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UBC Theses and Dissertations

VUV kinetic spectroscopy of ClO₂ and vibrational energy distributions in O₂ produced from NO₂ and ClO₂… Morse, Robert Donald 1972

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VUV KINETIC SPECTROSCOPY OF C10 2 AND VIBRATIONAL ENERGY DISTRIBUTIONS IN 0 2 PRODUCED FROM N0 2 AND C10 2 BY FLASH PHOTOLYSIS by ROBERT DONALD MORSE B . S c , U n i v e r s i t y of B r i t i s h Co lumbia , 1966 M . S c , U n i v e r s i t y of B r i t i s h Co lumbia , 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of Chemistry We accept t h i s t h e s i s as conforming to the r e q u i r e d s tandard THE UNIVERSITY OF BRITISH COLUMBIA January , 1972 In presenting th i s thesis i n p a r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the Library s h a l l make i t f reely avai lable for reference and study. I further agree that permission for extensive copying of th i s thesis for scholar ly purposes may be granted by the Head of my Department or by h i s representatives. I t i s understood that copying or publ ica t ion 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 wri t ten permission. Department of C h e m i s t r y The Univers i ty of B r i t i s h Columbia Vancouver 8, Canada Date March 5 , 1 9 7 2 . ABSTRACT A f l a s h p h o t o l y s i s apparatus s u i t a b l e f o r k i n e t i c spect roscopy in the vacuum u l t r a v i o l e t i s d e s c r i b e d . Four new Rydberg t r a n s i t i o n s of C10^ have been observed and t h e i r meas-urements g i v e n . Molar e x t i n c t i o n c o e f f i c i e n t s o f the p r e v i o u s l y repor ted bands of C10^ have been measured. Two of these e l e c t r o n i c t r a n s i t i o n s (and p o s s i b l y a t h i r d ) have been ass igned to a Rydberg s e r i e s converg ing to the f i r s t i o n i z a t i o n p o t e n t i a l (10.36 eV ) . The f l a s h p h o t o l y s i s of ClO^ f o l l o w e d by k i n e t i c spect roscopy in the vacuum u l t r a v i o l e t has y i e l d e d a new spectrum of CIO. S i x e l e c t r o n i c t r a n -s i t i o n s have been a s s i g n e d ; a l l are Rydberg in n a t u r e . The f i r s t four o f 2 2 these t r a n s i t i o n s are thought to be £ X IT. from which a s p i n o r b i t c o u p l i n g c o n s t a n t , A=-318 ± 5 cm \ is ob ta ined f o r the ground s t a t e . Hot bands in three of the above systems of CIO have been observed in a b s o r p t i o n . Th is has enabled the d i r e c t measurement of the ground s t a t e v i b r a t i o n a l constant ( a ^ = 859 ± 6 cm ') f o r the f i r s t t i m e . E x t i n c t i o n c o e f f i c i e n t s f o r a number of the CIO t r a n s i t i o n s have been measured. A k i n e t i c study of CIO recombinat ion by k i n e t i c s p e c -t roscopy of these very s t r o n g and w e l l reso l ved v i b r a t i o n a l bands of CIO and C10^ has been i n i t i a t e d . Th is p r e l i m i n a r y study showed that CIO decay i s second order over i t s f i r s t two h a l f l i v e s , in agreement w i t h p rev ious r e s u l t s ; a t longer t i m e s , however, a d e v i a t i o n was no ted . I n i t i a l v i b r a t i o n a l energy d i s t r i b u t i o n s in 0^ produced from the f l a s h p h o t o l y s i s of 1 ^ and ClO^ have been measured. A b s o l u t e p o p u l a t i o n s of the l e v e l s u"=l to 7 have been combined w i t h p r e v i o u s l y measured r e l -a t i v e p o p u l a t i o n s of the l e v e l s 6 to 13 f o r the 0^ d i s t r i b u t i o n from ClO^ to g i v e a complete i n i t i a l v i b r a t i o n a l energy d i s t r i b u t i o n . P o p u l a t i o n s of the l e v e l s u"=A to 13 of 0^ from N 0 2 have been measured. Both d i s t r i b -u t ions show that the i n i t i a l energy content in is a s i g n i f i c a n t p o r t i o n of the heat of r e a c t i o n . Ex tens ions of the NC^ d i s t r i b u t i o n to inc lude a l l of the l e v e l s observed , but not measured due to exper imenta l d i f f i c u l t i e s , a re d i s c u s s e d . TABLE OF CONTENTS CHAPTER PAGE 1 INTRODUCTION I Scope of the I n v e s t i g a t i o n 1 I I The CIO R a d i c a l 2 III The P r o d u c t i o n and Decay of V i b r a t i o n a 1 l y E x c i t e d Oxygen 7 2 EXPERIMENTAL I I n t r o d u c t i o n 1A I I Apparatus 16 III Photography and P l a t e Photometry 19 IV Wavelength Measurements 23 V Gas Handl ing Procedures and Exper imenta l Technique . . . 2k VI M a t e r i a l s 25 VI I E r r o r s 26 3 THE FLASH PHOTOLYSIS OF CHLORINE DIOXIDE FOLLOWED BY KINETIC SPECTROSCOPY IN THE VACUUM ULTRAVIOLET I The A b s o r p t i o n Spectrum of C 1 0 2 in the VUV 27 ( i ) R e s u l t s 28 ( i i ) Di scuss ion . . 35 II The A b s o r p t i o n Spectrum of CIO in the VUV 38 ( i ) R e s u l t s **0 ( i i ) D i s c u s s i o n 51 (a) Nature of the E l e c t r o n i c T r a n s i t i o n 51 (b) V i b r a t i o n a l A n a l y s i s 53 ( i i i ) Concl us ion 56 CHAPTER PAGE 3 THE FLASH PHOTOLYSIS OF CHLORINE DIOXIDE FOLLOWED BY KINETIC SPECTROSCOPY IN THE VACUUM ULTRAVIOLET (Cont 'd) III The Recombination of CIO Fo l lowed by K i n e t i c Spectroscopy in the VUV 57 ( i ) E x t i n c t i o n C o e f f i c i e n t s of CIO 58 ( i i ) Extended K i n e t i c R e s u l t s on CIO Decay 63 ( i i i ) D i scuss ion 65 k THE PRODUCTION OF VI BRAT IONALLY EXCITED OXYGEN FROM NITROGEN DIOXIDE AND CHLORINE DIOXIDE I I n t r o d u c t i o n 66 -t-II The P r o d u c t i o n of 0 2 from the F l a s h P h o t o l y s i s of N0 2 . . . 67 ( i ) Treatment of I n t e n s i t y Measurements 67 ( i i ) R e s u l t s 71 ( i i i ) Di scuss ion 79 III The P r o d u c t i o n of 0 2 from the F l a s h P h o t o l y s i s of C 1 0 2 . . . 8k ( i ) Treatment of I n t e n s i t y Measurements 8^ ( i i ) R e s u l t s 85 ( i i i ) D i scuss ion 95 IV The R e l a x a t i o n of 97 V I n i t i a l V i b r a t i o n a l D i s t r i b u t i o n s of the Product M o l e c u l e s . 99 BIBLIOGRAPHY 101 LIST OF TABLES TABLE PAGE 3 - 1 VUV Bands of C 1 0 2 3 1 ~ 3 2 3 - 2 Deslandres Tables of the E l e c t r o n i c Systems of CIO 4 3 - 4 8 3 - 3 CIO S p e c t r o s c o p i c Data . . . . 5 4 3 _ 4 Summary of R e s u l t s 6 2 4 - 1 R e l a x a t i o n of 0^ expressed in terms of h a l f l i v e s 7 6 4 - 2 Second o rder quenching cons tants of 0^ by N0 2 76 4 - 3 I n i t i a l P o p u l a t i o n s of 0 2 from N 0 2 81 4 - 4 Es t imates of the degree of v i b r a t i o n a l e x c i t a t i o n 81 4 - 5 R e l a x a t i o n of 0 2 produced from C10 2 32 4 - 6 P o p u l a t i o n d i s t r i b u t i o n s of 0 from C10 9 9 4 LIST OF FIGURES FIGURE PAGE 2-1 Vacuum U l t r a v i o l e t Apparatus 17 2- 2 Example of an e x p e r i m e n t a l l y determined c h a r a c t e r i s t i c curve f o r a g iven wavelength 22 3 - 1 The A b s o r p t i o n Spectrum of ClO^ 29 3-2 The O X System of C102 . 30 3-3 The CK-X and E+X Systems of C102 34 3 - 4 M i c r o d e n s i t o m e t e r t r a c e of remaining C10^ t r a n s i t i o n s 36 3 _ 5 The A b s o r p t i o n Spectrum of CIO 41 3-6 The C+X System of CIO 49 3-7 The v+X System of CIO 50 3-8 Decay of CIO produced from f l a s h p h o t o l y s i s of C102 60 3-9 Second Order P l o t of CIO Decay 61 3- 10 P l o t of a l l p o i n t s on f i g u r e 3"8 as 0D^^Q vs t and 1/0D vs t . . 64 4- 1 Ha l f path p l o t f o r ot l e v e l s produced in the f l a s h p h o t o l y s i s of N02 69 t 4-2 F i r s t o rder decay of 0 2 from 37% pr imary p h o t o l y s i s of WQ^. . . 74 4-3 F i r s t o rder decay of 0 2 {25% pr imary p h o t o l y s i s ) 75 4-4 0 2 R e l a x a t i o n as a f u n c t i o n of [N02] 77 4-5 E x t r a p o l a t e d energy d i s t r i b u t i o n s 80 4-6 Low v i b r a t i o n a l l e v e l s of 0 2 produced from C10^ ( f i l t e r e d ) . . . 86 4-7 Low v i b r a t i o n a l l e v e l s of 0 2 produced from C10,, (unf i 1 te red) . . 87 4-8 F i r s t o rder decay of 0* from C102 ( f i l t e r e d ) 90 4-9 F i r s t o rder decay of 0 2 from C102 ( u n f i l t e r e d ) 91 ACKNOWLEDGEMENTS I would l i k e to thank Dr. Norman Basco f o r h i s cont inuous a d v i c e and he lp throughout the course of t h i s i n v e s t i g a t i o n . I am g r a t e f u l to Dr. A . J . Merer f o r many h e l p f u l d i s c u s s i o n s regard ing i n t e r p r e t a t i o n of the s p e c t r a . A vote of thanks is a l s o due Dr . G .B . P o r t e r f o r h i s c r i t i s i m s of the rough d r a f t of t h i s m a n u s c r i p t . I am indebted to Miss G.H. Thomas f o r her generous help w i t h the t y p i n g and proof read ing of the f i n a l copy. F i n a l l y , I would l i k e to thank the members of the t e c h n i c a l s t a f f f o r t h e i r inva1uab le a s s i s t a n c e w i t h the c o n s t r u c t i o n and mainta inance of the a p p a r a t u s . CHAPTER 1 INTRODUCTION I S c o p e o f t h e I n v e s t i g a t i o n The s u b j e c t o f t h i s d i s s e r t a t i o n i n v o l v e s two m a i n t o p i c s w h i c h , i n p a r t , a r i s e f r o m an e x a m i n a t i o n o f t h e f l a s h p h o t o l y s i s o f c h l o r i n e d i -o x i d e f o l l o w e d by k i n e t i c s p e c t r o s c o p y i n t h e vacuum u l t r a v i o l e t (VUV) . The f i r s t t o p i c d e a l s w i t h t h e i d e n t i f i c a t i o n and c l a s s i f i c a t i o n o f new s p e c t r a w h i c h w e r e o b s e r v e d i n t h i s r e g i o n . As e x p e c t e d , t h e c a r r i e r o f one o f t h e new s p e c t r a i s t h e CIO r a d i c a l . A k i n e t i c s t u d y o f CIO d e c a y i s g i v e n b a s e d on i n t e n s i t y m e a s u r e m e n t s o f t h e s e new VUV s y s t e m s . The s e c o n d m a i n t o p i c i n v o l v e s p o p u l a t i o n and e n e r g y d i s t r i b u t i o n s o f v i b r a t i o n a l l y e x c i t e d o x y g e n (0^) p r o d u c e d f r o m t h e f l a s h p h o t o l y s i s o f n i t r o g e n d i o x i d e and o f c h l o r i n e d i o x i d e . B a c k g r o u n d m a t e r i a l r e l e v a n t t o t h i s i n v e s t i g a t i o n w i l l be r e v i e w e d i n t h e r e m a i n d e r o f t h i s c h a p t e r . The a p p a r a t u s and e x p e r i m e n t a l t e c h -n i q u e a r e d e s c r i b e d i n c h a p t e r 2 and t h e r e s u l t s p e r t a i n i n g t o t h e a b o v e two t o p i c s a r e p r e s e n t e d and d i s c u s s e d s e p a r a t e l y i n c h a p t e r s 3 and k. -2-I i The CIO R a d i c a l Numerous k i n e t i c s t u d i e s have been c a r r i e d out on t h i s r a d i c a l s i n c e i t s a b s o r p t i o n s p e c t r u m i n t h e u l t r a v i o l e t (UV) was f i r s t o b s e r -1 2 ved by P o r t e r (1950). The s p e c t r u m was a n a l y s e d by P o r t e r and by D u r i e •3 and Ramsay (1958) and was a s s i g n e d t o th e e l e c t r o n i c t r a n s i t i o n , 2 2 A JIj •«- X II.. A l l o f th e s t u d i e s on t h i s r a d i c a l by k i n e t i c s p e c t r o -s c o p y have been l i m i t e d t o u s i n g t h i s e l e c t r o n i c band s y s t e m t o mon-i t o r CIO decay. P o r t e r and W r i g h t (1953) f o u n d , f r o m a s t u d y o f t h e f l a s h p h o t o l -y s i s o f m i x t u r e s o f c h l o r i n e , oxygen and n i t r o g e n , t h a t CIO decayed i n a b i m o l e c u l a r r e a c t i o n whose r a t e was independent o f c h l o r i n e , o x y g e n , and t o t a l p r e s s u r e i n t h e range 55 t o 610 t o r r . I t was a l s o i ndependent o f t e m p e r a t u r e o v e r t h e range 293 t o 433 °K. The mechanism p r o p o s e d was: 2C10 t C 1 2 0 2 + C l 2 + 0 2 The absence o f an a c t i v a t i o n e n e r g y and t h e low f r e q u e n c y f a c t o r were t a k e n as e v i d e n c e f o r t h e i n t e r m e d i a t e C12^2* t' i e f ° r m a t ' o n °f CIO, t h e i r p r o p o s e d mechanism: CI +hv + 2C1 b CI + 0- + M t C100 + M 2 c CI + C100 2 2C10 was l a t e r s u p p o r t e d by Burns and N o r r i s h (1963)*' and by N i c h o l a s and -3-N o r r i s h (1968)^ An e x t e n s i o n of t h i s mechanism was proposed by Benson and Buss (1957)^ as an a l t e r n a t i v e f o r the decay p r o c e s s e s . 2C10 -_- C100 + CI d ci + cioo ^ c i 2 + o 2 S p e c t r o s c o p i c ev idence f o r the e x i s t e n c e of the C100 r a d i c a l in m a t r i c e s 8 was o b t a i n e d by A r k e l 1 and Schwanger (1967), in the gas phase by M o r r i s q and Johnston (1968). and k i n e t i c ev idence f o r i t s being i n v o l v e d in the decay of CIO was presented by Clyne and Coxon (1966)^? For the o v e r a l l b i m o l e c u l a r r e a c t i o n , 2C10 I C l 2 + 0 2 (1) the r a t e constant was d e f i n e d by the e q u a t i o n : - d [ClOj/dt = k ^ C l O ] 2 and the r a t i o of to the molar e x t i n c t i o n c o e f f i c i e n t (e) of CIO at h 4 - 1 257-7 nm was measured by P o r t e r and Wright to be 7-2 x 10 cm s F u r t h e r work on both the r a t e constant and the e x t i n c t i o n c o e f f i c i e n t has y i e l d e d c o n f l i c t i n g r e s u l t s . L ipscomb, N o r r i s h and Thrush (1956)]^ in a study of the f l a s h p h o t o l -y s i s of c h l o r i n e d i o x i d e , conf i rmed that the decay of CIO was second o r d e r ; but found that the apparent ra te constant inc reased w i t h f l a s h energy from 1.9 x 10^ l i t e r mole ^ s ^ (H ' 5 ' ) a t 240 J to a l i m i t i n g v a l u e of 6.2 x 10^ M ^s ^ a b o v e , c a . I k J . The va lue f o r the molar e x t i n c -- 4 -t i o n c o e f f i c i e n t o b t a i n e d a l s o v a r i e d w i t h f l a s h energy from 680 M ^cm ^ at the h i g h e s t energy to c a . 1100 M ^cm ^ f o r e n e r g i e s below 400 J . These v a r i a t i o n s were t e n t a t i v e l y e x p l a i n e d by p o s t u l a t i n g the f o r m -a t i o n , at low f l a s h e n e r g i e s , of an e q u i l i b r i u m c o n c e n t r a t i o n o f C^O^ from CIO and unphotolysed C10^. They t h e r e f o r e adopted the high f l a s h energy va lues f o r both and e. Benson and Buss (1957)? in support of t h e i r own mechanism f o r the decay of C 1 0 , suggested that the s lower decay of C10 a t low f l a s h e n e r g i e s cou ld be e x p l a i n e d by the r e a c t i o n of c h l o r i n e atoms produced from (e) w i t h C1O2: CI + C 1 0 2 + 2C10 12 Edgecombe, N o r r i s h and Thrush (1957) f o l l o w e d the decay of C10 produced in the f l a s h p h o t o l y s i s of c h l o r i n e monoxide and found k^  to be 2 .4 x 10^ M ^s ^ in c l o s e agreement w i t h the low energy va lue of Lipscomb et a l . They measured the e x t i n c t i o n c o e f f i c i e n t of C10 a t 292 nm; but from t h e i r p u b l i s h e d d a t a , i t can be c a l c u l a t e d that t h e i r va lue f o r e -1 -1 at 257-7 nm would have been c a . 540 M cm , t h i s in good agreement w i t h 1 o the h igh f l a s h energy va lue of Lipscomb et a l . They l a t e r , (1957) , b r i e f l y cons idered the p o s s i b i l i t y that the decrease in the apparent e x t i n c t i o n c o e f f i c i e n t of C10 w i t h i n c r e a s i n g f l a s h energy , repor ted by Lipscomb et a l , was due to the d e s t r u c t i o n of C10 "perhaps by oxygen atoms". They p o i n t e d o u t , however, that t h i s i n t e r p r e t a t i o n cou ld on ly widen the I) d i s c r e p a n c y between t h e i r r e s u l t s and those of P o r t e r and Wright and c i t e d the apparent p a r t i a l reappearance of C 1 0 2 du r ing the decay of C10 as e v i d -ence f o r the presence of C1^0^. N e v e r t h e l e s s , t h i s sugges t ion was taken - 5 -up by Clyne and C o x o n ^ who, us ing a d i s c h a r g e f l o w sys tem, showed that the r a t i o of the r a t e cons tants f o r the r e a c t i o n s of oxygen atoms w i t h ClO^ and CIO was l e s s than 7 and probably equal to 4. C lyne and Coxon suggested that c o m p e t i t i o n between these two r e a c t i o n s p rov ided a p l a u s i b l e e x p l a n a t i o n f o r the v a r i a t i o n in the apparent observed by Lipscomb et a l . T h i s e x p l a n a t i o n d i d n o t , however, apply to the C^O system s t u d i e d by Edgecombe 12 e t a l whose va lues of and e (257-7 nm) a l s o d i f f e r e d markedly from the va lues of 1.7 x 1 0 7 M ^s ^ and 1360 M ^cm ^ repor ted by Clyne and C o x o n ^ . In a f u r t h e r s t u d y , Clyne and Coxon ( 1 9 6 8 ) ^ found e to be 1270 M ^cm ^ at 257-7 nm and , which they i d e n t i f y w i t h r e a c t i o n (e) o f the Benson and Buss mechanism, to be 1.4 x 10^ M ^s ' at 298°K. They a l s o found an a c t i v a t i o n energy of 10.5 kJ/mole over the temperature range 294 to 495°K; but no p r e s s u r e dependence between 1.3 and 3-0 t o r r was found . They r e c o n c i l e d t h e i r r e s u l t s w i t h those o b t a i n e d from f l a s h p h o t o l y s i s by p o s t u l a t i n g that an u n s t a b l e i n t e r m e d i a t e , such as Cl202> i s formed at h igh p ressures by an e q u i l i b r a t e d o v e r a l l t h i r d o rder r e a c t i o n : % 2C10 + M t C l o 0 o + M n 2 2 w i t h the subsequent r e a c t i o n : c i 2 o 2 V c i 2 + o 2 J o h n s t o n , M o r r i s and Van den Bogaerde ( 1 9 6 9 ) ^ s t u d i e d the p r o d u c t i o n and decay of CIO produced by the p h o t o l y s i s of c h l o r i n e + oxygen and of c h l o r i n e + oxygen + argon mix tu res and f o l l o w e d the k i n e t i c behav io r of CIO by m o l e c u l a r modulat ion spec t romet ry . I n i t i a l l y , they intended to use the CIO i n t e r m e d i a t e as a c a l i b r a t i o n f o r the new modulat ion method. When some new f e a t u r e s , namely a v a r i a t i o n of CIO ra te o f recombinat ion w i t h t o t a l p r e s s u r e and a new spectrum which they a t t r i b u t e d to C100, were observed the c a l i b r a t i o n exper iments were expanded. The i r o b s e r v a t i o n of a l i n e a r v a r i a -t i o n o f the second o r d e r r a t e of recombinat ion of CIO w i t h t o t a l p ressure in the range 50 to 750 t o r r i s in d i r e c t c o n t r a d i c t i o n to the r e s u l t s o f P o r t e r and Wr ight . Johnston et a l proposed a t o t a l combined mechanism which 1 k d i f f e r s from Clyne and Coxon on ly in the i n c l u s i o n of a t h i r d body (M) i n r e a c t i o n ( i 1 ) . Dogra ( l 970) ^ ^ and Basco and Dogra ( 1 9 7 1 ) ^ 7 have r e c o n c i l e d a l l the p r e -v ious f l a s h p h o t o l y s i s r e s u l t s by showing that the f a s t r e a c t i o n : 0 + CIO + o 2 + CI 1^=7 x 1 0 9 M ' V 1 ^ 17^ o n l y becomes s i g n i f i c a n t at high f l a s h energ ies when more than c a . 