13 Elementary Combustion Reactions
The Chemistry of Combustion Processes Downloaded from pubs.acs.org by YORK UNIV on 12/08/18. For personal use only.
Laser Photolysis-Laser-Induced Fluorescence Kinetic Studies FRANK P. TULLY Sandia National Laboratories, Livermore, CA 94550 A new, laser-based, chemical kinetics technique has been demonstrated in studies of the reactions of the hydroxyl radical with ethane and ethylene. A widely-tunable, quasi-cw, ultraviolet laser source for exciting transient-species fluorescence in chemical kinetics experiments has been built and is described. The reaction between OH and C H is shown to proceed through both OH addition and Η-atom abstraction routes. 2
4
Combustion processes are d r i v e n by e n e r g y - r e l e a s i n g chemical r e actions. D e t a i l e d knowledge o f t h e c h e m i c a l k i n e t i c s o f these i n d i v i d u a l r e a c t i v e steps i s r e q u i r e d input t o combustion models. F o r more t h a n a d e c a d e , e l e m e n t a r y g a s - p h a s e r e a c t i o n k i n e t i c s has b e e n s u c c e s s f u l l y s t u d i e d w i t h t h e f l a s h p h o t o l y s i s / r e s o n a n c e f l u o r e s c e n c e technique ( 1 - 8 ) . T y p i c a l l y , f o l l o w i n g broadband p h o t o l y s i s o f a m o l e c u l a r p r e c u r s o r , r e a c t a n t d e c a y s h a v e been measured u n d e r p s e u d o - f i r s t - o r d e r k i n e t i c c o n d i t i o n s w i t h cw r e s o n a n c e lamp e x c i t a t i o n o f f r e e r a d i c a l f l u o r e s c e n c e . Increased u t i l i z a t i o n o f l a s e r probes i n k i n e t i c s t u d i e s i s e x e m p l i f i e d by the recent p u l s e d - l a s e r p h o t o l y s i s / p u l s e d - l a s e r - i n d u c e d f l u o r e s c e n c e e x p e r i m e n t s o f M c D o n a l d , L i n and c o w o r k e r s ( 9 - 1 3 ) . I n t h e p r e s e n t w o r k , a new k i n e t i c s c o n f i g u r a t i o n u t i l i z i n g a p u l s e d l a s e r f o r p h o t o l y s i s and a quasi-cw, u l t r a v i o l e t l a s e r f o r f l u o r e s c e n c e e x c i t a t i o n has been d e v e l o p e d . T h i s t e c h n i q u e c o m b i n e s t h e b e s t f e a t u r e s o f t h e two k i n e t i c methods m e n t i o n e d above. L a s e r p h o t o l y s i s g e n e r a l l y p e r m i t s g r e a t e r r e a c t a n t f o r m a t i o n s p e c i f i c i t y t h a n does f l a s h l a m p p h o t o l y s i s . Laser-induced fluorescence d e t e c t i o n outperforms resonance f l u o r e s c e n c e detec t i o n because o f i t s i n c r e a s e d f l u o r e s c e n c e e x c i t a t i o n f l u x , de c r e a s e d s c a t t e r e d l i g h t s i g n a l , and w a v e l e n g t h t u n a b i l i t y . Cw fluorescence e x c i t a t i o n i s d e s i r a b l e over pulsed f l u o r e s c e n c e e x c i t a t i o n due t o i t s f r e e d o m f r o m p u l s e - t o - p u l s e n o r m a l i z a t i o n c o n s t r a i n t s and, most i m p o r t a n t l y , b e c a u s e o f i t s e f f i c i e n t d u t y c y c l e and t h e c o n s e q u e n t i n c r e a s e d d e n s i t y o f p o i n t s o b t a i n a b l e 0097-6156/84/0249-0225S06.00/0 © 1984 American Chemical Society
