The Chemistry of Combustion Processes - American Chemical Society

ted fluorescence signal (S) while minimizing the detected scat tered light background .... He also wishes to thank R. D. Gay, S. C. Gray, A. R. Van Ho...
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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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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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Elementary Combustion

500

Reactions: Kinetic Studies

600

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227

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F i g u r e 1. L a s e r - i n d u c e d f l u o r e s c e n c e d e t e c t i o n o f U V - a b s o r b i n g free radicals. The v e r t i c a l l i n e s d e n o t e t h e -wavelengths t h a t a r e most u s e f u l i n f l u o r e s c e n c e e x c i t a t i o n .

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Table

I.

Second H a r m o n i c G e n e r a t i o n

f r o m V i s i b l e Dye L a s e r s

Pulse Repetition R a t e (Hz)

Peak Power Dens i t y (MW/cm )

Nd/YAG-pumped dye l a s e r

30

50, unfocused

10-25

cw dye l a s e r

cw

0.05, focused

0.1

q u a s i - c w dye laser

2.46 χ 1 0

2

8

20, focused

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

CHEMISTRY OF

COMBUSTION

PROCESSES

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.

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231

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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

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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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CHEMISTRY OF COMBUSTION PROCESSES

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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