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Submitted on 1 Jan 1985
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COLLISIONAL IONISATION OF EXCITED RUBIDIUM ATOMS
M. Chéret, L. Barbier
To cite this version:
M. Chéret, L. Barbier. COLLISIONAL IONISATION OF EXCITED RUBIDIUM ATOMS. Journal
de Physique Colloques, 1985, 46 (C1), pp.C1-193-C1-197. �10.1051/jphyscol:1985119�. �jpa-00224491�
JOURNAL DE PHYSIQUE
Colloque C l , suppl6ment au nO1, Tome 46, janvier 1985 page Cl-193
COLLISIONAL IONISATION OF EXCITED RUBIDIUM ATOMS
M.
Ch&ret and
L .Barbier
Centre drEtudes Nucl6aires de Saclay, Service de Physique des Atomes e t des Surfaces, 91 191 Gif-sur-Yvette Cedex, France
R6sun6 - -Dans une vapeur de rubidium,la n e s u r e des c o u r a n t s d ' i o n s Rb+, Rb+ e t Rb a ~ e r m i s de c a r a c t z r i s e r l e s processus c o l l i s i o n n e l s de forma- t i o n d ' i o n s 2 b p a r t i r d'atomes e x c i t e s e t de determiner l e s c o e f f i c i e n t s de t a u x d ' i o n i s a t i o n a s s o c i 6 s b c e s r e a c t i o n s .
+ +
A b s t r a c t - Ion c u r r e n t measurements f o r Rb , Rb and Rb- i o n s allowed t o c h a r a c t e r i z e i o n i z i n g c o l l i s i o n a l p r o c e s s e s i n v o l v i n g e x c i t e d atoms i n a 2 Rb vapour. Rate c o e f f i c i e n t s f o r t h e s e r e a c t i o n s a r e d e t e r n i n e d .
When an a l k a l i vapor i s i r r a d i a t e d with high power l a s e r beams a complex medium i s c r e a t e d and it i s not very e a s y t o s e p a r a t e l a s e r f i e l d s e f f e c t s from p u r e l y c o l l i - s i o n a l mechanisms. It i s t h u s important t o understand f i r s t t h e involved c o l l i s i o n a l r e a c t ions.
Low power l a s e r s allow t o s t u d y t h i s p r o c e s s e s i n a s i n p l e medium.
Two C.W. multimode dye l a s e r s c r o s s i n g a t t h e c e n t r e of a c y l i n d r i c a l c e l l a r e used t o pump , i n a f i r s t s t e p rubidium atoms i n a r e s o n a n t l e v e l Rb(5p) and i n a second s t e p i n a h i g h l y e x c i t e d l e v e l ~ b ( n ' L ) ( I = s o r d ) . The experimental s e t up has been d e s c r i b e d p r e v i o u s l y / l 1. Typical experimental c o n d i t i o n s correspond t o :p=l 0-4 T o r r ,
Rb(5s)=lO l 2 cm-3, Rb(5d)
=10 cm-3, Rb ( n l ) = 10' c n 3 . The main p r o c e s s e s a r e :
liornbeclc-blolnar a s s o c i a t i v e i o n i z a t i o n
Rb(nZ)+ Rb(5s)-Rb; + e-
Penning atomic i o n i z a t i o n
:Beside t h e s e r e a c t i o n s t h e follorving p r o c e s s e s have a l s o been observe0 : Energy p o o l i n g r e a c t i o n :
Peqning a s s o c i a t i v e i o n i z a t i o n :
Ion p a i r formation
R e s u l t s concerning energy pooling r e a c t i o n have been published f o r t h e Rb case / 2 / and M. A l l e g r i n i p r e s e n t s a review on t h i s s u b j e c t a t t h e p r e s e n t conference.
The o t h e r r e a c t i o n s correspond t o i o n i z i n g mechanisms i n v o l v i n g h i g h l y e x c i t e d Rb atoms. Ions a r e analyzed w i t h a mass spectrometer wired t o a l t e r n a t e l y d e t e c t e d po- s i t i v e - o r n e g a t i v e i o n s . A t y p i c a l r e s u l t of mass scanping i s p r e s e n t e d i n f i g . 1 . The Rb c u r r e n t i s 5000 t i n e s lower t h a n t h e Rb+ o r Rb 2 c u r r e n t and consequently ion p a i r formation does n o t c o n t r i b u t e s i g n i f i c a n t l y t o t h e Rb+ c u r r e n t . Photoionisa- t i o n by t h e l a s e r beams : Rb(n1) + h*--+RbC + e- i s an important mechanism c o n t r i - b u t i n g t o t h e Rb+ c u r r e n t . The c r o s s s e c t i o n s f o r t h i s p r o c e s s a r e w e l l known /3/
and t h e p h o t o i o n i s a t i o n c o n t r i b u t i o n i s used t o d e t e r n i n e d t h e e x c i t e d s t a t e densi- t i e s / l / .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985119
c l - 1 9 4 JOURNAL DE PHYSIQUE
I
R E L . U N I Tt
The p r e s e n t paper only r e p o r t s on Penning a t o n i c i o n i s a t i o n and i o n p a i r format i o n .