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226
i n measured m o l e c u l a r c o n c e n t r a t i o n v e r s u s t i m e p r o f i l e s . This high data point density f a c i l i t a t e s accurate slope determinations, r e a d i l y r e v e a l s even s u b t l e d e v i a t i o n s from p s e u d o - f i r s t o r d e r ex p o n e n t i a l d e c a y s , and o f f e r s i n f o r m a t i o n on s e c o n d a r y r e a c t i o n s by c a r e f u l l y mapping s u c h d e v i a t i o n s i n d u c e d b y c o n t r o l l e d p e r t u r bations of the i n i t i a l reactant conditions. Many m o l e c u l a r i n t e r m e d i a t e s o f i m p o r t a n c e t o c o m b u s t i o n and atmospheric c h e m i s t r y have p r i m a r y e l e c t r o n i c t r a n s i t i o n s i n t h e n e a r u l t r a v i o l e t r e g i o n o f t h e e l e c t r o m a g n e t i c s p e c t r u m . We have t h e r e f o r e constructed a w i d e l y - t u n a b l e , quasi-cw, u l t r a v i o l e t l a s e r s o u r c e f o r e x c i t i n g t r a n s i e n t - s p e c i e s f l u o r e s c e n c e i n chem i c a l k i n e t i c s experiments. A s s u m m a r i z e d i n F i g u r e 1, a model o c k e d A r l a s e r o p e r a t i n g a t 514.5 nm s y n c h r o n o u s l y pumps an e x t e n d e d - c a v i t y dye l a s e r , p r o d u c i n g , w i t h v a r i o u s dyes, t u n a b l e r a d i a t i o n f r o m 540 nm t o 900 nm. The dye l a s e r f u n d a m e n t a l o u t put c o n s i s t s o f a t r a i n o f p u l s e s o f 3-6 n J e n e r g y and 8-10 ps d u r a t i o n a t a r e p e t i t i o n r a t e o f 246 MHz. T h i s o u t p u t i s f r e q u e n c y d o u b l e d u s i n g t e m p e r a t u r e - and a n g l e - t u n e d s e c o n d h a r monic g e n e r a t i o n c r y s t a l s . The u l t r a v i o l e t l a s e r r a d i a t i o n p r o duced i n t h i s p r o c e s s i s then used t o e x c i t e f l u o r e s c e n c e i n t h e reactive r a d i c a l s of i n t e r e s t . Three f e a t u r e s of t h i s l a s e r source merit f u r t h e r d i s c u s s i o n . F i r s t , i n a t y p i c a l k i n e t i c e x p e r i m e n t , t h e 1/e c h e m i c a l l i f e t i m e o f t h e p h o t o l y t i c a l l y p r o d u c e d r a d i c a l s v a r i e s between 0.2 and 25 ms, a r e p r e s e n t a t i v e mean b e i n g t i / = 2 ms. F o r s t a t i s t i c a l r e a s o n s , one d e s i r e s t o c o l l e c t a minimum o f 20 c o n c e n t r a t i o n v e r s u s t i m e d a t a p o i n t s p e r 1/e c o n c e n t r a t i o n d e c a y p e r i o d . F o r multichannel s c a l i n g d e t e c t i o n , these t y p i c a l k i n e t i c c o n d i t i o n s i m p l y a maximum d w e l l p e r i o d p e r c h a n n e l o f 100 y s . The u l t r a v i o l e t l a s e r s o u r c e d e s c r i b e d above e m i t s 2.5 χ 10** p u l s e s p e r 100 y s i n t e r v a l ; t h u s , r e l a t i v e t o c h e m i c a l d e c a y s , t h i s r a p i d l y p u l s e d s o u r c e i s v i e w e d b y t h e e x p e r i m e n t a s a cw e x c i t a t i o n probe. S e c o n d , g i v e n t h a t a p u l s e d initiâtion/cw d e t e c t i o n k i n e t i c s c o n f i g u r a t i o n i s d e s i r e d , one may a s k why a cw l a s e r s o u r c e i s