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TRANSIENT ABSORPTION STUDIES IN PURE
RARE GASES FROM 2500 Å TO 4000 Å
L. Champagne, R. Chang
To cite this version:
JOURNAL DE PHYSIQUE CoZZoque C9, suppzgment au n o l l , Tome 41, novembre 1980, page C9-445
0 0
T R A N S I E N T ABSORPTION S T U D I E S
I N
PURE RARE GASES FROM 2500 A TO 4000 A L.F. Champagne and R.S.F. ChangNavaZ Research Laboratory, Washington, D. C. 20375, U. S. A.
A b s t r a c t . - The broad band a b s o r p t i o n i n e l e c t r o n beam e x c i t e d r a r e g a s plasmas was measured f o r neon, argon, k r y p t o n and xenon g a s . A broad continuum emisgion from a xenon f l a s h l a m p was used a s t h e pro- be s o u r c e . Absorption d a t a was o b t a i n e d o v e r a 1500 A r a n g e . A b s o r p t i o n peaks i n a r g o n , neon and xenon c o r r e l a t e w e l l w i t h t h e p r e d i c t e d peak a b s o r p t i o n c r o s s s e c t i o n s f o r t h e r e s p e c t i v e dimer i o n s . No a b s o r p t i o n peak i n k r y p t o n was observed. Secondary a b s o r p t i o n peaks of comparable magnitude t o t h e dimer a b s o r p t i o n peak were a l s o observed i n argon and xenon.
The e x i s t e n c e of t r a n s i e n t a b s o r p t i o n i n e l e c - t r o n beam p u m p e d r s r e g a s e s h a s l i m i t e d t h e p e r f o r - mance of h i g h energy l o n g - p u l s e l a s e r s by l o w e r i n g e x t r a c t i o n e f f i c i e n c y and l i m i t i n g s c a l a b i l i t y . S i n c e electron-beam pumped e x c i t e d r a r e g a s e s a r e used a s energy s t o r a g e media i n t h e s e systems a n u n d e r s t a n d i n g of t h e a b s o r p t i o n phenomena a s w e l l a s t h e a m p l i t u d e of t h i s a b s o r p t i o n a t any p a r t i c u - l a r wavelength i s of i n t e r e s t . T h i s r e q u i r e s t h e i d e n t i f i c a t i o n of t h e v a r i o u s a b s o r b i n g s p e c i e s which may i n c l u d e e x c i t e d atoms, e x c i t e d dimers, dimer and t r i m e r i o n s depending on t h e wavelength r e g i o n o f i n t e r e s t and t h e r a r e g a s e s used.
Absorption s t u d i e s i n t h e u l t r a v i o l e t and v i s i b l e r e g i o n s of t h e spectrum have been performed u s i n g l i n e t u n a b l e o r e ~ n t i n u o u s l y t u n a b l e probe t e c h n i q u e s . These measurements g i v e a q u a n t i t a t i v e measure of t h e a b s o r p t i o n a t any s i n g l e wavelength of i n t e r e s t . However, t h e s e measurements shed l i t t l e l i g h t on t h e c o n t r i b u t i o n t o a b s o r p t i o n made by v a r i o u s e x c i t e d s t a t e s p e c i e s . The c o n t r i b u t i o n by v a r i o u s s p e c i e s t o a b s o r p t i o n a t a p a r t i c u l a r wavelength i s a l s o e x p e c t e d t o change w i t h e l e c - 'tron-beam pumping c u r r e n t , g a s p r e s s u r e , tempera- t u r e and time. T h e r e f o r e , a n a b s o r p t i o n measure- ment, though q u a n t i t a t i v e , p r o v i d e s l i m i t e d i n f o r - mation a b o u t t h e a b s o r b e r s themselves. A wider
s p e c t r a l r e g i o n must b e probed i n o r d e r t o charac- t e r i z e b e t t e r t h e s e t r a n s i e n t a b s o r b e r s . We r e c e n t - l y r e p o r t e d t h e a p p l i c a t i o n of a broad band p r o b e t e c h n i q u e t o s t u d y t r a n s i e n t a b s o r p t i o n o v e r a 300
%
r e g i o n . ' Although t h i s t e c h n i q u e was v e r y u s e f u l i n i d e n t i f y i n g v a r i o u s a b s o r b i n g s p e c i e s and c h a r - a c t e r i z i n g t h e i r b e h a v i o r a s a f u n c t i o n of v a r i o u s p a r a m e t e r s , t h e 3002
band w i d t h probed r e p r e s e n t s o n l y a s m a l l p o r t i o n o f t h e spectrum o v e r which t h e dimer i o n s a r e expected t o a b s o r b . We r e p o r t h e r e t h e r e s u l t s of e x p e r i m e n t s i n which a xenon lamp s o u r c e i s used t o p r o b e 4 l a r g e wavelength r a n g e limi'ted o n l y b e t h e s p e c t r a l r a n g e of t h e monochro- mator which I n t h f s c a s e i s 1500%.
