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Reactions of iodine atoms, chlorine atoms and of chlorine oxides followed by Kinetic spectroscopy Hunt, James Edgar 1973

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cl REACTIONS OF IODINE ATOMS, CHLORINE ATOMS, AND OF CHLORINE OXIDES FOLLOWED BY KINETIC SPECTROSCOPY by JAMES EDGAR HUNT B . S c . ( H o n s . ) , U n i v e r s i t y o f W a t e r l o o , 1967 M . S c , U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1970 A THESIS SUBMITTED IN THE REQUIREMENTS DOCTOR OF PARTIAL FULFILMENT OF FOR DEGREE OF PHILOSOPHY i n t h e D e p a r t m e n t o f Chemi s t r y We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d /') J  D e c e m b e r , 1973 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of Ctt£M/37^J?y  The University of B r i t i s h Columbia Vancouver 8, Canada Date Z>j?<r- /7/j73 i i ABSTRACT The r e c o m b i n a t i o n o f I atoms i n m i x t u r e s o f NO and i n e r t gas was s t u d i e d by f l a s h p h o t o l y s i s . The m e c h a n i s m p r o p o s e d i s : I + NO + M t INO + M 1. K1>.l = Ki/.K_! I + INO -> I 2 + NO 2 . K 2 2 I N 0 - I 2 + 2N0 3 . K 3 21 + M -> I 2 + M 4 . Kh A new s y s t e m o f t h e INO s p e c t r u m was o b s e r v e d f r o m 310 t o 251 nm w i t h an e x t i n c t i o n c o e f f i c i e n t w h i c h i n c r e a s e s t o w a r d s s h o r t e r w a v e l e n g t h s f r o m 107 t o 1 .57 x ]0k tal^cm"1. A t low p r e s s u r e s o f N 0 ( P N Q < 53 Pa o r 0 . 4 t o r r ) , [INO] i s i n a s t e a d y s t a t e and r e a c t i o n (1) i s r a t e d e t e r m i n i n g . A t h i g h p r e s s u r e s o f NO ( 0 . 6 7 kPa o r 5 t o r r < P N Q < 106 kPa o r 800 t o r r ) , [ I ] i s i n a s t e a d y s t a t e and r e a c t i o n (3) d o m i n a t e s . The f o l l o w i n g r a t e c o n s t a n t s and a c t i v a t i o n e n e r g i e s w e r e m e a s u r e d : K : = 3 - 7 4 x 1 0 9 £ 2 m o r 2 s _ 1 ; E = - 3 . 4 k J m o l " 1 ( f o r M = A r ) 1 a c t ( l ) K 2 > 5 x 1 0 9 A r n o l d s " 1 K 3 = 3 - 9 5 x 1 0 7 A r n o l d s - 1 ; E = 13 k J m o l " 1 a c t ( 3 ) Kk = 4 . 7 2 x 1 0 9 £ 2 m o l _ 2 s - 1 ; E a c t ^ j = - 4 . 3 k J m o l " 1 ( f o r M = A r ) i i i 1 0 7 £ m o l - 1 < K l r l < 2.1 x 1 0 1 0 £ m o l _ 1 3.7^ x 1 0 2 imo]~l > K-i ^ 0.178 Imo]-1 U s i n g a c a l c u l a t e d v a l u e o f A S j = -100 J m o l - 1 K - 1 then 62 k J m o l - 1 <-AHx (I-NO bond s t r e n g t h ) < 81 k J m o l - 1 . C100 i s t h e p r e c u r s o r t o t h e CIO r a d i c a l f o l l o w i n g p h o t o l y s i s o f m i x t u r e s o f C l 2 and 0 2 . The mechanism pr o p o s e d i s : Cl + 0 2 + M t C100 + M 5. # 5 , - 5 = K 5/K - 5 ( f o r M = C l + C100 -> C l 2 + 0 2 6. K 6 C l + C100 -»• 2C10 7. K 7 The C100 s p e c t r u m was o b s e r v e d i n a b s o r p t i o n w i t h [C100] i n e q u i 1 i b r i u r n w i t h [ C l ] and [ 0 2 J . The f o l l o w i n g were measured: K 6 / K 7 = 16 K 5 , - 5 K 7 = 6 - 0 3 X 1 0 9 « , 2 m o r 2 s _ 1 The decay o f CIO was s t u d i e d u s i n g C 1 0 2 , C1 20 and ( C l 2 + 0 2 ) as s o u r c e s . The r e a c t i o n i s t h i r d o r d e r w i t h CIO d e c a y i n g t o C 1 2 0 2 . 2C10 + M t C 1 2 0 2 + M 8. K 8 ) _ 8 = K 8/K_ 8 (M = A r ) i v A new s p e c t r u m a t t r i b u t e d t o C 1 2 0 2 i s o b s e r v e d e x t e n d i n g from 292 to 232 nm 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 i n c r e a s i n g t o -wards lower w a v e l e n g t h s from 360 t o 2.61 x 1 0 3 £mor 1c(n" 1. The e q u i l i b r i u m i s e s t a b l i s h e d s l o w l y g i v i n g : K 8 = 3.74 x 10 9 J l 2 m o l " 2 s _ 1 #8,-8= 2.9 x 10 9 lmo\~l K_ 8 = 1 .29 A r n o l d s ' 1 U s i n g AS1 = -8h J m o l " 1 ^ 1 then AH I = -71 . 5 k J m o l " 1 . C 1 2 0 2 i s pr o p o s e d t o decay by a c h a i n mechanism i n v o l v i n g C l and C100. V TABLE OF CONTENTS Page T i t l e Page i 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 v L i s t o f F i g u r e s . . . . . . . . . i x L i s t o f T a b l e s x i i Acknowledgements x i i i CHAPTER I INTRODUCTION 1 1. R e c o m b i n a t i o n o f I o d i n e Atoms i n NO ... . l 2. C h l o r i n e O x i d e s 6 II EXPERIMENTAL . . . . . 18 1. INTRODUCTION . 18 2. FLASH PHOTOLYSIS APPARATUS . . . . . . . . 22 P h o t o l y t i c F l a s h Lamp 22 R e a c t i o n V e s s e l 23 3. DETECTION SYSTEMS 23 S p e c t r o g r a p h . . . . . 23 Wavelength C a l i b r a t i o n 24 P h o t o g r a p h i c D e t e c t i o n 29 - S p e c t r o s c o p i c Lamp 29 vi CHAPTER Page - P h o t o g r a p h i c P l a t e s . . . . . . . . . . 30 - P l a t e Photometry 31 - B e e r ' s Law 33 P h o t o e l e c t r i c D e t e c t i o n , 35 - L i g h t S o u r c e s 37 - P h o t o m u l t i p l i e r 39 - O s c i l l o s c o p e 43 - Camera 48 - S h u t t e r Assembly 48 - F i r i n g Sequences 49 - D i g i t i z i n g o f T r a c e s 53 4. REAGENTS AND GAS HANDLING 55 Vacuum Rack 57 Temp e r a t u r e C o n t r o l . . . . . . . . 58 5. LEAST SQUARE FI T S FOR EXPERIMENTAL DATA . . . . 59 I I I IODINE ATOM RECOMBINATION . . . . . . . . 60 1. RESULTS 60 Low P r e s s u r e s o f NO . . . . . . . . . . . . . 63 Medium P r e s s u r e s o f NO . . . . . . 71 Hig h P r e s s u r e s o f NO 71 v i i CHAPTER Page 2. DISCUSSION . . . . . . . 81 I d e n t i t y o f t h e T r a n s i e n t S p e c i e s 81 Summary o f Im p o r t a n t E x p e r i m e n t a l R e s u l t s . 82 P o s s i b l e Mechanisms 83 Case A: S t e a d y S t a t e f o r [ I ] Atoms . 84 Case B: S t e a d y S t a t e f o r INO . . . . 85 E x t i n c t i o n C o e f f i c i e n t o f INO 86 V a l i d i t y o f Mechanisms 87 3. SUMMARY 90 IV CHLORINE OXIDES 92 1. RATE OF FORMATION AND DECAY OF CIO FROM THE ( C l 2 + 0 2 ) SYSTEM . . . . . . . . . . . . 92 R e s u l t s • • • o » « * i » » » > » a » < » o " » » » 92 Fo r m a t i o n o f CIO 93 Decay o f CIO 94 D i s c u s s i o n . . . . . . . . . . . 104 2. CIO DECAY FROM C 1 0 2 and C 1 2 0 . . . . . . . . H O R e s u l t s H O E x t i n c t i o n C o e f f i c i e n t o f CIO . . . . 112 Decay o f CIO . . . . . . . . . . . . . I 1 5 a) P h o t o g r a p h i c R e s u l t s . . . . 115 b) P h o t o e l e c t r i c R e s u l t s . . . . 121 vi i i CHAPTER p a g e D i s c u s s i o n 133 I d e n t i t y o f t h e New Continuum 133 Mechanism f o r CIO Decay . . . 136 3. SUMMARY 142 F o r m a t i o n o f CIO fro m ( C l 2 + 0 2 ) . . 142 Decay o f CIO 143 BIBLIOGRAPHY . 144 APPENDIX . . . . . 149 i x LIST OF FIGURES Page 1 . S c h e m a t i c Diagram o f A p p a r a t u s 1 9 2 . A b s o r b a n c e o f C l 2 P l u s B r 2 F i l t e r C e l l 2 5 3 . T r a n s m i s s i o n C u r v e s f o r C o r n i n g G l a s s F i l t e r s 2 6 4 . D i s p e r s i o n Curve f o r Medium H i l g e r S p e c t r o g r a p h 2 7 5. C h a r a c t e r i s t i c Curve o f a P h o t o g r a p h i c P l a t e 3 4 6 . R e s i s t i v e V o l t a g e D i v i d e r f o r P h o t o m u l t i p l i e r 4 2 7 . C i r c u i t f o r U s i n g t he S l i d e - B a c k T e c h n i q u e t o O f f s e t a DC V o l t a g e L e v e l 4 6 8 . O s c i l l o s c o p e T r a c e f o r t h e F i r i n g Sequence: Main Sweep O n l y 5 0 9 . O s c i l l o s c o p e T r a c e f o r t h e F i r i n g Sequence: A l t e r n a t e Sweep 5 1 1 0 . O s c i l l o s c o p e T r a c e f o r t h e F i r i n g Sequence: A D e l a y e d by B 5 2 1 1 . F i r s t O r d e r P l o t o f t h e Reappearance o f I 2 A b s o r p t i o n a t Low P N Q 6 4 1 2 . Second O r d e r P l o t o f the Reappearance o f I 2 A b s o r p t i o n a t Low PJ^ Q 6 5 1 3 . K ^ S £ v e r s u s P^Q i n Low P r e s s u r e Region . . . . . . . . . 6 6 1 4 . K j s t v e r s u s P A r i n t h e Low P^Q Region 6 7 X Page 15. K l s t versus ( p A r ) ( p N C J i n t n e L o w P N 0 R e 9 i ° n 6 8 16. Spectrum of INO 72 17. Second Order Plot of the Reappearance of I 2 Absorption at High Pressures of NO 73 18. First Order Plot of the Reappearance of I 2 Absorption at High Pressures of NO 74 19. I<2n(j versus (1/P^Q) in the High P^Q Region . . . . . . . 75 20. Second Order Rate Constant for the Reappearance of I 2 Absorption versus 1/P^Q from Data Reported by PST. . . . 76 21. Time Dependence of Absorbance Due to CIO and New Continuum 96 22. Time Dependence of Absorbance of CIO and Cl00 Corrected for Differences in Extinction Coefficients at 251 nm . . 97 23. Spectrum of C100 . . . . . . . . . . . . . 98 24. Plot of [Cl]o/[C10]oo versus 1/P^. . . - 99 25. Oscilloscope Trace Showing CIO Decay with Oscillations . 102 26. Second Order Rate Constant for CIO Decay versus Total Pressure (Cl 2 + 0 2) 103 27. Second Order Decay of CIO to Apparent Base Line at 277.2 nm 116 28. Second Order Rate Constant for CIO Decay versus P r^ Measured to Apparent Base Line at 277.2 nm . . . . . . . 117 x i Paae_ 29. Second O r d e r P l o t f o r CIO a t 251 nm. . . . . . . . . . . 122 30. Decay o f CIO a t 292.2 nm 124 31. Decay o f CIO a t 251 nm 125 32. Decay o f CIO a t 240 nm 126 33. Time Dependence o f CIO and C 1 2 0 2 a t 240 nm and 267 J . . 128 34. Time Dependence o f CIO and C 1 2 0 2 a t 240 nm and 1066 J . . 129 35. Decay o f C10 fro m P h o t o l y s i s o f C 1 2 0 134 36. T e c h n i q u e s f o r Measurements t o A p p a r e n t Base L i n e . . . . 150 x i i LIST OF TABLES P a j e 1. F i r s t O r d e r Rate C o n s t a n t f o r I 2 Reappearance a t Low P r e s s u r e s o f NO 69 2. E x t i n c t i o n C o e f f i c i e n t o f INO 70 3. Second O r d e r Rate C o n s t a n t ( K 2 n ( j ) f o r INO Decay a t High P r e s s u r e s o f NO 77 4. [ C l ] /[CIO] v e r s u s 1/P n 100 0 0 0 U j 5. Second O r d e r Rate C o n s t a n t f o r CIO Decay a t 251 nm v e r s u s P^ i n t h e System C l 2 + 0 2 . . . 101 6. Second O r d e r Rate C o n s t a n t f o r CIO Decay a t 277.2 nm ve r s u s u s i n g P h o t o g r a p h i c Measurements i n t h e C 1 0 2 System. . . . . . . . . . . . . . . . . . 118 7. E x t i n c t i o n C o e f f i c i e n t v e r s u s Wavelength f o r C 1 2 0 2 . . . I 3 0 8. C o r r e l a t i o n i n t h e L o s s o f CIO A b s o r b a n c e w i t h t h e G a i n i n C 1 2 0 2 A b s o r b a n c e . . . . . 132 y i i i ACKNOWLEDGEMENTS I am s i n c e r e l y g r a t e f u l t o D r . N . B a s c o f o r h i s e n c o u r a g e m e n t , g u i d a n c e and g e n e r o u s s u p p o r t t h r o u g h o u t t h i s w o r k . I s h o u l d a l s o l i k e t o t h a n k M r . E . F i s h e r f o r h i s i n v a l u a b l e a i d i n t h e e l e c t r o n i c s . I am g r a t e f u l t o t h e W a l t e r C . Sumner F o u n d a t i o n f o r f i n a n c i a l s u p p o r t . I am p a r t i c u l a r l y i n d e b t e d t o M r . B . J e n n i n g s and M r . D. Coombe f o r t h e i r h e l p i n p r o o f r e a d i n g t h e m a n u s c r i p t . I e s p e c i a l l y want t o t h a n k my w i f e , E l a i n e , f o r t y p i n g t h e m a n u s c r i p t and f o r h e r s u p p o r t and u n d e r s t a n d i n g e n c o u r a g e m e n t o v e r t h e p a s t y e a r s . CHAPTER I INTRODUCTION 1. RECOMBINATION OF I ATOMS IN NO D i s c u s s i o n s o f t h e m e c h a n i s m o f atom r e c o m b i n a t i o n r e a c t i o n s have c e n t e r e d a r o u n d two p r o c e s s e s . The e n e r g y - t r a n s f e r mechan?sm i s r e p r e s e n t e d by 2A t A 2 A 2 + M -> A 2 + M w h e r e r e p r e s e n t s an e x c i t e d o r a c t i v a t e d s p e c i e s w h i c h w i l l d i s -s o c i a t e u n l e s s d e a c t i v a t e d by a t h i r d b o d y . The r a d i c a l - m o l e c u l e c o m p l e x m e c h a n i s m i s r e p r e s e n t e d by R + M Z RM* RM* + M •> RM + M RM + R -* R 2 + M The s e c o n d m e c h a n i s m w i l l p r e d o m i n a t e when t h e b i n d i n g e n e r g y o f RM - 2 -e x c e e d s t h e v a l u e (RT) and hence RM-'- c a n be d e a c t i v a t e d t o a r e l a -t i v e l y l o n g l i v e d s t a t e . NO has been f o u n d t o be an e f f e c t i v e t h i r d body f o r t h e r e c o m b i n a t i o n o f s e v e r a l d i f f e r e n t atoms w h e r e t h e r e l a t i v e l y s t a b l e i n t e r m e d i a t e ANO i s f o r m e d . F o r h y d r o g e n ^' ^ , o x y g e n ^ and c h l o r i n e a t o m s ^ ' 7 ^ t h e m e c h a n i s m w r i t t e n i s A + NO + M -»• ANO + M s l o w A + ANO -v A 2 + NO f a s t so t h a t ^ 1 = K[A][N0 ] [ M ] 10 112 -2-1 and i n t h e s e c a s e s K i s i n t h e r a n g e 10 - 10 Si mol s In t h e c a s e o f s u l p h u r a t o m s , t h e e f f e c t s o f s m a l l p r e s s -u r e s o f NO on t h e r a t e o f f o r m a t i o n w e r e c o n s i d e r e d b e s t e x p l a i n e d by t h e m e c h a n i s m * 7 7 ^ S + NO + M t SNO + M K S + SNO -*• S 2 + NO K x f r o m w h i c h 4 I | L L = X K . [ N0][ S ] 2 d t I w h e r e K K} > 2 x 101^ £ 2 m o l ~ 2 s - 1 . Engleman and D a v i d s o n ^ f r o m a s t u d y o f t h e r e c o m b i n a t i o n o f I atoms f r o m f l a s h p h o t o l y s i s r e p o r t t h a t t h e r e c o m b i n a t i o n i n - 3 -the presence of NO was too rapid to be studied. Three runs were made at 323 K using pressures of 5 8 . 8 kPa (442 torr) of NO, 36 kPa (27 torr) of NO with 19-4 kPa (146 torr) of argon, and 0 . 13 kPa (1 torr) of NO with 14.6 kPa (110 torr) of argon. They estimate that the third order rate constant for removal is almost 270 times 14 2 -2-1 larger than that for I 2 with a lower l imit of 4 .8 x 10 I mol s In a further study of the recombination of I atoms in NO by flash photolysis, Porter, Szabo and Townsend^(PST) covered a much wider range of NO pressures. They found that low pressures of NO (less than 0.13 kPa or 1 torr) along with a large excess of Ne were suff ic ient to produce an extremely rapid iodine atom re-combination. For pressures above 0.13 kPa (l torr) the rate became too fast to be measured. At pressures of approximately 2 .7 kPa (20 torr), or higher, the rate once more became slow enough to be measured and at the same time a new transient absorption which they identif ied with INO appeared from 400 to 460 nm. As the pressure of NO was increased further, the intensity of the tran-sient increased as did the in i t i a l decrease in intensity of iodine absorption measured at 520 nm. They obtained the extinction coef-f ic ient by equating the rate of loss of INO with the rate of for -mation of I5< Low pressure rate constants were obtained from an - k -iodine pressure of k Pa(0.03 torr) and from I measurements at 520 nm while high pressure results were obtained from an iodine pressure of 370 Pa (3.6 torr) and from the INO absorption at 430 nm. They explain their results in terms of the mechanism I + NO t INO K = K x/K_ 2 I + INO -*- I 2 + NO K 3 2IN0 -v I 2 + 2N0 K 4 in which equilibrium between I and INO is always maintained. Using the def init ion for the observed second order rate constant as = K. [I + INO] 2 dt b then at low pressures of NO they derive K b = K 3 X[N0] and at high pressures of [NO] K b = ITNOT + K 4 At low NO pressures they find that varies direct ly with [NO] giving K^K = 1.1 x 109 Jlmol~1s"1mm"1. At high pressures a plot of versus 1/[NO] gives K3 , r - , J 0 „ ,-1 -1 = 1.5 x 10 JLmol s mm and < k x 1 0 7 Arnolds" 1 - 5 -al l values being for the temperature 333 K. Combining these data g i ves = h x 109 J lmol"^" 1 and K = 6 x 1(T Jlmol -1 -1 By estimating AS for reaction (l) as -85 Jmol K and using the measured value of K the heat of formation of JNO from 1 and NO was found to be hi kJmol \ Then by assuming E for act reaction (3) is zero the observed activation energy should range from -kl kJmol 1 at low [NO] to +kl kJmol 1 at high [NO] . However their measurements gave values of -10 and 16.7 kJmol ' at low and high [NO] respectively. They suggest that reaction (k) is contr i -buting s ignif icant ly in the high pressure region and has an a c t i -vation energy less than k2 kJmol ^ as a partial explanation for this discrepancy. Later work by Porter and Hussain suggested minor revisions in the numerical constants obtained. The observed ac t i -vation energy at low [NO] was found to be -21 kJmol ' and a more detailed stat is t ica l mechanical calculation of partit ion functions gave AS for reaction (l) as -100 Jmol 'k 1 which when combined with the previously obtained K gave AH^  = -kS kJmol Thus the discrepancy between experiment and theory remained. (9) In a review article,Benson and DeMore suggested that - 6 " the mechanism could be incorrect. They point out that if reaction ( 1 ) requires a third body and if i t is assumed that [INO] is in stationary state rather than equilibrium then at low NO the rate 1 aw is d[I 2] = K 3£[N0J [ I ] 2 dt 1 + (K [I]/K_,[M]) At high [NO] if [I] is assumed in steady state then ~dT = V 1 N 0 ] j 1 + [N0] + (K 3[IN0]/K 1[M])} where no term is negl igible. In another review art ic le , Heicklen and Cohen^""^ suggest that, at high pressures of NO, i f reaction (h) dominates then the extinction coeff icient reported for INO by PST may be in error by a factor of two since PST equated the rate of decay of INO with formation. The value of K and would then be too large and too small by a factor of One purpose of this study was to further elucidate the mechanism for I recombination with NO and to resolve the discrep-ancies reported by PST. 2. CHLORINE OXIDES The absorption spectrum of CIO was f i r s t investigated - 7 -by P o r t e r ^ ' ) ( ^ ) ^ Subsequent work by D u r i e and R a m s a y ^ ^ e s t a b -2 2 (14) l i s h e d the t r a n s i t i o n as A II. -<- X II. . Basco and Morse have i i r e p o r t e d the vacuum u l t r a v i o l e t a b s o r p t i o n s p e c t r u m o f CIO and " -1 (15) f i n d AG, = 845 ±4 cm . R o c h k i n d and P i m e n t e l have r e p o r t e d a v i b r a t i o n a l f r e q u e n c y o f 970 ± 20 cm 1 f o r CIO i n t h e i r s t u d y o f t h e p h o t o l y s i s o f m a t r i x - i s o l a t e d Cl^O a t 20 K. O'Hare and W a h l ^ ^ c a l c u l a t e d a v a l u e o f 975 cm 1 based on a m o d i f i e d (13) B i r g e - S p o n e r e x t r a p o l a t i o n u s i n g D u r i e and Ramsay's v a l u e o f the CIO d i s s o c i a t i o n energy w i t h x g = 8.6 x 10 . Andrews and Raymond^' 7^ o b s e r v e d two i n f r a r e d bands a t 995 and 850 cm 1 i n t h e i r m a t r i x r e a c t i o n o f C^O w i t h a l k a l i metal atoms. They a s s i g n e d t h e f o r m e r t o the fundamental and t h e l a t t e r t o t h e f u n d a m e n t a l p e r t u r b e d by a n o t h e r m o l e c u l e i n the m a t r i x s i t e . They admit the o p p o s i t e a s s i g n m e n t c o u l d not be r u l e d o u t . (1 8) J o h n s t o n e , M o r r i s and Van den Bogaerde (JMV) u s i n g a molecu-l a r m o d u l a t i o n t e c h n i q u e f o r t h e p h o t o l y s i s o f m i x t u r e s o f (C1^ + 0^) o b s e r v e d C10 i n t h e u l t r a v i o l e t but not i n the i n f r a r e d . However they d i d not l o o k below 850 cm \ (19) P o r t e r and W r i g h t , i n t h e i r s t u d y o f the C10 r a d i c a l f r o m p h o t o l y s i s o f ( C ^ + 0^) t f ' n c ' n e a r l y a z e r o t e m p e r a t u r e dependence f o r t h e f o r m a t i o n and decay o f C10. They p o s t u l a t e d a s h o r t l i v e d p e r oxy r a d i c a l C100 as a p r e c u r s o r t o the C10 - 8 -radical in order to explain the observation that Cl atoms are removed 46 times faster in O2 to form CIO and C l 2 than in N 2 to orm 012' Direct evidence of the existence of C100 has been obtained only recently. C o l e ^ ^ detected an electron spin resonance ascribed to C100 in an irradiated perchlorate crysta l . In a matrix isolation study Rochkind and Pimentel^"^ assigned a pair of infrared absorption bands to C100 and a positive identif icat ion (21) of C100 in an argon matrix was made by Arkell and Schwager in an infrared study. Gaseous C100 was observed in the infrared and ultraviolet by JMV.