n o t u s e d . The r a t i o n a l e h e r e i s t h a t t h e v i s i b l e - t o u l t r a v i o l e t c o n v e r s i o n e f f i c i e n c y i s much h i g h e r when t h e q u a s i cw s o u r c e r a t h e r t h a n a cw s o u r c e i s u s e d . F r e q u e n c y d o u b l i n g e f f i c i e n c y v a r i e s i n p r o p o r t i o n t o t h e f u n d a m e n t a l peak power d e n s i t y present i n t h e second harmonic g e n e r a t i o n c r y s t a l , (P /Po))ot Ρ . T a b l e I l i s t s t y p i c a l p u l s e r e p e t i t i o n r a t e s , f u n d a m e n t a l peak power d e n s i t i e s and f r e q u e n c y d o u b l i n g e f f i c i e n c i e s obtainable with various v i s i b l e l a s e r sources. For the cw and q u a s i - c w dye l a s e r s o u r c e s , peak power d e n s i t i e s a r e e s t i m a t e d a s s u m i n g t h a t 1.0 w a t t o f v i s i b l e r a d i a t i o n i s f o c u s e d t o a 50 ym s p o t w i t h i n t h e f r e q u e n c y d o u b l i n g c r y s t a l . Because t h e beam e n e r g y i s b u n c h e d i n t o s h o r t d u r a t i o n p u l s e p a c k e t s w i t h t h e q u a s i - c w s o u r c e , t h e o b t a i n a b l e f o c u s e d peak power d e n s i t y and t h e r e s u l t a n t s e c o n d h a r m o n i c g e n e r a t i o n e f f i c i e n c y a r e much l a r g e r w i t h t h i s s o u r c e t h a n w i t h a cw dye l a s e r s o u r c e . From 1.0 w a t t o f dye l a s e r r a d i a t i o n a t 616 nm, f o r e x a m p l e , we +
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500
Reactions: Kinetic Studies
600
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Second H a r m o n i c G e n e r a t i o n
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30
50, unfocused
10-25
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0.05, focused
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2.46 χ 1 0
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8
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Second H a r m o n i c Gene r a t i o n E f f i c i e n c y (%)
8
o b t a i n a v e r a g e u l t r a v i o l e t l a s e r powers o f 80 mW and 1 mW upon f r e q u e n c y d o u b l i n g t h e q u a s i - c w and cw f u n d a m e n t a l beams, r e s p e c tively. The h i g h u l t r a v i o l e t f l u x s o o b t a i n e d w i t h t h e q u a s i - c w l a s e r s o u r c e w i l l p e r m i t e f f i c i e n t s t u d y o f many p r e v i o u s l y u n observable chemical processes. Finall" the uncertainty p r i n c i p l e d i c t a t e s t h a t s h o r t d u r a t i o n l a s e r a l s e s have wide s p e c t r a l b a n d w i d t h s . We measure FWHM l a s e r l i n e w i d t h s o f ~ 50 GHz f o r t h e f u n d a m e n t a l beam. T h i s w i d t h i s much l a r g e r t h a n a t y p i c a l Doppler-broadened a b s o r p t i o n l i n e i n a d i a t o m i c molecule, and, as d e s c r i b e d b e l o w f o r OH d e t e c t i o n , a b s o r p t i o n l i n e c o i n c i d e n c e s must be l o c a t e d and e x p l o i t e d t o o p t i m i z e t h e l a s e r - i n d u c e d f l u o rescence d e t e c t i o n e f f i c i e n c y of diatomic r a d i c a l s w i t h t h i s quasi-cw, u l t r a v i o l e t source. This constraint l a