO was observed. F u r t h e r , s i n c e [Br 2]>> [ClO], the f i r s t order p l o t f o r the decay of CIO should be l i n e a r w i t h a slope p r o p o r t i o n a l t o the bromine c o n c e n t r a t i o n . As seen from f i g . (37-a), t h i s i s not so. Thus the decay of CIO can be represen ted as - d [ C 1 0 ] = k . t C I O ] 2 + k. n[BrO] [CIO] d t 3 4 U -a[cio]-1 = v + k 4 n -0*21 (o) d t b 4U [ c l 0 ] ^ I f we assume t h a t [BrO]/[C10] r a t i o remains constant ( i . e . , 1) equation (o) can be w r i t t e n as a r c i o ] - 1 = ( k + k ' ) = k ' dt 5 4 0 Figure 37-a. A f i r s t order p l o t of CIO f o l l o w i n g the f l a s h p h o t o l y s i s of bromine i n the presence of C10 2. C10 2= 3.16 x 10~ 6 M, B r 2 = 137.5 x 10" 6 M, Argon= 200 t o r r , E= 1060 J , F i l t e r s 4400 % . 0.5 0.4 0.3 o rH t*> 0.2 o 0.1 I - J 1 I 1 1 I I I I l _ 0 200 400 600 800 1000 time (/Asec ) 171 [ C I O ] - 1 was p l o t t e d a g a i n s t time as shown i n f i g . ( 3 7 ) and i t was found t h a t the p l o t s obey a l i n e a r r e l a t i o n up to ^300 t o 400 ysec but then depart from l i n e a r i t y w i t h a s m a l l e r s l o p e , as expected from equation (o). I t can be c l e a r l y seen from f i g . (36) t h a t BrO decay i s f a s t e r than CIO and the r a t i o [BrO]/[C10] w i l l decrease from i t s i n i t i a l value of u n i t y . This r a t i o was c a l c u l a t e d from the experimental data and found t h a t the average value l i e s between 0.8*0.2, w i t h i n the whole p e r i o d of our study, i . e . , 0.630 t o 1.02 msec. The slope was measured from the l i n e a r p a r t and combined w i t h the e x t i n c t i o n c o e f f i c i e n t of CIO, k 1 was c a l c u l a t e d and l i s t e d i n Table XXI. From the r e l a t i o n k' = k 5 + k 4 Q and average value of [BrO]/[ClO] i n the r e s p e c t i v e experiment, k^ Q was c a l c u l a t e d and i s l i s t e d i n Table XXI The average , 9 _ i _ i of 11 measurements gave k^ Q = 1.43 I 0.1 x 10 1 mole sec The other p o s s i b l e r e a c t i o n of BrO can be the reverse r e a c t i o n (16), i . e . , BrO + CIO £ Br + O-Cl-0 + 3 K c a l which i s exothermic by 3 K c a l s . There are, however, two objec-t i o n s t o t h i s r e a c t i o n . F i r s t l y , the CIO2 should i n c r e a s e and secondly, as BrO c o n c e n t r a t i o n i s consumed by r e a c t i o n (39), the e q u i l i b r i u m should s h i f t t o the l e f t and thus the CIO con-c e n t r a t i o n should n e a r l y remain constant as the decay of CIO by r e a c t i o n (5) i s slow. But n e i t h e r C10~ c o n c e n t r a t i o n 172 Fi g u r e 38. A p l o t of l/ p 3 r 0 3 against time f o l l o v r i n g the f l a s h p h o t o l y s i s of bromine i n the presence ' o f C10 2. C10 2 =-3. . l6 x 10" 6 M, B r 2 = l 3 7 . 5 x 1 0 ~ 6 M, Argon =200 t o r r , E= 1 0 6 o J , F i l t e r 4400 A. CO -p £ 1.25 CD CO o •H -P ft O 1.0 0.75 on o r H X & 0 . 5 0 . 2 5 0 0 . 2 0.4' time ( msec ) 0 . 6 I 173 increase s (rather i t decreases slowly) nor the CIO concen-t r a t i o n remains constant during the BrO decay. F u r t h e r , r e a c t i o n (40) i s more s u i t a b l e t o account f o r the production of B r C l , because the r e a c t i o n i s exothermic e x a c t l y to the i 3 extent of the energy of the hi g h e s t l e v e l of B r C l [ TT Q+] observed by Clyne and C o x o n ^ ( i . e . , 18,153 cm x) , whereas 25 Clyne and Coxon reasonably suggested that' the E a f o r the r e a c t i o n (14) was a v a i l a b l e t o make up the 4 .Kcal discrepency between the e x o t h e r m i c i t y and the hi g h e s t l e v e l observed. Thus the BrO decay can be represented by _ djB^O]_ = k^g [BrO] 2 + k^Q [BrO] [ClO] d j B r p T i = k 3 9 + k 4 0 I « 2 L ( p , dt J y 4 U [BrO] This was t r e a t e d i n the same manner as equation (o) and a l/[BrO] p l o t a g a i n s t time gave a s t r a i g h t l i n e i n the beginning but departs from l i n e a r i t y w i t h i n c r e a s i n g slope as expected from the equation (p) s i n c e the ^ C x 0 ^ increases from i t s [BrO] i n i t i a l value of u n i t y . Slopes were measured from the l i n e a r p a r t of curves (fig.38) and the value of k^g was c a l c u l a t e d 9 - 1 - 1 usin g k^g = 1.25 x 10 1 mole sec ( c a l c u l a t e d from Br2/02 system) and using r e s p e c t i v e values of r a t i o , [CIO]/[BrO] the average value of which l i e s between 0.8 j l 0.2. The values are l i s t e d i n Table XXI and the average of k^ Q was found to be , 9 _ i _ i 1.76 I 0.35 x 10 1 mole sec . The agreement w i t h the value c a l c u l a t e d p r e v i o u s l y i s very s a t i s f a c t o r y . The r e l a t i v e l y 174 Table XXI j C a l c u l a t i o n of k. Q of Reaction CIO + BrO •* B r C l + 0 2 P l a t e [C10 2] [ B r 2 ] P T F i l t e r E k 1' k " k 4 Q x 10~ 9 n o - ( 1 0 - % ) (10"6M) t o r r A J (1 mole~ 1sec" 1) from k' from k' 1 248 3 .21 82 .5 200(N 2) 4400 1060 1. 06 4 .16 1. 50 2. 02 249a 2 .62 137 .5 200(N 2) 4400 1060 1. 0 2 4 .24 1. 52 1. 88 250 3 .16 137 .5 200 4400 1060 1. 02 3 .70 1. 22 2. 00 251 2 .54 41 .3 200 4400 1060 1. 28 2 .86 1. 60 1. 25 252a 3 .00 137 .5 100 4400 1060 1. 20 3 .90 1. 43 2. 18 252b 2 .96 137 .5 500 4400 1060 1. 19 4 .05 1. 43 ?. 20 253 3 .7 82 .5 200 4400 1060 1. 07 4 .00 1. 43 2. 00 254 3 .49 137 .5 200 5600 1060 1. 07 3 .28 1. 51 1. 40 255 3 .95 220 .0 200 5600 1060 1. 00 3 .28 1. 35 1. 46 256a 3 .30 137 .5 500 5600 1060 1. 07 3 .33 1. 33 1. 61 257b 3 .1 137 .5 100 5600 1060 _ 2 .88 _ 1. 11 Average 1.43 1.76 ±0.1 +0.35 l a r g e standard d e v i a t i o n s r e f l e c t t h a t k^ g and k^g are very s i m i l a r . The value obtained from the CIO measurements i s th e r e f o r e taken t o be the more accurate measurement and we adopt 9 - 1 - 1 the value of 1.5 x io 1 mole sec f o r k. n. 175 3) Reaction of Bromine Atoms w i t h CIO2 I f the r e a c t i o n of bromine atoms w i t h C10 2 can be w r i t t e n as Br + C10 2 -*• BrO + CIO (16) the r a t e of the r e a c t i o n can be represented as d[C10 2] d[Br] d[C10] = k l g [Br] [C10 2] (r) dt dt dt To t e s t t h i s simple mechanism k^g was determined from the decrease i n the C10 2 c o n c e n t r a t i o n and the inc r e a s e i n the ClO and BrO co n c e n t r a t i o n s . i ) From C10 2 Unlike the C10 2/C1 2 system, C10 2 i n the presence of B r 2 goes On decreasing s l o w l y a f t e r the f i r s t r a p i d decay f o r s e v e r a l msec or sec depending upon the B r 2 c o n c e n t r a t i o n . This i n d i -cated t h a t there are two types of r e a c t i o n s , of which only the f i r s t need be considered here. The decay of C10 2 was p l o t t e d a g a i n s t time as shown i n f i g . ( 3 9 ) and the slow p a r t of the curve was e x t r a p o l a t e d back to zero time i n order t o c a l c u l a t e the amount of C10 2 reacted w i t h bromine atoms. Thus [ B r ] Q = [C10 2] - o - [ C l O ^ = a-b [ B r ] t = [ C 1 0 2 ] t - [ C l 0 2 ] r o = x-b where [C102] o, [ C 1 0 2 ] t and [ C 1 0 2 ] o o are the c o n c e n t r a t i o n of CIO2 a t t = 0, a t time t and t = 0 0, obtained from the l i n e a r e x t r a p o l a t i o n . Equation (r) can be w r i t t e n F i g u r e 39• Decay o f C10 2 f o l l o w i n g the f l a s h p h o t o l y s i s of bromine i n the presence of C10 2. C10 2 = 3 . l 6 x 10 ^ M, B r 2 r l 3 7 . 5 x 10~ 6 M, Argon =200 t o r r , E —106o J , F i l t e r 4400 A. 4 . 0 '2 2.0 CM O H O 0 0 . 1 0.2 0.3 time ( msec ^ 0 . 4 0 .5 I 177 d ^ 1 0 2 J dx . , , « ! = - — = k ,x (x - b) dt d t 1 6 or l o g = a k16 t + l o g ^ 1 <r') 2.303 I l o g — w a s p l o t t e d a g a i n s t time as shown i n f i g . ( 4 0 ) and from the slope of the s t r a i g h t l i n e k^g was c a l c u l a t e d and i s l i s t e d i n Table XXII. The average value of k^ g obtained i s 6.4 ±. 0.9 * 10 9 1 mole 1 sec ^. ! i i ) From CIO and BrO Measurements I f there i s no other r e a c t i o n of bromine atoms w i t h C10 2, producing BrO or CIO except r e a c t i o n (16), the bromine atom c o n c e n t r a t i o n , i n terms of CIO and BrO can be represented as [Br] _ = [C10] Q = [ B r O ] Q = a [ B r ] t = [ C 1 0 ] q - [C10] t = [BrO] Q - [BrO] t = a-x [C10 2] t= [ C 1 0 2 ] o - [ C l O ] t = [ C 1 0 2 l o - [ B r O ] t = b-x 1 The [C10] Q or [BrO] _ are obtained from the ^ - ^ Q J O R — decay p l o t a g a i n s t time at t = 0. The equation (r) [BrO] can be w r i t t e n i n terms of CIO and BrO as dx — = k, f. (a - x) (b - x) dt 1 6 b - x (b - a)k^g b or l o g = t + l o g — a - x 2.303 a b - x l o g was p l o t t e d a g a i n s t time f o r both CIO and BrO as a - x shown i n f i g s . (41) and (42) and k^g was c a l c u l a t e d from the F i g u r e 40. A p l o t of log(x/x-b) against time. b z [ c i O p l , x - [ c i O ? l . . - f i - 6 C10 2= 3 .16 x 10 M, B r 2 = 137.5 x 10 M, Argon =200 t o r r , E =1060 J , F i l t e r 4400 A. 179 F i g u r e 4 l . A p l o t of log(a-x/b-x) against time. a = t C 1 0 2 ] o > b=l C 1°]o> *=[ci°]t-C 1 0 2 = 3 . l 6 x 1 0 ~ 6 M, B r 2 = 1 3 7 . 5 x 1 0 6 M, • Argon = 2 0 0 t o r r , E = 1 0 6 0 J , F i l t e r 4400 A. i tsD O 1 . 0 0 . 8 0 . 6 0.4 0 . 2 0 1 1 1 1 1 1 1 1 2 5 5 0 7 5 time ( yusee ) 1 0 0 125 180 F i g u r e 4 2 . A p l o t o f l o g ( a-x/b-x) a g a i n s t .time. a = & 1 0 2 3 o , .b = [ B r O ] Q , .x = [ B r O J t . —/r C10 2 = 3 . l 6 x 10 M, Br 2 = : 1 3 7 . 5 x 10 6 M, A r g o n =200 t o r r , E =1060 J , F i l t e r 4400 A. 1.0 1 1 1 1 0.75 — (a-x/b-x) 0 . 5 — log 0 . 2 5 — 0 1 I I 1 0 25 50 75 t i m e ( y u s e c ) 100 125 181 slope of the s t r a i g h t l i n e . The values obtained are l i s t e d i n Table XXII. The average value f o r k^g was found t o be 7.7 ± 1.3 x i o 9 1 m o l e - 1 s e c - 1 from CIO and 7.7 ± 1.3 x 10 9 1 m o l e - 1 s e c - 1 from BrO measurements. The values obtained agree w i t h each other and are i n s a t i s f a c t o r y agreement w i t h t h a t from the CIO2 measurement. The o v e r a l l average value was found t o be 7.2 ± '2.0 x i o 9 1 m o l e - 1 s e c - 1 . The d i f f e r e n c e between the C10 2 and (CIO and BrO) values i s probably w i t h i n experimental e r r o r . There are, however, two systematic e r r o r s . F i r s t l y , the e x t r a p o l a t i o n of [CIO-] t o zero time assumes t h a t the l a t e slow r e a c t i o n i s a l s o o c c u r r i n g during the f i r s t 200 t o 3.00 ysec. I f i t i s not, the value of CIO2 a t , say, 300 ysec i s used f o r [ C 1 0 2 ] o o , k^g (C10 2) i s inc r e a s e d by<-->±5% and the r a t i o ofA[cioy^rO] b y ^ l o % . Secondly, the e r r o r i n the c a l c u l a t i o n of k^g from CIO and BrO i s the neglec t of t h e i r decay during the p e r i o d of formation. I n c l u s i o n of co r -r e c t i o n f o r t h i s would i n c r e a s e the value obtained from CIO by ^20 t o 30%. Although t h i s treatment of the r e s u l t s appears t o be s u c c e s s f u l , the value of k^g obtained i s unacceptably high f o r a r e a c t i o n at room temperature endothermic by 3 K c a l s . The p o s s i b i l i t y t h a t e x c i t e d bromine atoms are i n v o l v e d can be r u l e d out because, using the r a t e constant f o r s p i n o r b i t r e -2 80 l a x a t i o n of Br ( Py2) by bromine obtained by Donovan and Hussain, the h a l f l i f e of the e x c i t e d atom i s c a l c u l a t e d t o be ^ 2 ysec. 182 Table XXII j C a l c u l a t i o n of k l g of the Reaction Br ; + ClC^+BrO + CIO P l a t e no. [cio 2] ( 1 0 - 6 M ) [ B r 2 ] (10"6M) P T t o r r F i l t e r o A E J k jxlO 9 From c i o 2 (1 mole' From CIO ~ x s e c ~ x ) From BrO 248 3.21 82.5 200(N 2) 4400 1060 6.24 7.6 7.6 249a 2.62 137.5 200(N 2) 4400 1060 — 8.9 8.5 250 3.16 137.5 200 4400 1060 7.75 8.7 7.8 251 2.54 41.3 200 4400 1060 5.3 8.3 9.0 252a 3.00 137.5 100 4400 1060 8.0 7.5 7.6 252b 2.96 137.5 500 4400 1060 6.3 7.7 8.5 253 3.7 82.5 200 4400 1060 6.4 6.6 6.4 254 3.49 137.5 200 5600 1060 6.3 6.6 6.9 255 3.95 220.0 200 5600 1060 5.1 7.2 6.6 256a 3.30 137.5 500. 