^ l 8^ Porter and Wright found that reactions by which CIO is removed are unaffected by the pressure of any gas other than CIO. The reaction was second order with respect to CIO and had no a c t i -vation energy. Chlorine atoms were excluded from the decay of CIO because they are rapidly removed at early times. The simple double decompos i t ion 2C10 -> C l 2 + 0 2 was rejected since a large activation energy would be expected for such a reaction. They therefore suggest the poss ib i l i ty of dimer-ization. 2C10 t C1 2 0 2 C , 2 ° 2 * C 1 2 + ° 2 2 " - 9 -If >> K_ j then reaction (l) becomes the rate determining step. However it would be d i f f i cu l t to explain such a slow reaction, with no activation energy, entirely by means of a ster ic factor. They mention the poss ib i l i ty that ^2^2 m ' 9 n t ' 3 e a "stable" molecule which would decompose very slowly to C1^ and 0^ . They therefore prefer to think that the reaction is best explained in terms of an equilibrium between CIO and C ^ C ^ J ^ J >> l^ .wi th the latter decomposing to C ^ and O2. The observed activation energy wi l l be(Ej -E_j + E2). The difference (E_^ - Ej) is equal to the heat of formation of ^2^2 from two CIO radicals and must nearly equal E2 , the activation energy for dissociation of t o ^2 a n c' 0^. The rate constant for CIO removal was reported as 7-2 x 10 e exp (0 ± 650/RT)£mol 's 1 where e is the molar extinction coeff ic ient at 2 5 7 • 7 nm. (22) Nicholas and Norrish investigated the formation of CIO from flash photolysis of C ^ and O2 mixtures. They also used the ClOO intermediate as a precursor to CIO via the mechanism Cl + 0 2 + M J ClOO + M 3. Cl + ClOO •* 2C10 4 c i + c i o o - > c i 2 + o 2 5 . Since the decay of CIO is very slow compared to its formation it was not considered. They assumed ClOO concentration to be stationary - 10 -during the period of CIO formation. Then the rate of Cl atom removal is direct ly proportional to the rate of formation of CIO. This implies that the half l i f e of Cl removal equals the half l i f e for CIO formation and =(ln2)/2K [0 2 ] [ M ] 8 2 — 2 — 1 From this was evaluated as 6 .2 x 10 1 mol s . Using this in the expression for CIO formation ^ir1- T 0 K ^ 1 0 , 1 1 0 2 1 [ M 1 (which seems to be in error by a factor of two) they evaluated K(./K^ from an estimate of [Cl] and measuring ^ ^ t ^ ' ^ 6 r e s u ' t s gave K c / K , = 7.7(enc^ -. = 680 A m o r ' c m " 1 ) or 15 (e„r-, n a ]-9 x 1 q 3 i> H lol• / i-Ol • I &mol c^m ^ ) where e 9 t- 7 7 i S the extinction coeff ic ient of CIO at (23) 2 5 7 - 7 as measured by Lipscomb, Norrish and Thrush and Clyne and (24) Coxon respectively. Following Porter and Wright further work on both the rate constant for CIO removal and its extinction coeff ic ient yielded confl ict ing results. (23) Lipscomb, Norrish and Thrush (LNT) in a study of the flash photolysis of ClO^ confirmed that the decay of CIO was second order (after correction for a weak continuous absorption which was observed mainly below 300 nm and attributed to C10^). The formation - 11 of ClO^ was explained by the reactions: C102 + hv -* CIO + 0 o + cio 2 c io 3 since the amount of ClO^ formed depended on flash energy (for a fixed amount of ClO^) and no inert gas effect was observed. The apparent rate constant varied from 1.9 x 10^ to 5.9 x 10^ Jlmol 's ' and extinction coeff icient at 257-7 nm from 1.14 x 10 to 690 £mol 'cm ' as the flash energy was increased from 240 to 1620 J . The extinction coeff icient was measured by the assumption that each molecule of C10^ decomposed wil l give one molecule of C10. The second order plots showed increasing slopes with a constant intercept as flash energy was increased. They suggest these observations could be explained in terms of the equilibrium cio + c io 2 t c i 2 o 3 3 -1 where K = 1.5 x 10 Atm at 293 K. However approximate values of the part it ion functions of C10, C12^3 a n c ' C10^ indicate the formation of C1 w o u ^ be about 62 kJmol ' exothermic for this value of K and it would be unexpected that equilibrium would be established rapidly enough to be important. They, therefore feel that this explanation is unl ikely. (78) Edgecombe, Norrish and Thrush ' investigated the photolysis - 12 -of C1^0 by the flash technique and found the second order rate constant for CIO removal to be l.k x 10^ Jlmol 's ' in close agree-ment with the low energy value of LNT. From their published data the extinction coeff icient can be ca lculated^ 2 ^ as 5^ 0 Jlmol 'cm ' in agreement with the high flash energy value of LNT. They later (26) suggest that the destruction of CIO radicals by oxygen atoms may account for the decrease in e with increase in flash energy reported by LNT. (27) This suggestion was taken up by Clyne and Coxon who, using a discharge flow system showed that the ratio of rate constants for the reaction of oxygen atoms with ClO^ and CIO was less than 7 and probably equal to k. However a recent report by Bemand, Clyne (28) -2 (25) and Watson give this ratio as 10 . Basco and Dogra have found it to be k. In any case the reaction 0 + CIO + Cl + o2 is important in the high, energy photolysis of C102 and provides an explanation for the variation in the apparent CIO extinction coe f f i -cient observed by LNT. This explanation did not apply to the C1^0 system studied (78 ) by Edgecombe et al . However an investigation of the C120 system (29) by Basco and Dogra gave an extinction coeff icient for C10 at (29) 257.7 nm i  agreement with that obtained by them in the C10^ system - 13 -3 -1 -1 Their value of I J x 10 £mol cm at 257-7 nm was in good agree--1 1 (2k) ment with the value of 1.9 £mol cm" reported by Clyne and Coxon from a flow system where the CIO concentration was determined by t i t rat ion with NO or 0 atoms. Al l investigators who have studied the kinetics of CIO agree that it decays by.a.second order process in CIO. However the rate constants obtained scatter over a wide range. In an (27 2k) attempt to explain these discrepancies Clyne and Coxon sug-gest at high pressures CIO decays via an intermediate w ' t h a chaperon molecule M involved while at low pressures it decays via the peroxy radical as proposed by Benson and Buss^*^ who studied the halogen catalyzed decomposition of UJ) and required a chain propagating step 2C10 •+ C100 + Cl Since Porter and Wright proposed the reverse of this reaction (for the formation of CIO from Cl,, and 0 2) to be thermoneutral it is reasonable that this may occur and be rate determining for the (2k) decay of CIO. Clyne and Coxon reported convincing evidence for this reaction by adding chlorine atom scavengers (such as Cl^O, Br 2 and 0 )^ to a flow system containing CIO and concluded (18) that Cl atoms were produced during the CIO decay. JMV (using their molecular modulation technique) proposed a ten step - 14 -mechanism involving both decay processes which are reproduced here: C l 2 + hv -> 2C1 a Cl + 0 2 t ClOO + M b,c (Kj) Cl + ClOO Z 2C10 d , e ( k 2 ) Cl + ClOO -> c i 2 + o 2 f 2C10 + M $ c]2°2 + M 9'h Cl 2 0 2 + M -»- C l 2 + 0 2 + M i 2C1+M^-C1 2 + M j In order to simplify the kinetics they assume equilibrium between (Cl + ClOO) and CIO, between CIO and C1 2 0 £ and between (Cl + 0 £) and ClOO. Two of the equilibrium constants were estimated from thermo-(13) dynamic properties of ClOO and CIO. Durie and Ramsay estimate by spectroscopic methods the dissociation energy of CIO and hence JMV calculate the AH .^ for CIO. From the rotational constants given (31) by Durie and Ramsay, the spin orbit coupling constant and an estimate of the vibrational frequencies the entropy is calculated for CIO. From their observed rate constant for reaction d and a (32) theoretical estimate of the preexponentia 1 factor they find that the activation energy for reaction d must be close to zero. Thus, making use of the activation energy of reaction e reported (27) by Clyne and Coxon , a value for AH .^ for ClOO is calculated. (21) From the fundamental frequences of ClOO and the geometry of - 15 -C100 an estimate of the entropy of C100 was calculated. These thermodynamic properties were used to evaluate Kj = 0.089 Atm 1 and K2 = 227- A value for b reported by Nicholas (22) 8 — 1 — 1 and Norrish (6.2 x 10 £mol s ) was used to determine c from Kj. JMV made direct measurements of d, f and ig/h. It was pointed out that ig/h can be identif ied with the third order rate constant for 2C10 + M->Cl 2 + 0 2 + M They make the point that at low total pressures Clyne and Coxon were in a region where the rate constant measured is largely given by the constant term e and thus their rate was largely independent of M. They include Clyne and Coxon's low pressure data with their high pressure points in a plot of rate constant versus pressure. However the data have suff ic ient scatter that extrapolation to zero pressure is not re l iab le . They speculate that the fa i lure of Porter to find an activation energy or inert gas effect on the rate constant for CIO decay may be due to the combination of an adiabatic temperature'rise, which might be expected in their system, and a pressure effect . Thus near cancel-lation of these two may have caused Porter to miss both of them. It may also be noted that the ratio of f:d was found to be 108 which is a factor of about 7 higher than that reported by Nicholas - 16 ~ and Norr ish^ 2 2 ) . (33) In a more recent paper Clyne and White redetermine a value for e and find no signif icant third body effect over the low pressure range of 60 to 1100 Pa. However, the scatter of their points is such that the third order rate constant (ig/h) reported by JMV lies within the limits of error. The value of e calculated by JMV is about a factor of 4 less than that found by Clyne and White. They point out that the method of calculating AH^ . of ClOO used by JMV could easily be in error by 10 kJmol \ Thus using the measurement of d from JMV and their value for e, AH^ . of ClOO is found to be 93 kJ mol ' . In a recent study Dogra^^ and Basco and D o g r a ^ ^ ^ " ^ found the decay of CIO to be second order and independent of third body pressure. The same rate constant (2.7 x 10 7 £mol 's ') was obtained by using (C1 2 + 0 2^ » C 1 0 2 ' a n d C 1 2 ° a s s o u r c e s o f c l ° radicals. This value is a factor of ten smaller than that predicted by JMV at 26.7 kPa (200 torr ) . The explanation of Cl cancellation of the temperature and pressure effects suggested by JMV to ration-a l ize the results of Porter and Wright were considered unconvincing. The activation energy in the high pressure region used by Porter and Wright must apply to reactions governed by ig/h and not reaction e. The activation energy of 2.5 kJ reported by Clyne must apply - 17 -to reaction e. In addition an increase in temperature would cause an increase in the rate constant (assuming a positive a c t i -vation energy) and hence the rate constant observed by Porter and Wright should be larger than that observed by JMV. In fact the reverse is the case. This study was undertaken in an attempt to resolve the confl icts between the results of JMV, Porter and Wright, and Basco and Dogra. It was also hoped to examine the difference of a factor of 7 between JMV and Nicholas and Norrish for the ratio f : d . - 18 -CHAPTER II EXPERIMENTAL 1. INTRODUCTION The technique of flash photolysis and kinetic spectros-( 3 6 ) ( 3 7 ) copy was f i r s t developed by Norrish and Porter in 1950. Since then many instrumental improvements have been made. This powerful and versati le experimental method has proved useful in a number of f i e lds . These include both gas phase and solution photochemistry, energy transfer processes, free radical spectros-copy, and various biological systems. The main features of the technique are essential ly the same for most applications. Excel 1 ent review art ic les on flash photolysis and kinetic spectroscopy have been published(38)(kk). The basic outline of the apparatus used in this invest-igation is shown in figure 1. A convenient lamp configuration wi FIGURE 1. SCHEMATIC DIAGRAM OF APPARATUS P A. E l e c t r o n i c Delay U n i t ; EL Lyman Tube; £ . Condenser(10 kv, 2 y F ) ; D_. Con d e n s e r ( 1 0 kv, 33.3 y F ) ; E_. I n d u c t i o n P i c k - u p ; f_. Spark Gap; G_. Condensing Lens; H_. H o l l o w E l e c t r o d e ; J _ . P h o t o l y s i s Lamp; J_, R e a c t i o n V e s s e l ; K. Brass C a s i n g ; L_. C o n n e c t i o n t o Vacuum System; M. S p e c t r o g r a p h ; P.. D e t e c t o r ( P h o t o g r a p h i c P l a t e o r P h o t o m u l t i p l i e r ) ; R^. R e s i s t o r - 50 ka (A, B_, C_, and E_ r e p l a c e d s t e a d y l i g h t s o u r c e f o r p h o t o e l e c t r i c work) I - 20 -t h e r e a c t i o n v e s s e l p a r a l l e l t o t h e p h o t o l y s i s lamp was u s e d . B o t h w e r e e n c a s e d i n a b r a s s c y l i n d e r l i n e d w i t h an a l u m i n u m f o i l r e f l e c -t o r , and f i t t e d w i t h s e v e r a l i n l e t t u b e s f o r t e m p e r a t u r e c o n t r o l by a i r f l o w . T h e r e was s u f f i c i e n t room t o p l a c e v a r i o u s g l a s s f i l t e r s between t h e r e a c t i o n v e s s e l and p h o t o l y s i s l a m p . A h i g h v o l t a g e power s u p p l y was u s e d t o c h a r g e a D u b i l i e r 33-3 yF, 10 kV r a p i d d i s c h a r g e c a p a c i t o r w h i c h , when c h a r g e d t o 8 k V , gave a d i s c h a r g e e n e r g y o f a b o u t 1066 J . A l l e x p e r i m e n t s w e r e c a r r i e d o u t i s o t h e r m a 1 1 y . T h a t i s , a l a r g e e x c e s s o f i n e r t gas ( u s u a l l y a r g o n ) was added t o t h e r e a c -t i o n m i x t u r e s t o i n c r e a s e t h e h e a t c a p a c i t y o f t h e s y s t e m s u f f i c i e n t -l y t o l i m i t t h e t e m p e r a t u r e r i s e t o l e s s t h a n a few d e g r e e s . O b s e r v a t i o n o f t r a n s i e n t s p e c i e s f o l l o w i n g d i s c h a r g e o f t h e p h o t o l y s i s lamp was c a r r i e d o u t by e i t h e r p h o t o g r a p h i c o r p h o t o -e l e c t r i c m e t h o d s . T h e s e a r e d e s c r i b e d i n more d e t a i l l a t e r i n t h i s c h a p t e r . In p h o t o g r a p h i c d e t e c t i o n , a b s o r p t i o n s p e c t r a o f t h e r e a c t a n t m i x t u r e w e r e r e c o r d e d a t v a r i o u s d e s i r e d d e l a y t i m e s f o l l o w -i n g t h e p h o t o l y t i c f l a s h . A s e r i e s o f s u c h s p e c t r a w e r e u s e d t o o b t a i n t h e t i m e d e p e n d e n c e o f any o b s e r v a b l e s p e c i e s . A Lyman t u b e ( s p e c t r o s c o p i c f l a s h lamp) was u s e d t o p r o d u c e a b u r s t o f l i g h t f o r t h e a b s o r p t i o n s p e c t r a . The l i g h t was f o c u s e d down t h e r e a c t i o n - 21 ~ vessel and on the s l i t of the spectrograph by a spherical quartz lens. The Lyman tube capacitor (Dubilier 2 yF, 10 kV rapid d is -charge) was usually operated at 9 kV giving a discharge energy of about 80 J . Photolytic changes caused by the spectroscopic flash were considered negl igible. Photoelectric detection was accomplished by monitoring the intensity of a continuous light source (Hg, Xe or quartz-iodine lamps) at a given wavelength after passing through the reaction vessel. Light intensity was monitored with a photo-multipl ier tube which was mounted at the exit focal plane of the spectrograph and could be tracked along this plane to any desired wavelength. It can thus be seen that the photoelectric technique gives a complete time dependence of the absorbance in one experi-ment, but only at one wavelength. The experiment must be repeated for each desired wavelength. In contrast, the photographic tech-nique gives, in each experiment, an absorption spectrum across a wavelength range but only at a single chosen time following the photolytic f lash. Several experiments are required to build up a time sequence. Both techniques are complimentary and were used in this investigation. It should be noted that photoelectric measurements - 22 -are more accurate and a great deal more sensitive than photographic measurements. There are, however, several limitations to photoelec-t r i c detection which wil l be discussed later. The development of a photoelectric apparatus was crucial to this investigation. 2. FLASH PHOTOLYSIS APPARATUS Photolytic Flash Lamp The photolysis lamp (Suprasil quartz) was 50 cm long with an 8 mm inside diameter and a wall thickness of 1.5 mm. The electrodes were made of tungsten and were soft soldered into brass cones which were black waxed into BIO sockets at the ends of the lamp. One terminal connected to the high voltage lead of the cap-acitor while the other was connected to ground through a spark gap. The ground electrode was hollow to permit f i l l i n g of the lamp with argon to a pressure of about 2 kPa (15 torr ) . A large resistor (50 kQ) from the positive end of the capacitor to the ground term-inal of the lamp maintained both ends of the lamp and one end of the spark gap at high voltage until the spark gap was triggered, allowing the capacitor to discharge through the lamp and spark gap to ground. The photoflash rise time and half peak width were about - 23 " 5 and 15 ysec respectively. Reaction Vessel The reaction vessel (Suprasil quartz, Englehard Industries Inc.) was k~J cm in length with an inside diameter of 20 mm and wall thickness of 1 mm. Plane windows (also Suprasil quartz) were fused to the ends. In i t i a l l y , the windows were fixed to each end with an epoxy resin, however the resin was not inert to halogens (especially iodine). A side arm was attached to allow f i l l i n g and pumping. There was suff ic ient clearance between the photoflash lamp and re-action vessel to allow insertion of strips of glass f i l t e r s . Thus, by use of the appropriate f i l t e r , it was possible to limit the photolysis light to desired wavelength regions. The transmission curves of the f i l t e r s used are shown in figures 2 and 3. 