15 3
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Measurement of the C (α П ) and C (X Σ ) Disappearance Rates with O from 298 to 1300 Kelvin 2
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2
STEVEN L. BAUGHCUM and RICHARDC.OLDENBORG Chemistry Division, Los Alamos National Laboratory, Los Alamos, ΝM 87545
3
The disappearance rates of C (a П , v=0, 1, and 2) and C (X Σ, v=0) in the presence of O have 2
u
1
2
2
been measured over the 298-1300 Κ temperature range. The C is produced by multiple-photon dissociation of CF3CCCF3 at 193 nm and probed by laser-induced fluorescence. The disappearance rate of C (a П , v=0) as a function of tempera ture can be extremely well represented by the Arrhenius expression k(T) = A exp (-E/RT), with A = 1.49 ± 0.03 x 10- cm molecule s and Ε = 0.98 ± 0.02 kcal/mole. The quality of the fit over such a large temperature range provides a test of previously proposed models of the C2 + O system, which do not predict simple Arrhen ius behavior. Our results are consistent with a model in which C and C are interconverted by O via a long range interaction and reaction occurs upon still closer approach, so that the identity of the initial state is lost before reaction occurs. Experiments with C (X Σ ) + O are consistent with this model. 2
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-1
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Although radical-molecule r e a c t i o n s p l a y an important r o l e i n combustion, s t a t e - s e l e c t e d measurements o f the r e a c t i o n rate constants a t high temperature have been made f o r only a few reactions. Data on the temperature dependence o f r a d i c a l molecule r e a c t i o n rate constants are v i t a l f o r assessing the importance of various r e a c t i o n s i n combustion and s i m i l a r high temperature processes and provide important i n s i g h t i n t o the d e t a i l s of the p o t e n t i a l energy surfaces i n v o l v e d . Radicals which are e l e c t r o n i c a l l y or v i b r a t i o n a l l y e x c i t e d may react with d i f f e r e n t rates and produce d i f f e r e n t products compared to the 0097-6156/ 84/0249-0257506.00/ 0 © 1984 American Chemical Society Sloane; The Chemistry of Combustion Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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258
CHEMISTRY OF COMBUSTION PROCESSES
ground s t a t e r a d i c a l . T h u s , i t i s i m p o r t a n t t o do s t a t e s e l e c t i v e m e a s u r e m e n t s , where p o s s i b l e . D e t a i l e d r e a c t i o n mechanisms and k i n e t i c s a r e b e s t s t u d i e d i n an e n v i r o n m e n t i n w h i c h s p e c i f i c r e a c t i o n s o r s e q u e n c e s o f r e a c t i o n s c a n be i s o l a t e d . I n o r d e r t o c o n d u c t s u c h s t u d i e s , we have c o n s t r u c t e d a c e l l i n w h i c h t h e t e m p e r a t u r e c a n be c o n t i n u o u s l y v a r i e d o v e r t h e 298-1300 Κ t e m p e r a t u r e r a n g e . R a d i c a l s a r e p r o d u c e d by p h o t o l y s i s o f s u i t a b l e p r e c u r s o r s w i t h a p u l s e d r a r e g a s - h a l i d e e x c i m e r l a s e r and t h e r a d i c a l p o p u l a t i o n s p r o b e d by laser-induced fluorescence (LIF). LIF i s a powerful tool f o r s t u d y i n g k i n e t i c p r o c e s s e s , s i n c e i t i s b o t h v e r y s e n s i t i v e and s t a t e s e l e c t i v e , a l l o w i n g the study of r a d i c a l - m o l e c u l e reactions u n d e r pseudo f i r s t o r d e r c o n d i t i o n s . Furthermore, since s p a t i a l l y i t i s a p o i n t d i a g n o s t i c , h i g h t e m p e r a t u r e s t u d i e s c a n be done i n a r e l a t i v e l y s m a l l v o l u m e , w h i c h m i n i m i z e s some o f t h e e x p e r i mental d i f f i c u l t i e s a s s o c i a t e d w i t h the h i g h temperatures, par t i c u l a r l y thermal gradients. A v a r i e t y o f i m p o r t a n t r a d i c a l s can be p r o b e d by L I F , i n c l u d i n g OH, HS, CH, C , C and C H 0 . Our i n i t i a l e x p e r i m e n t s have c e n t e r e d on t h e C radical, w h i c h i s known t o e x i s t i n h i g h c o n c e n t r a t i o n s i n f l a m e and o t h e r combustion environments. The p r e s e n c e o f a l o w - l y i n g e x c i t e d electronic state ( a ^ ) w i t h i n 610 cm" o f the ground state 2
3
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2
3
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(X !*)
(1)
requires
that
1
kinetic
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done on b o t h
states
