Cesium Hydride Formation from Cs(7P) - American Chemical Society

17 Cesium Hydride Formation from Cs(7P) and H Interactions

2

Downloaded by EAST CAROLINA UNIV on January 3, 2018 | http://pubs.acs.org Publication Date: March 8, 1982 | doi: 10.1021/bk-1982-0179.ch017

B. S A Y E R , M . F E R R A Y , J. L O Z I N G O T , and J. B E R L A N D E Centre d'Etudes Nucléaires de Saclay, Service de Physique Atomique, 91191 Gif-sur-Yvette, Cedex, France

Laser photoexcitation of cesium atoms to the 7P state in the presence of hydrogen gas leads to the forma tion of cesium hydride by a process which i s shown to be indirect. The concentration of the reactants is studied under various experimental conditions using a laser fluorescence technique. The rate coefficient of CsH formation is found to be proportional to the Cs(6S) density suggesting a two step process with the formation of an intermediate species which reacts with ground state cesium atoms. S e v e r a l a t t e m p t s have been made d u r i n g t h e l a s t few y e a r s t o s t u d y t h e f o r m a t i o n o f a l k a l i h y d r i d e s i n t h e v a p o r phase when a m i x t u r e o f a l k a l i m e t a l v a p o r and h y d r o g e n i s e x p o s e d t o l a s e r i r r a d i a t i o n w h i c h p h o t o e x c i t e s t h e a l k a l i a t o m . The p r e s e n t work i s devoted to the f o l l o w i n g o v e r a l l r e a c t i o n s : Cs(6S) + hv Cs(7P) + H

2

L

+ Cs(7P)

(1)

->• CsH + H

(.2)

T h i s p r o c e s s was r e p o r t e d f o r t h e f i r s t t i m e by Tam and Happer (1_). U s i n g t h e l i n e s o f an a r g o n i o n l a s e r t h e y p h o t o e x c i t e d t h e c e s i u m atoms t o t h e 7P s t a t e t h r o u g h t h e b r o a d e n e d 6S-7P l i n e s and observed the formation o f macroscopic c r y s t a l s o f cesium hydride ( " l a s e r snow"). More r e c e n t l y P i c q u l and c o w o r k e r s (2!) have r e p o r t e d t h e same o b s e r v a t i o n u s i n g a t u n a b l e dye l a s e r r e s o n a n t w i t h e a c h o f t h e f i n e s t r u c t u r e components o f t h e 6 S - 7 P t r a n s i tion. However, i n s p i t e o f t h e i m p o r t a n t f e a t u r e s a p p a r e n t i n t h e s e s t u d i e s , t h e d e t a i l e d mechanism o f t h e C s ( 7 P ) + H r e a c t i v e c o l l i s i o n i s s t i l l u n c e r t a i n . As p r o p o s e d by Tam and Happer ( T j , i t i s c o n c e i v a b l e t h a t e i t h e r a d i r e c t p r o c e s s ( e q u a t i o n 2) o r a t w o - s t e p p r o c e s s w i t h t h e f o r m a t i o n o f an i n t e r m e d i a t e s p e c i e s m i g h t be r e s p o n s i b l e f o r t h e CsH f o r m a t i o n . 2

0097-6156/82/0179-0255$05.00/0 ©

1982

A m e r i c a n Chemical Society

Gole and Stwalley,; Metal Bonding and Interactions in High Temperature Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

256

METAL BONDING AND INTERACTIONS

I t i s t h e p u r p o s e o f t h e p r e s e n t work t o t r y t o o b t a i n i n s i g h t i n t o CsH f o r m a t i o n and t o e s t i m a t e t h e e f f i c i e n c y o f t h e process. F o r t h i s r e a s o n , we p e r f o r m e d a s p e c t r o s c o p i c i n v e s t i g a t i o n o f the r e l a t i v e c o n c e n t r a t i o n s o f the r e a c t a n t s - C s ( 7 P ) , H and CsH - u n d e r v a r i o u s e x p e r i m e n t a l c o n d i t i o n s i n s t a t i o n a r y as w e l l as i n t r a n s i e n t r e g i m e s . A more d e t a i l e d d i s c u s s i o n o f t h e s e r e s u l t s w i l l be p u b l i s h e d i n J . Chem. P h y s . 2