The Penning i o n i z a t i o n occurs v i a c o l l i s i o n between a h i g h l y e x c i t e d atom and a r e s o n a n t s t a t e atom.
I n f a c t when a h i g h l y e x c i t e d s t a t e i s pumped t h e r a d i a t i v e cascading d e e x c i t a t i o n p o p u l a t e s t h e neigh- bouring l e v e l s . s and d l e v e l s es-
s e n t i a l l y decay t o p l e v e l s and consequently t h e s t u d i e d medium i s composed of che e x c i t e d s o r d le-.
v e l and of t h e v a r i o u s lower p l e v e l s . The t o t a l d e n s i t y of t h e s e p l e v e l s r e p r e s e n t s 80% of t h e den- s i t y of t h e pumped s l e v e l and 2CX of t h e pumped d l e v e l . The mea- sured q u a n t i t i e s correspond t o t h e c o n t r i b u t i o n of t h e pumped s t a t e and of t h e r a d i a t i v e l y populated p l e v e l s . t h e n a s s t a t e i s populated t h e p l e v e l d e n s i t i e s a l o n e can e x p l a i n t h e e x p e r i n e n t a l observa- t i o n s / l / - I dependency of t h e r a t e c o e f f i c i e n t s w i l l be known only i f t h e p c o n t r i b u t i o n i s c o r r e c t l y deduced.
Measured r a t e c o e f f i c i e n t s l i e i n t h e range 10-8 cm3 s - l . T h e o r e t i c a l - l y two mechanisms a r e u s u a l l y consi- Fig. l - Mass scanning of p o s i t i v e ( a ) and dered. The f i r s t one i s t h e d i p o l e - n e g a t i v e ions f o r t h e e x c i t a t i o n of t h e d i p o l e i n t e r a c t i o n which g i v e r a t e 6d 2 ~ l e v e l . ~ / ~ c o e f f i c i e n t s i n t h e range 10-9 a d s - l
f o r d o r p l e v e l s and i n t h e range 10-1
0cm3 s-l f o r s l e v e l . The second one i s t h e exchange t h e o r y ; Kimura f o r Rb atoms found r a t e c o e f f i c i e n t s 1 i n - dependent lying i n t h e range 10-8 cm3 s-l 141. It i s t h e n important t o determine t h e
1dependency of t h e Penning r a t e c o e f f i c i e n t .
T h i s h a s been done by p o p u l a t i n g t h e e x c i t e d s t a t e s w i t h p u l s e d l a s e r s . As an exam- p l e i f t h e 7 s l e v e l i s e x c i t e d i t s c o n c e n t r a t i o n decays e x p o n e n t i a l l y and p o p u l a t e s t h e 6p l e v e l . 200 n s a f t e r t h e l a s e r s h o t t h e 6p d e n s i t y becomes h i g h e r t h a n t h e 7 s d e n s i t y ( f i g . 2 ) . I f oile of t h e Penning r a t e c o e f f i c i e n t f o r 7 s o r 6d l e v e l i s s t r o n g l y predominant t h e ~ b + c u r r e n t w i l l vary a s t h e 7s o r a s t h e 6p d e n s i t y . I f t h e two l e v e l s have e q u a l r a t e c o e f f i c i e n t s t h e Rb+ c u r r e n t w i l l t h e n decay a s t h e sum of t h e 7 s and 6d d e n s i t i e s . Experimental v a r i a t i o n ( p o i n t s f i g . 2 ) accounts f o r t h i s l a s t hypothesis. The same r e s u l t s have been obtained when t h e 5d, 6d o r 8 s
l e v e l i s e x c i t e d : e x p e r i m e n t a l r e s u l t s c l e a r l y show t h a t t h e Penning r a t e c o e f f i - c i e n t i s l i n d e p e n d e n t . The r a t e c o e f f i c i e n t s o b t a i n e d with t h e C.N. system t h e n can be c o r r e c t e d from cascading r a d i a t i v e mechanisms. The r e s u l t s a r e p r e s e n t e d i n f i g . 3 t o g e t h e r with t h e d i p o l e - d i p o l e c a l c u l a t i o n s f o r s ( x ) , p ( o ) , d ( + ) l e v e l s and Kimura's c a l c u l a t i o n s
( 0 )f o r l e v e l s 6d, 8 s and 7d.