5600 1060 6.3 7.2 7.6 256b 3.1 137.5 100 5600 1060 5.8 7.6 7.7 Average 6.35 ±1.2 7.6 +1.3 7.7 + 1.3 Mean (7 2±2.0) 9 -1 -1 x 10 1 mole sec Moreover, the same value f o r k^g was found from experiments o o w i t h the 4400 A and the 5600 A f i l t e r s . The assumption inherent i n the c a l c u l a t i o n t h a t [ B r J o = A[C10 2] must t h e r e f o r e be re-examined. The assumption i s only v a l i d i f the r a t e of recombination of bromine atoms i s much l e s s than 183 I i t h a t of r e a c t i o n (16). Considering argon and bromine as the ^ 82 only t h i r d bodies and using the data of Strong e t a l . 83 84 85 Burns, C h r i s t i e e t a l . Burns and Horing and B a s i l a 86 ! and Strong, the minimum h a l f l i f e f o r bromine atoms i n the C10 2/Br 2/Ar mixtures used i s 0.75 msec. I t i s , t h e r e f o r e , necessary to p o s t u l a t e t h a t a much more r a p i d removal of bromine atoms can occur through complex formation. This mechanism has been used to e x p l a i n , f o r example, the e x c e p t i o n a l e f f i c i e n c y 87 of n i t r i c oxide as t h i r d body f o r the recombination of I , 88 89 90 C l , H and O atoms. Spectroscopic evidence f o r the e x i s -87 tence of the intermediate NOI has been obtained. The mechanism proposed i s NO + I t NOI NOI + I NO + I 2 together w i t h 2 NOI -*• 2 NO + I 2 a t high NO pressures. The formation of the complexes CS^ and COS 2 has been proposed t o e x p l a i n the high recombination r a t e 91 of sulphur atoms i n the presence of CS_ and COS. For bromine a z _ • atoms, complexes w i t h CIO, BrO and C10 2 appear reasonable and could l e ad t o a very r a p i d recombination. The p r e c i s e e f f e c t of these r e a c t i o n s on the apparent value of the r a t e constant k^g depends, of course, upon the d e t a i l e d mechanism which determines the r a t i o of the r a t e s of 184 removal of bromine atoms and ClO.,. I f t h i s i s c o n s t a n t then 2 I [Br] o = n ( [ C l 0 2 ] o - [ C K > 2 ] J = n(a - b) [ B r ] t = n ( [ C 1 0 2 ] t - [CK> 2]J = n ( x - b ) d[C10 2] dx dt d t = n k^gX(x - b) Thus a s t r a i g h t l i n e would s t i l l be o b t a i n e d from the p l o t of l o g and from the p l o t s f o r CIO and BrO but the s l o p e i s now g i v e n by k-^ g n r a t h e r than by k^g. The e f f e c t then i s t o reduce the measured r a t e c o n s t a n t by a f a c t o r n. One mechanism by which a c o n s t a n t v a l u e of n i s s t i l l p r e d i c t e d i n v o l v e s the complex C l 0 2 B r as the common i n t e r m e d i a t e f o r the p r o d u c t i o n of ClO and BrO. Both s t r u c t u r e s of t h i s complex, BrCl<^° and B r ^ - ^ C ^ were c o n s i d e r e d by Clyne and ^0 0 25 Coxon as p o s s i b l e t r a n s i t i o n s t a t e s i n the r e a c t i o n , the former a c c o u n t i n g much more n a t u r a l l y f o r the f o r m a t i o n of B r C l . Br + C10 2 -y C l O ^ B r (43) Br + C l 0 2 - B r -> B r 2 + CK> 2 (44) Br + C l 0 2 - B r BrO + CIO-Br (45) Br + CIO-Br -> CIO + B r 2 (46) A steady s t a t e treatment y i e l d s d t C l O l . d l B r O I = - a ' C 1 0 2 ' . " 4 3 ^ 4 5 [ c l B r ] d t d t d t k;,+ k '44' "45 [Br], 2 k. . and 2_ = 3 + = n A[C10 2] k 4 5 185 One other i n t e r e s t i n g f e a t u r e of t h i s type of mechanism i s t h a t i t allows the p o s s i b i l i t y t h a t another complex C10 2*Br would gi v e B r C l and 0 2 at low pres s u r e s , although r e a c t i o n (16) s t i l l seems to be".a b e t t e r e x p l a n a t i o n of p ^ o formation fallowed by r e a c t i o n (40) to give B r C l . F o r a mechanism by which the recom-b i n a t i o n of atoms occurs independently of r e a c t i o n (16), e.g. Br + C10 2 + BrO + CIO Br + CK> 2 ^ C10 2*Br k47 Br + C10 2'Br ^ C10 2 + B r 2 (47) d[Br] K k._ [Br] = 1 + 4 8 - d[C10 2] k 1 6 and the r a t i o w i l l depend on the i n i t i a l [Br] and w i l l change w i t h time. P r e l i m i n a r y experiments designed t o t e s t the p o s s i -b i l i t y t h a t the bromine atoms are removed by other r e a c t i o n s than r e a c t i o n (16) have been performed. Mixtures of bromine o (at pressures of 0.25 t o 0.75 t o r r when 4400 A f i l t e r and 2.5 o t o r r when 5600 A f i l t e r was used) w i t h C l 2 0 and argon have been f l a s h photolysed and at a f l a s h energy 1060 J . The c o n c e n t r a t i o n of bromine atoms produced was equal to the CIO c o n c e n t r a t i o n e x t r a p o l a t e d t o zero time. A comparison w i t h the C l 0 2 / B r 2 system then shows t h a t the bromine atom c o n c e n t r a t i o n i s much higher than the amount of C10 2 decomposed. For experiments w i t h d i f f e r e n t pressures of bromine than those used w i t h C l 2 0 , j 186 the bromine atom c o n c e n t r a t i o n can be taken as approximately p r o p o r t i o n a l to the pressure. The data so 'far obtained are not s u f f i c i e n t l y p r e c i s e t o help i n deducing the mechanism, but TBr 1 ' there i s no doubt t h a t 1 J o >> 1 and the t r u e value f o r A[C10 2] the o v e r a l l r a t e constant f o r r e a c t i o n (16) i s lower (by a t l e a s t a f a c t o r of 10) than t h a t obtained from the f i r s t treatment. CHAPTER VI VIBRATIONALLY EXCITED OXYGEN V i b r a t i o n a l l y e x c i t e d oxygen i n the ground e l e c t r o n i c s t a t e has been observed i n three d i f f e r e n t systems, v i z . , the f l a s h p h o t o l y s i s of C10 2, of C l 2 0 and of the CIO r a d i c a l . ! Though the f i r s t system has been s t u d i e d i n more d e t a i l , the other two have helped very much i n e l u c i d a t i n g the mechanism * f o r the production of 0 2, i t s p o p u l a t i o n d i s t r i b u t i o n and f i n a l l y , the decay of 0 2. This chapter has been d i v i d e d i n t o three s e c t i o n s . F i r s t w i l l i n c l u d e the formation, second, the p o p u l a t i o n d i s t r i b u t i o n , and i n the l a s t , the decay of 0 2 w i l l be d i s c u s s e d . A. Production of V i b r a t i o n a l l y E x c i t e d Oxygen 1) By F l a s h i n g C10 2 - The f l a s h p h o t o l y s i s of CK> 2 has been s t u d i e d at s e v e r a l C10 2 and t o t a l pressures and i n the presence of v a r i o u s a d d i t i v e e.g., C l 2 and C1 20. The C10 2 pressure was v a r i e d from 0.05 t o 0.25 t o r r , t o t a l pressure from 30 t o 500 t o r r and the pressure of C l 2 and C l 2 0 as. mentioned i n the r e s p e c t i v e t a b l e s . Quartz and pyrex r e a c t i o n v e s s e l s were used as w e l l as the l i g h t f i l -o o o t e r s r e s t r i c t i n g r a d i a t i o n above 3100 A, 3400 A or 3700 A. The f l a s h energy was v a r i e d w i t h i n the range corresponding to approximately 25 to 85% primary p h o t o l y s i s of CIO,. Under a l l 188 c o n d i t i o n s , the spectrum of v i b r a t i o n a l l y e x c i t e d oxygen was observed by i t s absorption i n the Schumann-Runge system 3 - 3 -(B Z -XT. ) . A t y p i c a l spectrum i s shown i n f i g . (43) . 11 CJ" This was observed 10 usee from the peak of p h o t o l y s i s f l a s h w i t h 0.25 t o r r of CIO,, i n 200 t o r r of argon using pyrex r e a c t i o n v e s s e l and f l a s h energy 1060 J . * The hi g h e s t l e v e l of 0 2 detected was v' 1 = 15, though only l e v e l s up t o v " = 14 were strong enough f o r p o s i t i v e * i d e n t i f i c a t i o n . The presence of 0 2 (V'=15) was confirmed by f l a s h p h o t o l y s i n g C10 2 at high energies i n the presence of a sm a l l amount of C1 20. Under these c o n d i t i o n s , f o r reasons to be discussed l a t e r , a strong spectrum of the higher l e v e l s i s observed and t h i s allowed the p o s i t i v e i d e n t i f i c a t i o n of the (0,15) l e v e l of 0 2. I t i s s i g n i f i c a n t t h a t no l e v e l higher than v 1 ' = 15 was observed under these more f a v o r a b l e c o n d i t i o n s . 21 These r e s u l t s confirm Lipscomb, N o r r i s h and Thrush's obser-v a t i o n of 0 2 (v*' = 4 t o v " = 8 ) , w i t h the important a d d i t i o n of v'-' = 9 t o v' ' = 15. it The amount of 0 2 formed a t each f l a s h energy was measured at 0,12 and 3,6 l e v e l s . I t was found t h a t w i t h the in c r e a s e i n * the primary p h o t o l y s i s of C10 2, the amount of 0 2 produced a l s o i n c r e a s e s . This has been shown i n f i g . ( 4 4 ) , where the amount of oxygen formed i s p l o t t e d a g a i n s t the primary p h o t o l y s i s of 21 C10 2. This i s q u i t e c o n t r a r y t o L.N.T. who observed t h a t the amount of oxygen formed decreases when the primary p h o t o l y s i s of C10 2 i s more than 50%. 3000 1,11 3100 0,1 1 JT2. 0,42- 1,1: CIO 3200 A 0,13 0 2 ,6 4,6 3,6 5,7 4,7 3,7 2,7 7,9 CIO F i g u r e 43. A p o r t i o n of the Schumann Runge System of 0 2 ( ^-«) observed i n the is o t h e r m a l f l a s h p h o t o l y s i s o f C10 2. C10 = 0.25 t o r r , Argon=200 t o r r , E=1060 J . CO 1 — : — i r: r 1 — — i 1 i r — — F i g u r e 4 4 . A p l o t of. 0 2 formed a g a i n s t p e r c e n t a g e p r i m a r y p h o t o l y s i s of CIO-C10 2=0.25 t o r r , Argon = 2 0 0 t o r r . 2) From the P h o t o l y s i s of C l o 0 j • i As w i t h CIC^/ C1 20 was f l a s h photolysed w i t h d i f f e r e n t f l a s h e n e r g i e s , w i t h C1 20 and argon pressure v a r y i n g from 0.25 t o 1.0 t o r r and 75 to 200 t o r r , r e s p e c t i v e l y . The quartz and pyrex r e a c t i o n v e s s e l s were used as w e l l as a f i l t e r t o r e s t r i c t o the l i g h t r a d i a t i o n above 3100 A. C h l o r i n e as an a d d i t i v e gas was a l s o used when C1 20 was f l a s h e d using pyrex and quartz r e -a c t i o n v e s s e l s . Under the above c o n d i t i o n s the f o l l o w i n g r e s u l t s were obtained: 1) When C1 20 was f l a s h photolysed r e s t r i c t i n g the r a d i a -o t i o n above 3100 A, no spectrum of v i b r a t i o n a l l y e x c i t e d oxygen was observed. 2) Using pyrex and quartz r e a c t i o n . v e s s e l s , the spectrum of v i b r a t i o n a l l y e x c i t e d oxygen was seen by i t s ab s o r p t i o n i n 3 - 3 -the Schumann-Runge system ( B E -XT ) . A t y p i c a l spectrum i s shown i n f i g . (45) w i t h c o n d i t i o n s as mentioned t h e r e , •k The h i g h e s t l e v e l of 0 2 detected was v' 1 = 14 though the spectrum was very weak as compared to those f o r l e v e l s up to v* * = 13. Observation of l e v e l s <. 8 was hindered by the over-l a p p i n g continuum of C l 2 0 and the l e v e l s seen depend on the degree of p h o t o l y s i s . I t was observed t h a t the lower l e v e l s ( v " = 8) could not be seen when pyrex r e a c t i o n v e s s e l was used. That i s the reason only those l e v e l s of 0 2 have been shown i n f i g . (45), which were q u i t e v i s i b l e even though the quartz r e a c t i o n v e s s e l was used. The i n t e n s i t y of a l l the l e v e l s ob-Figure 45. A p o r t i o n of the Schumann Runge System of (' — ^ observed i n the is o t h e r m a l f l a s h p h o t o l y s i s of C^O. Cl o0= 1.0 t o r r , Argon= 200 t o r r , E= 600 J . ro 193 served w i t h pyrex r e a c t i o n v e s s e l was very weak compared to t h a t when quartz r e a c t i o n v e s s e l was used. The minimum energy necessary f o r the ob s e r v a t i o n of 0 2 was 600 J when the pyrex r e a c t i o n v e s s e l was employed. These r e s u l t s are q u i t e d i f f e r e n t 22 from those of Edgecombe e t a l . but these have helped to e s t a -* b l i s h the mechanism of the formation of 0 2 from C10 2. 