LINE RADIATION OF ARGON PLASMA IN EARLY AFTERGLOW

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LINE RADIATION OF ARGON PLASMA IN EARLY AFTERGLOW

J. Labat, J. Vukićević, O. Labat, S. Djeniže

To cite this version:

J. Labat, J. Vukićević, O. Labat, S. Djeniže. LINE RADIATION OF ARGON PLASMA IN EARLY AFTERGLOW. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-9-C7-10.

�10.1051/jphyscol:1979704�. �jpa-00219063�

JOURNAL DE PHYSIQUE ColZoque C7, suppllment au no?, Tome 40, JuilZet 1979, vage C7- 9

LWE RADIATION OF ARGON PLASMA IN EARLY AFTERGLOW

J.M. Labat, J. Vukicevic?, 0. Labat and S. Djeni2e.

Department of Physics and Meteorology and I n s t i t u t e of Physics, Beograd.

S p e c t r a l l i n e r a d i a t i o n of a glow d i s c h a r g e argon plasma submited t o t h e s h o r t e l e c t r i c p u l s e p e r t u r b a t i o n was r e p o r t e d by Labat e t a 1 ( l ) . S e t of s p e c t r a l l i n e s t h a t predominantly o r i g i n a t e from 4p t o 4 s l e - v e l s t r a n s i t i o n s , a f t e r t h e p e r t u r b a t i o n have two c l e a r l y d i s t i n c t i v e maxima. I n i - t i a l r a p i d i n t e n s i t y peak d u r i n g t h e per- t u r b a t i o n p u l s e i s caused by t h e d i r e c t e x c i t a t i o n of atomic l e v e l s by e l e c t r o n s . Time t h e second i n t e n s i t y maximum appeared depends i n v e r s e p r o p o r t i o n a l on t h e i n i t i a l g a s p r e s s u r e and was same f o r a l l t h e li- nes observed. Decay o f t h e second peak con- s i s t s of t h r e e phases: i n i t i a l r a p i d d r o p was followed by two e x p o n e n t i a l decays i n

s u c c e s s i o n , w i t h time c o n s t a n t s t h a t sug- g e s t s d i s s o c i a t i v e recombination being r e s - p o n s i b l e f o r t h e 4p l e v e l s p o p u l a t i o n .

Experiment. Experimental a p p a r a t u s c o n s i s t s of U shaped d i s c h a r g e t u b e w i t h water cooled e l e c t r o d e s . Discharge c u r r e n t was mantained c o n s t a n t a t a v a l u e of 0.3 A f o r t h e whole r a n g e of i n i t i a l g a s pressu- r e (2-15 T o r r ) . P a r t of t h e p o s i t i v e co- lumn (4.5 cm l e n g t h ) w a s p e r t u r b e d by a s h o r t ( 3 n s ) high v o l t a g e ( 7 kV) p u l s e sup- p l i e d by a Blumlein l i n e . R a d i a t i o n was recorded by Zeiss SPM-2 monochromator-EM1 9569B p h o t o m u l t i p l i e r - Biomation 8100 wave- form r e c o r d e r system.

R e s u l t s . S p e c t r a l l i n e s o r i g i n a t i n g from 4p t o 4 s l e v e l s t r a n s i t i o n s i n a per- t u r b e d plasma have two i n t e n s i t y maxima.

F i r s t t h a t occured r i g h t a f t e r a p p l i c a t i o n of e l e c t r i c p u l s e , o r i g i n a t e d from d i r e c t e x c i t a t i o n of ground l e v e l atoms by e l e c t r o - n i c c o l l i s i o n s . Time t o r e a c h t h e f i r s t i n - t e n s i t y maximum c o n s i s t s of time f o r ava- lanche t o develop and t h a t of spontaneous d e e x c i t a t i o n a f h i g h e r l e v e l s t o l e v e l s 4 ~ " ) . Decay time of t h i s peak was found t o be e q u a l t o t h e l i f e time of t h e p a r t i c u l a r l e v e l .