Experimental

Set-Up

A CW t u n a b l e dye l a s e r ( S t i l b e n e 3) i s pumped by t h e u l t r a v i o l e t l i n e s o f an a r g o n i o n l a s e r . The l a s e r i s f o c u s e d i n a c e l l f i l l e d w i t h a m i x t u r e o f c e s i u m and h y d r o g e n . The c e l l (10 cm l o n g w i t h an i n s i d e r a d i u s o f 2.6 cm) i s c o n n e c t e d t o a r e s e r v o i r c o n t a i n i n g l i q u i d c e s i u m and t o a pumping s y s t e m o r a h y d r o gen gas t a n k . I t i s p l a c e d i n an oven h e a t e d i n d e p e n d e n t l y f r o m the r e s e r v o i r . The l a s e r beam e n t e r s t h e c e l l f r o m b e l o w t h r o u g h a f l a t window and i s f o c u s e d a t a p o i n t 1 cm above t h e window. The f l u o r e s c e n c e l i g h t i s o b s e r v e d a t r i g h t a n g l e s t o t h e l a s e r beam by an o p t i c a l s y s t e m and a s m a l l monochromator f o l l o w e d by a RCA Q u a n t a c o n p h o t o m u l t i p l i e r , t h e s i g n a l o f w h i c h i s s e n t t o an a m p l i f i e r and a c h a r t r e c o r d e r when t h e c e l l i s c o n t i n u o u s l y i r r a d i a t e d o r t o a p h o t o n c o u n t i n g s y s t e m and a m u l t i c h a n n e l a n a l y s e r when t r a n s i e n t e f f e c t s a r e s t u d i e d . Preliminary

Observations

The t y p i c a l e x p e r i m e n t a l c o n d i t i o n s a r e : [Cs] = 0.8 - 8 x 10 c m " , [ H ] = 8 - 80 x 1 0 c m " , T = 630 - 670 K. The l a s e r power i n t h e c e l l i s o f t h e o r d e r o f 20 mw. When t h e l a s e r w a v e l e n g t h i s t u n e d on t h e 6 S - 7 P ^ c e s i u m l i n e ( 4 5 9 . 3 nm), we o b s e r v e t h a t t h e c e s i u m atoms a r e p h o t o e x c i t e d t o t h e 7P^ s t a t e and t h a t e x c i t a t i o n t r a n s f e r t o t h e 7P^ s t a t e o c c u r s b e c a u s e o f i n e l a s t i c collisions. These C s ( 7 P ^ ) atoms f l u o r e s c e a t 4 5 5 . 5 nm. I f we now s h i f t t h e l a s e r l i n e by a few t e n s o f GHz t o w a r d s t h e s h o r t w a v e l e n g t h s i d e , we o b s e r v e i n a d d i t i o n t o t h e a t o m i c f l u o r e s c e n c e some d o u b l e t s w h i c h a r e c h a r a c t e r i s t i c o f t h e CsH (A ^" " + X ! " ) s p e c t r u m (_3). T h i s i n d i c a t e s t h a t t h e l a s e r i s b r o a d enough t o p h o t o e x c i t e c e s i u m atoms and t h e CsH m o l e c u l e s c r e a t e d by r e a c tion (2). The d a t a on t h e e n e r g y o f t h e r o v i b r a t i o n a l l e v e l s (4) and the i n t e n s i t i e s o f the A ! ( v ' = 19) + X E l i n e s (.3), l e a d us t o t h e c o n c l u s i o n t h a t t h e CsH t r a n s i t i o n r e s p o n s i b l e f o r t h e l a s e r l i g h t absorption is X E ( v " = 0 , J " = 11) + A ! * ( v = 19, J = 12). The CsH f l u o r e s c e n c e t h e r e f o r e c o r r e s p o n d s t o t h e v a r i o u s t r a n s i t i o n s t o t h e ( v \ J " = 1 1 , 13) l e v e l s o f t h e X E state. In c o n t r a s t t o t h e o b s e r v a t i o n s o f Tam and H a p p e r (1_) and P i c q u e e_t al_. ( 2 ) , no m a c r o s c o p i c CsH c r y s t a l s w e r e o b s e r v e d . T h i s i s a r e s u l t o f o u r e x p e r i m e n t a l c o n d i t i o n s where t h e c e s i u m d e n s i t i e s and t h e l a s e r power a r e l o w e r t h a n i n p r e v i o u s e x p e r i 1 6

3

1 6

3

2

1

1

X

+

4

1

-1

!