T h i s t e c h n i q u e a l l o w s u s t o s t u d y t h e e n t i r e broad band a b s o r p t i o n r e g i o n and i n c l u d e s t h e e f f e c t s of narrow l i n e a b s o ~ p t t o n a s w e l l as t h e broad a b s o r p t i o n a t t r i b u - t e d t o t h e dimer i o n s . B r i e f l y , we f i n d t h a t t h e c o n t r i b u t i o n t o a b s o r p t i o n by t h e e x c i t e d s p e c i e s i s broad and n o t s h a r p l y For a r g o n , neon, and xenon w eo b s e r v e a b s o r p t i o n peaks which a r e i n good a g r e e - ment w i t h t h e o r e t i c a l l y p r e d i c t e d a b s o r p t i o n c r o s s s e c t i o n peaks f o r t h e r e s p e c t i v e dimer i o n s . 1 3 2 However, i n k r y p t o n no peak a b s o r p t i o n which would c o r r e l a t e w i t h t h e t h e o r e t i c a l l y p r e d i c t e d peak due
C9-446 JOURNAL DE PHYSIQUE
t o ~ rof -3250 ~ +
2
i s observed. We a l s o observe secvndary peaks i n argon and xenon plasmas and a t t r i b u t e t h i s t o some e x c i t e d s t a t e s p e c i e s o t h e r than t h e dimer ionk.The medium probed was a one-meter l a s e r cham- b e r which has been d e s a r i b e d p r e v i o u s l y .
'
The probe s o u r c e was a xenon flashlamp which h a s an o p t i c a l p u l s e l e n g t h o f - 0 . 3 ~ ~(FWHM) and which
c l o s e l y approximated t h e 0 . 6 ~ s p u l s e used t o ex- c i t e t h e r a r e g a s e s . Emission i n t e n s i t y from t h e lamp was uniform over t h e e n t i r e wavelength r e g i o n i n v e s t i g a t e d . The lamp e x h i b i t e d a c h a r a c t e r i s t i c t a i l which i n c r e a s e d a t l o n g e r wavelengths. It was necessary t o c a l i b r a t e t h e r a t i o of emission i n t e n s i t y from t h e lamp which occured during t h e e x c i t a t i o n p u l s e a s a f u n c t i o n of wavelength. The wavelength i n t e g r a t e d lamp emission i n t e n s i t y s i g - n a l was monitored by an S-5 photodiode and, w i t h t h e c u r r e n t p u l s e , was d i s p l a y e d on a dual-beam o s c i l l o s c o p e f o r each s h o t . The lamp emission passed through t h e 1-meter long c e l l . The o u t p u tc o n v e r t d e n s i t y u n i t t o r e l a t i v e i n t e n s i t y a t each wavelength.
A s shown i n f i g u r e 1, i n electron-beam e x c i t e d argon plasmas a t 2 atm t r a n s i e n t a b s o r p t i o n extend- ed from 2400
8
t o 3600$
w i t h a g r a d u a l l y peaking a b s o r p t i o n a t 29802.
A broad b u t weaker absorp- t i o n peak was observed a t-
27002.
Strong l i n e and narrow band a b s o r p t i o n were observed f o rA
2
3500x.
