16 Chemiluminescence from the Gas Phase Reaction of
State-to-State Chemistry Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 11/06/18. For personal use only.
Atomic Boron with the Alkali Metal Fluorides U. C. SRIDHARAN, D. L. MC F A D D E N , and P. DAVIDOVITS Department of Chemistry, Boston College, Chestnut Hill, MA 02167
During the past few y e a r s t h e r e has been c o n s i d e r a b l e i n t e r e s t i n chemical r e a c t i o n s t h a t produce excited alkali metal atoms. Chemiluminescence from alkali atoms has been s t u d i e d r e c e n t l y i n both beam experiments (1-3) where s i n g l e collision p r o c e s s e s can be observed as w e l l as i n flames (4-8) where complex collisional energy t r a n s f e r p l a y s an i m p o r t a n t r o l e i n the e x c i t a t i o n p r o c e s s . These s t u d i e s were m o t i v a t e d i n p a r t by the possibility t h a t these r e a c t i o n s may produce i n v e r s i o n i n the e x c i t e d s t a t e s of the alkali atoms, s u i t a b l e f o r chemical l a s e r s . We are s t u d y i n g the chemiluminescence produced by the r e a c t i o n of boron with the alkali f l u o r i d e s (MF), that is Β + MF-->BF + M With the r e a c t a n t s i n the ground s t a t e the e x o e r g i c i t y of these r e a c t i o n s i s about 3 ev. T h i s amount of energy can e x c i t e only the first p s t a t e s in sodium and potassium but i n rubidium and cesium h i g h e r l y i n g e x c i t e d s t a t e s can be r e a c h e d . Our aim i s to d e t e r mine the distribution o f e x c i t e d s t a t e s produced i n these r e a c t i o n s and u l t i m a t e l y to understand the r e a c t i o n mechanism. It i s expected t h a t the r e s u l t s from experiments with these relatively simple r e a c t i o n s w i l l l e a d a l s o to a b e t t e r general u n d e r s t a n d i n g of energy partitioning in exoergic r e a c t i o n s . Both c r o s s e d flow and m o l e c u l a r beam t e c h n i q u e s are being used to study these chemi1uminescent reac tions. A simplified schematic drawing o f the flow apparatus i s shown i n Fig. 1. The flow c o n t a i n i n g the boron atoms i s produced i n s i d e the vacuum chamber by a microwave d i s c h a r g e i n a m i x t u r e of 1% B H , 99% helium. The alkali f l u o r i d e molecules are produced i n an oven heated between 925-1275 Κ (depending on the 2
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Chemiluminescence to Rotary Pump
t. Halide Source to Pressure Gauges Microwave Source Boron Source [ GAS FLOW STATIONS Flowmeters, Gauges, Valves, etc. Figure 1. Schematic of the crossedflowappa ratus
s a l t ) and a r e c a r r i e d i n t o t h e r e a c t i o n r e g i o n t h r o u g h a 1 mm h o l e i n t h e o v e n by h e l i u m c a r r i e r g a s . The t o t a l p r e s s u r e i n t h e r e a c t i o n chamber i s about 0.5 t o r r . I n t h e m o l e c u l a r beam a p p a r a t u s , an e f f u s i v e a t o m i c b o r o n beam i s p r o d u c e d by v a p o r i z a t i o n o f e l e m e n t a l b o r o n a t 2400 Κ i n a k n u d s e n c e l l . The a l k a l i f l u o r i d e beam s o u r c e c o n s i s t s o f an o v e n h e a t e d t o a b o u t 1000 K. Photon c o u n t i n g t e c h n i q u e s a r e used i n t h e low p r e s s u r e ( Ι Ο * t o r r ) beam e x p e r i m e n t s . We have p e r f o r m e d e x p e r i m e n t s i n t h e f l o w a p p a r a t u s w i t h t h e s a l t s C s F , R b F , KF a n d NaF ( 9 ) . The s p e c t r u m f r o m e a c h r e a c t i o n was r e c o r d e d a n d t h e r e l a t i v e number o f p h o t o n s f r o m t h e v a r i o u s transi-ι t i o n was c o m p u t e d . From t h e s e d a t a we o b t a i n e d t h e r e l a t i v e steady s t a t e population d i s t r i b u t i o n . We o b s e r v e e x c i t a t i o n o f s t a t e s w i t h e n e r g i e s up t o a n d i n some c a s e s a b o v e t h e e x o e r g i c i t y f o r t h e r e a c t i o n of boron w i t h the metal f l u o r i d e s . The r e s u l t s srhow t h a t w h i l e the p o p u l a t i o n of the e x c i t e d s t a t e s tends to d e c r e a s e w i t h energy t h e r e a r e p o p u l a t i o n i n v e r s i o n s b e t w e e n some o f t h e s t a t e s . Experiments indic ate that the f i r s t e x c i t e d Ρ s t a t e s are produced m a i n l y i n the primary c h e m i c a l r e a c t i o n o f boron w i t h t h e r m a l MF m o l e c u l e s . The p h o t o n y i e l d f r o m t h e f i r s t e x c i t e d s t a t e i s e s t i m a t e d t o be as h i g h as 0 . 5 p h o t o n s p e r b o r o n atom r e a c t e d . The m e c h a n i s m f o r t h e e x c i t a t i o n o f t h e s t a t e s l y i n g above t h e f i r s t Ρ l e v e l s c a n n o t be d e t e r m i n e d f r o m t h e f l o w e x p e r i m e n t s . We c a n o n l y c o n c l u d e t h a t s e v e r a l s e c o n d a r y e n e r g y t r a n s f e r processes c o n t r i b u t e to the p r o d u c t i o n of the higher excited s t a t e s . P r e l i m i n a r y r e s u l t s from the s i n g l e c o l l i s i o n beam s t u d i e s c o n f i r m t h e a b o v e r e s u l t s and a r e i n t h e p r o c e s s o f y i e l d i n g q u a n t i t a t i v e values f o r the chemiluminescence cross s e c t i o n s . 5
STATE-TO-STATE CHEMISTRY
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