3. DETECTION SYSTEMS Spectrograph Most of the work was done using a medium Hilger (model E7^2) prism instrument employing quartz optics. The spectral range covered from 200 to 660 nm over 20 cm and could be recorded on a single photographic plate measuring about 25 by 10 cm. This made it ideal for survey work. In addition, the good dispersion and - 2k ~ speed below 250 nm made it well suited for work in this region. The spectrograph could be f i t ted with either a plate holder or a photomultiplier housing as the exit focal plane. The plate holder held glass photographic plates up to 25 cm by 10 cm. A racking mechanism allowed the plate to be advanced, exposing a fresh str ip of the plate for each exposure. Typical ly , around 20 to 30 exposures were taken per plate depending on the str ip height. The photomu1tip1ier housing contained a gear mechanism and counter which allowed the photomu1tipiier to be reproducibly moved to any point along the focal plane. A mirror and s l i t assembly effect ively provided an exit s l i t for the spectrograph behind which the photomultiplier was situated. In addition to the medium Hilger instrument, a Jarrel l Ash 3.** meter, Ebert mounting grating spectrograph was used. These have been described el sewhere (^ 6)^  photoelectric work was carried out on the medium Hilger instrument. Wavelength Calibration A cal ibrat ion for the medium Hilger spectrograph was needed in order to position the photomultiplier at desired wave-lengths. A curve was drawn from 208 to 390 nm. Above 220 nm iron arc lines were used as a reference. Below 220 nm (where there - 25 " FIGURE 2. ABSORBANCE OF C 1 2 PLUS B r 2 FILTER CELL C l 2 : i = 3 cm, P = 93 kPa(700 t o r r ) B r 2 : £ = 5 cm, P = 32 kPa(200 t o r r ) I 1 1 1 1 1 1— 250 300 350 400 ^50 500 550 X (nm) FIGURE 3. TRANSMISSION CURVES FOR CORNING GLASS FILTERS 7-54 \ (nm) - 28 -are few iron emission lines) a copper arc, s i l i c a lines present in the continuum of the spectroscopic flash lamp, and several atomic lines of P, As and Sb (observed following the flash photolysis of (45) P H 3 , ASH3 and SbH3 ) were used. Iron standards were taken from (47) prints supplied by Adam Hilger Ltd. Standards for copper and s i l i c a were taken from the Chemical Rubber Company Handbook^^ . Accurate measurements of wavelength versus plate position were obtained using a Grant 1ine measuring comparator (Grant Instruments Inc.). The calibrated region was divided into 26 sections and the cal ibration lines within each section were f i t ted by the method of least squares to a cubic equation. These equations were then * used to generate a table of wavelength against plate posit ion. Above 400 nm cal ibration was carried out by placing a monochromator (Jarre 1.1 Ash model 82-410) in front of the spectro-graph s l i t . This is a calibrated grating monochromator with band-pass and wavelength accuracy comparable to or better than the medium Hilger in this wavelength region. Light from a quartz-iodine lamp was focused onto the s l i t of the monochromator from which a wavelength was chosen and its exit position from the spectrograph was noted. In this way the calibration was extended to 500 nm. - 29 " A comparison of the scales for the calibration curve and the position counter of the photomultiplier housing was done using a low pressure mercury arc (model SCT1I Ultra-Violet Products Inc., San Gabriel, C a l i f . ) . Only a linear shift was required since both were in position units of mm. The dispersion curve is shown in figure 4. Photographic Detection Spectroscopic Lamp: A Lyman tube provided the photographic flash from which an absorption spectrum could be recorded (see figure 1 ) . The flash was triggered at any desired delay time after the photolysis f lash, thus recording reactant spectra at a given time. The lamp output gave a continuous spectrum with a few s i l i c a absorption and emission l ines. The discharge through the lamp was directed along a 50 mm length of capi l lary quartz tubing which was opt ica l ly aligned with the reaction vessel and spectrograph. The light was focused down the reaction vessel onto the s l i t of the spectrograph. A f i l l i n g pressure of about 8 kPa (60 torr) of argon was used. The half peak width for the lamp discharge was k ys. The electrodes were made of tungsten and were soft soldered into brass cones which were black waxed into BIO sockets on the lamp. - 30 " One terminal was connected to the high voltage lead of the capaci-tor while the other was grounded through a thyratron. When the thyratron was triggered it became conducting and allowed the lamp to f i r e . Photographic Plates: In the region of 220 to 660 nm l l ford HP3 plates were used. They are fast panchromatic plates of medium grain and con-trast and were supplied with a mauve anti-halation backing which cleared on development. The high speed is part icular ly marked for short times of exposure. For 200 to 220 nm l l ford Q2 plates were used. They are intended for recording radiatjon normally absorbed by gelatin emulsions and hence have a very thin emulsion surface and are extremely pressure sensit ive. Below 200 nm l l ford HP4 f i lm sensitized with sodium salicylate(NaSal) was used. The HP4 emulsion type is highly sensitive to 450 nm light which is approximately the peak of NaSal fluorescence. The fluorescence quantum yield is essential ly unity from 60 to 340 nm^^ . The f i lm was sensitized by dipping it into a 0 .5 M solution of NaSal in 35% methanol and then drying with a warm air blower. Best results were obtained i f the solution was prepared just before sensi t iz ing the f i lm. - 31 -All photographic plates were developed in Kodak D19 developer for 5 minutes at 293 K with constant agitat ion. They were then rinsed in a stop bath of 3% acetic acid for 30 seconds, fixed in Kodak Rapid Fixer for 2 minutes, rinsed in flowing water for 20 to 30 minutes, rinsed with a wetting agent to eliminate water marks, and f ina l l y dried in a dust free container. Plate Photometry: Plate density. (PD) is defined as PD = log(D /D j 3 o t where D q is the intensity of a beam of light passing through an unexposed portion of the plate and D t is the transmitted intensity through an exposed portion of the plate. Measurements of plate density were made on a Joyce Loebel MK i l l c recording microdensi-tometer. The dependence of plate density on log (exposure) is called the characteristic curve for the photographic plate. Expos-ure equals intensity times exposure time. For a given light source intensity depends direct ly on the s l i t area and hence depends on s1i t wi dth. A typical characterist ic curve is shown in figure 5 . Since this curve varies with wavelength, emulsion type, and conditions - 32 -of development, characterist ic curves were plotted for the wave-length regions of interest for each plate used. One method of determining this curve was to record a series of exposures at different s l i t widths and plot PD versus log (s l i t width). This assumes reproduceable spectroscopic lamp f i r i ng , both in flash intensity and duration, and uniform s l i t i l lumination. A more convenient method was to use a seven step neutral density f i l t e r (model F l 2 7 3 , Hilger Watts Ltd. ) . The f i l t e r was a quartz disc onto which was deposited rhodiumised strips of varying density. The f i l t e r was placed close to the s l i t of the spectrograph with the dividing line between the clear and rhodiumised strips at 90° to the length of the s l i t . It was important to ensure that illumination of the f i l t e r was uniform over its height of about 10 mm. The reaction vessel was removed from the path length during this ca l ibrat ion. The absorbance of each str ip on the step wedge at various wavelengths was measured on a recording spectrophotometer (Cary, model 14). The results were in good agreement with the cal ibration at 450 nm supplied by Hilger and Watts Ltd. Thus from a single flash of the spectro-scopic lamp the characterist ic curve for the plate could be drawn. The absorbance of a species in the reaction vessel can be given as A = log(I /I) 33 where I is the i n t e n s i t y of l i g h t pass ing through the r ea c t i on vesse l w i t h no absorb ing spec ies and I i s the reduced i n t e n s i t y due to a b s o r p t i o n . For the l i n e a r p o r t i o n of the c h a r a c t e r i s t i c curve we can w r i t e PDj = y l o g ( I ) + C where C is a cons t an t , I is l i g h t i n t e n s i t y and y ( p l a t e gamma) is the s lope of the l i n e a r p o r t i o n of the c h a r a c t e r i s t i c cu r ve . The p l a t e gamma is sometimes r e f e r r e d to as the con t r a s t of the p l a t e . Thus we can w r i t e A = APD = y l o g d /I) where APD = (PD, - PDjs o ' A l l exper iments are c a r r i e d out w i th p l a t e d e n s i t i e s in the l i n e a r reg ion of the c h a r a c t e r i s t i c cu rve . Beer ' s Law: Beer ' s law connects absorbance w i t h concen t r a t i on and i s g iven by A A " e A C 4 where E is a constant of p r o p o r t i o n a l i t y c a l l e d the decad ic molar A e x t i n c t i o n c o e f f i c i e n t and is a f u n c t i o n of wave length , C is the concen t r a t i on of the absorb ing spec ies in moles per l i t r e and JI i s - 3^ ~ FIGURE 5, CHARACTERISTIC CURVE OF A PHOTOGRAPHIC PLATE LINEAR PART OF CURVE USED l o g ( E x p o s u r e ) - 35 -the absorbing pathlength in cm. Beer's law is generally val id provided only one species is absorbing and e does not change s i g -ni f icant ly over the bandpass of the detector. The two-path method of Norrish, Porter and Th rush^^ was used to test the val id i ty of Beer's law for a l l transients studied. The absorbing path length was varied by masking half of the reaction vessel from the photol-ysis l ight. Beer's law was found to be val id for a l l of the trans ients. Photoelectric Detection Generally, photoelectric measurements are more accurate and very much more sensitive than photographic determinations. The minimum detectable absorbance by plate photometry is about 0.03 while, under the best conditions, the photoelectric detection limit is about 0.002. Thus once preliminary survey work has been done photoelectric measurements are preferred. Plate work is i n i t i a l l y very important since it yields a fu l l spectrum of the species present. Examination of this information determines which wavelengths may or may not be suitable for kinetic observations (due to overlapping spectra of two or more species) and what species are present in the reactant mixture. In cases where a banded spec-trum of one species is fu l ly overlapped by a continuum of a second - 36 -species plate photometry can separate each while photoelectric measurements cannot. Despite the obvious advantages of photoelectric work there are some experimental problems which l imit its use. The main d i f f i cu l t y is that most continuous light sources are not intense enough to compete with the scattered light from the main f lash. Although the half peak width of the photolytic flash is only about 15 ps a long fluorescent ta i l persists. The intensity of this ta i l is s ignif icant compared to the analysing beam for up to about 200 us. This may be reduced somewhat by adding a trace of nitrogen to the photolysis lamp. Thus kinetic measurement cannot be started much before 200 us after the mai/i flash and the reactive species which can be observed must have correspondingly long 1i fet imes. There are some conditions where this restr ict ion can be overcome. If the photolysing and analyzing wavelengths are d i f f e r -ent and can be separated by the use of f i l t e r s then the scattered light problem can be eliminated. For example, i f photolysis takes place above 400 nm and analysis is carried out at 250 nm then a 350 nm cutoff f i l t e r placed between the reaction vessel and flash lamp wi l l eliminate scattered light at the monitoring wavelength. The only photolysis l ight reaching the photomultiplier wi l l be - 37 ~ l ight scattered within the spectrograph. Much of this can be removed by careful placement of baffles in the spectrograph. Another source of interference is e l ec t r i ca l . Due to the large and sudden flow of current in the photolysis lamp, double shielding of the photomu1tip1ier signal lead is needed. In addition, care must be taken to avoid ground loops, especially from the grounded path of the photolysis lamp current. F ina l ly , mechanical vibrations (mostly in the form of a shock wave from the photolysis lamp discharge) can cause long lived periodic fluctuation in the light intensity observed by the photomultipiier. These can be reduced by independent mounting of the light source, photolysis lamp, and spectrograph on antivibra-tion rubber mounts. The brass cylinder housing the reaction vessel and photolysis flash lamp should be large enough so there is no close contact between these tubes. If these precautions are followed measurements can be made during the flash in some cases. Light Sources: Most of the work was carried out using mercury short arc 100 watt discharge lamps (PEK Inc., C a l i f . ) . Some use was made of an xenon short arc 75 watt lamp (PEK Inc., Ca l i f . ) and a quartz-iodine tungsten 12 volt 100 watt lamp (Phi l ips, Holland). The tungsten lamp was operated from a 12 volt lead-acid storage battery. - 38 -This gave a smooth continuum and a very steady output. The intens-ity drops very rapidly below around 380 nm. The xenon lamp has a smoother spectral output than the mercury but its intensity in the ultra violet above about 220 nm is less than that of mercury. Both discharge lamps were operated with a Schoeffel Instrument Corp. model LPS 251 lamp power supply. The AC ripple ( 0 . 3 5 % RMS) was too large and an extra f i l t e r section on the output was added. With this addition no AC ripple could be detected from the lamps. It should be noted that performance and useful l ifetime of the lamps was variable. Often new lamps were very irregular in their output but settled down after a few hours of operation. This seems to be a characterist ic of short arc lamps an,d may be, in large part, due to wandering of the arc from one point to another on the electrode. The requirements here for short time high s tab i l i t y are very different than that for photolysis experiments using mercury lamps where long term dr i f t in intensity is important. A sudden change of only 1% in lamp output can cause a 10% error for an observed absorbance of 0 . 0 5 - Thus for measurements of weak absorb-ances a signal to noise ratio must be at least 1 0 0 . - 39 " Photomultiplier: An RCA IP28 photomultip1ier was used for a l l measurements. This is a nine stage tube, side view cage type structure with UV glass window material. The response (S-5 or 104) peaks at about 350 nm but remains quite high down to 230 nm below which response fa l l s off rapidly. The maximum anode current is 0.5 mA and maxi-mum supply voltage is 1250 V. The high-voltage source (John Fluke Mfg. Co. Inc., model 413C) was very stable as required since the output signal of a photomultipiier is extremely sensitive to variations in the supply voltage. The interstage voltage gradients for thet photomultiplier elements was provided by a resist ive voltage divider placed across the high-voltage source (see figure 6). Resistance values were chosen such that the current through the voltage-divider network (chain current) was about twenty times the maximum anode current used. This was done to prevent dynode voltages from varying as the photo current moved through the tube. This effect is especially important in the last two stages and can lead to a nonlinear tube response. Attempts to improve the over-all performance of the tube were made by setting the f i r s t cathode to dynode e lec t r i c f i e ld at a higher potential . Therefore a larger chain resistor - 4o -was added to this stage. In addition, a larger resistor was added as the second to last dynode stage to improve performance. It should also be noted that although a photomultiplier acts essent-ia l ly as a constant-current device, the signal voltage developed across the load is in series with the last-dynode-to-anode voltage and consequently opposes i t . Nonlinear operation may occur if the load voltage becomes signif icant compared to the last-dynode-to-anode voltage. From these considerations it is clear that the l inearity of the tube should be carefully checked under operating conditions. The output of the photomultipiier was measured with various c a l i -brated neutral density f i l t e r s (Oriel) placed between a steady light source and the spectrograph s l i t . The desired result is given by V = KI where V is the output signal voltage, I is the light intensity str iking the photomultipiier and K is a proportionality constant. Hence A = l o g ( I o / I F ) = log(V o/V F) where A is the absorbance of the neutral density f i l t e r and the subscripts "o " and "F" refer to conditions without and with the - 41 ~ f i l t e r respectively. Good l inearity was found for a supply vo l t -age as low as 650 volts and a 10 kfi load. Linearity was checked at several wavelengths. The signal-to-noise ratio was found to increase with increasing light intensity and decreasing supply voltage. Thus a supply voltage of as close to 650 volts as poss-ible (consistent with a reasonable output signal) was used and the light intensity maximized to give a maximum anode current of 0 . 4 mA where possible. When a very fast time response was not required a cap-acitor was placed in paral lel with the load resistor. This gives a greatly improved signal-to-noise rat io. The time constant of this arrangement is x = 1/RC The usual load resistor was 10 kfi and hence 1 nF capacitor gives a time constant of 10 us. This combination was used for measure-ments in the millisecond time scale and slower. Although the maximum anode current is 0 . 