UBC Theses and Dissertations

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UBC Theses and Dissertations

Flash photolysis of NO2 and SO2. Morse, Robert Donald 1969

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T H E F L A S H P H O T O L Y S I S O F N 0 2 A N D S O z B Y R O B E R T D O N A L D M O R S E B . Sc. , U n i v e r s i t y of B r i t i s h C o l u m b i a , 1 9 6 6 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F M A S T E R O F S C I E N C E in the Department of C h e m i s t r y We accept this thesis as conforming to the r equ i r ed standard T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A D E C E M B E R , 1 9 6 9 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree tha permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Depa rtment The University of British Columbia Vancouver 8, Canada A B S T R A C T V i b r a t i o n a l l y exci ted oxygen with up to 15 quanta of energy has been produced in the f lash photolysis of NO . When the NO was se lec t ive ly photolysed such that only radia t ion below 4000 A° was a d m i t -ted to the reac t ion vesse l the highest l eve l observed was reduced to v" = l l which i n t e r m s of v ib ra t iona l energy corresponds to the exo thermic i ty of the reac t ion O + N O z —> NO + Q, Re la t ive population ra t ios for the success ive v ib ra t iona l leve ls v" = 6 to 11 were ca lcula ted for both the f i l t e red and unf i l tered photolysis of N O ^ using two independently calcula ted a r r a y s of F r a n c k - C o n d o n F a c t o r s . In both cases the ra t ios showed that the population d i s t r ibu t ion was defini tely non Bo l t zmann . in that the levels were almost equally populated. The d i s t r i -bution for the unf i l tered exper iments gave a pronounced dip at v" = l l i n -dicat ing that the leve ls 12 £ v" ^ 15 are formed by a second react ion . Th i s reac t ion has been shown to be Q + N O * 2 - * 0 - : W , ^ ^ + N O where the N O ^ is probably e l ec t ron i ca l l y exci ted. The adiabatic f lash photolysis of both NO and SO has been i n -vestigated. These spec t ra show weakening of the i r banded s t ructure up to 100 mic roseconds after the f lash and then gradual strengthening unti l back to n o r m a l by 10 m i l l i s e c o n d s . Th i s effect is we l l known for SO^ and has been postulated to be the resul t of the format ion of a high temperature i s o m e r of SO^. Subsequent ly the postulated i s o m e r had a t ransient spec t rum A B S T R A C T ( C o n t ' d . ) i n the v a c u u m u l t r a v i o l e t a s s i g n e d to i t . We have found that the w e a k e n -i n g of both the N O ^ and SO„, b a n d e d s t r u c t u r e i s the r e s u l t of t e m p e r a t u r e b r o a d e n i n g . T h e s p e c t r u m a s s i g n e d to the S O 2 i s o m e r has been ex t ended . It has a l s o been p r o d u c e d f r o m the f l a s h p h o t o l y s i s of S O ^ u n d e r i s o t h e r m a l c o n d i t i o n s s h o w i n g that i t s a s s i g n m e n t to a h i g h t e m p e r a t u r e i s o m e r i s i n -c o r r e c t . F r o m k i n e t i c da ta t h i s s p e c t r u m i n the 1800 A ° r e g i o n and a n -o t h e r i i i the 1400 A ° r e g i o n has been t e n t a t i v e l y i d e n t i f i e d as two t r i p l e t s y s t e m s of S O . T A B L E O F C O N T E N T S P a g e C h a p t e r 1 I n t r o d u c t i o n -I T h e I s o t h e r m a l F l a s h P h o t o l y s i s of N 0 2 1 II T h e A d i a b a t i c F l a s h P h o t o l y s i s of S O ^ 5 III P r e s e n t I n v e s t i g a t i o n 7 C h a p t e r Z E x p e r i m e n t a l I M a t e r i a l s 9 II A p p a r a t u s 10 III F i l t e r s • ' 16 ' I V P h o t o g r a p h y 18 V P r e p a r a t i o n of a M i x t u r e and P r o c e d u r e Z l C h a p t e r 3 T h e I s o t h e r m a l F l a s h P h o t o l y s i s of N 0 2 Z3 I P o p u l a t i o n R a t i o s of 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 Z4 (a) R e s u l t s Z4 (b) D i s c u s s i o n 35 II T h e M e c h a n i s m fo r O * * P r o d u c t i o n i S e c o n d a r y P h o t o l y s i s of N O 3 38 i i A n E x c i t e d State of N O z (a) R e s u l t s 41 (b) D i s c u s s i o n 50 C h a p t e r 4 T h e A d i a b a t i c F l a s h P h o t o l y s i s of N O z and S O z I B r o a d e n i n g of the N o r m a l S p e c t r a 54 II T r a n s i e n t S p e c i e s P r o d u c e d f r o m the F l a s h P h o t o l y s i s of S O ^ O b s e r v e d i n the V a c u u m U l t r a v i o l e t 66 III D i s c u s s i o n 73 B i b l i o g r a p h y 75 I L L U S T R A T I O N S F I G U R E P A G E 1 Schematic D i a g r a m of a Conventional F l a s h 12 P h o t o l y s i s Apparatus 2 A u x i l i a r y L a m p T r i g g e r i n g C i r c u i t 14 3 F i l t e r T r a n s m i s s i o n C h a r a c t e r i s t i c s 17 4 P h o t o g r a p h i c P l a t e C u r v e s 19 5 S p e c t r a of V i b r a t i o n a l l y E x c i t e d Oxygen 24 6 Half P a t h P l o t for some of the O* Bands P r o d u c e d 29 7 Population R a t i o s and R e l a t i v e Populations of the O z L e v & l s v"= 6 to 14 32 8 I l l u s t r a t i o n of a Double L a m p E x p e r i m e n t 42 9 V a r i a t i o n of I as a F u n c t i o n of NO? E x c i t a t i o n • 46 10 A P o r t i o n of the V i s i b l e S p e c t r u m of NO^ F l a s h e d Under A d i a b a t l c Conditions 55 11 A P o r t i o n of the uv S p e c t r u m of N 0 2 F l a s h e d Under A d i a b a t l c Conditions 56 12 P h o t o m e t e r T r a c e s of a P o r t i o n of the NO-> uv s y s t e m 58 13 C o m p a r i s o n of P e a k Height M e a s u r e m e n t s vs A r e a 60 14 P h o t o m e t e r T r a c e s of the long Wavelength S y s t e m 62 of SO 2 15 Short Wavelength S p e c t r a of V a r i o u s P r e s s u r e s of 63 S 0 2 F l a s h e d Under A d i a b a t i c Conditions 16 P h o t o m e t e r T r a c e s of P l a t e 15 0. 04 t o r r of S 0 2 65 17 S p e c t r a O b s e r v e d in the A d i a b a t i c F l a s h P h o t o l y s i s 67 of 0. 04 t o r r of S 0 2 18 V a c u u m uv S p e c t r a of SO2 F l a s h e d (a) A d i a b a t i c a l l y 70 and (b) Isot.herma.lly 19 V a c u u m uv Bands O b s e r v e d in the F l a s h P h o t o l y s i s 71 of SO 2 I w i sh to express my s incere gratitude to D r . N o r m a n B a s c o for his advice and encouragement throughout the course of this work . I am also indebted to M r . M a r k Vagg and M r . E r i c F i s h e r for the i r invaluable ass is tance in construct ing the flash apparatus. C H A P T E R I  I N T R O D U C T I O N I T H E I S O T H E R M A L F L A S H P H O T O L Y S I S O F N O , The f lash photoly s is of N O o was f i r s t invest igated by L i p s c o m b , N o r r i s h and T h r u s h ^ i n 1955. They observed v ib ra t i on -a l l y exci ted oxygen (represented as 0 2 * ) with up to eight quanta of energy in the ground e lec t ron ic state. The mechanism, given for i ts format ion was the s imple two step p rocess : N O ' — 8 > NO + O (1) O (?P) + NOz—3>NO + Q 2 * AH= -48 k c a l (2) F r o m the i r work it i s c lear that the energy d i s t r ibu t ion dur ing o r ^ i m -mediately after the reac t ion is quite different f rom that given by a M a x w e l l Bo l t zmann d i s t r ibu t ion and i n addit ion a large amount of the energy may be found in the fo rm of v ibra t ion of the newly formed mole -cule. Because radia t ive t rans i t ions of O * to the ground v ibra t iona l state are only a l lowed as magnetic dipole or quadrupole t rans i t ions and hence are slow, c o l l i s i o n a l deact ivat ion in the mic rosecond range w i l l be the only process of impor tance . C o l l i s i o n a l deactivat ion of 0 2 * was found to be ve ry ineffective with A r and N 2 (one effective c o l l i s i o n 7 in 10 ) and quite effective with N 0 2 (one in less than 500). Aga in due to the t ime reso lu t ion of the apparatus and the p re s su res of both NO., and A r used, L i p s c o m b et a l bel ieved that the energy d is t r ibu t ion they observed at shortest t ime delays was very close to the i n i t i a l d i s t r ibu t ion I - P a g e 2 -T H E I S O T H E R M A L F L A S H P H O T O L Y S I S O F NO-> ( C o n t ' d . ) —• —— 1 • —• —— '—• ~~& p r o d u c e d f r o m the r e a c t i o n (2). T h e y e s t i m a t e d that at the s h o r t e s t d e l a y the v"=5, 6 and 7 l e v e l s of 0 2 w e r e 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 the v " = 8 w a s p o p u l a t e d to a l e s s e r ex tent . T h e s tudy of the N 0 2 r e a c t i o n as w e l l as a n u m b e r of s i m i l a r r e a c t i o n s s t u d i e d b y 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 to the f o l l o w i n g g e n e r a l i z a t i o n (2, 3). " W h e n 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 of the g e n e r a l f o r m A + B C D — & A B + C D o c c u r s , the m o l e c u l e A B w i t h the n e w l y f o r m e d b o n d t a k e s a h i g h p r o -p o r t i o n of the e x o t h e r m i c e n e r g y of the r e a c t i o n i n the f o r m of u n e q u i l i " 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 s o m e doubt that a l a r g e p o r t i o n of the heat of r e a c t i o n i s found i n the f o r m of v i b r a t i o n of the n e w l y f o r m e d bond fo r a l l of the r e a c t i o n s s t u d i e d , t h e r e ha s b e e n o n l y one r e p o r t to date i n w h i c h the e n e r g y i n v i b r a t i o n of the n e w l y f o r m e d bond e x c e e d s the (4 ,5) e x o t h e r m i c i t y of the r e a c t i o n f o r m i n g i t . B a s c o and N o r r i s h r e p o r t e d s e e i n g O ^ * w i t h up to t h i r t e e n quan ta of v i b r a t i o n a l e n e r g y . T h i s e x c e e d e d the e x o t h e r m i c i t y of r e a c t i o n (2) by a p p r o x i m a t e l y 4 k c a l . S i n c e the (0, 13) was s een o n l y v e r y f a i n t l y and the (0, 12) was o n l y 1 k c a l i n e x c e s s , w h i c h c o u l d be a c c o u n t e d fo r by the e n e r g y of a c t i v a t i o n , the m a t t e r w a s not p u r s u e d . O n t h i s ex t ended s e r i e s of v i b r a t i o n a l l e v e l s of O ^ 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 e s t i m a t e i n w h i c h the p o p u l a t i o n of the l e v e l s w e r e found to d e c r e a s e by about a f a c t o r of t h r e e t o w a r d s - P a g e 3 -T H E I S O T H E R M A L F L A S H P H O T O L Y S I S O F N O s u c c e s s i v e h i g h e r l e v e l s . 2 ( C o n t ' d . ) T h e f l a s h p h o t o l y s i s o f N 0 2 w a s a g a i n i n v e s t i g a t e d i n 1962 b y H u s a i n a n d N o r r i s h ^ i n o r d e r t o d e t e r m i n e t h e r o l e , i f a n y , p l a y e d b y t h e N O 3 r a d i c a l . U n d e r c o n d i t i o n s o f r e l a t i v e l y h i g h d i l u e n t t o N 0 2 r a t i o s a n d o f h i g h N O 2 p a r t i a l p r e s s u r e s , N O ^ w a s s e e n (7) w e a k l y a t 6 6 2 0 a n d 6 2 4 0 ° A a t s h o r t t i m e d e l a y s . T h u s f o r t h e s e c o n d i t i o n s , a t l e a s t , t h e m e c h a n i s m o f t h e f l a s h p h o t o l y s i s o f N 0 2 h a d t o b e a l t e r e d t o i n c l u d e N O ^ a s a n i n t e r m e d i a t e . T h e e f f e c t o f t h i s o n t h e p r e s e n t i n v e s t i g a t i o n o f N O 2 w i l l b e d i s c u s s e d l a t e r . K a n e e t a l ^ e x t e n d e d t h e k i n e t i c s p e c t r o s c o p y o f N 0 2 . i n t o t h e v a c u u m u l t r a v i o l e t . T h e y w e r e t h e n a b l e t o o b s e r v e t h e o x y g e n b a n d s o f l o w v " n u m b e r , b e c a u s e t h e d i s c r e t e p o r t i o n o f t h e S c h u m a n n R u n g e 3 ~ 3 — s y s t e m o f 0 2 ( 2 . u ^ - £. g ) , w h i c h i s t h e s y s t e m u s e d t o o b s e r v e O 2 * , o e x t e n d s f r o m a b o u t 1 7 5 0 A i n t o t h e v i s i b l e r e g i o n . P r e v i o u s l y , o n l y l e v e l s g r e a t e r t h a n v " = 4 h a d b e e n o b s e r v e d . T h e y 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 f o r t h e l e v e l s v " = 0 t o 8 a n d f o u n d t h a t t h e l e v e l s 0 t o 4 f o l l o w e d a B o l t z m a n n d i s t r i b u t i o n f o r 7 0 0 ° K . T h e h i g h e r l e v e l s t h e n d e v i a t e d m a r k e d l y f r o m t h i s d i s t r i b u t i o n . T h e p o p u l a t i o n o f t h e l e v e l s w e r e f o u n d t o d e c r e a s e a s t h e v i b r a t i o n a l q u a n t u m n u m b e r i n c r e a s e d a n d t h e p o p u l a t i o n r a t i o 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 ( v " : v " + 1) r a n g e d f r o m 5 0 : 1 t o 2 : 1 . F r o m t h i s to n n ° d i s t r i b u t i o n t h e y c a l c u l a t e d t h a t t h e t o t a l m o l a r v i b r a t i o n a l e n e r g y r e p r e -s e n t e d o n l y 0 . 47c o f t h e h e a t o f r e a c t i o n . , . . . . 