3 ) . . From the P h o t o l y s i s of CIO CIO r a d i c a l s were generated from the p h o t o l y s i s of CIO2 as has been described i n Chapter IV, sec. fl-2. . The mixture thus obtained containing ClO, C l 2 and 0 2 (from the secondary r e a c t i o n s ) and c h l o r i n e atoms l e f t from the r e a c t i o n of oxygen atoms w i t h CIO, was f l a s h photolysed w i t h the a u x i l i a r y lamp u s i n g two f l a s h energies at d i f f e r e n t time d e l a y s . The delays f o r the a u x i l i a r y lamp were s e l e c t e d i n such a manner t h a t a l l the v i -b r a t i o n a l l y e x c i t e d oxygen produced i n f l a s h i n g C10 2 had d i s a p -peared and a l l the C10 2 had been decomposed. In some e x p e r i -ments, a very s m a l l amount of C10 2 remained a t the time of the a u x i l i a r y f l a s h . . Mixtures c o n t a i n i n g t h i s amount were f l a s h e d * i n separate experiments and no 0 2 was found. Both pyrex and quartz r e a c t i o n v e s s e l s were used. In a l l cases, 0 2 was observed w i t h v i b r a t i o n a l l e v e l s up t o v*' = 13 (14 ?) . The spectrum of oxygen was very weak i n a l l . the l e v e l s as compared to t h a t when the same pressure of C10_ (between 0.1 t o 0 . 15 t o r r ) was photolysed w i t h primary 194 p h o t o l y s i s approximately 30%. The reasons f o r t h i s w i l l be discussed i n S e c t i o n C. The i n t e n s i t y of each band does depend upon the i n i t i a l pressure of C10 2 and the amount of CIO present a t the time of r e f l a s h i n g . Only the r i s e and decay of 0,12 l e v e l has been shown i n f i g . (46), when CIO r a d i c a l was f l a s h e d w i t h f l a s h energy 1325 J and delay times 100 and 166 usee r e s -p e c t i v e l y . Mechanism * The p r i n c i p a l mechanism f o r the production of 0 2 proposed by L.N.T. 2 1 C10 2 + hv -> CIO + 0 (27) ClO,, + O CIO + O* ( X 3 ! - ) + 59 Kcals (7) 2 2 g * r e c e i v e s immediate support from our ob s e r v a t i o n of 0 2 ( v 1 ' = 15), the energy of which corresponds e x a c t l y t o the e x o t h e r m i c i t y of the r e a c t i o n (7). However, t h i s mechanism r e q u i r e s t h a t the * maximum c o n c e n t r a t i o n of 0 2 should be produced when the primary p h o t o l y s i s of C10 2 i s 50%. Whereas we have observed a continued 21 i n c r e a s e up t o 85% p h o t o l y s i s ( f i g . 44) co n t r a r y t o L.N.T. The second mechanism proposed i n v o l v e d the prod u c t i o n and p h o t o l y s i s of ClO^, O + C10 2 •* C10 3 (9) C10 3 + hv CIO + 0* (31) * and i s open to same o b j e c t i o n . Unless 0 2 i s a very minor product of p h o t o l y s i s of C10 2, t h i s mechanism r e q u i r e s an extremely high 195 Blank N.D. 5 / A s e c 11 2 1 3 3 53 N.D. 5 /Lsec 11 2 1 0 , 1 2 F i g u r e 46. Ri s e and Decay of 02 . G10 2— 1-Otorr, A r g o n r 200 t o r r , E m= 1060 J E a u x . : r l 3 2 5 J . a) Delay time = 24l/xsec b) Delay time - l66/U_sec 196 r a t e c o n s t a n t f o r the f o r m a t i o n of ClO^ i n c o m p e t i t i o n w i t h 74 r e a c t i o n (7). U n l i k e the analogous f o r m a t i o n o f NO^, no t h i r d body c o u l d be invoked s i n c e the p r o d u c t i o n of 0^ i s independent of t o t a l p r e s s u r e a t l e a s t i n the range of 30 t o 500 t o r r . T h i s mechanism a l s o r e q u i r e s the i n t e g r a t e d a b s o r p t i o n of Clog i n the wavelength r e g i o n used t o p h o t o l y s e ClO^ t o be extremely h i g h . The s t r o n g e s t a b s o r p t i o n of ClO^ i s , however, o • . _i _i xn t h e . r e g i o n 2000 to 3000 A (e ^  1200 1 mole cm ), the e x t i n c t i o n c o e f f i c i e n t then decreases t o ^100 1 mole 1 cm 1 o * near 3600 A. The amount of produced would be then markedly dependent on wavelength and the r e l a t i v e p o p u l a t i o n of d i f f e r e n t l e v e l s and the h i g h e s t l e v e l produced v e r y p r o b a b l y would va r y * w i t h wavelength. In p r a c t i c e , the p r o d u c t i o n o f i s the o same wit h q u a r t z and pyrex r e a c t i o n v e s s e l s and wit h 3100 A, o o * 3400 A and 3700 A l i g h t f i l t e r s . The c o n c e n t r a t i o n o f C<2 i s reduced o n l y by the amount expected from the decrease i n the C1C>2 p h o t o l y s i s . We t h e r e f o r e conclude t h a t t h i s mechanism may be n e g l e c t e d . Although any mechanism i n v o l v i n g ClO^ i s u n l i k e l y , a b e t t e r case can be made f o r the p r o d u c t i o n o f 0^ i n the r e a c t i o n 0 + C10 3 -»• C10 2 + O* (28) Evidence a g a i n s t t h i s , however, i s the absence o f l e v e l s h i g h e r than v 1 1 = 15 and the f a c t t h a t the presence of c h l o r i n e atoms i n a f l a s h p h o t o l y s e d mixture of CIC^ and C l 2 o n l y seems t o 197 * a f f e c t the r a t e of r e l a x a t i o n of 0 2 r a t h e r than i t s p r o d u c t i o n . Another p o s s i b i l i t y worth c o n s i d e r i n g i s the d i r e c t pro-du c t i o n of 0 2 i n the primary process C10 2 + hv -»• C l + 0* t (37) A s i m i l a r mechanism has been proposed t o e x p l a i n the presence * • 75 of SO f o l l o w i n g the f l a s h p h o t o l y s i s of SO^. The formation of CIO i n the f a s t r e a c t i o n C l + CK> 2 -»• 2 CIO (8) makes the o v e r a l l y i e l d of both the products the same as t h a t obtained from the f i r s t primary process ( r e a c t i o n 27) , provided t h a t £ 50% of the C10 2 i s photolysed. The decreased y i e l d of ClO a t higher energies would e x p l a i n the apparent f a l l i n i t s 21 e x t i n c t i o n c o e f f i c i e n t observed by L.N.T., w h i l e the continued * l i n e a r dependence of 0 2 production on f l a s h energy f o l l o w s auto-m a t i c a l l y . The e f f e c t of C l 2 0 i n decreasing the amount of CK> 2 decomposed at low f l a s h energies i s accounted f o r by the r e a c t i o n C l + C1 20 -»• CIO + C l 2 (19) which competes w i t h the C10 2 f o r c h l o r i n e atoms. The apparent i n c r e a s e i n CIO pr o d u c t i o n (as measured a t long delays or by l i n e a r e x t r a p o l a t i o n of the second order p l o t ) caused by C1 20 at high energies i s e q u a l l y w e l l e xplained by the same r e a c t i o n (19) when there i s an excess of c h l o r i n e atoms over unphotolysed C10 2. This excess of c h l o r i n e atoms causes the r a p i d r e l a x a t i o n of 0 2 at high f l a s h energies and t h e i r removal by C1 20 produces the r e q u i r e d e f f e c t , i . e . the r a p i d r e l a x a t i o n of 0_ i s drama-198 t i c a l l y r e p l a c e d by a slow decay. While t h i s mechanism i s q u a l i t a t i v e l y a t t r a c t i v e our r e s u l t s provide convincing reasons f o r b e l i e v i n g t h a t "^10% of t o t a l CIO i s i n f a c t produced i n the r e a c t i o n (8). F i r s t l y , the same value of the e x t i n c t i o n c o e f f i c i e n t of ClO has been obtained at low and high f l a s h energies and t h i s value i s very c l o s e to those obtained from C1 20 and C1 20/C1 2 and tjo the (non-photolytic) 25 value of Clyne and Coxon. Secondly, the i n i t i a l r a p i d decay of CIO cannot reasonably be e x p l a i n e d by t h i s mechanism. T h i r d l y , the t o t a l c o n c e n t r a t i o n of CIO produced a t high f l a s h energies i n the presence of C l 2 0 exceeds the i n i t i a l C10 2 con-c e n t r a t i o n ( f i g . 19). This can only be explained by the reac-t i o n of oxygen atoms w i t h C l 2 0 as has been suggested i n the comparison of the r a t e constants of oxygen atoms w i t h ClO and C1 20 (Chapter IV, S e c t i o n B), i . e . , O + C10 2 -> CIO + O* (7) 0 + C1 20 + 2 CIO (35) F o u r t h l y , the decrease i n C10 2 decomposition i n presence of C1 20 can be e x p l a i n e d by the competition of C10 2 and C1 20 f o r c h l o r i n e atoms. But i f we accept t h i s e x p l a n a t i o n , there should be, consequently, a decrease i n the CIO p r o d u c t i o n too. On the other hand, our r e s u l t s suggest a corresponding i n c r e a s e i n the CIO production but a decrease i n the C10 2 decomposition. This can n i c e l y be e x p l a i n e d i f we assume a competition of C10 2 and C1 90 f o r oxygen atoms. 199 F i n a l l y , the r a t e s of p r o d u c t i o n of CIO i n r e a c t i o n s (35) and (8) are i n s u f f i c i e n t t o account f o r the observed maxi-mum CIO c o n c e n t r a t i o n a t 1 0 t o 2 0 ysec. For example, assuming instantaneous 50% p h o t o l y s i s of 0.25 t o r r of C I O 2 , 75% of t o t a l c o n c e n t r a t i o n of CIO would be produced i n ^60 ysec by 9 -1 -1 the r e a c t i o n (8), i f our value of 5 * 10 1 mole sec f o r 31 kg i s accepted. Only 5 ysec are r e q u i r e d through r e a c t i o n (7) and the f i n i t e f l a s h d u r a t i o n e x p l a i n s the observed maximum. * The t o t a l 0 2 produced by t h i s mechanism i s , t h e r e f o r e , l e s s than 5% of the C I O 2 decomposed. A f u r t h e r r e s t r i c t i o n i s imposed by the f a c t t h a t a t low f l a s h energies and high C^O * to C I O 2 r a t i o s , l e s s O 2 i s produced i n the presence of C^O. In view of the c o r r e l a t i o n between t h i s and the i n c r e a s e i n the CIO p r o d u c t i o n and the decrease i n the C K ^ decomposition, t h i s can only be e x p l a i n e d by the competition of oxygen atoms between C I 2 O and C 1 0 2 as s a i d above. Therefore, even i f 0 2 i s produced by t h i s mechanism i t accounts only f o r a s m a l l f r a c t i o n of the * t o t a l 0 2 . * We conclude t h a t O 2 i s produced i n the r e a c t i o n s 0 + C 1 0 2 •> CIO + 0 * + 59 Kcals (7) 0 + CIO ->• C l + 0 * + 55 Kcals (6) The f i r s t predominates at low f l a s h e n e r g i e s , but i s superseded , by the second f o r a primary p h o t o l y s i s exceeding 67%. The e v i -dence f o r the production of v i b r a t i o n a l l y e x c i t e d species i n 200 92—98 r e a c t i o n s of t h i s type i s strong and i n view of the s i m i -l a r i t y between them, i t i s reasonable to make the assumption * t h a t the 0 2 i s produced w i t h a s i m i l a r energy d i s t r i b u t i o n f o r * v*' 2. 12. On t h a t assumption, the l i n e a r dependence of 0 2 production on f l a s h energy i s a n a t u r a l consequence of the r e -a c t i o n s (7) and (6) as has already been shown i n f i g . (44). * Independent evidence f o r the production of 0 2 by r e a c t i o n (6) has been obtained by the f l a s h p h o t o l y s i s of C1 20 and ClO r a d i c a l s which w i l l be d i scussed i n the next s e c t i o n s . F l a s h P h o t o l y s i s of Cl.,0 The p r o d u c t i o n of e x c i t e d oxygen i n the f l a s h p h o t o l y s i s of C1 20 can be explained i n terms of r e a c t i o n (6) but the other p o s s i b i l i t i e s w i l l a l s o be considered. From the r e s u l t s of C1 20 experiments, i t i s c l e a r t h a t oxygen atoms are necessary * f o r the production of 0 2 and thus the p o s s i b l e r e a c t i o n of oxygen atoms i n the C1 20 p h o t o l y s i s could be 0 + C1 20 -»• C l 2 + 0* + 83 Kcals (36) 0 + CIO C l + O* . +_ 55 Kcals (6) 0 + C1 20 -> 2 CIO (35) .The immediate o b j e c t i o n to r e a c t i o n (36) i s t h a t the l e v e l s higher than v' 1 = 14 should be formed, but we could not observe them. 201 From s t e r i c c o n s i d e r a t i o n s and the geometry of the a c t i v a t e d complex i t appears t h a t r e a c t i o n i ( 3 6 ) should have a high a c t i v a t i o n energy and s m a l l r a t e constant. The r a t e constant of the r e a c t i o n of oxygen atoms w i t h C^O was found to be 8 . 3 x 10 9 1 m o l e - 1 s e c - 1 by P h i l l i p s e t a l . 7 3 and 5 . 3 x 9 -1 -1 73 10 1 mole sec by us. I t was shown by them t h a t ( 3 5 ) i s the probable r e a c t i o n and a s i m i l a r c o n c l u s i o n was a l s o drawn by us from the measurement of the CIO c o n c e n t r a t i o n when C^O and CK>2 was f l a s h photolysed w i t h v a r i o u s r a t i o s and d i f f e r e n t f l a s h energies. I f r e a c t i o n ( 3 6 ) does occur t o a reasonable e x t e n t , the t o t a l amount of CIO produced should be l e s s when CIO2 i s f l a s h e d i n the presence of C^O than f l a s h e d without C^O, unless the r e a c t i o n ( 3 6 ) and ( 3 5 ) occur w i t h equal r a t i o . Whereas i f r e a c t i o n ( 3 5 ) i s predominant, the r a t i o of CIO produced when CIO2 i s f l a s h e d w i t h and without-the presence of C^O should vary between 1:1 .5 depending upon the r a t i o of CI2O/CIO2, w i t h the assumption t h a t r e a c t i o n (6) i s neglected i n both the cases. I t was found from our r e s u l t s t h a t the r a t i o of CIO w i t h and without C^O when CIO2 i s fl a s h e d at low f l a s h e n e r g i e s , does l i e between 1 t o 1 . 