r g e l y disappears when m o n i t o r i n g p o l y a t o m i c r a d i c a l s , a s t h e s e p a r a t i o n s b e t w e e n t h e i r r o - v i b r o n i c t r a n s i t i o n l i n e s a r e comparable t o Doppler l i n e w i d t h s , thereby making a l l o f t h e source u l t r a v i o l e t r a d i a t i o n a b s o r b a b l e by t h e r a d i c a l . These c o n s i d e r a t i o n s a r e d i s c u s s e d i n d e t a i l b y Inoue e t a l ( 1 4 ) i n t h e i r c o m p a r i s o n o f t h e laser-induced fluorescence spectroscopies of the molecular homologs OH and C H 0 . The a p p l i c a t i o n o f t h e q u a s i - c w , u l t r a v i o l e t l a s e r s o u r c e t o k i n e t i c s t u d i e s was d e m o n s t r a t e d i n t h e l a s e r p h o t o l y s i s / l a s e r i n d u c e d f l u o r e s c e n c e e x p e r i m e n t s shown s c h e m a t i c a l l y i n F i g u r e 2. C h e m i c a l r e a c t i o n s were i n i t i a t e d by 193-nm p h o t o l y s i s o f N 0 i n N 0 / H 0 / h y d r o c a r b o n / h e l i u m gas m i x t u r e s . The 0(*D) atoms formed by p h o t o d i s s o c i a t i o n r a p i d l y c o n v e r t e d t o OH t h r o u g h r e a c t i o n w i t h H 0 , and t i m e - r e s o l v e d OH c o n c e n t r a t i o n s were measured a s f u n c t i o n s o f h y d r o c a r b o n number d e n s i t y b y l a s e r - i n d u c e d f l u o rescence. H y d r o x y l r a d i c a l f l u o r e s c e n c e was e x c i t e d b y pumping t h e n e a r l y c o i n c i d e n t P j l , 0^3, and Q ^ ( 0 , 0 ) band Χ Π > Α Σ * t r a n s i t i o n s a t 308.16 nm, ( 1 5 ) and r a d i a t i o n e m a n a t i n g f r o m t h e r e a c t i o n volume i n a downward d i r e c t i o n was skimmed by b l a c k a n o d i z e d c o l l i m a t o r s , f o c u s e d by q u a r t z l e n s e s , s e l e c t e d by a b a n d p a s s f i l t e r (308.3 nm p e a k , 8 nm FWHM), and d e t e c t e d b y an RCA 8850 p h o t o m u l t i p l i e r o p e r a t i n g i n t h e p h o t o n - c o u n t i n g mode. P
3
2
2
2
2
1
2
2
PDP 11/23 MINI COMPUTER
Figure 2. Schematic diagram o f the l a s e r p h o t o l y s i s / l a s e r - i n d u c e d fluorescence chemical k i n e t i c s apparatus. (Reproduced w i t h permission from Ref. 20. Copyr i g h t 1 9 8 3 , North-Holland.)
ANALYZER
MULTICHANNEL
230
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The p h o t o m u l t i p l i e r o u t p u t p u l s e s were a m p l i f i e d , d i s c r i m i n a t e d , and f e d i n t o a m u l t i c h a n n e l s c a l e r , and OH f l u o r e s c e n c e d e c a y s w e r e s i g n a l a v e r a g e d o v e r 25-250 e x c i m e r l a s e r s h o t s . The f i r s t k i n e t i c s e x p e r i m e n t s p e r f o r m e d w i t h t h i s a p p a r a t u s d e a l t w i t h t h e a b s t r a c t i o n o f h y d r o g e n atoms by OH f r o m methane and e t h a n e , OH + RH • H 0 + R. R e l i a b l e rate c o e f f i c i e n t data f o r t h e s e r e a c t i o n s had p r e v i o u s l y been o b t a i n e d i n f l a s h p h o t o l y s i s / r e s o n a n c e f l u o r e s c e n c e s t u d i e s , (7,16) and agreement w i t h t h e s e p u b l i s h e d d a t a s e r v e d a s a r e q u i r e d c h e c k on t h e p e r f o r m a n c e o f t h e new k i n e t i c s c o n f i g u r a t i o n . The r e s u l t s f o r t h e r e a c t i o n between OH and C H a r e shown i n F i g u r e 3. The r a t e c o e f f i c i e n t s measured i n t h e p r e s e n t w o r k a l l f a l l a b o u t 5% b e l o w t h o s e o f R e f . 