50% of the C 1 0 2 i s decomposed in the pr imary s tep g i v i n g an ' e x c e s s ' of 0 atoms a f t e r the r e a c t i o n : 0 + C 1 0 2 I CIO + 0 2 k 3=3 x 1 0 1 0 M _ 1 s " 1 ^ 1 7 ^ i s complete . Under c o n d i t i o n s where r e a c t i o n (4) i s not s i g n i f i c a n t and there i s no p o s s i b i l i t y of secondary p h o t o l y s i s of CIO, they f i n d the va lue of kj to be 2 .65 ± 0 .3 x 1 0 7 M ^s \ which i s independent of t o t a l p ressure over the range 75 to 200 t o r r . At 200 t o r r t o t a l p r e s s u r e , t h i s v a l u e of k^  i s more than a f a c t o r o f ten lower than the va lue repor ted by Johnston et a l ( 1 9 6 9 ) ^ Basco and Dogra^ 7 proposed as a mechanism a combinat ion of the P o r t e r -7-and Wright and the Benson and Buss mechanisms us ing C 1 a s a c o m m o n i n t e r m e d i a t e : I 2 C 1 0 ^ C 1 2 0 2 C 1 2 ° 2 " C , 0 ° + C 1 C1 2 0 2 + M # C l 2 + 0 2 + M ClOO + CI I c i 2 + o 2 Th is mechanism i d e n t i f i e s k^  w i t h ( l ) and the low k^  va lues of Clyne and Coxon were e x p l a i n e d i f the r a t e cons tants f o r Cm) and Cn) a re about e q u a l . 18 Clyne and White (1971) have found no t h i r d body e f f e c t on CIO r e -combinat ion w i t h e i t h e r Ar or SF^ in the p ressure range 0.5 to 9 t o r r in t h e i r d i s c h a r g e f l ow exper iments i n v o l v i n g C10^ + C I . From these r e s u l t s they conclude that the t o t a l p ressure of argon at which second and t h i r d o rder r e a c t i o n s c o n t r i b u t e e q u a l l y to the removal of CIO i s c o n s i d e r a b l y g r e a t e r than that t e n t a t i v e l y suggested by Johnston et a l . They p o i n t out that the r e s u l t s of both s t u d i e s are n e v e r t h e l e s s c o n s i s t e n t w i t h one another i f the e a r l i e r e s t i m a t e of the d i s s o c i a t i o n energy of the ClOO r a d i c a l ( 2 6 k j / m o l e ^ ) i s r e v i s e d by a few kJ/mole to 29-III The P r o d u c t i o n and Decay of V ib ra t iona11y E x c i t e d Oxygen The chemical p r o d u c t i o n of v i b r a t i o n a 1 1 y e x c i t e d oxygen i s now one of the c l a s s i c examples of a growing number of s t u d i e s on r e a c t i o n s i n v o l v i n g i n i t i a l n o n - e q u i l i b r i u m d i s t r i b u t i o n s of i n t e r n a l energy in product m o l e c u l e s . An e x t e n s i v e l i s t i n g of a l a r g e number o f these r e a c t i o n s has been compi led 19 by C a r r i n g t o n and G a r v i n ( 1 9 6 9 ) . Most of the r e a c t i o n s s t u d i e d to date have invo lved o b s e r v a t i o n o f the e x c i t e d s p e c i e s by means of v i b r a t i o n a l chemi1uminescence or by e l e c t r o n i c a b s o r p t i o n s p e c t r o s c o p y . The study of v i b r a t i o n a l l y e x c i t e d oxygen (0^) i s r e s t r i c t e d to the use of the l a t t e r method s i n c e oxygen i s homonuclear. From an examinat ion of the many r e a c t i o n s that have been s t u d i e d , p a r -t i c u l a r l y by f l a s h p h o t o l y s i s , i t i s ev ident that very l i t t l e q u a n t i t a t i v e work has been done to determine i n i t i a l d i s t r i b u t i o n s of e x c i t e d product molecules and hence the amount of energy in the form of v i b r a t i o n compared to the o v e r a l l r e a c t i o n energy. Such data can be used as a i d s in the c o n s t -r u c t i o n of p o t e n t i a l energy s u r f a c e s w h i c h , in t u r n , can be used to d e s c r i b e 20 the dynamics of the r e a c t i o n s , P o l a n y i ( 1 9 ^ 7 ) -The i so thermal f l a s h p h o t o l y s i s of C 1 0 ^ , and N 0 2 was f i r s t i n v e s t i g a t e d by Lipscomb e t a l (1956). They observed 0^ by p h o t o g r a p h i c a l l y r e c o r d i n g 3 - 3 -the Schumann-Runge e l e c t r o n i c a b s o r p t i o n spectrum (B X Z^) a t p r e d e t e r -mined d e l a y s a f t e r f l a s h p h o t o l y s i s of the parent m o l e c u l e . T h e i r e x p e r -iments revea led that oxygen w i t h up to e i g h t quanta of v i b r a t i o n a l energy in the ground e l e c t r o n i c s t a t e was produced from both parent m o l e c u l e s . Because of the very e x t e n s i v e nature of the Schumann Runge band system (from the VUV f o r u"=0 to the near UV f o r u"=15) and the r e s u l t i n g l i m i t a t i o n s of t h e i r spect rograph below 210 nm, they were o n l y a b l e to observe 0^ as low as u"=5. t The mechanism g i ven for the f o r m a t i o n of. 0 2 from N02 was the s imp le two s tep p r o c e s s : - 9 -N0 2 + hv -> NO + 0 0 + N 0 2 -> NO + 0 2 AH=-201 k J + The m e c h a n i s m p r o p o s e d f o r t h e p r o d u c t i o n o f 0 2 f r o m C10^ w a s : C10 2 + hv •*• CIO + 0 0 + C10 2 I CIO + 0 2 AH=-247 k J 0 + cio2 -*• cio3 C10 3 + hv -y C10 + 0 2 They f o u n d t h a t t h e amount o f o x y g e n p r o d u c e d d e c r e a s e s i f p r i m a r y p h o t o l y s i s i s more t h a n 50% b e c a u s e t h e r e was n o t enough C10^ l e f t t o r e a c t w i t h o x y g e n atoms a f t e r t h e p r i m a r y p r o c e s s . T h i s e f f e c t was n o t f o u n d f o r N0 2 s i n c e i t was n o t p o s s i b l e t o p h o t o l y s e more t h a n 50% o f t h e H0^. T h e i r w o r k has shown t h a t t h e e n e r g y d i s t r i b u t i o n d u r i n g o r I m m e d i a t e l y a f t e r t h e r e a c t i o n i s q u i t e d i f f e r e n t f r o m t h a t g i v e n by a M a x w e l l B o l t z m a n n d i s t r i b u t i o n . F o r e a c h r e a c t i o n t h e y e s t i m a t e d t h a t , a t t h e s h o r t e s t d e l a y , t h e u " = 5 , 6 , and 7 l e v e l s were a p p r o x i m a t e l y e q u a l l y p o p u l a t e d w h i l e t h e u"=8 was p o p u l a t e d t o a much l e s s e r e x t e n t . T h i s d i s t r i b u t i o n was c o n c l u d e d t o + be t h e i n i t i a l one i n t h a t c o l l i s i o n a l d e a c t i v a t i o n o f 0 2 was r e l a t i v e l y s l o w . The s t u d y o f t h e s e two r e a c t i o n s as w e l l as a number o f s i m i l a r r e a c t i o n s 21 s t u d i e d by f l a s h p h o t o l y s i s , l e d M c G r a t h and N o r r i s h (1958) t o t h e f o l l o w i n g g e n e r a 1 i z a t i o n : "When an e x o t h e r m i c a t o m i c r e a c t i o n o f t h e g e n e r a l f o r m A +BCD ->• AB + CD -10-o c c u r s , t h e m o l e c u l e AB w i t h the newly formed bond t a k e s a h i g h p r o p o r t i o n o f the e x o t h e r m i c energy o f t h e r e a c t i o n i n the f o r m o f u n e q u i 1 b r a t e d v i b -r a t i o n a l e n e r g y . " A l t h o u g h t h e r e i s some doubt t h a t o n l y t h e AB bond can be v i b r a t i o n a l l y e x c i t e d (e.g. Basco and N o r r i s h (1960) 2 2 and Smith(1967) 2 3) and a l s o t h a t a l a r g e p o r t i o n o f t h e heat o f r e a c t i o n can be f o u n d i n t h e form o f v i b r a t i o n a l e n e r g y , o n l y a few c a s e s have been r e p o r t e d i n w h i c h the v i b r a t i o n a l e n e r g y o f the newly formed m o l e c u l e exceeds t h e e x o t h e r m i c i t y o f t h e r e a c t i o n f o r m i n g i t . In a l l o f t h e s e c a s e s t h e h i g h e s t v i b r a t i o n a l l e v e l o b s e r v e d was o n l y one h i g h e r t h a n the h i g h e s t p o s s i b l e l e v e l based on t h e heat o f r e a c t i o n as t h e o n l y s o u r c e o f e n e r g y . No a t t e m p t has been made t o s t u d y t t h e p o s s i b i l i t y o f energy t r a n s f e r t o 0^ i n t h e c a s e s w h i c h have ' e x c e s s ' e n e r g y , s i n c e , i n a l l c a s e s , t h i s e n ergy was a s m a l l amount o f t h e t o t a l and i n most c a s e s t h e a s s i g n m e n t o f t h e s e h i g h l e v e l s was u n c e r t a i n . 2 2 t One such r e p o r t was by Basco and N o r r i s h (1960) who o b s e r v e d 0^ w i t h up t o t h i r t e e n q u a n t a o f v i b r a t i o n a l energy f o l l o w i n g t h e f l a s h p h o t o l y s i s o f HO^- T h i s exceeded the e x o t h e r m i c i t y o f t h e r e a c t i o n : 0 + N 0 2 -*• NO + 0 * A H = - 2 0 1 k j by a p p r o x i m a t e l y 17 k j and they s u g g e s t e d t h a t t h i s was t h e f i r s t c a s e where th e k i n e t i c energy of the a p p r o a c h i n g atom p a r t i c i p a t e s i n v i b r a t i o n a l e x c i t -a t i o n . From t h i s e x t e n d e d s e r i e s o f v i b r a t i o n a l l e v e l s o f o x y g e n , they o b t a i n e d a t e n t a t i v e p o p u l a t i o n d i s t r i b u t i o n i n w h i c h t h e p o p u l a t i o n s o f the l e v e l s were f o u n d t o d e c r e a s e by about a f a c t o r o f t h r e e toward s u c c e s s i v e h i g h e r l e v e l s o v e r t h e range u"-6 t o 13. 2k Kane, McGarvey and McGrath (1963) ex t e n d e d the k i n e t i c s p e c t r o s c o p y - 1 1 -of the f l a s h photolysis of NO^ into the VUV in order to measure populations of the low v i b r a t i o n a l l e v e l s . They obtained r e l a t i v e populations for the lev e l s u"=0 to 8 and found that the l e v e l s 0 to 4 followed a Boltzmann d i s -t r i b u t i o n for a v i b r a t i o n a l temperature of 700°K. The higher le v e l s then deviated markedly from t h i s d i s t r i b u t i o n . They estimated that the tota l molar v i b r a t i o n a l energy represented only 0.k% of the heat of reaction. 25 This communication was followed by one from Bass and Garvin (1964) who had c a r r i e d out an independent study in the same spectral region and had obtained quite d i f f e r e n t r e s u l t s . They could not assign any bands a r i s i n g from the 0 and 1 le v e l s and they suggested that Kane et al confused strong NO bands in that region f o r oxygen bands. As a r e s u l t , Bass and Garvin's d i s t r i b u t i o n covered the range u"=2 to 6. They found that these l e v e l s were approximately equally populated within a factor of two and that as much as 10% of the oxygen produced is v i b r a t i o n a l l y excited. The energy content in these l e v e l s , however, would only amount to ca. k% of the heat of reaction. Morse (1969)^ and Basco and Morse^ 7 have observed 0^ with up to f i f t e e n quanta of v i b r a t i o n a l energy following the f l a s h photolysis of UO^- In t h i s case, the amount of 'excess' energy over the heat of reaction was as much as 61 k J . They found that the production of these high le v e l s (12 to 15) was c l e a r l y the r e s u l t of energy transfer from HO^ excited in the photolysis f l a s h . When the f l a s h was f i l t e r e d such that only photolysing radiation was admitted to the reaction vessel containing a mixture of NO^ and argon, no l e v e l s above u"=11 could be observed. They obtained r e l a t i v e population d i s -t r i b u t i o n s f o r the f i l t e r e d and u n f i l t e r e d cases (u"=6 to 11 and u"=6 to 12 r e s p e c t i v e l y ) . d i s t r i b u t i o n s were not s i g n i f i c a n t l y d i f f e r e n t from each -12-o t h e r . The p o p u l a t i o n s were found t o d e c r e a s e by about a f a c t o r o f 1.4 toward s u c c e s s i v e h i g h e r l e v e l s . 16 In t h e i r s t u d y o f t h e f l a s ' h p h o t o l y s i s o f c h l o r i n e d i o x i d e , Dogra and Basco and D o g r a ^ 7 o b t a i n e d a d i s t r i b u t i o n f o r t h e l e v e l s 5 t o 13- A l t h o u g h u"= 14 and 15 were o b s e r v e d they were t o o weak t o measure q u a n t i t a t i v e l y . They e s t i m a t e t h a t t h e r a t e s o f p r o d u c t i o n o f oxygen i n t o t h e l e v e l s 5 t o 13 a r e a p p r o x i m a t e l y e q u a l , but w i t h a t r e n d toward i n c r e a s i n g r a t e s w i t h uV T h i s t r e n d a p p e a r e d t o be u n i f o r m and t h e r a t i o o f the r a t e s f o r s u c c e s s i v e h i g h e r l e v e l s i s p r o b a b l y 1.25 ± 0.15- T h i s d i s t r i b u t i o n was based on quan-t i t a t i v e measurements o f t h e 6, J, and 12 l e v e l s o n l y and eye e s t i m a t e s on t h e r e s t s i n c e t h e r e was s t r o n g CIO and C l O ^ band o v e r l a p on a l l o f the oxygen bands e x c e p t f o r some members o f t h e above t h r e e p r o g r e s s i o n s . Lipscomb e t a l ^ d e t e r m i n e d t h a t c o l l i s i o n a l d e a c t i v a t i o n o f 0^ was v e r y i n e f f e c t i v e f o r a r g o n and n i t r o g e n (one e f f e c t i v e c o l l i s i o n i n 1 0 7 ) . They a l s o o b s e r v e d t h a t t h e h a l f l i f e o f t h e s i x t h l e v e l v a r i e d between 200 t o 700 us and was i n v e r s e l y p r o p o r t i o n a l t o the i n i t i a l p r e s s u r e o f ClO^. I t was t h u s c o n c l u d e d t h a t C l O ^ and C10 were a p p r o x i m a t e l y e q u a l l y e f f e c t i v e i n d e a c t i v a t i n g 0^ (one e f f e c t i v e c o l l i s i o n i n 2000) and t h a t a v a l u e o f 8 -1 -1 k=1.0 x 10 M s c o u l d be a s s i g n e d f o r t h e p r o c e s s : 0 2(u"=6) + C 1 0 2 ( o r C10) -* 0 2(u"=5) + C 1 0 2 ( o r C10) 28 17 Basco and Dogra (1968, 1971) ' have c o n f i r m e d t h e s e r e s u l t s o n l y f o r low f l a s h e n e r g i e s when no more than c a . 50% o f the C 1 0 2 i s decomposed by p r i m a r y p h o t o l y s i s . They f o u n d t h a t t h e r a t e o f d e a c t i v a t i o n o f t h e t w e l f t h l e v e l i s somewhat h i g h e r t h a n t h e s i x t h but not by a l a r g e f a c t o r . Above -13-50% pr imary p h o t o l y s i s they found tha t the h a l f l i v e s of the 0^ l e v e l s are decreased by about a f a c t o r of 30. Th is r e s u l t was a t t r i b u t e d to the p r o c e s s e s : CI + O ^ u ' - n ) 1 ^ CI + 0 2 (u"<n) k n (n=12)=7 x 1 0 9 M _ 1 s 0 + 0 2 ( u " = n ) 1 ^ 0 + 0 2 (u"<n) k 1 2(n=12)=2 x 1 0 1 0 M _ 1 s which are very f a s t but predominate on ly when there is an excess of oxygen atoms a f t e r r e a c t i o n (3) i s complete or when secondary p h o t o l y s i s of CIO i s poss i b l e . 11 t Lipscomb et a l have g iven a r a t e of d e a c t i v a t i o n of 0^ f o l l o w i n g the f l a s h p h o t o l y s i s of NO^ (one e f f e c t i v e c o l l i s i o n in l e s s than 500) cor respond 8 -1 -1 ing to a second o r d e r quenching constant by N0 2 of g r e a t e r than 4 x 10 M s . They observed no e f f e c t on the r a t e of r e l a x a t i o n when NO was added to the mi x t u r e . CHAPTER 2 EXPERIMENTAL I I n t r o d u c t i o n F l a s h P h o t o l y s i s l i n k e d w i t h k i n e t i c spect roscopy was developed in 1950 1 29 by N o r r i s h and P o r t e r . ' Th is technique enables the d i r e c t o b s e r v a t i o n , as a f u n c t i o n of - t ime, of many t r a n s i e n t in te rmed ia tes produced by a photo -l y t i c f l a s h . S ince 1950 improvements and m o d i f i c a t i o n s of t h i s techn ique have proved i t to be a powerful and v e r s a t i l e exper imenta l method in the f i e l d s 30 31 32 33 of gas phase and s o l u t i o n photochemist ry and in f r e e r a d i c a l s p e c -34 t r o s c o p y . The main f e a t u r e s of the f l a s h p h o t o l y s i s technique are e s s e n t i a l l y the same no matter what the p a r t i c u l a r a p p l i c a t i o n , but the f o l l o w i n g b r i e f o u t -l i n e a p p l i e s d i r e c t l y to t h i s work. A p h o t o c h e m i c a l l y respons i ve substance conta ined in a r e a c t i o n vesse l i s sub jec ted to a h igh i n t e n s i t y shor t d u r a t i o n p u l s e of cont inuous l i g h t which is produced from the r a p i d d i s c h a r g e of c a p a c i t o r s through argon f i l l e d q u a r t z f l a s h lamps. Induced c u r r e n t from the rap id d i s c h a r g e i s used as an -15 -input t r i g g e r pu lse to an e l e c t r o n i c de lay u n i t . In the delay u n i t , the t r i g g e r p u l s e i s delayed e l e c t r o n i c a l l y over a p reset p e r i o d (1 ps to 100 ms) and is then a p p l i e d to the g r i d of a hydrogen t h y r a t r o n which makes the tube c o n d u c t i n g . The t h y r a t r o n i s i n c o r p o r a t e d as an e l e c t r o n i c s w i t c h in the f i r i n g c i r c u i t of a second f l a s h lamp of lower power which i s the s p e c t r o s c o p i c l i g h t source and f o r that reason i s r e f e r r e d to as the s p e c t r o s c o p i c lamp. The l i g h t from the s p e c t r o s c o p i c lamp passes through the r e a c t i o n v e s s e l a long i t s a x i s , p a r a l l e l to the p h o t o l y s i s lamps, and i s focused onto the s l i t of the s p e c t r o g r a p h . If a t r a n s i e n t i s present at that p rese t time and i t has an a b s o r p t i o n spectrum wi th a s u f f i c i e n t l y la rge e x t i n c t i o n c o e f f i c i e n t in a wavelength reg ion a c c e s s i b l e to the s p e c t r o g r a p h , i t s spectrum i s recorded p h o t o g r a p h i c a l l y . When an absorb ing s p e c i e s i s f l a s h e d in the absence o f an i n e r t d i l u e n t , the energy absorbed may be s u f f i c i e n t to r a i s e the temperature by s e v e r a l thousand degrees w i t h i n a few u s . By t h i s , the a d i a b a t i c method, p y r o l y s i s and combustion r e a c t i o n s can be s t u d i e d . In c o n t r a s t , the i sothermal method invo l ves keeping the r e a c t i o n at constant temperature ( u s u a l l y ambient) by adding a la rge excess of i n e r t gas to the p h o t o c h e m i c a l l y respons ive r e a c t -a n t . The o v e r a l l heat c a p a c i t y of the system is thus r a i s e d to a s u f f i c i e n t l e v e l that energy absorbed by the reac tant and heat l i b e r a t e d by the r e a c t i o n causes on ly a minimal temperature r i s e . Examples of both the a d i a b a t i c and the i sothermal methods, as w e l l as a review of some r e a c t i o n s p r e v i o u s l y s t u d i e d by f l a s h p h o t o l y s i s and k i n e t i c s p e c t r o s c o p y , a re g i ven by R.G.W. N o r r i s h (1966). - 1 6 -I I