5 mA this rating refers to a 30 second average current. Short pulses many times this rating can be applied without photomultiplier damage. Thus scattered light during the photolytic flash may drive the photo-multipl ier into a nonlinear response during the flash but recovery to l inearity is very rapid following the f lash. - hi -FIGURE 6, RESISTIVE VOLTAGE DIVIDER FOR PHOTOMULTIPLIER Ri Ri RL CL RN E x c e p t R10 R, • A W -± P < DY, N-2 < DY, < DY, -C DY X K 10 kn 2 x 10"6 F 10" n R2 = 2 x l O " n P C L RL •DYN R N N Anode Anode Load C a p a c i t o r Anode Load R e s i s t o r Dynode S t a g e s V o l t a g e D i v i d e r R e s i s t o r s T o t a l Number o f Dynode S t a g e s = 9 K - Cathode - 43 " Osci1loscope: A Tektronix Inc. type 5^ 7 oscilloscope was used for the photoelectric work. It is an extremely versati le instrument designed for use with a l l lettered, or 1-series Tektronix plug-in units. The instrument features two identical time-base generators that can be used singly or electronical ly alternated. The time-base generators can also be used in a delayed sweep operation. Delayed Trigger - The delay trigger output supplies a sharp positive-going trigger spike of about 10 V at the end of the delay period as set by the TIME/CM or DELAY TIME switch and the DELAY-TIME MULTIPLIER control. The delay period is measured from the start of the B time base. This trigger was used to f i re the main photolysis lamp in photoelectric work and the spectroscopic lamp in photographic work. It was found that the trigger pulse was too narrow to break down the thyratron or spark gap. There-fore a pulse generator was bui l t which was triggered by the delay pulse signal. The output from this unit was then applied to the thyratron or spark gap. Thus for photographic work the spark gap for the photol-ysis flash was manually triggered. An induced signal in a coil around the ground lead of the flash lamp triggered the main time base (B). After a preset delay period the delayed trigger breaks - hh -down the thyratron allowing the spectroscopic lamp to f i r e . In photoelectric work, the delayed pulse was used to f i re the photol-ysis lamp. It should be noted that the delay trigger is tied to the main time base (B) and this sweep must be used to obtain a delayed pulse. Alternate Sweep Mode - In this mode of operation both time bases are observed in succession. Usually the main time base is set in the trigger mode and time base A is set in the auto s tab i l i t y mode. The main time base is started either by an exter-nal trigger or by rotating its trigger level control through zero. Once the main time base is finished A time base can start . It wi l l wait for a short period of time for a trigger s ignal . If none is received it wi l l self-tr igger. This period of time be-tween sweeps was undesirable and hence a modification was carried (50) out which allowed A time base to start immediately following completion of the main time base. In this way two successive sweeps are obtained at different sweep speeds. A Delayed by B - Under this mode of operation time base A produces the observed sweep but starts some time after the start of B sweep. This time delay is set by the delay time set by the B sweep rate and delay-time mult ipl ier . B time base is set in trigger - hS -mode and A at auto s tab i l i t y . At the appropriate time B sweep is triggered but not observed. After the preset delay period A sweep begins and is observed. Thus the observed trace can be delayed by any desired amount after a particular trigger event. 1A1 Plug-In Unit - The Type 1A1 Dual-Trace plug-in unit (Tektronix Inc.) contains two identical high-gain fast rise c a l i -brated preamplifier channels. Either channel can be used independ-ently to produce a single display or e lectronical ly switched to produce dual-trace displays. Each channel has its own controls. In the chopped mode an internal multivibrator switches the channels at a free-running rate of about 1 MHz. Due to possible errors from low frequency attenuation by AC coupling of the input s ignal , DC coupling was used for a l l experiments. For large changes in the signal this was sat isfactory; however, when the input signal change was small, suf f ic ient ly accurate measurements could not be made. It was not possible to increase the input sensi t iv i ty because the input changes were im-pressed on a relat ively large DC voltage level . This problem was overcome by building a f loating DC compensator which "balanced o f f " the DC level and permitted observations at much higher input sensi-t i v i t i e s (see figure 7 ) . In this case the chopped mode of operation was used. The - 46 -FIGURE 7, C I R C U I T FOR U S I N G T H E S L I D E - B A C K T E C H N I Q U E TO O F F S E T A DC V O L T A G E L E V E L O U T ( j > V i = 1.5 V v2 = 7.5 V s i • s w i t c h e s Ri = 10 R 2 = 10 kQ R 3 = 0 - 5 0 k n ( c o u r s e c o n t r o l ) Rf = 0 - 0 . 5 k n ( f i n e c o n t r o l ) In p r a c t i c e s w i t c h S i was always i n p o s i t i o n b. W i t h . b o t h S 2 and S 3 open t h e i n p u t and o u t p u t v o l t a g e s were t h e same. When S 2 and S 3 were c l o s e d t h e l e v e l s d i f f e r e d by an amount dependent on t h e s e t t i n g s o f t h e v a r i a b l e r e s i s t o r s . - hi -input to channel 1 was the signal direct from the photomultiplier while the balanced-off voltage was applied to channel 2 at higher sensi t iv i ty . In this way the voltage output before f i r ing the photolysis lamp and the change after f i r ing can both be measured simultaneously at different sens i t i v i t i es . Frequent checks on the gain of each channel are important to ensure accurate results. Type W Unit - This unit was used later in the course of the study. As a di f ferent ia l comparator, voltage measurements using the slide-back technique can be made. The high accuracy and stab i l i ty of the DC comparison voltage added d i f ferent ia l l y to the input signal makes precise voltage measurements possible. This plug-in unit is equivalent to using^ the 1A1 and the DC compensator except that with the type W the compensator voltage is precisely known. Hence there is no need for two input channels. As with the 1A1 frequent checks of amplifier gain were made. It is a good idea to check this gain against the comparison voltage output terminal. In cases where the scattered light from the photoflash is large the input amplifiers may be severely overloaded. Although both units show good overdrive recovery certain signals can show a slow (thermal) shift in the reference level (overdrive DC sh i f t ) . This was observed and hence scattered light signals were limited - 48 -to no more than about 10 volts. Camera: A Tektronix C-27 camera was used. The optical system of the camera permits displays to be simultaneously viewed and photographed. An Ilex number 3X shutter was used. For the sur-vey parts of this study an Ilex f l . 3 lens with a magnification of 1 : 0 . 5 was used. This allowed three pictures to be recorded on a single photo. Most measurements were made using an Ilex f l.9 lens with a magnification of 1 : 0 . 8 5 which increased the p ic -ture area by almost a factor of three. Only one picture could be recorded per photo. Polaroid Land rol l f i lm type 4 7 , 3000 ASA speed was used. Shutter Assembly: Since some of the reagents (in particular CIO2) were s ignif icant ly decomposed by the analysing beam over a period of seconds a shutter and bulb release were used between the light source and reaction vessel. When the shutter (number 3X, Ilex Optical Co.) is opened it triggers a pulse generator. This pulse is then used to start the f i r ing sequence. - ks -Firing Sequences: Since the detailed f i r ing sequence may not be obvious, three modes of operation wi l l be discussed. Traces obtained in the various modes are shown in figures 8 , 9 and 1 0 . Main Sweep Only - In this mode only one sweep (B time base) is used. When the shutter is opened the generated pulse is used to trigger the B time base externally. The sweep starts and after the delay setting (usually one major division on the g ra t i -cule) the delayed pulse from the oscil loscope triggers a pulse generator which in turn triggers the spark gap allowing the main lamp to f i r e . Thus one major division on the graticule is used to measure the output before photolysis and nine divisions are used to measure transient changes. Alternate Sweep - In this mode both sweeps are used, f i r s t B then A. B time base is set at external trigger while A is set at auto-stabil i ty. The sequence is the same as for the previous Main Sweep Only mode except once the B sweep is completed A sweep begins (usually at a slower speed). Thus the f i r s t major divis ion of B sweep is used to measure the output before photolysis and nine division at B sweep speed plus ten divisions at the A time base sweep speed are used to measure transient changes. - 50 -FIGUE 8, OSCILLOSCOPE TRACE FOR THE FIRING SEQUENCE: MAIN SWEEP ONLY b) Type 1A1 P l u g - I n U n i t V = 0 " 51 " FIGURE 9, OSCILLOSCOPE TRACE FOR THE FIRING SEQUENCE: ALTERNATE SWEEP a) Type W P l u g - I n U n i t ( V r = C a l i b r a t e d O f f s e t V o l t a g e ) V = 0 b) Type 1A1 P l u g - I n U n i t - 52 -FIGURE 10. OSCILLOSCOPE TRACE FOR THE FIRING SEQUENCE: A DELAYED BY B i V = 0 a) Type W P l u g - I n U n i t ( V r = C a l i b r a t e d O f f s e t V o l t a g e ) i b) Type 1A1 P l u g - I n U n i t V = 0 - 53 " A Delayed by B - In this mode only the A time base is observed. The pulse generated on opening the shutter is used to trigger the B time base externally. This pulse is also used to f i r e the main photolysis lamp. After the desired delay time, as set by the B sweep rate and the delay time multiplier, the A sweep begins and is observed. A second sweep is generated by rotating the B trigger level control through zero as quickly as possible after the transients have disappeared in order to record the vo l t -age before the photoflash. This assumes no lamp fluctuations over this period and complete transient decay. This mode of operation is useful where the scattered light from the main flash makes measurements impossible over the f i r s t three or four divisions on this sweep and a slower sweep rate is not desirable. Digi t iz ing of Traces: Accuracy of measurements was usually limited by trace thickness, photomultiplier noise, or lamp s tab i l i t y . Hence, hand measurements of traces were suitably accurate. However many of the traces were analysed using an Instronics Gradicon d ig i t i ze r . This was found to be convenient when a large number of traces were to be analyzed. The d ig i t izer can be used for converting graphic - 54 -representations to numerical representations with the output, in this case, on IBM keypunched cards. The Gradicon is a two-dimensional coordinate measuring system with an absolute accuracy of 0 . 0 0 1 inches. The position of the cursor is detected by an electro-magnetic closed loop servo system located below the workboard. Any convenient out-put format may be specified by the user. A computer program was written to handle the output from the d ig i t i ze r . Given control data such as the osci l loscope's ve r t i -cal sens i t iv i ty , sweep speed, and the off-set voltage, the raw data was converted into absorbance versus time. Data for either f i r s t or second order plots is generated and the points plotted by machine. The slopes and intercepts of the plots may also be. calculated. Unless carefully set up, the axis system of the d ig i t izer and photo wi l l not coincide although the origins w i l l . In order to avoid this alignment for every trace, part of the program was written to correct the raw data for the axis rotation. This was part icular ly useful when several photos were digit ized at one time. Plotting routines and best f i t s for various curves by the method of least squares (including f i t s to nonlinear expressions) were carried out using l ibrary programs supplied by the University of Br i t ish Columbia computing centre. Those used were UBC PLOT, UBC TRIP, and UBC BMD:P3R. - 55 " k. REAGENTS AND GAS HANDLING Argon - Argon at 99•9995% purity (Matheson grade) was used for f i l l i n g flash lamps and di lut ing reactant mixtures. It was taken direct ly from the cylinder after passing through a trap f i l l e d with glass wool at 195 K-Oxygen - Oxygen was obtained at 99-95% purity (Ultra pure, Matheson). It was taken direct ly from the cylinder after passing through a trap f i l l e d with glass wool at 195 K. Chlorine - Chlorine used for synthesis of C102 and C120 was obtained at 99-5% purity (Matheson, high purity) and was used direct from the cylinder as described in the synthesis of C102 and C l 2 0 . Chlorine used in reaction mixtures was obtained at 99-95% purity- The major impurities were C0 2 , N 2 , 0 2 and a trace of water. Purif icat ion was carried out by repeated d i s t i l l a t i on from an ethanol slush (156 K) to l iquid nitrogen. Only the middle fractions were retained. Iodine - Iodine crystals were obtained at 99-997% mini-mum purity for volat i le matter (Ma 11inckrodt Chemical Works). They were dried over P205, d i s t i l l ed under vacuum and stored in a trap f i t ted with a greaseless teflon tap (Kontes Glass Co.). - 56 -The section of the vacuum rack used to handle iodine was isolated from the rest of the system by teflon taps. Al l ground joints were waxed using an inert halocarbon wax (series 15-00, Halocarbon Products Corp., N.J.). Ni t r ic Oxide - Ni t r ic Oxide was obtained at 33-5% purity (Matheson). It was f i r s t outgassed by repeatedly freezing at 77 K pumping and warming. An in i t i a l d i s t i l l a t i on was carried out from 1^3 K (n-pentane slush) to 77 K. Only the middle fraction was re-tained. Final d i s t i l l a t i on was carried out from 90 K ( l iquid oxygen) through a trap at 65 K (cooled l iquid nitrogen). Liquid nitrogen was cooled to about 2 K above its freezing point by injection of a stream of helium rather than the us^ial vapor-pumping (79) technique . The pure NO appeared light blue in the condensed phase. Chlorine Dioxide - C l 2 gas and nitrogen carrier gas were mixed after passing through bubblers f i l l e d with concentrated sulphuric acid. This mixture was then run through a column packed with NaC102 (analytical grade, 20 mesh, Matheson Coleman and Bel l ) . The product was collected at 195 K. This synthesis is essential ly that of Derby and Hutchinson ^ 3 ) ^ The product was br ief ly pumped on at 195 K then d i s t i l l ed from 175 K (methanol slush) through 156 K (ethanol slush) to l iquid - 57 -nitrogen. The fraction at 156 K was retained. Since CIO2 decomposes in room l ight, a l l work with C102 was carried out in a darkened room. C102 was stored in a darkened trap at 77 K. C102 may decompose violently in the vapour phase at pressures above h kPa (30 torr ) . In practice, pressures were a l -ways kept below 2 . 5 kPa (20 torr ) . Chlorine Monoxide - C120 preparation was the same as for C10 2^^ except the column was packed with mercuric oxide (yellow, technical, J .T . Baker Chemical Co.) and glass chips. The column was freshly packed for each preparation. The product was br ief ly pumped on at 195 K then d i s t i l l ed from 156 K (ethanol slush) to 1*»3 K (n-pentane slush). The same handling precautions were taken as for C102. Vacuum Rack: Al l mixtures were made on a standard high vacuum rack. Pumping was carried out using a single stage rotary o i l pump in series with a two stage si l icone o i l diffusion pump. A working pressure of 10~5 torr was obtained. Sil icone grease was used on a l l taps except that section used to handle iodine. Here grease-less teflon taps were used. Gas pressures (up to 120 kPa or 900 torr) were measured - 5 8 -using a Crosby metal bourdon gauge. Lower pressures (as low as 0 . 4 kPa or 3 torr) were measured on a quartz spiral gauge. Press-ures below . 4 kPa ( 3 torr) were obtained by expansion into pre-determined volume rat ios. A l l mixtures were allowed to mix for at least three hours to ensure homogeneity. Temperature Control: Although the apparatus was not designed for observations at different temperatures, crude temperature control was attempted for some experiments. Hot air was run into the reaction vessel enclosure through the air inlets. In addition the, outside of the enclosure was wrapped with heating tape. Temperature measurements were made using a copper:constantan thermocouple. Reaction mixtures were allowed to equil ibrate at the desired temperature for at least one hour before using. The maximum temperature which could be obtained by this method was 3 2 8 K. A measurement of the extinction coeff icient of iodine was made by obtaining various pressures of iodine by temperature control on some iodine crystals. Vapour pressure curves were obtained from Gi l lespie and F r a s e r T e m p e r a t u r e of the iodine trap was main-tained by a constant temperature circulator (model NB, Lauda) while - 59 -room temperature was kept above that of the temperature c irculator. 