5 . At the same f l a s h e n e r g i e s , the r a t i o goes more towards 1 . 5 i f the r a t i o of C l 2 0 t o C10 2 i s i n c r e a s e d . These r a t i o s agree s a t i s -f a c t o r i l y w i t h the t h e o r e t i c a l ones c a l c u l a t e d by the known values of k ^ and k^ f o r d i f f e r e n t percentage p h o t o l y s i s of 202 C l O j . As already mentioned i n the case of C10 2, t h i s can only be explained i f oxygen atoms r e a c t w i t h C1 20 v i a r e a c t i o n (35) r a t h e r than (36). I f the oxygen atoms r e a c t w i t h C1 20 v i a r e a c t i o n (36), * the t o t a l amount of formed i n the presence of C1 20 should be equal t o t h a t formed i n the absence of C l 2 0 when C10 2 i s I ° f l a s h photolysed using r a d i a t i o n s above 340,0 A to prevent the p h o t o l y s i s of C l 2 0 . But i t has been found t h a t the amount of e x c i t e d oxygen formed decreases w i t h the i n c r e a s e of C l 2 0 t o C10 2 r a t i o . I t can be seen from f i g . (24) when the C1 20 t o * C10 2 r a t i o i s 20, the 0 2 can h a r d l y be seen. * The other p o s s i b l e reason f o r the production of 0 2 could have been due to the r e a c t i o n O + C10 2 CIO + 0* + 59 Kcals (7) This does seem to be reasonable as the delay i n the appearance of C10 2 has been observed when C l 2 0 i s f l a s h e d . But t h i s can e a s i l y be r u l e d out s i n c e a s i m i l a r delay has been observed o when C1 20 i s f l a s h e d w i t h r a d i a t i o n above 3100 A where C l 2 0 does not g i v e any oxygen atoms i n the primary process and a l s o the oxygen atoms cannot stay long enough i n the presence of C l j O and CIO whose r a t e constants f o r the r e a c t i o n s w i t h oxygen atoms are very h i g h . 3 1 F i n a l l y , the r e s u l t s obtained i n the case of C l 2 0 t o -gether w i t h those observed i n the f l a s h p h o t o l y s i s of C10 2, * i . e . , the amount of 0 2 produced incre a s e s l i n e a r l y w i t h the 203 increase of primary photolysis, suggests that reaction (6) i s * responsible for the production of 0,,. The reaction mechanism can be written as C1 20 + h y 4 CIO + C l C1 20 + hv •> 2 C l + 0 (22) CIO + hv -»• C l + 0 O + CIO -*- C l + 0* (6) This mechanism agrees with the r e s u l t s that the amount * of 0 2 produced with the pyrex reaction vessel i s less than that when quartz reaction vessel was used. This i s because the C1 20 concentration i s much greater than that of CIO and hence .'-1-,„ „ „ — A. ~ •» •> i - - -. — i ... •>-.. • - - - - - .. •• - ..... . .c„ ,, L-iie O A y C j C i l d w j - j - x J_»e: u u n o u i u c u yjy icau(, iuu v ~> -J / J-ii jJiCLCiCilC to reaction (6), whereas i n the case of quartz, the r a t i o of CIO to C1 20 i s increased. There are many reactions of atoms with stable molecules known which are s i m i l a r to (35), e.g. C l + C10 2 2 CIO Br + C10 2 * CIO + BrO Br + C1 20 ~r CIO + BrCl and so on. Thus i t i s reasonable to assume that reaction (35) i s more probable for reaction of oxygen atoms with C l 2 0 . 204 F l a s h P h o t o l y s i s of ClO * The production of 0 2 when CIO was f l a s h photolysed can be e x p l a i n e d by the simple mechanism CIO + hv -»• C l + O O + CIO -> C l + 0* + 55 Kcals (6) because the hi g h e s t v i b r a t i o n a l l e v e l (v* 1 = 13, 14?) observed almost e x a c t l y agrees w i t h the e x o t h e r m i c i t y of r e a c t i o n (6). The only doubt can be the e l e c t r o n i c s t a t e of the oxygen atoms i n v o l v e d s i n c e the primary p h t o l y s i s of ClO r a d i c a l s can take place according t o the f o l l o w i n g r e a c t i o n s : CIO + hv (>2800 A) -»• C l + 0 ( 3P) (6-1) CIO + hv (<2800 A) •> C l + 0 (1D) (6-2) Reaction (6-1) occurs when the l i g h t absorbed by ClO i s i n the re g i o n of d i f f u s e bands and as suggested by Durie and 16 Ramsay t h a t t h i s t r a n s i t i o n leads t o the formation of both atoms i n the ground s t a t e . This can be achieved when ClO i s f l a s h e d using pyrex r e a c t i o n v e s s e l only. The t r a n s i t i o n 2 2 ~ ( TT «- TT ), i . e . , l i g h t absorbed i n the continuum leads t o the formation of ground s t a t e c h l o r i n e atoms and e x c i t e d (XD) oxygen atoms as i n r e a c t i o n (6-2). But i t has been seen by many workers 1*^ x ^ 3 t h a t r a d i a t i v e l i f e time of the t r a n s i t i o n 0 ( XD) - O ( 3P) + hv 205 i s very long as compared to the c o l l i s o n a l d e a c t i v a t i o n by other gases. Even 200 t o r r of argon or n i t r o g e n was enough f o r the competition. Thus the primary r e a c t i o n s can be f o l l o w e d by G (^ D) + CIO + C1( 2P) + 0 ? ( X 3 E - ) (6-3) O (^ D) + M -> O ( 3P) + M (6-4) 0 ( 3P) + CIO + c l ( 2P) + 0 2 ( X 3 E " ) (6-5) .. * Reaction (6-3) can be neglected as the h i g h e s t l e v e l of 0 2 c o r -responds t o only 55 K c a l s , whereas l e v e l s higher than 14 should be observed. A l s o the pressure of argon or n i t r o g e n used was enough to quench the O (XD) t o the ground s t a t e . Therefore, i t * seems l i k e l y t h a t the 0 2 i s formed by r e a c t i o n (6-5). This can a l s o be seen from the r e s u l t s obtained i n the case of a pyrex r e a c t i o n v e s s e l . * Thus the formation of 0 2 when CIO i s f l a s h e d has confirmed the f o l l o w i n g p o i n t s . 1) The l i n e a r i n c r e a s e i n the amount of e x c i t e d oxygen formed w i t h the i n c r e a s e i n the primary p h o t o l y s i s of C10 2. 2) The formation of e x c i t e d oxygen i n the case of C1 20 photo-l y s i s . -n 3) This i s an a d d i t i o n of another r e a c t i o n t o the type of 79 r e a c t i o n s p o s t u l a t e d by P o l a n y i , A + BC ->• AB* + C where AB* i s a v i b r a t i o n a l l y e x c i t e d molecule. 206 B. V i b r a t i o n a l Energy D i s t r i b u t i o n i n E x c i t e d Oxygen The accurate measurement of the r e l a t i v e p o p u l a t i o n of * a l l the l e v e l s of 0 2 observed i n the f l a s h p h o t o l y s i s of C10 2 i s d i f f i c u l t because every band i s , t o some ext e n t , overlapped by the spectrum of C10 2 or of CIO. An a d d i t i o n a l problem f o r the lower l e v e l s , w i t h the r e s o l u t i o n used, i s the overlap of * the v a r i o u s 0 2 bands. However, the i n t e n s i t i e s of the (3,6), (5,7) and (0,12) bands were measured by p l a t e photometry and the r e l a t i v e p o pulations were obtained by using the t r a n s i t i o n 77 - p r o b a b i l i t i e s g i ven by N i c h o l l s . Since eye e s t i m a t i o n of the other bands showed t h a t the change i n p o p u l a t i o n between suc-c e s s i v e l e v e l s i n the range v" 1 = 5 t o v " = 13 was sm a l l and constant, these measured values f o r the 6th, 7th, and 12th l e v e l s f a i r l y represent the trend over t h i s range. Between the 13th, 14th and 15th l e v e l s the i n t e n s i t y d i f f e r e n c e was much more marked and the hi g h e s t l e v e l was b a r e l y d e t e c t a b l e under most c o n d i t i o n s . * To o b t a i n the r e l a t i v e r a t e s of prod u c t i o n of 0 2 i n t o the 6th, 7th, and 12th l e v e l s , the r a t i o s of the measured con-c e n t r a t i o n s were p l o t t e d a g a i n s t time and e x t r a p o l a t e d to zero time d e l a y , as can be seen i n f i g . (47-a). The r e s u l t s obtained by t h i s method are compared, i n Table X X I I I , t o those c a l c u l a t e d from the l i n e a r e x t r a p o l a t i o n of the f i r s t order decay p l o t s f o r each l e v e l t o zero delays ( f i g . 48). The d i f f e r e n c e between the r e s u l t s obtained by these procedures a r i s e s from the more F i g u r e 4 7 . A p l o t of the r a t i o s of N n 1 9 / N 5 r against time. a) Curves are drawn w i t h the assumption that 0 2 obeys Beer Lambert's law, b) Curves are drawn by r a i s i n g peak height t o the power 1.75-0 C10 2-0 . 2 5 t o r r , Argon =200 t o r r . T i r E - 250 J E= 600 J E - lObO J 2 0 4o 10 8 0 0 time (^JLsec) T 1 1 — • E - 260 J —e- E - 600 J *» E - lOoO J 2 0 (b) T 4o 209 r a p i d change i n the d i s t r i b u t i o n at very s h o r t delays ( f i g . 47-a). The f i r s t method takes t h i s i n t o account because * measurements made at sh o r t delays (while the c o n c e n t r a t i o n i s s t i l l i n c r e a s i n g ) can a l s o be used. The e x t r a p o l a t e d popu-l a t i o n r a t i o s , t h e r e f o r e , more a c c u r a t e l y represent the i n i t i a l d i s t r i b u t i o n . This d i s t r i b u t i o n i s seen to be independent of C1C"2 pressure, t o t a l pressure and f l a s h energy and i s the same w i t h quartz as w e l l as w i t h pyrex r e a c t i o n v e s s e l s . The same o o r e s u l t s were obtained w i t h l i g h t f i l t e r s of 3100 A, 3400 A or o 3700 A cut o f f wavelength and the i n i t i a l d i s t r i b u t i o n was a l s o u n a f f e c t e d by the presence of C l 2 or of C1 20, though these gases had marked e f f e c t s on the r a t e of r e l a x a t i o n on * °2' To what extent t h i s measured i n i t i a l d i s t r i b u t i o n r e p r e -* sents the r e l a t i v e r a t e s of production of 0 2 i n t o v a r i o u s l e v e l s depends on three f a c t o r s . The f i r s t concerns the experimental accuracy of the determination i t s e l f . C l e a r l y the steepness of the curves at s h o r t delays and small u n c e r t a i n t y i n the def-i n i t i o n of zero time preclude high accuracy. Probably more * important i s n o n l i n e a r i t y i n the dependence of 0 2 c o n c e n t r a t i o n * on the p l a t e d e n s i t y when the 0 2 bands were overlapped by other s p e c t r a . For example the two path method has shown t h a t under our experimental c o n d i t i o n s , the power 'n' to which the change i n p l a t e d e n s i t y should be r a i s e d i n the r e l a t i o n s h i p [C^l os (change i n p l a t e d e n s i t y ) 1 1 , l i e s c l o s e r to 1.5 ± 0.25 than u n i t y . 210 When t h i s n o n l i n e a r i t y i s taken i n t o account, the average value f o r t h i s i n i t i a l r a t i o of populations of the l e v e l s 12th * t o 6th of C>2 i s found to be approximately f i v e ( f i g . 47-b) . The second f a c t o r i s the p o s s i b i l i t y of a very r a p i d change i n the r e l a x a t i o n r a t e at s h o r t delays so t h a t the e x t r a -p o l a t i o n i s not v a l i d . The f a c t t h a t the l i n e a r f i r s t order p l o t y i e l d s lower values f o r the i n i t i a l p o p u l a t i o n r a t i o s than the p l o t of r a t i o s i n d i c a t e s t h a t species other than C10 2 and CIO c o n t r i b u t e to the r e l a x a t i o n . The oxygen atoms i n the e a r l y stage are the only other species present i n s u f f i c i e n t concen-t r a t i o n and evidence t h a t they are e x c e p t i o n a l l y e f f i c i e n t w i l l be presented i n the next s e c t i o n . However, the minimum h a l f l i f e f o r oxygen atoms i n our system i s approximately 4 usee, so t h a t the r a t e of r e l a x a t i o n would change by l e s s than a f a c t o r of two. This change i s already r e f l e c t e d i n the e x t r a p o l a t i o n . The t h i r d f a c t o r and the one which could d r a s t i c a l l y change the d i s t r i b u t i o n w i t h i n a microsecond, i s the importance of o p t i c a l pumping. A r a t h e r extreme example of t h i s i s known 78 i n the f l a s h p h o t o l y s i s of cyangen and cyanogen h a l i d e s . In t h i s case the CN r a d i c a l i s produced i n the zeroth v i b r a -t i o n a l l e v e l and very r a p i d l y reaches a high degree of v i b r a t i o n a l e x c i t a t i o n through a b s o r p t i o n of r a d i a t i o n from the p h o t o l y s i s lamp. - . CN .