Apparatus Most of the r e s u l t s to be presented were ob ta ined from s p e c t r o s c o p i c measurements in the VUV. Some work on h i g h l y v i b r a t i o n a 1 l y e x c i t e d oxygen was done on a J a r r e l l Ash 3-4 meter Ebert s p e c t r o g r a p h . T h i s l a t t e r e x p e r i -16 26 mental arrangement has been d e s c r i b e d in d e t a i l e l sewhere . ' The d e s c r i p t i o n of the VUV apparatus is best done in two s e c t i o n s : (a) the e x t e r n a l f l a s h p h o t o l y s i s equipment and (b) the vacuum s p e c t r o g r a p h , (a) E x t e r n a l Equipment For work in the VUV the o p t i c a l path must n e c e s s a r i l y be evacuated , as a r e s u l t , the des ign of the system is not as s t r a i g h t f o r w a r d as that of a c o n v e n t i o n a l a p p a r a t u s . The s a l i e n t f e a t u r e s our o p t i c a l arrangement a re g i ven in f igure 2 - 1 . A L i F lens focused f o r 140 nm r a d i a t i o n was sea led onto the end of a 108 mm long (22 mm od) s i l i c a o r Pyrex r e a c t i o n v e s s e l , s e p a r a t i n g i t from the s p e c t r o s c o p i c lamp. A L i F o p t i c a l f l a t separated the r e a c t i o n vesse l and the s p e c t r o g r a p h . A be l lows was a t tached to each end of the r e a c t i o n v e s s e l case in o rder to f a c i l i t a t e o p t i c a l a l ignment which was very c r i t i c a l as a r e s u l t of the r i g i d l y mounted l e n s . The s p e c t r o s c o p i c lamp was made from a s i l i c a c a p i l l a r y a t tached in a h o r i z o n t a l p o s i t i o n to the be l lows by means of a tapered h o l l o w cathode and a B14 s i l i c a socket c o n n e c t i o n . The lamp was f i l l e d w i t h 30 t o r r of argon and was operated at 100 J from a 1 pF low inductance c a p a c i t o r . The h a l f w id th l i f e t i m e was 0.5 p s . S i x 85 mm (12 mm od) S u p r a s i l p h o t o l y s i s lamps were p o s i t i o n e d around the r e a c t i o n v e s s e l . The f i r i n g arrangement c o n s i s t e d of th ree groups of F igure 2 -1 Vacuum U l t r a v i o l e t Apparatus (a) neoprene o - r i n g s ; (b) e l e c t r o d e and c o a x i a l h igh v o l t a g e c a b l e ; (c) 108 mm S u p r a s i l or Pyrex (22 mm od) r e a c t i o n v e s s e l ; (d) one of s i x S u p r a s i l 85 mm (8 mm id) p h o t o l y s i s lamps surrounding the r e a c t i o n v e s s e l ; (e) ho l low e l e c t r o d e f o r lamp f i l l i n g ; ( f ) a d j u s t -a b l e r i g i d bel lows suppor t ; (g) b e l l o w s ; (h) gas f i l l i n g i n l e t ; ( i ) s i l i c a s p e c t r o s c o p i c lamp; ( j ) B19 ho l low e l e c t r o d e as par t of o p t i c a l p a t h ; (k) L i F l e n s ; ( l ) brass h o u s i n g ; (m) L iF o p t i c a l f l a t ; (n) threaded compression r i n g ; (o) spect rograph s l i t - 1 8 -two lamps connected to a common ground v i a a mechanical high v o l t a g e s w i t c h . The two lamps of each group were connected in s e r i e s and operated from two 10 uF c a p a c i t o r s which were connected in p a r a l l e l . The t o t a l energy input to the lamps was u s u a l l y about 1 kJ and the l i g h t output had a h a l f w i d t h l i f e t i m e of 15 u s . To ensure spontaneous f i r i n g of the p h o t o l y s i s lamps, the argon f i l l i n g was changed every run (=20 f l a s h e s ) . There was s u f f i c i e n t space between the r e a c t i o n vesse l and the p h o t o l -y s i s lamps to a l l o w the i n s e r t i o n of s e m i - c i r c u 1 a r g l a s s f i l t e r s . For the most p a r t , a delay u n i t w i t h about seventy de lays rang ing from 1 us to 100 ms was used to t r i g g e r the breakdown of the s p e c t r o s c o p i c lamp. For the k i n e t i c study of CIO, however, a T e k t r o n i x Model 5^7 o s c i l l o s c o p e , w i t h a c o n t i n u o u s l y v a r i a b l e delay c a p a b i l i t y over the range 1 ps to 10 seconds was used. In t h i s c a s e , an induced t r i g g e r p u l s e was found to be u n s u i t a b l e and a p h o t o c e l l was used to g i v e the t r i g g e r p u l s e from the o u t -put of the p h o t o l y s i s lamp to the de lay mode of the o s c i l l o s c o p e . An added f e a t u r e of t h i s arrangement was the ease w i t h which the output of both the p h o t o l y s i s lamps and the s p e c t r o s c o p i c lamp cou ld be monitored f o r spontan -eous breakdown o f the former and f o r the c o r r e c t de lay of the l a t t e r . (b) The Spectrograph The spect rograph used i s a comple te l y s e l f - c o n t a i n e d vacuum instrument b u i l t by J a r r e l l Ash . It employs a Czerny Turner mounting which u t i l i z e s three i n t e r n a l r e f l e c t i o n s of the i n c i d e n t l i g h t . The main use made of such a mounting i s that of a scanning spect rometer b u t , through the use of a f o u r t h r e f l e c t i n g s u r f a c e , the e n t i r e e x i t beam can be d e f l e c t e d onto a photograph ic -19-p l a t e . The e x c e l l e n t VUV r e f l e c t i n g p r o p e r t i e s o f MgF 2 c o a t e d o p t i c s i n the range 200 t o 125 nm has overcome t h e o b j e c t i o n t o u s i n g f o u r r e f l e c t i n g s u r f a c e s i n t h i s r e g i o n . The ad v a n t a g e s o f such a mounting a r e good r e s o l -u t i o n , h i g h l i g h t g a t h e r i n g power, ease o f w a v e l e n g t h s e l e c t i o n and t h e use o f i n t e r c h a n g e a b l e p l a n e g r a t i n g s . These f a c t o r s p l u s i t s compact o p t i c a l a r rangement make i t v e r y s u i t a b l e f o r VUV i n v e s t i g a t i o n s . The s p e c t r o g r a p h was o r i g i n a l l y an f0.75 meter i n s t r u m e n t but was changed t o f 2 . 0 m by a d d i n g an e x t e n s i o n t a n k and c h a n g i n g t h e c o l l i m a t i n g and o b j e c t i v e m i r r o r s . T h i s m o d i f i c a t i o n was done a t an e a r l y s t a g e o f the p r e s e n t s t u d y and a l l o f t h e f o l l o w i n g r e s u l t s were o b t a i n e d on t h e f 2 . 0 m i n s t r u m e n t . Two g r a t i n g s were u s e d ; t h e f i r s t has 590 lines/mm and i s b l a z e d f o r 140 nm. A t f 2 . 0 , i t g i v e s a l i n e a r r e c i p r o c a l d i s p e r s i o n o f 0.842 nm/mm and a w a v e l e n g t h c o v e r a g e o f c a . 84 nm p e r e x p o s u r e on a 10 cm p l a t e o r f i l m . The s econd g r a t i n g has 1180 lines/mm b l a z e d a t 140 nm g i v i n g a l i n e a r r e c i -p r o c a l d i s p e r s i o n o f 0.412 nm/mm and a w a v e l e n g t h c o v e r a g e o f c a . 41 nm. The c h o i c e o f w h i c h g r a t i n g t o use depended on whether b e t t e r r e s o l u t i o n o r a l a r g e r w a v e l e n g t h c o v e r a g e was the more i m p o r t a n t f a c t o r i n t h e ex-p e r i m e n t . T y p i c a l l y , t h e s p e c t r o g r a p h was e v a c u a t e d t o l e s s than 5 x 10 ^  t o r r d u r i n g an e x p e r i m e n t . I l l P h o t o g r a p h y and P l a t e Photometry l l f o r d HP, and 0_~ p l a t e s were used w i t h t h e 3 - 4 meter s p e c t r o g r a p h . -20-The f i r s t i s a f a s t p a n c h r o m a t i c p l a t e s u i t a b l e f o r r e c o r d i n g s p e c t r a i n t h e w a v e l e n g t h r a n g e 230 t o 550 nm. The s e c o n d i s a p l a t e e m p l o y i n g a t h i n e m u l s i o n i n o r d e r t o m i n i m i z e g e l a t i n a b s o r p t i o n i n t h e w a v e l e n g t h r e g i o n b e l o w 230 nm. The l a t t e r p l a t e s a r e s l o w e r , g r a i n i e r and more f r a g i l e t h a n HP^ -T h r e e d i f f e r e n t p l a t e s have been used i n t h e VUV; l l f o r d Q^ , s o d i u m s a l i c y l a t e ( N a S a l ) s e n s i t i z e d l l f o r d HP^ f i l m , and 35 mm Kodak S p e c i a l F i l m , Type 101-01. F o r most w o r k , t h e s e n s i t i z e d HP^ f i l m was f o u n d t o be t h e most u s e f u l . The HP^ e m u l s i o n t y p e i s h i g h l y s e n s i t i v e t o 450 nm l i g h t w h i c h i s a p p r o x i m a t e l y t h e peak o f N a S a l f l u o r e s c e n c e . The <j>^. o f N a S a l i s e s s e n t i a l l y u n i t y f r o m 60 t o 340 nm. ' The f i l m i s s e n s i t i z e d by d i p p i n g i t i n t o a 0.5 M s o l u t i o n o f N a S a l i n 35% m e t h a n o l and t h e n d r y i n g i t w i t h a warm a i r b l o w e r . I t was f o u n d t h a t t h e b e s t r e s u l t s w e r e o b t a i n e d i f t h e s o l u t i o n was p r e p a r e d j u s t b e f o r e s e n s i t i z i n g t h e f i l m . The N a S a l does not a p p e a r t o a f f e c t t h e g r a i n o f t h e HP^ f i l m ; a s a r e s u l t , t h e images a r e w e l l d e f i n e d and t h e p l a t e r e s o l u t i o n i s g o o d . p l a t e s were n o t a s f a s t a s H P ^ , t o o k l o n g e r t o degas and were p r e s s u r e s e n s i t i v e . They were u s e d o n l y f o r w a v e l e n g t h c a l i b r a t i o n s s i n c e i t was t h o u g h t more a d v i s a b l e t o use a r i g i d p l a t e r a t h e r t h a n f l e x i b l e f i l m . The Kodak S p e c i a l F i l m was u s e d a s a l a s t r e s o r t t o c o v e r t h e w a v e l e n g t h r e g i o n b e l o w 135 nm. I t i s so f r a g i l e t h a t t h e e d g e s o f t h e f i l m a r e l i n e d w i t h p r o t r u d i n g ' r a i l s ' t o p r e v e n t t h e r o l l e d l o o p s o f f i l m f r o m t o u c h i n g . T h i s f i l m i s v e r y f a s t and e x t r e m e l y c o n t r a s t y ( s e e f o r e x a m p l e f i g u r e 3"4 where t h e p l a t e d e n s i t y c h a n g e s v e r y r a p i d l y o v e r c a . 5 n m ) . As a r e s u l t o f i t s r a p i d c h a n g e i n c o n t r a s t and i t s s u s c e p t i b i l i t y t o p r e s s u r e m a r k s , i t i s n o t s u i t a b l e f o r q u a n t i t a t i v e -21-measurements. A l l o f the p l a t e s or f i l m s d e s c r i b e d above were developed in Kodak D19 developer at 20°C. For a l l q u a n t i t a t i v e runs f r e s h developer was used each t i m e . Development t imes were f i v e minutes f o r a l l but the s e n s i t i z e d HP^ f i l m s which were g i ven 6 m inutes . From then o n , the p r o c e s s i n g invo l ved a 30 second r i n s e in an a c e t i c a c i d stop b a t h , two minutes in Kodak Rapid F i x e r and a r i n s e at 20°C in c o n t i n u o u s l y f l o w i n g water f o r 20 to 30 m i n u t e s . The p l a t e s were then r i n s e d w i t h a w e t t i n g agent to e l i m i n a t e water marks and d r i e d in a dust f r e e c o n t a i n e r . On each p l a t e intended f o r i n t e n s i t y measurements, a s e r i e s of ex -posures at d i f f e r e n t s l i t widths was r e c o r d e d . From t h i s , c h a r a c t e r i s t i c curves f o r the wavelength reg ions of i n t e r e s t were p l o t t e d . The c h a r a c t e r -i s t i c curve d e f i n e s the p l a t e d e n s i t y reg ion i n which q u a n t i t a t i v e measure-ments may be made, namely the l i n e a r r e g i o n . The s l o p e of that l i n e (gamma) g i v e s the convers ion f a c t o r from p l a t e d e n s i t y to o p t i c a l d e n s i t y (absorb -a n c e ) . These terms a re i l l u s t r a t e d in f i g u r e 2-2. A c t u a l l y , the a b s c i s s a of the c h a r a c t e r i s t i c curve i s p l o t t e d in terms of log exposure , where exposure i s i n t e n s i t y / u n i t a r e a / u n i t t i m e . By v a r y i n g the s l i t w id th we are assuming uni form i l l u m i n a t i o n of the s l i t and constant s p e c t r o s c o p i c lamp f i r i n g both in d u r a t i o n and in i n t e n s i t y . These assump-t i o n s have been checked us ing s o l u t i o n s of v a r y i n g o p t i c a l d e n s i t y (0D) at the same wavelength and by us ing a c a l i b r a t e d H i l g e r Watts n e u t r a l d e n s i t y 38 39 wedge ' and are found to be v a l i d . P l a t e d e n s i t y measurements, d e f i n e d as log(DO/D-|.) where D q i s the i n t e n s i t y of a beam o f l i g h t pass ing through an unexposed p o r t i o n of the -22-Plate Densi ty b' a log(exposure) Figure 2-2 Example of an experimentally determined characteristic curve for a given wavelength For optical density measurements to be valid, the background continuum cannot have a density greater than (a) and the strongest absorption cannot have a plate density less than (b) , since (a) and (b) define the linear portion of the characteristic curve. The slope (b-a)/(b 1-a 1) gives the plate gamma (y) for the particular conditions used. Thus the plate density difference within the range a - b is a linear function of absorbed intensity, i.e. (b-a)/Y=b'-a'=loglT-log I = log(I /l Q)=0D -23 -p l a t e and D-j. i s the t r a n s m i t t e d i n t e n s i t y through an exposed p o r t i o n of the p l a t e , were made on a Joyce Loebl Mk I l i e r e c o r d i n g m i c r o d e n s i t o m e t e r . IV Wavelength Measurements An advantage in us ing a g r a t i n g inst rument i s the a lmost l i n e a r dependence of p l a t e p o s i t i o n on wave length . Thus to a good a p p r o x i m a t i o n , a constant v a l u e of the d i s p e r s i o n and a s i n g l e re fe rence l i n e on the p l a t e were a l l that was necessary to l o c a t e and c o n f i r m the presence of known s p e c t r a . In o rder to o b t a i n maximum p o s s i b l e accuracy in r e p o r t i n g new s p e c t r a , however, spect rograph d i s p e r s i o n curves were determined by f i t t i n g the p l a t e p o s i t i o n s of atomic re fe rence l i n e s to a c u b i c equat ion by the 40 method o f l e a s t squares . The re fe rence l i n e s are S i , N, 0, and C, a l l of which are produced in the d i s c h a r g e of the s p e c t r o s c o p i c lamp. P l a t e p o s i t i o n s were measured on a Grant L i n e Measuring Comparator. Th is i n s t r u -ment i s capab le of measuring p l a t e p o s i t i o n w i t h an accuracy of one m i c r o n , which is more than s u f f i c i e n t f o r our purposes . The d i s p e r s i o n curves were then used to c a l c u l a t e the wavelengths of the v i b r a t i o n a l bands of the new s p e c t r a from the p l a t e p o s i t i o n of t h e i r i n t e n s i t y maximum r e l a t i v e to an atomic r e f e r e n c e l i n e . The d i s p e r s i o n curves and one set of wavelength measurements were made from p l a t e s to e l i m i n a t e e r r o r due to the f l e x i b i l i t y of HP^ f i l m . No s i g n i f i c a n t d i f f e r -ences between these and o ther se ts o f measurements were n o t e d , however. From the type of s t r u c t u r e o b s e r v e d , i . e . on ly the band p r o f i l e i s o b t a i n e d , and the r e s u l t s of numerous measurements, the accuracy of the - 2 4 -wavelength measurements i s w i t h i n 0.01 nm (3 cm at 180 nm and 7 cm at 125 nm). V Gas Handl ing Procedures and Exper imenta l Technique Standard h igh Vacuum g lassware was used as the gas hand l ing a p p a r a t u s . P ressu re measurements of the reagents were made on c a l i b r a t e d g l a s s s p i r a l gauges , s i n c e mercury manometers are r e a d i l y contaminated by ox ides of n i t r o g e n and c h l o r i n e . The s p i r a l gauges were s u f f i c i e n t l y a c c u r a t e to p e r -mit p r e s s u r e measurements as low as 3 t o r r . For p ressures below t h i s , p r e -determined expansion volumes were used . Because both N0 2 and C10^ are l i g h t s e n s i t i v e , they were s to red at l i q u i d N 2 temperature and were p r o t e c t e d from the room l i g h t . Reac t ion mix tu res were made up in blackened s to rage bulbs and were a l lowed to mix f o r approx imate ly three hours . C h l o r i n e d i o x i d e exper iments were conducted in the absence of f l u o r e s c e n t l i g h t i n g as an added p r e c a u t i o n a g a i n s t p h o t o l y s i n g some of the mix tu re p r e m a t u r e l y . S p e c i a l p r e c a u t i o n s taken in conduct ing a k i n e t i c run were as f o l l o w s : (a) the s p e c t r o s c o p i c lamp windows were c leaned of s i l i c a dust a t the beginning of each r u n ; - in the VUV case the window was the L i F lens a n d , as s u c h , had to be c leaned in s i t u w i t h a d i l u t e s o l u t i o n of HF f o l l o w e d immediately by a l i b e r a l r i n s e w i t h a b s o l u t e e t h a n o l , (b) the r e a c t i o n vesse l was pumped down to a good vacuum a f t e r each f l a s h and then r e f i l l e d w i t h a f r e s h sample , and (c) the s p e c t r o s c o p i c lamp in the VUV apparatus was pumped out a f t e r -25-e a c h f l a s h and t h e n r e f i l l e d w i t h a r g o n s i n c e CO i s p r o d u c e d i n t h e d i s c h a r g e and i t s s p e c t r u m i s s t r o n g enough t o c a u s e i n t e r f e r e n c e i n t h e w a v e l e n g t h r e g i o n b e l o w 155nm. VI M a t e r i a l s N i t r o g e n D i o x i d e was p r e p a r e d f r o m n i t r i c o x i d e and o x y g e n . M a t h e s o n Canada L t d . 98.9% p u r e n i t r i c o x i d e was d i s t i l l e d be tween 90°K ( l i q u i d 0 2 ) and 77°K ( l i q u i d N 2 ) . L i q u i d A i r L t d . $8% p u r e o x y g e n was p a s s e d t h r o u g h a g l a s s wool f i l l e d t r a p a t 195°K ( s o l i d C0 2 / m e t h a n o l ) and t h e n d r i e d o v e r P 20^. A l a r g e e x c e s s o f 0 2 was r e a c t e d w i t h NO t o f o r m N0 2- The N0 2 (mp= -10°C) was t h e n f r o z e n down and t h e e x c e s s o x y g e n was pumped a w a y . The r e m a i n i n g s o l i d was i n most c a s e s p u r e w h i t e (used a s a c r i t e r i o n o f p u r i t y ) . I f a s l i g h t b l u e t i n g e r e m a i n e d , i n d i c a t i n g t h e p r e s e n c e o f a s m a l l amount o f N 2 0 3 , NO + N0 2 « w N £ 0 3 t h e N0 2 was c o o l e d t o 195°K and pumped on u n t i l t h e NO was r e m o v e d . C h l o r i n e D i o x i d e was p r e p a r e d by S . K . D o g r a ^ u s i n g t h e method o f Derby and H u t c h i n s o n w h i c h r e s u l t e d i n a c h l o r i n e f r e e p r o d u c t . Ozone - A f l o w o f d r y oxygen c o n t a i n i n g o z o n e f r o m a c o n v e n t i o n a l o z o n -i s e r was p a s s e d a t a t m o s p h e r i c p r e s s u r e t h r o u g h a g l a s s U - t u b e p a c k e d w i t h s i l i c a g e l and c o o l e d i n a s o l i d C0 2 / m e t h a n o l b a t h . The g e l and t h e U - tube had been d r i e d by h e a t i n g t o c a . A00°C u n d e r v a c u u m . As r e p o r t e d e l s e w h e r e ^ 2 t h e c o l o r l e s s g e l became d a r k b l u e when o z o n e was a d s o r b e d . -26-The U-tube was a t tached to a vacuum apparatus and the remaining oxygen was pumped o f f . The U-tube and contents s l o w l y pumped down to a p ressure of c a . 