4 - P a g e A -T H E I S O T H E R M A L F L A S H P H O T O L Y S I S O F N O ^ , ( C o n t ' d . ) T h i s c o m m u n i c a t i o n w a s i m m e d i a t e l y f o l l o w e d b y o n e f r o m B a s s ' a n d G a r v i n ^ w h o h a d c a r r i e d o u t a n i n d e p e n d e n t s t u d y i n t h e s a m e s p e c t r a l r e g i o n a n d h a d o b t a i n e d m a r k e d l y d i f f e r e n t r e s u l t s . T h e y c o u l d a s s i g n n o b a n d s a r i s i n g f r o m t h e v " = 0 o r 1 l e v e l s . I n f a c t u s i n g t h e s a m e c o n d i t i o n s a s t h e p r e v i o u s w o r k e r s t h e y f o u n d t h a t t h e s m a l l e s t d e t e c t a b l e s a m p l e o f 0 2 (v"=0) w a s t w i c e a s m u c h a s t h e m a x i m u m p o s s i b l e y i e l d o f O ^ p r o d u c e d f r o m f l a s h i n g N 0 2 , a n d t h e y c o n c l u d e t h a t K a n e et a l m u s t h a v e c o n f u s e d s t r o n g N O b a n d s i n t h a t r e g i o n f o r 0 2 (v"=0 a n d 1). B a s s a n d G a r v i n f o u n d t h e l e v e l s v " = 2 t o 6 t o b e 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 a n d t h a t t h e v i b r a t i o n a l e n e r g y o f t h e e x c i t e d o x y g e n c o u l d i n v o l v e a s m u c h a s 10% of t h e h e a t o f r e a c t i o n . T h e s e 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 w e r e d e t e r m i n e d f o r a 60 f j s e c . s p e c t r o s c o p i c l a m p d e l a y w h i c h (1) i s c l o s e t o t h e h a l f l i f e (75 fj sec. ) r e p o r t e d f o r t h e v " = 6 l e v e l w h i c h i n -d i c a t e s t h a t t h e d i s t r i b u t i o n g i v e n i s u n l i k e l y t o b e t h e o n e i n w h i c h t h e p r o d u c t m o l e c u l e s a r e i n i t i a l l y f o r m e d . T h e t o t a l e n e r g y d i s t r i b u t i o n o f p r o d u c t s a s w e l l a s a n g u l a r m o m e n t u m c h a n g e s c a n d e t e r m i n e t h e s h a p e o f t h e p o t e n t i a l e n e r g y h y p e r -( 1 0 , 1 1 ) s u r f a c e s o f t h e r e a c t i n g s p e c i e s a s s h o w n b y P o l a n y i e t a l . A s a r e s u l t a c c u r a t e i n i t i a l v i b r a t i o n a l e n e r g y d i s t r i b u t i o n s o f p r o d u c t s f r o m s u i t a b l e r e a c t i o n s c a n b e o f f u n d a m e n t a l i m p o r t a n c e i n d e t e r m i n i n g t h e r e a c t i o n p a t h w a y . ; - Page 5 -II T H E A D I A B A T I C F L A S H P H O T O L Y S I S O F SO? B e c a u s e the near adiabatic f l a s h photolysis of N 0 2 has been found to have features c l o s e l y p a r a l l e l i n g those of S 0 2 some background on the l a t t e r s p e c i e s i s included here. T h e adiabatic f l a s h p h o t o l y s i s of a species i s c a r r i e d out i n the absence of excess diluent, thus the p h o t o c h e m i c a l l y r e s p o n s i v e species i s subjected to an adiabatic shock which can i n c r e a s e the temperature of the system to s e v e r a l thousand degrees in a t i m e of the o r d e r of a few m i c r o -seconds. On the other hand, the i s o t h e r m a l method in v o l v e s the use of a l a r g e excess of moderating gas such as N 2 or A r which has a r e l a t i v e l y l a r g e heat capacity. Within the time r e s o l u t i o n of the ap-paratus the energy except for v i b r a t i o n a l degrees of f r e e d o m i s t h e r m a l l y . _•„-. e q u i l i b r a t e d by c o l l i s i o n s with the moderating gas. It was f i r s t r e p o r t e d by N o r r i s h and Z e e l e n b e r g ^ ^ that when ' 1 t o r r of S 0 2 i s f l a s h e d a d i a b a t i c a l l y the v i b r a t i o n a l s t r u c t u r e of its longer wavelength s y s t e m disappears'Completely then reappears over a .period of s e v e r a l m i l l i s e c o n d s . N o r r i s h and O l d e r s h a w ^ ^ extended this work to s h o r t e r wavelengths and found that i m m e d i a t e l y after the f l a s h the short wavelength s y s t e m of S 0 2 was r e p l a c e d by a strong continuous absorption. Bands belonging to the v"=0, 1, 2 and 3 p r o g r e s s i o n s of SO were also o b s e r v e d i n this r e g i o n but the amount of S 0 2 decomposition n e c e s s a r y for its produc-tion was v e r y slight. T h e degree of S 0 2 disappearance was dependent on the p r e s s u r e 6 Page 6 -II T H E A D I A B A T I C F L A S H P H O T O L Y S I S O F SO, (Cont'd.) of moderating gas added. The effect was greatest with SO^ alone and a l m o s t non-existent with a ratio: of 100:1 (N£:S02). N o r r i s h and O l d e r -shaw est i m a t e d the t e m p e r a t u r e r i s e i n the undiluted s e r i e s to be of the o r d e r of 3000°C. T h e y concluded that (a) the n o r m a l sulphur dioxide s p e c t r u m i s weakened on heating and completely e l i m i n a t e d at the high t e m p e r a t u r e generated by the f l a s h , (b) the effects were t h e r m a l r a t h e r than photochemical, and (c) the continuum generated in the far uv must be a s s o c i a t e d with a new s p e c i e s of sulphur dioxide. T h i s s p e c i e s was designated as (SO,,) and was postulated to be a high t e m p e r a t u r e i s o m e r of normal' S 0 2 . • , - • (14),,. , - - ^ . :.. ., .., • M c G a r v e y and M c G r a t h f u r t h e r extended the SO-, •absorp"-"'.-' tion m easurement into the vacuum uv. When low p r e s s u r e s (0. 1 t o r r ) of SO^ were flashed, the short wavelength r e g i o n of the SO2 s p e c t r u m was seen to weaken but no new continuous absorption was found. In the 1840 to 1780°A r e g i o n two groups of strong t r a n s i e n t bands appeared. T h e s e bands were also present in the f l a s h photolysis of H^S + O2 where the product SO,, has been r e p o r t e d to take a r e m a r k a b l y long time to appear (12, 15)^ T h e y concluded that the S 0 2 i s o r i g i n a l l y f o r m e d i n an i s o m e r i c f o r m of sulphur dioxide (SO2) and the t r a n s i e n t bands o b s e r v e d at short t i m e s are a r e s u l t of this s p e c i e s . H e r m a n et a l ^ ^ heated SO^ to 700°K and found that its banded s t r u c t u r e in the longer wavelength s y s t e m 2700 to 3200°A d i m i n i s h e d but - Page 7 -II T H E A D I A B A T I C F L A S H P H O T O L Y S I S O F SC>2 (Cont'd.) continuous a b s o r p t i o n remained. T h e o v e r a l l absorption co e f f i c i e n t r e m a i n e d s u b s t a n t i a l l y the same between 2500 and 3500°A over the t e m p e r a t u r e range 300 to 700°K although the s p e c t r u m appeared to change s i g n i f i c a n t l y . T h e y attributed the apparent continuum to a g r e a t e r complexity of the r o t a t i o n a l fine s t r u c t u r e at higher t e m p e r a -ture s . (17) K i n e t i c a l l y Gaydon et a l have found no evidence to support the i s o m e r hypothesis of SO^ in t h e i r shock tube studies of that species. At shock t e m p e r a t u r e s of around 1200°K they found that the short wavelength s y s t e m changes f r o m banded to continuous absorp-tion but the l a t t e r i s of comparable strength, to and in the same r e g i o n a.s the S 0 2 banded s t r u c t u r e of the short wavelength system. HI P R E S E N T I N V E S T I G A T I O N An attempt was made to obtain the i n i t i a l 0 2 v i b r a t i o n a l energy d i s t r i b u t i o n produced by the f l a s h p h o t o l y s i s of N 0 2 using an ap-paratus of i m p r o v e d e f f i c i e n c y . In this work, the s p e c t r u m of oxygen with up to fifteen quanta of v i b r a t i o n a l energy i n the ground e l e c t r o n i c state was observed. The highest l e v e l c o r r e s p o n d s to an energy 14 k c a l i n e x c e s s of the e x o t h e r m i c i t y of r e a c t i o n (2). T h i s apparent anomaly (with the r e s u l t s of M c G r a t h and N o r r i s h ) has been investigated f u r t h e r and we r e p o r t a s a t i s f a c t o r y explanation consistant with t h e i r r e s u l t s . F o r the near adiabatic f l a s h photolysis of NO an effect was C* found that i s v e r y s i m i l a r to that shown by the adiabatic f l a s h p h o t o l y s i s - Page 8 -P R E S E N T I N V E S T I G A T I O N (Cont'd.) of SO^- Both s p e c i e s have been investigated and the cause of the effect in each case has been found to be the same; namely a s p e c t r a l t e m p e r a t u r e effect r a t h e r than the f o r m a t i o n of a high t e mperature (13) i s o m e r as suggested by N o r r i s h and O l d e r s h a w for SO . - Page 9 " C H A P T E R 2 - E X P E R I M E N T A L  I - M A T E R I A L S Ni t rogen dioxide was prepared f rom M a t h e s o n 98. 9% pure n i t r i c oxide and L i q u i d A i r L t d . 98% pure oxygen. Both gases were d r i ed over P 2 0 ^ after pass ing through a t rap f i l l e d with glass wool and cooled to - 7 8 ° C . A l a rge excess of oxygen was reacted with n i t r i c oxide to f o r m NO-, . The NO., (MP = - 1 0 ° C ) was then frozen out and the O^ pumped off. The r ema in ing so l id was in most cases pure white (used as a c r i t e r i o n of pur i ty) . If a slight blue tinge remained , indicat ing the presence of N ^ O ^ , NO + NO_ — » N O , (3) the N 0 2 was cooled to " 7 8 ° C and pumped on unt i l the NO ( M P - l 6 3 ° C ) was removed . The N O ^ was s tored in a darkened f lask at l i qu id n i t rogen tempera tures unt i l used. Matheson ion iza t ion grade argon (99- 999% pure) was taken straight f r o m the cy l inder and passed through a glass wool f i l l e d t rap cooled to _ 7 8 ° C . It was used as the diluent gas in most of this work . Sulphur dioxide (98. 9% pure) was obtained f rom a cy l inde r , d r i e d over P 2 0 ^ and r e d i s t i l l e d i n vacuo. N i t r i c ac id was prepared by reac t ing concentrated su lphur ic ac id with anhydrous potass ium ni t ra te . The ac id was co l lec ted in a t rap cooled to - 7 8 ° C by flushing the reac t ion f lask with n i t rogen. It was then pur i f i ed by f rac t ional d i s t i l l a t i on i n vacuo. The p repared samples were used the same day. Because of the ve ry react ive nature of the ma te r i a l s used, Dow 10 - Page 10 - . - M A T E R I A L S (Cont'd.) C o r n i n g high vacuum s i l i c o n e g rease was used on a l l taps in the vacuum system. With the use of this g rease it was s t i l l p o s s i b l e to pump the - 5 system to l e s s than 10 t o r r , T o avoid NC^ contact with m e r c u r y , two g l a s s s p i r a l gauges of different s e n s i t i v i t y were c a l i b r a t e d against a m e r c u r y manometer and used to m e a s u r e p r e s s u r e s g r e a t e r than 4 t o r r . F o r p r e s s u r e s l e s s than that c a l i b r a t e d expansion volumes were used. II - A P P A R A T U S The methods of f l a s h p h o t o l y s i s were developed by N o r r i s h and P o r t e r ^ ^ ' i n 1950. In g e n e r a l the method in v o l v e s the production of v e r y high concentrations of t r a n s i e n t i n t e r m e d i a t e s r e s u l t i n g f r o m the p h o t o c h e m i c a l d e c o m p o s i t i o n of an a p p r o p r i a t e species by a v e r y intense short duration f l a s h . F o r this work, the f l a s h i s c r e a t e d by the r a p i d d i s -charge of 500 to 1600J (joules) through an i n e r t gas f i l l e d 50 cm fused s i l i c a tube. T h e concentrations of t r a n s i e n t s produced are, i n c e r t a i n cases, high enough that r a d i c a l - r a d i c a l r e a c t i o n s a r e m o r e important than those between r a d i c a l s and stable m o l e c u l e s . T h e y ar e also high enough i n many cases to allow the photography of t h e i r absorption spec-t r u m by a second f l a s h lamp e l e c t r o n i c a l l y t r i g g e r e d f r o m the f i r s t f l a s h 5 at s p e c i f i e d time delays ranging f r o m 5 to 10 m i c r o s e c o n d s f r o m the beginning of the p h o t o l y s i s f l a s h . T h us f r o m a s e r i e s of experiments the growth and decay of t r a n s i e n t s p e c i e s in the r e a c t i n g system may be ob-served. - P age 11 -H - A P P A R A T U S (Cont'd.) A b l o c k d i a g r a m of the a r rangement of components of such a conventional f l a s h p h o t o l y s i s apparatus i s given in F i g u r e (1). T h e operation of the a p p a r a t u s e s as follows: -The p h o t o l y s i s lamp A i s mounted p a r a l l e l to the 50 cm r e a c t i o n v e s s e l B which i s in o p t i c a l alignment with the s p e c t r o s c o p i c lamp F at one end and-a s p e c t r o g r a p h G at the other. Both A and B are encased i n a b r a s s c y l i n d e r C l i n e d with an aluminum f o i l r e f l e c t o r . The f l a s h tube u s u a l l y contains 10 to 30 t o r r of argon. A33 JJ F capacitor D j and a 2 JU F capa c i t o r are charged by a high voltage charging unit (I). When the manual high voltage switch (L) is c l o s e d the ca p a c i t o r is d i s c h a r g e d a c r o s s the tungsten e l e c t r o d e s (E) loc a t e d at opposite ends of the f l a s h tube .creating an intense f l a s h . A s the ca p a c i t o r d i s c h a r g e s , a pulse, p i c k e d up f r o m an induction c o i l (J) wound around the high voltage l e a d i s fed into an e l e c t r o n i c delay unit (H) c o n s i s t i n g of a c o l l e c t i o n of c a l i -b r a t e d R C c i r c u i t s . T h e delayed pulse i s strengthened by a s m a l l t h y r a -t r o n - c a p a c i t o r cir.cuit and i s d e l i v e r e d to the g r i d of a l a r g e hydrogen t h y r a t r o n (K) which in t u r n d i s c h a r g e s the s m a l l capacitor (D^) through the s p e c t r o s c o p i c lamp. Thus a s p e c t r o s c o p i c examination of the p r i m a r y products produced f r o m the photolys i s f l a s h i s made p o s s i b l e for a c c u r a t e l y d e t e r m i n e d t i m e s after the i n i t i a t i o n of the r e a c t i o n . A s w e l l as the conventional f l a s h p h o t o l y s i s apparatus a second f l a s h s y s t e m was developed which allowed the photolysis of t r a n s i e n t species. i ' 12 - Page 12 -H L E G E N D A -B -C -D r D 2 -E -F -G -Photolysis Lamp Reaction Vessel Brass Container 33 and 2 jj F Capacitor Respectively. Tungsten Electrodes Spectroscopic Lamp Spectrograph Slit H ~ Electronic Delay Unit I ~ High Voltage Charging Unit J ~ Induction Coil Pick _ up K - Hydrogen Thyratron L - High Voltage Switch M - Gas Inlets from Vacuum System FIG. (1) SCHEMATIC DIAGRAM OF A CONVENTIONAL F L A S H PHOTOLYSIS A P P A R A T U S  - P age 13 -I I - A P P A R A T U S (Cont'd. ) T h e arrangement for this s y s t e m over and above what i s r e q u i r e d for the conventional apparatus i s shown in F i g u r e (2). A second photol y s i s lamp ( a u x i l i a r y ) , which can be e l e c t r o n i c a l l y delayed i n much the same manner as the s p e c t r o s c o p i c lamp, i s p l a c e d p a r a l l e l to the f i r s t photo-l y s i s lamp (main) on the opposite side of the r e a c t i o n v e s s e l . T h e s p e c t r o -s c o p i c lamp i s then t r i g g e r e d f r o m the a u x i l i a r y lamp. T h e t r i g g e r i n g c i r c u i t used to f i r e the a u x i l i a r y lamp was so designed that only a s m a l l amount of the total energy of the d i s c h a r g e went through a hydrogen t h y r a t r o n which i s used as an e l e c t r o n i c switch. T h i s was done because it was b e l i e v e d that the t h y r a t r o n could not withstand the high energy of the f u l l d i s c h a r g e . However recent experiments have shown that it w i l l and a c i r c u i t of the same type as the s p e c t r o s c o p i c lamp c i r c u i t i s now used to f i r e the a u x i l i a r y lamp. T h e c i r c u i t u sed f or this work was one that r e q u i r e d the argon i n the lamp to hold the voltage (V) i m p r e s s e d a c r o s s the e l e c t r o d e s E ^ and E^. The t r i g g e r i n g e l e c t r o d e E 2 which was held at V/2, was dropped to ground potential by d i s c h a r g i n g the two 0. 00 3 fjF c a p a c i t o r s through the t h y r a t r o n when a t r i g g e r i n g pulse f r o m the a u x i l i a r y delay unit H, was applied. T h e argon i n the lamp became conducting since V was then applied over'only half the length of the lamp and allowed the d i s c h a r g e of the 33 (j F ca p a c i t o r between E ^ and E ^ . A s can be i m a g i n e d the argon p r e s s u r e i n the a u x i l i a r y lamp was quite c r i t i c a l for s u c c e s s f u l f i r i n g at a - Page 14 -PULSE P U L S E TO F FROM A JL F I G . (2) " A U X I L I A R Y T R I G G E R I N G C I R C U I T C O M P O N E N T S W I T H S U B S C R I P T 1 A R E D U P L I C A T E C O M P O N E N T S O F ' T H E C O N V E N T I O N A L S Y S T E M , O T H E R -WISE T H E L A B E L I N G IS T H E S A M E AS IN F I G . (1).  - P a g e 1 5 - ' I I - A P P A R A T U S ( C o n t ' d . ) p r e d e t e r m i n e d d e l a y . T o o m u c h a r g o n i n t h e l a m p w o u l d p r e v e n t i t s f i r i n g a t a l l a n d t o o l i t t l e w o u l d c a u s e s p o n t a n e o u s b r e a k d o w n . It w a s f o u n d t h a t t h e s p o n t a n e o u s b r e a k d o w n v o l t a g e o f t h e l a m p s h o u l d e x c e e d t h e w o r k i n g v o l t a g e b y n o t m o r e t h a n 1. 5 K v . I f t h i s l i m i t w a s e x c e e d e d a n d t h e l a m p s t i l l d i d f i r e i t c o u l d b e a t d e l a y s a s h i g h a s t w o h u n d r e d 20 jj sec. o v e r t h e d e s i r e d d e l a y . C h r i s t i e a n d P o r t e r a l s o n o t i c e d t h i s e f f e c t w h e n t h e y t r i e d t o f i r e a s e r i e s o f p h o t o l y s i s l a m p s s i m u l t a n e o u s l y . T h e y a t t r i b u t e d t h e c a u s e o f t h i s i n d u c t i o n p e r i o d t o o x y g e n o r o t h e r i m -p u r i t i e s w h i c h a r e s t r i p p e d f r o m t h e w a l l s o f t h e l a m p o n f l a s h i n g . T h e c u r e f o r t h i s s i t u a t i o n w a s f o u n d t o b e c o n d i t i o n i n g t h e l a m p b y p r e f l a s h i n g a n d p u m p i n g o n i t b e f o r e u s i n g . S i n c e a c c u r a t e t i m e d e l a y s o f t h e a u x i l i a r y l a m p w e r e r e q u i r e d , • o s c i l l o g r a p h i c t r a c e s o f t h e l i g h t o u t p u t o f t h e l a m p s w e r e r e c o r d e d f o r e a c h d o u b l e l a m p e x p e r i m e n t . T h e d e l a y t i m e s q u o t e d a r e m e a s u r e d f r o m t h e p e a k o f t h e f i r s t f l a s h to t h e p e a k o f t h e s e c o n d . T h e c h a r a c t e r i s t i c l i f e t i m e s o f t h e l a m p s a r e u s u a l l y r e p o r t e d a s t h e w i d t h a t h a l f t h e p e a k l h e i g h t . v . . T h e l i f e t i m e f o r t h e p h o t o l y s i s l a m p s w a s 18 m i c r o s e c o n d s o v e r a w o r k i n g v o l t a g e range ' : o f 7 t o 9 K v . (800 t o 1 3 0 0 J ) a n d 4 m i c r o s e c o n d s f o r t h e s p e c t r o s c o p i c l a m p a t 10 K v . ( 1 0 0 J ) . T w o s p e c t r o g r a p h s w e r e u s e d i n t h e s e i n v e s t i g a t i o n s . T h e f i r s t w a s a J a r r e l l A s h 3. 4 m e t e r E b e r t m o u n t i n g s p e c t r o g r a p h h a v i n g t h e f o l l o w i n g c h a r a c t e r i s t i c s : (a) 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 16 " Page 16 -II - A P P A R A T U S (Cont 'd. ) (b) two plane gratings each with 15,000 l ines per inch one b l azed for 3300A° and the other for 6000A° . (c) r e c i p r o c a l l inea r d i spe r s ion of 5. l A ° / m m i n f i r s t o rder (d) 2500A° range covered i n f i r s t o rder by two 10 x 4 inch photographic plates . The second spectrograph was a medium H i l g e r p r i s m i n -strument employing quartz opt ics . It was used for low d i spe r s ion work i n the v i s i b l e p r i m a r i l y for the detection of N O 3 . The spec t rum obtained f rom this ins t rument extends f rom 2000A° to 10, 000A° and i s 221 cm long, enabling i t to be r eco rded on a single plate. The r e c i p r o c a l l inear d i s p e r -sion at 2200A° i s approx imate ly 5 A ° / m m whereas at 6000 it i s about 150 A ° / m m . T h i s ins t rument i s most useful for exp lora tory work because of the wide range covered . A s w e l l , it i s useful for studies invo lv ing weak continuous absorpt ion i n the v i s i b l e reg ion since i ts low d i spe r s ion f a c i l i -tates observa t ion of such species . III - F I L T E R S A major por t ion of th is work r equ i red the se lect ive photolysis of NO, , . T h i s was accompl i shed by p lac ing 3 m m th ick C o r n i n g G l a s s f i l t e r s between the photolys is l amp and the reac t ion vesse l . Two f i l t e r s were used; C o r n i n g glass f i l t e r 7~54 .and 3~73 having the t r a n s m i s s i o n p r o -per t ies shown i n F i g u r e (3). The curves are reproduced f rom the C o r n i n g (21) G l a s s W o r k s c i r c u l a r . The t r a n s m i s s i o n of the f i l t e r s was checked on a C a r y 14 uv and v i s i b l e spect rometer after T uany f lashes. No changes in - P a g e 17 -WAVELEN6 TH (millip) F I G U R E (3) - TRANSMISSION CHARACTERISTICS.OF T H E . : CORNING 7~54 A N D 3-73 F I L T E R S . - P age 18 -III - F I L T E R S (Cont'd.) the t r a n s m i s s i o n c h a r a c t e r i s t i c s were found. The 0°/c t r a n s m i s s i o n a r e a s of the f i l t e r s had a m e a s u r e d o p t i c a l density of gr e a t e r than 5. 0. Two r e a c t i o n v e s s e l s of different t r a n s m i s s i o n c h a r a c t e r i s t i c s were used. One was made of S u p r a s i l ( E n g l e h a r d Industries, Inc. ) having \0°/c t r a n s m i s s i o n at l e s s than 1700A 0 and the other was made of P y r e x o(22) (Corning 7~74) with 10% t r a n s m i s s i o n at 2800A . The P y r e x r e a c t i o n v e s s e l was used almost e x c l u s i v e l y with the NO experiments to r u l e out any p o s s i b i l i t y of O(^D) atom contribution in r e a c t i o n (2) f r o m N O z NO ( 2 7T ) + O ( XD) < 2 4 0 0 A ° IV - P H O T O G R A P H Y * . O v e r the l a r g e s p e c t r a l region c o v e r e d i n t h i s investigation j four different types of plates of different s p e c t r a l s e n s i t i v i t y were used. E a c h plate and its c h a r a c t e r i s t i c s a r e d e s c r i b e d . T h e most ex t e n s i v e l y used type was I l f o r d HP3 which i s a fast p a n c h r o m a t i c plate of medium g r a i n and contrast. Its useful working o o range i s 2400 to 6600A . F o r the r e g i o n 2400 to 5000A adequate plate density was obtained with one f l a s h u s i n g a sl i t width of 30 m i c r o n s where-as 50 m i c r o n s was r e q u i r e d for the re g i o n 5000 to 6600A°. C h a r a c t e r i s t i c c urves ( F i g u r e 4) were plotted for three s p e c t r a l r e g i o n s of inte r e s t , f o r the development conidtions d e s c r i b e d . T he v a r i a t i o n i n exposure for these plots was obtained by v a r y i n g the sl i t width since the a r e a i l l u m i n a t e d i s p r o p o r t i o n a l to exposure i f a l l else i s kept constant. 1 ' 19 "Page 19 " R E G I O N , A ° P L A T E G A M M A (Plate d e n s i t y / l o g exposure) O 3 2 5 0 H P 3 1 . 0 " © 3 1 0 0 H P 3 1 . 2 O 2 8 0 0 . HP 3 1 . 2 A 2 2 7 0 Q 2 1. 1 lo ! L_ —I I ' I J t I o.[ o.2 0.4 0 .6 o.g l,o Log Slit Wf dth F I G U R E 4 - C H A R A C T E R I S T I C C U R V E S O F H P 3 AND Q 2 P L A T E S F O R S E L E C T E D W A V E L E N G T H S  - P a g e 20 -IV - P H O T O G R A P H Y (Cont'd.) P l a t e d e n s i t i e s were m e a s u r e d on a Joyce L o e b l double beam 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 using an a c c u r a t e l y c a l i b r a t e d n e u t r a l density wedge. The c u r v e s show a l i n e a r r e l a t i o n s h i p between plate density and l o g ex-posu r e for the 0. 2 to 0. 8 plate density region. A l l intensity m e a s u r e -ments a r e confined to this region. The plates were developed i n Kodak D19 developer for five minutes and f i x e d i n " A m f i x " high speed f i x e r (May & B a k e r L t d . ) for at l e a s t two minutes. T he plates were then washed for 30 minutes and d r i e d i n a dust f r e e container. F o r the re g i o n 2000 to 2 2 0 0 A ° I l f o r d Q 3 and then Q 2 plates were used. T h e s e plates a r e intended f o r r e c o r d i n g r a d i a t i o n which i s p a r t i a l l y a b s o r b e d by the gelatin base in the emulsion. F o r th i s r e a s o n the emxilsion must be v e r y thin and hence they are quite f r a g i l e . T h e (23) Q-j plate was r e p o r t e d to be a fast g r ainy plate and the Q_2 slower and l e s s grainy. When the c h a r a c t e r i s t i c s of the la t t e r were g r e a t l y i m p r o v e d by the I l f o r d Company it was found that they were m o r e suitable f o r this work. T h e s e plates were p r o c e s s e d i n the same manner as HP^ except the p r o c e s s i n g t i m e s were halved. T he Q 2 gamma was found to be v e r y s e n s i t i v e to development time. Hence t h e r e were m i n o r v a r i a t i o n s i n its value. However, the c h a r a c t e r i s t i c c u r v e s were found to be l i n e a r between a plate density r e g i o n of 0. 2 and 0. 8 and that i s a l l that i s r e q u i r e d f o r follow-ing f i r s t o r d e r p r o c e s s e s . 21 - Page 21 -IV - P H O T O G R A P H Y (Cont 'd. ) Kodak spec t roscopic plates type 103-F and 1~N were used for the 6500 to 7000A° and the 6500 to 8500A° regions respec t ive ly . Both plates are ve ry slow reqxuring five to ten flashes per exposure (using a f resh charge of sample for each flash) with wide (0. 4 mm) s l i t s on the J a r r e l l A s h spectrograph. Only two exposures with a 50 m i c r o n s l i t were r e q u i r e d for the med ium H i l g e r because of i ts greater speed and low d i s p e r s i o n in the red . The 103~F plates were . found to be more contras ty and s l ight ly faster than the 1~N plates. A s a resu l t they were more suitable for photographing the 6620A° band of N O ^ which was just out of range of the H P ^ plates. In o rde r to extend the N O ^ spec t rum to as long a wavelength as poss ib le it was neces sa ry to use the 1~N plates . Development t imes for these two plates were 4 minutes in undiluted Kodak D~19. The res t of the p rocess ing was the same as for the H P ^ plates. V - P R E P A R A T I O N O F A M I X T U R E A N D P R O C E D U R E  M i x t u r e s of reactant species and argon were made up at least three hours before commencing the run to ensure homogeneous t h e r m a l m i x i n g . The mix tu r e was made up i n a l a rge blackened f lask at the same argontreactant ra t io to be used i n the exper iments but in suff icient ly higher to ta l p r e s su re to a l low the exper iments to be completed. The re are twenty s t r ips avai lable on each plate which a l lows at least seventeen of these to be t ime exper iments . The other three s t r ips are 22 - P age 22 " V - P R E P A R A T I O N O F A M I X T U R E A N D P R O C E D U R E (Cont'd. ) r e s e r v e d for a blank, a b e f o r e and after exposure. The blank i s s i m p l y an exposure taken with no sample in the r e a c t i o n v e s s e l , the before is the ab s o r p t i o n s p e c t r u m of the r e a c t i o n m i x t u r e i n the c e l l b e f o r e photo l y s i s and the after i s an a b s o r p t i o n s p e c t r u m of products and reactants r e m a i n -ing at a r e l a t i v e l y long t i m e (30 seconds) after the photolysis f l a s h . E a c h of these exposures i s n e c e s s a r y on e v e r y plate to act as a standard for the p a r t i c u l a r conditions used. - Page 23 -C H A P T E R 3 T H E I S O T H E R M A L F L A S H P H O T O L Y S I S O F N O , Spect ra of v ib ra t iona l ly exci ted oxygen ( O 2 * ) in the ground e lec t ron ic state, obtained f rom the f lash photolysis of N O 2 , were r e -3 3 corded through the Schuman Runge sys tem ( £ u *~ 2 g)- F i g u r e 5 shows the major por t ion of the spec t rum that was used for this work . L e v e l s wi th up to 15 quanta of v ib ra t iona l energy corresponding to about 62 k c a l / m o l e , were observed but the two highest l eve ls were too faint to reproduce on the pr in t . The opt imum conditions found for producing the spec t rum, e spec ia l ly for the highest l eve l s (v" = 12, 13, 14, and 15) were 1 to 2 t o r r of N 0 2 f lashed at moderate to high photoflash energies (800 to 2200J). Such high l eve l s of O 2 , which w i l l be represented as O 2 * * , could not be produced under the above conditions when the photoflash was f i l t e red wi th a C o r n i n g 7~54 glass f i l t e r having the t r a n s m i s s i o n cha rac t e r i s t i c s shown in F i g u r e 3. T h i s f i l t e r quite conveniently cuts o\it radiant energy below the p red i s soc i a t i on l i m i t of the N O ^ v i s i b l e sys tem (25, 130 cm ), yet t r a n s m i t s above the l i m i t where the measured quantum y ie lds for the reac t ion : hO 2 3 o N 0 2 — * NO ( TT ) + 0 ( P ) < 4000 A (24) are c lose to unity. . On compar ing the spect ra obtained with and w i t h -out the f i l t e r , care was taken to use runs in which equal amounts of N O 2 were decomposed ra ther than runs with equal total f lash energies , since - P a g e 2 4 -T H E I S O T H E R M A L F L A S H P H O T O L Y S I S O F N Q 2 ( C o n t ' d . ) e n e r g y l o s s e s d u e t o t h e f i l t e r m a y h a v e b e e n a s m u c h a s 2 0 % i n t h e 2 8 0 0 t o 4 0 0 0 A ° r e g i o n . T h e N O ^ c o n c e n t r a t i o n w a s m e a s u r e d f r o m a c a l i b r a t i o n c u r v e o f c o n c e n t r a t i o n v s . p e a k h e i g h t o f t h e 4 3 5 0 A ° b a n d . T h i s c u r v e s h o w e d o n l y a s l i g h t n e g a t i v e ' d e v i a t i o n f r o m l i n e a r i t y o v e r t h e N 0 2 p r e s s u r e r a n g e u s e d . ~~ I - P O P U L A T I O N R A T I O 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 (a) R E S U L T S I n o r d e r t o o b t a i n 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 o f 0 2 f r o m r o t a t i o n a l l i n e i n t e n s i t y m e a s u r e m e n t s c e r t a i n a p p r o x i m a t i o n s t o t h e f u n c t i o n a l r e l a t i o n s h i p o f t h e s e t w o a r e n e c e s s a r y . T h e a p p r o x i m a t i o n t h a t t h e i n t e n s i t y o f r o t a t i o n a l l i n e s i n a b s o r p t i o n i s p r o p o r t i o n a l t o t h 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 o f t h e g r o u n d e l e c t r o n i c s t a t e o f t h e m o l e c u l e i s g i v e n b y t h e f o l l o w i n g (27) ' e q u a t i o n s * . T h e e q u a t i o n f o r t h e i n t e n s i t y i n a b s o r p t i o n o f a n i n d i v i d u a l r o t a t i o n a l l i n e , a s s u m i n g t h e r m a l e q u i l i b r i u m , i s : ^ ' K ' ^ c o n s t . ( N v n / Q ) E v 1 v l l R 2 e ( ; v I v l l ) q v , v „ S K t K M e x p f - E ^ / k T ) (1) w h e r e t h e c o n s t a n t d e p e n d s o n t h e u n i t s a n d t h e g e o m e t r y o f t h e c e l l , N , , i s t h e p o p u l a t i o n o f t h e v i b r a t i o n a l l e v e l f r o m w h i c h t h e r o t a t i o n a l t r a n s i t i o n o r i g i n a t e s , E ^ i ^ n i s t h e e n e r g y - c o r r e s p o n d i n g t o t h e t r a n s i t i o n K " t o K ' , R ( r , , , ) i s t h e e l e c t r o n i c t r a n s i t i o n m o m e n t , q , , , i s t h e F r a n c k ' C o n d o n e v v v v f a c t o r f o r t h e p a r t i c u l a r b a n d ( v ' , v " ) , Q i s t h e r o t a t i o n a l p a r t i t i o n f u n c t i o n , £ > K ' K " i s t h e r o t a t i o n a l l i n e s t r e n g t h , E ^ ^ i s t h e r o t a t i o n a l e n e r g y t e r m a n d T i s t h e e f f e c t i v e r o t a t i o n a l t e m p e r a t u r e . R e l a t i v e p o p u l a t i o n s o f t h e v a r i o u s l e v e l s c a n t h e n b e c o m p u t e d , i n t h e f o r m o f r a t i o s , b y d i v i d i n g t h e a b o v e e q u a -t i o n f o r t h e v " + 1 l e v e l b y t h e s a m e e q u a t i o n f o r t h e v " l e v e l a n d t h e n r e -a r r a n g i n g t o g i v e : N v „ + 1 = I K , K „ ( v " + P Q ( v " ) E K , K „ ( v " +' 1 ) R 2 q ( r v ' v " + l ) q ( v ' v " + l ) S K , K „ ( v " + l )  N v " : IK' K" ( v , , ) Q ( v " + 1 ) E K . K H ( v M ) R 2 e ( r v ' v " ) q ( v ' v " ) S ^ , , ( v n ) e x p ( - ( E ( v " ) - E ( v " + l ) / k T ) (2) r o t r o t . . . 