1

1

X


1

17.

SAYER ET AL.

Cesium

Hydride

257

Formation

m e n t s , l e a d i n g t o l o w e r CsH c o n c e n t r a t i o n s , w h i c h do n o t p e r m i t crystal formation. M o r e o v e r , i n t h e two p r e v i o u s s t u d i e s , a CsH f l u o r e s c e n c e i n t e n s i t y d e p e n d i n g on t h e l a s e r i n t e n s i t y I|_ t o t h e power \ was o b s e r v e d ; t h i s dependence i s i n t e r p r e t e d i n terms o f a d e s t r u c t i o n p r o c e s s m a i n l y due t o d i m e r i z a t i o n . In o u r e x p e r i ment we f i n d a f l u o r e s c e n c e i n t e n s i t y w h i c h v a r i e s as l £ i n d i c a t i n g t h a t t h i s p r o c e s s i s o f m i n o r i m p o r t a n c e i n t h e l o s s o f CsH m o l e c u l e s , and t h a t d i f f u s i o n o u t o f t h e i r r a d i a t e d r e g i o n i s p r o b a b l y the dominant l o s s p r o c e s s .


Experimental

Method

The f i r s t s t e p c o n s i s t s o f e s t a b l i s h i n g t h e method f o r mea s u r i n g t h e r a t i o between t h e C s ( 7 P ) and CsH p o p u l a t i o n s . The C s ( 7 P ) c o n c e n t r a t i o n i s r e l a t e d t o t h e 1^-6%^ l i n e i n t e n s i t y . In o r d e r t o do t h i s , we c a l c u l a t e d t h e f r a c t i o n Y o f C s ( 7 P ) atoms w h i c h a r e i n t h e 7P^ s u b l e v e l and 3 t h e f r a c t i o n o f t h e 7 P ^ - 6 S ^ l i n e i n t e n s i t y which i s not reabsorbed. The 7Pi^ and 7P^ s t a t e s have s t e a d y s t a t e p o p u l a t i o n s d e t e r mined by t h e f o l l o w i n g c o m p e t i t i v e c o n t r i b u t i o n s : (a) radiative transitions: Cs(7P! (b)

)

+ C s ( 7 S , 5D, 6S)

+ hv

e x c i t a t i o n t r a n s f e r by c o l l i s i o n w i t h Cs C s ( 7 P i ) + (Cs

and

3

(c)

quenching

or H )

-

2

collisions

Cs(7P

x

3

)

+ H

C s ( 7 P ) + (Cs 3

with H 2

-

2

(3)

(5)

and H

2

(6):

or H ) 2

molecules

(4)

(6):

Cs(6S) + H (v)

(5)

2

where H ( v ) i s a v i b r a t i o n a l l y e x c i t e d H m o l e c u l e . The f r a c t i o n Y = [ C s ( 7 P ^ ) ] / [ C s ( 7 P ) ] has been o b t a i n e d f r o m t h e r a d i a t i v e t r a n s i t i o n p r o b a b i l i t i e s and t h e r a t e c o e f f i c i e n t s o f r e a c t i o n s (4) and (5) ( 5 , 6 ) . R e a b s o r p t i o n o f t h e c e s i u m f l u o r e s c e n c e l i n e has a l s o been c a l c u l a t e d f r o m t h e c e s i u m d e n s i t y and t h e l i n e p r o f i l e . Under o u r e x p e r i m e n t a l c o n d i t i o n s , l i n e b r o a d e n i n g by c o l l i s i o n s w i t h H m o l e c u l e s i s t h e p r e d o m i n a n t e f f e c t (50 MHz p e r T o r r a c c o r d i n g to (7)). The r e l a t i o n s h i p between [ C s ( 7 P ) ] and I , the i n t e n s i t y of t h e 4 5 5 . 3 nm f l u o r e s c e n c e l i n e , i s t h e n : 2

2

3

I

3

= G A

3

3 y

[Cs(7P)]

