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Submitted on 1 Jan 1979
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EXCIMER FORMATION IN RARE GAS DISCHARGE AFTERGLOWS
W. Wieme, J. Lenaerts
To cite this version:
W. Wieme, J. Lenaerts. EXCIMER FORMATION IN RARE GAS DISCHARGE AFTERGLOWS.
Journal de Physique Colloques, 1979, 40 (C7), pp.C7-37-C7-38. �10.1051/jphyscol:1979718�. �jpa-
00219157�
JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment n07, Tome 40, J u i l Z e t 1979, nage C7- 37
EXCIMER FORMATION IN RARE GAS DISCHARGE AFTERGLOWS
W. Wieme, J. Lenaerts.
Laboratoriwn uoor Natuurkunde, Ri j k s u n i v e r s i t e i t , Rozier 44 Gent, BeZgiwn.
I . I n t r o d u c t i o n .
T h e V U V c o n t i n u u m i n r a r e g a s e s h a s b e e n e x t e n s i v e l y s t u d i e d w i t h d i f f e r e n t m e t h o d s .
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D i s c h a r g e a f t e r g l o w s w e r e m a i n l y s t u d i e d a t l o w p r e s s u r e s ( 0 , l - 2 0 T o r r ) . A b s o r p t i o n m e a s u r e m e n t s o n t h e 3 ~ m e t a s t a b l e s t a t e 2e s t a b l i s h e d t h e r o l e o f t h r e e - b o d y c o l l i - s i o n s : R ( ~ P , ) + 2 R ( ' S o ) + R, + R ( 1 R e a c t i o n r a t e s f o r p r o c e s s ( 1 ) a r e : A r : 1 8 . 5 p 2 [ 1 1 K r : 4 4 p 2 [ 2 1 X e : 8 7 p 2 [ 3 1
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E x c i t a t i o n by h i g h l y e n e r g e t i c p a r t i c l e s ( a s o u r c e o r e - b e a m ) , u s u a l l y a t h i g h e r p r e s s u r e s ( 1 0 0 - 1 0 0 0 T o r r ).
A s p r e c u r s o r s a l l 4 s t a t e s o f t h e n p 5 ( n + l ) s c o n f i g u r a t i o n h a v e b e e n i n v o l v e d . R e a c t i o n r a t e s f o r ( 1 ) w e r e s t a t e d a s :A r : 10 p 2 [ 4 1 Kr : 4 6 p 2 [ 9 ] X e : 4 0 p 2 [ 6 1 F o r t h e 3 ~ 1r e s o n a n t s t a t e r e a c t i o n ( 2 ) h a s b e e n e s t a b l i s h e d w i t h r a t e c o n s t a n t s :
R ( ~ P ~ ) + 2 R ( ' S o ) + R; ( 2 ) A r : 2 1 p 2 C 8 1 Kr : 8 , 4 5 p 2 C 5 1 Xe : 4 6 p 2 [ 6 1
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O p t i c a l e x c i t a t i o n u s i n g s y n c h r o t r o n r a - d i a t i o n h a s b e e n a p p l i e d t o K r a n d Xe [ 7 ] . T h i s y i e l d s f o r r e a c t i o n ( 2 )K r : 2 3 p 2 Xe:36 p 2 w h e n R: i s t h e 0: s t a t e K r : 4 6 p 2 Xe:63 p 2 w h e n R: i s t h e '3~' s t a t e . T h e 0: s t a t e c a n e i t h e r d e c a y r a d i a t i v e l y , g o r c o l l i s i o n a l l y t h r o u g h :
R : ( o ~ ) + I l ( l S o ) R:('r3~=) + R ( ' S ~ ) ( 3 ) R e a c t i o n c o n s t a n t s a r e :
Kr : 3 , 3 1 0 6 p Xe : 2 , 8
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O t h e r e x p e r i m e n t a l t e c h n i q u e s a r e n o t r e - v i e w e d f o r l a c k o f s p a c e . G e n e r a l l y , we f i n d t h a t l o w p r e s s u r e a f t e r g l o w d a t a t e n d t o g i v e r e a c t i o n r a t e s r o u g h l y t w i c e t h o s e o b t a i n e d a t h i g h p r e s s u r e s w i t h o t h e r me- t h o d s . Low p r e s s u r e a b s o r p t i o n m e a s u r e m e n t s b e i n g t h e o n l y o n e s w h e r e t h e p r e c u r s o r s c a n b e p o s i t i v e l y i d e n t i f i e d , we r e p o r t m e a s u r e - m e n t s o f t h e V U V e m i s s i o n i n d i s c h a r g e a f - t e r g l o w s a t p r e s s u r e s b e t w e e n 1-150 T o r r ,i n a n a t t e m p t t o e l u c i d a t e t h i s d i s c r e p a n c y . 2 . T h e o r y .