5. LEAST SQUARE FITS FOR EXPERIMENTAL DATA The evaluation of rate constants from raw data was done by the method of a weighted least squares f i t . The weighting fac-tor was determined by estimating the expected errors in each meas-urement. For example, consider a plot of the rate constant (K) versus pressure. Although the percentage error in K is constant the absolute value of the error increases direct ly with K. Hence, if no weighting factors were used, a least square f i t would give too much weight to the higher values of K. To prevent this a weighting factor (1/K)2 should be used. The absolute errors in measurements of absorbances from photographic plates were constant. However, since f i r s t and second order plots use the variables In A and 1/A respectively in a kinetic plot, the errors are not constant for these plots. In fact, the expected error is not even symmetrical about each point. To avoid this problem the rate expression was written in the nonlinear form A = F(A Q, K, t) and a nonlinear least squares f i t was carried out using unity as a weighting factor for a l l points. The estimated accuracy of most of the rate constants is about ±10%. - 60 -CHAPTER I I I IODINE ATOM RECOMBINATION 1. RESULTS All experiments using I 2 were carried out in a pyrex reaction vessel and a Corning glass f i l t e r (3 _73> ^00 nm cutoff) was used to f i l t e r the photolysis l ight . This prevented any photolysis of transients absorbing below 400 nm and reduced the scattered light reaching the photomultiplier from the main photo-flash at wavelengths below 400 nm. The analysing beam was f i l te red using a Corning 7~54 glass f i l t e r which transmits between 240 and 400 nm. This was used to prevent any possible effects from photoly-sis of I 2 by the monitoring l ight . Except for preliminary survey work a l l iodine measurements were made using photoelectric detection at 500 nm. This is in the continuum of the I 2 spectrum and was chosen because the extinction - 61 " coeff icient is a maximum here and is pressure independent. As a check on the newly bui lt apparatus the rate of iodine atom recombination was measured at 300 K and kO kPa (300 torr) of argon and 17 Pa (0.13 torr) of iodine. The extinction coeff icient was measured by determining the absorbance at various vapour pressures of iodine. Beer's law was obeyed and gave e = 522 £mol _ 1 cm - 1 . This is in good agreement with the previously reported value of 560 £mol' (75) For a l l measurements the lifetime of the flash was short compared to the time scale of observations. Zero time (t=0) is defined as that time just following the "instantaneous" f lash. At this time define the iodine concentration as [ I 2 ] 0 « Since a l l the I 2 reappears then [^l^ (the iodine concentration at inf in i te time) is equal to the iodine present just before the f lash. Hence [ I ] T - 2 { [ I 2 ] M - [ I 2 ] T > The recombination rate is defined by =4$- = K R [ I ] 2 [ M ] This integrates to give 1 1 m ^ T n ; = V M ] t If Ij. is the light intensity at the photomu 11ip 1 ier at time t to - 62 -produce a s i g n a l o f V v o l t s and F i s t h e i n t e n s i t y f o r an empty r e a c t i o n v e s s e l then Thus a p l o t o f l / l o g ( V t / V j v e r s u s t s h o u l d be l i n e a r w i t h a s l o p e o f 2K [M]/eA. Such a second o r d e r p l o t was o b t a i n e d and was l i n e a r o v e r two h a l f l i v e s . The s l o p e gave a v a l u e f o r K 2 o f 4 . 7 2 x 1 0 9 £ 2 m o l _ 2 s - 1 . T h i s a g r e e s s a t i s f a c t o r i l y w i t h t h e v a l u e o f 5-40 x 1 0 9 £ 2 m o l _ 2 s - 1 g i v e n by Ip and B u r n s ^ ^ . The r a t e c o n s t a n t measured a t a p p r o x i -m a t e l y 328 K was 6 . 0 0 x 1 0 9 £ 2mol 2 s _ 1 g i v i n g a n e g a t i v e a c t i v a t i o n e n ergy o f about 4 k J m o l - 1 compared t o 5.51 k J m o l - 1 c a l c u l a t e d from d a t a g i v e n by Ip and Burns f o r a t e m p e r a t u r e , range o f 298 t o 423 K. T h i s agreement i s s a t i s f a c t o r y s i n c e a 28 K t e m p e r a t u r e r i s e o n l y p roduces about a 15% d e c r e a s e i n the o b s e r v e d r a t e con-s t a n t . I t i s c o n v e n i e n t t o d i v i d e t h e r e s u l t s f o r NO as a chaperon gas i n t o low p r e s s u r e s o f NO (0 t o 65 Pa o r 0 . 5 t o r r ) , - 63 " medium pressures of NO (65 Pa to 0.65 kPa or 0 . 5 to 5 torr ) , and high pressures of NO (0.65 to 110 kPa or 5 to 800 torr ) . Low Pressures of NO In this region the reappearance of I2 was measured at 500 nm. Excess argon (to a total pressure of 40 kPa or 300 torr) was added to maintain isothermal conditions. The rate of reappear-ance of I2 f i t s f i r s t order kinetics (figure 11) better than second order (figure 12). The f i r s t order rate constant (K^ s t) was found to depend direct ly on both NO and argon pressures (figures 13, 14 and 15). The order of the decay rate and its dependence on argon and NO pressures are c r i t i ca l in distinguishing between possible mechanisms. These results are in direct conf l ic t with those of PST and are discussed later in this chapter. Temperature dependence of the rate constant gave an average negative activation energy of 3-4 kJmol - 1 . The values ranged from 2.1 to 6.2 kJmol" 1. Although good accuracy is not claimed for these measurements, the discrepancy between this value and the value of 10 kJmol - 1 given by Porter, Szabo and Townsend ^ (PST) is s ignif icant and is discussed later in this chapter. Table 1 summarizes the results of low NO pressure runs. No other transient was observed from 250 nm to 500 nm. - 6h -FIGURE 11, F I R S T ORDER PLOT OF THE REAPPEARANCE OF I g ABSORPTION AT LOW P •4.2 H OM L U 0 8 LO 1.2 • IM L 6 t(ms) - 65 -FIGURE 12, S E C O N D O R D E R P L O T O F T H E R E A P P E A R A N C E O F I 2 A B S O R P T I O N . A T LOW P K , N P,.n = 26.7 Pa(0.2 t o r r ) n 1 1 1 1 1 r— 0.4 0.6 0.8 1.0 1.2 I A 1.6 t(ms) - 68 -- 69 " TABLE 1. FIRST ORDER RATE CONSTANTS FOR I 2 REAPPEARANCE AT LOW PRESSURES OF NO Run # W P a > P A r ( k P a ) K l s t ( l 0 3 s - i ) T(K ; 103 13.3 40.0 0.689 300 104 13.3 40.0 0.655 328 97 26.6 40.0 1.40 300 98 26.6 40.0 1.38 300 99 26.6 40.0 1.23 300 100 26.6 40.0 1.26 300 101 26.6 40.0 1.07 328 102 26.6 40.0 1.03 328 105 40.0 40.0 2.16 300 106 40.0 40.0 1.75 328 107 40.0 13.3 0.606 300 108 40.0 13.3 0.631 300 109 40.0 60.0 2.76 300 110 40.0 60.0 2.95 300 111 40.0 26.7 1.17 300 112 40.0 26.7 1.28 300 ( F l a s h E nergy = 1066 J , P T = 17 Pa, X = 500 nm) - 70 -TABLE 2, EXTINCTION COEFFICIENT OF INO Run # x(nm) e ( £ m o l " 1 c m - 1 ) * ( m ; / V = K 2 n d C o 3 460 118 131 4 440 270 118 5 430 284 129 6 430 266 134 7 430 282 123 16 430 251 146 8 420 303 126 9 400 257 157 10 390 282 136 11 380 193 167 12 370 170 151 13 360 120 142 17 310 107 150 18 300 240 156 19 290 598 161 20 280 1490 159 21 280 1410 -22 270 2730 143 23 270 2620 -24 270 2690 -34 251 1570 121 *Where P^Q o r f l a s h e n e r g i e s d i f f e r e d t hen w a v e l e n g t h s were o v e r l a p p e d t o c o r r e c t f o r the d i f f e r e n t C . - 71 -Medium P r e s s u r e s o f NO In t h i s p r e s s u r e r e g i o n t h e r a t e o f r e a p p e a r a n c e o f i o d i n e was t o o f a s t t o be o b s e r v e d . In f a c t , t h e r e c o m b i n a t i o n was so f a s t t h a t i t appeared as i f no p h o t o l y s i s had t a k e n p l a c e . T h i s was a l s o o b s e r v e d on p h o t o g r a p h i c p l a t e s where e a r l i e r t i m e o b s e r v a t i o n s c o u l d be made. No o t h e r t r a n s i e n t s were o b s e r v e d . However, as the NO p r e s s u r e a p proached 0.65 kPa (5 t o r r ) a weak t r a n s i e n t a b s o r p t i o n was d e t e c t e d a t 251 nm. Below 0.65 kPa (5 t o r r ) o f NO the i n t e n s i t y o f t h i s t r a n s i e n t was t o o weak and i t s decay t o o s l o w t o be f o l l o w e d . Above 0.65 kPa (5 t o r r ) k i n e t i c measurements c o u l d be made on t h i s t r a n s i e n t . * High P r e s s u r e s o f NO F o r p r e s s u r e s o f NO above hO kPa (300 t o r r ) t h e r e a p p e a r -ance o f I 2 c o u l d a g a i n be f o l l o w e d . The r a t e o f r e c o m b i n a t i o n f o l l o w e d second o r d e r k i n e t i c s w i t h the same r a t e c o n s t a n t a t AO kPa (300 t o r r ) as a t 110 kPa (800 t o r r ) o f NO. However, w i t h i n c r e a s i n g NO p r e s s u r e the a p p a r e n t p h o t o l y s i s o f I 2 i n c r e a s e d and t h e a b s o r p -t i o n o f the t r a n s i e n t s p e c i e s a l s o grew. The s p e c t r u m o f t h i s s p e c i e s i n t h e r e g i o n o f 36O nm t o 460 nm was i n f a i r agreement w i t h t h a t r e p o r t e d by P o r t e r , Szabo, and Townsend ( P S T ) ^ as shown i n f i g u r e 16. In o r d e r t o o b t a i n such s p e c t r a from p h o t o e l e c t r i c - 72 -- 73 -FIGURE 17, SECOND ORDER PLOT OF THE REAPPEARANCE O F i 2 ABSORPTION AT HIGH PRESSURES OF NO P N Q = 36 kPa(270 t o r r ) A = 500 nm i : 1 1 r 20 10 60 80 t(ms) -Ik -FIGURE 18, FIRST ORDER PLOT OF THE REAPPEARANCE OF I ? ABSORPTION AT HIGH PRESSURES OF NO P N Q = 36 kPa(270 t o r r ) X = 500 nm 0 20 10 60 t(ms) 80 - 75 ~ 100-FIGURE 19. K 2 N D V E R S U S ( I / P N O ) I N T H E H I G H P N Q R E G I O N 120 X = 251 nm o = 300 K A = 328 K '2nd SO-l s - 1 ) 601 401 o o o 20-0.4 0.8 l / P N 0 ( k P a - i ) 1.2 - 76 -FIGURE 20, SECOND ORDER RATE CONSTANT FOR THE REAPPEARANCE OF I , ABSORPTION VERSUS 1 / P N Q FROM DATA REPORTED BY PST PST L e a s t S q u a r e F i t W e i g h t e d as ( 1 / K ) 2 / / / / / / / / / / / / / V / / o o 50 100 150 l/PN 0(kPa-i) 200 250 - 77 -TABLE 3, SECOND ORDER RATE CONSTANT (K* ,) FOR INO DECAY AT HIGH PRESSURES OF NO Run # F l a s h Energy ( J ) P N 0 ( k P a ) P A r ( k P a ) P T (Pa) ! M ( 1 0 3 s-1) T(K) 80 1066 107 - 17 58.7 300 81 1066 107 - 17 58.7 300 82 1066 107 - 17 82.3 328 83 1066 107 - 17 91.2 328 22 416 36 - 17 57.0 300 23 204 36 - 17 59.5 300 24 204 36 - 17 57.9 300 30 416 14.7 - 17 55.2 300 33 416 14.7 - 6.8 60.1 300 34 1066 14.7 - 6.8 54.4 300 31 1066 6.0 - 2.7 51.2 300 35 1066 6.0 - 2.7 53.8 300 36 1066 6.0 - 2.7 58.4 300 37 1066 6.0 - 2.7 55.5 300 38 1066 6.0 - 2.7 49.7 300 39 204 5.33 34.7 17 49.6 300 40 204 5.33 34.7 17 51.1 300 - 78 " TABLE 3, (CONTINUED) Run # F l a s h E n ergy ( J ) P N 0 ( k P a ) P A r ( k P a ) P T (Pa) % t do 3 T(K) 41 204 5.33 34.7 17 80.4 328 42 204 5.33 34.7 17 78.7 328 43 204 5.33 34.7 17 50.2 328 50 1066 2.67 37.3 17 52.7 300 51 1066 2.67 37.3 17 88.3 328 52 1066 2.67 37.3 17 79.4 328 53 1066 2.67 37.3 17 56.8 300 44 416 2.27 14.4 6.8 58.9 300 46 1066 2.27 14.4 6.8 54.0 300 47 1066 2.27 14.4 6.8 55.9 300 48 1066 2.27 14.4 6.8 92.4 328 49 1066 2.27 14.4 6.8 82.8 328 58 1066 1.33 38.7 17 50.6 300 59 1066 1.33 38.7 17 50.0 300 60 1066 1.33 38.7 17 81.9 328 61 1066 1.33 38.7 17 89.2 328 62 1066 1.33 38.7 17 57.7 300 63 1066 0.667 39.3 17 55.6 300 64 1066 0.667 39.3 17 51.4 300 67 1066 0.667 39.3 17 86.1 328 ( V a l u e s f o r X = 251 nm) - 7 9 -measurements it was necessary to repeat the experiment at each wavelength. A new continuous absorption system was also observed bel ow 310 nm which strengthens towards lower wavelengths such that at 251 nm it is about kO times stronger than at 420 nm. Since both systems decayed at the same rate relative to their intensi -ties they were attributed to INO. Other reasons for this assign-ment are discussed later in this chapter. The extinction coeff icient of INO was determined by correlating the decay of INO with the reappearance of I 2 . The va l id i ty of this approach wil l also be discussed later. Figure 16 shows a plot of e versus X and includes data reported by PST (see also t ab l e2 ) . As a further check that the observed spectrum belonged to only one species the slope divided by the intercept of the second order plots was compared at different wavelengths for the same mixture. Corrections were made for the different con-centrations of INO produced by different flash energies. A second order plot for INO decay (l/A versus t) gives a slope m x = K 2 n d / e x * (where £ is the path length) and the intercept is where C is the concentration at zero time - 80 -Hence m x / b A = K 2 n d C Q T h e r e f o r e (m,/b-,) s h o u l d be independent o f w a v e l e n g t h . S a t i s f a c t o r y A A r e s u l t s were o b t a i n e d ( t a b l e 2 ) . Below kO kPa (300 t o r r ) t he r e a p p e a r a n c e o f I 2 was t o o s m a l l t o be measured, however t h e t r a n s i e n t s p e c i e s c o u l d be s t u d i e d down t o NO p r e s s u r e s o f 0.65 kPa (5 t o r r ) . The second o r d e r r a t e c o n s t a n t was measured as a f u n c t i o n o f NO p r e s s u r e from 110 kPa t o 0.65 kPa (800 t o 5 t o r r ) . Second o r d e r k i n e t i c s was c l e a r l y b e t t e r than f i r s t o r d e r ( f i g u r e s 17 and 18). The r a t e was independent o f NO p r e s s u r e (0.65 t o 110 kPa o r 5 t o 800 t o r r ) , I 2 p r e s s u r e (k t o 17 Pa o r 0 . 0 3 t o 0.13 t o r r ) , f l a s h e n e r g y (150 t o 1065 J ) , o r p r e s s u r e o f d i l u e n t argon gas (0 t o 36 kPa o r 270 t o r r ) , a s 4 s h o w n i n f i g u r e 19 and t a b l e 3- A s e r i e s o f e x p e r i m e n t s were a l s o c a r r i e d o u t a t 328 K. The independence o f the second o r d e r r a t e c o n s t a n t on the v a r i o u s p a r a m e t e r s was the same as a t 300 K and t h e change i n r a t e c o n s t a n t w i t h t e m p e r a t u r e i n d i c a t e s a p o s i t i v e a c t i v a t i o n e n e r g y o f about 13 k J m o l - 1 . T h i s i s i n s a t i s f a c t o r y agreement w i t h 17 k J m o l - 1 r e p o r t e d by PST. However, t h e dependence o f the second o r d e r r a t e c o n s t a n t on p r e s s u r e o f NO r e p o r t e d by PST i s i n c o n f l i c t w i t h t h e s e r e s u l t s . T h i s i s d i s c u s s e d l a t e r i n t h i s c h a p t e r . - 81 " 2. DISCUSSION Identity of the Transient Species The spectrum of C1N0 in the gas phase reported by Goodeve and Katz^ 7 ^ shows two main peaks in the ultra-violet . These broad bands are centered at 330 and 196 nm with maximum extinction coe f f i -cients of about 30,and 1.6 x 103 £mol "^m" 1 f respective1y. Although the BrNO spectrum has not been studied in detail it has been reported (rg) by Basco and Norrish as being continuous in the ul tra-violet , becoming stronger towards shorter wavelengths but somewhat more intense and red shifted compared to C1N0. The transient observed following the photolysis of I2 in the presence of bjgh NO pressures has a similar spectrum to C1N0 and BrNO. This spectrum is continu-ous from 460 nm down to at least 251 nm and has a peak at 420 nm which is about 40 times weaker than the absorbance at 251 nm. That this spectrum is more intense and red shifted compared to the C1N0 spectrum would be expected for the series C1N0, BrNO, and INO. Since the recombination of iodine atoms at room tempera-ture has been shown to occur via the formation of intermediate complexes between chaperon and iodine atom^^ ^'^ , and, since FNO, C1N0, and BrNO are suf f ic ient ly stable to be observed under equ i l i -brium conditions (with decreasing s tab i l i t y of the nitrosyl halide - 82 -in the series) INO can be expected to have a transient existence. The spectrum was only observed when both I 2 and NO were present. As a result of these considerations the transient spectrum is assigned to INO. Summary of Important Experimental Results a) At low pressures of NO the rate of reappearance of I 2 was f i r s t order with respect to [ I ] . The f i r s t order rate constant, ^ lst ' ' n c r e a s e c ' direct ly with [NO] and [Ar] and the activation energy was - 3 - 4 kJmol - 1 . It may be noted that PST found a second order dependence on [ I ] with an activation energy of - 1 0 kJmol - 1 . The second order rate constant varied direct ly with [NO] but the dependence on argon pressure was not measured. b) At high pressure of NO both the rate of reappearance of I 2 and the decay of INO followed second order kinet ics. The rates were independent of NO, Ar, and I 2 pressures and of flash energy with an activation energy of 13 kJmol 1 . By contrast, PST reported an inverse dependence of the second order rate constant on NO pressures (figure 20) and did not investigate the effect of third body pressure. - 83 -P o s s i b l e M e c h a n i s m s C o n s i d e r t h e m e c h a n i s m I + NO + M t INO + M 1. (K = K i / K _ i ) INO + I -> NO + I 2 2. ( K 2 ) 2IN0 I 2 + 2N0 3. ( K 3 ) 21 + M + I 2 + M 4. ( K J T h e g e n e r a l e q u a t i o n s a r e ^ 1 ^ 1 = - K j t l J t N O J t M ] + K_![IN0][M] + K 2 [ I ] [ I N 0 ] + K 3 [ I N 0 ] 2 3-1 K 2 [ I ] [INO] + ^ [ I N O ] 2 + ^ [ I ] 2 [ M ] 3_ 2 =$p-= K i [ I ] [ N 0 ] [ M ] - K_x [ INO] [M] + K 2 [ I ] [ I N 0 ] + M I ] 2 [ M ] 3-3 A l s o u s e i s m a d e o f t h e d e f i n i t i o n ^ J - = K 2 n d { [ I 2 l . - [ I 2 ] t > - h " ± { [ I ] + [ I N O ] } 2 3-1, T h e c o n s e q u e n c e s o f t w o s i m p l i f y i n g a s s u m p t i o n s a r e now c o n s i d e r e d . T h e s e a r e A . I o d i n e a t o m s a r e i n a s t e a d y s t a t e c o n c e n t r a t i o n . B . INO i s i n a s t e a d y s t a t e c o n c e n t r a t i o n . - 8k -Case A: Steady State for [I] Atoms The steady state approximation holds where [INO] >> [ I ] . Then From equation 3~3 then K - J I N O H M ] [ I ] s s = M N O H M ] + K 2 [ I N 0 ] (reaction %-k can be neglected at very low [I]) •'• K ! [ I ] s s [ N 0 ] [ M ] + K 2 [ I ] $ s [ I N 0 ] = K_ 1[IN0] [ M ] Comparing this with e q u a t i o n 3 " l gives - . d L I N O i | ^ i ^ t M ] j dt " | K i [NO] [M] + K 2 [ I N 0 ] + K 3 j U N U J Combining equations 3 "1 , 3~2 and 3~3 - 2 d l l ? ] = d[I] , d[IN0] dt dt dt Hence for = 0 - d [ I 2 ] _ 1 d[IN0] dt 2 dt Then d [ I 2 ] _ f _ K . iK 2 [ M ] . K 3 ) r T N n l 2 ~dt J K j N O l t M ] + K 2 [ I N 0 ] + T] [ I N 0 ] If KitNOJtM] » K 2 [ I N 0 ] (which might be expected at high NO pressu and the relation (for [I] << [INO]) that [INO] = 2 { [ I 2 ] a > - [ I 2 ] t > - 8 5 " t h 6 n d t \K[HO] + K 3 [INO] : K 2 n d [ I N 0 ] 2 and - d [ I 2 ] dt - \ A + 2K3I { [ I z l » " [ l 2 , t } 2 - " a n d ' " * 1 - - ' ^ t ' 2 The s t e a d y s t a t e a s s u m p t i o n f o r I i m p l i e s r e a c t i o n (3) i s much s m a l l e r t h a n r e a c t i o n ( l ) . C o m b i n i n g t h i s w i t h t h e above a s s u m p t i o n c o n c e r n i n g t h e r e l a t i v e r a t e s o f r e a c t i o n s ( ] ) and (2) i s e q u i v a l e n t t o a s s u m i n g e q u i l i b r i u m between I , NO and INO. T h i s i s t h e b a s i c a s s u m p t i o n made by P S T . Case B: S t e a d y S t a t e f o r INO The s t e a d y s t a t e a p p r o x i m a t i o n h o l d s w h e r e [ I ] >> [ I N O ] . Then d[INQ] _ n d t ' U From e q u a t i o n ( 3 ~ 0 rTNim - K ^ N 0 ] [ M ] [ l ]  [ I N 0 ] s s ' K _ i l M J + k 2 [ I J S u b s t i t u t i n g i n e q u a t i o n (3~2) d [ I 2 ] _ K ! K 2 [ I ] 2 [ N 0 ] [ M ] , Kit r n 2 r M 1 d t " K_j [M] + K 2 [ I ] + T U J m - 86 -If K_j[M] « K 2 [ I ] and | M H « K J N O ] then ^ i a i = Id [ I ] [NO][M] For low [INO] i t follows that [i] = 2{[\2\m - [ i 2 y Then 2 K 1 [ N 0 ] [ M ] { [ I 2 ] e o - [ I 2 ] Q } and K j s ^ . = 2Kd[N0][M] where is the f i r s t order rate constant measured by following [ I 2 ] . Extinction Coefficient of INO At high pressures both I 2 and INO follow second order kinetics. This indicates that Case A applies. The independence i ° ^ ^2nd ° r ^2nd ° n ^ pressure indicates reaction (3) is the domi-nant step in the decay of INO. The slope (m) of the second order plots of 1/A versus t is given by 2K3/eJ2£ K 3 / e I N 0 £ ~T ( m I 2 / m I N 0 ) Using this r e l a t i one for INO was calculated at various wavelengths (figure 16 and table 2 ) . K3 was found to be 3-95 x 1 0 7 £mo l " 1 s " 1 . and m X m hence e i , n INO - 87 ~ Validity of Mechanisms At low pressures of NO the rate of I 2 reappearance is f i r s t order in I. This indicates Case B applies and gives Kx = 3 . 7 4 x 1 0 9 £ 2 m o l " 2 s _ 1 . No detectable amount of INO was seen and this gives a minimum ratio of [I] to [INO] of about 2 . 5 x 1 0 3 . This shows that the steady state approximation for INO is val id for these conditions. Additional assumptions made in Case B are a) M I ] « Ki [NO] In the range of NO from 13 to 65 Pa ( 0 . 