(6)

where G i s a f a c t o r w h i c h i n c l u d e s t h e c o n t r i b u t i o n o f b o t h s e n s i t i v i t i e s and g e o m e t r y and A t h e E i n s t e i n c o e f f i c i e n t o f t h e t r a n sition. In o r d e r t o r e l a t e t h e CsH c o n c e n t r a t i o n t o t h e CsH f l u o r e s c e n c e i n t e n s i t y , we have assumed t h a t t h e CsH ( X ! * ) m o l e c u l e s 3

1


258


p r o d u c e d , w h i c h u n d e r g o many c o l l i s i o n s b e f o r e a b s o r b i n g a p h o t o n , are d i s t r i b u t e d over the r o v i b r a t i o n a l s t a t e s according t o the t h e r m o d y n a m i c p a r t i t i o n f u n c t i o n a t t h e gas t e m p e r a t u r e , an i s the f r a c t i o n o f these molecules i n t h e s t a t e (v" = 0, J " = 1 1 ) . L e t us c o n s i d e r t h e a b s o r p t i o n o f t h e CsH l i n e : CsH [ X E 1

+

(v

M

= 0, J " = 11)] + h v

L

(7)


+ CsH [ A ^

+

(v' = 19, J

= 12)]

1

The l a s e r l i n e i s l o c a t e d a t a w a v e l e n g t h between t h o s e o f t h e CsH and C s ( 7 P i J a b s o r p t i o n l i n e s s u c h t h a t t h e CsH f l u o r e s c e n c e i s a maximum, i . e . t h e p r o d u c t o f t h e a b s o r p t i o n by t h e l i n e s i s maximum ( s e e F i g u r e 1 ) . The p h o t o e x c i t a t i o n r a t e i s c a l c u l a t e d : *CsH

=

M

W

"p

C

C

s

H

]

'o

k

(

v

)

d

V

(

8

)

where I|_( CsH) t h e l a s e r i n t e n s i t y a t t h e w a v e l e n g t h o f t h e CsH line. The i n t e g r a l o v e r k ( v ) i s r e l a t e d t o t h e E i n s t e i n A coef f i c i e n t of the corresponding absorption t r a n s i t i o n . By a n a l o g y w i t h what i s o b s e r v e d f o r C s * a t o m s , we have c o n s i d e r e d t h a t q u e n c h i n g o f C s H * by H m o l e c u l e s c a n n o t be a p r i o r i n e g l e c t e d w i t h r e s p e c t t o r a d i a t i v e d e p o p u l a t i o n and we have t e n tatively written: [CsH*] - /(x- , + k [ H ] ) (9) v

i

s

a

2

C s H

H

Q

2

where kn i s t h e q u e n c h i n g r a t e c o e f f i c i e n t w h i c h i s unknown. The i n t e n s i t y I p o f t h e two u n r e s o l v e d f l u o r e s c e n c e l i n e s c o r r e s p o n d ing to the t r a n s i t i o n s : CsH [ A ! * ( v ' = 1 9 , J 1

CsH [ X ! 1

1

= 12)] +

(v" = 4, J " = 11, 13)] + h v

p

i s t h e n g i v e n by Ap [ C s H * ] , where Ap i s t h e t r a n s i t i o n p r o b a b i l i t y o f the observed p a i r o f f l u o r e s c e n c e l i n e s . From e q u a t i o n s ( 7 ) , ( 9 ) and t h e r e l a t i o n w h i c h e x i s t s between k and A , t h e t r a n s i t i o n p r o b a b i l i t y f o r t h e CsH a b s o r p t i o n l i n e , we h a v e : a

where g / g j i s t h e r a t i o between t h e s t a t i s t i c a l w e i g h t s o f u p p e r and l o w e r l e v e l s o f t h e a b s o r b e d t r a n s i t i o n a n d 6 ' i s a f a c t o r s i m i l a r t o G i n e q u a t i o n ( 6 ) . R e c e n t c a l c u l a t i o n s o f CsH [ A S •> X I ] t r a n s i t i o n p r o b a b i l i t i e s (8) a r e used f o r t h e p r e s e n t c a l c u l a t i o n i n s p i t e o f t h e s l i g h t d i f f e r e n c e between t h e J v a l u e s o f t h e t r a n s i t i o n c o n s i d e r e d h e r e and t h o s e f o r w h i c h c a l c u l a t i o n s s

1

1

+


+

17.