16; s u c h agreement i s w e l l w i t h i n t h e e s t i m a t e d 10% a c c u r a c y l i m i t s o f t h e two s t u d i e s . I n b o t h s e t s o f e x p e r i m e n t s , t h e OH e x c i t a t i o n and f l u o r e s c e n c e w a v e l e n g t h s were r e s o n a n t , and o p t i m i z i n g the [0H]-time p r o f i l e s r e q u i r e d maximizing the d e t e c t e d f l u o r e s c e n c e s i g n a l (S) w h i l e m i n i m i z i n g t h e d e t e c t e d s c a t t e r e d l i g h t background ( B ) . For s i m i l a r v a l u e s of [0H] _Q, the r a t i o S/B i n t h e p r e s e n t work e x c e e d e d t h a t o f R e f . 16 b y more t h a n an o r d e r o f m a g n i t u d e . Two f a c t o r s c o n t r i b u t e d t o t h i s marked improvement i n S/B. F i r s t , the absorbable photon f l u x generated w i t h the quasi-cw, u l t r a v i o l e t l a s e r source exceeded t h a t f r o m an OH r e s o n a n c e lamp by a f a c t o r o f 2-3. S e c o n d , and most s i g n i f i c a n t , t h e d e t e c t e d s c a t t e r e d l i g h t s i g n a l f r o m t h i s c o l l i m a t e d l a s e r s o u r c e was 10-25 t i m e s l e s s t h a n t h a t t y p i c a l l y o b t a i n e d w i t h a v o l u m e - s o u r c e , OH r e s o n a n c e lamp 08). Further i m p r o v e m e n t s i n S/B a r e e x p e c t e d i n f u t u r e e x p e r i m e n t s i n w h i c h OH f l u o r e s c e n c e w i l l be e x c i t e d by s i n g l e - f r e q u e n c y u l t r a v i o l e t l a s e r r a d i a t i o n o b t a i n e d by i n t r a c a v i t y f r e q u e n c y - d o u b l i n g an a c t i v e l y s t a b i l i z e d cw r i n g dye l a s e r . E n c o u r a g e d by t h e s e r e s u l t s , we began t o s t u d y h y d r o x y l r a d i c a l r e a c t i o n s f o r which only l i m i t e d k i n e t i c i n f o r m a t i o n i s available. A d e t a i l e d i n v e s t i g a t i o n of the r e a c t i o n 2
2
6
t
(1) i s i n progress. A t p r e s e n t , k i n e t i c measurements have b e e n made at 600 T o r r h e l i u m t h r o u g h o u t t h e t e m p e r a t u r e r a n g e 291-796 K, and a t 291 Κ o v e r t h e p r e s s u r e r a n g e 50-600 T o r r h e l i u m . Abso l u t e r e a c t i o n r a t e c o e f f i c i e n t s k j were d e t e r m i n e d , o r , i n some c a s e s , a p p r o x i m a t e d , as d e s c r i b e d b e l o w . We c a r r i e d out a l l e x p e r i m e n t s u n d e r p s e u d o - f i r s t - o r d e r k i n e t i c c o n d i t i o n s w i t h [0H]«[t^H^ ] . E x c l u d i n g s e c o n d a r y r e a c t i o n s t h a t s i g n i f i c a n t l y d e p l e t e o r r e f o r m OH, [OH] v a r i e d exponentially w i t h time: [0H]
t
=
[ 0 H ] exp q
-(kJCjHj
+ k )t = [0H] d
q
exp(-k't)
w h e r e k' i s t h e m e a s u r e d p s e u d o - f i r s t - o r d e r d e c a y r a t e , k j i s t h e b i m o l e c u l a r r a t e c o e f f i c i e n t f o r R e a c t i o n ( 1 ) , [0 Η^] i s 2
13.