1 m t o r r . At t h i s p ressure very l i t t l e 0^ remained as observed from the absence of any 0^ a b s o r p t i o n in the VUV in a sample of 5 t o r r of 0^. C h l o r i n e was ob ta ined from a l e c t u r e b o t t l e (Matheson Canada L t d . ) . It was degassed at l i q u i d temperature , d r i e d by pass ing over a n c ' f i n a l l y . d i s t i 1 led from 195 to 77°K. Argon , 99-999% U l t r a h igh p u r i t y (Matheson) , was used f o r f i l l i n g the f l a s h lamps and f o r d i l u t i n g the reagents . It was taken d i r e c t l y from the c y l i n d e r a f t e r pass ing through a g l a s s wool f i l l e d t rap at 195°K. VI I E r r o r s The e r r o r l i m i t s a re g i ven as ± one standard d e v i a t i o n . CHAPTER 3 THE FLASH PHOTOLYSIS OF CHLORINE DIOXIDE FOLLOWED BY KINETIC SPECTROSCOPY IN THE VACUUM ULTRAVIOLET I The Absorption Spectrum of C10 2 in the VUV The f l a s h photolysis of chlorine dioxide followed by k i n e t i c spectro-scopy in the VUV has yielded a very extensive spectrum which is attributed to CIO and w i l l be discussed in the next section. For reasons of spectral and k i n e t i c analysis of CIO, the absorption spectrum of the parent molecule in this region was f i r s t investigated. The only previous investigation on C10^ absorption in the VUV was by Humphries, Walsh and Warsop (1963)**^ who reported seeing three band systems ( C-«-X, D^ -X, and E«-X) extending from 182.9 to ca. 1^ 9 nm, below which they observed only continuous absorption. They assigned each of the systems to a Rydberg transion; the f i r s t two systems converge to the f i r s t ionization potential and the thi r d system to the second IP. LL Recently Cornford, Frost, Herring and McDowell (1971) published the -28-p h o t o e l e c t r o n spectrum of ClO^ from which a v a l u e of 10.36 eV was ob ta ined as the v a l u e of the f i r s t a d i a b a t i c IP. V i b r a t i o n a l s t r u c t u r e w i t h a sep -a r a t i o n of 980 cm ^ was ev ident on the f i r s t band. The second band l e a d i n g to an IP of 12.32 was u n r e s o l v e d . We have observed the systems repor ted by Humphries et a l and have measured e x t i n c t i o n c o e f f i c i e n t s f o r them. Fur thermore , we have found some new t r a n s i t i o n s extending to at l e a s t 128 nm. Two of these new systems and the O X repor ted by Humphries et a l f i t a Rydberg s e r i e s f o r an IP of 10.36 eV in e x c e l l e n t agreement w i t h Cornford e t a l . ( i ) R e s u l t s The a b s o r p t i o n spectrum of ClO^ was photographed from 200 to 125 nm. A major p o r t i o n of the spectrum observed in t h i s reg ion i s shown in f i g u r e 3"1. The f i r s t system observed by Humphries e t a l i s not shown but a t r a c e of i t s a b s o r p t i o n spectrum i s shown in f i g u r e 3"2 and the two most in tense bands are seen in f i g u r e s k-$ and k-G. Below 128 nm the spectrum appeared to be complete ly cont inuous a l though t h i s may be the r e s u l t of i n s t r u m e n t a l l i m i t a t i o n s . Table 3"1 l i s t s the wavelengths of the i n t e n s i t y maxima of a l l the bands observed . The measurements of Humphries e t a l are inc luded f o r the sake of compar ison . It can be seen from the t a b l e that the two se ts of measure-ments are in good agreement. In the E-X system, where the bands are r e l a -t i v e l y sharp and hence the p o s i t i o n of each l m a x c a n be more p r e c i s e l y measured, the agreement of the two sets i s a lmost e x a c t . Only one of t h e i r 133 45 130.60 F i g u r e 3~1 The a b s o r p t i o n spectrum of 0.25 t o r r of ClO^ in the VUV. A background continuum is recorded above the spectrum. Note -Two abrupt changes in d e n s i t y on the continuum are the r e s u l t of o v e r l a p p i n g p r i n t s . W A V E L E N G T H / n m 184 182 180 178 176 174 172 - 1 1 1 -i 1— i i F R E Q U E N C Y / c m ' 1 x I O " 3 F i g u r e 3"2 - The O X system of C I O . -1 E i s the molar e x t i n c t i o n c o e f f i c i e n t (M cm - 3 1 -TABLE 3-1 VUV Bands of C10£ Label v a(cm )^ AG(cm )^ b V AG X(nm)k b e max c-x (182.9 nm) asyst em f= 2. 6 x 10~ 3 A 54 689 54 671 182.91 943 1 020 1 021 B 55 709 55 692 179.50 529 1 011 1 008 C 56 720 (56 6 9 9 ) C 176.4 125 D-X (162.8 nm)asystem f- 2 .5 x 10 - 2 D E F G H J K' 61 430 61 951 62 481-521 63 007 63 572 64 087 64 640 526 515 1 051 1 091 1 068 61 429 (61 930) 62 460 (63 011) 63 589 : (64 621)-501 551 1 031 1 129 1 032 162.79 I61 .5 160.10 158.50 157.26 154.8 4.12 x 10-2.88 x 10" E-X (156.8 nm)asystem f= 1.9 x 10 i 63 774 , 508 63 770 511 156.81 — K 64 282 496 64 281 492 155-57 L 64 778 503 64 773 506 154.38 M 65 281 489 65 279 493 153-19 3.0 x 10^ N 65 770 511 65 772 492 152.04 2.9 " " 0 66 281 486 66 264 484 150.91 2.3 " " P 66 767 513 66 748 519 149.82 1.8 " " Q 67 280 (67 267) 148.66 1.2 " " (a) - from Humphries et al (b) - present work (c) - measurements in brackets are for weak or d i f fuse bands -32-TABLE 3-1 (Cont 'd) VUV Bands of CIO 2 Label v(cm ^) AG X (nm) F-X system R 71 004 140.84 991 S 71 995 138.90 936 U 72 931 137-12 1 008 W 73 939 135.43 G-X system X 74 186 134.79 602 a 74 788 133.71 600 d 75 388 132.65 629 f 1 (76 017) 131.55 H-X system Y 74 324 134.54 632 b 74 956 133.41 633 e 75 589 132.29 637 g 76 226 131.19 l - X z 74 490 134.25 619 c 75 109 133-14 603 f 75 712 132.08 613 h 76 325 131-02 T h i r d Rydberg? i (76 800) 130.2 970 j (77 770) 128.6 Unass igned Bands V 73 478 136.09 V ' 73 567 135-93 -33-bands (J) was too f a i n t to be measured by us. The C1C«2 spectrum in the 200 to 149 nm region was recorded on NaSal sensitized HP^ f i l m . This allowed the measurement of absolute molar extinc-tion c o e f f i c i e n t s of the f i r s t three band systems. The method was to ensure that absorption is occurring within the linear region of the ch a r a c t e r i s t i c curve for the plate at the appropriate wavelength. Plate density at that wavelength was then converted to optical density (OD) by the slope of the ch a r a c t e r i s t i c curve (y). A plot of (OD/l) vs IC10^] where £ is the absorp-tion path length in cm and IC10^ 3 is in units of M, yielded a straight line the slope of which was the molar extinction c o e f f i c i e n t (e) for that par-t i c u l a r wavelength. The range of CIO2 p a r t i a l pressures covered was 0.05 to 0.5 to r r . Figures 3"2 and 3~3 show the absorption spectrum of the f i r s t three VUV systems of C10,. in terms of e. The e values of the f u l l y 2 max 1 resolved bands of these systems are l i s t e d in Table 3"1- The f values of the systems were obtained by integrating the above curves with a planimeter to give /edv ( a small correction for the fact that the curve was plotted on a linear wavelength scale was not computed) which was then applied to the formula: 2303m ec 2 /ODdv 3-1 f = —..I—2— /edv and /edv=——r nm TTN I C where me is the mass of an electron, e i t s charge, N is Avogadro's number, c the speed of l i g h t , I is the absorption path length and C i s the concentra-tion of the absorbing species. The integration is carried out over the entire band system for the electronic t r a n s i t i o n from states m to n. The shorter wavelength spectrum, below ca. 150 nm, was recorded on 35 mm F i g u r e 3~3 _ The O X and E«-X systems of C10 2 The v i b r a t i o n a l t r a n s i t i o n s are 1i s ted in Table 3-1. -35-Kodak S p e c i a l F i l m , Type 101-01 w h i c h , as d i s c u s s e d in the E x p e r i m e n t a l , i s not s u i t a b l e f o r making e x t e n s i v e q u a n t i t a t i v e measurements. As a r e s u l t , on l y the wavelength measurements and a mic rodens i tometer t race of the bands uncor rec ted f o r background are g i v e n , Table 3"1 and f i g u r e 3"4 r e s p e c t i v e l y . In t h i s r e g i o n , there appear to be f o u r e l e c t r o n i c band systems w i t h constant v i b r a t i o n a l spac ing w i t h i n each system. The f i r s t , s t a r t i n g at 140.8 nm, i s d i f f u s e and has a v i b r a t i o n a l spac ing of c a . 990 cm \ The second system s t a r t i n g at 13^.8 nm is sharper and has v i b r a t i o n a l spac ing of c a . 600 cm.^ The f i n a l g r o u p i n g , which can be measured w i t h reasonable a c c u r a c y , appears to c o n s i s t of two o v e r l a p p i n g systems of approx imate ly the same v i b r a t i o n a l spac ing ( = 620 cm The beg inn ing of a f i f t h system which i s j u s t at the f r i n g e of d e t e c t -a b i l i t y has v i b r a t i o n a l spac ing 970 cm \ The methods by which the wavelength and i n t e n s i t y measurements were o b t a i n e d and were subsequent ly t r e a t e d i s f u l l y d i s c u s s e d in the e x p e r i -mental s e c t i o n . ( i i ) D i s c u s s i o n The assignment of a l l the observed bands to c h l o r i n e d i o x i d e is q u i t e s t r a i g h t f o r w a r d f o r a number of reasons . The most important o f these i s the h igh p u r i t y of the sample as ev idenced by the method which was used to prepare and p u r i f y i t (exper imental s e c t i o n ) . No new bands other than those observed by Humphries et a l were seen in the 200 to 149 nm r e g i o n . These bands have been p o s i t i v e l y ass igned to ClO^ by them. Under c o n d i t i o n s where the C10 i s p a r t i a l l y decomposed by f l a s h p h o t o l y s i s (measured from -36-W A V E L E N G T H / n m 140 135 130 — i 1 I V ' X Z a c d f f ' h j ? F R E Q U E N C Y / c m - 1 x l O 3 F i g u r e 3-4 - M ic rodens i tometer t r a c e , w i t h background, of the remaining CIO t r a n s i t i o n s which are l i s t e d in Table 3-1. -37-the 160.1 band) , the i n t e n s i t i e s of the bands below 1^ 9 nm change by approx -imate ly the same f a c t o r as the known ClO^ bands. Thus the new s t r u c t u r e can q u i t e c o n f i d e n t l y be ass igned to C lO^. Based on the h igh i n t e n s i f y of the new bands and on t h e i r v i b r a t i o n a l spac ing i t i s p robab le that they are a l l members of Rydberg systems. The f i r s t and f o u r t h systems ( O X and F-*-X) have been f i t t e d to the Rydberg fo rmula which g i v e s a convergence l i m i t of 10.36 eV (83 530 cm ^) and a quantum d e f e c t of 2.05- The c h o i c e of the two systems as two members of the same Rydberg s e r i e s i s based on the s i m i l a r i t y of t h e i r v i b r a t i o n a l spac ing (1 020 and 990 cm ^) and on the appearance of the bands. For both of these t r a n s i t i o n s to be members of the same Rydberg s e r i e s , however, they should a l s o have approx imate ly the same r e l a t i v e i n t e n s i t y . T h i s c o n d i t i o n was d i f f i c u l t to check because of the r a p i d l y changing back-ground d e n s i t y below 1^ 9 nm which prevented the d e t e r m i n a t i o n of r e l a t i v e i n t e n s i t i e s f o r the F-*-X system. From the nature of the Rydberg f o r m u l a , at l e a s t two members of the s e r i e s are necessary in order to o b t a i n the c o n s t a n t s , but to be meaningful a t h i r d system should be observed where p r e d i c t e d by the f o r m u l a . There i s a f i f t h system (and perhaps a s i x t h ) at the s h o r t e s t wavelengths observed which cannot be measured to b e t t e r than c a . 60 cm ^ a c c u r a c y . The f i r s t member of t h i s system f a l l s w i t h i n 200 cm ' of the p r e d i c t e d v a l u e of the t h i r d member of the Rydberg s e r i e s . The v i b r a t i o n a l spac ing appears to be about 970 cm ^. F u r t h e r support of t h i s assignment of a Rydberg s e r i e s i s the e x c e l l e n t agreement between our v a l u e and the va lue o f the f i r s t IP from the photo --38-e l e c t r o n spect rum, as w e l l as the v i b r a t i o n a l spac ing of the ion and the ion c o r e . The quantum d e f e c t of 2 . 0 5 means deep p e n e t r a t i o n of the Rydberg o r b i t a l i n t o the core i n d i c a t i n g that the t r a n s i t i o n i s to a sa^ o r b i t a l as suggested 4 3 f o r the f i r s t member by Humphries et a l . The v i b r a t i o n a l assignments in the D-X and E-X systems are those o f 4 3 Humphries et a l . The G -X , H-X, and |-X systems of ClO^ appear to be a complex of Rydberg t r a n s i t i o n s i n v o l v i n g the bending f requency (v^) as does the E-X sys tem, which Humphries et a l have a s s i g n e d to a Rydberg t r a n s i t i o n l e a d i n g to the second IP. If these f o u r are Rydberg t r a n s i t i o n s converg ing to the second 4 4 IP , then i t would e x p l a i n the i n a b i l i t y of Cornford et a l to r e s o l v e v i b -r a t i o n a l s t r u c t u r e on the second IP band s i n c e an o v e r l a p p i n g bending complex such as t h i s would not be reso l ved by t h e i r ins t rument . U n t i l the a b s o r p t i o n spectrum in the VUV i s ob ta ined under much h igher r e s o l u t i o n , l i t t l e more can be s a i d about the spect roscopy o f ClO^ in t h i s wavelength r e g i o n . The main purpose of t h i s s t u d y , however, has been ach ieved in that a l l of the bands of ClO^ in the VUV reg ion a c c e s s i b l e to us have been i d e n t i f i e d and the molar e x t i n c t i o n c o e f f i c i e n t s of a number o f the bands have been determined . These w i l l be used l a t e r in a b r i e f k i n e t i c study of the f l a s h p h o t o l y s i s of t h i s m o l e c u l e . II The A b s o r p t i o n Spectrum of CIO in the VUV The p h o t o l y s i s of c h l o r i n e d i o x i d e i s of c o n s i d e r a b l e i n t e r e s t s i n c e i t s -39-products are the r e l a t i v e l y long lived free radical CIO and highly vibra-t i o n a l ^ excited oxygen (0^) • The recombination of CIO and the energy d i s t -ribution of 0^ from CIO2 have been studied extensively as seen from the Introduction. The production of v i b r a t i o n a l l y excited oxygen w i l l be d i s -cussed in Chapter^. The A2II.-0(2II. band system of CIO was f i r s t observed by Porter^ and 2 3 analysed by Porter and by Durie and Ramsay. There have been no other reports of e l e c t r o n i c band systems of CIO. Porter's v i b r a t i o n a l analysis of the above t r a n s i t i o n r e l i e d on the interpretation of the flame emission bands which Gaydon and Pannetier at-tributed to CIO. The measurements of these l a t t e r bands had a reported accuracy of 0.2 nm, at best, to 0.5 nm. Furthermore, the upper state pro-gressions involved rather large u" numbers for which anharmonicity effects were high. The u>y thus obtained by Porter was reported as 868 ± 26 cm. ^  Subsequent work by Durie and Ramsay3 has established the electronic trans-i t i o n as that given above. 46 Rochkind and Pimentel(1967) have reported a vibrational frequency of 970 ± 20 cm ^  for CIO from thei r study of the photolysis of matrix-isolated C120 at 20°K. Carrington, Dyer and Levy (1967)^ have analysed the EPR spec-trum of CIO and have reported a value of the spin o r b i t coupling constant (A=-282± 9 cm ^ ) . It must be noted here that their f i e l d free Hamiltonian does not include a centrifugal d i s t o r t i o n term. 48 Amano, Saito, Hirota, Morino, Johnson, and Powell(1969) have analysed the microwave spectrum of CIO. They found that the value of A= -282 is ap-proximately right within about 50 cm ' and point out that omission of - 4 0 -c e n t r i f u g a l d i s t o r t i o n e f f e c t s can cause a s i g n i f i c a n t e r r o r in the va lue of the s p i n o r b i t c o u p l i n g c o n s t a n t . In t h i s s e c t i o n , the very e x t e n s i v e spectrum of CIO in the vacuum u l t r a -v i o l e t i s r e p o r t e d . Th is spectrum c o n s i s t s of more than s i x e l e c t r o n i c t r a n s -i t i o n s from which a v a l u e of A can be d i r e c t l y measured. Fur thermore , the ground s t a t e v i b r a t i o n a l constants g iven by P o r t e r have been conf i rmed w i t h on ly a s l i g h t a l t e r a t i o n (=10 cm ^) . ( i ) R e s u l t s The f l a s h p h o t o l y s i s of ClO^ f o l l o w e d by k i n e t i c spect roscopy in the VUV showed r a p i d d isappearance of the C10^ spectrum concur rent w i t h the appear -ance of an in tense t r a n s i e n t spectrum which extends from 174 to a t l e a s t 124 nm. The k i n e t i c spect roscopy of the c a r r i e r of t h i s spectrum l e a d i n g to molar e x t i n c t i o n c o e f f i c i e n t s of a number of the VUV bands w i l l be d i s -cussed in the next s e c t i o n . F i g u r e 3"5 i s a photograph ic copy of the t r a n s i e n t spectrum. It d i s p l a y s r e l a t i v e l y s imp le v i o l e t degraded v i b r a t i o n a l s t r u c t u r e to about 140 nm. At wavelengths s h o r t e r than t h a t , the s t r u c t u r e becomes much more complex. C o n d i t i o n s s u i t a b l e fo r the o b s e r v a t i o n of the spectrum through a 10 cm a b s o r p t i o n path length were: p ressures of 0.05 to 0.25 t o r r of C lO^ , d i l u t e d w i t h 10 to 200 t o r r of a rgon , and a f l a s h energy of 750 J through a Sup-r a s i l r e a c t i o n v e s s e l . Under these c o n d i t i o n s the ClO^ was complete ly de -composed a f t e r approx imate ly 50 us . In o rder to observe the low v i b r a t i o n a l t l e v e l s of 0., p ressures in the order of 1 to 2 t o r r of C10- were necessary . 170.8 nm I 164.8 nm I ,,, c-x2rii/2 c-x*n.3/z QJ L2.0 L3,o 4,1 14942 N(I) 14968 R E F E R E N C E LINES /nm ~"3SS8 , 133.45 C (11)133.57 130.60 O(I) 130.48 130.22 i D -x 2!"! i / 2 D - x 2 n o,o 3/21 1.1 1 I.O 2,1 3,2 |2,Q 1.1 F - X o,o 2 . 