(X2Z„ v=0) + hv—>CN(B 2E, v=0, 1, 2...) CN (B 2Z, v=0, 1 , 2...)—* CN (X 2Z, v=0, 1, 2...) 211 These processes are repeated many times and the e x c i t e d l e v e l s a l s o absorb r a d i a t i o n . Convincing evidence t h a t the d i s t r i -* b u t i o n of C»2 produced from ClOj has not been a l t e r e d by a s i m i l a r mechanism i s provided by the f a c t t h a t the same r e s u l t s are obtained w i t h quartz and pyrex r e a c t i o n v e s s e l s and w i t h o o o 3100 A, 3400 A and 3700 A l i g h t f i l t e r s . Supporting evidence has been obtained from s i m i l a r l i g h t f i l t e r experiments u s i n g •64 * NC>2 as the source of O^i the d i s t r i b u t i o n again being u n a f f e c -t e d . We conclude t h a t i n the r e a c t i o n of oxygen atoms w i t h ClC^f the r a t e s of production of i n t o the l e v e l s v " = 5 to v'' = 13 are approximately equal, but w i t h a trend i n favour of the higher l e v e l s . This trend appears to be uniform and the r a t i o of the r a t e s f o r successive l e v e l s i s probably 1.3 ± 0.2. * From the l i n e a r i n c r e a s e i n the C>2, c o n c e n t r a t i o n i n v a r i o u s l e v e l s w i t h f l a s h energy, we conclude t h a t a s i m i l a r d i s t r i b u t i o n * of C>2 i s produced i n the r e a c t i o n of oxygen atoms w i t h CIO r a d i c a l , although the d i r e c t measurement i n t h i s case i s not p o s s i b l e due t o the very s m a l l c o n c e n t r a t i o n produced. A * s i m i l a r c o n c l u s i o n can be drawn i n the case of O2 produced when CI2O i s f l a s h photolysed, because i t has been shown t h a t i t i s the r e a c t i o n of oxygen atoms w i t h CIO which i s r e s p o n s i b l e f o r * the production of . * For l e v e l s v 1 1 = 15 and 14, no d e f i n i t e c o n c l u s i o n i s p o s s i b l e but i t seems t h a t the apparent r e v e r s a l of the t r e n d i s , 212 at l e a s t i n p a r t , due t o the high r a t e of depopulation of these l e v e l s . I f the l e v e l s v*' = 12 t o v 1 ' = 15 were i n i t i a l l y e q u a l l y populated, then w i t h stepwise r e l a x a t i o n , the apparent h a l f l i v e s of these l e v e l s would be approximately i n the r a t i o 7:5:3:1. The d i f f e r e n c e appears t o be s u f f i c i e n t t o account f o r the r e l a t i v e l y low concentrations of the 14th and 15th l e v e l s observed. In t h i s connection, i t i s i n t e r e s t i n g to note t h a t 21 L.N.T. a l s o estimated the populations of the 5th, 6th and 7th l e v e l s t o be e q u a l l y populated, whereas t h a t of the 8th l e v e l , h i g h e s t observed by them, was a p p r e c i a b l y l e s s . This can be explained by the f a s t e r r a t e of depopulation of the 8th l e v e l i n the absence of higher l e v e l s . F urther evidence f o r the importance of the r e l a x a t i o n i n determining the observed r e l a t i v e populations of the hi g h e s t l e v e l s i s provided by the e f f e c t of C l 2 0 . When C10 2 i s f l a s h e d at high f l a s h ener-g i e s i n the presence of C l 2 0 , the r a t e of r e l a x a t i o n of a l l l e v e l s i s decreased and the r e l a t i v e c o n c e n t r a t i o n of the 15th * l e v e l of 0 2 i s s i g n i f i c a n t l y i n c r e a s e d , as i s , t o a l e s s e r degree, t h a t of the 14th l e v e l , although no 15th l e v e l of 0 2 was observed when Cl-0 was f l a s h photolysed alone. Table XXIII C a l c u l a t i o n of R e l a t i v e Populations of E x c i t e d Oxygen P l a t e [ c i o 2 J Argon Energy F i l t e r N 1 2 / N 7 N l 2 / N 6 - N l 2 / N 6 2 no. t o r r t o r r J 0 A [ o * i 1 [o * , 1 . 7 5 2 J [ O * ]1 [0*] 1.75 40 0.25 200 1060 — 1.6 4.7 1.3 3.3 0.66 1.1 •34 0.25 75 1060 — 1.4 3.3 1.1 2.2 0.65 1.1 35 0.1 30 1060 — 1.5 4.7 — — — — 30 0.25 500 1060 — 1.6 4.8 1.6 4.4 0.9 2.0 25 0.1 30 1060 3100 — — 1.3 4.5 0.68 1.1 152 0.25 200 1060 3100 — — 1.9 6.4 0.6 ' 1.0 61 0.25 200 1060 3700 — — 1.7 6.3 0.62 1.0 62* 0.25 200 1060 3700 — — 1.6 6.3 0.58 0.8 36 0.25 75 .'600 — 1.5 4.0 1.1 3.2 0.7 1.2 27 0.25 200 600 . — 1.7 4.6 1.7 5.8 1.3 v 3.5 37 0.1 30 600 — 1.6 3.7 2.0 5.5 0.7 1.4 26 0.1 150 600 1.9 4.8 • : — — — — 29 0.1 500 600 — 1.3 3.8 1.6 5.1 0.8 1.7 25 0.1 30 600 3100 — — 1.4 4.9 0.6 1.1 153 0.25 200 600 3100 — — 2.0 7.5 1.0 2.5 154 0.1 200 600 3100 — 1.9 7.5 0.8 1.54 39 0.25 200 260 — 1.7 5.2 2.2 7.0 0.7 1.2 38 0.25 75 260 1.7 3.8 1.7 6.2 0.9 1.9 44 0.25 500 260 — 2.2 5.9 2.2 7.0 0.9 2.1 164 0.25 200 260 3100 — — 1.6 6.2 0.7 1.1 118 0.25 200 260 3100 — — 1.7 5.7 . 0.6 1.1 119 0.25 75 260 3100 — -- 1.7 6.0 0.6 1.0 - 1 . 6 ± 0 .2 4.4 I . . 7 ± 0 . 3 5. 7 ± 1 . 4 0.73+0.12 ! 1.4 Note: N 1 2 / N ^ and N 1 2 / N 1 are values obtained from the p l o t s of r a t i o s of r e l a t i v e p o p u l a t i o n vs. time. N i 2 / N 6 2 i s o b t a i n e d from the e x t r a p o l a t i o n of f i r s t M order p l o t of [ O 2 ] . W 214 C. R e l a x a t i o n of E x c i t e d Oxygen The r e l a x a t i o n o f . t h e l e v e l s v 1 ' = 6 and v ' 1 = 12 was measured f o r s e v e r a l f l a s h e n e r g i e s , u s i n g a pyrex r e a c t i o n v e s s e l and two mm of g l a s s f i l t e r A. The r e s u l t s are l i s t e d i n T able XXIV. The e f f e c t of the a d d i t i o n of C1 20 and C l 2 was a l s o s t u d i e d and i s i l l u s t r a t e d i n Tables XXVIIIand XIX r e s p e c t i v e l y . The decay of 0 2 was found t o be f i r s t o r d e r over the e n t i r e p e r i o d of measurement f o r the 12th l e v e l a t a l l f l a s h e n e r g i e s and f o r the 6th l e v e l a t h i g h f l a s h e n e r g i e s . At lower e n e r g i e s , the f i r s t o r d e r p l o t f o r the 6th l e v e l became l i n e a r o n l y a f t e r about 100 ysec, the i n i t i a l r a t e of decay being a p p r e c i a b l y f a s t e r . For a l l experiments, the r e s u l t s are c o n v e n i e n t l y expressed i n terms of h a l f - l i v e s f o r each l e v e l measured from the l i n e a r f i r s t o r d e r p l o t g i v e n i n T a b l e XXIV. For the 6th l e v e l , the i n i t i a l h a l f - l i v e s , i . e . , * those measured from the time of maximum 0 2 c o n c e n t r a t i o n , are g i v e n i n parentheses. 1) Low F l a s h Energy ~ A t low f l a s h e n e r g i e s the r e s u l t s f o r the 6th l e v e l are 21 s i m i l a r t o those r e p o r t e d by L.N.T. These r e s u l t s may be p r o p e r l y compared s i n c e t h e i r measurements began 150 ysec a f t e r the b e g i n n i n g of the p h o t o f l a s h , by which time the decay was found to be f i r s t o r d e r i n both s t u d i e s . Again a t low f l a s h e n e r g i e s , the h a l f - l i v e s of both l e v e l s are independent of the 215 f r a c t i o n of C10 2 photolysed over the s m a l l range where the measurements were p o s s i b l e . This i s c o n s i s t e n t w i t h the con-21 e l u s i o n by L.N.T. t h a t CIO and C10 2 have a s i m i l a r e f f i c i e n c y * i n the r e l a x a t i o n of 0 2- Since they found only a 10% decrease i n the h a l f - l i f e of the 6th l e v e l when the C10 2 decomposition was reduced from 90 - 100% to 50%, the r a t e constant f o r ClO 21 and C10 2 may be assumed to be equal. We f i n d , as d i d L.N.T. th a t the e f f e c t of the moderating gas i s n e g l i g i b l e so th a t the r a t e constant k,-g f o r the decay of 0 2 may be c a l c u l a t e d as f o l l o w s . d l n [ 0 * ] — = k E. n{[C10 9] + [CIO]} dt 50 2 si n c e the decomposition of C10 2 i s e s s e n t i a l l y complete before the beginning of the h a l f - l i f e p e r i o d and the decay of CIO i s s t r i c t l y second order over t h i s p e r i o d . d l n [ 0 * ] c 9 " - = k 5 0 [ C l + " ] dt u 1 + k 5 C 2 t where C^ and C 2 are the concentrations of C10 2 and CIO r e s -p e c t i v e l y at the beginning of the h a l f - l i f e . Thus, l n ( l + krC-t ) , k 5 Q = m 2 [ c x t + — ^ ] " K5 8 —1 —1 and our r e s u l t s y i e l d k^Q(12) = 1.8 x 10 1 mole sec and 8 —1 —1 kejg(6) = 0.87 x 10 1 mole sec , the l a t t e r r e s u l t s being 8 —1 —1 i n good agreement w i t h the value of 1.0 x 10 1 mole sec 21 found by L.N.T. f o r the r e l a x a t i o n of the 6th l e v e l by ClO. These decay r a t e constants cannot, however, be d i r e c t l y i d e n t i f i e d w i t h the r a t e constants f o r the process 216 # Table XXIV H a l f - L i v e s f o r theO ( f o l l o w i n g the Plash P h o t o l y s i s - of.CIO, P l a t e [C10 2] Argon Energy % F i l t e r ty' (usee) n n ° t o r r t o r r J Photo A 0,12 3,6 0.25 200 26 100 - 7 — 34 0.25 75 1060 100 - 10 26 40 0.25 200 1060 100 - 10 15 35 0.1 30 1060 100 - 13 — 30 0.25 500 1060 100 - 10 28 36 0.25 75 1060 100 - 20 29 27 0.25 200 600 100 - 15 30 26 0.1 150 600 100 - 20 41 37 0.1 30 600 100 - 24 — . 29 0.1 500 600 100 - 20 29 152 0.25 200 1060 100 A 12 18 155 0.1 200 1060 100 A 20 50 153 0.25 200 600 100 A 18 35 154 0.1 200 600 100 A 20 40 120 0.1 200 600 100 A 35 60 25 0.1 200 600 100 A 36 69 63 0.25 200 1060 90 3400 15 — 61 0.25 200 1060 90 3700 130 380 118 0.25 200 260 80 A 100 650(140) 164 0.25 200 260 80 A 125 675(100) 119 0.25 200 260 80 A 115 600(110) 118 0.25 200 160 65 A 320 540 111 0.1 200 160 72 A 280 770 121 0.25 200 160 70 A 244 580 110 0.25 200 160 65 A 250 680 Note: A = gl a s s f i l t e r . 217 O^Cv'^n) + C10(C10 2) -+ 0*(v"=n-l) + C10(C10 2) s i n c e , i f the r e l a x a t i o n i s stepwise, the presence of the l e v e l s higher than t h a t f o l l o w e d must be taken i n t o account. 21 L.N.T. noted t h a t h a l f - l i v e s of l e v e l s lower than 6th were " s l i g h t l y longer" and those f o r higher l e v e l s " s l i g h t l y s h o r t e r " than t h a t of the 6th l e v e l . On the b a s i s of p u r e l y stepwise r e l a x a t i o n an ap p r e c i a b l e d i f f e r e n c e would have been expected i n the h a l f - l i v e s . I f we assume t h a t the populations of the 8th and higher l e v e l s were n e g l i g i b l e at the s t a r t of the h a l f - l i f e there should have been a f a c t o r of approximately three between the h a l f - l i v e s of the 6th and 7th l e v e l s . On the same b a s i s , we should have expected our value f o r the decay constant f o r the 6th l e v e l to have been lower than t h a t 21 of L.N.T. by a f a c t o r of approximately f i v e s i n c e we f i n d an e s s e n t i a l l y equal p o p u l a t i o n of a l l l e v e l s up t o the 13th. Furthermore, the d i f f e r e n c e between k^p(12) and k,-Q(6) should be a t l e a s t twice t h a t observed i f , as expected, the t r u e r e l a x a t i o n r a t e f o r the 12th l e v e l i s at -least equal t o t h a t f o r the 6th l e v e l . I t appears e i t h e r t h a t r e l a x a t i o n occurs by a m u l t i -quantum process or t h a t i t i s slow enough f o r a near resonance exchange process °2 (v = n) + 0 2 (v = m) -»• 0 2 (v=n-l) + 0 2 (v = m+1) to be co m p e t i t i v e . 