I n o u r a n a l y s i s we u s e a f o u r - l e v e l m o d e l ? L e t R r e p r e s e n t t h e 3 ~ 1r e s o n a n c e s t a t e , M t h e ' P 2 m e t a s t a b l e s t a t e ; P a n d S i n d i c a -
t e t w o m o l e c u l a r l e v e l s w h i c h c a n b e p o p u - l a t e d t h r o u g h c o l l i s i o n p r o c e s s e s . We g i v e n o a s s i g n m e n t t o t h e s e l e v e l s , a s t h e y may s t a n d f o r a m u l t i t u d e o f v i b r a t i o n a l l e v e l s b e l o n g i n g t o d i f f e r e n t e x c i m e r s t a t e s . We
d e f i n e :
8 d e c a y c o n s t a n t f o r t h e e s c a p e o f i m p r i - s o n n e d r e s o n a n c e r a d i a t i o n
aRSaRMaMRaMPCXSPaPS r a t e c o e f f i c i e n t s f o r c o l l i s i o n - i n d u c e d t r a n s i s t o r s R t o S,R t o M, M t o R , M t o P , S t o P , P t o S, r e s p e c t i v e -
l y . E x c i m e r s S a n d P d e c a y w i t h t i m e c o n - s t a n t
e
a n d r r e s p e c t i v e l y .T h e f o l l o w i n g s e t o f d i f f e r e n t i a l e q u a - t i o n s r e s u l t s :
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dR =d t - ( B + h R M + a R S ) R + a MR M
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dS d t = - ( e + a S p ) s + a ~ + ~'psP RWe n e g l e c t b a c k w a r d t r a n s i t i o n s R+M a n d S+R.
We o b t a i n f o r P a n d S :
P = ~ , e ' ~ ~ + ~ ~ e - P ~ + ~ , e - ~ f t + ~ , , e - d s t
s
= c , ~ - ~ ~ + c , ~ - P ~ + c , ~ - ~ ~ t + ~ , e - d s5
,
= [ - ( k ?J
(m-k) '+ 4 a S p a p g I2
a = f l + aRM + a R S
,
b = aMR + a MP m = B+ aP S , k = 0 + a S P
W i t h r e a c t i o n r a t e s known f r o m l i t e r a t u r e , we f i n d t h a t f o r p r e s s u r e s up t o 6% T o r r , w i t h i n 5 X : d f = a , d s = b S i m p l i f i c a t i o n o f a a n d p i s more d i f f i - c u l t , a s n o r e l i a b l e v a l u e s o f t h e c o e f f i - c i e n t s B , p , a p S a s p a r e k n o w n . I t s e e m s how- e v e r t h a t a t p r e s s u r e s up t o a b o u t 5 0 T o r r
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979718
we have df, ds < < m. k.
3. Experimental.
The discharge tube is discribed in [3]. It is followed by a McPherson 2 1 8 monochroma- tor. The detector is a EMI-GENCOM G-26E315.
The P M signal is analysed with a DATALAB DL 920 transient recorder with a time resolu- tion of 5 0 ns. The intensity decay is fit-
ted to a curve : I = A ~ - P ~ + B ~ - ~ ~ ~ + c ~ - ~ s ~ . The fast decay ~ e may include several - ~ ~
transient processes and has not been analy- sed, df and ds have been defined earlier.
4. Results.
We distinguish between :
a) The wavelengths adjacent to the 3 ~ 1 - ' ~
resonance line (first continuum).
b) The so-called second continuum with a maximum at 170nm(Xe),150nm(Kr)& 13Onm(Ad No significant wavelength dependence was found throughout this second continuum. A typical result i.s .given for Xe in Fig.1.
The full line is a curve fit to the d va- lues from the second continuum at pressures
< 2 0 Torr.
In complete agreement with absorption s tu- dies, we find : Ar : ds = 45p + 18p2 (4)
Kr : ds = 82p + 41p2 (5) Xe : ds = 113p + 8 7 p 2 (6) At higher pressures the observed ds is al- ways slower than predicted by (4) (5) or (6).
At the same time, ds becomes highly depen- dent on the discharge current. This implies that interactions involving electrons beco- me important. The points presented in Fig. 1 are taken in a diffuse discharge at the lo- west possible current.
A computer simulation, including dimeric ion formation and recombination was attemp- ted. As n o precise values of electron den- sity and temperature were available, these were treated as a parameter. The computer predicted decays show an exponential decay,
corresponding to ds, with a time constant 10 to 200 % below the full line of Fig.1, the lower n giving the best approximation.
The first continuum has only been measured in Xe. It decays with rate constant df which which is close to the measured decay of the
3 ~ ,resonance line, reported in [IO](dashed line).
4. Discussion.
Our results indicate that reaction rates obtained from 3 ~ absorption measurements 2
in Ar, Kr and Xe, are indeed reliable. The discrepancy with high pressure results is probably partly due to other precursors, partly to neglect of electron collision ef- fects. Collision induced transitions such as (3) also become increasingly important at higher pressures.
5. References.
[ I ] W . W i e m e , J . W i e m e - L e n a e r t s , P h y s . L e t t . 4 7 A , 37,1974.
[21 R.Turner,Phys.Rev.l58,121,1967.
[31 W.Wieme,J.Phys.B,7.850,1974.
C41 N.Thonnard,G.S.Hurst,Phys.Rev.A5,1110, 1972.
[51 P.K.Leichner,R.J.Ericson,Phys.Rev.A9, 251,1974.
[61 P.K.Leichner et al.,Phys.Rev.A13,1787, 1976.
[71 R.Brodmann,G.Zimmerer,J.Phys.B,10,3395, 1977.
181 M.BourSne et al.,J.Chem.Phys.63,1668,75.
C91 R . B o u c i q u 6 , P . M o r t i e r , J . P h y s .D3, 1905,1970.