SAYER ET AL.

Cesium

Hydride

Formation

259

were c a r r i e d o u t . The r a d i a t i v e l i f e t i m e o f t h e u p p e r s t a t e ( i n fact A E ( v ' = 1 9 , J ' = 10) i n s t e a d o f J ' = 12) has a l s o been c a l c u l a t e d ( 8 ) (85 n s ) . From e q u a t i o n s ( 6 ) and ( 1 1 ) , i t i s t h u s p o s s i b l e t o r e l a t e [CsH]/[Cs(7P)] t o I /I : 2

+

F

3

[CsH]/[Cs(7P)] = ( B I / I ) p

3

• (1 + k

Q

[H ] 2

x

C

s

H

J

(12)

where B i s a c o e f f i c i e n t w h i c h can be c a l c u l a t e d f r o m t h e e x p e r i m e n t a l c o n d i t i o n s and does n o t depend on [ C s ] and [ H ] c o n c e n t r a tions.


2

Experimental

Results

In a f i r s t s e t o f e x p e r i m e n t s , t h e h y d r o g e n d e n s i t y was k e p t c o n s t a n t and t h a t o f c e s i u m v a r i e d . The q u a n t i t y B I p / I i s p l o t t e d on F i g u r e 2 a s a f u n c t i o n o f [ C s J and we o b s e r v e a l i n e a r v a riation. Two i n t e r p r e t a t i o n s o f t h i s v a r i a t i o n a r e p r o p o s e d : a CsH f o r m a t i o n p r o c e s s r e q u i r i n g a g r o u n d s t a t e c e s i u m atom i n a d d i t i o n t o C s ( 7 P ) and H m o l e c u l e s ; a CsH d e s t r u c t i o n p r o c e s s i n v e r s e l y proportional t o [Cs]. C o l l i s i o n c o n t r o l l e d d i f f u s i o n out o f the i r r a d i a t i o n r e g i o n w i t h c e s i u m atoms c o u l d be s u c h a p r o c e s s . In a s e c o n d s e t o f e x p e r i m e n t s b o t h [ C s ] and [ H ] have been varied. A s l i g h t increase o f BfCs]" I p / I w i t h [ H ] i s observed (see F i g u r e 3 ) . As [ C s H ] / [ C s ( 7 P ) ] s h o u l d be p r o p o r t i o n a l t o [ H ] , we have p l o t t e d on t h e same g r a p h t h e q u a n t i t y B [ C s J " (1 + k g [H J T * ) I / I w i t h k g = CTQ SO as t o have t h i s q u a n t i t y proportional t o [H J. The a p p r o x i m a t e q u e n c h i n g c r o s s s e c t i o n OQ o f C s H * b y H t h a t we f i n d i s e q u a l t o 4 A , a v a l u e w h i c h i s o f t h e same o r d e r o f m a g n i t u d e a s t h a t o f C s ( 6 P ^ ) ( 9 ) . N e i t h e r t h e p r e c i s e n a t u r e o f t h e CsH f o r m a t i o n p r o c e s s n o r t h e g l o b a l r a t e o f f o r m a t i o n c a n be d e r i v e d f r o m t h e d e t e r m i n a t i o n o f [CsH] ( [ C s ( 7 P ) ] [ H ] ) i n a s t a t i o n a r y regime because the l o s s r a t e f o r CsH m o l e c u l e s i s unknown. T h i s f a c t has l e a d t o i n v e s t i g a t i o n s o f t r a n s i e n t l a s e r i r r a diation. T h i s i s o b t a i n e d b y c h o p p i n g t h e l a s e r beam w i t h a r o t a t i n g wheel a t a f r e q u e n c y o f 1 kHz. I t i s observed t h a t the r i s e t i m e o f I p i s o f t h e o r d e r o f a few h u n d r e d m i c r o s e c o n d s w h i l e I f o l l o w s c l o s e l y the l a s e r i r r a d i a t i o n . I t i s p o s s i b l e t o show t h a t t h e i n i t i a l d l p / d t i s r e l a t e d t o t h e t i m e c o n s t a n t o f CsH f o r m a t i o n , b e c a u s e C s H * m o l e c u l e s have a v e r y s h o r t l i f e t i m e and b e c a u s e d i f f u s i o n does n o t p l a y a n i m p o r t a n t r o l e a t t h e v e r y b e g i n n i n g o f t h e l a s e r i r r a d i a t i o n . Thus t h e r i s e t i m e c o n s t a n t T = Ip ( d l p / d t ) " " i s a s s i g n e d t o [ C s H ] ( d [ C s H ] / d t ) and has been mea sured under v a r i o u s e x p e r i m e n t a l c o n d i t i o n s ( [ C s ] , [ H ] ) . We have d e r i v e d f r o m t h e s e measurements t h e q u a n t i t y k = ( [ C s ( 7 P ) ] [ H ; , ] ) " d[CsH]/dt = ( x [ H ] ) " [CsH]/[Cs(7P)] which represents the r a t e o f CsH f o r m a t i o n a c c o r d i n g t o t h e g l o b a l r e a c t i o n ( 2 ) . This rate co e f f i c i e n t has been f o u n d t o be i n d e p e n d e n t o f b o t h [ C s ( 7 P ) ] and 3