2 5 - P a g e 25 -2400 W a v e l e n g t h (A°) 250 0 2600' 2700 I -ww [ B e f o r e 10 fjsec. 20 40 60 (71) 100 138 166 H P U l 1 M i — i — J — J J (4,6) (3,6) (4,8) (3,8) (2,8) (1,8) (5,7) (4, 7) (3, 7) (2, 7) (3, 9) (2, 9) (1, 9) 290 0 3000 I 3100 I 3200 I 3300 I I L J [ I 0, 10 (1, 11) (0, 11) (1, 12) (0, 12) 0, 13 T r a n s i t i o n s ( H e r z b e r g a n d P e a r s e a n d G a y d o n ) B e f o r e 10 f j s e c . 20 40 60 (71) 100 138 166 A f t e r 25, 26 F i g u r e 5 A p o r t i o n o f t h e S c h u m a n _ R u n g e s y s t e m o f 3 < 3 I £ 0.) o b s e r v e d i n t h e i s o t h e r m a l f l a s h p h o t o l y s i s o f N O z . P N O -1 t o r r a r g o n T 150 t o r r - Page 2 6 -I - P O P U L A T I O N R A T I O 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 (Cont'd. ) (a) R E S U L T S Since the same r o t a t i o n a l l e v e l s are m e a s u r e d i n each case, the last two t e r m s of the above equation w i l l cancel. A l s o , the p a r t i t i o n function, the t e r m value, the e l e c t r o n i c t r a n s i t i o n moment and the t r a n s i t i o n energy r a t i o s w i l l be v e r y n e a r l y one due to the v e r y s m a l l change i n the i n t e r -n u c l e a r s e p a r a t i o n of the O-, m o l e c u l e between the two s u c c e s s i v e v i b r a -t i o n a l l e v e l s . T h i s i s not t r u e for the F r anck"Condon f a c t o r r a t i o s because the o v e r l a p i n t e g r a l s of which they are c o m p r i s e d are s e n s i t i v e to s m a l l changes i n i n t e r n u c l e a r distance. T hus there r e m a i n s only: NV " + i = q ( v ' v " ) W"^ " + l ] N v " q(v'v"+ 1) ""K'K" ^V") Intensity m easurements were made on a Joyce L o e b l M a r k III double beam 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 for the strongest bands of each of the v" p r o g r e s s i o n s 6 to 13. T h e s e oxygen bands were the (3,6), (3,7), (2,8), (1,9), (1,10), (0,11), (0,12), and (0, 13). Both the (0,14) and the (0, 15) bands were e a s i l y o b s e r v e d at short t i m e delays but they were too weak to photometer. However, they were m e a s u r e d roughly by eye estimate. Since the r e s o l u t i o n and d i s p e r s i o n of the s p e c t r o g r a p h used was such that the oxygen r o t a t i o n a l l i n e s were r e s o l v e d , s e v e r a l r o t a t i o n a l l i n e i n t e n s i t i e s near the band head were m e a s u r e d for each of the above bands for a number of delays and f l a s h e n e r g i e s . B y repeating a number of e x p e r i -ments s e v e r a l times, these m e a s u r e m e n t s were found to be r e p r o d u c i b l e 27 - Page 27 -I - P O P U L A T I O N R A T I O S O F V I B R A T ION A L L Y E X C I T E D O X Y G E N (Cont 'd. ) (a) R E S U L T S wi th in 10% for the l eve l s greater than v" = 8 and wi th in 15% for the lower l eve l s . Re la t ive v ib ra t iona l l e v e l populations were ca lcula ted as fo l lows; The peak heights of the f i r s t three rota t ional l ines f rom the band head were measured . These were assumed to be p ropor t iona l to the integrated absorpt ion of the ro ta t ional l ines since the photometer t r aces of these l ines r e sembled sharp peaks that could be represented as t r i ang les . P r o v i d e d the base remained reasonably constant peaks of this shape have the i r height p ropor t iona l to the a rea under them (Area= 1/2(base t imes height). To test whether using peak heights was jus t i f ied in de termining re la t ive I j ^ 'K" values, half the reac t ion ves se l was masked off in order that (19) only half the number of O * molecu les would be produced . Seve ra l iden t ica l exper iments were c a r r i e d out with the mask cover ing different por t ions of the reac t ion v e s s e l . The O2* in tensi t ies were found to be iden t ica l for a l l posi t ions of the mask indicat ing that l ight absorpt ion from the spec t roscopic lamp is uniform over the entire length of the react ion ves se l . If the peak heights were propor t iona l to the integrated absorpt ion (I , 1 ) of the ro ta t ional l ine , then, for this type of exper iment , the peak heights (h) for a se r i e s of exposures plotted against the peak heights f rom a s i m i l a r s e r i e s with half the reac t ion vesse l masked off (h ) should 1/2 28 ~ P a g e 28 -I - P O P U L A T I O N R A T I O 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 ( C o n t ' d . ) (a) R E S U L T S g i v e a s t r a i g h t l i n e o f s l o p e 2 , e q u a t i o n (4 ) . h I ( v " ) N v" = K ' K " v ' = v " 2 (4) h l / 2 V " I 1 / 2 K ' K " ( V " ) 1 / 2 ( N V . - ) S i n c e t h e h a n d w e r e m e a s u r e d f o r t h e same b a n d , t h e F r a n c k -C o n d o n f a c t o r s w e r e n o t n e c e s s a r y f o r t h i s p l o t . P l o t s o f t h i s t y p e w e r e m a d e f o r t h e b a n d s ( 3 , 6 ) , (4, 6), (4, 7 ) , ( 2 , 8 ) , ( 0 , 9 ) , ( 1 , 9 ) , a n d ( 1 , 1 0 ) a s s h o w n i n F i g u r e (6) . A l l , e x c e p t t h o s e f o r t h e (4, 6) a n d (4 , 7) h a d s l o p e s r a n g i n g b e t w e e n 1. 9 a n d 2 . 0 . T h e o t h e r t w o h a d s l o p e s o f 2 . 4 a n d 2 . 3 r e s p e c t i v e l y . T h i s w a s a t t r i b u t e d t o o v e r l a p p i n g w i t h m u c h w e a k e r b a n d s o f O a n d N O - . W i t h t h e k n o w l e d g e t h a t p e a k h e i g h t s o f i n d i v i d u a l r o t a t i o n a l l i n e s c o u l d b e u s e d to a g o o d a p p r o x i m a t i o n to d e t e r m i n e r e l a t i v e b a n d i n -t e n s i t i e s o f t h e 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 p r o d u c e d i n t h e f l a s h p h o t o l y s i s o f N O ^ , r e l a t i v e p o p u l a t i o n s o f t h e s e l e v e l s w e r e c a l c u l a t e d u s i n g e q u a t i o n 3 a n d t w o i n d e p e n d e n t l y c a l c u l a t e d a r r a y s o f F r a n c k - C o n d o n f a c t o r s f o r t h e S c h u m a n R u n g e s y s t e m o f O ^ . O n e o f t h e a r r a y s w a s c a l c u l a t e d b y P i l l o w i n 1 9 4 9 a n d t h e o t h e r b y N i c h o l l s ^ ^ i n I 9 6 0 . T h e f i r s t a r r a y w a s t h a t u s e d b y K a n e et a l i n t h e i r d e t e r m i n a t i o n o f t h e p o p u l a t i o n r a t i o s f o r (•8) (9) t h i s s y s t e m * ' a n d t h e s e c o n d w a s u s e d i n p a r t b y B a s s a n d G a r v i n C o m p a r i s o n o f t h e s e t w o a r r a y s s h o w e d v e r y l i t t l e d i f f e r e n c e i n t h e i r r e l a t i v e v a l u e s , i n d i c a t i n g t h a t t h e l a r g e d i s c r e p a n c y b e t w e e n t h e t w o 29 (4.6) o (4.7) 0 ( 3 ,6 ) 0 (2,8) O (0,9) A (1,9) V (1, 10) F I G U R E 6 - Hal f path plot for the O^* leve ls produced in. the f lash photolysis of NO . P 2 NO, 1 t o r r P =' 150 t o r r argon 70% decomposi t ion , h and ^^/z a r e ^ e P e a ^ heights for the ful l and half path se r i e s r e spec t ive ly . Slope = 2 - P a g e 30 -I - P O P U L A T I O N R A T I O 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 ( C o n t ' d . ) (a) R E S U L T S p u b l i s h e d p o p u l a t i o n d i s t r i b u t i o n s c a n n o t b e d u e t o u s e o f d i f f e r e n t F r a n c k - C o n d o n f a c t o r s . T h e F r a n c k ~ C o n d o n a r r a y o f N i c h o l l s i s p r o b a b l y m o r e a c c u r a t e , h o w e v e r . H i s v a l u e s w e r e c a l c u l a t e d i n t h e s a m e m a n n e r a s P i l l o w ' s b u t w i t h t h e a v a i l a b i l i t y o f c o m p u t e r s f o r h i s w o r k , s u m m a t i o n o v e r m u c h s m a l l e r i n t e r v a l s i n t h e n u m e r i c a l e v a l u a t i o n o f t h e o v e r l a p i n t e g r a l s w a s p o s s i b l e . I n f a c t , P i l l o w ' s v a l u e s w e r e f o u n d t o l o s e i n t e r n a l c o n s i s t e n c y f o r t h e h i g h e r l e v e l s . T h a t i s , t h e e x p e r i m e n t a l l y d e t e r m i n e d i n t e n s i t y r a t i o f o r t w o b a n d s o r i g i n a t i n g f r o m t h e s a m e v " l e v e l s h o u l d a g r e e v e r y c l o s e l y w i t h t h e i r F r a n c k -C o n d o n f a c t o r r a t i o . F o r t h e h i g h l e v e l s ( v " > 10) , w h i c h K a n e et a l d i d -n o t o b s e r v e , P i l l o w ' s v a l u e s d i f f e r e d b y a s m u c h a s a f a c t o r o f t e n f r o m t h e r e s p e c t i v e i n t e n s i t y r a t i o s , w h e r e a s N i c h o l l ' s v a l u e s w e r e f o u n d t o b e c o n s i s t e n t . T h e r e l a t i v e p o p u l a 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- , w e r e c o m p u t e d f o r p h o t o f l a s h e n e r g i e s o f 800 a n d 1300 j o u l e s a t t i m e s r a n g i n g f r o m 10 to 100 m i c r o s e c o n d s a f t e r i n i t i a t i o n o f t h e p h o t o f l a s h . T h e r e s u l t s f o r t h e r a t i o s N ^ , + ^/N n ( 7 / 6 to 1 3 / 1 2 ) a r e g i v e n i n T a b l e s I a n d I I f o r b o t h a r r a y s . It c a n b e s e e n t h a t t h e r e i s n o s i g n i f i c a n t d i f f e r e n c e i n t h e r a t i o s f o r t h e t w o e n e r g i e s o r t h e q v a l u e s u s e d . F i g u r e 7 s h o w s a v ' v p l o t o f t h e r a t i o v a l u e s f o r u n f i l t e r e d e x p e r i m e n t s a l o n g w i t h t h o s e f i l t e r e d b y t h e 7 - 5 4 C o r n i n g f i l t e r . I n c l u d e d a s w e l l a r e t h e r a t i o s e s t i m a t e d b y - Page 31 -I - P O P U L A T I O N R A T I O 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 (Cont 'd.) B a s s and G a r v i n and those est imated by Kane et a l . Both groups did thei r exper iments i n this energy range though s t r i c t l y speaking that i s i m m a t e r i a l since it i s the amount of energy absorbed by the species which i s important . T h i s i s dependent on the eff iciency of the apparatus which i n turn is a function of the geometry and composi t ion of both the c e l l and re f lec to r . F o r this reason it is much better to report the amount of reactant decomposed. F o r 1300 joules 75% of the 1 t o r r sar-nple of NO,, i s decomposed whereas 50% i s decomposed for an 800 joule f lash in this work . Bes ides the different a r r a y s , a second point on which the three d i s t r ibu t ion measurements differ i s the t ime delay at which they were made. B a s s and G a r v i n measured the i r d i s t r ibu t ion at s ix ty mic roseconds , Kane et a l .measured the i r s at twenty. However , it appears that the t ime at which the dis t r ibut ion, i s measured is not too important since the O.^* re laxa t ion p rocess involves a constant ra t io of populations between l eve l s , except for the highest observed. T h i s i s seen by compar ing the low l eve l r a t ios given in Tab les I and II with t i m e . Whether the d i s t r ibu t ion desc r ibed here corresponds to the i n i t i a l d i s t r ibu t ion or not i s another matter . Because the ra t ios change ve ry l i t t l e with t ime it i s v e r y l i k e l y that this population d i s t r ibu t ion of the v" l eve l s i s the i n i t i a l d i s t r ibu t ion and the only question r emain ing i s : of the total O^ produced, how much is formed in a v ib r a t i ona l l y exci ted state? - Page 32 -1. 0 1 ' 5. . 6 7. 8 J£ J L Q . i l JLi • J l 14 3 4. 7 6 7 8 9 10 11 12 13 (a) (v" I- l ) / v " . 7 8 9 10 11 12 13 14 (b) v" ' F I G U R E 7 (a) Popu la t ion ra t ios of v i b r a t i o n a l l y exci ted oxygen produced i n the f lash photolys is of 1 t o r r . NG*2 in 150 t o r r argon, measured at 20 p sec. , o a re the ra t ios for the unf i l te red exper iments and 0 for those f i l t e r ed by the C o r n i n g 7~54 f i l t e r ; us ing N i c h o l l s a r r a y of F r a n c k _ C o n d o n fac tors . •• . . are those es t imated from the work of B a s s and G a r v i n and ^ are given by Kane et a l . (b) R e l a t i v e populations of each v ib ra t i ona l l e v e l 7 to 14 - Page 3 3 -T A B L E I P O P U L A T I O N R A T I O S O F O z V I B R A T I O N A L L E V E L S W I T H O U T F I L T E R USING P I L L O W ' S A R R A Y O F F R A N C K - C O N D O N F A C T O R S F O R 50% T O T A L NO„ D E C O M P O S I T I O N Rat io Timewec 7/6 8/7 9/8 10/9 11/10 12/11 13/12 14/13 11 0. 8 0. 9 0. 7 0. 7 0. , 8 0. 1 20 0. 9 0. 9 0. 7 0. 7 0. 8 0. 1 40 0. 9 0. 9 0. 6 0. 6 0. , 6 0. 1 62 0. 8 1. 0 0. 7 0. 6 0. . 5 75 0. 8 1. 0 0. 7 0. 6 (• 2) 96 0. 8 1. 0 0. 6 0. 6 F O R 75% N 0 2 D E C O M P O S I T I O N Rat io 7/6 8/7 9/8 10/9 11/10 12/11 13/12 14/13 11 0. 9 0. 7 0. 7 0. 6 0. 7 0. 2 0. 6 20 0. 9 0. 7 0. 7 0. 6 0. 7 0. 1 0. 8 (0. 8) 48 0. 8 0. 7 0. 8 0.6 0. 6 0. 1 71 0. 8 0. 7 0. 7 0. 6 0. 4 100 0. 9 0. 7 0. 7 0. 6 ( • 1) The ra t ios i n parentheses are obtained f rom eye est imate of the band in tens i t ies . - Page 34. " T A B L E II P O P U L A T I O N R A T I O S O F 0 2 V I B R A T I O N A L L E V E L S W I T H O U T F I L T E R USING N I C H O L L S A R R A Y O F F R A N C K ~ C O N D O N F A C T O R S F O R O z 50% T O T A L N 0 2 D E C O M P O S I T I O N Rat io T i m e 7/6 8/7 9/8 10/9 11/10 12/11 13/12 14/13 ptsec 11 1. 2 0. 7 0. 6 0. 6 0. 7 0. 2 • 20 1. 2 0. 7 0. 7 0. 6 0. 7 0. 2 0. 8 (0. 8) 40 1. 3 0. 6 0. 6 0. 6 0. 5 0. 2 62 1. 1 0. 8 0. 7 0. 6 0. 3 75 1. 1 0. 8 0. 7 ' 0. 6 (• 1) 96 1. 1 0. 8 0. 6 0. 6 75% T O T A L N 0 2 D E C O M P O S I T I O N Rat io Time, , , 7/6 8/7 9/8 10/9 11/10 12/11 13/12 14/13 11 1. 1 0. 6 0. 6 0. 5 0. 6 0. 2 0. 7 20 1. 1 0. 6 0. 6 0. 5 0. 6 0. 2 0. 8 48 1. 0 0. 5 0. 6 0. 6 0. 6 0. 2 71 1. 0 0. 6 0. 6 0. 5 0. 4 100 1. 0 0. 5 0. 5 0. 5 (0. 