0 I.O 3P 2 , 0 4,0 3,0 G - X 2 3 4 5 [2.0 3,0 7 8 9 10 11 12 F i g u r e 3"5 The a b s o r p t i o n spectrum of CIO in the VUV. A background continuum is recorded above the spectrum. Note - Two a r t i f i c a l d e n s i t y changes on the continuum are due to o v e r l a p p i n g p r i n t s . -42-As before, the spectra in the wavelength region 200 to 140 nm were recorded on sensitized HP^ f i l m . Below that, they were recorded on Kodak Special Film, Type 101-01. The spectra were photographed both under 0.412 and 0.842 nm/mm linear reciprocal dispersion. The short wavelength l i m i t is the result of instrumental lim i t a t i o n s rather than the onset of a con-tinuum. The spectrum represented in figure 3"5 shows regular spacings that are attributed to multiplet s p l i t t i n g . A large part of the spectrum has been assigned to s i x el e c t r o n i c band systems, Table 3"2. The prominent vibrational spacing in the upper electronic states i s about 1000 to 1070 cm,^  although a few hot bands with a vibra t i o n a l spacing of 845 ± 4 cm ' have been assigned. These hot bands only occurred when the ClO^ was flashed with low diluent pressures (0.25 torr of ClO^ in 10 torr Ar). When produced under isothermal conditions (0.25 torr of C10^ in 200 torr A r ) , no hot bands were observed. The complexity of the short wavelength transitions was noticeably reduced under the l a t t e r conditions. Table 3"2 l i s t s the wavelengths(X) and frequencies (v) of the vibrational t r a n s i t i o n s . Included in these Deslandres tables are the frequency separations o f the vibrational levels observed and the multiplet s p l i t t i n g . Spectra recorded under higher dispersion show f a i n t l y resolved double heads in each sub-band of the f i r s t two VUV electronic t r a n s i t i o n s . Figures 3 _ 6 and 3"7 show the absorption spectrum of the f i r s t two band systems in terms of e. The method by which these values were obtained is described in d e t a i l in the next section. -43-TABLE 3-2 Deslandres Tables of the E l e c t r o n i c Systems of CIO C - X System 0 -1 1 -1 X(nm) v(cm ) X(nm) v(cm ) 171-72 58 234 „ n 845 174.25 57 389 „, 170.78 58 554 ^ 844 173.28 57 710 ^ 1 058 1 057 168.66 59 292 167.75 59 611 ^ 1 055 1 052 165.71 60 347 -. 164.84 60 663 3 846 167.18 59 817 1 041 1 044 (1 035) 162.91 61 388 " " -162.07 61 707 5 y (855) 164.3 (60 852)a a - measurements in b rackets are f o r weak or over lapped bands - 4 4 -TABLE 3-2(Cont'd) Deslandres Tables of the Electronic Systems of CIO D - X System °' 0 -1 1 -1 X(nm) v ( c m ) X(nm) v ( c m ) 0 155.60 64 269 154.84 64 582 * 1 5 842 156.89 63 740 1 046 1 047 (1 037) 1 153.10 65 315 " _ 152.37 65 629 * 852 154.4 64 777. 1 042 1 042 (1 038) 2 150.70 66 357 " " 1 4 9 . 9 9 66 671 3 (856) 1 5 2 . 0 (65 815) b - ( 3 , 2 ) 151.5 nm (66 023) c m " 1 -45-TABLE 3-2(Cont'd) Deslandres Tables of the Electronic Systems of CIO E - X System X (nm) v(cm )^ X (nm) v(cm S 148.99 148.27 146.65 145.95 144.42 143.74 67 120 67 445 1 069 1 073 68 189 68 518 1 053 1 054 69 242 69 572 325 329 330 (835) 844 148.47 147.8 (67 354) 67 674 (320) 047 141.60 70 619 1 037 139-56 71 656 -46-TABLE 3-2(Cont'd) Deslandres Tables of the Electronic Systems of CIO F - X System u" 0 ' 0 _j X(nm) v(cm ) 0 145.20 68 869 , 1 9 144.55 69 181 i U 999 002 1 143.13 69 868 . 142.48 70 183 * * 947 953 141.21 70 815 140.58 71 163 983 977 139.28 71 798 138.67 72 113 321 315 -47-TABLE 3-2(Cont'd) Deslandres Tables of the Electronic Systems of CIO G - X System u" X(nm) v(cm ) 0 135 .66 73 705 135 .34 73 986 1 055 1 057 1 133 .76 74 760 133 .26 75 043 1 030 1 031 2 131 .94 75 790 131 .45 76 074 (988) 1 014 3 130 .25 (76 778) 129 • 72 77 088 281 283 284 (310) -48-TABLE 3-2(Cont'd) H - x(2n3/2) Assignment X(nm) v(cm ') 0,0 134.75 74 212 1,0 132.92 75 235 2,0 131.12 76 263 CIO Bands not yet a s s i g n e d L a b e l 0 X (nm) v(cm 1 148.63 67 281 2 136.37 73 329 3 136.17 73 436 4 134.33 74 445 5 133-59 74 858 6 132.38 75 539 7 129.43 77 260 8 129.29 77 345 9 128.74 77 677 10 128.08 78 076 11 127.16 78 643 12 126.45 79 080 c - unass igned bands are numbered in f i g u r e 3"5« -49-W A V E L E N G T H / n m 1 7 0 1 6 5 1 6 0 - J : I 1 F R E Q U E N C Y / c m - 1 x I O - 3 Figure 3"6 The C X system of CIO. -50--51-( i i ) Di scuss ion The assignment of t h i s spectrum to CIO is q u i t e s t r a i g h t f o r w a r d f o r a number of reasons ; (a) the i n i t i a l products o f the f l a s h p h o t o l y s i s of c h l o r i n e d i o x i d e a re oxygen, some of which i s v i b r a t i o n a 1 l y e x c i t e d , and C I O ^ ; (b) from the appearance of the f i r s t few sys tems , f i g u r e s 3"5, 3"6 and 3-7, i t i s r e a d i l y seen that the t r a n s i t i o n s i n v o l v e s p i n o r b i t comp-2 onents that at l e a s t i nvo l ve a ground s t a t e n (case a) sys tem; (c) the AG '^ o f the hot bands was 845±4 cm ^ which i s in very good agreement w i t h the va lue of AG^ = 853±25 cm ^ c a l c u l a t e d from the v i b r a t i o n a l c o n s t a n t s f o r CIO which were repor ted by P o r t e r ; (d) the k i n e t i c behav ior of the bands i s the same f o r v a r y i n g i n i t i a l c o n c e n t r a t i o n s of C lO^ , ( the k i n e t i c behav ior of a number of these new bands has been shown to be the same as that observed by Basco and D o g r a ^ in the A-«-X system of CIO (next s e c t i o n ) ) ; .39 have y i e l d e d the same t r a n s i e n t spectrum (e) p r e l i m i n a r y r e s u l t s on the f l a s h p h o t o l y s i s of Cl^O in the VUV (a) Nature of the E l e c t r o n i c T r a n s i t i o n 2 The ground s t a t e of CIO has been i d e n t i f i e d as a n. s t a t e w i t h the f o l l o w i n g e l e c t r o n i c c o n f i g u r a t i o n from which the e x c i t e d s t a t e s are 49 determined a s : - 5 2 -Electron Configurations of CIO State KKL a a* a TT _ a. —. J -TT ** 0 " x 2 n . 2 2 2 k 3 0 A 2IIj 2 2 2 3 A 0 B 2 I + 2 2 1 k 0 (not known) Rydberg States 2 2 2 4 2 + kso kpakp-n kdokd-nkdS ton core + Rydberg o r b i t a l s The ion core is isoelectronic with SO and the order of i t s states is ^Z , ^ A, 1 Z + in terms of increasing electronic energy. These states are 4 - 2 - 2 + 2 transformed to Z , Z , Z and A with the addition of an electron to 2 2 the kso o r b i t a l . Thus, since a Z - II. i s the f i r s t f u l l y allowed t r a n s i t i o n of the four i t is expected to be the f i r s t band system observed for CIO. With the exception of the G-«-X and the H-*-X systems, the spin o r b i t s p l i t t i n g in the new VUV systems is 318 ± 5 cm.^  The l i m i t s on this value are one standard deviation of fourteen measurements and is within the error l i m i t s of our measurements. Thus, i t appears l i k e l y that the 2 -1 upper states C, D, E, and F are Z and that A = -318 ± 5 cm for the ground state. The multiplet s p l i t t i n g in the G«-X system is ca. 284 cm \ This value cannot be associated with the values for the other states since i t i s outside the error l i m i t s of the low wavelength measurements. 2 2 Thus, i t appears that the G state is either a n. or A., and the agree-ment of this value of the multiplet s p l i t t i n g and the 282 cm * value -53-47 reported by Carrington et al for -A of the ground state is purely coi nci denta1. I f , on the other hand, the value for the.ground state reported by Carrington et al is accepted, i t would be necessary to account for the 2 2 f i r s t four Rydberg states of CIO as regular n or A. As noted e a r l i e r the multiplet bands of the f i r s t two systems are double headed. This 2 2 2 2 p r o f i l e is expected for both A - II and Z - H transitions but not 2 2 50 for II - n since in this case the Q head is very weak . Hence, using the value of A =-282 cm ^ reported by Carrington et al would 2 2 require that the two strongest VUV transitions be assigned as A r - II. t r a n s i t i o n s . We believe that the former assignments are more l i k e l y and, as a r e s u l t , the constants given in Table 3 -3 are based on these. No multiplet s p l i t t i n g was observed for the H-f-X system although i t is quite possible that these subbands were obscured by the increasing complexity of the spectrum in the low wavelength region. In Table 3"4 is found a l i s t of the observed bands that have not yet been assigned to any vibrati o n a l scheme. These bands are not well resolved and in many cases are weaker than the assigned bands. It is pos-2 2 s i b l e that some belong to II. - II. transitions for which l i t t l e or no s p l i t t i n g may be observed. Because of the number of bands in the short wavelength region, multiplet members may be e n t i r e l y overlapped. Since there appeared to be no long progressions, no attempt was made to group these short wavelength bands. Attempts to assign some of these t r a n s i -tions to a Rydberg series were not successful. (b) Vibrational Analysis Except for the F-*-X system, a l l of the vibrational schemes appear -54-TABLE 3-3 CIO S p e c t r o s c o p i c Data (cm )^ S t a t e T - A u UJ x e e e e H -74 131 A G ^ I 023 G 73 782 35 1 066 5 F 2 E -69 110 0 002 E 2 E 67 338 0 1 070 a 4 D2E 64 484 0 1 052 3 C 2 E B V 58 451 ? 0 1 062 3 A 2 n . i 31 050b 126° 557 d 11 d x 2 n. • 0 318 859 7.5(6 a - assumes v=0 l e v e l p e r t u r b e d , hence not used in c a l c u l a t i n g v i b r a t i o n a l cons tants b - based on our e x t r a p o l a t e d V q q from the v"=0 p r o g r e s s i o n , 30899 cm ^ ( in good agreement w i t h P o r t e r ) 2 2 c - based on P o r t e r ' s measurements of the A n . - X II. m u l t i p l e t s p l i t t i n g and our v a l u e of the sp in o r b i t c o u p l i n g constant (A) f o r X 2 d - G. P o r t e r(l950) - the va lue of to x in b rackets i s our e e r e v i s e d va lue as e x p l a i n e d in the t e x t . It was used to c a l c u l a t e u . e -55-to be unambiguously a s s i g n e d . Th is one e x c e p t i o n cou ld be the r e s u l t o f two shor t p r o g r e s s i o n s be longing to d i f f e r e n t upper s t a t e s but probably i s one p r o g r e s s i o n in which the 1,0 and 2,0 bands are p e r t u r b e d . Th is same e x p l a n a t i o n cou ld be t rue f o r the s l i g h t l y l a r g e r s e p a r a t i o n of the 0,0 and 1,0 bands in the E - X system. The ground s t a t e v i b r a t i o n a l cons tants repor ted in Tab le 3"3 make use o f our va lue of AG1,1 and a va lue of co x which has been r e v i s e d i e e 2 s l i g h t l y over tha t g iven by P o r t e r as d e s c r i b e d below. The v a l u e s f o r 2 2 the v i b r a t i o n a l cons tants f o r the A II. s t a t e are those g i ven by P o r t e r . 3 Dur ie and Ramsay have g iven an equat ion which p r e d i c t s the f r e q u e n c i e s . . 2 2 of the v"=0 p r o g r e s s i o n of the A II^^^X n^/2 t r a n s i t i o n . It a p p e a r s , however, that t h i s equat ion i s the best numerical f i t of t h e i r d a t a . As s u c h , the c o e f f i c i e n t s of t h i s equat ion cannot n e c e s s a r i l y be a s s o c -i a t e d w i t h v i b r a t i o n a l cons tants f o r the upper s t a t e s i n c e the v 00 band o f t h i s e l e c t r o n i c t r a n s i t i o n has not been l o c a t e d w i t h any degree of c e r t a i n t y . (The lowest band tha t P o r t e r observed was the 4,0 and Dur ie and Ramsay observed on ly as low as the 5,0.) 17 39 From the k i n e t i c study of C10 in our l a b o r a t o r y , numerous p l a t e s on which the spectrum of h igh c o n c e n t r a t i o n s of C10 have been recorded have enabled us to observe as low as the 2,0 band in the A -X system. Our measurements of the 2,0 and 3,0 bands (31975 and 32480 cm ^ r e s p e c t i v e l y ) are in good agreement w i t h P o r t e r ' s e x t r a p o l a t e d v a l u e s . Our e x t r a p o l a t e d va lue of v and the measurements of the v 00 2,0 and 3,0 bands p lus the f i r s t s i x t e e n bands of the v"=0 p r o g r e s s i o n -56-which were measured by Porter, were f i t t e d of a cubic equation by the method of least squares. The co e f f i c i e n t s were, within our experimental 2 e r r o r , the same as those given by Porter . Thus, since we have a -1 " r e l i a b l e v Q 0 ± 10 cm and AG^, i t is possible to check the accuracy of the value of wgx^ reported by Porter using these data to reproduce 45 the frequencies of the emission bands measured by Pannetier and Gaydon . Poor agreement was obtained with Porter's value of to x "=7.5 cm ^ but e e when a value of 6.8 cm ^ was used the f i t was almost exact. Values of u e x e < 6 or > '8 cm ' produced very poor f i t s . Hence, our value of (859 cm ) should have a maximum error of ±2 cm as a result of inaccuracy in w e x e -( i i i ) Conclus ion It is evident from the absorption curves in figures 3"6 and 3 _ 7 that the vibrat i o n a l structure of CIO is well resolved and that under our dispersion the structure is sharp. The spectrum is free , for the most part, from CIO2 spectral interference and is e n t i r e l y free of the Schumann-Runge (B +• X E ) system of 0^. Thus conditions for a CIO kin e t i c study in the VUV are very favourable. The lack of CIO overlap with 0^ also indicates that a low vibrational level energy d i s t r i b u t i o n deter-mination is possible with this system. -57-III The Recombinat ion of CIO Fol lowed by K i n e t i c Spectroscopy in the VUV Because o f the t r a n s i e n t nature o f CIO, i t s molar e x t i n c t i o n c o e f f i c i e n t s can on ly be determined by k i n e t i c a n a l y s i s . Values of have been d e t e r -mined p r e v i o u s l y on the banded s t r u c t u r e and in the continuum at 257-7 nm by two d i f f e r e n t methods; one employing f l a s h p h o t o l y s i s and the o ther a f a s t f l o w r e a c t o r . The former method was used by Lipscomb et a l ^ and was l a t e r r e f i n e d by D o g r a ^ and Basco and Dogra^ 7 who showed that the method was v a l i d on ly under very l i m i t e d exper imenta l c o n d i t i o n s . E s s e n t i a l l y the same method 1 2 was used by Edgecombe e t a l , a l though they d i d not e x p l i c i t l y repor t a va lue of EQJQ- T h i s method was l a t e r r e f i n e d by Basco and D o g r a ( 1 9 7 1 b ) t o de -termine £QJQ from the f l a s h p h o t o l y s i s of C ^ O . The second method was used by Clyne and C o x o n ^ ' ^ in t h e i r study of the r e a c t i o n of c h l o r i n e atoms w i th c h l o r i n e d i o x i d e . They o b t a i n e d £QJQ by t i t r a t i n g NO or 0 w i t h CIO having f i r s t shown that these r e a c t i o n s were f a s t and s t o i c h i o m e t r i c . The va lues ob ta ined by these two methods are in A J A.17 good agreement. The f o l l o w i n g is a b r i e f summary of the f l a s h p h o t o l y s i s method d e s c r i b e d by Basco and D o g r a ! 7 Two b a s i c assumptions are i n v o l v e d ; (a) that the change in CIO2 c o n c e n t r a t i o n f o l l o w i n g f l a s h p h o t o l y s i s is the i n i t i a l CIO concen-t r a t i o n , AfClO^] = [C IO] . ; and (b) that the i n i t i a l CIO c o n c e n t r a t i o n is de -termined by the shor t e x t r a p o l a t i o n of the second order p l o t of CIO decay back to zero r e a c t i o n t i m e . These assumptions are o n l y v a l i d under c e r t a i n s p e c i f i c c o n d i t i o n s when the f o l l o w i n g three processes predominate : -58-C10 2 + hv -> CIO + 0 C10 2 + 0 -> CIO + 0 2 k3=3.0 x 1 0 1 0 M~ 1s _ 1 ^ 1 7 ^ 2C10 + 0 2 + C l 2 k 1 = 2.7 x IO 7 M _ 1 s" 1 ^ 1 7 ^ The c o n d i t i o n s r e q u i r e d are low f l a s h energy and the use of p h o t o l y s i n g r a d i a t i o n of wavelengths g r e a t e r than 300 nm. The former c o n d i t i o n i s nec -essary in o rder to l i m i t the amount of pr imary p h o t o l y s i s o f C 1 0 2 to less than 50% and yet leave s u f f i c i e n t C10 2 a f t e r f l a s h p h o t o l y s i s to enable a c c u r a t e measurement. Th is prevents the process o + cio -> b 2 + ci k/,=7.o x i o 9 M _ 1 S _ 1 from i n t e r f e r r i n g s i g n i f i c a n t l y . The l a t t e r c o n d i t i o n ensures that there i s no secondary p h o t o l y s i s of C10. Th is above method was used in t h i s study to o b t a i n the e x t i n c t i o n c o e f -f i c i e n t s of some of the VUV bands of CIO. ( i ) E x t i n c t i o n C o e f f i c i e n t s of CIO To meet the above c o n d i t i o n s , 0 . 2 to 0 .3 t o r r of C10^ was f l a s h photo -l ysed w i t h a f l a s h energy of 750 J (480 J in one c a s e , Table 3 - 4 ) . To avo id any h e a t i n g , a l a r g e excess of d i l u e n t was added to the CIO,,. In p a r t i c -u l a r the two r a t i o s of Ar to C10^ which were used were 1400:1 and 8 0 0 : 1 . In o rder to f i l t e r out r a d i a t i o n w i t h wavelengths less than 300 nm, a Pyrex r e a c t i o n vesse l was wrapped w i t h a 1 mm layer of c l e a r c e l l u l o i d which has an o p t i c a l d e n s i t y of g rea te r than 2 at 300 nm, 0.1 at 320 nm and e s s e n -t i a l l y 0 at 3^0 nm. The o p t i c a l d e n s i t y measurements were made on a Cary -59-Model 14 scanning spec t rometer . F i g u r e 3"8 shows the decay of both CIO and ClO^ as a f u n c t i o n of t i m e . P l o t s of r e c i p r o c a l o p t i c a l d e n s i t y of th ree CIO bands (0,0; 1,0; and 2 2,0 o f the C-<-X 1^/2 s V s t e m ) v s time are l i n e a r over the range 0 to c a . 