2

2

1

3

2

2

1

2

c

§

h

F

3

2

2

2

- 1

2

3

1

- 1

2

1

1

2


260


Journal of Chemical Physics

Relative positions of laser line and absorption lines in absorption region (7).


Figure 1.

*

[Cs](cm" ) 3

— i

1

5

10

16

i

1

I

2

5

10

17


Figure 2.

Variation of BI /I with Cs density at a given [H ] f ~ 9 X 10 This quantity proportional to [CsH]/[Cs(7P)] (Eq. 12) (7). F

3

2

16


cm' ). 3

SAYER ET AL.

Cesium Hydride

261

Formation


17.


Figure 3. Experimental values of S fitted using various quenching cross sections of CsH* by H . The upper straight line (O) shows the linear variation of a quantity proportional to [CsH]/[Cs(7P)] [Cs] with [H ] (Eq. 12) (1). 2

2


262


[ H ] , b u t a p p r o x i m a t e l y p r o p o r t i o n a l t o [ C s ] as shown i n F i g u r e 4 , where t h e e x p e r i m e n t a l v a l u e s o f k c o r r e s p o n d i n g t o v a r i o u s [H ] have been p l o t t e d as a f u n c t i o n o f [ C s ] . The b e s t s t r a i g h t l i n e corresponding to k a [Cs] i s presented. T h i s r e s u l t i n d i c a t e s t h a t t h e dependence o f [ C s H ] on [ C s ] f o u n d i n t h e s t a t i o n a r y r e g i m e o r i g i n a t e s f r o m t h e CsH f o r m a t i o n i t s e l f and n o t f r o m t h e n a t u r e o f t h e l o s s p r o c e s s as m i g h t be e n v i s a g e d . 2

2


D i s c u s s i o n and

Conclusion

U s i n g a m u l t i m o d e l a s e r , we have s i m u l t a n e o u s l y p h o t o e x c i t e d , i n a m i x t u r e o f c e s i u m and h y d r o g e n , c e s i u m atoms t o t h e 7F^ s t a t e and t h e r e s u l t i n g CsH m o l e c u l e s . The p r e s e n t e x p e r i m e n t a l o b s e r v a t i o n s can be e x p l a i n e d as f o l l o w s : - The C s ( 7 P ) p o p u l a t i o n i s e s t a b l i s h e d by a b a l a n c e between l a s e r p h o t o e x c i t a t i o n and q u e n c h i n g p l u s e x c i t a t i o n t r a n s f e r by c o l l i s i o n between Cs and H m o l e c u l e s . - I t i s a l s o p r o b a b l e t h a t p h o t o e x c i t a t i o n and q u e n c h i n g by H a r e r e s p o n s i b l e f o r e s t a b l i s h i n g the steady s t a t e CsH* p o p u l a t i o n . - By v a r y i n g t h e c e s i u m and h y d r o g e n c o n c e n t r a t i o n , i t was f o u n d t h a t t h e e x p e r i m e n t a l r e s u l t s were c o n s i s t e n t w i t h a CsH f o r m a t i o n mechanism w h i c h can be w r i t t e n as a s t e p w i s e p r o c e s s , i n v o l v i n g c o l l i s i o n s w i t h cesium atoms. T h i s seems t o e l i m i n a t e t h e p o s s i b i l i t y o f d i r e c t CsH f o r m a t i o n by a C s ( 7 P ) + H p r o c e s s , a p o s s i b l e mechanism i n i t i a l l y s u g g e s t e d by Tam and Happer (1_). On t h e c o n t r a r y t h e o t h e r mechanism p r o p o s e d : 2