218 2) High F l a s h Energies * The r a t e of r e l a x a t i o n of 0 2 was found to be h i g h l y * dependent on f l a s h energy/ the h a l f - l i f e of 0 2 decreasing by a f a c t o r of ^35 where the energy was increased over the range 160 t o 1060 J . This can be seen from f i g . ( 5 0 ) . This v a r i a t i o n i s i l l u s t r a t e d f o r an i n i t i a l pressure of 0.25 t o r r of C10 2 i n f i g . ( 5 1 ) where the sharp decrease i n the h a l f - l i f e i s seen to occur over the range of energies corresponding t o 40 t o 60% primary p h o t o l y s i s . The other data are presented i n Table XXIV. A l s o shown i n f i g . (51), are two t h e o r e t i c a l curves,, the c a l c u -l a t i o n of which w i l l be discussed below. * The r a p i d r e l a x a t i o n of 0 2 observed at higher energies i s c l e a r l y due to species other than CIO and C10 2 or 0 2 and we propose t h a t the species r e s p o n s i b l e f o r the r a p i d r e l a x a t i o n are c h l o r i n e and oxygen atoms. The complete mechanism f o l l o w i n g p h o t o l y s i s i s thus 0 + C10 2 -> CIO + 0* (7) 0 + CIO -> C l + 0* (6) C l + C10 2 -> 2 CIO (8) C10(C10 2) + O j f v ' ^ n ) ' + C10(C10 2) + 0*(v"< n) (50) C l + 0*(v"=n) - C l + O ^ v ' ^ n) (52) O + O^v'^n) 0 + O ^ v 1 ^ n) (53) F i g u r e 49. R i s e and Decay of 0«, a) Lower l e v e l s b) v"- 12 C10 2-'0 .25 t o r r , A r g o n r 7 5 t o r r , E - 600 J (a) (JO 4,>6—3J6L-5 ,7 Blank Before N.D. 5 >Usec 11 19 28 43.5 64 88 124 — a s s 4,7 0,12 H V D F i g u r e 5 0 . Decay of 0 2 (v"=12) when C10 2 Is f l a s h e d at high and low f l a s h e n e r g i e s . a) C10 2 = 0 . 2 5 t o r r , Argon =200 t o r r , E = 1 0 6 0 J . b) C10 2= o . 2 5 t o r r , Argon = 200 t o r r , E = £ 6 0 J . 0 , " J l Blank Before 5 /Usee 5 8 . 8 1 0 . 6 1 5 2 1 2 8 40 6 0 8 0 C10, CIO, 5 . 9 msec 3 . 2 1 . 6 q40 6 2 0 1 0 . 6 40 110 1 9 5 4 0 5 Before /LL se< t o t o o F i g u r e 51. A p l o t of t L of 0 (v"=. 12) against percentage primary p h o t o l y s i s of CIO i 1 1 1 1 1 1 1 1 1 r Percentage Primary p h o t o l y s i s 2 2 2 Q u a l i t a t i v e l y t h i s mechanism p r e d i c t s the f o l l o w i n g sequence of events. At low energies when [C10 2] > [CIO] and r e a c t i o n (6) i s unimportant, those c h l o r i n e atoms which are produced r e a c t very r a p i d l y w i t h the remaining C10 2 w h i l e the oxygen atoms r e a c t even more r a p i d l y , so t h a t n e i t h e r c o n t r i -butes to the r e l a x a t i o n of 0 2 over the p e r i o d used f o r measure-ments. As the energy incre a s e s both the t o t a l c o n c e n t r a t i o n of c h l o r i n e atoms produced and t h e i r l i f e - t i m e s i n c r e a s e . T h e i r presence begins t o have an a p p r e c i a b l e e f f e c t on the * r e l a x a t i o n of 0 2. This can be seen i n f i g . ( 4 8 ) i n t h a t the e a r l y measurements depart from the f i r s t order p l o t of 0 2 . Above 50% primary p h o t o l y s i s , the c o n c e n t r a t i o n of c h l o r i n e atoms produced i n r e a c t i o n (6) exceeds t h a t of the C10 2 remain-i n g and stays e s s e n t i a l l y constant during the r e l a x a t i o n . This c o n c e n t r a t i o n t h e r e a f t e r i n c r e a s e s almost l i n e a r l y w i t h energy. At s t i l l h igher e n e r g i e s , the oxygen atoms must be taken i n t o account. At low and moderate en e r g i e s , i t i s reason able to consider the production of oxygen atoms by p h o t o l y s i s and t h e i r removal by r e a c t i o n w i t h CIO and C10 2 t o occur at comparable r a t e s (tj^(o)< 3 ysec f o r 0.25 t o r r of C10 2) and the c o n c e n t r a t i o n of oxygen atoms i s very low. At very high ener-g i e s , a b e t t e r approximation would be to consider the p h o t o l y s i to occur v i r t u a l l y i n s t a n t a n e o u s l y f o l l o w e d by chemical reac-t i o n s . The t o t a l c o n c e n t r a t i o n of oxygen atoms present during * the formation of 0~ i s thus i n c r e a s e d , both by v i r t u e of the 223 t o t a l p h o t o l y s i s and the i n c r e a s e of ty% of oxygen atoms. Under * these c o n d i t i o n s , the h a l f - l i f e of i s comparable w i t h t h a t * of oxygen atoms so t h a t [ 0 ] > [Cl] during the decay of and r e a c t i o n (53) i s r a t e determining. The evidence i n favour of t h i s e x p l a n a t i o n i s c o n v i n c i n g . The r a t e constant f o r r e a c t i o n (6) i s known to be very high and the independent evidence f o r the importance of t h i s r e a c t i o n i n the f l a s h p h o t o l y s i s of CIC^ i s discussed i n connection w i t h the v a r i a t i o n of the apparent e x t i n c t i o n c o e f f i c i e n t of ClO and of the i n i t i a l r a t e of decay of CIO w i t h f l a s h energy. Our abso-l u t e values f o r k_ and k,. c a l c u l a t e d on t h i s b a s i s are c o n s i s t e n t 7 6 23 w i t h the lower l i m i t values of Clyhe and Coxon and, more important f o r the present purpose, our value f o r k^/kg =4.4 i s i n e x c e l l e n t agreement w i t h the value of 4 found by Clyne 23 and Coxon. The f a c t t h a t the i n i t i a l r a p i d decay of CIO at the h i g h e s t energies occurs e s s e n t i a l l y a f t e r p h o t o l y s i s i s completed, a l s o supports the assumption t h a t , at these e n e r g i e s , the oxygen atoms are major cause of r a p i d r e l a x a t i o n of 0 * . Further evidence can a l s o be seen i n t h a t atoms have been proposed as very f a s t quenchers. I t has been found t h a t n i t r o g e n atoms have a very high e f f i c i e n c y f o r the r e l a x a t i o n 3 + of N 2(A Z ) i n low l e v e l s (v = 0,1), the r a t e constants found to be 3 x i o 1 0 1 m o l e - 1 s e c - 1 , 1 0 4 - 1 0 6 and . 3 x 10 9 1 mole x sec x , x 0 ^ where f o r higher l e v e l s an upper l i m i t of 8 —1 —1 108 3 xio 1 mole sec has been found. Oxygen atoms have 2 2 4 a l s o been found t o be very e f f i c i e n t f o r the v i b r a t i o n a l r e l a x -a t i o n of oxygen 1^ 9 and n i t r o g e n 1 1 ^ although both these systems have been s t u d i e d w i t h shock tubes at very high temperatures. * The r a p i d r e l a x a t i o n of 0 2 by atoms produced from C l 2 0 and CIO * and the e f f e c t of C l 2 and C l 2 0 on the r e l a x a t i o n of 0 2 provide f u r t h e r evidence f o r the high e f f i c i e n c y of c h l o r i n e and oxygen atoms. These r e s u l t s obtained from these systems w i l l be discussed i n t h i s chapter. * Thus the r e l a x a t i o n of 0 2 by atoms may be represented i n terms of a strong i n t e r a c t i o n . The oxygen atoms may be forming an 0-0 2 complex and thus resembling the ozone molecule, which i s q u i t e s t a b l e . With c h l o r i n e atoms, the Cl-O-0 r a d i c a l which has been proposed i n the p h o t o l y s i s of C l 2 / 0 2 , has been 18 observed i n U.V. spectrum. Since both the c h l o r i n e and * oxygen are paramagnetic, t h e i r a t t r a c t i o n f o r 0 2 i s q u i t e l i k e l y . The mechanism was a l s o t e s t e d q u a n t i t a t i v e l y i n the f o l l o w i n g manner. * a) R e l a x a t i o n of 0 2 by c h l o r i n e atoms The t o t a l c o n c e n t r a t i o n of c h l o r i n e atoms produced f o r each percentage primary p h o t o l y s i s was c a l c u l a t e d at i n t e r v a l s of 10%. For these c a l c u l a t i o n s , the r a t i o of k^/kg was taken to be 4 . 4 and f o r each, the c a l c u l a t i o n i n c l u d e d the e f f e c t of changing C10 2 and CIO c o n c e n t r a t i o n as the p h o t o l y s i s and the oxygen atom r e a c t i o n proceeded. The f i n a l C10 2 and CIO concen-t r a t i o n were thus c a l c u l a t e d . For the 0 t o 60% p h o t o l y s i s , the 225 r a t e of p r o d u c t i o n of oxygen atoms by p h o t o l y s i s was assumed to be s u f f i c i e n t l y slow f o r the oxygen atom to be removed by r e a c t i o n w i t h ClC^ and ClO as f a s t as they are produced. The c a l c u l a t i o n was repeated f o r another extreme assumption, i . e . , a l l oxygen atoms are produced i n s t a n t a n e o u s l y and then r e a c t . I t can be seen from the Table XXV t h a t there i s only a d i f f e r -ence of three t o four percent between the two. For 70 to 100% p h o t o l y s i s , the l a t t e r assumption had to be used t o o b t a i n 70% p h o t o l y s i s and i n f a c t 85% p h o t o l y s i s was achieved experimen-t a l l y , which i s f u r t h e r evidence f o r the importance of oxygen atoms at the h i g h e s t energies. The r e s u l t s of these c a l c u l a -t i o n s are shown i n Table XXV, the c o n c e n t r a t i o n of a l l species being given as a percentage of the i n i t i a l C10~ c o n c e n t r a t i o n . Table XXV Concentrations of CK^, ClO and C l Atoms Present A f t e r the Primary P h o t o l y s i s and Chemical Reactions % P h o t o l y s i s [C10 9] 'a [CIO] 4r [Cl] % [C10 9] [CIO] 0 [Cl] 10 80.2 19.5 0.3 ~ 80.2 19.5 0.3 20 61.3 37.3 0.4 61.1 37.8 1.1 30 43.7 52.6 3.7 42.8 54.4 2.8 40 27.8 64.4 7.8 25.8 68.4 5.8 50 14.7 70.7 14.6 11.0 78.0 11.0 60 5.4 69.2 25.4 0.5 79.0 20.5 70 1.0 58.1 40.9 80 ~ 40.0. 60.0 90 — 20.0 80.0 100 — — — 100.0 - — I . - n i; 226 * The h a l f - l i f e of 0 2 f o r the 12th l e v e l was c a l c u l a t e d .=1 as f o l l o w s . For l e s s than 40% p h o t o l y s i s , the removal of c h l o r i n e atoms by C10 2 i n the r e a c t i o n (8) was taken t o be psuedo f i r s t o rder, using average value of C10 2 c o n c e n t r a t i o n and our value 9 - 1 - 1 of kg =5.0 x 10 1 mole sec . For 50% p h o t o l y s i s , the second orderequation was used. For 60% p h o t o l y s i s , the c h l o r -i n e atom c o n c e n t r a t i o n i s given by equation ( s ) . The appro-p r i a t e equations are then [ C 1 ] = C 0 ] o " h k 6 at , +C C 13 0 ( S ) D e -1 where a = [ c l 0 \ o ~ t 0 ] o b = t c l olo For l e s s than 40% p h o t o l y s i s k52 -kg ty l n 2 = k , ( 1 - e ) + k£0ti/2 (t) 8 ¥hen theprimary p h o t o l y s i s = 50%, k52 .. In 2 = k l n (1 + kg t^) + k 5 Q t ( / l (u) 8 and i f the primary p h o t o l y s i s h. 60%, k 6 a T , A k 5 2 b e _ 1 l n 2 = k 5 2 t i / l ( [ 0 ] o + a + [C1] Q) — l n ( b _ x ) (v) k 6 226 d/ where k^ 2 = k g 2 [ C l ] e k 8 = k 8 [ C l 0 2 ] kg' = k 8 [ C 1 0 2 ] o k50 = k 5 0 ( [ C 1 O 2 ] o + [ C l 0 ] o ) The d e t a i l s of the above equations have been discussed i n Appendix 2. „ / 9 The r e s u l t s f o r the t r i a l values of k^ 2 = 5 x 10 and 9 - 1 - 1 10 x 10 1 mole sec were c a l c u l a t e d and l i s t e d i n Table XXVI. The t y was p l o t t e d i n f i g . (51). The value of 320 usee was assumed to correspond t o the lowest p o s s i b l e energy. A 9 - 1 - 1 value of k^ 2 = 7 x io 1 mole sec i s thus obtained from the f i t a t moderate energies where the e f f e c t of oxygen atoms i s u n l i k e l y t o be important. The value of k^ 2 was calculated, at the h i g h e s t energy corresponding to 82% primary p h o t o l y s i s assuming t h a t oxygen atoms do not p l a y any p a r t i n the r e l a x a t i o n . A value of 2 x 10 x 0 1 mole x sec x was obtained f o r k^ 2 but t h i s value does not s a t i s f y the experimental p l o t and was thus r e -j e c t e d . b) R e l a x a t i o n by oxygen atoms At the hi g h e s t energy, when the primary p h o t o l y s i s i s around 85%, the oxygen atoms are i n much g r e a t e r c o n c e n t r a t i o n * than c h l o r i n e atoms. An equation f o r the r e l a x a t i o n of 0 2 by oxygen atoms was de r i v e d by assuming t h a t a l l the C10 2 has been 227 Table XXVI * C a l c u l a t e d H a l f - l i f e of 0 2 at Various % Primary P h o t o l y s i s of C10 o Using Equations ( t ) , (u), (v) Prim. Photo. % t. / 2(12,0) A ytisec B 0 320 320 30 r 240 200 40 165 90 50 50 30 60 38 24 70 30 18 80 20 13 90 13 10 100 11 8 Note: A = h a l f - l i v e s are c a l c u l a t e d w i t h = 5.0 x 10 1 mole~l s e c - 1 ; B = h a l f - l i v e s are c a l c u l a t e d w i t h k c o = 10.0 x 10 -1 -1 5 1 mole x sec decomposed by the time (10 ysec) measurements were s t a r t e d . I t has been a c t u a l l y observed t h a t at t h i s time the CIC^ concen-t r a t i o n was always <5%. The oxygen atom's c o n c e n t r a t i o n was c a l c u l a t e d i n the same manner as de s c r i b e d i n Chapter IV, s e c t i o n p, i . e . [ C 1 0 ] q - [ C 1 0 ] q . The equation obtained ( d e t a i l s as described i n Appendix 2) i s : 228 dt [O] = = k 5 3 [ 0 ] + k 5 2 ( [ c i ] 0 + r c i ] t ) k c a t b e 6 - 1 [Cl] = [01 - k,a t ° b. e 6 - 1 kga ty^ In 2 = a ( k 5 3 ~ k52 ) ' [ l n b 6 —• - t./J k 6 k g a (b - 1) [0]tu + k c o [ C l ] t 52 l u Jo T : ,/2 + K 5 2 L C X J o r , / a where a = [ C l O ^ - [0]^ b = [ C l 0 \ o S u b s t i t u t i n g the experimental values of ti^, k^, kg, k^Q and k^ 2/ the values of k^ 3 obtained are l i s t e d i n Table XXVII. The average of three experiments i s 1 . 