1 to 0 . 5 torr) the maximum pressure of I is about 3-3 Pa. This gives [ N 0 ] / [ I ] £ h. Using K4. = 4 . 7 2 x 1 0 9 £ 2 m o l " 2 s _ 1 (as measured in this study) then the f i r s t term in the inequality is at least 3 . 2 times smaller than the second. b) K_![M] « K 2 [ I ] Here a minimum value of K 2 is estimated. Since at intermediate flash energies v i r tua l ly a l l the [ I 2 ] photolysed recombines within about 100 us or less, then the half l i f e for [ I ] must be about 20 us. Assuming this rapid recombination is due to reaction(2) and using [ I ] = 1 0 " 5 moU" 1 then K? £ 5 x 1 0 9 £mo l " 1 s " 1 . This o high value is not unexpected since the rate constant for Cl + C1N0 is 2 x 1 0 9 J lmol^s" 1 An estimate of K_j is obtained from a - 88 -lower limit for K. This is deduced later in this chapter when the results at high NO pressures are discussed. It is found that K > 1 0 7 A m o r 1 and hence K_i< k x 1 0 2 Arnolds" 1 with [I] . = 2 x 1 0 " 7 x mm m o i r 1 (which is the smallest [I] over the period measured) and [M] = 1.6 x 1 0 - 2 mol£ - 1.Then the inequality holds since the second term is at least 160 times larger than the f i r s t . At high pressures of NO both I2 and INO follow second order kinetics. Neither showed a dependence on NO pressure. This indicates Case A applies. The very rapid recombination of I at intermediate press-ures of NO is presumably due to reaction (2) - Therefore [I] is expected to be very low at high pressures of NO. That is [I] << [INO]. This is the condition for the steady state hypothesis of Case A. The additional assumption in Case A is K a[N0][M] » K 2[IN0] From the experiments at low pressures of NO, Ki = 3 - 7 4 x 1 0 9 £ 2 m o l _ 2 s - 1 . As NO varied from 0 . 6 5 to 110 kPa (5 to 800 torr) the in i t i a l [INO] varied from about 0 . 2 2 to kO Pa ( 0 . 0 0 2 to 0 . 3 torr ) . Thus [N0]/[IN0] is nearly constant at 3 x 1 0 3 . Using K2 = 5 x 1 0 9 £mo l " 1 s " 1 and [M] . = 1.5 x 1 0 ~ 2 mol£ - 1 then the f i r s t term in the inequality is mm 35 times the second. Thus at high NO pressures Case A applies. Using INO - 89 " measurements (which are more accurate and cover a greater range of NO pressures than I2 measurements) a plot of versus 1 /[NO] (figure 19) gives an intercept K3 and slope 2K2/K. K3 was found to be 3-95 x 107 £mol~ 1s~ 1. A least square analysis gives a slope of -1 .0 x 103 s - 1 ± 7-5 x 102 s _ 1 . Using this slope plus two standard deviations as a maximum value then K2/K < 5 X 1 0 2 S _ 1 Using K 2 I 5 X 1 0 9 £ m o 1 ~ 1 S ~ 1 gives K ^ 1 0 7 i i m o l " 1 Using the reasonable value for AS for this type of reaction estimated by PST ^ as -100 J m o l " 1 ^ 1 then at 300 K, AHX < - 6 2 kJmol" 1 and hence the minimum I-N0 bond strength is 62 kJmol - 1 . This gives a minimum AH3 of - 2 5 kJmol l . Since AH < E for any reaction then for reaction 3,AH3 < 13 kJmol" 1. Hence the I-N0 bond strength is - 81 kJmol" 1. This gives an upper limit for K of 2.1 x 10 1 0 ilrnol"1 and lower l imit for K_i of 0.18 A m o l " ^ " 1 . PST give a value of K = 6 x 103 ^mol - 1 at 333 K. Using their estimate of AS as -100 J m o l " 1 ^ 1 then at 300 K, K = k x \0 k Arnol" 1. It can now be seen why the measured activation energies and equilibrium constant given by PST were inconsistent with their mechanism. At low pressures of NO, I2 follows f i r s t order kinetics and not second order as supposed by PST. This gives an activation - 90 " energy for K^ , not K2#. It should also be noted that the "second order" rate constants reported by PST for low NO should, from the results of this study, increase with both [NO] and [M]. This [M] dependence is not predicted by their mechanism and they did not investigate the third body effect here. At high pressures of NO,reaction (3) dominates and hence i the activation energy measured from ^ ^ is the activation energy for reaction (5) and not (E 2 - AH^). PST suggested the poss ib i l i ty that reaction (3) may interfere but did not expect it to dominate. With high pressures of NO PST used high pressures of I 2 (about 400 Pa or 3 torr ) . With such high pressures of I 2 a s ignif icant adiabatic temperature rise can occur in the system which wi l l affect the results. As they report "at such high concentrations of iodine, appreciable thermal effects occur which make the absolute measure-ment of rate constants unrel iable". 3. SUMMARY The mechanism for the recombination of I atoms in the presence of NO is proposed to be: I + NO + M t INO + M 1. U = Kj/K.i) I + INO - I 2 + NO 2. (K2) 2IN0 I 2 + 2N0 3. ( K 3 ) 21 + M -> I 2 + M 4. ( K J - 91 " At low pressures o f NO, I 2 reappears by f i r s t order kinetics with directly dependent on [NO] and [Ml, and [INO] is at a steady state concentration. At high pressures of NO, I 2 and INO follow second order kinetics. The second order con-i stant measured from ^ (^nd^ o r * ^ ^ 2 n d ^ ' S ' n c ' e P e n c ' e n t : ° f ^ 0 ] or [M]. Both steady state and equilibrium hold for [ I ] . A new absorption system is found for INO in the ultraviolet with e reaching 1.6 x 10^ Arnol^cm - 1 at 251 nm. The following rate constants and activation energies were measured: Kx = 3.74 x 10 9 A 2 m o l ' 2 s " 1 ; E a c t ( 1 ) ="3.4 kJmol" 1 K3 = 3-95 x 10 7 £mo l " 1 s " 1 ; E a c t ( 2 ) = '3 kJmol - 1 Ku = 4.72 x 10 9 £ 2 m o r 2 s - 1 ; E ==-4.3 kJmol' 1 act l j ) The equilibrium constant 10 7 £mol _ 1 t K < 2.1 x 1 0 1 0 £,mol_1 from a calculated value of AS for reaction (l) as -100 Jmol 1 K - ] g i ves 62 kJmol" 1 f -AHi (I-NO bond strength) 5 81 kJmol"1 - 92 -CHAPTER IV CHLORINE OXIDES 1. RATE OF FORMATION AND DECAY OF C10 FROM THE (Cl7+Q ?) SYSTEM Resu1ts Flash photolysis of mixtures of C1 2 and Q 2 produced the CIO radical . Typical reaction mixtures contained 0 . 6 7 kPa ( 5 torr) of C l 2 and 53 kPa (400 torr) of 0 2 and were photolysed using a flash energy of 600 or 1066 J . A pyrex reaction vessel and Corning 0 - 5 ^ glass f i l t e r (300 nm cutoff) was used to prevent photolysis of the C10 in its continuous absorption region (below 266 nm) and through predissociation from the ( 7 . 0 ) and ( 8 , 0 ) bands at 2 9 2 . 2 and 2 8 8 . 5 nm by the flash lamp and to reduce scattered l ight . During the rise of C10 a fast signal response time from the photomu1tip1ier was needed and hence the f i l t e r capacitor was removed from across the load resistor. - 93 -From measurements of the photolysis of C12 in argon the in i t i a l photolysis of C l 2 could be estimated. However, since the percentage photolysis is low and scattered l ight interferes before 150 us after the f lash, an estimate of [C1] Q may not be more accur-ate than ±50%. For a flash energy of 600 and 1066 J the photolysis of C l 2 is about k% and S% respectively. However, only about 5% of this amount undergoes a net reaction to form CIO. Hence the CIO yield is quite low. Except for survey work,measurements were made using the photoelectric technique. CIO was measured at 277.b and 251 nm. The ( 1 2 , 0 ) band of the A 2n 3/z + X 2 n 3 / 2 transit ion has a head at 2 7 7 - 2 nm. In order to avoid the poss ib i l i ty that the bandpass of the detection system might overlap beyond this band head, measure-ments were made at the s l ight ly longer wavelength of 2 7 7 - ^ nm. The other wavelength (251 nm) l ies in the continuum of the A 2n. •<- X2II. system. The extinction coeff icient of CIO at these wavelengths was determined from the results of experiments with C102 which are discussed later in this chapter. Formation of CIO: By comparing the formation at both 251 and 277-k nm - Sk -shown in figure 21 it was clear that another transient species was interfering. The absorbance of 277-4 nm rises to a peak in about 4 0 us and then decays slowly (on a millisecond time scale). At 251 nm the absorbance rises very rapidly and peaks by about 50 us. There is then a decay to a steady level in about 250 u s . This is followed by a slow decay on the millisecond time scale. By 250 us the ratio of absorbances at both wavelengths equals the correspond-ing ratio of the extinction coefficients of CIO. If the interfering species does not absorb at 211.h nm then it may be separated from the CIO absorbance at 251 nm. The expected absorbance due to CIO at 251 nm was calculated using the measured absorbance at 2 7 7 . 4 nm and then, by difference, the absorbance of the interfering species was found. This is shown in figure 2 1 . The ratio of the in i t i a l chlorine atoms formed ( [C 1 ] q ) to the maximum [CIO] produced ([ClO]^) was found and plotted against l / [ 0 2 j . These results are summarized in table 4 and figure 2 4 . Decay of CIO: Reliable measurements of the decay of CIO were d i f f i cu l t for the following reason. When the reaction mixture was photolysed there resulted a series of irregular osc i l lat ions (on a millisecond time scale) in the monitoring light intensity reaching the photomultiplier - 95 -(figure 2 5 ) . These osci l lat ions were most easily observed at a wavelength region outside the CIO spectrum (eg. 330 nm). They were not observed with an empty reaction vessel or with oxygen or argon only in the reaction vessel. When oxygen was replaced by argon in the reaction mixture (thus no CIO was formed) the osci l lat ions were present but if the photolysis light was f i l tered using a Corning 3"73 glass f i l t e r (kOO nm cutoff) none were observed. They were roughly the same at 2 5 1 , 277-^ and 330 nm and decreased with decreasing flash energy. Thus it seems that the osc i l lat ions were dependent on the photolysis of C^- The only reasonable conclusion is that a thermal effect is responsible for the observat ions Assuming k% or 3% photolysis of C l 2 a maximum adiabatic temperature rise of about 9 or 20 K may be expected. Although this temperature rise is not large it should be remembered that since the yield of CIO is quite low then measurements must be made at high sens i t iv i t ies where small effects may become important. The maximum adiabatic temperature rise from other systems studied in this work is ex-pected to be less than 2 or 3 K. Since the osc i l lat ions were not reproducible a correction for them could not be carried out. A plot of the second order rate constant for CIO removal against total pressure is shown in figure 26 and summarized in FIGURE 21. T I M E DEPENDENCE OF ABSORBANCE DUE TO CIO AND NEW CONTINUUM (Assumes no i n t e r f e r e n c e a t 277.4 nm) P r 1 = 0.67 kPa(5 t o r r ) 0 50 100 t ( u s ) 150 200 250 FIGURE 2 2 , TIME DEPENDENCE OF ABSORBANCE OF CIO AND ClOO CORRECTED FOR DIFFERENCES IN EXTINCTION COEFFICIENTS AT 251 nm 0.08 H 5 0 100 t (Ps) 150 200 250 - 98 -(1/P n )(kPa-i) - 100 -TABLE 4. [ C l ] /[CIO] VERSUS 1/P 0 0 0 1 Run # F l a s h E nergy ( J ) CIO max ( 10 6 mol a-1) [cu 0 597 1066 .759 66.9 1.88 580 1066 .868 58.5 •1.49 593 1066 1.37 37.1 0.75 594 1066 1.46 34.8 0.75 577 1066 1.70 29.9 0.50 578 1066 1.63 31.2 0.50 590 1066 2.40 21.2 0.25 591 1066 2.51 20.2 0.25 574 1066 2.22 22.9 0.15 575 1066 2.50 20.3 0.15 587 1066 2.85 17.8 0.093 588 1066 2.96 17.2 0.093 602 1066 2.56 19.8 0.093 598 600 3.68 58.9 1.88 581 600 3.86 56.2 1.49 595 600 6.32 34.3 0.75 579 600 7.41 29.3 0.50 592 600 9.86 22.0 0.25 576 600 9.07 23.9 0.15 [ c i ] 0 = 2.17 x 10~ 5 m o U - 1 f o r F l a s h Energy = 600 J - 101 -TABLE 5. SECOND ORDER RATE CONSTANT FOR CIO DECAY AT 251 nm VERSUS IN THE SYSTEM C l 2 + 0 2 ' j (kPa) F l a s h Energy ( J ) K/e(10k : 10.0 1066 • 0.91 10.0 1066 3.00 10.0 1066 2.75 20.0 1066 2.88 20.0 1066 3.46 20.0 1066 2.30 *30.0 1066 3.39 30.0 600 4.37 *30.0 600 5.68 36.7 1066 4.70 *36.7 .600 6.08 36.7 600 7.72 50.0 1066 7.00 *50.0 1066 9.27 50.0 1066 4.30 63.4 1066 8.00 63.4 1066 9.00 63.4 1066 10.2 70.0 1066 7.46 70.0 600 9.13 70.0 1066 11.3 80.0 1066 10.8 85.0 1066 7.20 ( P n = 10 kPa) u 2 *Measured a t 277.2 nm and c o r r e c t e d t o 251 nm. FIGURE 25. O S C I L L O S C O P E T R A C E S H O W I N G C I O D E C A Y W I T H O S C I L L A T I O N S t(ms) - 103 -FIGURE 2 6 . SECOND ORDER RATE CONSTANT FOR CIO DECAY VERSUS TOTAL PRESSURE ( C l 2 + 0 2 ) L i n e shown i s from C 1 0 2 d a t a PQ 2 = 10 kPa(75 t o r r ) 2 0 40 P T ( kPa ) 60 80 " l e t -table 5- The line drawn in this figure was taken from results of the flash photolysis of C l O g which is discussed later in this chap-ter. Although there is a large scatter in the results the following important observations can be made. a) The second order rate constant for the decay of C I O increases with increasing total pressure over the range 20 to 100 kPa (150 to 750 torr) and K_ , is consistent with results from C l O o . 5 rd b) Results at both 251 and I'll.b nm are the same, indicat-ing that there is no signif icant absorption of any other species than C I O at these wavelengths during the C I O decay. These observations are important in interpreting the CIO2 results which are discussed later. D i scuss ion Consider the following mechanism for the formation of CIO. Cl + 0 2 + M t C100 + M 1. (K = Ki/K_i) C l + C100 + C l 2 + 0 2 2. (K2) C l + C100 2C10 3. (K3) 2C1 + M •+ C l 2 + M 4. (K4) (19) This mechanism was f i r s t proposed by Porter and Wright and later supported by other workers ^ ^ 7 ^ 8 ) T ^ e ; n f r a r e d spectrum (21) of C100 was f i r s t identified by Arkell and Schwager in an argon - 105 -matrix at h K from photolysis of 0C10. The f i r s t detection of ClOO in the gas phase was reported by Johnston, Morris and Van den Bogaerde^ ' (JMV) from photolysis of C l 2 and 0 2 mixtures using their new technique of molecular modulation kinetic spec-trometry. They observed the infrared spectrum and a new system in the ultraviolet between 230 and 260 nm. Extinction coe f f i -cients for the ultraviolet system were found and the spectrum is reproduced in figure 23. In addition, they reported values for K2 and K3. Since the ClOO spectrum reported by JMV peaks at about 250 nm with an extinction coeff ic ient of 3-48 x 103 £mol - 1 cm _ 1 and the reaction mixture used in this study is similar to that used by JMV, the observed " in ter fer ing" transient is assigned to ClOO. The absorption of ClOO at 211 .k nm is signif icant com-pared to CIO and hence the difference between absorbances at 211.k and 251 nm (when corrected for the differences in the extinc-tion coeff icient of CIO at these wavelengths) is not an accurate, measurement of ClOO. Using the extinction coeff icients of both CIO and ClOO at the two wavelengths the growth of CIO and the growth and decay of ClOO were calculated (figure 22). Over the period where CIO is formed its decay is negl igible. - 106 -The general equations are: = 2K3[C1] [ClOO] h-] d [ C ^ ° ] = K1[C1][02][M] - (K2 + K3) [Cl] [ClOO] + k-2 - K_x[ClOO][M] -d[Cl] d t = K1[C1][02][M] - K_![C100][M] + (K2 + K3)[C1][C100] + k~3 + MC1] 2 [M] ^ ^ 1 = K2[C1][C100] + (KV2 ) [C1] 2 [M] k-k (22) Nicholas and Norrish (NN) studied the formation of CIO from the flash photolysis of C12 + 0 2 mixtures. They point out that since the bond energy of C1-00 had been estimated to be 3 3 - 5 kJmol - 1 by Benson and Buss then reaction (3) is approxi-mately thermoneutral, while reaction (2) is highly exothermic and hence reaction (2) is probably faster than reaction ( 3 ) . Thus only a small amount of ClOO reacts to form C10. However, since about 5% of the chlorine atoms produced end up as C10 they conclude that most ClOO radicals react rapidly with Cl rather than undergo a unimolecular decomposition by react ion (-]) and that the concentra-tion of ClOO remains low compared to that of Cl atoms and C l 2 . Therefore ClOO is considered by them to be in a stationary state during the C10 formation. Reaction (k) is not considered important - 107 -in their mechanism. They derive the rate expressions for CIO formation and Cl removal using steady state for [C100], (note that they did not include a factor of 2 in equation 4-1). This gives: [ClOO]^ = Kx [02] [M] / (K2 + K3) ••• ^ - K^tC.][. 2][M] and ^ I |H » 2K1[C1][02][M] hence ~ d [ C l ] = 2 ( K ' + K2) d[C10]  n e n c e dt K3 dt Thus they conclude that the ha l f- l i fe for chlorine atom removal equals that for CIO formation and is given by t, = On2)/2K 1[0 2] [M] Kj was determined to be 6 . 1 8 x 1 0 8 £,2mol 2 s ~ 1 from this relat ion. The rate of CIO formation immediately following the flash was determined and as the rate of Cl atom removal was proportional to the rate of CIO formation the [Cl] could be found. Then, using equation 4 - 5 the ratio K2/K3 was calculated to be 15-On the other hand JMV,as part of their analysis^assume that [ClJ and [C100] are in chemical equilibrium through reactions (1) and ( - 1 ) . In order to estimate K the thermodynamic properties of C100 were largely inferred from kinetic data. - 108 -The preexponentia 1 factor for reaction (3) was estimated (32) by theoretical methods • and from the measured value of K3 its activation energy was found to be close to zero. Clyne and Coxon ^ found the activation energy for the reverse of reaction (3) to be 10 kJmol Thus JMV estimated the standard heat of formation for ClOO to be 95-4 kJmol" 1. This gave K = 2.18 lmo\~l and by using the value of K j ' = 6.18 x 108 £ 2 m o r 2 s - 1 reported by NN, K_i is found to be 2.81 x 108 Arnolds -}. The ratio K 2/K 3 was measured as 108-At high pressures of 0 2 where reaction (k) may be neg-lected it is obvious from inspection of the mechanism that the ratio of chlorine atoms in i t i a l l y formed from the flash to the maximum [CIO] produced is given by (K 2+K 3)/K 3. This is true regardless of any simplifying assumptions made. In this study [C100] is formed rapidly (during the flash) and peaks before a signif icant amount of CIO has been formed (figure 22). It is therefore reasonable to assume that reaction (1) is considerably faster than reaction (2) or (3). Hence ClOO is rapidly formed in an equilibrium concentration with Cl and 0 2 . From equations k-\ and2j-2» i d [C 1 2 ] = K2 [Cl ] [C 100] + KV2[C1]2[M] d[C10] 2K3[Cl][C100] - 109 -Equilibrium assumption for [C100] gives [C100]E = Ki[0 2 ] [Cl] K. d[Cl 2 ] = _Kz_ M M ] d[C10] 2K3 kKiK[02] [ C I 2 I 0 0 K 2 _ . M M ] Tc l o r 2K3 kK3K[0z\ Mass balance gives [Cll = 2[C12] + [C10]( o [Cl ip = TcToT K 2 + K 3 K 3 + Kit [M] 2K 3Z[0 2] Tc 11 A plot of i i2. versus 1/Pn is shown in figure 2k and summarized [ClO]^ ° 2 in table k. From this it is found that K 2/K 3 = 16 and K4/K3Z = 0.882 . Using a recent value, for Kk of 5 - 3 2 x 109 £ 2 m o r 2 s _ 1 then the. value for K3K is found to be 6.03 x 109 £ 2 m o r 2 s _ 1 . This may be compared with the value of 1.89 x 109 i ! 