A t 29808,
t h i s peak a b s o r p t i o n i s i n good agreement w i t h t h e p r e d i c t e d peak a b s o r p t i o n+
f o r t h e dimer i o n A r 2
.
The c a l c u l a t e d c r o s s sec- t i o n f o r t h i s a b s o r p t i o n peak was determined by measuring t h e a b s o r p t i o n w i t h a CMX-4 dye l a s e r tundd t o 30002.
Using a number d e n s i t y preddcted by a numerical model which included e l e c t r o n kin--!-
e t i c s , t h e c r o s s s e c t i o n f o r a b s o r p t i o n by A r 2 was t o be -0.7
g2.
T h i s v a l u e i s i n r e a s o n a b l e agreement w i t h t h e o t h e r e ~ p e r i r n e n t s ~ ' ~ and calcu- l a t i o n ~ ~ - ~ . The second peak ( a t 2700%)
i s a t t r i - buted t o a n o t h e r e x c i t e d - s t a t e s p e c i e s i n argon.spectrum was photographed on Kodak Royal-X Pan
$
nsoodI = 6.. A.m-2 p=2.,,"
Film u s i n g a Minuteman 0.5 meter s p e c t r o m e t e r w i t h I I ' t i t
a 1200 grooves/mm g r a t i n g blazed a t 2000
2
( - 32
t i t + + i f f t+ ~ i i i 1 f i 1 1 t
r e s o l u t i o n ) . I n r e c o r d i n g t h e a b s o r p t i o n spec- trum, a s i n g l e s h o t provided exposure on t h e f i l m . I n o r d e r t o compare p r o p e r l y a b s o r p t b n i n e l e c - tron-beam e x c i t e d r a r e g a s e s , one of t h e c r i t e r i a of t h e s e measurements i s t h a t t h e p r e s s u r e of t h e r a r e g a s e s w a s v a r i e d such t h a t energy d e p o s i t i o n i n t h e g a s i n each c a s e was comparable.
The s e n s i t i v i t y of t h e Royal-X Pan f i l m over t h e wavelength range i n v e s t i g a t e d v a r i e d by l e s s t h a n a f a c t o r of 2. A Grant microdensitometer was used t o c o n v e r t t h e i n f o r m a t i o n contained on t h e photographic f i l m i n t o a d e n s i t y exposure a s a f u n c t i o n of t h e wavelength. A c h a r a c t e r i s t i c c u r v e was formulated which provided a means t o
F i g u r e 1: T r a n s i e n t a b s o r p t i o n i n an argon plasma a s a f u n c t i o n of wavelengths.
I n neon plasma ( f i g u r e 2) a t 4 atm, a broad band a b s o r p t i o n weaker i n amplitude t h a n argon was observed. The a b s o r p t i o n band extends from 2400
2
t o 36002
w i t h a peak at-25302.
T h i s a b s o r p t i o n peak i s i n r e a s o n a b l e agreement w i t h t h e t h e o r e t - i c a l l y p r e d i c t e d peak a b s o r p t i o n c r o s s s e c t i o n f o r+
Ne2
.
Superimposed on t h i s broad a b s o r p t i o n band were severaT l i n e a b s o r p t i o n s . Of p a r t i c u l a ra r e a t t r i b u t e d t o t h e 3s-4p t r a n s i t i o n s i n neon. A b s o r p t i o n l i n e s were a l s o o b s e r v e d i n t h e r e g i o n around 3080
2.
These l i n e s a r e a t t r i b u t e d t o t h e 3s-5p t r a n s i t i o q i n neon. A s e l e c t i v e l y i n t e n s e background e m i s s i o n f r a m t h e e x c i t e d neon plasma was obsrved and s u b t r a c t e d o u t when t h e d a t a was reduced. NEON m 1 - M A s m - 'E
- p - 4 m m 5 m - i - F i g u r e 2: T r a n s i e n t a b s o r p t i o n i m a neon plasma a s a f u n c t i o n o f waveleng,th.When p u r e xenon a t a p r e s s u r e of 467 t o r r was e x c i t e d by a n e l e c t r o n beam, w e o b s e r v e d a band a b s o r p t i o n r e g i o n which extended f r ~ a 2500
2
t o 40001
w i t h a peak i n t h e v i c ; i n i t y of 34502.