A s p r e s e n t e d i n f i g . 1 mass scanning of n e g a t i v e i o n s i n d i c a t e s t h e presence of a low Rb- c u r r e n t . Molecules a r e commonly considered a s r e s p o n s i b l e f o r n e g a t i v e i o n formation 151, t h e main r e a c t i o n s being :
Rb, + e -Rb(5s) + ~ b -
l d e n s i t i e s
( R . L . U ~ T )F i g . 2 - E x c i t a t i o n of t h e 7 s l e v e l . Tine v a r i a t i o n of t h e 7s and 6p d e n s i t i e s calcu-
l a t e d with measured l i f e t i m e , Experimental p o i n t s deduced from t h e ~ b + c u r r e n t .
Fig.3 - Rate c o e f f i c i e n t f o r Penning i o n i z a t i o n k a g a i n s t t h e e f f e c t i v e quantum number n* of
p i
t h e e x c i t e d s t a t e .
Under our experimental c o n d i t i o n s ( c e l l temperature 4 5 0 ' ~ ) t h e r a t i o between t h e dimer d e n s i t y and t h e atom d e n s i t y i s e q u a l t o 4 x 1 0 - ~ Experimental checks show t h a t : - n e g a t i v e i o n f o r n a t i o n r e q u i r e s both l a s e r s t o be r e s o n a n t . - n e g a t i v e i o n c u r r e n t does n o t vary when t h e Rb2 c o n c e n t r a t i o n i s i n c r e a s e d by a f a c t o r of 10 (by v a r y i n g t h e c e l l temperature a t
c o n s t a n t Rb p r e s s u r e ) .
- n e g a t i v e i o n c u r r e n t v a r i e s l i n e a r l y w i t h b o t h l a s e r powers.
These o b s e r v a t i o n s r u l e o u t r e a c - t i o n s i n v o l v i n g any of t h e f o l l o - wing p a r t i c l e s
:molecules, mole- c u l a r i o n s , photons, e l e c t r o n s . Ion p a i r formation by a c o l l i s i o n between a h i g h l y e x c i t e d atom and a ground s t a t e atom, namely Rb(n1) + Rb(5s) Rb'
+~ b -
i s a r e a c t i o n t h a t f u l l f i l l s a l l our experimental checks. T h i s r e a c t i o n i n v o l v e s t h e same p a r t n e r s Rb(n1) and Rb(5s) than t h e Hornbeck- Molnar a s s o c i a t i v e i o n i z a t i o n . The r a t e c o e f f i c i e n t f o r i o n p a i r f o r - n a t i o n , nk-(nl) , can be deduced from t h e r a t e c o e f f i c i e n t f o r as- s o c i a t i v e i o n i z a t i o n kAI(nl) by comparing t h e n e g a t i v e Ion c u r r e n t I(Rb-) t o t h e molecular i o n c u r r e n t I(Rb3) :
When t h e 6d D ' l e v e l i s e x c i t e d ,
0
b e t - t h e c e l l terape?&ure varyin, ween 383°K and 4 5 3 O ~ a t c o n s t a n t Rb p r e s s u r e , t h e k-(6d) v a l u e s , r e p o r t e d on f i g . 4 a r e measured. As t h e e l e c t r o n a f f i n i t y of t h e Rb atom is e q u a l t o 3919.2
t h e r e a c t i o n i s e x o e r g i c by 1082.4 cm-l f o r t h e 6d 2 ~ l e v e l . The ~ , ~ experimental v a l u e s of t h e r a t e c o e f f i c i e n t can be f i t t e d with a c r o s s s e c t i o n Q - e q u a l t o 1.3
X10-1 6 m 2 above t h e t h r e s h o l d and e q u a l t o z e r o below t h i s l i m i t and w i t h a maxwellian v e l o c i t y d i s t r i - b u t i o n f ( v ) being assumed, leading
t o
Wk-
=v ~ ( v ) dv.
JOURNAL DE PHYSIQUE
Fig.4 - k-(6d 2 D5,2) a g a i n s t c e l l temperature - .
experimental p o i n t s
-16 2 f i t t e d curve w i t h - = I .3x10 cm .
{E
cm"-f
- - - - --- -
I Sd
2s