2 mol- 2 s - 1 measured by JMV. Ki was calculated by JMV from thermo-dynamic data to be 2.18 Jlmol"1. As previously mentioned this calculation depended on a value for the AH of formation for C100 (33) from Cl and 0 2 . In a recent report Clyne and White evaluated - 1 1 0 -this quantity by comparing their measured value of K3 with JMV's rate constant for the reverse reaction. They calculate the standard heat of formation of ClOO to be 93 kJmol - 1 which is in remarkably good agreement with the value of 95 kJmol - 1 used by JMV. To obtain the value of 108 for K 2/K 3 reported by JMV would require 23% photolysis of C l 2 for a flash energy of 600 J in this study. However only about k% is photolysed. From figure 3 in NN1s paper [C10]^ for 2.73 kPa (50 torr) of 0 2 can be estimated as 3-03 x 10~6 moR" 1 . At this pressure of 0 2 the ratio ([Cl lytClO]^) is close to (K2+K3VK3 as seen in figure 2k of this study. Hence an estimate of K2/K3 can be made using [C1] q = 6.6 x 10 - 5 moU" 1 as given by NN, giving ^/K^ = 22. Con-sidering the accuracy in estimating [C1] q is this study this is in satisfactory agreement with the value of 16 obtained. 2. C10 DECAY FROM C102 and C120 Results C102(0C10) and Cl 20(C10Cl) were used as sources of C10 radicals in order to study the decay of C10. This study involved - Il l -both photographic and photoelectric detection methods and the results from both are needed in order to interpret the results. Since a small but signif icant amount of photolysis resulted from the mercury lamp over a period of seconds, two f i l t e r cel ls (one with C12 and one with Br 2) were used to f i l t e r the output of the arc (see figure 2 for the transmission curves of these f i l t e r s ) . These f i l t e r s prevented monitoring above 300 nm and hence when C102 was measured at 3 4 3 - 7 nm the halogen f i l t e r was replaced with a neutral density f i l t e r (A = 1.8) to cut down the light intensity of the arc. As an added precaution a shutter assembly was also used in front of the arc. Opening the shutter generated a trigger pulse which init iated the. f i r iog sequence for the photolysis lamp. Photographic measurements of CIO were made in the 2 n 3 / 2 + 2 n 3 / 2 transition at the (7,0) band (292 .2 nm) , (12,0) band (277-2 nm) and in the continuum at 257-7 nm. Photoelectric measurements in the CIO continuum were made at 251, 240, 2 3 5 - 5 and 232 nm. The intensity of the mercury arc was too low to make measurements at 257-7 nm and a group of s i l i c a absorption lines prevented photographic measurements at 251 nm. In order to avoid overlapping of the band heads, the (12,0) and (7,0) bands of C10 were measured at the s l ight ly longer wavelengths of 277-4 and 112 292.2 nm respectively. Data at 2kO, 235-5 and 232 nm is less accurate due to the low output of the mercury lamp at these wave-lengths and was only used to measure the spectrum of the interfer-ing species. C10 2 was photographically measured at 351-5 nm and photoelectrically at 3^3-7 nm. The wavelength of 3^3-7 nm was used for photoelectric measurements since a drop in light intensity of about 70% (for a pressure of 26.7 Pa or 0.2 torr of C10 2) could be more accurately measured than the 97% drop at 351-5 nm. This is in contrast with photographic work where the best accuracy is obtained for the largest change in absorbance(within the linear portion of the characteristic curve of the plate). The measured extinction coefficient at 351.5 nm of 2.97 x 10 3 £mol - 1 cm - 1 agrees well with the value of 3.00 x 10 3 £mol - 1 cm _ 1 reported by Clyne and Coxon^^ and 3-09 x 10 3 £mol - 1 cm - 1 reported by Basco and Dogra^"^ A correction for the small absorbances by C10 2 was necessary for photoelectric work at 292.2 and 277-4 nm. Extinction Coefficient of C10: In order to measure the extinction coeff icient of C10, the loss of C10 2 was compared to the amount of C10 produced. This (25) was the method used by Basco and Dogra . The assumptions involved are: - 113 -a) The change in ClOg concentration following flash photolysis is the in i t i a l CIO concentration. b) The in i t i a l CIO concentration is determined by the short linear extrapolation of the second order plot of CIO decay back to zero time. These assumptions are valid when the primary photolysis of C102 is less than 50%. The important reactions are C102 + hv -*• CIO + 0 1 0 + C102+ CIO + of 2 0 + C l O - v C l + O * 3 Cl + C102 2C10 li 2C10 -y C l 2 + 0 2 5 where 0 2 represents vibrational ly excited oxygen. Because reaction (28)(65) (k) is very fast the assumption doesn't depend on the re la -tive rates of reaction (2) and ( 3 ) . Reaction (5) is slow compared to the others. It may be noted here that the vibrational bands are not resolved to the base line and hence when measuring CIO using the photographic technique, reference must be made to a blank in order to position the base l ine. An alternative method is to use an "apparent" base line formed by joining the minima on either side of the band to be measured. The apparent extinction coeff icient - 114 -to this base line can be determined in the same manner as the actual extinction coeff icient and the two are related by a con-stant factor. This procedure was found to be valid and is j u s t i -fied in the appendix. This method has the great advantage that a blank is not needed to measure the absorbance. In addition, any underlying continuum is not included as CIO in the measurements. At 277.2 nm the actual extinction coeff icient was found by photographic measurements to be 1-98 x 1 0 3 Arnol^cm - 1 which compares well with the value of 1.90 x 1 0 3 £mol _ 1cm 1 reported by Clyne and Coxon ^ ) and the value of 1 -70 x 1 0 3 £mol - 1 cm _ 1 reported by Basco and Dogra (25). The apparent extinction coeff icient was found to be 1 .41 x 1 0 3 Jlmol^cm - 1 (a factor of 1.40 smaller than the actual extinction coeff ic ient ) . The values of the extinction coeff icient at 299.2 nm and 257-7 nm were found to be 1.12 x 1 0 3 and 1.3 x 1 0 3 £mol 1cm 1 in good agreement with the values given by Basco and Dogra (25) of 1.05 x 1 0 3 and 1.15 x 10 3 Arnol^cm"1 and at 257.7 nm the value of 1.27 x 1 0 3 £mol 1cm~ 1 by Clyne and Coxon (^4) j^y measured relative e values for CIO from 280 to 225 nm. This data was normalized to the value of 1.27 x 1 0 3 £mol ^m 1 reported by Clyne and Coxon^) . JMV 's data was plotted and the values for e at 251, 240, 235-5 and 232 nm were 980, 500, 360 and 260 Arnolds" 1 respectively. - 115 -Photoelectric measurements gave a value of the extinction coeff icient of CIO at 251 nm of 995 Jlmol^cm - 1 which is in good agreement with the value of 980 Arnol^cm*"1 from JMV. The values from JMV at 2k0, 235 - 5 and 232 nm were used since an interfering species (to be discussed later) affected the measurements at these wavelengths. Decay of CIO: a) Photographic Results The decay of CIO was measured over a range of pressures of C 1 0 2 and argon of 2 6 . 7 to 80 Pa (0.2 to 0 . 6 torr) and 6 . 6 7 to 80 kPa (50 to 600 torr) respectively. CIO decay followed second order kinetics (figure 27) and the rate constant was l inearly dependent on argon pressures (figure 2 8 ) , independent of CIO,, pressures, and independent of flash energy from 150 to 1066 J . Table 6 summarizes the results where a l l data were obtained by using the apparent base l ine. The third order rate constant was found to be 3 -7^ x 10 9 £ 2 m o l - 2 s _ 1 . At high flash energies where almost a l l the C10^ is destroyed rapidly (either by primary photolysis or by fast second-ary reactions) the second order decay rate was the same whether the apparent or actual base line was used. This rate was also - 1)6 -FIGUE 27, SECOND ORDER DECAY OF C l 0 TO APPARENT BASE LIME AT 2 7 7 . 2 nm P r i n = 40 P a ( 0 . 3 torr) L I U 2 - l 1 1 1 i r ~ 0.05 0.10 0,15 0,20 0.25 0.30 t(ms) - 118 -TABLE 6 . SECOND ORDER RATE CONSTANT FOR CIO DECAY AT 277.2 nm VERSUS P USING PHOTOGRAPHIC MEASUREMENTS IN THE C 1 0 2 SYSTEM Run # P h o t o l y s i s Energy ( J ) P A r ( k P a ) p c i o 2 ( P a ) K / e d O4 s 262 320 6.67 26.7 1.08 263 320 6.67 26.7 1.04 253 320 13.3 66.7 1.26 264 320 13.3 66.7 1.40 251 320 13.3 53.3 1.78 250 320 13.3 40.0 2.34 252 320 13.3 26.7 * 2.38 268 320 13.3 26.7 2.08 261 320 26.7 66.7 3.02 259 320 26.7 53.3 3.88 258 320 26.7 40.0 3.40 260 320 26.7 26.7 3.68 257 320 40.0 66.7 4.44 255 320 40.0 53.3 4.67 254 320 40.0 40.0 5.71 265 320 40.0 40.0 5.14 270 320 40.0 40.0 4.60 - 119 -T A B L E 6 . (CONTINUED) *un # P h o t o l y s i s Energy ( J ) P A r ( k P a ) p c i o 2 ( P a ) K/edO 1* s" 271 150 40.0 40.0 5.31 273 504 40.0 40.0 4.55 274 1066 40.0 40.0 4.51 256 320 26.7 26.7 5.53 242 416 53.3 53.3 5.31 247 416 53.3 53.3 5.54 280 416 53.3 53.3 5.96 281 1066 53.3 53.3 . 6.10 248 416 53.3 40.0 5.90 243 416 53.3 26.7 6.36 244 416 53.3 26.7 5.35 246 266 53.3 26.7 6.80 266 320 53.3 26.7 5.77 267 320 80.0 26.7 9.15 272 320 80.0 26.7 8.41 - 120 -found to be the same using the ( 7 , 0 ) band, the ( 1 2 , 0 ) band or at 2 5 7 - 7 nm (in the CIO continuum). At low flash energies however, the rate of decay measured to the actual base line was slower than that measured to the apparent base line at both 2 7 7 - 2 and 292 nm. In addition, after 3 or h ms the rate of decay measured to either the actual or apparent base lines became very slow with detectable amounts of CIO present even after several hundred milliseconds. Although the absorbance after about h ms was too low for quantita-tive measurements it is clear that the resolution of the CIO bands from the actual base line had dropped considerably. This indicates that a continuum underlying the CIO spectrum forms during the CIO decay. A similar effect is observed at 292 nm. At 2 5 7 - 7 nm the decay of CIO is very nearly offset by the growth of the continuum. Thus at low flash energies measurements made to the base line include both CIO and the interfering species and hence second order plots measured to the base line wil l not agree with those measured to the apparent base l ine. The ratios of absorbances (measured to either the apparent or actual base lines) at 2 9 2 . 2 , 2 7 7 - 2 and 2 5 7 - 7 nm are the same immediately following the photo flash for a l l flash energies or ClO^ pressures used. This ind i -cates the interfering continuum is not present to a signif icant amount at these wavelengths and early times. - 121 -The important observation was also made that, even at the lowest flash energies used (150 J ) , the ClO^ which was not i n i t i a l l y decomposed eventually decayed over a period of several seconds. b) Photoelectric Results The interfering continuum was further studied using the photoelectric method because of its greater sens i t iv i ty . Most of this work was carried out with 5 3 - 3 Pa (0.4 torr) of C10 2, 5 3 . 3 kPa (400 torr) of argon, and flash energies from 266 to 1 0 6 6J. It should be noted that photoelectric measurements yield the actual absorbances. The "apparent" absorbance as defined for photographic work cannot be measured. Thus the effect of an under-lying continuum cannot be separated by measurements at only one wavelength. At high flash energies about 35% of the ClO^ was decom-posed and the decay of CIO followed second order kinetics. A negative curvature was noticed in the second order plot beyond about 4 ms (figure 2 9 ) . This complication is not considered here and its effects are avoided by measuring rate constants at times shorter than 4 ms. These agreed with photographic results. At low flash energies the decay at 277.4 and 2 9 2 . 2 nm was second order for the f i r s t 2 or 3 ms then the rate of decay - 122 -- 123 -slowed rapidly and became very slow with a detectable absorbance for several seconds. At 251 nm there is less than a 10% decay in the absorbance over the f i r s t few milliseconds. This is followed by a very slow decay over a period of seconds. The decay at 2k0, 235-5 and 232 nm was similar to 251 nm except at 2k0 nm the absorb-ance increases over the f i r s t two milliseconds. The effect of flash energy on the decay of absorbance at 2 9 2 . 2 , 251 and 2h0 nm is shown in figures 30, 31 and 32 respectively. From the figures it is clear that the interfering con-tinuum affects the measurements more strongly at shorter wave-lengths. For example at 240 nm a growth in absorbance is i n i t i a l l y observed following the photo flash indicating that, the loss in absorbance expected from CIO decay is more than offset by the growth in the new continuum. As has previously been mentioned, the second order rate constant for the decay of CIO measured using photographic detec-tion to the apparent base l ine, is independent of flash energy. Since this is a measurement of CIO only (that is the interfering species is not included) it is inferred that the interfering species does not affect the decay of CIO over the f i r s t 3 or k ms. Thus, given an in i t i a l concentration of CIO at low flash energy, the expected decay of CIO can be calculated using the observed rate - 125 -FIGURE 31, DECAY OF CIO AT 251 nm P c l 0 ? = 53.3 Pa(0.4 torr ) P. = 53.3 kPa(400 torr ) 1 1 1 1 1 : i r 0 1 2 3 1 5 6 7 t(ms) - 126 -- 127 -of decay for high flash energy. Figure 30 shows the decay of absorbance at both high and low flash energies at 292.2 nm. It is fortuitous that both energies produce the same intial concen-trations of CIO. The reason for this is that if much more than 50% of the C10^ is decomposed by reaction (1) there are more oxy-gen atoms produced than ClO^ remaining. Hence there wil l be a reduced yield of CIO due to the net results of reaction (2) followed by ( 3 ) ; or (3)followed by (4). This is true regardless of the relative rates of reactions (2) and ( 3 ) . Taking advantage of the equality of the in i t i a l concen-tration of CIO for the two flash energies, then the difference between the two equals the strength of the interfering continuum. * Using the ratio of extinction coefficients of CIO the expected decay of CIO is calculated at 277-4, 251, 240 and 235-5 nm. The difference between the calculated, and observed decays at low and high flash energies gives the continuum strength at these wavelengths. Figures 33 and 34 show the time behaviour of CIO and the interfering continuum at 240 nm for low and high flash energies respectively. The interfering continuum is tentatively attributed to C1 2 U2* T n ' s w ' ' ^ ^ e discussed later. Table 7 gives the re la -tive extinction coefficients of C1,0„ at several wavelengths where t(ms) - 129 -FIGURE 34.- TIME DEPENDENCE OF CIO AND C l ^ AT 240 nm AND 1066 J t ( m s ) - 1 3 0 -TABLE 7. EXTINCTION COEFFICIENT VERSUS WAVELENGTH FOR ClgOg A(nm) R e l a t i v e e e ( 1 0 3 A m o l ^ c n f 1 ) 292.2 0.210 0.360 277.4 0.449 0.772 251.0 1.00 1.72 240.0 1.15 1.98 235.5 1.37 2.36 232.0 1.52 2.61 - 131 -e at 251 nm has been set at unity. This was arrived at by compar-ing the strength of the C '2^2 continuum at each wavelength. If it is assumed in figure 3^ that a l l CIO lost decays to C 1 o v e r the time period followed and no 01^02 decays, then the ratio (R) of the change in absorbance due to loss of CIO to the change in absorbance from the formation of £^2®2 ' S 9' v e n R - 2- £ C 1 ° e c i 2 o 2 Oe c io e „ , n = 2-' C 1 2 0 2 R From figure 33 it is found that R is constant within experimental error and equal to 0 . 5 0 5 (table 8 ) . Hence at 2k0 nm n ~ 1 -98 x 1 0 3 £mol - 1 cm" 1 . From the relative values of the L I 2 U 2 extinction coefficients the absolute values may be calculated and are also give in table J. By estimating the accuracies of each step in the calculation the relative values of the extinction coefficients are not expected to be better than ±35%. It was c r i t i c a l to establish whether ClO^ played any role in the formation of the transient - both to identify 0 ^ 0 2 as the transient species absorbing and to determine a mechanism. For this reason C^O was used as another source of C10 radicals. A pyrex reaction vessel was used with flash energies - 13?. -TABLE 8. CORRELATION IN THE LOSS OF CIO ABSORBANCE WITH THE GAIN IN C l f l z ABSORBANCE t(ms) A A r i n AA r L' 2 U 2 X I 0 " " C 1 2 0 2 AA CIO 1.0 0.128 0.281 4.55 1.2 0.139 0.297 4.57 1.4 0.147 0.305 4.83 1.6 0.155 0.318 4.88 1.8 0.161 0.324 4.98 • 2.0 0.163 0.326 5.00 2.5 0.172 0.335 5.13 3.0 0.177 0.340 5.22 3.5 0.183 0.346 5.29 4.0 0.186 0.349 5.32 5.0 0.191 0.354 5.40 6.0 0.194 . 0.356 5.43 7.0 0.197 0.360 5.50 8.0 0.199 0.362 5.50 - 133 -ranging from 320 to 1066 J . The pressure of Cl^O used was from 67 Pa to 0 . 2 7 kPa (0.5 to 2 torr) and of argon was 53-3 kPa {k00 torr) for a l l experiments. When Cl^ was added to the mixture 0 . 6 7 kPa (5 torr) was used. The important reactions in C10 formation are: C l 2 0 + hv->-Cl+C10 Cl + C1 2 0 -> C l 2 + C10 Since C^O does not absorb as strongly as CIO2 only a small per-centage was decomposed (about 2% at 1066 J). In order to increase the decomposition of C^O and increase the yield of C10, C]^ was added to the mixture. This gives a higher yield of Cl atoms from the f lash. Although a detailed quantitative study was not carried out it was found that when a low percentage of C^O was decomposed the CI2O2 continuum was formed (figure 35 ) . Under conditions of increasing decomposition of Cl^O (by adding C ^ and increasing the flash energy) decreasing amounts of ^ 2 ^ 2 w e r e formed. D i scuss ion I dent i ty of the New Continuum: An attempt to identify the new continuum with the spectra - )3b -- 135 -(66) of known chlorine oxides was made. The spectra of C1^0 and C l C ^ k ^ ^ ^ a r e known and were observed in this study. The new continuum is clearly different from these. ^'2^6 ' n t' 1 e 9 a s e o u s state exists almost entirely as C10^. The spectrum of C l O ^ ^ ^ shows a drop in extinction coeff icient by 50% from 277.k nm to 251 nm while the new continuum increases by a factor of two over this range. Thus C10^ can be rejected. The spectrum of C^O^ has been reported^^ and has an extinction coeff icient of only 158 ilmol"1cm~1 at 251 nm which is too low to account for the new continuum. C100 was given particular attention since it has been observed from the photolysis of mixtures of and Q^^^^ From figure 23 it can be seen that the extinction •coefficient of C100 drops by a factor of two while the new continuum (table 7) increases by 50%. Thus C100 is not responsible for the new con-t i nuum. McHale and E l be^ 7 1 ^ 7 2 ^ report production of C^O^ from the steady photolysis of C10^ vapour at 228 K. No spectrum was reported. In this study the new continuum was produced in experi-ments involving the low percentage photolysis of both C10^ and C^O. Since it is d i f f i cu l t to imagine any probable mechanism by which C^O^ c o u l d be formed from the photolysis of C^O , then - 136 -Cl^O^ is not accepted as the carrier of the new continuum. Thus, since neither ClO^ or Cl^O are prerequisites for the formation of the new continuum which forms as CIO decays, it seems reasonable that it is a product resulting from this decay. The only probable carrier is t^^z' Mechanism for CIO Decay: In their study of the flash photolysis of C10^, Lipscomb, (23) Norrish and Thrush (LNT) report a weak continuous absorption, mainly below 300 nm. They attribute this to a small amount of ClO^ which is formed during the flash and stays roughly constant through the more rapid radical decays and disappears slowly over a period of about a minute. The amount formed was independent of inert gas pressure but is greater with lower flash energies. This was explained by the reaction 0 + C102 •+ C103 Using a measurement of C10^ at 2 9 4 . 