2) - P a g e 35 -I - P O P U L A T I O N R A T I O 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 ( C o n t ' d . ) S i n c e a b s o l u t e 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 O , , * a r e n o t a v a i l a b l e o n l y r e l a t i v e p o p u l a t i o n s c o u l d b e o b t a i n e d f o r t h e m e a s u r a b l e l e v e l s o b s e r v e d . H e n c e t h i s q u e s t i o n m u s t r e m a i n o p e n u n t i l m o r e a c c u r a t e m e t h o d s a r e f o u n d t o e s t i m a t e 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 o r u n t i l t h e v " =0 o r 1 l e v e l s o f O ^ c a n b e p o s i t i v e l y i d e n t i f i e d i n t h i s s y s t e m a n d c o m p a r e d w i t h k n o w n a m o u n t s o f s a m p l e s w h i c h w o u l d t h e n p u t a l l t h e r e l a t i v e p o p u l a t i o n s o n a n a b s o l u t e s c a l e . (b) D I S C U S S I O N A n u m b e r o f O * b a n d s b e t w e e n 2 1 0 0 a n d 2 3 0 0 A ° w h i c h w e r e i m p o r t a n t t o t h i s s t u d y w e r e i n a c c e s s i b l e u n d e r p r e s e n t c o n d i t i o n s a s a r e s u l t o f a m a r k e d d e c r e a s e i n p l a t e c o n t r a s t a s w e l l a s s e v e r e O a n d N O - , b a n d o v e r l a p i n t h a t r e g i o n . B e c a u s e o f t h e s e i n h e r e n t d i f f i -c u l t i e s , i n t e n s i t y m e a s u r e m e n t s c o u l d o n l y b e m a d e o n b a n d s o r i g i n a t i n g f r o m t h e l e v e l s v " = 6 t o 13 ( w i t h r o u g h e y e e s t i m a t e s f o r t h e (0 , 14) a n d (0 , 15) b a n d s ) . S i n c e t h i s g i v e s v i r t u a l l y n o o v e r l a p w i t h t h e p r e v i o u s w o r k e r s d i s t r i b u t i o n s , t h e o n e g i v e n h e r e c a n o n l y b e c o n s i d e r e d a s a n e x t e n s i o n o f m e a s u r e m e n t s t o h i g h e r l e v e l s . H o w e v e r , t h e g e n e r a l t r e n d , F i g u r e 7, s h o w s t h e r a t i o s i n c r e a s i n g w i t h d e c r e a s i n g v " n u m b e r , r a t h e r t h a n r a p i d l y d e c r e a s i n g , w h i c h a g r e e s w i t h t h e r e s u l t s o f B a s s a n d G a r v i n r a t h e r t h a n t h o s e o f K a n e et a l . i A s i t t u r n s o u t , t h e m o s t i n t e r e s t i n g f e a t u r e o f t h i s O- , p o p u l a t i o n w o r k i s t h e d i s c o n t i n u i t y f o r t h e 1 2 / 1 1 r a t i o , F i g u r e 7. - P a g e 36 -I - P O P U L A T I O N R A T I O 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  ( C o n t ' d . ) T h i s b r e a k i s t o o l a r g e a n d t o o r e p r o d u c i b l e to b e e x p e r i m e n t a l e r r o r o r a d e f e c t i n t h e a r r a y s s i n c e i t i s p r e s e n t o n b o t h . F u r t h e r -r r i o r e i t o c c u r s a t t h e l e v e l t h a t t h e e n e r g y a v a i l a b l e i n t h e f o r m o f v i b r a t i o n ( v " = l l ) j u s t m a t c h e s t h e e x o t h e r m i c i t y o f r e a c t i o n (2 ) . T h e f o r m o f t h e d i s t r i b u t i o n c u r v e f o r t h e h i g h v " l e v e l s c a n b e e x p l a i n e d t h e n , a s a r e s u l t o f p r o d u c t i o n o f 0 2 b y t w o d i f f e r e n t m e c h a n i s m s . (1) T h e f i r s t , g i v e n b y L i p s c o m b et a l , i s f o r t h e l e v e l s l e s s t h a n v " = 12 w h i l e t h e s e c o n d a c c o u n t s f o r t h e l e v e l s 12 ^ v " ^ 1 5 . T h e n a t u r e o f t h i s s e c o n d m e c h a n i s m i s d i s c u s s e d i n t h e n e x t s e c t i o n . H o w e v e r , a s s e e n i n F i g u r e 7 t h e s e c o n d m e c h a n i s m i s r e m o v e d w h e n t h e C o r n i n g 7 " 5 4 f i l t e r s t o p s m o s t o f t h e v i s i b l e l i g h t p r o d u c e d b y t h e p h o t o l a m p f r o m e n t e r i n g t h e r e a c t i o n v e s s e l , s i n c e i n t h a t c a s e n o l e v e l s a b o v e v " = l l a r e s e e n a n d t h e r a t i o s 1 1 / 1 0 t o 7 / 6 a p p r o a c h u n i t y w i t h n o a p p a r e n t d i s c o n t i n u i t y . T h e r a t h e r f a s t r e l a x a t i o n p r o c e s s o f O . , * i s c o n s i s t e n t w i t h a v i b r a t i o n - v v i b r a t i o n e n e r g y t r a n s f e r b e t w e e n t h e 1 3 2 0 c m ^ f u n d a m e n t a l v i b r a t i o n f r e q u e n c y o f N O , , a n d t h e O * f r e q u e n c i e s o f 1450 t o 1300 c m ^ d e p e n d i n g o n t h e v " n u m b e r i n v o l v e d T h e e f f i c i e n c y o f q u e n c h i n g b y N O i s k n o w n t o b e q u i t e h i g h (one e f f e c t i v e c o l l i s i o n i n l e s s t h a n 500)^ \ T h i s g i v e s a q u e n c h i n g c o n s t a n t ^ Q ) 8 - 1 f o r 0 ? * b y N O , o f g r e a t e r t h a n 6 x 1 0 l i t e r ( m o l e s e c . ) u s i n g a - 1 (34) c o l l i s i o n f r e q u e n c y ( Z ) o f 3 x 1 0 l i t e r ( m o l e s e c . ) F o r N O 2 - Page 37 -I - P O P U L A T I O N R A T I O 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 (Cont 'd.) p a r t i a l p r e s su re s used in this -work a half l i fe of O * (v" = 10) in the order of s ix ty mic roseconds i s expected and observed. Recent ly Basco and D o g r a found that the oxygen atom can play a signif icant ro le in the deact ivat ion of O ^ * produced in the f lash photolysis of C l O ^ . They repor ted an oxygen atom quench-ing constant k ^ (O) of approximate ly 1 0 ^ l i t e r (mole sec . ) . T h i s effect has not been seen with the O atoms produced from NO . T h i s 2 resu l t i s not, however , inconsis tent with the C l O work . F o r the NO Ci Ci sys tem, it has not yet been poss ib le to decompose more than 40% of it i n the p r i m a r y step, reac t ion (1), so that the O atoms react with.the r e s i -dual NO before they can quench 0 ? * to an appreciable extent. However , wi th C IO it has been poss ib le to decompose as much as 80% in the p r i m a r y step, due to its much higher ext inct ion coefficient, leaving an excess of O atoms, which react more s lowly with other products , to quench O^* . A s w e l l , NO i s known to be a more efficient quencher of O * than C I O by c. 2 2 at least a factor of four ^ \ making any s m a l l change in the O ^ * l i fe t ime m o r e diff icul t to observe . Hence it is only when an excess of O atoms i s produced by decomposing more than 50% of the NO in the p r i m a r y step that a d ramat i c change in the O * l i fe t ime would be expected. At present the O ^ * l i fe t ime i s found to inc rease with i nc rea s ing amount of decomposi t ion as would be expected for NO-, being the predominant deact ivator . 38 - Page 38 -II - T H E M E C H A N I S M F O R O '** P R O D U C T I O N 2 • The resu l t s of the f i l t e red exper iments on the production of O ** f rom the f lash photolysis of N O ^ c l e a r l y show that the process i n -volved i n generating the high v ib ra t iona l l eve ls is dependent on r a d i a -t ion above 4000 A° and hence i s not explained by the L i p s c o m b et a l . mechanism^ \ T h i s resul t i s not too s u r p r i s i n g since the v" = 12 to 15 l eve l s of 0 2 a l l cor respond to energies above the 48 k c a l exo thermic i ty of reac t ion 2 (v" = 15 is approx imate ly 62 kca l of v ib ra t iona l energy) . ,(31) Hence the heat of reac t ion and the act ivat ion energy (580+500 ca l /mo le ) of r e a c t i o n 2 cannot alone explain the product ion of O **. The reg ion above 4000 A ° w h i c h i s f i l t e red out by the* Corn ing 7"5 4 f i l t e r corresponds to the bound {ZByt-K- ( 2 A ^ ) system of N 0 2 - Thus it is poss ib le that the ext ra 14 k c a l of 0 2 * * v ib ra t iona l energy above the exo the rmic i ty of reac t ion (2) i s gained from an exci ted state of NO . When the photoflash i s f i l t e red by the C o r n i n g 7 - 5 4 f i l t e r this exci ted state would not be produced. A s w e l l , secondary photolysis of N O ^ is poss ib le in this region and i t s products could resu l t in O ** format ion. These two explanations that are consistent with the above resu l t s have been tested as fo l l ows : -(i) Secondary P h o t o l y s i s of N O ^ i 6 Husa in and N o r r i s h have repor ted observing NO as a t ransient i n the f lash photolys is of N O ^ . They base thei r kinet ic resu l t s on the mechan i sm: NO —$> NO + O (1) • • • •39 - P a g e 39 ~ I I - T H E M E C H A N I S M F O R O * * P R O D U C T I O N ( C o n t ' d . ) 2 (i) S e c o n d a r y P h o t o l y s i s o f N O ^ ( C o n t ' d . ) O + N O — * O + N O (2) 2 2 O + N O z —s> N 0 3 ( 4 ) N O + N O » 2 N O (5) (32) 9 - 1 (k^ = 2 . 1 x 10 l i t e r ( m o l e s e c . ) 11 2 - 2 - 1 ( 3 3 ) (k^=10 l i t e r ( m o l e s e c . ) 10 - i (6) k =9 x 10 l i t e r ( m o l e s e c . ) 5 (33) w h i c h i s e s s e n t i a l l y t h e s a m e m e c h a n i s m g i v e n b y F o r d a n d E n d o w f o r t h e s t e a d y s t a t e p h o t o l y s i s o f N O . T h e p o s s i b i l i t y o f o t h e r s t e p s i n v o l v i n g O ^ * * p r o d u c t i o n f r o m N O ^ w a s i n v e s t i g a t e d . T h e t w o , s t e p s c o n s i d e r e d m o s t l i k e l y w e r e : - ' O + N O 3 — t . N 0 2 + 0 2 * * A H = - 6 9 k c a l / m o l e (6) N O - M - N O + O *# (7) 3 2 V U n d e r t h e c o n d i t i o n s u s e d i n t h i s w o r k n o N O ^ h a s b e e n o b s e r v e d . O n u s i n g t h e h i g h e r P - ^ Q a n d h i g h e r a r g o n t o N O , , r a t i o o f H u s a i n a n d N o r r i s h ' s w o r k a w e a k N O ^ b a n d w a s s e e n at 6 6 2 0 A ° a t t h e s h o r t e s t t i m e d e l a y s u s e d (5 a n d 12 m i c r o s e c o n d s ) . S c h o t t a n d (7) + D a v i d s o n h a v e e s t i m a t e d a m o l a r e x t i n c t i o n c o e f f i c i e n t o f 3 0 0 0 ~ 1000 l i t e r ( m o l e c m ) ^ f o r N O ^ a t t h i s w a v e l e n g t h . T h i s v a l u e w a s u s e d t o e s t i m a t e t h e NO_^ c o n c e n t r a t i o n p r o d u c e d b y t h e f l a s h . T h e a m o u n t o f N O p r o d u c e d a t a g i v e n t i m e f o r a p p r o x i m a t e l y 50% N O d e c o m p o s i t i o n 2 a g r e e d q u i t e f a v o u r a b l y w i t h t h a t f o u n d b y H u s a i n a n d N o r r i s h f o r t h e s a m e - Page 40 -II - T H E M E C H A N I S M F O R C U * * P R O D U C T I O N (Cont 'd .) conditions both with and without f i l t e r s . They repor ted 4. 5 x 10 t o r r of NO at m i n i m a l t ime delay with no f i l t e r (Py rex reac t ion vessel) and 3 - 3 1Z. 6 x 10 with a cuprammonium sulphate solut ion f i l t e r which is opaque i n the 5000 to 10,000 A° reg ion . The p a r t i a l p re s su res of N O ^ found i n -3 ~ 2 this study were 5. 3 x 10 t o r r with no f i l t e r and 10 t o r r with the Corn ing 7 - 5 4 f i l t e r between the photo lamp and the P y r e x reac t ion ves se l . On the bas is of the known rate constants k-> and k^ it i s poss ib le to say that contr ibut ion of N O ^ to O ** f rom either reac t ion (6) or (7) w i l l not be significant s ince N O ^ product ion is t h i r d body dependent. Under the conditions vised in these exper iments k = 5k because of the lower iner t 2 4 gas p r e s su re s used. Reac t ion (6) can be ru l ed out complete ly since the N O ^ con-centra t ion is i nc rea sed with the 7 - 5 4 f i l t e r present which prevents second-a r y photolysis of N O ^ . Thus i f reac t ion (6) were a contributing factor to O^** product ion one would expect to see s t ronger O^** bands rather than none at a l l . To ru le out the p o s s i b i l i t y of reac t ion (7) as a contr ibuting factor , H N O ^ was f lashed as another source.of N O ^ . Husa in and N o r r i s h found that NO_ was produced from HNO in a manner consistent with the mechan i sm H N O s O H + NO (8) O H + H N 0 o — > N O + H O (9) 3 3 2 Because of the p r i m a r y product ion and a slight i m p u r i t y of NO-, in the 41 - P a g e 41 -I I - T H E M E C H A N I S M F O R O » * P R O D U C T I O N ( C o n t ' d . ) H N O ^ , r e a c t i o n (7) c o u l d o n l y b e r u l e d o u t i f n o n e o f t h e h i g h e r l e v e l s o f O ^ a p p e a r e d b u t c o u l d n o t b e c o n f i r m e d i f t h e y d i d s i n c e t h e y m i g h t a r i s e f r o m s e c o n d a r y p h o t o l y s i s o f t h e N O ^ . T h e r e s u l t s o f t h i s e x p e r i m e n t w e r e a n N O ^ s p e c t r u m a p -p r o x i m a t e l y t e n t i m e s s t r o n g e r t h a n t h a t o b s e r v e d w i t h N O f o r t h e s a m e t i m e d e l a y a n d a w e a k l o w v i b r a t i o n a l l e v e l s p e c t r u m o f O ^ . N o t r a c e w a s s e e n o f a n y l e v e l a b o v e v " = 8 a n d t h e r a t e o f d e c a y f o r t h e v " = 6 l e v e l w a s s e e n to b e s l o w e n o u g h t h a t t h e h i g h e r l e v e l s c o u l d n o t h a v e b e e n r a p i d l y q u e n c h e d b e f o r e b e i n g o b s e r v e d . T h u s , b o t h o f t h e m o s t l i k e l y p o s s i b i l i t i e s o f N O ^ c o n t r i -b u t i o n to t h e p r o d u c t i o n o f O c a n be ' e f f e c t i v e l y r u l e d o u t . ( i i ) A n E x c i t e d S t a t e o f N O 2 (a) R e s u l t s I n o r d e r t d d e t e r m i n e w h e t h e r a n e x c i t e d s t a t e o f N O - , i s r e s p o n s i b l e f o r t h e O * * p r o d u c t i o n , t h e d o u b l e l a m p s y s t e m w a s u s e d . T h i s f l a s h u n i t a l l o w e d t h e e x c i t a t i o n a n d p h o t o l y s i s p r o c e s s e s o f N O ^ to b e s e p a r a t e d i n t i m e b y f l a s h i n g t h e m a i n l a m p f i l t e r e d b y t h e C o r n i n g 3 - 7 3 ( 4 0 0 0 A ° c u t o f f f i l l e r ) f o l l o w e d a t k n o w n t i m e d e l a y s b y t h e a u x i l i a r y l a m p w h i c h w a s e i t h e r f i l t e r e d b y t h e C o r n i n g 7 - 5 4 f i l t e r o r l e f t u n f i l t e r e d . T h e s p e c t r o s c o p i c l a m p w a s t h e n f i r e d a t a c o n s t a n t d e l a y a f t e r t h e a u x i l i -a r y l a m p , F i g u r e 8. A s t h e p h o t o l y s i s p r o c e s s w a s s e p a r a t e d f u r t h e r i n t i m e f r o m t h e e x c i t a t i o n p r o c e s s , t h e O * * s p e c t r u m w a s s e e n to w e a k e n CJ a n d f i n a l l y v a n i s h . •• . 