P r i n c i p a l o b j e c t i v e s of t h i s r e p o r t a r e t h e p r o p e r t i e s of second i n t e n s i t y maxi- mum. By r e c o r d i n g of i n t e n s i t y c u r v e s f o r

l a r g e number of s p e c t r a l l i n e s o r i g i n a t i n g from 4p l e v e l s , it was found t h a t i n s t a n t t2 t h e second maximum occured does n o t de- pend on t h e p a r t i c u l a r l i n e . I t was always i n v e r s e l y p r o p o r t i o n a l t o t h e i n i t i a l argon p r e s s u r e , t h e r e s u l t s a r e given i n f i g u r e 1.

F i g u r e 1.

This p r o p o r t i o n a l i t y suggested t h a t t h e se- cond maximum i s c r e a t e d by t h e p r o c e s s of i o n i s a t i o n , s i n c e t h e time c o n s t a n t f o r i o n i s a t i o n

l/'ti = nol.S i .p

i s i n v e r s e l y p r o p o r t i o n a l t o p r e s s u r e . Va- l u e nol i s t h e n e u t r a l atom d e n s i t y a t p = l T o r r , si i s t h e r a t e c o e f f i c i e n t f o r c o l l i -

e

s i o n a l i o n i s a t i o n and p i s t h e g a s p r e s s u r e i n T o r r . Taking f o r si v a l u e s f o r c o l l i s i o - n a l i o n i s a t i o n from ground l e v e l o n l y , one o b t a i n s t h e r e s u l t s t h a t match very w e l l w i t h t h e experiment. The i o n i s a t i o n p r o c e s s

i s followed by t h e p r o c e s s of molecular i o n s formation, i n t h i s c a s e it i s only Hornbeck-Molnar p r o c e s s of a s s o c i a t i v e i o n i s a t i o n " ) , t h e i o n conversion being n e g l i g i b l e under c o n d i t i o n s of t h e expe- riment. I t was suggested by L o r e n t s ( 3 ) and i n d i c a t e d by t h e experimental r e s u l t s ( 4 ) (7) t h a t t h e argon 4p l e v e l s i n a decaying

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979704

plasma are populated by the dissociative recombination of electrons and molecular ions. Further it was suggested that the Becay constant of noble gases in the later

+ (511 (6) afterglow is proportional to n2.ne

where n2 and n are densities of molecular +

e

ions and electrons respectively. Typical time decay of the second intensity maximum peak is shown in figure 2.

Figure 2.

Records for all the lines studyed were prac- tically equal at equal pressures. Loga- ritmic plot of intensity versus time have three distinct sections. First, that lasted some two microseconds, has decay constant of about 4 ps. It was followed by the se- aond section that lasted for next 10 ps with time constant of 7-9 ps. After about

4 ps plateau follows the third exponential decay with even longer time constant 8 to

11.5 ps. Values of three time constants versus initial pressure are given in figu- re 3 for 7514 2 ^line.

Since the pressure dependence of the time constants is rather weak, the decay is obviously caused by the recombination processes and definitely not by the diffu- sion.

while the first intensity drop is not .easily explicable, the other two are clear-

ly exponential and suggest that the decay process is governed by the dissociative recombination. Taking the value of disso- ciative recombination coefficient being

Figure 3.

aDR = 9 . 1 . 1 0 - ~ ( 3 0 0 / ~ ~ ) ~ ' ~ ~ c ~ n ~ / s ( ~ ) one can obtain the corresponding values of electron concentration for the pressure range 2 to 15 Torr being 1.8.10~~ to 1 . 1 0 ~ ~ cm-3 for

T~ and 1 . 6 ~ 1 0 ~ ~ to 7.9.10~~ cm-3 for r 3 - Slight alteration of electron concentration during the plateau of decay is caused possi- bly by some dynamical processes in the plasma.

References

1. O.Labat, J.Labat, S.~jeniHe,~j.~irkovi6, 2. 0.A.Malkin: Pulse Discharge and Relaxa-

tion in Gases (in Russian) Atomizdat, Moscow, 1974

3. D.C.Lorents, Physica 82C (1976) 19

4. G.N.Gerasimov, S.Ia.Petrov, I.L.Sabirova, Optika i Spektroskopia 42 (1977) 1035 5. G.E.Veatch, H.J.Oskam, Phys.Rev.A 1

(1970) 1498

6. L.Fromhold and M.Biondi, Phys.Rev. 185 (1969) 244

7. Y.J.Shiu and M.A.Biondi, Phys.Rev. 17

(1978) 863.