8 nm they applied a correction at 2 5 7 - 7 nm to obtain the absorption due to CIO alone. The result -ing "corrected" CIO absorption gave linear second order plots. By assuming the loss in CIO2 equals the formation of (C10^  + CIO) then e for CIO and the second order rate constant could be obtained. However with increasing flash energies the second order plots gave - 137 -higher slopes and constant intercepts instead of constant slopes and lower intercepts as expected. Since the values of ('/^nd^ were l inearly dependent on the pressure of C10^ remaining the equi1ibrium CIO + C1.02 t C1 20 3 was suggested to explain the inhibition of CIO decay by C1O2. If, however, the continuum attributed by them to C10^ was the same as observed in this study then their corrections at 2 5 7 - 7 nm are too low and a signif icant amount of the interfering species would be included in their plots (especially at lower flash energies). From a study of CIO decay from photolysis of C ^ and (19) 0^ plus ^ Porter and Wright found that CIO decayed in a bimolecular reaction whose rate was independent of C ^ , O2,total pressure and temperature. They ruled out a direct decomposition of the type 2C10 * C l 2 + 0 2 because of the high activation energy associated with such a reac-tion and suggested that the overall reaction could proceed via the dimer C^C^ by 2C10 t C1 20 2 C1 20 2 C l 2 + 0 2 They mention that their experimental evidence does not eliminate - 138 -the poss ib i l i ty of the formation of a "stable" C'2^2 which might then decompose slowly to plus O^. However i f such were the case the decay of CIO would be expected to be third order, con-trary to their results. They prefer to consider 0^02 formed rapidly in equilibrium with CIO and a slower decay of to C1^ and O2. Then the observed activation energy would be equal to the difference between the heat of formation of ^^2^2 ^ r o m 2C10 and the activation energy for the decomposition of ^2^2 t 0 C1^ and O2. The near cancellation of these two quantities is considered by them to be not unl ikely. At low pressures (67 Pa to 1.12 kPa or 0 . 5 to 8.4 torr) (24) (33) Clyne and Coxon and Clyne and White obtained evidence that C10 radicals are removed predominantly by the free radical mechan-ism proposed by Benson and B u s s ^ ^ . 2C10 Cl + C100 C l+C100- * - C l 2 + 0 2 A dist inct ion between the two mechanisms for C10 was made by intro-duction of a substrate which reacts rapidly with Cl atoms; ozone, CIO2 and Br2 were used. This work was carried out in a discharge flow system where a pure stream of C10 was generated by the rapid reaction of Cl with C1O2. To reconcile their results with the high pressure flash work, they postulate that at high pressures C10 decays, -139-via C 1 ' ^ a n e t l u ' 1 ' Crated third order reaction 2C10 + M t C1 2 0 2 + M fo11 owed by C1 2 0 2 -»- C l 2 + 0 2 From their study of the C1^ + ° 2 s y s t e m by molecular modulation, JMV include both the a n c * the CI + ClOO mech-anisms, except to add a third body for the decay of 0^02° . C1 2 0 2 + M -> C l 2 + 0 2 + M This work was carried out at high total pressures (6.67 to 100 kPa or 50 to 760 torr ) . Consider the following mechanism proposed for the decay of C10 at the pressures used in this study. 2C10 + M 2 C1 2 0 2 + M 1 . C1 2 0 2 +, M -*• ClOO + Cl + M 2. C1 2 0 2 + M-*C l 2 + 0 2 + M 3. Cl + C1 2 0 2 -*- C l 2 + ClOO k. ClOO + C1 2 0 2 -> C l 2 + ClOO + 0 2 5 . ClOO + M Cl + 0 2 + M 6. Cl + ClOO •+ C l 2 + 0 2 7 . Cl + ClOO -> 2C10 8. i Cl + Cl + M -*• C l 2 + M 9. 2C100 -»• C l 2 + 20 2 10. - 140 -C l + C 1 0 2 -> 2 C 1 0 11. C l + C l 2 0 - > - C l 2 + C 1 0 1 2 . C 1 0 0 + C 1 0 2 -> 2 C 1 0 + 0 2 13. C 1 0 0 + C 1 2 0 -»• C l 2 + C I O + 0 2 14. T h e d e c a y o f C I O p r o c e e d s b y r e a c t i o n ( 1 ) f o r m i n g C I 2 O 2 . R e a c t i o n ( 2 ) p r o d u c e s a C 1 0 0 r a d i c a l a n d C l a t o m w h i c h i n t u r n r e a c t r a p i d l y b y r e a c t i o n s (4 a n d 5) t o d e c o m p o s e m o r e C ^ ^ ' R e a c t i o n s (4, 5 a n d 6) c o n s t i t u t e a c h a i n w h i c h i s i n i t i a t e d b y r e a c t i o n ( 2 ) . P o s s -i b l e t e r m i n a t i o n s t e p s a r e g i v e n b y r e a c t i o n s (7 t o 14). W h e n C I O i s p r o d u c e d f r o m C 1 0 ^ a n d C ^ O a n d u n d e r c o n d i -t i o n s w h e r e m o s t o f t h e C 1 0 ^ o r C ^ O a r e p h o t o l y s e d r e a c t i o n s (11 a n d 12) w i l l b e u n i m p o r t a n t a n d t h e t e r m i n a t i o n s t e p s w i l l b e r e a c t i o n s (7 t o 1 0 ) . I f t h e s e r e a c t i o n s a r e r e a s o n a b l y a s s u m e d t o b e s l o w c o m p a r e d t o r e a c t i o n s (4 a n d 5 ) , t h e n t h e l e n g t h o f t h e c h a i n w i l l b e l a r g e a n d 0 ^ 0 2 w i l l d e c a y r a p i d l y . H e n c e , a l a r g e b u i l d u p o f C I 2 O 2 f r o m r e a c t i o n ( l ) i s u n e x p e c t e d . R e a c t i o n ( 1 1 ) i s f a s t ^ 2 8 ^ 6 5 ^ l = 5 x 1 0 9 A r n o l d s " 1 o r 3.6 x 1 0 1 0 £ m o l 1 s * ) a n d a l t h o u g h t h e r a t e c o n s t a n t f o r r e a c t i o n ( 1 2 ) i s a t l e a s t a n o r d e r o f m a g n i t u d e s l o w e r ( 7 3 ' ( K 1 2 = 4 . 1 x 1 0 8 £mol ^ s " 1 ) , t h e r a t e i s r a p i d s i n c e m u c h l a r g e r p r e s s u r e s o f C ^ O t h a n C I O 2 a r e u s e d . I f t h e s e r e a c t i o n s a r e s i g n i f i c a n t c o m p a r e d t o (4 a n d 5 ) , t h e n t h e c h a i n l e n g t h may b e v e r y s h o r t a n d a b u i l d u p o f C I 2 O 2 b y r e a c t i o n ( l ) t o a n e q u i l i b r i u m c o n c e n t r a t i o n c a n o c c u r . - 141 -This mechanism is consistent with the results at high and low percentage decomposition for ClO^ and C^O. The third order rate constant measured at high flash energy is then a meas-ure of Kj and is equal to 3-7^ x 1 0 9 A2mol 2 s _ 1 . JMV report a value of 2.4 x 1 0 1 0 £2mol 2 s 1 for Kj. The high flash energy runs of LNT should also give a measure of . They report K/e = 9 . 1 2 x \Q k s - 1 as the maximum slope at high flash energies. Using e = 1 . 2 7 x 10 3 Amol ^xm^and assuming a total pressure of 77.4 kPa (580 torr) then Kj = 3-68 x 10 9 ^ f f l o l ^ s " 1 which is in excellent agreement with this study. The equilibrium constant K = K J / K - J can be obtained by measurements of the concentration of CIO and at times greater than 10 ms when equilibrium has been reached and assuming reaction (l) is the only reaction by which CIO decays. Using the extinction coeff icient for d 2 ^ 2 a t "^^  n m 9 ' v e n 'n t a L , l e 7 then K = 2.9 x 10 9 A m o r 1 . Since = 3-74 x 1 0 9 A 2 m o l " 2 s _ 1 then K_! = 1 . 2 9 Arnolds"" 1. If AS^  is estimated to be -84 Jmol^K^then AH =-71-5 kJmol 1 . Alternatively we can use the relation K_i = Z exp(-AH/RT) where Z = 2 x lO i : iAmol 1s~l. Then using Kj = 1.29 Amol _ 1 s" 1 the value for AHj is-62.7 kJmol" 1. - 142 -(74) Gray and Ip estimate a value for the standard heat of formation of C^*^ t 0 ^ e ' ^ kJmol" 1. This was estimated from a value of the C1-00C1 bond strength suggested by McHale and (72) Von Elbe . These authors note that the bond strengths of F-OOF and F-00 have close to the same value (71 kJmol - 1 and 63 kJmol - 1 respectively). Since Benson and Buss^ u)have estimated C1 -00 to be 33-4 kJmol 1 , then C1-00C1 is assumed to be about the same. From Gray and Ip's data, then, AHx = -35.2 kJmol 1 . (34) Remeasurement of some of the data obtained by Dogra has revealed both a pressure dependence of CIO decay and an inter-fering continuum in the CIO continuum which as been attributed in this study to C ^ ^ - ^ence a reanalysis of this data may be required, 3. SUMMARY Formation of CIO from (Cl 2 + 0 2) The formation of CIO goes via the intermediate ClOO which was observed at 251 nm. C 1 + 0 2 + M ? C 1 0 0 + M 1. Cl + ClOO -»• C l 2 + 0 2 2. Cl + C100 2C10 3. The ratio of to is 16, in agreement with a value of 22 calculated - 143 -from data presented by NN but lower than the value of 108 reported by JMV. K^K is found to be 6.03 x 109 £ 2 m o l - 2 s - 1 which can be com-pared with the value of 1 -89 x 109 £ 2 mol~ 2 s _ 1 as measured by JMV. During the CIO decay no other species was observed and the rate increased with increasing total pressure in agreement with the results from the photolysis of C10^ and Cl^O. Decay of CIO At pressures above 6 . 6 7 kPa (50 torr) CIO decays by the ^^•pl intermediate. A chain mechanism is proposed for the decom-position of CI2O2 which is carried by Cl and C100. If there are no scavengers for Cl or C100 present then the chain length is suf-f i c ient ly long to prevent build up of C 1 " ' n t ' 1 e P r e s e n c e °^ scavengers ^2^2 builds to an equ i 1 ibriurn concentration with CIO. 2C10 + M t C 1 2 0 2 + M The equilibrium constant was found to be 2 . 9 x 10 9 £mol 1 and Kj = 3 . 7 4 x 10 9 £ 2 mol - 2 s - 1 . Then K_] = 1 .29 Amol " 1 s" 1 . Using AS, = -84 J m o l - 1 ^ 1 then AH. = -71-5 kJmol - 1 . - 144 -BIBLIOGRAPHY 1. M.A.A. C l y n e and B.A. T h r u s h ; T r a n s . F a r a d . S o c , 57, 1305 (1961). 2. M.A.A. C l y n e and B.A. T h r u s h ; T r a n s . F a r a d . S o c , 33_, 139 (196 2 ) . 3. E.M. B u l e w i c z and T.M. Sudgen; P r o c Roy. S o c , A277, 143 (196 4 ) . 4. F. Kaufman; P r o c Roy. S o c , A247, 123 (1 9 5 8 ) . 5. P.G. Ashmore and M.G. B u r n e t t ; T r a n s . F a r a d . S o c , 5_7, 1315 (196 1 ) . 6. R. Engleman and N.R. D a v i d s o n ; J . Am. Chem. S o c , 82, 4770 (1960). 7. G. P o r t e r , Z.G. Szabo and M.G. Townsend; P r o c . Roy. S o c , A270, 493 (19 6 2 ) . 8. G. P o r t e r and H u s s a i n ; T e c h . R e p o r t F.T.R. No. 6, U.S. Army C o n t r a c t DA-91-591-EIC-2801 ( 1 9 6 3 ) . 9. S .W. Benson and W.B. DeMore; Ann. Rev. Phys. Chem., 16, 399 (1 9 6 5 ) . 10. J . H e i c k l e n and N. Cohen; Adv. i n Photochem., 5_, 255 ( 1 9 6 8 ) . 11. G. P o r t e r ; P r o c . Roy. S o c . , A200, 284 (1 9 5 0 ) . 12. G. P o r t e r ; D i s c . F a r a d a y S o c , 9_, 60 (195 0 ) . 13. R.A. D u r i e and D.A. Ramsay; Can. J . Ph y s . , 36, 35 (1 9 5 8 ) . 14. N. Basco and R.D. Morse; J . M o l . S p e c t . , 45_, 35 ( 1 9 7 3 ) . 15. M.M. R o c h k i n d and G.C. P i m e n t e l ; J . Chem. Phys., 46, 4481 (196 7 ) . 16. P.A.G. O'Hare and A.C. Wahl; J . Chem. Phys., 54, 3770 ( 1 9 7 1 ) . 17. L. Andrews and J . I . Raymond; J . Chem. Phys., 55_, 3087 (19 7 1 ) . 18. H.S. J o h n s t o n , E.D. M o r r i s and J . Van de Boae r d e ; j . Am. Chem. S o c . , 91, 7712 (196 9 ) . - 145 -19. G. P o r t e r and F . J . W r i g h t ; D i s c . F a r a d a y S o c , 14_, 23 (1953 ) . 20. T. C o l e ; P r o c . N a t l . A cad. Soc. U.S., 46, 506 (1 9 6 0 ) . 21. A. A r k e l l and I . Schwager; J . Am. Chem. S o c , 89, 5999 ( 1 9 6 7 ) . 22. J . E . N i c h o l a s and R.G.W. N o r r i s h ; P r o c . Roy. S o c , A307, 391 (1 9 6 8 ) . 23. F . J . Li p s c o m b , R.G.W. N o r r i s h and B.A. T h r u s h ; P r o c Roy. S o c , A233, 455 (1 9 5 6 ) . 24. M.A.A. C l y n e and J.A. Coxon; P r o c . Roy. S o c , A303, 207 (1 9 6 8 ) . 25. N. Basco and S.K. Dogra; P r o c . Roy. S o c , A323, 29 ( 1 9 7 1 ) . 26. F.H.C. Edgecombe, R.G.W. N o r r i s h and B.A. T h r u s h ; Chem. S o c Spec. Pub., 9, 121 (1957 ) . 27. M.A.A. C l y n e and J.A. Coxon; T r a n s . F a r a d a y S o c , 62, 1175 (196 6 ) . 28. P.P. Bemand, M.A.A. C l y n e and R.T. Watson; F a r a d a y T r a n s . I , 69, 1356 (1973 ) . 29. G. Burns and F.S. D a i n t o n ; T r a n s . F a r a d a y Soc,..48, 39 (1952 ) . 30. S. Benson and J.H. Bu s s ; J . Chem. Phys., 27, 1382 (1 9 5 7 ) . 31. A. C a r r i n g t o n and D.H. L e v y ; J . Chem. Phys.,44, 1298 (1 9 6 6 ) . 32. D.R. H e r s c h b a c h , H.S. J o h n s t o n , K. S. P i t z e r and R.E. P o w e l l ; J . Chem. Phys., 47_, 1756 (1 9 6 7 ) . 33. M.A.A. C l y n e and I.F . W h i t e ; T r a n s . F a r a d a y S o c , 67,2068 ( 1 9 7 1 ) . 34. S.K. Dogra; Ph.D. T h e s i s , U n i v e r s i t y o f B r i t i s h C o l u m b i a , ( 1 9 7 0 ) . 35. G. P o r t e r and J.A. S m i t h ; P r o c . Roy. S o c , A261, 28 (1 9 6 1 ) . - 146 -36. R.G.W. N o r r i s h and G. P o r t e r ; N a t u r e , 164, 658 (1949 ) . 37. G. P o r t e r ; P r o c . Roy. S o c , A200, 284 (1950 ) . 38. R.G.W. N o r r i s h ; P r o c . Chem. S o c , 247, ( 1 9 5 8 ) . 39. R.G.W. N o r r i s h ; C h e m i s t r y i n B r i t a i n , 289 ( 1 9 6 5 ) . 40. G. P o r t e r ; " T e c h n i q u e s o f O r g a n i c C h e m i s t r y " , e d i t e d by A. W e i s s b e r g e r , 2nd e d i t i o n , Volume V I I I , P a r t I I , C h a p t e r XIX, ( I n t e r s c i e n c e , New Y o r k , 1963). 41. R.G.W. N o r r i s h and B.A. T h r u s h ; Q u a r t . Revs., 10, 149 (1 9 5 6 ) . 42. G. P o r t e r ; " P h o t o c h e m i s t r y and R e a c t i o n K i n e t i c s " , e d i t e d by P.G. Ashmore, F.S. D a i n t o n and T.M. Sudgen, C h a p t e r 5, (Cambridge U n i v e r s i t y P r e s s , 1967). 43. B.A. T h r u s h ; i b i d . , C h a p t e r 6. 44. A.B. C a l l e a r ; i b i d . , C h a p t e r 7. 45. J . E . Hunt; M.Sc. T h e s i s , U n i v e r s i t y o f B r i t i s h C o l u m b i a , ( 1 9 7 0 ) . 46. R.D. Morse; Ph.D. T h e s i s , U n i v e r s i t y o f B r i t i s h C o l u m b i a , ( 1 9 7 2 ) . 47. Twyman and S m i t h ; "Wavelength T a b l e s f o r Spectrum A n a l y s i s " , 2nd e d i t i o n , (Adam H i l g e r , London, E n g l a n d , 1931). 48. "Handbook o f C h e m i s t r y and P h y s i c s " , e d i t e d by C D . Hodgman, 43rd e d i t i o n ( C h e m i c a l Rubber P u b l i s h i n g Co., C l e v e l a n d , O h i o , 1961). 49. F.S. J o h n s o n , K. Watanabe and R.Tousey; J . Op. Soc. Amer., 41_, 702 ( 1 9 5 1 ) . 50. R. A l l i s o n and J . B u r n s ; i b i d . , 55, 574 (1 9 6 5 ) . - 147 -51. R.G.W. N o r r i s h , G. P o r t e r and B.A. T h r u s h ; P r o c . Roy. S o c , A216, 169 ( 1 9 5 3 ) . 52. D e t a i l s o f t h e s e m o d i f i c a t i o n s a r e a v a i l a b l e from t h e E l e c t r o n i c s Shop, The U n i v e r s i t y o f B r i t i s h C o l u m b i a . 53. R.I. Derby and W.S. H u t c h i n s o n ; " I n o r g a n i c S y n t h e s i s " , Volume 4, p. 152 (McGraw H i l l Book C o . ) . 54. G.H. Cady; i b i d . , Volume 5, p. 156. 55. L . J . G i l l e s p i e and L.H.D. F r a s e r ; J . Am. Chem. S o c , 58, 2260 (19 3 6 ) . 56. J.K.K. Ip and G. Bu r n s ; J . Chem. Phys., 56, 3155 (1 9 7 2 ) . 57. C F . Goodeve and S. K a t z ; P r o c . Roy. S o c , A172, 432 (1939 ) . 58. N. Basco and R.G.W. N o r r i s h ; P r o c . Roy. S o c , A268, 291 (1962 ) . 59. J.A. B l a k e and G. Bu r n s ; J . Chem. Phys., 54, 1480 (1971 ) . 60. G. B u r n s , R.F. LeRoy, D.J. M o r r i s s and J.A. B l a k e ; P r o c . Roy. So c . , A315, 81 (1 9 7 0 ) . 61. G. P o r t e r ; D i s c . F a r a d a y S o c , 33, 198 (1962 ) . 62. G. P o r t e r and H u s s a i n ; T e c h . R e p o r t F.T.R. No. 6, U.S. Army C o n t r a c t DA-91-591-EIC-2801 ( 1 9 6 3 ) . 63. Acknowledgement i s g i v e n t o M i s s C. Chye f o r h e l p i n c o l l e c t i n g some o f t h e d a t a i n t h i s s e c t i o n . 64. R.P. Widman and B.A. D e G r a f f ; J . Phys. Chem., 77, 1325 ( 1 9 7 3 ) . 65. N. Basco and S.K. Dogra; P r o c . Roy. S o c , A323, 417 (1 9 7 1 ) . - 148 -66. C F . Goodeve, J . I . W a l l a c e ; T r a n s . F a r a d a y S o c , 26, 254 ( 1 9 3 0 ) . 67. C F . Goodeve and C P . S t e i n ; T r a n s . F a r a d a y S o c , 25, 738 (192 9 ) . 68. C F . Goodeve and F.D. R i c h a r d s o n ; T r a n s . F a r a d a y S o c , 33_, 453 ( 1 9 3 7 ) . 69. C F . Goodeve and B.A.M. Win d s o r ; T r a n s . F a r a d a y S o c , 32_, 1518 ( 1 9 3 6 ) . 70. T h i s S t u d y . 71. E.T. McHale and G. V o n E l b e ; J . Am. Chem. S o c , 89, 2795 (1967). 72. E.T. McHale and G. V o n E l b e ; J . Phys. Chem., 72, 1849 (196 8 ) . 73. N. Basco and S.K. Dogra; P r o c Roy. S o c , A323, 401 ( 1 9 7 1 ) . 74. P. Gray and J.K.K. I p ; Combustion and Flame, 18, 361 ( 1 9 7 2 ) . 75. J.G. C a l v e r t and J.N. P i t t s ; " P h o t o c h e m i s t r y " , (John W i l e y and Sons I n c . , 1966). 76. T.C. C l a r k and M.A.A. C l y n e ; Chem. Comm., 287 . ( 1 9 6 6 ) . 77. N. Basco and A.E. P e a r s o n ; T r a n s . F a r a d . S o c , 63, 2684 (1967). 78. F.H.C. Edgecombe, R.G.W. N o r r i s h and B.A. T h r u s h ; P r o c Roy. S o c , A243, 24 ( 1 9 5 7 ) . 79. F.W. L y t l e and J.T. S t o n e r ; S c i e n c e , 148, 1721 ( 1 9 6 5 ) . - 149 -APPENDIX It is required to show that when measuring the absorbance of a vibrational band which is not resolved to the base line the apparent absorbance is direct ly proportional to the actual absorb-ance. The following proof makes use of figure 36. Required to prove: AH = KAE where K = constant Proof: Since the wavelength for the band maxima and minima are constant, then (EF/DF) is a constant. AH = AE - CF - GH GH = EFtane t a n 9 = (BD - CF)/DF AH = AE - CF - (EF/DF)(BD - CF) AH = AE - CF(1 - K ) - K BD where K = (EF/DF) = a constant AH/AE = 1 - (CF/AE)(1 - K*) K (BD/AE) Since AE, BD, and CF are the respective absorbances at ^A' B^ a n d ^C' t h e n t h e r a t ! o ° f a n y t w o ' s a constant equal to the ratios of the corresponding extinction coeff ic ients. AH/AE = K where K = a constant - 150 -FIGURE 36, TECHNIQUES FOR MEASUREMENTS TO APPARENT BASE LINE PUBLICATIONS N. Basco, James E. Hunt; "Absorption Spectra Attributed to AsP, AsSb and PSb". In preparation. N.. Basco, James E. Hunt; "Absorption Spectrum Attributed to PCI". In preparation. N. Basco, James E. Hunt; "Recombination of Iodine Atoms In the Prescence of NO". In preparation. N. Basco, James E. Hunt; "Formation of C100 from the Flash Photolysis of Mixtures of and O^"- In preparation. N. Basco, James E. Hunt; "Evidence for C 1 P r o < ^ u c t ' o n in the Bimolecular Reaction of CIO". In preparation. 

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