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Submitted on 1 Jan 1979
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VISIBLE LINE AFTERGLOW EMISSION OF A PULSED XENON DISCHARGE
M. Vanmarcke, W. Wieme
To cite this version:
M. Vanmarcke, W. Wieme. VISIBLE LINE AFTERGLOW EMISSION OF A PULSED XENON DISCHARGE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-35-C7-36.
�10.1051/jphyscol:1979717�. �jpa-00219146�
JOURNAL DE PHYSIQUE ColZoque C7, supplBment n07, Tome 40, J u i l l e t 1979, vaae C7- 35
VISIBLE LINE AFTERGLOW EMISSION OF A WLSCD X M O N DIXHARGE
M. Vanrnarcke, W. Wierne.
Laboratoriwn voor Natumkunde R i j k s u n i v e r s i t e i t , Rosier 44, Gent Belgium.
1 . I n t r o d u c t i o n .
A l t h o u g h i n r e c e n t y e a r s much w o r k h a s b e e n d o n e o n r a r e g a s a f t e r g l o w s i n t h e VUV r e - g i o n , t h e a f t e r g l o w e m i s s i o n o f t h e v i s i b l e l i n e s p e c t r u m h a s o n l y o c c a s i o n a l l y b e e n i n - v e s t i g a t e d [ 1 ] [ 2 ] [ 3 ] . I n t h i s p a p e r a s t u d y o f t h e s p e c t r a l r e g i o n 4 0 0 - 9 0 0 nm i n t h e a f - t e r g l o w o f a x e n o n d i s c h a r g e i s d e s c r i b e d ( p r e s s u r e r a n g e 0 , l - 4 0 T o r r ) .
2 . M e t h o d .
T h e d i s c h a r g e t u b e i s a c y l i n d r i c a l p y r e x v e s s e l 4 0 cm l o n g , d i a m e t e r 3 8 mm. The d i s -
c h a r g e i s s t r u c k b e t w e e n t w o n i c k e l r i n g e l e c t r o d e s p l a c e d a t 3 cm f r o m t h e e n d w i n - d o w s . A s i d e a r m i s p r o v i d e d a t o n e e n d t o r u n t h e d i s c h a r g e a t l o w p r e s s u r e w i t h o u t s p u t t e r i n g e l e c t r o d e m a t e r i a l o n t h e w i n - d o w s . A r e c t a n g u l a r v o l t a g e p u l s e w i t h a f a l l t i m e l e s s t h a n 1 u s i s a p p l i e d w i t h a 8 0 7 p e n t h o d e c o n n e c t e d p a r a l l e l t o t h e d i s - c h a r g e t u b e . T h e t u b e i s pumped down o n a UHV s y s t e m a n d f i l l e d w i t h h i g h p u r i t y x e - n o n ( L ' A i r L i q u i d e N47). T h e l i g h t i s c o l -
l e c t e d a t t h e e n d w i n d o w w i t h a l e n s a n d f o c u s s e d o n t h e e n t r a n c e s l i t o f a m o n o c h r o - m a t o r . The d e t e c t i o n s y s t e m c o n s i s t o f a O p t i c a l M u l t i c h a n n e l A n a l y s e r (PAR OMA I
s y s t e m ) w i t h a S I T d e t e c t o r . T h i s d e t e c t o r c a n b e g a t e d t o o b t a i n a t i m e r e s o l u t i o n down t o 1 0 0 n s . T h e s p e c t r a l r e s o l u t i o n i s d e t e r m i n e d b y t h e 1 2 0 0 l / m m g r a t i n g a n d w a s b e t t e r t h a n 0 , 5 nm. I n t h i s way a l l t h e p r i n c i p l e l i n e s o f t h e X e n o n s p e c t r u m w e r e c o m p l e t e l y s e p a r a t e d . T h e OMA s y s t e m was u s e d t o o b t a i n s p e c t r a i n s e g m e n t s o f 2 0 0 nm a t s e l e c t e d t i m e s i n t h e a f t e r g l o w , d e t e r m i - n e a b y a d e l a y g e n e r a t o r . T h e s e s p e c t r a we- r e r e c o r d e d w i t h a XY r e c o r d e r a n d r e a d o u t s u b s e q u e n t l y a s n o d i g i t a l r e a d o u t f a c i l i - t i e s a r e a v a i l a b l e a t t h e moment.
3 . R e s u l t s .
A t y p i c a l r e s u l t i s s h o v n i n F i g . ] . T h e l i g h t i n t e n s i t y I a t f i r s t i n c r e a s e s d u r i n g t h e a f t e r g l o w p e r i o d , t h e n d e c a y s e x p o n e n - t i a l l y , a n d f i n a l l y s h o w s a n o n - e x p o n e n t i a l v e r y s l o w d e c a y .