6 ms. A r e p r e s e n t a t i v e p l o t i s shown in f i g u r e 3"9 ( p l a t e 220 L X I l ) . The r e s u l t s of a l l the runs used to o b t a i n E ^ J Q a re l i s t e d in Table 3~A. From the manner in which the p l o t s were drawn, the s l o p e of the l i n e i s k^/eZ, where kj is the second order ra te c o n s t a n t , e i s the molar e x t i n c -t i o n c o e f f i c i e n t and I i s the a b s o r p t i o n path length in cm. The i n i t i a l C10 o p t i c a l d e n s i t y (OD^^) i s the r e c i p r o c a l of the i n t e r c e p t of the p l o t , and A f c i O ^ ] i s o b t a i n e d by measuring the r e s i d u a l ClO^ c o n c e n t r a t i o n by means of e „ , „ at 160.1 nm. Severa l of these l a t t e r measurements were a v a i l a b l e CIO2 s i n c e the r e s i d u a l ClO^ c o n c e n t r a t i o n remained e s s e n t i a l l y cons tant over the range 0.1 to 500 ms. From these d a t a , va lues of the e x t i n c t i o n c o e f f i c i e n t s of the i n d i v i d u a l C10 bands a re determined as w e l l as independent measurements of k^  ( independ-ent of the UV systems of C10 and 0102). The v a l u e of k^  was found to be 3.0k ± 0.22 x 10 7 M ' s ^ in good agreement w i t h Basco and Dogra^ 7 and the va lues of e^]Q were measured as 3820, 3790 and 1900 M 1cm 1 ±5% f o r the 0,0; 1,0; and 2,0 bands of the C X system r e s p e c t i v e l y . These e x t i n c t i o n c o e f -f i c i e n t s were used to put the a b s o r p t i o n curves in f i g u r e s 3~6 and 3"7 on an a b s o l u t e s c a l e . The assumption that B e e r ' s law i s obeyed i s i n c o r p o r a t e d i n t o the c o n -v e r s i o n of i n i t i a l C10 o p t i c a l d e n s i t y to G ^ J Q - T h i s assumption was b e l i e v e d to be v a l i d f o r a number of reasons . The bands are w e l l r e s o l v e d and are -60-170.8 nm 160-1 F i g u r e 3 _8 Decay of CIO produced from the f l a s h p h o t o l y s i s of 0.3 t o r r of C10 2; 80% t o t a l d e c o m p o s i t i o n ; f i l t e r e d The apparent low wavelength continuum is the r e s u l t of s i l i c a d e p o s i t on the l e n s . -61-Figure 3"9 Second order plot of CIO decay for the f i r s t 6 ms. From plate 220 LXI I -62-TABLE 3-4 Summary of £QJQ R e s u l t s P l a t e Band S lope 0 D r 1 . A[C10.](M) E _ , _ X 10~ 3 k. x 1 0 ~ 7 220 O X 2 n 3 / 2 cm s " 1 C 1 ° x l O 5 2 ^ V w 1 1 M ^ s " 1 XLIX 0,0 800 0.411 1.08 3.81 3.05 1,0 858 0.387 1.08 3-58 3.07 2,0 1 820 0.198 1.08 1.75 3.17 LX 0,0 819 0.363 0.95 3.84 3.14 1,0 708 0.377 0.95 3-98 2.82 2,0 1 403 0.182 0.95 1.92 2.69 LXI 0,0 727 0.344 0.91 3-77 2.73 1,0 776 0.332 0.91 3.63 2.82 2,0 1 446 0.179 0.91 1.96 2.82 LXI 1 0,0 846 0.365 0.95 3.85 3.26 1,0 828 0.376 0.95 3-97 3.29 2,0 1 650 0.185 0.95 1.95 3.22 LV 0,0 840 0.470 1.23 3.82 3-21 1,0 875 0.468 1.23 3.79 3-31 2,0 1 555 0.238 1.23 1.92 2.98 0 , 0 1 ,0 2 , 0 Condi t ions P l a t e p c i o 2 PAr A l C 1 0 2 ] F i 1 t e r F l a s h 220 t o r r t o r r t o r r Energy (J) XLIX 0.28 420 0.20 P+Ca 750 LX 0 .20 280 0.18 P+C 750 LXI 0 .20 280 0.17 P+C 750 LXI 1 0 .20 280 0.18 P+C 750 LV 0 . 3 0 240 0.23 P o n l y 480 Average E ^ J Q Average k^  3.82 ± 0.03 x 10 3 3.04 ± 0.22 x 10 7 3.79 ± 0.18 x 10 3 1.90 ± 0.09 x 10 3 a - P (Pyrex r e a c t i o n v e s s e l ) C ( l mm l a y e r of c l e a r c e l l u l o i d ) -63-f r e e of i n t e r f e r e n c e from any o ther s p e c i e s . C h l o r i n e d i o x i d e bands in the E -«-X system, which are sharp and have s i m i l a r band widths as CIO, have been shown to obey B e e r ' s law. F i n a l l y , the £ Q J Q f o r the VUV bands are constant w i t h i n exper imenta l e r r o r f o r d i f f e r e n t va lues of A[C102]> Tab le 3"4. ( i i ) Extended K i n e t i c R e s u l t s on CIO Decay A p r e l i m i n a r y k i n e t i c study of the CIO r a d i c a l produced from CIO2 was undertaken by m o n i t o r i n g the VUV bands over an extended time range (by us ing the delay t r i g g e r mode on the T e k t r o n i x Model 5^ 7 o s c i l l o s c o p e as d e s c r i b e d in Chapter 2). T h i s study has shown that the k i n e t i c behav ior o f CIO in t h i s system is more compl i ca ted than p r e v i o u s l y thought . B r i e f l y , the second o rder nature o f the CIO recombinat ion is no longer obeyed above c a . 10 ms. Th is i s shown in a p l o t o f O D ^ Q V S t and 1/O D ^ J Q in f i g u r e 3~ 10 ( the p o i n t s on the l i n e a r p a r t of the 1/OD vs t p l o t are those shown in f i g u r e 3"9). I t can a l s o be seen in f i g u r e 3 _8, where the CIO spectrum i s s t i l l v i s i b l e a f t e r one second. Fur thermore , an attempt to l i n k the r e s u l t s of f l a s h work w i t h those of the f l ow d i s c h a r g e work of Clyne and C o x o n ^ ' 1 ^ and Clyne and 1 g White by f l a s h i n g C10^ + Ar mix tu res a t low t o t a l p r e s s u r e has shown that there i s p o s s i b l y a t h i r d body dependence below c a . 100 t o r r but the degree and ex tent of t h i s dependence i s s t i l l u n c e r t a i n . Needless to s a y , these r e s u l t s a re not c o n c l u s i v e and much f u r t h e r w o r k ' i s needed before the problem is s u f f i c i e n t l y w e l l d e f i n e d . 50 0.32-I 0.28-0.24-0.20 OQ 0 . 1 6 -C L O 0.12-0.08 0.04-200 300 t /ms 400 500 o 600 vs t (•) Figure 3-10 A plot of a l l the points on figure 3~8 (plate 220 LXIl) both as 0D C 1 Q and as 1/0D C 1 Q vs t (O). The so l i d l i n e and the dashed line predict the behavior of 0D C ] Q and 1/0D C ] Q respectively for the rate expression 1/ [C1 Oj -1 / [C103 .=3x1 0 7t. -65-( i i i ) D iscuss ion A l though i t appears that the mechanism fo r the decay of CIO in the presence of CIO2 i s not f u l l y unders tood , the method by which i t s e x t i n c -t i o n c o e f f i c i e n t s are o b t a i n e d does seem to be v a l i d s i n c e constant va lues of EQJQ are c o n s i s t e n t l y o b t a i n e d when the c o n d i t i o n s d e f i n e d by Basco and D o g r a 1 7 a r e met, Table 3"4. Th is is not unreasonable when one c o n s i d e r s that EQJQ ' s determined from the i n t e r c e p t of the second order p l o t o f CIO decay which i s l i n e a r from 100 ys to at l e a s t 6 ms. Even i f f u t u r e r e s u l t s show that the ra te of decay i s not s t r i c t l y second o rder a t constant t o t a l p r e s s u r e and s h o r t t i m e s , i t is s u f f i c i e n t l y c l o s e that i t a c c u r a t e l y d e f i n e s the shor t e x t r a p o l a t i o n to zero r e a c t i o n t i m e . The assumption that A[C1O^J = [ C 1 0 J . has been shown by Basco and D o g r a 1 7 to be a good one on the b a s i s that EQJQ does not change f o r v a r y i n g AfClO^J and that CIO2 c o n c e n t r a t i o n remains e s s e n t i a l l y constant a f t e r the photo -l y t i c f l a s h . These f i n d i n g s have been conf i rmed in t h i s study which uses the 160 .1 nm band as a measure of the C10^ c o n c e n t r a t i o n . Study of t h i s i n t e r e s t i n g r a d i c a l w i l l no doubt cont inue on the b a s i s of these r e s u l t s . P r e l i m i n a r y s t u d i e s in the VUV have shown that C^O has d i s c r e t e s t r u c t u r e in a wavelength reg ion a c c e s s i b l e to our spect rograph (160 to 175 nm) and that CIO s u f f e r s no severe i n t e r f e r e n c e from the C^O cont inuum. Th is w i l l a l l o w a study of CIO in the VUV from another source f r e e of C102• In a d d i t i o n , a s p e c t r o p h o t o m e t r i c technique is now in o p e r a t i o n •30 in our l a b o r a t o r y which permi ts cont inuous m o n i t o r i n g of one wavelength over the e n t i r e l i f e t i m e of the t r a n s i e n t . Use of t h i s technique on the CIO r a d i c a l would g r e a t l y f a c i l i t a t e a study of i t s k i n e t i c behavior at long t imes, CHAPTER 4 THE PRODUCTION OF VI BRAT IONALLY EXCITED OXYGEN FROM NITROGEN DIOXIDE AND CHLORINE DIOXIDE I I n t r o d u c t i o n The background l e a d i n g to t h i s study o f v i b r a t i o n a 1 1 y e x c i t e d oxygen has been reviewed in Chapter 1. In t h i s c h a p t e r , the exper imenta l method of d e t e r m i n i n g p o p u l a t i o n d i s t r i b u t i o n s from HO^ w i l l be d e s c r i b e d and both p o p u l a t i o n and energy d i s t r i b u t i o n s g i v e n . The same procedure w i l l be f o l l o w e d in r e p o r t i n g the d i s t r i b u t i o n of the u"=1 to 7 l e v e l s t of 0^ produced from the f l a s h p h o t o l y s i s of C lO^. T h i s study extends and complements those done by S . K . Dogra(1970) 1^ and Basco and Dogra (1971 ) 1 7 who ob ta ined r e l a t i v e p o p u l a t i o n e s t i m a t e s f o r v i b r a t i o n a l l e v e l s g r e a t e r than f i v e from the f l a s h p h o t o l y s i s of C lO^ ; 26 and by R.D. Morse(l969) who o b t a i n e d r e l a t i v e p o p u l a t i o n s of the l e v e l s 6 to 13 from the f l a s h p h o t o l y s i s of N0„ . -67-II The P r o d u c t i o n o f 0^  from the F l a s h P h o t o l y s i s of NO^ ( i ) Treatment of I n t e n s i t y Measurements Most of the f o l l o w i n g NO^ r e s u l t s were o b t a i n e d on a c o n v e n t i o n a l f l a s h p h o t o l y s i s apparatus employing a J a r r e l1 Ash 3-4 meter s p e c t r o -g raph . The spect rograph s l i t w idth was kept s u f f i c i e n t l y narrow to a l l o w the o b s e r v a t i o n of the t rue co l 1 i s i o n a l l y broadened r o t a t i o n a l l i n e s (approx imate ly 1/3 of the h a l f w i d t h of the co l 1 isiona11y broadened 1 ine) . C h a r a c t e r i s t i c curves of the photograph ic p l a t e were p l o t t e d f o r the band p o s i t i o n s of the e x c i t e d oxygen o b s e r v e d . As shown e a r l i e r , these curves set the l i m i t s o f r e l i a b l e measurements by t h e i r range and degree of l i n e a r i t y . T h i s , coupled w i t h the incomplete p h o t o l y s i s range of NC^j r e s t r i c t e d the c o n d i t i o n s under which a d i s t r i b u t i o n cou ld be o b t a i n e d . R e l a t i v e v i b r a t i o n a l l e v e l p o p u l a t i o n s were o b t a i n e d by measuring the peak o p t i c a l d e n s i t y of the band head r o t a t i o n a l l i n e s f o r a t l e a s t two bands of each u" p r o g r e s s i o n and then w e i g h t i n g them by t h e i r r e s p e c t i v e Franck Condon f a c t o r s . These va lues were repor ted p r e v i o u s l y as p o p u l a t i o n r a t i o s : Nvj"+1 = qu',u" 'u^vV'+l N u " qu{,u"+1 'o^.u" where N r e f e r s to the p o p u l a t i o n of the u" and uM+1 l e v e l s , q , ,, i s -68-the Franck Condon f a c t o r and I , ,, i s the o p t i c a l d e n s i t y of the band ( u ' , u " ) . u u The s u b s c r i p t s 1 and 2 on u 1 a re s imp ly l a b e l s to i n d i c a t e that the u 1 va lues are not n e c e s s a r i l y the same fo r the two bands being measured. The v a l i d i t y of us ing peak he ights o f the band head r o t a t i o n a l l i n e s was determined by the h a l f path method. Th is invo l ves comparing the peak he ights of the l i n e s obta ined through the f u l l path length of the r e a c t i o n v e s s e l w i t h those ob ta ined through h a l f the path length ( N o r r i s h , P o r t e r C O and T h r u s h , 1953). For p l a t e d e n s i t y , measured as peak h e i g h t s (h) o f the r o t a t i o n a l l i n e s , to be d i r e c t l y p r o p o r t i o n a l to the v i b r a t i o n a l p o p u l a t i o n (N m ) , the s l o p e of a p l o t of h vs h, should be 2. U 2 h u ' , u " _ ' u ' , u " _ N u " _ 2  h i u \ u " ' i u ' . u " * N u " A r e p r e s e n t a t i v e p l o t of the va lues ob ta ined f o r the (3,6), (4,6), (2,8), (0,9) and (1,10) bands i s shown in f i g u r e 4-1. Except f o r the (4,6) and the (4,7) bands, a l l had s lopes ranging between 1.9 and 2.0. The two excep -t i o n s had s lopes of 2.3 and 2.4 r e s p e c t i v e l y . Th is i s probably due to 0^ and NO2 band o v e r l a p . T r a n s i t i o n p r o b a b i l i t i e s f o r the l e v e l s 9 to 18 were determined by 53 Treanor and Wurster (i960) by shock h e a t i n g oxygen and Franck Condon f a c t o r s were c a l c u l a t e d by N icho l l s (1960)^ These were used to conver t f O2 o p t i c a l d e n s i t y to r e l a t i v e p o p u l a t i o n s . Treanor and W u r s t e r ' s a r r a y of t r a n s i t i o n p r o b a b i l i t i e s was used because i t s va lues cou ld be expressed in terms of a b s o l u t e o s c i l l a t o r s t r e n g t h s . In a d d i t i o n , t h e i r a r r a y gave b e t t e r i n t e r n a l agreement f o r the two or th ree bands of each u" p r o g r e s s i o n - 6 9 -0 ( 3 , 6 ) © ( 4 , 6 ) • (2,8) 1 3 (4,7) A ( 0 , 9 ) A ( 1 , 9 ) + (1,10) 0 . 5 - i h 0 0 .05 0 .10 0 .15 0 .20 h i f F i g u r e 4-1 Ha l f path p l o t f o r some 0^ l e v e l s produced in the f l a s h p h o t o l y s i s of HO^. h and h, are the peak h e i g h t s of the f u l l and h a l f 2 paths in terms of p l a t e d e n s i t y . The s o l i d l i n e represents a s lope of 2. - 7 0 -measured, i . e . the t r a n s i t i o n p r o b a b i l i t i e s f o r the bands of a u" p r o g r e s s i o n should vary in the same r a t i o as the i n t e n s i t i e s of the bands. U n f o r t u n a t e l y , t h e i r a r ray does not cover the whole range of l e v e l s measured in our work and , as a r e s u l t , the Franck Condon a r r a y of N i c h o l l s was used to complete the range covered . The i n t e r n a l agreement of t h i s second a r r a y was not a p p r e c i a b l y d i f f e r e n t from the f i r s t . A second e x t e n s i v e a r ray of Franck Condon f a c t o r s c a l c u l a t e d by H a r r i s , B l a c k l e d g e and Generosa (1969)^^ became a v a i l a b l e d u r i n g p a r t of t h i s study of 0 ^ . I t i s based on a more r e a l i s t i c p o t e n t i a l model ( R y d b e r g - K l e i n -Rees) f o r h igh v i b r a t i o n a l quantum numbers than the N i c h o l l s a r r a y c a l c u l a t e d on the b a s i s of a Morse p o t e n t i a l . Comparison of the two a r r a y s , in the form of r a t i o s , fo r the range of l e v e l s measured in t h i s work show that there i s no s i g n i f i c a n t v a r i a t i o n , w i t h i n 10%, between the two. The p o p u l a t i o n r a t i o s us ing N i c h o l l s va lues were thus not r e c a l c u l a t e d us ing the newer a r r a y . A b s o l u t e p o p u l a t i o n s were determined by c o n v e r t i n g Treanor and W u r s t e r ' s t r a n s i t i o n p r o b a b i l i t i e s (Re^ ,^ , , ) 2 to band o s c i l l a t o r s t r e n g t h s through the r e l a t i o n s h i p s g i ven in 3 " 1 - As b e f o r e , the i n t e g r a t i o n i s c a r r i e d out by measuring the area of the e n t i r e band w i t h a p l a n i m e t e r . The i n t e g r a t e d a b s o r p t i o n of the (1,9) and (0,12) bands was measured and the a b s o l u t e c o n c e n t r a t i o n of these two l e v e l s o b t a i n e d . Using the c o n -c e n t r a t i o n of the n i n t h l e v e l and r e l a t i v e p o p u l a t i o n s , o b t a i n e d from the peak he ight measurements, a b s o l u t e p o p u l a t i o n s were o b t a i n e d f o r the remain -ing l e v e l s , i n c l u d i n g the t w e l f t h . An i n d i c a t i o n of the accuracy of the method was o b t a i n e d by comparing - 7 1 -the above v a l u e f o r the p o p u l a t i o n o f t h e t w e l f t h l e v e l w i t h the one de-t e r m i n e d d i r e c t l y from the i n t e g r a t e d a b s o r p t i o n . These two v a l u e s were found t o a g r e e w i t h i n 30% w h i c h i s q u i t e a c c e p t a b l e when one c o n s i d e r s the e r r o r s i n v o l v e d i n both i n t e n s i t y measurements and e x p e r i m e n t a l l y d e t e r -m i n i n g t r a n s i t i o n p r o b a b i l i t i e s . T h i s l a s t r e s u l t c o n f i r m s t h e h a l f p a t h r e s u l t s , t h a t peak h e i g h t i n t e n s i t i e s a r e t o a good a p p r o x i m a t i o n a measure 4-o f r e l a t i v e [ 0 ' ] . ( i i ) R e s u l t s When m i x t u r e s o f NO,, and a l a r g e e x c e s s o f argon were f l a s h photoly-sed i n a P y r e x r e a c t i o n v e s s e l , the s p e c t r u m o f v i b r a t i o n a 1 l y e x c i t e d oxygen (u"=4 t o 15) was o b s e r v e d by means o f i t s Schumann—Runge a b s o r p t i o n system. t t M o l e c u l e s i n l e v e l s u"=12 t o 15, w h i c h a r e denoted as 0^ > p o s s e s s v i b r a t i o n a l e n e rgy i n e x c e s s o f t h e heat o f r e a c t i o n : 0 + N 0 2 NO + 0 2 AH=-201 kJ/mole by up t o 61 k J / m o l e . A d e t a i l e d d e s c r i p t i o n o f t h i s has been g i v e n e l s e -• 26 where. t t 0 2 c o u l d not be produced when t h e p h o t o l y t i c f l a s h was f i l t e r e d by a C o r n i n g g l a s s f i l t e r (C.S. 7"54) w h i c h q u i t e c o n v e n i e n t l y a b s o r b s r a d i a t i o n below the d i s s o c i a t i o n l i m i t o f the N0 2 v i s i b l e s ystem (25130 cm ^) y e t has a p p r o x i m a t e l y 80% t r a n s m i s s i o n above t h e l i m i t where t h e measured quantum y i e l d s f o r t h e r e a c t i o n : NO + hv •* NO + 0 hv=25000 cm" 1 (kOO nm) -72-are c lose to unity (Leighton, 1961). Experiments with and without the Corning 7"54 f i l t e r were compared on the basis of an equal degree of photo-l y s i s of NCy The amount of oxygen produced, as measured by i t s op t i ca l dens i ty , was d i r e c t l y proport ional to the amount of NC^  photolysed, whereas the rate of re laxat ion of 0^ was proport ional to the amount of undecomposed NO^ remaining a f te r the f l a s h . There appeared to be no pressure ef fect of argon on the rate of 0^ re laxat ion over the pressure range 50 to 600 t o r r . No pressure broadening of the 0^ spectrum over t h i s range was noted. The condit ions under which the d i s t r i b u t i o n s were obtained were as f o l l o w s : a mixture of 1 tor r of N0 2 and 200 tor r of Ar was f lashed in a Pyrex reaction vessel (to avoid 0( 1D) production in the primary step) using f l a s h energies of 1.3 or 2.1 kJ g iv ing 25 or 37% primary photolys is respec-t i v e l y . Another d i s t r i b u t i o n was obtained for 25% primary photolysis in which the photolys is lamps were f i l t e r e d (C.S. 7~5k). These were found to be the optimum condit ions for measuring the concen-t rat ions of the oxygen levels with t ime, over the v ib ra t iona l quantum range u"=6 to 12 in the u n f i l t e r e d experiments and 6 to 11 with the f i l t e r i n . The u"=6 level was the lowest measurable on the 3-4 meter spectrograph. A number of prel iminary experiments using the VUV instrument enabled the ex-tension of the 25% primary photolys is d i s t r i b u t i o n (unf i l tered) to u"=4, below which the spectrum of NO and undissociated N0 2 interferes with the measurements. Only th i s d i s t r i b u t i o n could be extended to u"=A due to experimental d i f f i c u l t i e s . The e f f i c i e n c y of the VUV apparatus is not as great as the conventional apparatus which was used and, hence, i t was only -73-possible to decompose a maximum of 25% of the NO^ - In order to f i l t e r the VUV experiments, semi-circular f i l t e r s were required. These were not a v a i l -able commercially and attempts to bend the 7"54 f i l t e r were not successful. Thus, the f i l t e r e d f lash photolysis of N0 2 was not possible on the VUV apparatus. No quantitative intensity measurements could be made for the levels u"=13 to 15 because they were weak and decayed rapidly. The rest of the levels followed a f i r s t order decay between 20 and 100 us, as shown in f i g -ures 4-2 and 4 - 3 . In Table 4-1 are given the decay half li v e s for the levels 6 to 12(11). Plots of loglO^] vs t of the levels 7 and 8 for two pressures of N0 2 (0 . 