8 — 0 . 8 x 10 x^ 1 mole x sec x . Though there i s a v a r i a t i o n of a f a c t o r of two among the r e s u l t s themselves, c o n s i d e r i n g the measurements of * the h a l f - l i f e of 0 2 the agreement i s not bad. I t was found t h a t * an e r r o r of 10% i n tw of 0„ would make an e r r o r of 25% i n Ix 2 the f i n a l value of k ^ . 229 Table XXVII * Rate Constants f o r Quenching of 0 2 (0,12) by CIO, C10 2, C l and 0 atoms P l a t e no. [ c i o 2 ] t o r r Argon t o r r Energy J Prim. Photo. % k 5 0 x l 0 - 8 k 5 2 x l 0 - 9 (1 mole 1 sec k 5 3 x l 0 - 9 107 .08 75 1060 85 — — 18 152 .25 200 1060 80 — . 10 154 .09 200 600 62 — — . 25 7.0 — 118 0.25 200 160 30 1.6 — 121 0.1 200 160 30 1.8 — 110 0.1 200 160 30 2.0 . — Average -1.8+.2 7.0 1 8*7 3) E f f e c t of Cl^O on the R e l a x a t i o n o f O * C10 2 was f l a s h e d i n the presence of C l 2 0 w i t h the r a t i o s v a r i e d from 1:4 t o 1:20. The t o t a l pressure of argon was kept constant at 200 t o r r . The f l a s h energies used were enough t o cause primary p h o t o l y s i s of C10 2 from 22 t o 85%. A Corning 0-52 f i l t e r was used to prevent the p h o t o l y s i s of C1 20. The 230 h a l f - l i f e of the 12th l e v e l was measured and i s l i s t e d i n Table XXVIII. The t y 2 i n comparable experiments without C l 2 0 are a l s o l i s t e d f o r comparison. The lower l e v e l s could not be measured due to the C1 20 continuum. The d i f f e r e n c e i n the behaviour of 0 2 w i t h and without Cl.,0 i s i l l u s t r a t e d i n f i g . (52). I t can be seen from Table XXVIII t h a t t y2 of 12th l e v e l has been increased d r a m a t i c a l l y by C1 20, the extent depending upon i t s pressure. No q u a n t i t a t i v e measurements were made but the r e s u l t s can be explained q u a l i t a t i v e l y w i t h the help of r e s u l t s d escribed i n the case of C10 2 alone. I t i s c l e a r t h a t C1 20 acts as a scavenger f o r both oxygen and c h l o r i n e atoms and thus helps i n decreasing the r e l a x a t i o n r a t e . This can be seen from the f o l l o w i n g example. For an i n i t i a l pressure of 0.1 t o r r of C10 2 and 85% primary p h o t o l y s i s , approximately 70% of oxygen atoms w i l l r e a c t w i t h ClO to giv e c h l o r i n e atoms. * The h a l f - l i f e of oxygen atoms i s n e a r l y equal to t h a t of 0 2« But i n the presence of 0.4 t o r r of C1 20, the h a l f - l i f e of oxy-gen atoms i s reduced from 8 t o M usee and at the same time only 14% i n s t e a d of 70% of the oxygen atoms w i l l r e a c t w i t h CIO to g i v e c h l o r i n e atoms. In the presence of C l 2 0 , the h a l f - l i f e of c h l o r i n e atoms i s a l s o reduced to 30 to 60 usee, depending upon the pressure of C1 20. At low f l a s h e n ergies, the C l 2 0 , as expected from t h i s argument, i s found t o have much l e s s e f f e c t . F i g u r e 52. R i s e and Decay o f 0 2(v"=: 12) when ClO^ i s flashed w i t h and without C1 20, a) C10 2=0 . 2 5 t o r r , Argon=200 t o r r , E —1060 J . b) C10 2 o .25 t o r r , C l 2 0 = 1 . 0 t o r r , Argon - 200 t o r r , E =• 1060 J . b a JL CIO' Blank Before 5 /Usee 5 8.8 ' 10*6 15^  21 2 8 4 © 6 b-80 ! 110 620 I JL n o 2 0, Blank Before N.D. 5 A s e c 10.6 28 4o 60 80 110 140 195 240 270 C10, to Table XXVIII * Comparison of t y of 0 2 (0,12) l e v e l With and Without Cl-O Using 3400 A F i l t e r P l a t e no. [cio2] (10~6M) [ci2o] (10~ 6M) Energy J Prim.Photo. % t t^(usec) 63 13.7 — 1060 85 15 64 13.7 55.0 1060 120 155 5.5 — 1060 85.0 19 65 5.5 22.0 1060 230 66 1.4 27.5 1060 250 177 4.0 — 600 40.0 103 177 4.0 44.0 600 236 180 4.7 — 600 40.0 130 180 4.7 36 .2 600 200 179 4.8 — 260 30 206 179 4.8 36.2 260 315 176 4.1 — 260 22 200 176 4.1 57.0 260 270 4) E f f e c t of C h l o r i n e on the R e l a x a t i o n of 0 2 Mixtures of C10 2 and C l 2 ( i n the r a t i o s of 1:5 to 1:10) were f l a s h e d using f l a s h energies of 1060, 260 and 160 J . Two o f i l t e r s , 3700 and 3100 A, were. used. The h a l f - l i f e of the 12th l e v e l was measured and i s l i s t e d i n Table XXIX. For compari-son t without C l j , i s a l s o l i s t e d i n Table XXIX. The d i f f e r -* ence i n the behaviour of 0 2 w i t h and without C l 2 i s i l l u s -t r a t e d i n f i g . (53). I t can be seen from Table XXIX t h a t at each f l a s h energy the h a l f - l i f e i s decreased i n the presence of C l 2 . I t a l s o decreases w i t h i n c r e a s e of C l 2 pressure at the same energy The above r e s u l t s can e a s i l y be expl a i n e d w i t h the help of the previous r e s u l t s , i . e . , c h l o r i n e atoms are e f f i c i e n t i n remov-in g the 0 2. I t i s , however, a l s o p o s s i b l e t h a t the e f f e c t i s due t o the c h l o r i n e molecule r a t h e r than atoms, s i n c e i t was not p o s s i b l e to vary the c h l o r i n e atom c o n c e n t r a t i o n at the same C l 2 pressure without simultaneously changing the degree of p h o t o l y s i s of C10 2 which i t s e l f changes the r a t e of r e l a x a -nt t i o n of 0 2. An attempt was made to d i f f e r e n t i a t e between the e f f e c t due to c h l o r i n e atoms and c h l o r i n e molecules by v a r y i n g o the f l a s h energy over a 4 - f o l d range using 3700 A f i l t e r . Since the abso r p t i o n of C l 2 and C10 2 are i n the same r e g i o n , the r e s u l t s of these experiments were e q u i v o c a l , being e q u a l l y w e l l e x p l a i n e d by r e l a x a t i o n by- c h l o r i n e moleculeswith a r a t e 7 _ i - i constant 6.5 x 10 1 mole sec or by c h l o r i n e atoms w i t h a 9 - l - l r a t e constant of 2.8 x 10 1 mole sec . The l a t t e r p o s s i -b i l i t y i s reasonably c o n s i s t e n t w i t h our previous estimate though no f i r m c o n c l u s i o n can be drawn u n t i l the r e l a x a t i o n of * C<2 by c h l o r i n e has been independently measured, no data on t h i s has yet been seen i n the l i t e r a t u r e . W i t h C l , No C l 2 Tl o 2 J L c i o 2 Blank Before 940 /*sec 10.6 40 n o 195 270 405 620 1.6 msec 3.2 F i g u r e 53. Comparison of Decay of 0 2 ( 0 , 1 2 ) l e v e l f o l l o w i n g the f l a s h p h o t o l y s i s o f C10 2 l n the presence and absence of Cl^> C10 2= 0.25 t o r r , C l 2 = 2.5 t o r r , Argon = 2 0 0 t o r r , E r 2 6 o J , F i l t e r = G l a s s f i l t e r A. to L O 235 Table XXIX Comparison of t yz of O 2(0, 12) L e v e l With and Without C l P l a t e no. [ c i o 2 ] t o r r [ c i 2 ] t o r r F i l t e r Prim. Photo. % Energy J t ^ y s e c ) 40 0.25 — — 85 1060 10 46, 0.25 2.5 — 85 1060 7 61 0.25 — 3700 45 1060 130 62 0.25 2.5 3700 45 1060 55 164 0.25 — Pr 40 260 125 164 0.25 2.5 P 40 260 45 118 0.25 — P 30 160 320 59 0.25 1.25 •P 30 160 95 j 59 0.25 2.5 P 30 160 \ 60 5) Decay of E x c i t e d Oxygen i n Case of C l 2 0 and ClO P h o t o l y s i s The r e l a x a t i o n of the h a l f - l i f e of 12th l e v e l was mea-sured f o r four f l a s h e n e r g i e s , using both pyrex and quartz r e a c t i o n v e s s e l s . Measurements of lower l e v e l s were not p o s s i b l e because the continuum of C1 20 was always present. The r e s u l t s are l i s t e d i n Table XXX. it No attempt was made to c a l c u l a t e the r a t e constant of quenching of 0 2 by va r i o u s s p e c i e s . Because of the two primary processes i t i s d i f f i c u l t to c a l c u l a t e the amount of oxygen and c h l o r i n e atoms formed. The r e s u l t s confirm the theory developed 236 from the CIO,,, C 1 0 2 / C l 2 0 systems ( i . e . , c h l o r i n e and oxygen atoms pl a y the important r o l e i n the r e l a x a t i o n of O,,). For the same f l a s h energy and pre s s u r e , the h a l f - l i f e of 0 2 pro-duced from C l 2 0 i s g r e a t e r than t h a t produced from C10 2. This can be expl a i n e d as f o l l o w s . F i r s t l y , the e x t i n c t i o n c o e f f i c i e n t of C1 20 i s much l e s s than the C10 2 and t h e r e f o r e there i s a lower percentage of primary p h o t o l y s i s and lower c o n c e n t r a t i o n of c h l o r i n e and oxygen atoms. Secondly, the c o n c e n t r a t i o n of C1,,0 present a f t e r the primary p h o t o l y s i s i s always higher than ClO., and the l a r g e r p r o p o r t i o n of atoms w i l l r e a c t w i t h C l 2 0 r a t h e r than * quenching 0 2 , I t has a l s o been observed t h a t at the same i n i t i a l pressure of C1 20, the h a l f l i f e decreases w i t h i n c r e a s e i n the f l a s h energy as can be seen from f i g . (54). The in c r e a s e of f l a s h energy w i l l i n c r e a s e the production of atoms and decrease the C l 2 0 c o n c e n t r a t i o n and hence i n c r e a s e the r a t e of r e l a x a t i o n of 0,,. In the f l a s h p h o t o l y s i s of CIO r a d i c a l , only a low con-c e n t r a t i o n of 0 2 i s formed and the decay was r a p i d . The spec-trum was too weak to do any q u a n t i t a t i v e measurements. Q u a l i -t a t i v e l y , however, the r e s u l t s support the c o n c l u s i o n drawn p r e v i o u s l y . The r a p i d r e l a x a t i o n can be ex p l a i n e d by the presence of c h l o r i n e atoms produced i n the primary process as 238 Table XXX H a l f - L i v e s f o r the 0 (0,12) f o l l o w i n g the f l a s h p h o t o l y s i s of Cl^O. P l a t e R.V. [ c i 2 o ] Argon Energy t i / 2 (0,12) no. t o r r t o r r J (ysec) 5 P 1.0 100 800 125 70 P 1.0 200 1060 120 71 P 0.5 200 1060 150 3 Q 1.0 200 800 17 6 Q 1.0 100 800 15 7 Q 0.5 75 800 27 77 Q 0.5 200 600 35 79 Q 1.0 200 260 35 80 Q 0.5 200 260 60 81 Q 0.25 200 260 85 102 Q 1.0 200 600 38 w e l l as by the c h l o r i n e atoms remaining from the f i r s t f l a s h and formed by r e a c t i o n (6) . The c h l o r i n e atom c o n c e n t r a t i o n was c a l c u l a t e d d i r e c t l y from r e a c t i o n s (6-•1) and (6-2) and th a t . remaining from the f i r s t f l a s h t a k i n g i n t o account the recombination 79 of c h l o r i n e atoms found by L i n n e t t and Booth and Bader and ^ n 29,30 Ogryzlo. ' Taking 1.8 x 10 10 i n - l 1 mole sec 1 239 9 —1 -1 and 7 x 10 1 mole sec f o r k 5 3 and k 5 2 , r e s p e c t i v e l y , and the concentrations of atoms found above, the h a l f - l i f e f o r the 12th l e v e l was found to be between 5 to 20 usee, depending upon the i n i t i a l CK> 2 pressure and CIO c o n c e n t r a t i o n before the a u x i l i a r y f l a s h . The experimental value f o r the t o t a l l i f e time was found to vary from 20 t o 60 msec and thus,the agreement i s q u i t e good. CHAPTER V I I CONCLUSION The f l a s h p h o t o l y s i s of C10 2 at room temperature and o w i t h r a d i a t i o n of 2800 to 3700 A has been i n v e s t i g a t e d . The f o l l o w i n g r e a c t i o n s have been shown to occur and to account completely f o r a l l o b s e r v a t i o n s . C10 2 + hv -»• CIO + 0 * 1 0 - 1 -1 O + C10 2 -> CIO + 0 2 ( v , , < 15) k ? = 3.1 x 10 1 mole sec O + CIO C l + 0%{v'_ 14) k, = 7 x IQ-1 1 mole A s e c 9 , , -1 -1 2 CIO -»• Cl„ + 0„ k c = 2.7 x 10 7 1 mole 1 sec -1 '2 V V - ~6 2 ' ~2 k 5 C10(C10 2)+ 0*(v»' = 12) + C10(Cl0 2)+0 2 (V' '<12) 8 —1 —1 k 5 Q = 2 x 10 1 mole sec C10(C10 2)+0* (v* '=6)^ ClO(C10 2)+0*(v' '<6) k 5 Q = 0.8'x 10 8 1 m o l e ~ 1 s e c " 1 C l + 0*(v''=12)-*- C l + 0*(v"<12) k 5 2 = 7 x i o 9 1 m o l e " 1 s e c - 1 C l + 0* (v* '=6) •* C l + 0* (v' • <6) k 5 2 = 2 * 10 9 1 m o l e - 1 s e c " 1 O + O* (v''=12)-> 0 + 0*(v"<12) k 5 3 = 2 x 1 0 1 0 1 m o l e - 1 s e c - 1 O + 0*0'* =6) 0 + 0*(v ' ' <6 ) k 5 3 = 9 V 10 9 1 m o l e - 1 jsec." 1 f • The mechanism d i f f e r s from the mechanism o r i g i n a l l y pro-posed by the a d d i t i o n of r e a c t i o n s 6, 52 and 53 and by the omission of r e a c t i o n s i n v o l v i n g C10 3 and C l 2 0 3 and i t extends e a r l i e r work by the measurements of the r a t e constants k^, kg, k 5 2 ' k53 a n d k 5 0 ^ v ' • A f u r t h e r important d i f f e r e n c e between the present study and previous work i s the demonstration 241 t h a t the energy of hig h e s t v i b r a t i o n a l l e v e l of 0 2 produced i n r e a c t i o n s 7 and 6 i s t h a t corresponding to the e x o t h e r m i c i t y of the r e a c t i o n s (59, 55 Kcals) r a t h e r than t o v " = 8 (34 K c a l s ) . Measurements of the r e l a t i v e values f o r k^ f o r the l e v e l s v'' = 5 t o v' 1 = 13 have shown t h a t these are a p p r o x i -mately equal. Unless there i s a d i s c o n t i n u i t y i n the values f o r k 7 f o r the l e v e l s v " = 0 to v*' = 5, i t f o l l o w s t h a t the major p a r t of the energy of the r e a c t i o n appears i n i t i a l l y i n the form of v i b r a t i o n a l e x c i t a t i o n of 0 2. The e x c e p t i o n a l l y high values of k,-2 a n d kj.