42 1 - P a g e 42 -V a r i a b l e D e l a y s Constant D e l a y F I G U R E 8 . Output of (a) M a i n lamp - i s f i l t e r e d by a 4000 A ° cut off f i l t e r . It can only p r o v i d e NO^ excitation. (b) A u x i l i a r y lamp - f i l t e r e d by the 7~54 f i l t e r wbi,c.h p r o v i d e s NO p h o t o l y s i s only or i s not f i l t e r e d at a l l i n which case both p r o c e s s e s a r e p o s s i b l e . F o r this double l a m p experiment the s p e c t r o s c o p i c lamp i s held constant with r e s p e c t to the a u x i l i a r y l a m p i n o r d e r that the photo-l y s i s products a r e present i n a constant amount while the concentration of the e x c i t e d product i s v a r i e d by v a r y i n g the a u x i l i a r y l a m p delay with r e s p e c t to the main lamp. - P a g e 4 3 -II - T H E M E C H A N I S M F O R Q * » P R O D U C T I O N ( C o n t ' d . ) T h e i d e a l f i l t e r a r r a n g e m e n t f o r t h e s e e x p e r i m e n t s w o u l d h a v e i n -v o l v e d u s i n g t h e t w o c o m p l i m e n t a r y f i l t e r s 3 - 7 3 a n d 7 _ 5 4 , F i g u r e 3 , i n o r d e r t o c o m p l e t e l y s e p a r a t e e x c i t a t i o n a n d p h o t o l y s i s i n t i ^ e . H o w e v e r , q u a n t i t a t i v e m e a s u r e m e n t o f t h e a p p a r e n t r a t e o f 0 2 * * d e c a Y r e q u i r e d a s t r o n g e r s p e c t r u m . - "• B e c a u s e t h e o p a q u e r e g i o n s o f t h e t w o f i l t e r s c o m p l i m e n t e a c h o t h e r o n l y o n e t r a v e r s a l o f t h e r e a c t i o n v e s s e l w a s p o s s i b l e f o r l i g h t o f a g i v e n w a v e l e n g t h . T h a t i s , a l l r e f l e c -t i o n s o f l i g h t t h r o u g h t h e r e a c t i o n v e s s e l , w h i c h h a d p r e v i o u s l y a c c o u n t e d f o r a n a p p r e c i a b l e a m o u n t o f N O ^ d e c o m p o s i t i o n , w e r e n o w c o m p l e t e l y f i l t e r e d o u t b y t h e c o m b i n a t i o n o f t h e t w o f i l t e r s . F o r t h i s r e a s o n a s e c o n d a r r a n g e m e n t w a s u s e d , i n w h i c h t h e C o r n i n g 7 " 5 4 f i l t e r w a s r e m o v e d m a k i n g i t p o s s i b l e f o r t h e p h o t o l y s i s l a m p t o e m i t r a d i a t i o n a b o v e 4 0 0 0 A ° a n d t h u s p r o d u c e O 2 * * a s w e l l . H o w e v e r , t h e a m o u n t o f p h o t o l y s i s w a s s t i l l l o w s i n c e r a d i a t i o n b e l o w 4 0 0 0 A ° s t i l l h a d o n l y o n e o t r a v e r s a l o f t h e r e a c t i o n v e s s e l b u t r e f l e c t i o n a b o v e 4 0 0 0 A w a s n o w p o s s i b l e t o e f f e c t m o r e e x c i t a t i o n . W i t h t h i s d o n e , t h e O ^ * * w a s s e e n o n l y w e a k l y o n s t r a i g h t p h o t o l y s i s e v e n a t t h e h i g h e s t f l a s h e n e r g i e s u s e d . W h e n t h e m a i n ( e x c i t a t i o n ) l a m p w a s d i s c h a r g e d a n d t h e a u x i l i a r y ( p h o t o -l y s i s ) l a m p f o l l o w e d i t a t s h o r t d e l a y s w i t h a c o n s t a n t s p e c t r o s c o p i c d e l a y o n e s e e s t h e O * * i n t e n s i t y i n c r e a s i n g t o a m a x i m u m a t a p p r o x i m a t e l y 10 m i c r o s e c o n d s ( a u x i l i a r y d e l a y ) a n d t h e n d e c a y i n g r a p i d l y t o t h e v a l u e o b t a i n e d w i t h s t r a i g h t p h o t o l y s i s a l o n e . 4 4 - Page 45 -II - T H E M E C H A N I S M F O R O P R O D U C T I O N (Cont 'd. ) Th i s p rocess was o r i g i n a l l y thought to be p ressure depen-dent but on re inves t iga t ion there was found to be a temperature effect at the lower iner t gas p re s su re s used causing anomalous r e su l t s . In fact, over the p r e s su re range 50 to 200 t o r r of argon with 1 t o r r of N O ^ the exci ted species producing the O^ ** has an exponential decay with a constant half l i fe of approx imate ly 10 mic roseconds . The r i s e and decay of the O ^ * * species i s shown in F i g u r e 9 as a function of the l i fe t ime of the exci ted species . A l s o included in the f igure i s the prof i le of the main lamp as p icked up f rom a photomul t ip l ie r tube ( R C A 1P28) 'moni tor ing the f lash and d i sp layed on an o sc i l l o scope . The O ^ * * intensi t ies were mea-sured i n the same manner as those in the previous sect ion. Those plotted against t ime i n the above figure are co r r ec t ed for the amount of Out-produced on straight photolys is (aux i l i a ry lamp alone). The t imes given are measured from the peak intensi ty of the main lamp to the peak of the a u x i l i a r y l amp , which were obtained from photographical ly r eco rded o s c i l l o scope t races of each exper iment . Although attempts were made to obtain d i rec t spec t roscopic evidence of an exci ted state cf NO either e lec t ron ic or v ib ra t iona l , none 2 was found. T h i s i s not too s u r p r i s i n g , however , s ince the extent and c o m -plex i ty of the ground state NO spec t rum, which is d i scussed in the next i • chapter, would l i k e l y preclude any observat ion of a weak diffuse t ransient spectrum in its midst . ...46 - Page 46 -Oo6H 20 30 40 t(^s) Auxiliary Delay F I G U R E 9 V a r i a t i o n of I _ * * w i t h i n c r e a s i n g a u x i l i a r y (JO O I l a m p d e l a y . T h e band m e a s u r e d was the (0, 13). 50% N O d e c o m p o s i t i o n . T h e d a s h e d c u r v e s h o w s the p r o f i l e of the e x c i t a t i o n l a m p . - Page 47 ~ n - T H E M E C H A N I S M F O R O P R O D U C T I O N (Cont 'd. ) A n attempt was also made to find molecules suitable for quenching exci ted NO-, . Two methods of approach were poss ib le . The f i r s t involved detection of a decrease in O^** intensi ty while leaving that of the O ^ * unaffected. A n approach such as this was s e r ious ly compl ica ted by the fact that quenchers suitable for NO * deactivat ion 2 also gave appreciable O * deact ivat ion. A n example of this was SF 2 6 which was repor ted to be the most efficient N O ^ f luorescence quencher (34) of th i r teen studied . When va ry ing amounts of SF were used with o argon to make a total diluent gas p re s su re of 150 t o r r , O,,** was seen only ve ry weakly at the shortest delay but O^* was also cons iderab ly weakened, making in terpre ta t ion of th is resul t diff icul t . The second approach was to observe d i r e c t l y excitation.that had been t r an s f e r r ed from N O ^ * to the quenching molecule . T h i s approach p r a c t i c a l l y necessitated the use of d ia tomic molecules since in general they give w e l l defined exci ted state spect ra . A n d of the stable d ia tomics about the only suitable one i s NO which absorbs in a region where NO absorpt ion i s not p a r t i c u l a r l y strong. A mix tu re of 1 t o r r N O 2 , 5 t o r r N O , and 200 t o r r of argon was o f lashed with radia t ion above 4000 A which resu l ted in the inc rease of the (0, 1) band of NO to a m a x i m u m in 20 mic roseconds and subsequent r ap id decay to zero by 100 mic roseconds . Th i s exci ted NO can only a r i s e from t ransfer f rom NO'^ under the above conditions since there i s no NO absorp-t ion above 4000 A° nor i s there any appreciable photolysis of NO (only . . . 48 - Page 48 -II - T H E M E C H A N I S M F O R O ** P R O D U C T I O N (Cont 'd. ) 2 excitat ion). A s w e l l , the diluent p res su re is so large that the sys tem i s v i r t u a l l y i s o t h e r m a l which would ru le out t h e r m a l l y populated NO (0, 1). When a s i m i l a r sample without added NO was flashed under the same condit ions, no detectable amount of NO was produced from the pho to ly t i c r eac t ion (1). A f inal attempt was made to charac te r ize the nature of the O ** product ion by inves t iga t ing the poss ib le reac t ion sequence: N ° 2 N ° 2 * ^ N ° ^ * + ° ( l D ) ( 1 0 ) O (TJ) + NO —> O * * + NO (-94 kcal) (11) 2 where the N O ^ * stands as an unspecif ied exci ted state, was invest igated us ing hydrogen as the diluent gas. If O (*D) was - produced, r ap id O H product ion would occur but not with O(^P) : 0 ( 3 P ) + H o O H + H ' ( + 2 k c a l . ) (12) where k 1 2 = 1 0 1 0 exp ( -8800/RT) l i t e r (mole s e c ) _ 1 ^ \ s ince it i s not energe t i ca l ly favourable. Thus under i s o t h e r m a l conditions (150 t o r r of H : 1 t o r r of NO ) i f O H was observed using a P y r e x f i l t e r , O(^D) could only be produced by a double quantum process reac t ion (10). fol lowed by reac t ion 11. U s i n g these conditions no O H could be detected. €> ** was p r o -duced quite s t rongly under the same conditions except that argon was used as the diluent instead of hydrogen since the la t ter was found to be quite ef-fective in quenching both O * and O **. Use of a quartz reac t ion ves se l 2 permi t ted the product ion of O(^D) as a single quantum event and in this case . . . 49 - Page 49 " II - T H E M E C H A N I S M F O R O ** P R O D U C T I O N (Cont'd.) . . 2  a/strong s p e c t r u m of OH(0,-0) was o b s e r v e d at short delays, also the O H (2, 1) band was seen weakly, p r o v i n g that O H was produced f r o m ( 0 * 0 ) . T h i s l a s t r e s u l t shows that O(^D) i s a d i s s o c i a t i v e product of the s h o r t e r wavelength system of NO^ which in t u r n allows another method of checking the above mechanism. That i s , with a quartz r e a c t i o n v e s s e l , O(^D) i s produced by a means that does not r e q u i r e the excitation of NO,, through its v i s i b l e system. T h u s if O^D is an i n t e r m e d i a t e in f o r m i n g . O**, m o re O ** should be produced i n a quartz r e a c t i o n v e s s e l ^ 2 than in a P y r e x r e a c t i o n v e s s e l under i d e n t i c a l conditions. However, c o m p a r i s o n of the O ** spectrum for these two experiments showed no d i f f e r e n c e i n the amount produced. T h e c o n c l u s i o n that can be drawn from these r e s u l t s i s that O ** i s not produced as the result, of secondary photolysis of an excited NO^ sp e c i e s . A n i n t e r e s t i n g feature of the r a p i d H quenching of e x c i t e d O 2 2 was the o b s e r v a t i o n of the (0, 1) band of NO which was p r e v i o u s l y o b s c u r e d (4) by the O-,* but tentatively r e p o r t e d by B a s c o and N o r r i s h . . T h i s excited l e v e l was produced under i s o t h e r m a l conditions using the C o r n i n g 7 _54 f i l t e r which e l i m i n a t e d excited NO^ and hence energy t r a n s f e r to NO. The NO (0, 1) produced under these conditions could only be produced f r o m NO-, photolysis either by r e a c t i o n 1 or 2. T h e l a t t e r r e a c t i o n i s a c o n t r a d i c t i o n of the A + BCD.-* A B + C D e m p i r i c a l r u l e of M c G r a t h and N o r r i s h ^ in that, the CD 50 - Page 50 -II - T H E M E C H A N I S M F O R O ** P R O D U C T I O N (Cont 'd.) _ 2 bond should r e m a i n v ib ra t i ona l ly cold . NO (0, 1) being produced in the fo rmer reac t ion i s s i m i l a r to the NO produced i n the f lash photo" (37) l y s i s of the n i t r o s y l hal ides . The only ce r ta in way to d i s t inguish between these two p o s s i b i l i t i e s i s to to ta l ly decompose the N O ^ in the p r i m a r y photolytic step. If under these conditions no NO (0, 1) i s observed then it must a r i s e f rom reac t ion 2. However , so far only a maximum, of 35% of NO has been decomposed in the p r i m a r y step due to i ts r e l a t i v e l y weak absorpt ion. (b) D i s c u s s i o n It i s by ind i r ec t means that the p rocess respons ib le for the product ion of O ^ * * has been shown to be the resul t of an exci ted state of NO,, which has a ve ry short l i f e t ime . The nature of the exci ted state can only be i n f e r r ed from these resu l t s and the ra ther l i m i t e d know" ledge of the exci ted states of NO . The natura l radia t ive l i fe t ime of NO B 2 2 f luorescence in the v i s i b l e reg ion i s known to be 44 microseconds (38 39, 40) ' . T h i s i s one of the longest f luorescent l i fe t imes known for a t r i a t o m i c molecu le . No other rad ia t ive p rocesses of a different l i fe t ime have been observed for NO, ,* . Because of this long radia t ive l i f e t ime , the d i rec t reac t ion of e l e c t r o n i c a l l y exci ted N O ^ with a tomic oxygen m U s t be cons idered . E s s e n t i a l l y the p rocesses that can occur under the condi" t ions used for the double l amp exper iments are: . . . 51 - Page 51 -II - T H E M E C H A N I S M F O R 0 > * P R O D U C T I O N (b) D i s c u s s i o n (Cont 'd. ) NO ^ NO * 2 2 < e ) (12) NO * M M 2 (e) —j> N O * — * NO (13) 2 (v) 2 + O — > O z * * + NO (14) NO * + O — » 0 , * * + NO (15) 2 (e) 2 where the subscr ip t s (e) and (v) refer to e lec t ron ic and v ib ra t iona l exci ta t ion r e spec t ive ly and M is the diluent gas plus added quencher, i f any. That i s , the N C ^ ' ^ e ) can r e l a x to the ground e lec t ron ic state (but not n e c e s s a r i l y to i t s ground v ib ra t iona l state) by either a rad ia t ive o r c o l l i s i o n a l p rocess and then react with a tomic oxygen. The resu l t s have shown that the l i fe t ime of the in termediate species invo lved in the product ion of O ^ * * i s insens i t ive to diluent p r e s -sure va r i a t i on over the range studied as a resul t of i ts rate of re laxa t ion being so fast that it decays wi th in the pe r iod of the f lash, F i g u r e 9. The extent of the p r e s su re va r i a t i on for this study was l i m i t e d by the fact that at least 50 t o r r of diluent to 1 t o r r of N O ^ was needed to keep the system r e l a t i v e l y i s o t h e r m a l . NO f luorescence quenching studies have been c a r r i e d out by CJ (34) M y e r s et a l using a va r i e ty of common quenchers inc luding NO. , , N O , N ^ , and A r . They repor t a quenching constant for argon, k ^ (argon), of 1. 2 x 1 0 ^ l i t e r (mole sec. ) ^ and constants of only a factor of three higher than that for both N 0 2 and N O . In fact they found that a l l of the - Page 52 -II - T H E M E C H A N I S M F O R O ** P R O D U C T I O N . , . 2 (b) D i s c u s s i o n (Cont 'd. ) th i r teen quenchers studied had a kQ between D. 5 and 0. 004 of the i r c o l l i s i o n number . They admit that the absolute value of the i r constants may be i n e r r o r by as much as a factor of ten but are confident that the re la t ive rates of the var ious quenchers are accurate . Hence the rate of quenching of NO * for a d i v e r s i f i e d se lec t ion of quenchers appears to 2 (e) be ve ry fast and essen t i a l ly constant wi th in a factor of ten. Since at least 50 t o r r . of argon is neces sa ry to keep the system r e l a t i v e l y i s o t h e r m a l , a m i n i m u m ha l f l i fe of NO * , for th is study can be ca lcula ted f rom the 2(e) y kQ (argon) to be 0. 1 m ic ro second (or 1 mic ro second i n the l i m i t of e r r o r for the rate constant). Thus the two step mechan ism NO NO * (12) 2 2(e) N ° 2 ( e ) .+ O —*• N O + . O ** (15) can expla in these resu l t s us ing the p r ev ious ly de termined quenching constants. These previous r e su l t s predic t what has been observed for the conditions used i n this study; namely , that the N O * , . w i l l r e l a x at 2 (e) a rate that i s wi th in the pe r iod of the photo f lash even for the least eff ic-ient quencher. Hence, there w i l l be no detectable difference in the re laxa-t ion rate with p r e s su re . The p o s s i b i l i t y of a v ib r a t i ona l l y exci ted N O ^ species , which has been op t i ca l ly pumped through i ts v i s i b l e sys tem, being respons ib le . . . 