2

2

Cs(7P) + H

2

-

H (v) 2

Cs(6S) + H (v) 2

-

(13)

+ Cs(6S) CsH + H

(14)

i s c o n s i s t e n t w i t h our r e s u l t s . I t i s t o be n o t e d t h a t f r e e h y d r o g e n atoms c a n r e a c t w i t h c e s i u m atoms t o f o r m o t h e r CsH m o l e cules. - I f r e a c t i o n s 13 and 14 a r e t h o s e w h i c h o c c u r i n t h e medium, t h e r o l e o f C s ( 7 P ) atoms w o u l d be o n l y t h a t o f an e n e r g y t r a n s d u c e r c o n v e r t i n g the l a s e r photon energy i n t o the v i b r a t i o n a l energy o f H molecules. We have t h e f o l l o w i n g scheme: the energy o f e x c i t e d c e s i u m atoms i s t r a n s f e r e d t o H m o l e c u l e s , some o f them h a v i n g a v i b r a t i o n a l e n e r g y h i g h enough (v = 6) t o r e a c t w i t h g r o u n d s t a t e cesium atoms. I t s h o u l d be n o t e d t h a t t h e s e H ( v ) m o l e c u l e s have a l o n g l i f e t i m e as r e g a r d s d i f f u s i o n o u t o f t h e o b s e r v a t i o n region (10). - A n o t h e r mechanism w h i c h c o u l d e x p l a i n t h e p r e s e n t e x p e r i m e n t a l o b s e r v a t i o n s i s t h e f o r m a t i o n o f a t r a n s i e n t c o m p l e x (11_). In t h e present case t h i s would correspond t o : 2

2

2

Cs(7P) + H

2

->

(CsH )* 2


(15)

17.

Cesium

SAYER E T A L .

Hydride

263

Formation

B [Cs]" | /l = S (x) F

3


1

I

1

10

17

1

1

1

2

5

10

1 8


Figure 4.

Variation of the rate coefficient of CsH formation with Cs density. Key: ,k~N (7). Cs



264

( C s H ) * + Cs + 2CsH o r CsH + Cs + H 2

(16)

I f ( C s H ) * has a l o n g enough l i f e t i m e , t h i s p r o c e s s c a n p l a y a role. H o w e v e r , n o t h i n g i s known a b o u t t h e p o s s i b l e e x i s t e n c e o f such a complex. 2

Literature Cited


1. 2.

Tam, A. C . ; Happer, W. Phys. Rev. Lett. 1975, 35, 1630. Picque, J . L.; Vergès, J.; Vetter, R. J. Physique Lett. 1980, 41L, 305. 3. Tam, A. C . ; Happer, W. J. Chem. Phys. 1976, 64, 2456. 4. Hsieh, Y. K . ; Yang, S. C . ; Tam, A. C . ; Stwalley, W. C. J. Chem. Phys. 1978, 68, 1448. 5. Pace, P. W.; Atkinson, J. B. Can. J. Phys. 1974, 52, 1641. 6. Lukaszewicz, M. Bull. Acad. Polon. 1975, 23, 501. 7. Ferray, M . ; Sayer, B . ; V i s t i c o t , J. P . ; Lozingot, J., to be published. 8. Yang, S. C . ; Hsieh, Y. K . ; Tam, A. C . ; Zemke, W. T . ; Verma, K. K . ; Stwalley, W. C. J. Chem. Phys. (in press). 9. M c G i l l i s , D. A.; Krause, L. Can. J. Phys. 1968, 46, 1051. 10. Bernstein, R. B. "Atom-Molecule C o l l i s i o n Theory"; Plenum Press: New York, 1979. RECEIVED August 26,

1981.


Cesium Hydride Formation from Cs(7P) - American Chemical Society

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