5 and 1 .6 torr) are included to show the dependence of 0* decay on [N0 2]J: (concentration remaining after the f lash), figure 4 -4 . It was found that only these two levels could be accurately measured for a l l the p a r t i a l pressures of N0 2 used since high [N0 2] and low primary photolysis gives N0 2 spectral interference with u"=6; and low [N0 2] does not y i e l d s u f f i c i e n t excited oxygen in the levels greater than 8. Table 4-2 l i s t s the t second order quenching constants of 0 2 by N0 2 for each level measured. For times less than 20 us,(i.e. times within the l i f e t i m e of the photo-l y t i c flash) population ratios of the levels ( N u M + j ) / N y n remained e s s e n t i a l l y invariant, indicating that no other fast relaxation process is occuring dur-ing the f l a s h . Based on (a) the length ( p a r t i c u l a r l y in the 37% primary photolysis case) of the half l i f e compared to the time over the extrapolation, (b) the direct dependence of 0 2 relaxation on N0 2 > and (c) the invariance of the ratios at short times within the duration of the photolytic f l a s h , the extrapolation of the f i r s t order decay curves in figures 4-2 and 4-3 -76-TABLE 4-1 Relaxation of 0^ expressed in terms of half 1 i ves s _ l x 1 0 6 ) Flash INOJ %PPb u"=4 5 6 7 8 9 10 11 12 Energy / c -(J) x 1 0 5 (M)a 2100 5-38 37 122 105 100 75 58 47 29 1300 5.38 25 84 50 50 46 45 42 38 27 <25 1300 2.68 30 - 144 135 - - -1300 8.61 22 - 33 34 - - -aM = moles/1i ter b% primary photolysis TABLE 4-2 Second order quenching constants of 0^  by NO^ k = o.69/(x, I N OJ ) (rfV1) q ZE 2 Flash [N0 2] f k „ u"=4 5 6 7 8 9 10 11 12 Energy K (J) xlO^ (M) x10 ° 2100 1.4 - - 4.0 4.7 4.9 6.6 8.5 10.4 16.9 1300 2.7 3 5-0 5-0 5.6 5-7 6.1 6.7 9.4 <9 1300 1.1 - 4.8 4.5 -1300 4.9 - 4.2 4.3 - -Average k q ft u" = 7 and 8 4.8 + 0.5 x 10° -77-l o g ( l 0 t ) 2.9 -| 1 1 — i — i 1 — i — i — i 1 1 — i 1 1 — i — T — i 1 — i 0 20 40 60 80 100 120 140 160 180 t (us) F i g u r e 4 - 4 0^ R e l a x a t i o n - v a r i a t i o n of r e l a x a t i o n ra tes w i t h [NO,,] fo r approx imate ly the same degree of pr imary p h o t o l y s i s -78-to zero r e a c t i o n t ime i s expected to be a reasonable measure of . the i n i t i a l d i s t r i b u t i o n of v i b r a t i o n a l l y e x c i t e d oxygen. g These d i s t r i b u t i o n s , l i s t e d in T a b l e 4-3 in u n i t s of M x 10 , are used to c a l c u l a t e the number of v i b r a t i o n a l l y e x c i t e d m o l e c u l e s . Th is i s compared to the t o t a l amount of 0^ produced (measured as [NO^] decrease) and i s repor ted as a f r a c t i o n in Tab le 4-4. As w e l l , the amount of v i b -r a t i o n a l energy i n i t i a l l y present i s expressed as a f r a c t i o n of the heat of r e a c t i o n . Only the f i n a l r e s u l t i s g iven f o r the t h i r d d i s t r i b u t i o n {25% pr imary p h o t o l y s i s , f i l t e r e d ) s i n c e the h a l f l i v e s and c o n c e n t r a t i o n s of the l e v e l s (except the t w e l f t h ) very c l o s e l y p a r a l l e l e d those f o r the 25% pr imary p h o t o l y s i s u n f i l t e r e d c a s e . These f r a c t i o n s are g iven in the f i r s t two columns o f Table 4-4 and represent a l l the l e v e l s measured in t h i s s tudy . The remaining columns (a and b in Table 4-4) g i v e f r a c t i o n s that would r e s u l t i f our va lues were extended to u"=0. Thus, us ing our d i s t r i b u t i o n and the one measured by O C Bass and G a r v i n (1964) , who es t imated that the l e v e l s 2 to 6 were approx -imate ly e q u a l l y populated w i t h i n a f a c t o r of two, one can c a l c u l a t e the t t o t a l c o n c e n t r a t i o n of 0^ and the t o t a l energy in v i b r a t i o n w i t h the e x c e p t i o n of the u=1 l e v e l . Th is one e x c e p t i o n cou ld s i g n i f i c a n t l y a f f e c t t the t o t a l c o n c e n t r a t i o n of 0^ i f there i s a la rge o v e r p o p u l a t i o n of the f i r s t v i b r a t i o n a l l e v e l b u t , because of the low energy content of that l e v e l , i t would not g r e a t l y a l t e r the f r a c t i o n of v i b r a t i o n a l energy . A second way in which one cou ld extend our d i s t r i b u t i o n i s to assume that i t i s a Boltzmann d i s t r i b u t i o n and e x t r a p o l a t e the best s t r a i g h t l i n e through the p o i n t s of a log[0_] ys E ., p l o t to u=0, f i g u r e 4-5. From the - 7 9 -TABLE 4 - 3 P o p u l a t i o n s of the V i b r a t i o n a l Leve ls of 0 E x t r a p o l a t e d to Zero React ion Time [0\] x 1 0 8 (M) % P h o t o l y s i s u"=4 5 6 7 8 9 10 11 12 13 31% 70 82 55 41 31 26 11 (10) 25% ' (hO) 51 5h h2 3h 26 16 13 (h) (<1) 25%* (hO) ho 32 22 12 8 TABLE h-h Est imates of the Degree of V i b r a t i o n a l E x c i t a t i o n of 0, % Primary Z N ^ , Z E ^ , u" range Z N ^ , ZE^,, P h o t o 1 y s i s T _ r _ - ^ — ^ T _ ] _ _ - ^ — ^ 37% 0. 16 0 . 1 2 6 - 1 3 0 . 8 5 0 . 3 9 0 . 2 3 0 . 17 25% 0.21 0 . 1 3 4 - 1 3 0 . 8 2 0 . 3 8 0.21 0 . 16 25%" 0.11 0 . 0 8 6-11 0 . 7 3 0 . 2 9 0 . 1 8 0. 12 a - These f r a c t ions i nc lude the va lues f o r the leve l Is 0 to 5 of l o g [ 0 p vs E^| 1 . b - These va lues are based on the e s t i m a t e of Bass and G a r v i n (1963) that the l e v e l s 2 to 6 are approx imate ly e q u a l l y popu la ted , With 7 -54 f i l t e r - 8 0 -[0^]x10 8 (M) % Primary Photolysis O 37 • 25 £ 25 ( f i l t e r e d ) Figure 4-5 Extrapolation of log[0^] vs E ., plot corresponding V l 4 o to a vibra t i o n a l temperature of 2 x 10 K, ; Bass and Garvin's d i s t r i b u t i o n , - 8 1 -s l o p e of t h i s p l o t , a Boltzmann temperature of approx imate ly 2 x 10 K was o b t a i n e d . The r e s u l t s l i s t e d in Table 4 - 3 a re a c c u r a t e to w i t h i n 15% w i t h respect to the i n t e n s i t y measurements except f o r the va lues in b rackets which are much more u n c e r t a i n f o r the f o l l o w i n g reasons . These e r r o r e s t i m a t e s a re based on the d e v i a t i o n of a number of measurements made under i d e n t i c a l c o n d i t i o n s . The l a r g e d e v i a t i o n of the measurements in b rackets i s due to the low o p t i c a l d e n s i t y of these l e v e l s . If more NO^ was used to i n c r e a s e the 0D n t f o r the h igh l e v e l s , then the ra te of r e l a x a t i o n 2 would be inc reased f o r the same percentage of pr imary p h o t o l y s i s , and there would not be s u f f i c i e n t accuracy to e x t r a p o l a t e log lO^] vs- t to zero r e a c t i o n t i m e . I f more UO^ was used in the case of the low l e v e l ( u 1 - 4 ) , the bands be long ing to t h i s p r o g r e s s i o n were obscured by a NO^ cont inuum. ( i i i ) D iscuss ion The r e s u l t s in the f i r s t two columns of Table 4 - 4 c l e a r l y i n d i c a t e that a s i g n i f i c a n t p o r t i o n of the heat of r e a c t i o n is in the form of v i b r a t i o n in the newly formed bond and a l s o that the number of molecu les c o n t a i n i n g v i b r a t i o n a l e x c i t a t i o n is l a r g e compared to the t o t a l number produced. The e f f e c t of p o s s i b l e e x t e n s i o n s of the d i s t r i b u t i o n on the f r a c t i o n of v i b r a t i o n a 1 l y e x c i t e d molecules is .shown in the remainder of Table 4 - 4 (columns a and b ) . 26 t t E a r l i e r r e s u l t s have shown that the amount of 0^ produced from e x c i t e d N0 o in the u n f i l t e r e d exper iments is smal l compared to the amount -82-of 0^ produced d i r e c t l y . Hence, no attempt was made to c o r r e c t the d i s -t t t r i b u t i o n f o r the 0^  produced. OCT The d i s t r i b u t i o n repor ted p r e v i o u s l y by Bass and Garv in (1964) was used as one of the ex tend ing d i s t r i b u t i o n s . It must be p r e f e r r e d over that measured by Kane et a l (1963)^\ s i n c e in t h i s case Kane e t a l e s t i m a t e tha t the p o p u l a t i o n of the u"=2 l e v e l is 2 x 10 t imes g r e a t e r than the u"=7. For t h i s to be t r u e and s t i l l w i t h i n the l i m i t s set by the t o t a l p o s s i b l e c o n c e n t r a t i o n of oxygen produced from the f l a s h p h o t o l y s i s o f one t o r r of NO^, the va lue of the band o s c i l l a t o r s t r e n g t h of the (4,7) band must be g r e a t e r than 10 f o r the i n t e g r a t e d a b s o r p t i o n measured by us . Th is v a l u e is o b v i o u s l y f a r too h igh f o r any band o s c i l l a t o r s t r e n g t h , w i t h i n the exper imenta l e r r o r of our i n t e g r a t e d a b s o r p t i o n measurements. By assuming a Boltzmann d i s t r i b u t i o n f o r the e n t i r e set of v i b r a -t i o n a l l e v e l s , i t is p o s s i b l e to account f o r a l l o f the oxygen produced w i t h a smooth d i s t r i b u t i o n (dashed l i n e s in f i g u r e 4-5), w i t h i n e x p e r i -mental e r r o r . A l though the number of e x c i t e d molecu les in t h i s d i s t r i b -u t i o n (Table 4-4 (a)) i s la rge compared to the o ther e x t e n s i o n (Table 4-4 ( b ) ) , the energy content has not been a l t e r e d by more than 20%. As seen in f i g u r e 4-5, the t rend toward a Boltzmann d i s t r i b u t i o n i s broken by the f o u r t h l e v e l which has a lower measured p o p u l a t i o n than the h igher l e v e l s , f i g u r e 4-3- The p o p u l a t i o n of u"=4 i s , however, much more u n c e r t a i n than those f o r the h igher l e v e l s f o r reasons g i ven e a r l i e r . In f a c t , there may be a n o n - l i n e a r d e v i a t i o n of o p t i c a l d e n s i t y due to the non-adherence of B e e r ' s law as a r e s u l t of s p e c t r a l i n t e r f e r e n c e by the N0o cont inuum. If t h i s i s t r u e , and i t i s d i f f i c u l t to check in the - 8 3 -VUV for reasons given in the next section, the effect would be to raise the points at short times, figure 4 -3, r e l a t i v e to those at longer times. Thus, the measurements of I O ^ u ' - A a r e dubious a n c' a nY conclusion drawn on i t s apparent k i n e t i c behavior should be very q u a l i t a t i v e . Experiments involving observation of 0^ in the VUV were intended to lead to an independent check of the d i s t r i b u t i o n in that a l l of the levels could then be measured. Unfortunately, severe band interference from the NO (y, 3, and E) systems and strong H0^ absorption obscured the u"=0 to 3 25 l e v e l s . This problem was noted previously by Bass and Garvin but they were able to measure the u"=2 and 3 levels. Presumably, this was a result of t h e i r achieving a greater percentage primary photolysis which gave a 'window1 through the NO^ - Below u"=2 i t was mainly NO interference that obscured the oxygen. The observation that the population ratios of the levels at short times remain constant may at f i r s t seem to disagree with Basco and Dogra 28 17 (1968, 1971) ' who have found atomic oxygen to be very e f f e c t i v e in -f quenching 0^ produced from the flash photolysis of C102. This change was only observed, however, when conditions were such that a high concentration of 0 atoms was produced by secondary photolysis of CIO and/or by photo-lysing more than 50% of the i n i t i a l ClO^ concentration. In our case, neither of these conditions existed since we have only been able to decom-pose a maximum of 37% of the N0 2 in the primary process and NO photolysis is not possible with l i g h t f i l t e r e d by a Pyrex reaction vessel. Thus, in the f l a s h photolysis of NO^  the conditions for rapid 0^ deactivation by 0 atoms could not be met. -84-III The Production of 0^ from the Flash Photolysis of Chlorine Dioxide (i) Treatment of Intensity Measurements A l l of the intensity measurements obtained in this study were made in the far or vacuum u l t r a v i o l e t regions of the Schumann-Runge band system of oxygen. As a r e s u l t , the vacuum spectrograph and i t s associated apparatus were used exclusively. The method of treating the measurements was the same as in the NO^  case with the following two exceptions: (1) The band o s c i l l a t o r strength used to put r e l a t i v e 0^ concentrations on an absolute scale was f^p ^ ^he reasons for this choice are: (a) only f values for the u"=0 (Bethke, 1 9 5 9 ) a n d u"=1 and 2 (Carter and Hudson, 1968) p levels have been determined experimentally for 0^ (except for Treanor and Wurster's values for high u"). Of these three l e v e l s , those f values for the 2 level are the highest and f^g ^ ' s used because the 10,2 band is strong and r e l a t i v e l y free of any overlap from any other bands. In f a c t , the ra t i o of the f values in decreasing u" (2 :1 :0) is ca. 30:9:1 -4 with f j g 2 = l u - 2 9 ± 0.77 x 10 . Thus p r o h i b i t i v e l y high concentrations of CIO2 would be required to produce s u f f i c i e n t oxygen for measurement in the 0 and 1 levels at short times. (b) As w i l l be shown in the next section, a transient continuum p a r t i a l obscures these last two levels for most of the l i f e of the excited oxygen. (2) The f l a s h photolysis apparatus does not easily lend i t s e l f to half path experiments since the reaction vessel length is quite short (10 cm) and the diameter large (2.2 cm od), thus masking of half of the reaction vessel is not p r a c t i c a l . Hence another means of checking the -85-v a l i d i t y of the i n t e n s i t y measurement method was n e c e s s a r y . Th is invo l ved o b t a i n i n g a c a l i b r a t i o n curve of i n t e g r a t e d a b s o r p t i o n of the 13,0 and 12,0 bands of 0^ vs [O J^ in the range 2 to 10 t o r r . Over t h i s range the c a l i b r a -t i o n curve was l i n e a r . From the s l o p e of t h i s p l o t , va lues of the band o s c i l l a t o r s t r e n g t h s were o b t a i n e d and were found to agree q u i t e f a v o u r -a b l y w i t h those of Bethke ( 1 9 5 9 ) ( i . e . w i t h i n 10%). These two r e s u l t s i n d i c a t e that B e e r ' s law i s obeyed and that reasonable agreement i s ach ieved w i t h the a b s o l u t e va lue of the o s c i l l a t o r s t r e n g t h measured p r e -v i o u s l y . It i s assumed t h a t , having shown B e e r ' s law to ho ld f o r the u"=0 bands, i t a l s o holds t rue f o r the h igher l e v e l s . ( i i ) R e s u l t s f The optimum c o n d i t i o n s f o r the p roduc t ion of the low l e v e l s of 0^ (u"=1 to 4) in the f l a s h p h o t o l y s i s of C10^ were: 2 t o r r of C10 2 f l a s h e d in a Pyrex r e a c t i o n v e s s e l f i l t e r e d by the Pyrex - c e l l u l o i d combinat ion used p r e v i o u s l y to determine e Q j g . Under these c o n d i t i o n s , r e f e r r e d to as the f i l t e r e d e x p e r i m e n t s , on l y 80% o f the C10^ was decomposed and secondary p h o t o l y s i s of C10 was not p o s s i b l e . The optimum ClO^ c o n c e n t r a t i o n f o r f the p roduc t ion of 0^ in the l e v e l s 5 to 7 was found to be 1 t o r r w i t h the r e s t of the above c o n d i t i o n s remaining the same. For the sake o f compar i son , exper iments were performed in which the above c o n d i t i o n s were not met. In p a r t i c u l a r , 2.5 t o r r of ClO^ was f l a s h e d in a q u a r t z r e a c t i o n vesse l w i t h s u f f i c i e n t energy to comple te l y decompose the ClO^ by p h o t o l y s i s and secondary r e a c t i o n s ( u n f i 1 t e r e d e x p e r i m e n t s ) . F igu res 4-6 and 4-7 show the r i s e and decay of the 0 o l e v e l s 1-4 under the two se ts of c o n d i t i o n s . ZOOnm • 180nm • -7,4 8 , 4 9 , 4 10,4 11,4 9 3 10,3 11,3 12,3 10,2 11,2 12,2 10,1 •TM T T 12,0 13,0 B l a n k B e f o r e 1 s e c l O O m s 5 0 I O 3 2 . 5 2 . 