53 - Page 53 -II - T H E M E C H A N I S M F O R O ** P R O D U C T I O N (b) D i s c u s s i o n (Cont 'd. ) for the product ion of O ** cannot be comple te ly e l imina ted since there has apparently been no successful study made on NO v ib ra t iona l quench-2 ing by any technique. However , on the bas i s of v ib ra t iona l quenching (41) studies made on s i m i l a r molecu les one would expect the quenching rate to be observable wi th in the t ime reso lu t ion of the apparatus used for th is work . The observat ion of energy t ransfer to NO (as NO .. ,) f rom an exci ted state of N O ^ demonstrated that there is ' excess ' energy a v a i l -able i n the f lash photolysis of NO which could account for the product ion of O-,**. The mechan ism by which the NO exci ta t ion energy i s t r ans fe r -red to NO i s somewhat obscure due to the l ack of informat ion on the poten-t i a l surfaces of NO, , . In conc lus ion , v ib ra t i ona l ly exci ted oxygen with a m a x i m u m of 15 quanta of v ib ra t iona l energy i s produced in the f lash photolysis of 3 NO~ by the d i rec t reac t ion of ground state oxygen atoms (O P) with NO £ 2 which i s i n ei ther i ts ground or f i r s t exci ted e lec t ron ic state. The exci ted state p rocess i s capable of contr ibut ing 14 kca l more energy than the ground state p roces s . F r o m these resu l t s it i s apparent that e m p i r i c a l ru le of M c G r a t h and N o r r i s h , that energy up to the exo the rmic i ty of the reac t ion can be found in the f o r m of v ibra t ion i n the newly formed bond s t i l l holds in this case since a second reac t ion is respons ib le for the p ro -duction of O^** . . . . 54 C H A P T E R 4 T H E A D I A B A T I C F L A S H P H O T O L Y S I S O F N O , A N D S O . . , 2 2 I B R O A D E N I N G O F T H E N O R M A L S P E C T R A T h e n e a r a d i a b a t i c f l a s h p h o t o l y s i s o f a m i x t u r e o f 1 t o r r N O 2 a n d 5 t o r r a r g o n w i t h r a d i a t i o n a b o v e 4 0 0 0 A ° ( c o r r e s p o n d i n g t o ( 2 4 ) e n e r g y b e l o w i t s d i s s o c i a t i o n l i m i t g i v e s a n N O s p e c t r u m i n t h e *-* v i s i b l e r e g i o n w h i c h a p p a r e n t l y r a p i d l y d i s a p p e a r s w i t h t i m e , t h e m i n i m u m b e i n g r e a c h e d b y a p p r o x i m a t e l y 5 0 m i c r o s e c o n d s , a n d r e t u r n s t o n o r m a l a f t e r 10 m i l l i s e c o n d s , F i g u r e s 10 a n d 1 1 . A s t h e a r g o n : N O 2 r a t i o w a s i n c r e a s e d f r o m 5 : 1 ( b e l o w w h i c h t h e r m a l d e c o m p o s i t i o n b e -c o m e s a p p a r e n t ) t h e s p e c t r u m b e c a m e l e s s w e a k e n e d u n t i l a t 1 0 0 : 1 n o d i f f e r e n c e w a s o b s e r v e d f o r a l l t i m e d e l a y s . N o e v i d e n c e o f a n y o t h e r s p e c i e s a p p e a r i n g i n t h e f o r m o f c o n t i n u o u s o r d i s c r e t e a b s o r p t i o n t h a t w o u l d c o m p l e m e n t t h e N 0 9 d i m i n u t i o n - h a s b e e n o b s e r v e d i n t h e s p e c t r a l r e g i o n 2 0 0 0 t o 8 0 0 0 A C 2 o T h e d i s a p p e a r a n c e o f t h e N O ^ s p e c t r u m a t l o w p r e s s u r e s o f i n e r t g a s s u g g e s t s t h a t t h e s p e c t r u m m a y s i m p l y b e t e m p e r a t u r e b r o a d e n e d u n d e r t h e n o n _ i s o t h e r m a l c o n d i t i o n s . O t h e r m o r e i n t e r e s t i n g p o s s i b i l i t i e s i n c l u d e t h e p o p u l a t i o n o f a l o n g l i v e d q u a r t e t s t a t e o f N O w h i c h h a d p r e v i o u s n o t b e e n o b s e r v e d o r t h e f o r m a t i o n o f a t h e r m a l i s o m e r o f N O ^ s i m i l a r t o (13) t h a t s u g g e s t e d b y N o r r i s h a n d O l d e r s h a w v ' f o r S O . I n o r d e r t o d e t e r m i n e t h e d e g r e e o f d i s a p p e a r a n c e o f t h e s p e c -t r u m , a r e a u n d e r t h e a b s o r p t i o n c u r v e s w a s m e a s u r e d w i t h a p l a n i m e t e r . E x c e p t f o r t h e N O 2 v i s i b l e s y s t e m t h e c h a n g e i n a r e a w i t h t i m e w a s o v e r a W A V E L E N G T H (A°) 4500 I 4480 4600 -4580 4630 4700 I 4800 I 4900 5000 1 4740 4795 4880 P R O M I N E N T B A N D E D G E S (OR M A X I M A ) (26) Be fo re 20 fJS. 100 241 B l a n k 431 867 1. 68 "ns 10 m S B l a n k Af te r TO CO U l U l I F I G U R E 10 - The adiabat ic f lash photo lys is of NO showing the d i m i n i s h e d v i b r a t i o n a l s t ruc ture at in ^termediate t i m e s for a p o r t i o n of the A •*— X system •p N O 2 = 1 t o r r P argon = 4 t o r r E= 1300J. i - Page 56 -W A V E L E N G T H (A°) 2300 2400 2500 n i — i — | — 2351 2372 2419 2459 2491 B l a n k B e f o r e 103 sec. 720 900 1.08 msec. 1.3 1.67 31 83 A f t e r (0-0) (26) P ROMINENT BANDS OF T H E B( B 2 ) ^ - X S Y S T E M F I G U R E 11 The adiabatic flash photolysis of NO^ showing the apparent diminishing vibr a t i o n a l intensity of the system. P = 1 t o r r N O z P =4 t o r r arson - Page 57 -T H E A D I A B A T I C F L A S H P H O T O L Y S I S O F NO A N D SO, I B R O A D E N I N G O F T H E N O R M A L S P E C T R A (Cont 'd. ) suff icient ly na r row range that a convers ion of wavelength to wave-number was not neces sa ry to give reasonably accurate re la t ive in te-grated absorpt ion of the s y s t e m s studied. The N 0 2 spect rum i n the v i s ib l e and u l t rav io le t region i s ' o (42) ve ry extensive and so far has defied ana lys i s , even at 4 K . How-ever , there do appear to be two sys tems which can be separated, the1 2 2 o < 4 3) v i s i b l e system A( B ^ 4 X ( A ^ between 3200 and 10, 300 A and 2 o < 4 4 ) the u l t r av io le t system B( B 2 ) « X between 2350 and 2580 A . The a rea under mic rodens i tomete r t r aces of a major port ion of the v i s i b l e sys tem, cons is t ing of diffuse i r r e g u l a r l y spaced bands, showed that whi le most of the v ib ra t iona l s t ructure disappeared, v e r y l i t t l e of the under ly ing continuum did so. In fact only 30°/c of the a rea under the t r aces from.3200 to 6650 A could not be accounted for i n the t ime delay range 10 to 800 mic roseconds whereas by eye one would be led to be l ieve that the ent i re spectrum, had disappeared. .• The NO (B <! X) absopr t ion a lso appears to d i m i n i s h with 2 t ime and then reappear , although it i s not quite as d ramat ic as the v i s i b l e sys tem, F i g u r e s 10 and 11. P l a t e photometry of this short wavelength sys tem gave the t r aces shown in F i g u r e 12. Al though the bands appear to d i m i n i s h , the a rea under the t r aces remains constant wi th in 6% for a l l t imes used, i m p l y i n g that the bands are only broadened. Since this region is be-l i eved to be a second separate sys tem of N 0 2 , . t h e fact that the absorpt ion co 2250 aSOO 2 3 5 0 2 4 0 0 2 4 5 0 F I G U R E 12 T r a c e of the near adiabatic f l a s h p h o t o l y s i s of 1 t o r r NO^ : 4 t o r r A r g o n of a major p o r t i o n of 1 2 the ( B 2 « — .'Aj) system. - B e f o r e (150), ; 1 0 0 / / J , ( 1 4 5 ) a r e a s i n b r a c k e t s . - P a g e 5 9 " T H E A D I A B A T I C F L A S H P H O T O L Y S I S O F NOu A N D SGu I B R O A D E N I N G O F T H E N O R M A L S P E C T R A ( C o n t ' d . ) r e m a i n s c o n s t a n t o v e r a l l t i m e s m u s t m e a n t h a t t h e c h a n g e i n a r e a o f t h e u p p e r s y s t e m i s a r e s u l t o f n o t b e i n g a b l e to i n t e g r a t e o v e r t h e e n t i r e r a n g e o f t h e s y s t e m ( i t w a s o n l y p o s s i b l e t o c a r e f u l l y m e a s u r e a b s o r p t i o n t o t h e I l f o r d H P 3 p l a t e c u t o f f a t 6 6 5 0 A ) . A s a c h e c k o n t h i s a b o v e r e s u l t , a d o u b l e l a m p e x p e r i m e n t w a s c a r r i e d o u t i n o r d e r t o d e t e r m i n e w h e t h e r t h e a m o u n t o f N O p r e s e n t 2 i n i t s g r o u n d s t a t e w a s r e d u c e d at t h e t i m e i t s s p e c t r u m w a s t h e w e a k e s t . L o w a r g o n : N O ^ m i x t u r e s w e r e f l a s h e d o n t h e d o u b l e l a m p s y s t e m s u c h t h a t p h o t o l y s i s ( b e l o w 4 0 0 0 A ° ) w a s o n l y p o s s i b l e 100 m i c r o s e c o n d s a f t e r t h e N 0 2 h a d b e e n f l a s h e d w i t h r a d i a t i o n a b o v e 4 0 0 0 A ° . T h i s w a s d o n e b y p u t t i n g t h e C o r n i n g 3 " 7 3 f i l t e r b e t w e e n t h e r e a c t i o n v e s s e l a n d t h e m a i n l a m p a n d d e l a y i n g t h e u n f i l t e r e d l a m p b y 100 m i c r o s e c o n d s w h i c h i s t h e t i m e t h a t s h o w s m a x i m u m w e a k e n i n g o f t h e N O s p e c t r u m . T h e s p e c t r o s c o p i c l a m p d e l a y s w e r e m e a s u r e d f r o m t h e m a i n l a m p . F i g u r e 13 s h o w s t h e a m o u n t o f N O ^ d e c o m p o s e d w i t h t i m e f o r t h e m a i n a n d t h e a u x i l -i a r y (at 100 m i c r o s e c o n d s ) a n d f o r t h e a u x i l i a r y a l o n e , f i r s t a s a f u n c t i o n o i t h e a r e a o f a b s o r p t i o n o f t h e N O a n d s e c o n d l y a s a f u n c t i o n o f t h e 4 3 5 0 A ° p e a k h e i g h t o f N O . I n b o t h c a s e s t h e C o r n i n g 3 _ 7 3 f i l t e r w a s i n p l a c e s i n c e m o r e d e c o m p o s i t i o n w o u l d b e e x p e c t e d o n s t r a i g h t p h o t o l y s i s w i t . h -o u t t h e f i l t e r d u e t o m u l t i p l e r e f l e c t i o n s o f r a d i a t i o n b e l o w 4 0 0 0 A ° . I n b o t h c a s e s t h e a m o u n t o f N O ^ r e m a i n i n g i s v i r t u a l l y t h e s a m e a s s e e n b y c o m p a r i n g t h e b e f o r e a n d a f t e r m e a s u r e m e n t s i n F i g u r e 13 . 60 - P a g e 60 100 80H ^OQ O — O ^ — " ! — 1 1 S3 ro M P < ro ro (a) 10-f Delayed Photolysis t ( y t / s e c ) FIGURE 13 T h e c u r v e s O a n d A a r e f o r s t r a i g h t a n d d e l a y e d p h o t o l y s i s r e s p e c t i v e l y o f 1 t o r r . N O ^ i n 5 t o r r a r g o n . ( a ) s h o w s t h e b e h a v i o u r o f N O ^ a s a f u n c t i o n o f t h e a r e a u n d e r i t s a b s o r p t i o n c u r v e a n d ( b ) a s a f u n c t i o n o f t h e p e a k h e i g h t o f t h e 4350 A ° b a n d , s h o w i n g t h a t m e a s u r e m e n t s o f p e a k h e i g h t s f o r t h i s w o r k h a s n o s i g n i f i c a n c e . so ro P c+-< ro •0 ro P ro V - Page 61 " T H E A D I A B A T I C F L A S H P H O T O L Y S I S O F NO A N D SO I B R O A D E N I N G O F T H E N O R M A L S P E C T R A (Cont 'd. ) Since the NO effect was found to be ve ry s i m i l a r to that found for SO,, by N o r r i s h and Oldershaw i t was thought advisable to r e -examine the adiabatic f lash photolys is of SO^. F o r this species i t was found that a l l the absorpt ion could be accounted for wi th in 10% 1 i (45) i n the A ( B ^ < X ( X A X ) (2500 - 3400 A.°) sys tem of SO^ i f the areas under the mic rodens i tomete r t r aces for numerous t imes inc luding the before and after, are compared. A n example of this is shown i n F i g u r e 14 where the v ib ra t iona l s t ructure of SO-, i s quite apparent on the before but had d imin i shed cons iderably by 12 mic roseconds and i s to ta l ly m i s s i n g by 100 yet has comple te ly re turned on the after. Th i s i s essen t ia l ly the same as that found by L . H e r m a n et a l although the effect at 700 °K was not as d ramat ic as that given by tempera tures in the order of 3000 °K produced by the f lash. The continuous absorption seen in the low wavelength reg ion , F i g u r e 15a coincides a lmost exact ly with the D <— X( A^) and C <—X(^A^) sys tems of SO (1850 - 2300 A ° ) . • In this region SO i s a ve ry strong absorber as seen f rom the plates of the adiabatic f lash photolysis of 1, 0. 1, and 0. 04 t o r r of SO-,, F i g u re 15. One can see that when the SO,, p r e s su re i s great ly reduced d iscre te s t ructure appears at lower w ;avelengths. How-ever , the. t ime sequence shows that these bands become broadened at t ime delays in the o rder of 100 microseconds and then rever t to n o r m a l in the m i l l i -second range. -This suggests that the bands are m e r e i y spread out by a shift 62 JQ CD' F I G U R E 14 A b s o r p t i o n in the A( B ) « — X system of 1 t o r r of SO flashed a d i a b a t i c a l l y . The lower s o l i d l i n e i s the blank and . the upper s o l i d curve corresponds to the before and after, .~" at 12 and — at 100 m i c r o s e c o n d s . T h e a r e a s under the c u r v e s i n a r b i t r a r y units are 600, 570, and 560 r e s p e c t i v e l y 2 1 0 0 2 2 0 0 - P a g e 6 3 -W a v e l e n g t h ( A ° ) = 11 I I IW^M 1 • • BBS II I I 11 I I i I I 1 ! • , 1 1 t o r r S O „ S i m i l a r t o N o r r i s h a n d 2 O l d e r s h a w ( P l a t e 2 3 ) R e f . (11) B l a n k B e f o r e 5 ^ s e c . 2 0 3 5 1 0 0 4 3 0 5 3 0 8 7 0 3 . 2 m s e c . A f t e r 2 0 5 0 2 1 5 0 2 0 5 0 2 2 0 0 I B l a n k B e f o r e 5ysec ' 2 0 3 5 4 3 1 5 3 0 8 7 0 3 2 0 0 A f t e r 1 B e f o r e 3 . 2 m s e c . 11 i n I t 1 870 1 . 6 7 m s e c . A f t e r 0. 1 t o r r S O - 0 . 0 4 t o r r S O F I G U R E 1 5 T h e a d i a b a t i c f l a s h p h o t o l y s i s o f v a r i o u s p r e s s u r e s • o f S O - A s t h e p r e s s u r e i s d e c r e a s e d t h e o v e r l y i n g " c o n t i n u u m " b e c o m e s l e s s a p p a r e n t . - Page 64 -T H E A D I A B A T I C F L A S H P H O T O L Y S I S O F NO A N D SO _ z 2 I B R O A D E N I N G O F T H E N O R M A L S P E C T R A (Cont 'd.) i n ro ta t ional in tens i t ies due to an increase in temperature . 'J. The approximate temperature of the sys tem was deter-mined by measur ing the ro ta t ional temperature of the s m a l l amount of SO that was produced in the f lash. Th i s i s expected to ref lect the e q u i l i b r i u m tempera ture of the sys tem since rota t ional re laxa t ion to - 9 i ts equ i l i b r i u m tempera ture takes place i n the order of 10 sec. The SO rota t ional temperature was measured by p ick ing the most intense ro ta t ional l ine of the (1,6) band and then de termining i t s K number f rom the ro ta t ional ana lys i s by E . V . M a r t i n ( 1 9 3 2 ) ^ ^ . Use of the fo rmula 1/2 1/2 ( 4 ? ) K = (kT/2B.hc) - 1/2 =0.590 ( T / B ) ' - 1 / 2 max. where T i s the ro ta t ional tempera ture and B i s the ro ta t ional constant of ground state SO, gave a tempera ture of approximate ly 3000°K, for K max. =36 When the a rea of the mic rodens i tomete r t r aces for spect ra of the lowest p re s su re of SO^ at var ious t ime delays , F i g u r e 16, were measu red over the range 2100 to 2250 A° ah effect s i m i l a r to the upper wavelength system i s seen. That is the bandsbroaden out but the area under the t r ace r emains the same. It i s in this reg ion where the continu-um is supposed to be i ts strongest. The continuous absorpt ion (on the 1 t o r r S 0 2 plate) above 2300 A° is probably pa r t ly due to the SO-, spectrum . . . 