Elementary Combustion Reactions: Kinetic Studies
TULLY
231
1
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3
12
ϊ> i o - H
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10
-13 π 1.5
1 2
1 1 2.5 3 1000/T(K)
3.5
F i g u r e 3. A r r h e n i u s p l o t o f r a t e c o e f f i c i e n t d a t a f o r t h e r e a c t i o n OH + CpHg — - H 0 + CgH . Ο , R e f . l 6 , f l a s h p h o t o l y s i s / r e s o nance r l u o r e s c e n c e ; Δ , t h i s work, l a s e r p h o t o l y s i s / l a s e r - i n d u c e d fluorescence. E r r o r s p e c i f i c a t i o n s a r e +2 p r e c i s i o n l i m i t s i n both cases. 2
232
CHEMISTRY OF COMBUSTION PROCESSES
t h e ( c o n s t a n t ) e t h y l e n e c o n c e n t r a t i o n , and i s the f i r s t - o r d e r r a t e c o n s t a n t f o r OH r e m o v a l i n t h e a b s e n c e o f [C^H^] due t o d i f f u s i o n f r o m t h e r e a c t i o n volume and t o r e a c t i o n w i t h b a c k g r o u n d i m p u r i t i e s . We o b s e r v e d e x p o n e n t i a l [OH] d e c a y s , s u c h as t h a t shown as l n [OH] v e r s u s t i m e i n F i g u r e 4 a , f o r a l l e x p e r i m e n t s a t Τ = 291 and 361.5 K. However, as we v a r i e d Τ f r o m 438 t o 666 K, n o n - e x p o n e n t i a l f e a t u r e s i n t h e [OH] p r o f i l e s became i n c r e a s i n g l y apparent. A t y p i c a l [OH] p r o f i l e o b t a i n e d a t 591 Κ i s p l o t t e d f o r c o m p a r i s o n i n F i g u r e 4b. F o r e x p o n e n t i a l [OH] p r o f i l e s , - k was e q u a t e d t o t h e c a l c u l a t e d l e a s t - s q u a r e s s l o p e of t h e d e c a y t a k e n o v e r a t l e a s t a f a c t o r o f t e n v a r i a t i o n i n [OH]. When a n a l y z i n g n o n e x p o n e n t i a l [OH] p r o f i l e s , we e s t i m a t e d - k from the steep i n i t i a l s l o p e of the decay. I n e i t h e r case, the k v a l u e s o b t a i n e d a t a g i v e n t e m p e r a t u r e and p r e s s u r e were p l o t t e d , as shown i n F i g u r e 5, as a f u n c t i o n o f t h e c o r r e s p o n d i n g ethylene concentration. Bimolecular rate c o e f f i c i e n t s kj(T,P) were then determined from the s l o p e of the l e a s t - s q u a r e s s t r a i g h t l i n e t h r o u g h t h e ( [ C H j , k ' ) d a t a p o i n t s . The h i g h - p r e s s u r e l i m i t e d r a t e c o e f f i c i e n t s k ( T ) m e a s u r e d i n t h i s w o r k and i n p r e v i o u s s t u d i e s a r e p l o t t e d , a l o n g w i t h v a r i o u s summary a n a l y t i c r e p r e s e n t a t i o n s , i n F i g u r e 6. A t 291 K, k j was f o u n d t o be p r e s s u r e - d e p e n d e n t , and i t r e a c h e d a h i g h - p r e s s u r e - l i m i t e d v a l u e o f (8.47±0.24) χ 1 0 ~ cm molecule" s " above 400 T o r r h e l i u m . T h i s v a l u e f o r k j ( 2 9 1 K) i s i n e x c e l l e n t agreement w i t h t h e r e s u l t s o f p r e v i o u s s t u d i e s (17-19). From 291-438 K, t h e r e a c t i o n mechanism i s d o m i n a t e d by e l e c t r o p h i l i c a d d i t i o n o f OH t o t h e e t h y l e n e d o u b l e b o n d , and t h e t e m p e r a t u r e dependence o v e r t h i s r a n g e o f t h e h i g h - p r e s s u r e l i m i t e d r a t e c o e f f i c i e n t may be r e p r e s e n t e d by t h e e x p r e s s i o n k j ( T ) = (1.74±0.53) χ ΙΟ"" exp (918±214)/RT cm m o l e c u l e " s" , w h e r e q u o t e d e r r o r s r e p r e s e n t ±2σ v a l u e s and = Ασ^^ (20,21). Our o b s e r v a t i o n o f n o n e x p o n e n t i a l [OH] p r o f i l e s i n t h e t e m p e r a t u r e r a n g e 438-666 Κ can o n l y be i n t e r p r e t e d i n t e r m s o f a c h e m i c a l p r o c e s s w h i c h r e f o r m s OH d u r i n g t h e 1-20 ms d u r a t i o n of the experiment. This process i s the decomposition back t o r e a c t a n t s o f t h e t h e r m a l i z e d a d d u c t H 0 C H . I n d e e d , a t 591 K, we o b s e r v e d t h a t a t v e r y l o n g t i m e s t h e [OH] d e c a y s a g a i n became e x p o n e n t i a l w i t h a slope of - k (see F i g u r e 4b). T h i s s i t u a t i o n r e s u l t s o n l y when t h e OH + C H + M ^ Η 0 0 Η ^ + M r e a c t i o n has e s t a b l i s h e d dynamic e q u i l i b r i u m , w i t h the t h e r m a l i z e d adduct s e r v i n g , i n e f f e c t , as a t e m p o