The peak a b s o r p t i o n observed i s i n good agreement w i t h t h e p r e d i c t e d a b s o r p t i o n c r o s s s e c t i o n peak f o r-I-
Xe2
.
No e v i d e n c e of l i n e a b s o r p t i o n was observed o v e r t h e wavelength r a n g e i n v e s t i g a t e d ( f i g u r e 3 ) . ,.
0 L I I L 200 Jm, m, Y O O 3 9 m = U)m I lAwslronrl F i g u r e 3: T r a n s i e n t a b s o r p t i o n i n a xenon plasma a s a f u n c t i o n of wavelength. P r o b i n g of e x c i t e d k r y p t o n ( E i g u r e 4) a t one atmos- p h e r e r e s u l t e d i n a n asymmetrical band o f absorp- t i o n e x t e n d i n g from 24002
t o 3700g,
w i t h no w e l l pronounced peak which c o u l d b e c o r r e l a t e d t o t h e+
t h e o r e t i c a l l y e x p e c t e d peak f o r K r 2 a t 3200
2.
These p r e l i m i n a r y r e s u l t s i n d i c a t e t h e p o s s i b i l i t y t h a t s p e c i e s o t h e r t h a n t h e d ~ m e r i o n s may c o n t r i b - u t e t o a b s o r p t i o n i n t h i s p o r t i o n o f t h e wavelength spectrum, The peak c r o s s s e c t i o n f o r a b s o r p t i o n a t 32002
a g r e e d w e l l w i t h t h e p r e d i c t e d c r o s s sec- F i g u r e 4: T r a n s i e n t a b s o r p t i o n i n a k r y p t o n plasma a s a f u n c t i o n o f wavelength. Many r a r e g a s h a l i d e l a s e r c a n d i d a t e s e x h i b i t s t r o n g f l u o r e s c e n c e b u t do n o t o p e r a t e e f f i c i e n t l y as l o n g p u l s e l a s e r s . The d i f f i c u l t y i n improving l a s e r performance is n o t w i t h bhe f o r m a t i o n o f t h e e x c i t e d - s t a t e s p e c i e s b u t r a t h e r w i t h i n c r e a s M g t h e e x t r a c t i o n e f f i c i e n c y . By p r e s e n t i n g a n a c c u r a t e p h y s i c a l p r e p r e e n t a t i o n of t h e s e a b s o r b e r s and by i d e n t i f y i n g them, we hope t o f i n d ways 06 r e d u c i n g t h e a b s o r p t i o n a t t h e l a s e r wavelength. We are a t p r e s e n t improving t h e d a t a a n a l y s i s t e c h n i q u e and w i l l expand t h e q u a n t i t i a t i v e d a t a b a s e . We w i s h t o t h a n k P r . H. Powell of Lawrence Livermore L a b o r a t o r y f o r t h e l o a n o f h i s xenon f l a s h l a m p and f o ~ many h e l p f u l d i s c u s s i o n s . REFERENCES :1. R.S.F. Chang and L.F. Champagne, Appl. Phys. L e t t .
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879 (1980).2. W . J . S t e v e n s , 11. Gardner, A. Karo and P. J u l i e u n e , J . Chem. Phys.
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2860 (1977). 3 . W.R. Wadt, D.C. C a r t w r i g h t and J . S . Cohen,Appl.. Phys. L e t t ,
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672 (1977).4. H.H. M i c h e l s , R.H. Hobbs and L.A. Wright, J.
Chem. Phys.
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5053 (1979).5 . W.R. Wadt ( t o b e p u b l i s h e d ) J. Chem. Phys. (1 980)
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6. R.O. Hunter, J . O l d e n e t t e l , C. Howton and M.V. IlcCusker, F i n a l T e c h n i c a l R e p o r t , February
1977-November 1977, Maxwell L a b o r a t o r i e s (un- p u b l i s h e d ) .
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