^ a r e e x p l a i n e d * by the strong i n t e r a c t i o n between C l and 0 atoms and 0 2 which may lead t o the t r a n s i e n t formation of the intermediates Cl-O-0 and 0-0-0. The f l a s h p h o t o l y s i s of C l 2 0 and the c h l o r i n e and bromine p h o t o s e n s i t i s e d decomposition of C l 2 0 have been s t u d i e d . The r a t e constants f o r a l l the r e a c t i o n s i n v o l v e d have been measured. C1 20 + hv -»- CIO + C l C l 2 + hv -*• 2 C l B r 2 + hv -> 2 Br C l + C1 20 CIO + c i 2 Br + C1 20 + B r C l + CIO CIO + c i 2 o -> c i o 2 + c i 2 CIO + c i 2 o •+ C l + c i 2 + o 2 CIO + CIO -> c i 2 + o 2 k19 = 4 .1 X 10 8 1 , -1 mole -1 sec k 2 5 = 6 .1 X 108 1 , -1 mole -1 sec 5 -1 -1 k20 = 2 .6 X 10° 1 mole sec , ~5 , - l -1 k 2 1 = 6 .5 X 10 1 mole sec k 5 = 2 .9 X 107 1 mole 1 -1 sec 242 The values obtained f o r r a t e constants k 2 g and k 2 ^ are higher by f a c t o r 3 and 10 than those p r e v i o u s l y r e p o r t e d , w h i l e the value f o r k^g agrees w i t h the value p r e v i o u s l y g i v e n as a lower l i m i t . Reaction 25 i s of i n t e r e s t i n t h a t the a l t e r n a -t i v e r e a c t i o n Br + C1 20 -> BrO + C l 2 does not occur. The quantum y i e l d f o r the p h o t o l y s i s and c h l o r i n e photo-s e n s i t i s e d decomposition of C1 20 has been measured and found t o agree w i t h t h a t p r e d i c t e d from the values of the r a t e constants found. The f l a s h p h o t o l y s i s of mixtures of C l 2 0 and C10 2 has been used to measure the value of the r a t e constant 9 -1 -1 0 + C1 20 -»• 2 CIO k 3 5 = 5.3 x 10 1 mole sec and i t has been shown t h a t the a l t e r n a t i v e r e a c t i o n O + ci2o o2 + c i 2 does not occur to a measurable extent. A s i g n i f i c a n t new r e s u l t of the study of C l 2 0 i s the o b s e r v a t i o n t h a t 0 2 i s pro-duced by r e a c t i o n 6 f o l l o w i n g the secondary p h o t o l y s i s of CIO. The r a t e constants f o r the r e a c t i o n s C l + C10 2 * 2 CIO kg = 5.0 x 10 9 1 mole" 1 s e c - 1 Br + C10 9 BrO + ClO ' ^ , k16 = 7 - 2 * 1 0 1 m o l e sec" 243 have been measured by f l a s h p h o t o l y s i n g mixtures of C l 2 and Br2 w i t h C10 2. I t has been e s t a b l i s h e d t h a t n e i t h e r of the r e a c t i o n s C l + C10 2 + C l 2 + 0 2 (12) Br + C10 2 -*• B r C l + Q>2 are s i g n i f i c a n t . This c o n t r a d i c t s previous r e s u l t s (obtained by a flow technique) i n which the absence of BrO as an obser-vable product of r e a c t i o n 16 and the presence of e l e c t r o n i c a l l y e x c i t e d B r C l lead to the proposal t h a t r e a c t i o n 14 predominated. The e x t i n c t i o n c o e f f i c i e n t of BrO r a d i c a l has been measured and using t h i s value and the measured r a t e of decay of BrO pro-duced by f l a s h i n g B r 2 / 0 2 and B r 2 / C l 0 2 m i x t u r e s , absolute r a t e constants f o r the r e a c t i o n s of BrO w i t h BrO and CIO have been measured: 9 -1 -1 BrO + CIO -*• B r C l + 0_ k.. = 1.5x10 1 mole sec 2 40 9 -1 -1 BrO + BrO •> B r 2 + 0 2 k 3 9 = 1.3*10 1 mole sec Reaction 40 could e x p l a i n the production of B r C l * (3ir° v £ 8) . Another i n t e r e s t i n g f e a t u r e of these r e a c t i o n s i s t h a t t h e i r r a t e constants are so much higher than k p. kg i t s e l f i s i n t e r e s t i n g i n t h a t there appears to be two mechanisms f o r the r e a c t i o n . The values p r e v i o u s l y obtained by f l a s h p h o t o l y s i s of C10 2, C l 2 0 and of C l 2 / 0 2 mixtures l i e 7 - 1 - 1 i n the range 1.9 to 8.3 x 10 1 mole sec . These values have been measured and the r e a c t i o n a l s o s t u d i e d using C1 20/C10 2 244 and cn^/Cl^O as sources of ClO. The same v a l u e has been ob-t a i n e d from a l l systems and i s independent of t o t a l p r e s s u r e 7 -1 i n the range 75 to 200 t o r r . The v a l u e of 1.4 * 10 1 mole s e c - i o b t a i n e d a t low (1 t o 4 t o r r ) p r e s s u r e by a f a s t flow technique appears t o be r e l i a b l e and the mechanism a t these p r e s s u r e s i n v o l v e s the f o r m a t i o n of peroxy r a d i c a l 2 CIO + C100 + C l C l + ClOO -* c i 2 + o 2 The p o s s i b l e involvement of a common i n t e r m e d i a t e (CIO)2 i s d i s c u s s e d . The o v e r a l l mechanism f o r the bromine p h o t o s e n s i t i s e d decomposition of CIO2 appears t o be complex w i t h evidence t h a t a v e r y r a p i d recombination of Br atoms occurs i n the presence of C10 2/ p o s s i b l y due t o the f o r m a t i o n of C10 2'Br complex. 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APPENDICES 253 APPENDIX I Quantum Y i e l d o f C 1 2 0 D e c o m p o s i t i o n by CIO A f t e r t h e p h o t o l y t i c f l a s h and t h e r e a c t i o n o f c h l o r i n e atoms w i t h C 1 2 0 , t h e r e a c t i o n s t a k i n g p l a c e i n t h i s s y s t e m a r e CIO + CIO C l 2 + 0 2 (5) CIO + C 1 2 0 .-»• C 1 0 2 + C l 2 (20) CIO + C 1 2 0 -»• C l + C l 2 + 0 2 (21) C l + C 1 2 0 -> CIO + C l 2 (19) and t h e d e c a y o f CIO and C l 2 0 c a n be w r i t t e n as d[.C10] j = k 5 [ C 1 0 ] ^ + k 2 ( ) [ C 1 0 ] [ C 1 2 0 ] d t d[.Cl~0] = k 2 Q[CIO] [ C 1 2 0 ] + • 2 k 2 1 [ c i o ] [ C 1 2 0 ] d t = ( k 2 Q + 2 k 2 1 ) [ C I O ] [ C 1 2 0 ] D i v i d i n g t h e two e q u a t i o n s d [C10] k 5 [ C 1 0 ] 2 + k 2 Q [ C 1 0 ] [ C 1 2 0 ] k 1 0 k 5 [CIO] d [ C l 2 0 ] ( k 2 Q + 2 k 2 1 ) [CIO] [ C 1 2 0 ] k 2 0 + 2 k 2 1 k 2 0 + 2 k 2 1 [ C 1 2 0 ] L e t x = [CIO] y = [ C 1 2 0 ] 2 5 4 •20 k 2 0 + 2 k 2 1 = a and = b k 2 0 + 2 k 2 1 So the above e q u a t i o n becomes or dx dy dx dy a t b ^ b — = a y which i s a standard form o f the eq u a t i o n o f type dx dy - P ( y ) x = Q(y) where P.(y) = b Y and Q(y) = a The s o l u t i o n o f the above eq u a t i o n i s b . y a where c = cons t a n t of i n t e g r a t i o n when [CIO] = 0 , [C1 20] = [ClpO]^ I .e, x = 0, y = y and c = - y«>a d - b ) y." ,b t h e r e f o r e x = a y* (b-l)y, b-1 ( b - U y . y a (b-1) b-1 y a ; b - l or * = y y x b - 1 b-1 y » 255 i f x = x o, y = y Q [CIO] = [ c i o l 0 [ c i 2 o ] = [ C I 2 O I O b - i y n or or o ) c = _ c x. + 1 y a v v x^ 1/c J n . _ C O [b-1 = c] c o v-l/c — - < 1 + = £ > y ^O y Q - ( y Q - V = , , + « !o j-i/c c X o v - 1 / C -j A y r o = AICI.O]^ = y Q [ 1 - d + I y - ' ] A [ C 1 2 0 ] ( [CIO] [ c i 2 o ] [C10] , o [CIO] [ 1 - (1 + o j - l / c ^ [ c i 2 o ] 256 APPENDIX I I C h l o r i n e Atom Concentration a) For l e s s than 40% p h o t o l y s i s , the removal of c h l o r i n e atoms by CK> 2, r e a c t i o n (8), was taken as psuedo f i r s t order [Cl] + C10 2 + 2 CIO (8) Thus the c h l o r i n e atoms c o n c e n t r a t i o n can be given as - "51211 = k f l [ C l 0 9 ] [ C l ] = k' [Cl] dt 8 2 8 where kg '= kg[C10 2] - k l t [Cl] = [ C 1 ] Q e b) For 50% primary p h o t o l y s i s , the second order equation was used because the amount of c h l o r i n e atoms produced w i l l be n e a r l y same as [C10 2] l e f t . So the c h l o r i n e atom _ d[£l]_ = k [ c l ] [ c l 0 j = k . f C l ] 2 [Cl] = [CIO,] dt 8 . 8 2 c o n c e n t r a t i o n can be represented i n terms of C10 2 [ c i ] r c l 0 2 ] o [Cl] = ° ' ° l + k 8 [ c i ] o t l + k 8 [ c i o 2 ] o t c) For _ 60% p h o t o l y s i s , the c h l o r i n e atom produced by r e a c t i o n (6) 0 + CIO -*• 0* + C l (6) i s g i ven by 257 ii£iL = - a i ° L = k roncio] dt d t 6 s i n c e the decay of c h l o r i n e atoms i n the absence of C1C>2 i s n e g l i g i b l e over the p e r i o d concerned. The other assumption i n v o l v e d i s t h a t by the time the measurements were s t a r t e d , CIO2 has been decomposed and the oxygen atom c o n c e n t r a t i o n • j can be c a l c u l a t e d as described i n Chapter IV-A, i . e . , [ 0 ] Q = [ C 1 0 ] Q - [C10]j A l s o , a t the time of measurements (10 y s e c ) , some of the oxygen atoms must have been reacted w i t h CIO t o giv e c h l o r i n e atoms. This can be c a l c u l a t e d from f i g . ( 1 2 ) as f o l l o w s : [ci]o = [ 0 ] o - [ 0 ] ^ where [ 0 ] 1 = co n c e n t r a t i o n of oxygen atoms at 10 ysec and t h i s can be determined as f°lo " t C 1 O ] 1 0 " [ C 1 O ] 1 0 Now [CIO] = [ C 1 0 ] 1 Q - ([O]'o - [0]) = [ClO]10-[O]£ + [O] th e r e f o r e - d_L°JL = k, [O] ([CIO]. . - "[0]' + [0] ) d t 6 1 0 ° [0] = k c a t b e 6 -1 [C10] , where b = r-^- and a = [CIO] n n - [0] [0]' Q 10 o and [Cl] = [ 0 ] 1 - k » , 6 , b e -1 25© Thus the t o t a l c o n c e n t r a t i o n of c h l o r i n e atoms i s [ci] = [ c u o + [o/ o - k a a t b e 6 - 1 * R e l a x a t i o n of N e g l e c t i n g the e f f e c t of oxygen atoms, the r e l a x a t i o n of 0 2 can be represented as d [ l n 0*] dt = k 5 2 [ C l ] + k 5 Q ( [ C 1 0 ] + [C10 2]) a) For £ 40% p h o t o l y s i s , d [ l n O*] -k' t Z • = k . 5 2 [ C l ] 0 e 0 + k 5 Q ( [ C 1 0 ] + tC10 2]) dt , " k8 t - k 5 2 e + k 5 Q where k^ 2 = k 5 2 [Cl] and k ^ = k 5 Q ([CIO] + [C10 2]) k* -k' t * - 52 8 ' or - l n [ 0 2 ] = - ~ e + k 5 Q t + c k 8 when t = 0, [0 2] = [ 0 2 ] Q k' or - l n [ 0 * ] = - — "+ C 2. o , 1 8 k ' * 52 or c = - l n [ 0 2 ] o + *8 I [ 0 2 ] o k52 ~ k8 fc t h e r e f o r e I n — - — = ( 1 - e ) + k C f t t [ o 2 ] k 8 259 when t = t\/z, [0*] = j [0*] In 2 = ^ ( 1 == e ^ U/2i + t , / a  k 8 b) For 50% p h o t o l y s i s , d[lnO*] [C10-1 I , - — = k „ — +| k,.n d t " 1 + [ C 1 0 2 ] o k 8 t ! when t = 0, [0*] = [ 0 * J o * 1 * and a l s o t = t i / z , [0 2] = — [0 2] k t h e r e f o r e In 2 = — — l n ( l + t C 1 0 2 ] Q k g t i ^ ) + k ^ t y k 8 c) For i 60% p h o t o l y s i s , d[lnO*] I a i ~ = k 5 2 ( t C l ] o + [ 0 ] o k T i t - } + k50 d t b e 6 -1 / k,.at » 2J - iV 5 2VL-a. J q • L - ] Q ) t " k 5 2 c - l n [ 0 * = k„([Cll + [ 0 ] J t - r o a [ l n ( b e 6 - l ) - t ] *6< + k 5 Q t + c when t = 0, [0 2] = [ 0 2 ] Q l n ( b - l ) . \ c 2 or c = - ln[0,1 + o c - K 6 * 1 * and a l s o when t = t y , [0 2] = ^ t°2^o i k c a t\/. I k,-9 h _ 6 n. In 2 = k„([Cll + t O ] ) t w - - 2 ± l n ( 2-= _Z± ) 52 o o / i k 6 b - 1 + ty^a k 5 2 + k; 0 t V l 250 4 k k 6 a tth = k „ [ [ C l ] n + [ 0 ] o + a ] t , A - — l n ( b e ~ 1) 5 2 ° ° '* k 6 b - 1 + k 5 0 t l l x R e l a x a t i o n o f by oxygen atoms * The equation f o r r e l a x a t i o n of 0 2 by oxygen atoms was der i v e d i n the s i m i l a r manner as t h a t d e r i v e d f o r c h l o r i n e atoms when p h o t o l y s i s 1. 60%. The r a t e of r e l a x a t i o n of 0 2 i s given by d[ln0*] - — = k„[0] + k-,[Cl] dt 5 3 5 2 i The term k r Q can be neglected, being very s m a l l . S u b s t i t u t i n g the values of oxygen and c h l o r i n e atoms d[ln0*] a / a — = k,, • k c a t + k„[ [ C l l + [01 - k c a t ] dt b e -1 b e -1 • ( k 5 3 - k52> k J t + k 5 2 ( [ C l l o + [ 0 ] o } b e 6 -1 k ca t y 2 1 b e 6 -1 or l n 2 = (k 5 3 - k 5 2) — [ l n b _ ± -kf-tyj 6 + k 5 2 ( t C 1 I o + [°0 t'Jx 

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