r a r y s i n k f o r OH. The r a t e c o e f f i c i e n t d a t a d e r i v e d f r o m n o n e x p o n e n t i a l [OH] p r o f i l e s must t h u s be c o n s i d e r e d a p p r o x i m a t e , and t h e y a r e i n c l u d e d i n F i g u r e 6 o n l y t o show t h e d e c r e a s i n g t r e n d i n " n e t " r e a c t i v i t y b e t w e e n OH and C H w i t h i n c r e a s i n g t e m p e r a t u r e . These r e s u l t s and t h e i r i n t e r p r e t a t i o n are e n t i r e l y analogous to those obtained i n s t u d i e s o f h y d r o x y l r a d i c a l a d d i t i o n t o a r o m a t i c h y d r o c a r b o n s (22-25). We have a l s o made v e r y p r e l i m i n a r y k i n e t i c measurements on R e a c t i o n (1) a t Τ = 704 and 796 K. The [OH] p r o f i l e s c o l l e c t e d a t t h e s e t e m p e r a t u r e s show a l o w e r d e g r e e o f n o n e x p o n e n t i a l f
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F i g u r e k. T y p i c a l [OH] c o n c e n t r a t i o n p r o f i l e s o b t a i n e d i n k i n e t i c measurements o f t h e r e a c t i o n OH + C^H^ —·»products; a , Τ = 2 9 1 K , Ρ = 6θΟ t o r r h e l i u m , a n d [CpH. ] = 3.W χ 1 0 m o l e c u l e cm" ; b , Τ = 5 9 1 K , Ρ = 600 t o r r h e l i u m , a n d [ C ^ ] = 2 . 7 8 χ 10 molecule cm" . (Reproduced w i t h p e r m i s s i o n from Ref. 2 0 . C o p y r i g h t 1983, North-Holland.) 3
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F i g u r e 5· M e a s u r e d d e c a y r a t e k' a s a f u n c t i o n o f e t h y l e n e c o n c e n t r a t i o n f o r e x p e r i m e n t s a t Τ = 291K a n d Ρ = 100 t o r r h e l i u m . T h r e e d i f f e r e n t C2H^/He s o u r c e g a s m i x t u r e s were s a m p l e d . The e s t i m a t e d a c c u r a c y o f t h e [C^Hi] measurements i s 5%. (Reproduced w i t h p e r m i s s i o n from Ref. 20. C o p y r i g h t 1983, North-Holland.)
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behavior than those measured i n the range 515-666 K. However, k* values at f i x e d ethylene concentrations v a r i e d somewhat when p h o t o l y s i s pulse energies were changed; we have not yet i d e n t i f i e d the source of t h i s e f f e c t . Nevertheless, s e m i - q u a n t i t a t i v e estimates of k (704-796 K) may be d e r i v e d from these measure ments, and the r e a c t i o n r a t e c o e f f i c i e n t appears to grow with i n c r e a s i n g temperature over t h i s range. We b e l i e v e t h i s increase in k j ( T ) to be caused by the onset of the hydrogen a b s t r a c t i o n channel f o r Reaction (1), OH + C H •H 0 + C H . Twoparameter expressions f o r t h i s a b s t r a c t i o n channel r a t e c o e f f i c i e n t have p r e v i o u s l y been derived from complex, high-temperature k i n e t i c s t u d i e s i n three reviews (26-28). As seen i n Figure 6, our p r e l i m i n a r y measurements y i e l d poor agreement with these recommendations. Further d i r e c t , " s i n g l e - r e a c t i o n s t u d i e s of t h i s a b s t r a c t i o n process w i l l be needed to c l a r i f y these d i s crepancies. In summary, we have demonstrated that p h o t o l y s i s / f l u o r e s cence chemical k i n e t i c s techniques must e x p l o i t ongoing advances i n l a s e r technology. A h i g h l y - s e n s i t i v e , quasi-cw, u l t r a v i o l e t l a s e r source was constructed and used i n d e f i n i t i v e chemical k i n e t i c s experiments. OH-ethylene r e a c t i o n s have been charac t e r i z e d i n terms of OH a d d i t i o n and hydrogen atom a b s t r a c t i o n channels, and questions have been r a i s e d concerning both the importance of the a d d i t i o n process and the accuracy of p r e s e n t l y recommended k i n e t i c parameters f o r the a b s t r a c t i o n process at combustion temperatures. 2
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Acknowledgment s T h i s research was supported by the O f f i c e of Basic Energy Sciences, U. S. Department of Energy. The author g r a t e f u l l y acknowledges h e l p f u l d i s c u s s i o n s with J . Ε. M. Goldsmith, J . S. B i n k l e y , M. L. Koszykowski, C. F. Melius and R. E. Palmer. He a l s o wishes to thank R. D. Gay, S. C. Gray, A. R. Van Hook and G. D. Cosgrove f o r t h e i r c o n t r i b u t i o n s to t h i s work.