T h i s l a s t d e c a y h a s b e e n o b s e r v e d b y o t h e r a u t h o r s [ 3 1 [ 4 1 a n d i d e n t i f i e d a s r e c o m b i n a - t i o n l i g h t , o b t a i n e d t h r o u g h t h e d i s s o c i a - t i v e r e c o m b i n a t i o n p r o c e s s :
~ e : + e -+ Xe, + Xe + Xe
T h e d i r n e r i c i o n s ~ e : a r e known t o b e a b u n - d a n t l y f o r m e d i n t h e e a r l i e r p a r t o f t h e a f t e r g l o w t h r o u g h t h e t h r e e - b o d y p r o c e s s :
~ e + + 2Xe + ~ e : + Xe ( 3 )
T h e r e c o m b i n a t i o n p h e n o m e n a a r e g o v e r n e d b y t h e r a t e e q u a t i o n
w h e r e a r e p r e s e n t s t h e r e c o m b i n a t i o n c o e f - i
f i c i e n t f o r t I ~ e a p p r o p r i a t e i o n s p e c i e s
mi.
T h e r e f o r e i n t e n s i t y d e c a y s d u e t o r e c o m b i n a - t i o n f o l l o w t h e l a w
T h i s i n t e n s i t y d e c a y i s o b s e r v e d i n o u r e x - p e r i m e n t s o n l y f o r t h e l a t e r p a r t o f t h e a f - t e r g l o w . T h i s m e a n s o b v i o u s l y t h a t i n t h e e a r l i e r a f t e r g l o w o t h e r p r o c e s s e s h a v e t o b e r a t e - d e t e r m i n i n g .
We s h a l l now c o n c e n t r a t e on t h i s e a r l i e r p a r t . T h i s d e c a y c a n b e f i t t e d t o a c u r v e
A p l o t o f a , r e s p e c t i v e l y 6 a g a i n s t p i s g i v e n i n F i g . 2 a n d F i g . 3 . The p o i n t s up t o 2 0 T o r r h a v e b e e n l e a s t - s q u a r e f i t t e d , t o g i v e : a = - 2 2 + 137 p + 3 0 9 p 2
P
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979717
We suggest the following explanation :
Dimeric ions are formed according to (31, with rate constant
a.
a then represents the conversion coefficient of atomic ions into dimeric ions. This coefficient is given in literature by several authors between 190p2 and 450p2C51[61.The pseudo-lifetime 6 can be explained as follows : the Xe: ion is formed in an exci- ted state from which it cannot readily re- combine. I t decays to a lower vibrational level, possibly down to v=O by collisions with neutral atoms.
The reaction sequence thus reads + :: ::
Xe*
+
Xe + (Xe,) rate : 137 p (9)and + + ::::
Xe + 2xe + (Xe,) rate : 309 p 2 (10) followed by
(xe;) ::::
+ Xe + (xe:)"
+
~e + K.E. (1 1) rate : 5330 pand
(xe:)" + e + (~e,)" + xe"+ xe (12)
+ X X
Here (Xe,) represents one of possibly se- veral higher vibrational states whereas (xe:lX represents a lower vibrational state, possibly v=O.
Process (9) has not been mentionned i n li- terature before.
4. Comment.
Turner and Riccius [I] have performed a si- milar investigation in krypton. They found only an exponential decay with a rate con- stant 9 0 0 p for which they suggest a pro- cess similar to (11). However they failed to observe the initial formation processes (9) and/or (10). Experiments are planned in krypton to verify these results under our
experimental conditions. A computer program was written to obtain the variation of the light intensity during the afterglow, ta- king account all forementionned processes.
The shape of the intensity versus time cur- ve is indeed nearly exponential, as obser- ved. A t low pressures (tl Torr) diffusion effects can be expected to show up. This process can be characterized by an exponen- tial decay; we find a ambipolar diffusion
2 2
coefficient
-
i n agreement with literatu-P
'
re [31. At pressures above 20 Torr, the ex-
perimentally obtained a tend to be signifi- cantly lower than predicted by (7). This could mean that other processes become im- portant, which is now further investigated.
References.
1 1 1
R.Turner,H.D.Riccius,J.Chem.Phys.48,4351,1968.
[2] A.Montei1 et al.,J Q R S T 18,573,1977.
[31 A.Barbet et al.,J.Phys.B,8,1785,1975.
[41 Y.J.Shiu et al.,Phys.Rev.A15,494,1977.
[5] R.L.Fitzwilson, J.App1.Phys. ,44,5337,1973 [ 6 1 A.Barbet et al.,J.Phys.B,8,1785,1975.
Fig. 1
Fig. 2
Fig. 3