65 TO rs F I G U R E 16 The adiabatic f l a s h p h o t o l y s i s of 0. 04 t o r r SO T h i s portion of the short wavelength system shows the v i b r a t i o n a l bands broadening at an i n t e r m e d i a t e time. " B e f o r e and after (225) and 100 ftste. delay (210) a r e a s i n bra c k e t s . - P age 66 -T H E A D I A B A T I C F L A S H P H O T O L Y S I S O F NCU AND SO . 2 2 I B R O A D E N I N G O F T H E N O R M A L S P E C T R A (Cont'd.) broadening as w e l l as the f o r m a t i o n of a s m a l l amount of SO. At low SO p r e s s u r e s seven t r a n s i e n t bands in the 2000 to 2200 A ° r e g i o n d can be observed, F i g u r e 17. A l l seven can be a s s i g n e d to SO bands of the v"=0 p r o g r e s s i o n (14, 48,49). T h e r e s t of the bands present at a l l t i m e s in the short wavelength r e g i o n a r e l i s t e d i n T a b l e III and a r e (50,51) as s i g n e d to SO^ . E v e n with the added abs o r p t i o n f r o m the s m a l l amount of SO in this r e g i o n the total a r e a under the photometer t r a c e s i s constant for a l l t i m e s within 10%, which i s a p p r o x i m a t e l y the a r e a v a r i a t i o n between s e v e r a l blanks repeated on the same plate. II T R A N S I E N T S P E C I E S 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 -LY S I S O F S O ? O B S E R V E D IN T H E V A C U U M U L T R A V I O L E T An explanation of the features o b s e r v e d in the v i s i b l e and u l t r a v i o l e t on a d i a b a t i c a l l y f l a s h i n g both NO_ and SO seems quite straight 2 2 f o r w a r d f r o m the p r e v i o u s r e s u l t s . However, there s t i l l r e m a i n s the s i x (14) t r a n s i e n t bands o b s e r v e d by M;Grath and M c G a r v e y i n the vacuum u l t r a v i o l e t on a d i a b a t i c a l l y f l a s h i n g S O ^ O C S + and CS^ + O-,. T h e y found that these bands fitted into a D e l a n d r e s scheme showing that the bands f o r m e d two sequences. The average separation of the v i b r a t i o n a l l e v e l s i n the upper e l e c t r o n i c state i n v o l v e d in the t r a n s i t i o n was 1280 c m and the two values obtained f o r the lower state were 1311 and 1084 c m Since these two ground state f r e q u e n c i e s were not consistent with the 6 7 - Page 67 -Wavelength (A°) 2000 2050 2100 2150 2058 Si I 2124 Si I I -11 2200 2 3 5 6 1 11 9 10 12 13 14 15 B l a n k B e f o r e Bands of SO L i s t e d in T a b l e III 2 15,0 14,0 12,0 11,0 10,0 9,0 8,0 100 /i/sec Bands of SO L i s t e d i n T a b l e III FIGURE 17 Bands o b s e r v e d i n the adiabatic f l a s h p h o t o l y s i s of 0. 04 t o r r of SO_ - Page 68 -T A B L E III M E A S U R E M E N T S O F SO AND S Q 2 IN T H E 2000 T O 2200 A ° R E G I O N Band 15, 0 14, 0 12, 0 11, 0 10, 0 9,0 8, 0 SO (a) Wavelength (A°) 2013. 3 2033. 2 2074. 8 2096. 3 2119. 9 2144. 2 2169.4'" (a) (b) (47) (b) (48) •6 M c G r ath k M c G a r v e v C o l l i n 49653(cm 1) 49663 (cm" l) 49649 49168 . 49180 49182 48184 48198 48197 47688 47687 47688 47157 47166 47162 46623 46636 46626 46081 46099 46076 S ° 2 ( a ) ( a ) (b) (49) Wavelength (A°) 0) Chow (b)(50) Duchesne k Rosen 2016. 9 49565 ( c m - 1 ) 49550 2019- 8 49494 49477 20 34. 1 49146 49133 2048. 6 48797 48788 2051. 5 48729 48724 2065. 8 48392 48, 390 2081. 8 . 48019 48, 015 2098. 7 47633 47, 627 2100.7 47588 , 47576 2118. 4 47189 47198 2133. 0 46867 46866 2134. 7 46829 46829 2151. 5 46464 46464 2170.0 46068 46077 2186. 6 45718 45710 (a) this work (b) p r e v i o u s l y published measurements. - Page 69 -II T R A N S I E N T S P E C I E S 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 - S O O B S E R V E D IN T H E V A C U U M U L T R A V I O L E T (Cont 'd.) species being n o r m a l SO^ they assigned these bands to the t h e r m a l (13) i s o m e r of SO,, postulated by N o r r i s h and Oldershaw Just recent ly it has been poss ib le to extend this work on S 0 2 into the vacuum u l t rav io le t . Spectra obtained f r o m the adiabatic f lash photolys is of SO^ over the region 1850 t o 2100 A° (the remainder of the C 4—- X sys t em of SO-,) have been photometered and the t races show the same features as that observed in the 2100 to 2250 A ° reg ion . That i s , the SO,, spec t rum i s seen to weaken and then re tu rn to the o r i g i n a l banded s t ructure after s eve ra l m i l l i s e c o n d s , F i g u r e 18a, but the a rea under the t races remained essen t ia l ly constant (within 10%) for a l l delays used. The s ix bands repor ted by M c G r a t h and MzGarvey were also observed. A s w e l l three addi t ional diffuse bands were observed o in the 1400 A region. F u r t h e r invest igat ion of the f lash photolysis of SO i n this region gave another se r i e s of three bands around 1800 A° and three more around 1400 A ° . These bands are diffuse and r e g u l a r l y spaced wi th each group of three having approximate ly the same re la t ive intensi ty suggesting that they are two se r i es of t r i p l e t s . F u r t h e r m o r e these bands have been produced both under i s o t h e r m a l and adiabatic conditions (0. 25 t o r r of SO to 400 t o r r of argon and 0. 25 t o r r of SO respec t ive ly . In b o t h c a s e s the re la t ive in tens i t ies of the se r ies of t rans ient bands remained constant, F i g u r e 18. T h i s resul t alone is . . . 70 2000 Page 7 0 -Wavelength (A°) 1800 1600 1400 (a) 0. 5 t o r r SC>2 (1000 J f lash energy) 2000 1800 1600 I l l l l l l l l l Before 12 f/sec 82 1 msec Af te r B l a n k 1400 u J I I L J 0, 15 O, 16 S O z SO (49) (b) 0.5 t o r r S O z +• 400 t o r r A r (1000 J) re 18 The f lash photolysis of S 0 2 .under (a) adiabatic and (b) i s o t h e r m a l condit ions. Under adiabatic conditions the SO spec t rum i s seen to broaden at in termedia te t imes whereas the spec t rum under i s o t h e r m a l conditions does not. However , the trans-ient bands observed by McGra th and M c G a r v e y plus another low wavelength sys t em are observed in both cases . Measurements are given in Table IV . S. L i n e s (A ) 1 1842.5 172Z. 5 1614.5 S ( 3 P ) 1533.5 1485.5 J _ L B l a n k 1 0 0 fjjei F i g u r e 19 T r a n s i e n t bands in the f lash photo lys is of SO,, . 0. 25 t o r r of S O z + 100 t o r r of argon (1000J). Measu remen t s are given i n Tab le I V . - Page 72 " T A B L E IV T R A N S I E N T B A N D S O B S E R V E D IN T H E V A C U U M U L T R A V I O L E T _^ Re la t ive Re la t ive Wavelength (A°) _ _ c m ) Intensity Wavelength X^> (cm ) Intensity 1843. 3 54250 712 ( a ) 1841. 8 ^ 54295 9 ( c ) 1837. 5 54421 777 1836. 9 54440 8 1830.7 54624 786 1830. 3 54636 10 1801. 0 • 55525 45 3 1799- 5 55571 4 1795.9 55682 444 1794. 7 55720 7 1789. 3 55888 433 1788. 1 55920 5 1761. 8 56760 254 1756. 9 56917 245 1750.5 57127 239 1475.5 67773 933 1471. 8 67945 951 1467. 8 68129 942 1449. 3 69000 669 1445.6 69176 739 1441. 8 69358 713 (a) I max obtained i n this work f r o m a l inea r comparator having a scale p r e c i s i o n Of 3 m i c r o n s . (b) Band measurements made p rev ious ly by M c G r a t h and M c G a r v e y (c) P r e v i o u s worke r s band intensi ty measurements made on a scale va ry ing f r o m 1 to 10 (probably eye est imate) . II T R A N S I E N T S P E C I E S 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 SO-, O B S E R V E D I N T H E V A C U U M U L T R A V I O L E T (Cont 'd. ) enough to d isprove the assignment of these bands to a t h e r m a l i s o m e r of S Q 2 . P r e l i m i n a r y k inet ic resu l t s involv ing the known B < X sys t em of SO and the two t r ip l e t sys tems show that the rate of decrease in in tensi ty with t ime is the same for a l l three sys tems. These measu re -ments have been made for a va r i e ty of conditions which have a l te red the rate of r e m o v a l of SO by as much as a factor of ten and in each case the rate of disappearance of the two t r ip l e t systems was a l te red by the same amount. F o r this reason the t ransient spect ra are be l ieved to be two (V * - X ( 3l) ) sys tems of SO. j.:: co ..11 n'conc 1 us.i;6n the bands assigned by M c G r a t h and M c G a r v e y to a t h e r m a l i s o m e r have been produced under i so the rma l condi-t ions in this work indica t ing that the i r assignment is i nco r r ec t . P r e l i m i n a r y resu l t s show that the extended se r ies of t ransient bands observed belong to two separate t r ip l e t systems of SO. . Work on their assignment is continuing. I l l - DISCUSSION It i s c lea r f r o m the exper imenta l resu l t s given here that the weakening of the SO^ spec t rum, on ad iaba t ica l ly flashing, i s a resul t of temperature broadening and not of the format ion of a high temperature i s o m e r as suggested by N o r r i s h and Oldershaw. T h i s i s found to be the same cause of the ve ry s i m i l a r effect shown by N 0 2 . The ma in argument supporting the SO,, t h e r m a l i s o m e r hypothesis was the appearance of a . . . 74 - Page 74 -i n - DISCUSSION (Cont 'd.) continuum in the short wavelength region of the spec t rum as the long wavelength sys t em d imin i shed . However , the short wavelength sy s t em i s quite intense re la t ive to the longer one. A twenty-five fold reduct ion in p r e s su re f r o m that used by N o r r i s h and Oldershaw is r equ i red before the bands in that reg ion are reduced to measurable opt ica l density. It has been shown here that these bands broaden with temperature at p r e s su re s as low as 0. 04 t o r r . Thus at 1 t o r r p res su re they w i l l give the appearance of a continuum fo rming in that region. The reason N O ^ did not give this appearance in i ts short wavelength region is that it does not absorb s t rongly re la t ive to i ts v i s i b l e sy s t em and i ts bands are not r e g u l a r l y spaced. - Page 75 -B I B L I O G R A P H Y 1. F . J . L i p s c o m b , R . G . W . Nor r i sh . and B . A . T h r u s h ; P r o . Roy . Soc. A . 233. 455 (1956) 2. W . D . M c G r a t h and R . G . W . N o r r i s h ; P r o . Roy. Soc. A254 , 317 (I960) Z . P h y s . C h e m , 15., 246 (1958). 3. R . G . W . N o r r i s h ; Chem. i n B r i t a i n , J,, 289 (1965) a general r ev iew of f lash photolysis studies c a r r i e d out before 1965. 4. N . B a s c o and R . G . W. N o r r i s h ; Can . J . C h e m . J j ^ 1769 (1960) 5. N . B a s c o and R . G . W. N o r r i s h ; D i s c . F a r a d a y Soc. 33. 99 (1962) 6. D . Husa in and R . G . W. N o r r i s h ; P r o . Roy . Soc. A273 . 165 (1962) 7. G . S c h o t t a n d N . Davidson; J . A m . Chem. Soc. , 80. 1841 (1958) 8. R . A . Kane, J . J . M c G a r v e y and W. D . M c G r a t h ; J . Chem. P h y s . , .39_> 840 (1963) 9. A . M . B a s s and D . G a r v i n ; i b i d 40, 1772(1964) 10. J . C . P o i a n y i ; J . C h e m . p h y s . 3J., 1 338 (1.958) 11. P . J . Kuntz , E . M . Nemeth, J . C . P o i a n y i , S^D. Rosner and C . E . Young; J . C h e m . P h y s . JA^ H 6 8 (1966). 12. R . G . W . N o r r i s h and A . P . Ze l l enbe rg ; P r o . Roy . Soc. A.240, 293 (1957) 13. R . G . W . N o r r i s h and G . A . Oldershaw; i b i d A249, 498 (1959) 14. J . J . M c G a r v e y and W . D . M c G r a t h ; i b i d A278, 490 (1964) 15. A . L . M y e r s o n , F . R . T a y l o r and P . L . Hanst; J . Chem. P h y s . , 26, 1309 (1957) 16. L . H e r m a n , J . A k r i c h e and H . Grenat ; J . Quant. S p e c t r o s c Rad . Trans f . , 2, 255 (1962) 17. A . G . Gaydon, G . H . K i m b e l l and H . B . P a l m e r ; P r o . Roy . Soc. A 2 7 6 , ' 461 (1963). 18. R . G . W . N o r r i s h and G . P o r t e r ; Nature (Lond.) 164, 684 (1949): G . P o r t e r ; P r o . Roy . S o c A200, 284 (1950). 19. R . G . W . N o r r i s h , G . P o r t e r , and B . A . T h r u s h ; P r o . Roy . Soc. A216. 165 (1953). - Page 76 -B I B L I O G R A P H Y 20. M . I . C h r i s t i e and G . P o r t e r ; P r o . Roy . Soc. A212. 398 (1952). 21. C o r n i n g G la s s W o r k s , B u l l e t i n C F - 1 ; C o r n i n g , New Y o r k . 22. J . G . C a l v e r t and J . N . P i t t s ; Pho tochemis t ry ; John W i l e y & Sons, Inc. (1966), Table 7-12, p. 748. 23. I l fo rd T e c h n i c a l Information Sheet B40 . 1, I l ford L t d . , England. 24. P . A . Le igh ton ; P h o t o c h e m i s t r y of A i r P o l l u t i o n , A c a d e m i c P r e s s , New Y o r k (1961), P 4 7 . 25. G . H e r z b e r g ; Spect ra of D ia tomic M o l e c u l e s , D . V a n Nos t rand (1965), p. 560. 26. R . W. B . P e a r s e and A . G . Gaydon; The Identification of Moleular Spect ra , Chapman & H a l l L t d . (1963). 27. G . R . Hebert and R . W. N i c h o l l s ; P r o c . P h y s . Soc. 78, 1024(1961). 28. M . E . P i l l o w ; P r o c . P h y s . S o c A 6 3 . 940 (1950). 29. R . W . N i c h o l l s ; Can . J . P h y s . 38, 1705 (I960). 30. N . fiasco and S. K . Dogra ; Chem. C o m . , 1071 (1968) 31. A . A . Westenberg and N . De Haas; J . C h e m . P h y s . 5_0, 707 (1969). 32. S. W. Benson and A . E . Axwor thy ; J . Chem. P h y s . 26_ 1718 (1957) 33. H . W . F o r d and N . Endow; Ibid. 27, 1156 (1957). 34. G . H . M e y e r s , D . M S i lve r and F . Kaufman; Ibid 44, 718 (1966) 35. N . Basco and R . G . W. N o r r i s h ; P r o . Roy . Soc. A260. 293 (I960) 36. I. M . C a m p b e l l and B . A . T h r u s h ; T r a n s . F a r a d a y Soc. 64, 1265 (1968) 37. N . B a s c o and R. G . W. N o r r i s h ; P r o . Roy . S o c A268, 291 (1962) 38. D . Neuberger and A . B . F . Duncan; J . C h e m . P h y s . 22, 1693 (1954). 39. A . E . Douglas ; Ib id . 45, 1007 (1966). 40. K . Saura i and H . P . B r o i d a ; Ibid. 50, 707 (1969). - Page 77 -B I B L I O G R A P H Y 41. A . B . C a l l e a r ; Pho tochemis t ry and Reac t ion K i n e t i c s ; P . G . A s h m o r e , F . S . Dainton; and T . M . Sugden (Ed. ); Cambr idge U n i v e r s i t y P r e s s (1967), Chapt. 7. 42. G . W. Robinson , M . M c C a r t y , and M C. Kee l ty ; J . C h e m . P h y s . _ _ _ 972 (1957). 43. A . E . Douglas and H . P . Huber , Can . J . P h y s . 43, 74 (1965). 44. G . H e r z b e r g ; V o l . I l l , E l e c t r o n i c Spectra of P o l y a t o m i c M o l e c u l e s , V a n Nos t rand (1966), p. 509-45. G . H e r z b e r g , Ibid, p. 605. 46. E . V . M a r t i n ; P h y s . Rev . 41, 167 (1932) 47. G . H e r z b e r g ; V o l . II, op. C I T , p. 124. 48. W. D. M c G r a t h and J . J . M c G a r v e y ; J . Chem. P h y s . 3 _ 1574 (1962) 49. R . C o l i n ; Can . J . P h y s . 47, 979 (1969) 50. J . Duchesne and B . Rosen ; J . C h e m . P h y s . JJ5,' 631 (1947). 51. T . C . Chow; P h y s . Rev . 44, 638 (1933). 52. R . K . R i t c h i e , A . D . Wa l sh and P . A . Warsop; P r o . Roy . Soc. A266. 257. 

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