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8. Ravishankara, A. R.; Nicovich, J. M.; Thompson, R. L.; Tully, F. P. J. Phys. Chem. 1981, 85, 2498. 9. McDonald, J. R.; Miller, R. G.; Baronavski, A. P. Chem. Phys. 1980, 30, 133. 10. Lin, M. C.; McDonald, J. R. in "Reactive Intermediates in the Gas Phase"; Setser, D. W., Ed.; Academic: New York, 1979; p. 233. 11. Sanders, N.; Butler, J. E.; Pasternack, L. R.; McDonald, J. R. Chem. Phys. 1980, 48, 203. 12. Butler, J. E.; Fleming, J. W.; Goss, L. P.; Lin, M. C. Chem. Phys. 1981, 56, 355. 13. Nelson, H. H.; McDonald, J. R. J. Phys. Chem. 1982, 86, 1242. 14. Inoue, G.; Akimoto, H.; Okuda, M. J. Chem. Phys. 1980, 72, 1769. 15. Dieke, G. H.; Crosswhite, H. M. J. Quantum Spectry. Rad. Transfer 1962, 2, 97. 16. Tully, F. P.; Ravishankara, A. R.; Carr, K. Intern. J. Chem. Kinet. 1983, 15, 1111. 17. Atkinson, R.; Perry, R. Α.; Pitts, J. N., Jr. J. Chem. Phys. 1977, 66, 1197. 18. Lloyd, A. C.; Darnall, K. R.; Winer, A. M.; Pitts, J. N., Jr. J. Phys. Chem. 1976, 80, 789. 19. Cox, R. A. Intern. J. Chem. Kinetics Symp. 1975, 1, 379. 20. Tully, F. P. Chem. Phys. Lett. 1983, 96, 148. 21. Gordon, S.; Mulac, W. A. Intern J. Chem. Kinetics Symp. 1975, 1, 289. 22. Perry, R. Α.; Atkinson, R.; Pitts, J. Ν., Jr. J. Phys. Chem. 1977, 81, 296. 23. Perry, R. Α.; Atkinson, R.; Pitts, J. N., Jr. J. Phys. Chem. 1977, 81, 1607. 24. Tully, F. P.; Ravishankara, A. R.; Thompson, R. L.; Nicovich, J. M.; Shah, R. C.; Kreutter, Ν. M.; Wine, P. H. J. Phys. Chem. 1981, 85, 2262. 25. Nicovich, J. M.; Thompson, R. L.; Ravishankara, A. R. J. Phys. Chem. 1981, 85, 2913. 26. Warnatz, J. in "Chemistry of Combustion Reactions"; Gardiner, W. C., Jr., Ed.; Springer-Verlag: New York, 1983; Chap. 5. 27. Westley, F. "Table of Recommended Rate Constants for Chemical Reactions Occurring in Combustion"; NBSIR 79-1941, November 1979. 28. Westbrook, C. K.; Dryer, F. L. "Chemical Kinetics Modeling of Hydrocarbon Combustion"; Lawrence Livermore National Laboratory Report No. UCRL-88651; February, 1983. RECEIVED November 30, 1983