0 1.2 6 0 0 ^ . s 3 0 0 1 0 0 4 0 A f t e r B l a n k - C I O * i OO F igure 4-6 Low v i b r a t i o n a l l e v e l s of 0^ produced from 2 t o r r of CIO, - 80% t o t a l decomposi t ion of CIO P y r e x - c e l 1 u l o i d f i l t e r mmmm 2 0 0 n m I 180nm I 8,4 9,4 9,3 1 0 , 2 10,1 Figure k-1 Low vibrational levels produced from 2.5 torr of CIO, - >50% primary photolysis - quartz reaction vessel Blank Before No Delay 3 3 IJLS CIO, -88-In both cases the onset of a continuum in the region of the u"=0 and 1 levels is apparent. The c a r r i e r of the continuum was assumed to be either an inter-mediate containing a chlorine atom in the fla s h photolysis of ClO^ or an extension of the Schumann-Runge dissociation continuum from vibrat i o n a l states greater than u"=0. The f i r s t p o s s i b i l i t y was ruled out to a large extent by the k i n e t i c behavior of the continuum. This continuum was obtained in both f i l t e r e d and u n f i l t e r e d experiments and was found to ri s e and decay at about the same rate as the u"=2 level of 0^- If one compares this behavior with what would be expected from the decay of ClO^' 1 (slow, one minute) and ^ 2 ^ 2 ^ ' ^ (no difference in rate of C^C^ decay with either low or high fl a s h energy), i t is seen that even allowing a large margin of error, these previously proposed species do not f i t the k i n e t i c behavior of the observed continuum. The second p o s s i b i l i t y was tested by obtaining 0^ from a source other than C10^• Since, as already shown, NO and NO^  absorption obscure the region of interest when 0^ is produced from the f l a s h photolysis of C O NO2, ozone was used as such a source. Relative to ClO^, ozone was found to be either a very poor source or a very e f f i c i e n t quencher of 0^ since 0^ was only barely v i s i b l e at the shortest time (20us). This was true at pressures as high as 5 t o r r , above which the 0^ continuum in this region began to interfere. No transient continuum was noted in the 0^ experiments, but because of the low observable y i e l d of 0^ from ozone and the inherent d i f f i c u l t y in detecting the presence of weak continua, the p o s s i b i l i t y of an extended - 8 9 -Schumann Runge continuum cannot be c o n c l u s i v e l y e s t a b l i s h e d . A l though the r e s u l t s of the ozone exper iments were i n c o n c l u s i v e , the most p l a u s i b l e e x p l a n a t i o n of the observed continuum s t i l l remains that i t a r i s e s from a s h i f t i n g of the d i s s o c i a t i o n continuum of B 3 £ u to longer wavelengths as a r e s u l t of the l a r g e f r a c t i o n of oxygen that i s produced w i t h v i brat iona 1' exc i t a t i o n . t t F i g u r e s 4 -8 and 4 -9 show the r i s e and decay of 0 ^ , p l o t t e d as logtO^] vs t , f o r the l e v e l s 1 to 7 and 1 to 4 r e s p e c t i v e l y . The r e s u l t s f o r the u"=5,6 and 7 l e v e l s in f i g u r e 4 -8 were ob ta ined by f i t t i n g the o p t i c a l d e n -s i t y measurements of the f i f t h l e v e l which were o b t a i n e d from exper iments u t i l i z i n g 1 t o r r of C lO^ . to the c o n c e n t r a t i o n of the f i f t h l e v e l which was determined from the P Q J Q =2 t o r r exper iments . In t h i s way, the p o p u l a t i o n s of the l e v e l s 6 and 7 were put on an a b s o l u t e s c a l e . It was necessary to do t h i s in order to remain on 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 f o r the photograph ic p l a t e . Table 4 -5 l i s t s the h a l f l i v e s of the l e v e l s o b t a i n e d from the l i n e a r p o r t i o n of the f i r s t o rder decay c u r v e s , in f i g u r e 4 - 8 . In a d d i t i o n , h a l f l i v e s from the i n i t i a l s teep decay shown in f i g u r e 4 -9 are inc luded in Table 4 - 5 . From the f i r s t two sets of h a l f l i v e s , quenching cons tants of 0^ by ClC^ and CIO are c a l c u l a t e d f o r the v a r i o u s l e v e l s . Lipscomb et a l 1 1 and Basco and D o g r a 1 7 have shown that the e f f i c i e n c y •f of CIO2 i s about the same as CIO in d e a c t i v a t i n g 0 ^ . Thus the decay cons tants f o r 0 2 may be c a l c u l a t e d a s : d l n [ 0 * ] = k ( [ C 1 0 J + [CIO]) dt , q ' L 2 J -90--91-0.1 0.2 0.4 0.6 0.8 t (ms) F i g u r e 4-9 0^ r e l a x a t i o n ; P C ] Q =2.5 t o r r , 100% d e c o m p o s i t i o n , q u a r t z r e a c t i o n vesse l -92-since the 80% decomposition of ClO^ i s e s s e n t i a l l y complete before the beginning of the half l i f e period and the decay of CIO i s second order over t h i s period. d lnloy dt = k c l + 1 + k j C 2 t where c^ and c 2 are the concentrations of C10^ and CIO respectively at the beginning of the half l i f e , and k^, as before, is the rate constant for CIO recombination. Thus, k = In 2 q V i + 1n(1 + k^c 2T^) (4-1) TABLE 4-5 Relaxation of 0 2 Produced from C10^ Condi tions u"= 1 a >3000 2000 1400 620 390 b 580 200 190 c* 100 70 50 40 k x 10 M s using Eq'n 4-1 and these x, a <0.51 b 0.67 0.86 1.5 2.2 2.8 7.1 7-5 T| is measured from the i n i t i a l sharp decay of [0^] k is given for the f i r s t two sets of half lives o Conditions-(a) 2 torr of ClO^ f i l t e r e d f l a s h ; (b) 1 torr of C10 o, f i l t e r e d f l a s h ; (c)2.5 torr of C10 2, unfi l t e r e d . -93-t Two f e a t u r e s apparent in Tab le 4-5 a re the marked change in 0^ h a l f l i v e s between the two c o n d i t i o n s (a and c) and the change in of the u"= 5 l e v e l f o r the two pressures of C10^ used (a and b ) . The f i r s t f e a t u r e w i l l be d i s c u s s e d in d e t a i l l a t e r . The second f e a t u r e i s a r e s u l t of v a r y i n g the CIC^ p r e s s u r e . The two va lues of k^(u"=5) in Tab le 4-5 a r e , w i t h i n +15%, the same. Thus kq(u"=5) = 2.5 ± 0.3 x 10 7 M'V1 The va lue of the u"=l p o p u l a t i o n ob ta ined from f i g u r e 4-8 i s some-what doubt fu l due to the l a r g e f a l l - o f f in t r a n s i t i o n p r o b a b i l i t i e s be -t o tween u"=2 and 1 (3:1) . For t h i s reason e r r o r bars were g i ven in o rder to demonstrate that the e x t r a p o l a t i o n f o r the f i r s t l e v e l is r a t h e r a r b i t r a r y w i t h i n c a . 20%. f Table 4-6 l i s t s the p o p u l a t i o n d i s t r i b u t i o n of 0^ o b t a i n e d from the curves in f i g u r e 4-8 a t zero r e a c t i o n t i m e . The d i s t r i b u t i o n from f i g u r e 4-9 i s l i s t e d in Tab le 4-6 as w e l l b u t , because of the l a r g e e r r o r invo l ved in the e x t r a p o l a t i o n to zero t ime due to the i n i t i a l sharp decay , the d i s -t r i b u t i o n is g i ven f o r the s h o r t e s t t i m e , 33 us . An e s t i m a t e of the f i n a l oxygen c o n c e n t r a t i o n , [ u ^ f P r °duced from f l a s h p h o t o l y s i s is a l s o i n c l u -ded in t h i s t a b l e . Th is was determined by e s t i m a t i n g the t o t a l p o s s i b l e y i e l d of 0^ under the g i ven c o n d i t i o n s , i . e . , c a l c u l a t e d from A t C l O ^ ] , and was a l s o ob ta ined by measuring the i n t e g r a t e d a b s o r p t i o n of the 13,0 and 12,0 bands of 0^ a t long de lays a f t e r the f l a s h (1 m i n u t e ) , us ing the f va lues '* 7 to c a l c u l a t e the [0^] • The e r r o r invo l ved in the i n t e g r a t e d a b s o r p t i o n measurements i s q u i t e h igh due to the low t r a n s i t i o n probab-i l i t i e s from the u"=0 l e v e l . It should be noted at t h i s p o i n t that a c c o r d i n g to the m e c h a n i s m : 1 7 - 9 4 -C l 0 2 + hv->-C10 + 0 0 + C10 2 i CIO + 0 2 cio + cio X c i 2 + o 2 a maximum of on ly one h a l f of the t o t a l y i e l d of oxygen can be i n i t i a l l y t v i b r a t i o n a l l y e x c i t e d . No 0 2 in the l e v e l s g r e a t e r than 5 was observed from r e a c t i o n ( l ) in the f l a s h p h o t o l y s i s of Cl^O^. P r e l i m i n a r y e x p e r i -f ments in the VUV have shown that no o b s e r v a b l e amount of 0 2 in the l e v e l s l ess than 5 have been produced from r e a c t i o n (1) under c o n d i t i o n s where [CIO] i s h igh and no secondary p h o t o l y s i s of CIO is p o s s i b l e . TABLE 4-6 f P o p u l a t i o n D i s t r i b u t i o n s of 0 2 from C10^ [ 0 j ] u „ x 10 5 (M) C o n d i t i o n s " u"=1 2 3 ^ 5 6 7 t ° 2 ^ f + f ° 2 ] f + t a and b 0 . 9 0 . 3 0 0 .19 0.19 0.28 0.25 0 .19 8.2 8.7 c 4.4 1.6 0.8 0.5 11.8 13-5 " c o n d i t i o n s from Table 4-5; a and b e x t r a p o l a t e d to t=0; c taken at t=33 ps measured f i n a l [0,,] •ft t o t a l p o s s i b l e y i e l d of 0 2 c a l c u l a t e d from A[C10 2 ] The t o t a l p o p u l a t i o n of v i b r a t i o n a l l y e x c i t e d s t a t e s in the f i r s t case (a and b, Table 4-6) i s the sum of the p o p u l a t i o n s of the l e v e l s meas-ured (2 .4 x 10 M) and the f r a c t i o n of molecules i n i t i a l l y e x c i t e d in the -95-l e v e l s 1 to 7 from the process 0 + c i o 2 ^ of2 + CIO i s 0.56 of the t o t a l . The f r a c t i o n of the heat of r e a c t i o n in the form of v i b r a t i o n a l energy f o r these l e v e l s is c a . 0.12. Th is d i s t r i b u t i o n shows that the l e v e l s 2 to 7 a re approx imate ly e q u a l l y populated w i t h u"=1 some-what h i g h e r . The form of the second d i s t r i b u t i o n (c , Table 4-6) i s c o n s i d e r a b l y d i f f e r e n t in that the p o p u l a t i o n s of a l l the l e v e l s inc rease w i t h d e c r e a s -ing u " . It must be noted that t h i s d i s t r i b u t i o n does not represent an i n i t i a l d i s t r i b u t i o n . It is inc luded o n l y to i l l u s t r a t e the consequences of us ing c o n d i t i o n s such tha t atomic oxygen and c h l o r i n e can s e r i o u s l y com-t p l i c a t e 0^ r e l a x a t i o n . It should a l s o be noted that in t h i s case the t o t a l y i e l d of molecules c o n t a i n i n g v i b r a t i o n a l e x c i t a t i o n is 7'3 x 10 ^ M which may be more than h a l f of the c a l c u l a t e d t o t a l y i e l d of oxygen. ( i i i ) D iscuss ion The r e s u l t s of Table 4-6 show that as much as 56% of the 0^ produced from 0 + c i o 2 -> o 2 + C10 i s v i b r a t i o n a l l y e x c i t e d in the l e v e l s 1 to 7- Because t h i s d i s t r i b u t i o n covers on ly the lower l e v e l s , however, the f r a c t i o n of the heat of r e a c t i o n in the form of v i b r a t i o n a l energy is s m a l l , c a . 12%. Basco and Dogra (1971) 1 7 have reported that the l e v e l s 6 to 13 a re approx imate l y e q u a l l y populated w i t h the h igher l e v e l s being favoured so that the f a c t o r by which the l e v e l p o p u l a t i o n inc reases w i t h i n c r e a s i n g u" i s 1.25 ± 0.15-- 9 6 -Using t h e i r d i s t r i b u t i o n f o r the l e v e l s 8 to 13 and the i n i t i a l d i s t r i b u -t i o n in Tab le 4 - 6 g i v e s a t o t a l y i e l d of [0^] f o r the l e v e l s 1 to 13 which is a f a c t o r of 1 . 3 of the t o t a l p o s s i b l e y i e l d . The c a l c u l a t e d energy content of t h i s o v e r a l l d i s t r i b u t i o n i s 8 3 % of the heat of r e a c t i o n . When one c o n s i d e r s the l a r g e accumulated e r r o r that can r e s u l t from summing t h i r t e e n measurements, the above r e s u l t is not too d i f f i c u l t to unders tand . If i ns tead of a f a c t o r of 1 . 2 5 , the lower l i m i t of t h e i r d i s t r i b u t i o n , 1 . 1 0 , i s used a l l (1 .0) of the molecules can be accounted f o r in the l e v e l s 1 to 13 w i t h an energy content of 4 5 % o f the t o t a l . F i n a l l y , to account 1* - 5 f o r a l l of the 0^ i n c l u d i n g a va lue of [ 0 ^ ] u i I _ Q o f 0 . 6 x 10 M , i t i s on ly necessary to use a r a t i o of 1.0 f o r the l e v e l s g r e a t e r than seven which g i ves an energy content of 3 7 % of the t o t a l . As e x p l a i n e d e a r l i e r , the d i s t r i b u t i o n ob ta ined f o r the h igh l e v e l s (Basco and Dogra) invo lved a number of exper imenta l d i f f i c u l t i e s . The most s e r i o u s was o v e r l a p between high u" bands of oxygen w i t h C10 and C10^• As a r e s u l t , on l y the 6 , 7 » and 12 l e v e l s cou ld be measured and even these d i d not l i n e a r l y obey B e e r ' s law but cou ld be f i t t e d to i t by r a i s i n g the i n t e n s i t y measurements by an e x p e r i m e n t a l l y determined exponent . Needless to say the e r r o r invo l ved in us ing an exponent to c o r r e c t the data cou ld be l a r g e . W i t h i n exper imenta l e r r o r , any one set of the above f r a c t i o n s o f the t o t a l s would be a c c e p t a b l e , but f o r the sake of c o n s i s t e n c y the l a s t set is favoured s i n c e i t accounts f o r a l l of the oxygen produced in a smooth d i s t r i b u t i o n over the l e v e l s 0 to 13. The second d i s t r i b u t i o n in Tab le 4 - 6 (measured at 3 3 us) i s o b t a i n e d from exper iments where more than 50% of the C10 i s decomposed in the p r i --97-mary s tep and a l s o where secondary p h o t o l y s i s of CIO i s p o s s i b l e . Under ] 7 these c o n d i t i o n s , as shown by Basco and Dogra , atomic oxygen and c h l o r -ine are not r a p i d l y removed by ClO^ and the p r o c e s s e s : 0 + °2u"=n * ° + °2u"=n-1 k 1 2 = 2 * ^ ^ CI + 0 2 u l l = n - CI + 0 2 u l l = n_ 1 k l f 7 x l 0 9 M'V1 ( 1 7 ) t are r e s p o n s i b l e f o r the r a p i d r e l a x a t i o n o f 0,,. The r e s u l t s of Table 4-5 c o n f i r m these o b s e r v a t i o n s , (compare f i g u r e s 4-5 and 4-6). The y i e l d of v i b r a t i o n a l l y e x c i t e d oxygen in t h i s case i s g r e a t e r than the t o t a l p o s s i b l e y i e l d f rom: 0 + cio2 $ CIO + o2 i n d i c a t i n g that o ther processes a re c o n t r i b u t i n g to the p r o d u c t i o n of 0 2. These are p r o b a b l y : C 1 0 + h v + C I + 0 C10 + 0->-02 + Cl AH = -230 kJ as suggested by Basco and Dogra who found a l i n e a r i nc rea se in l 0 2 ] f o r the l e v e l s u"=6 and 12 w i t h f l a s h energy when a q u a r t z r e a c t i o n v e s s e l was used . + IV The R e l a x a t i o n of 0 2 The r e l a x a t i o n of 0 2 produced from NO,, has been shown to depend l i n -e a r l y on the N0 2 c o n c e n t r a t i o n remaining a f t e r i n i t i a l decompos i t ion and the quenching cons tants are g iven in Table 4-2. The va lue of the quenching constant f o r the u"=6 l e v e l is in good agreement w i t h that found by Lipscomb et a l ' 1 . They do not e x p l i c i t l y s p e c i f y a quenching constant but i t can be 8 -1 -1 c a l c u l a t e d to be approx imate ly 4 x 10 M s from t h e i r d a t a . -98-f The h a l f l i v e s of 0^ r e l a x a t i o n by CIO and ClO^ were used to c a l -c u l a t e quenching constants f o r 0^ by CIO and ClO^ f o r the f i r s t se t of (a and b, Tab le 4-5)• The quenching cons tants f o r the u"=6 and 7 l e v e l s are in good agreement w i t h Lipscomb et a l 1 1 and Basco and D o g r a ! 7 The marked change in in Table 4-5 f o r 0^ o b t a i n e d under the two s e t s of c o n -d i t i o n s ( f i l t e r e d and u n f i l t e r e d ) has been r e a d i l y e x p l a i n e d above. These decay cons tants cannot , however, be d i r e c t l y i d e n t i f i e d w i t h the r a t e cons tants f o r the p r o c e s s e s : O ^ u ' ^ n ) £ ( ^ ( u ^ n - l ) where A is CIO, ClO^ or HO^', s i n c e , i f the r e l a x a t i o n i s s t e p w i s e , as i t f appears to be f o r the low l e v e l s of 0^ produced from CIO2, the presence of l e v e l s h igher than the one being measured must be taken i n t o a c c o u n t . T h i s has e f f e c t i v e l y been done in our case by e x t r a p o l a t i n g the decay curves back to zero r e a c t i o n t i m e . As can be seen from f i g u r e 4-7, the low u" l e v e l s are a t f i r s t populated from the h igher l e v e l s more r a p i d l y than r e l a x -a t i o n out of them. It cou ld be seen from the r e s u l t s of exper iments r e p r e -sented in f i g u r e s 4-2 and 4-3 that t h i s t rend was d e v e l o p i n g f o r the low f l e v e l s of 0 2 re laxed by NO^. In t h i s c a s e , however, i t was not as obvious as i t was w i t h CIO2 s i n c e the lowest l e v e l s were not observed w i t h NO2 and the quenching e f f i c i e n c y of N0 2 i s c a . a f a c t o r of 4 g r e a t e r than e i t h e r CIO o r C 1 0 2 . Al though the e x t r a p o l a t i o n method takes the cascad ing e f f e c t of the l e v e l s i n t o account in de te rmin ing the i n i t i a l d i s t r i b u t i o n s , i t does not d i r e c t l y g i v e the e f f e c t on the e x p e r i m e n t a l l y determined quenching cons tants -99-To do t h i s r e q u i r e s the s imul taneous s o l u t i o n of a number of equat ions equal to the number of l e v e l s p o p u l a t e d ; t h i s in p r i n c i p l e can be accom-p l i s h e d but i t r e q u i r e s an a c c u r a t e l o c a t i o n of the maxima of the decay c u r v e s . Such an a n a l y s i s has been attempted by Fi tzs immons and B a i r (1964)' t f o r the d i s t r i b u t i o n from ozone. T h e i r r e s u l t s were s e r i o u s l y com-p l i c a t e d by the f a c t that most of the oxygen r e l a x a t i o n took p l a c e w i t h i n the 250 us l i f e t i m e of t h e i r p h o t o l y t i c f l a s h . Even s o , t h e i r a n a l y s i s showed that the r a t e constants so ob ta ined depended c r i t i c a l l y on the l o c a t i o n of the decay curve maximum. Thus such an a n a l y s i s was not attempted f o r t h i s study s i n c e maxima are on ly observed fo r the low l e v e l s and even these are d i f f i c u l t to l o c a t e a c c u r a t e l y . (F i tzs immons and B a i r f were c o n t i n u o u s l y m o n i t o r i n g the l e v e l s s p e c t r o p h o t o m e t r i c a l l y . ) V I n i t i a l V i b r a t i o n a l D i s t r i b u t i o n s of the Product Molecu les The d i s t r i b u t i o n s g i ven in Table 4-3 and the f i r s t g iven in Table 4-6 must be c l o s e to the i n i t i a l d i s t r i b u t i o n s of (the new bond) f o r reasons g i ven in the d i s c u s s i o n s of the separate d i s t r i b u t i o n d e t e r m i n a t i o n s . It has been shown that the amount of energy in the form of v i b r a t i o n in the new bond is a s i g n i f i c a n t p o r t i o n of the t o t a l and p o p u l a t i o n d i s t r i b u t i o n s have been found that account f o r a l l of the oxygen produced. These d i s t r i b u t i o n s have r e q u i r e d i n t e r p o l a t i o n or e x t r a p o l a t i o n of the p o p u l a -t i o n s of a number of l e v e l s due to exper imenta l l i m i t a t i o n s . Even i f the ex tens ions of these d i s t r i b u t i o n s are d i s r e g a r d e d , the r e s u l t s p r e -sented here s t i l l i n d i c a t e that ,the r e a c t i o n pathway of the two p r o c e s s e s : -100-o + cio2 -> 0 £ + CIO 0 + N0 2 ^ 0 2 + NO must account f o r a la rge p o r t i o n of v i b r a t i o n a l e x c i t a t i o n in the newly formed bond. There i s some ev idence to i n d i c a t e that the ' o l d bond 1 in the f l a s h 22 p h o t o l y s i s of N0 2 has some v i b r a t i o n a l energy (Basco and N o r r i s h , I960 26 and Morse , 1969 ) but the r e s u l t s are not c o n c l u s i v e . There is no ev idence to suggest that CIO is produced w i t h any v i b r a t i o n a l e x c i t a t i o n and the c o n d i t i o n s r e q u i r e d f o r i t s o b s e r v a t i o n are i d e a l (Chapter 3). BIBLIOGRAPHY 1. G. Porter; Proc. Roy. Soc. A200, 284 (1950) 2. G. Porter; Disc. Faraday Soc. % 60 (1950) 3. R.A. Durie S D.A. Ramsay; Can. J. Phys. 36, 35 (1958) 4. G. Porter & F.J. Wright; Disc. Faraday Soc. ^ 4, 23 (1953) 5. G. Burns £ R.G.W. Norrish; Proc. Roy. Soc. A27J_, 289 (1963) 6. J.E. Nicholas & R.G.W. Norrish; Proc. Roy. Soc. AJ07, 391 (1968) 7. S.W. Benson S J.H. Buss; J. Chem. Phys. 27, 1382 (1957) 8. A. Arke l l S I. Schwanger; J. Am. Chem. Soc. 89, 5999 (1967) 9. E.D. Morris & H.S. Johnston; ibid 90, 1918 (1968) 10. M.A.A. Clyne £ J.A. Coxon; Trans. Faraday Soc. 62, 1175 (1966) 11. F.J. Lipscomb, R.G.W. Norrish & B.A. Thrush; Proc. Roy. Soc. A233, 455 (1956) 12. F.H.C. Edgecombe, R.G.W. Norrish S B.A. Thrush; ibid A24_3_, 24 (1957) 13. F.H.C. Edgecombe, R.G.W. 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