s i n c e b o t h w i l l be s i g n i f i c a n t l y p o p u l a t e d a t t h e t e m p e r a t u r e s o f interest. The k i n e t i c s o f C w i t h a v a r i e t y o f r e a c t a n t s have b e e n i n v e s t i g a t e d a t room t e m p e r a t u r e (2-7) and o v e r t h e 300600 Κ t e m p e r a t u r e range ( 8 - 1 0 ) . We c h o s e t o s t u d y t h e C + 0 r e a c t i o n as a t e s t o f o u r a p p a r a t u s and t o f u r t h e r e v a l u a t e t h e model p r o p o s e d by M a n g i r and c o w o r k e r s (6) w h i c h p r e d i c t e d nonArrhenius behavior of the disappearance rate c o n s t a n t s . 2
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Experimental The h i g h t e m p e r a t u r e c e l l ( F i g u r e 1) i s b a s e d on a d e s i g n by F e l d e r and c o w o r k e r s (1_1).. A c e n t r a l h i g h - p u r i t y a l u m i n a t u b e i s h e a t e d r e s i s t i v e l y i n two zones by P t / 4 0 % Rh r e s i s t a n c e w i r e . T h e r m a l i n s u l a t i o n i s p r o v i d e d by an a l u m i n a h e a t s h i e l d s u r r o u n d e d by z i r c o n i a f i b e r i n s u l a t i o n , w i t h t h e w h o l e a s s e m b l y e n c l o s e d i n a w a t e r - c o o l e d b r a s s vacuum chamber. The t e m p e r a t u r e i s measured b y t h e r m o c o u p l e s i n s e r t e d t h r o u g h 0 - r i n g s e a l s t o p r o b e v a r i o u s r e g i o n s o f t h e oven. The t h e r m o c o u p l e o u t p u t s a r e sent to a M i c r i c o n microprocessor which a u t o m a t i c a l l y regulates the h e a t e r c u r r e n t . The r e a c t a n t and b u f f e r g a s e s a r e i n t r o d u c e d a t t h e b o t t o m o f t h e c e l l and a r e h e a t e d as t h e y f l o w s l o w l y (~0.15 s£m) t h r o u g h t h e c e n t e r t u b e . A s m a l l amount o f t h e r a d i cal p r e c u r s o r i n a helium mixture i s introduced v i a a waterc o o l e d v a r i a b l e l e n g t h i n l e t s y s t e m w i t h i n a few cm o f t h e o p t i c a l p o r t s t o m i n i m i z e p y r o l y s i s and p r e - r e a c t i o n p r o b l e m s . The p r e s s u r e i s measured with a c a p a c i t a n c e manometer and gas
Sloane; The Chemistry of Combustion Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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15.
BAUGHCUM AND OLDENBORG
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C (a Il ) 2
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and C (X X ) Disappearance Rates 2
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flows with c a l i b r a t e d Tylan mass flow meters. The gas flow i s s u f f i c i e n t l y f a s t to assure a f r e s h gas mix f o r each l a s e r shot. At the pressures (20 t o r r ) and flow c o n d i t i o n s used, the thermal gradients i n the region probed were l e s s than 1%. The p h o t o l y s i s l a s e r i s a Lambda Physik EMG-102 rare gash a l i d e excimer l a s e r operating at 193 nm. The r a d i c a l popula t i o n s are probed using e i t h e r Quanta-Ray Nd:YAG laser-pumped dye l a s e r s with n o n - l i n e a r mixing c r y s t a l s where appropriate or a Molectron n i t r o g e n - l a s e r pumped dye l a s e r . The excimer l a s e r beam i s focused to a 3 x 3 mm beam and the probe l a s e r s are combined using s u i t a b l e d i c h r o i c mirrors to probe the center of t h i s spot. The fluorescence i s imaged with a lens through a s u i t a b l e d i e l e c t r i c f i l t e r onto the element of an RCA 31034A p h o t o m u l t i p l i e r tube. The s i g n a l i s a m p l i f i e d and processed with a PAR Model 162 boxcar averager. The output of the boxcar i s then sent to a computer f o r s o p h i s t i c a t e d data a n a l y s i s . To measure the chemical l i f e t i m e of the r a d i c a l , the time delay between the excimer l a s e r and the probe l a s e r i s scanned while monitoring the t o t a l fluorescence i n the wavelength region of the band of i n t e r e s t . For s p e c t r a l scans, the time delay i s f i x e d and the dye l a s e r i s scanned over the region of i n t e r e s t . The C r a d i c a l i s produced by multiple-photon d i s s o c i a t i o n of CF3CCCF3 at 193 nm. Approximately 20 t o r r of helium i s used to t r a n s l a t i o n a l l y and r o t a t i o n a l l y e q u i l i b r a t e the C with the bath gas. The d i s s o c i a t i o n produces both *C and C and a s i g n i f i c a n t amount of the C i s v i b r a t i o n a l l y e x c i t e d . Our exper iments i n d i c a t e that at these pressures helium i s not e f f e c t i v e at v i b r a t i o n a l l y quenching the C , although the r a p i d r o t a t i o n a l t h e r m a l i z a t i o n i s c l e a r l y evident. For measurements of the r e a c t i o n rates of C , 1-2 mtorr of C F 3 C C C F 3 i s used with 20 t o r r of 99.99% helium and 0-0.8 t o r r of 99.99% 0 . The C ( a i l , v=0) i s probed by e x c i t a t i o n of the Swan bands 2
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