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ULTRAVIOLET ABSORPTION OF REFRACTORY ELEMENTS BY A DUAL LASER PLASMA METHOD
J. Costello, W. Lynam, P. Carroll
To cite this version:
J. Costello, W. Lynam, P. Carroll. ULTRAVIOLET ABSORPTION OF REFRACTORY ELE-
MENTS BY A DUAL LASER PLASMA METHOD. Journal de Physique Colloques, 1988, 49 (C1),
pp.C1-243-C1-246. �10.1051/jphyscol:1988151�. �jpa-00227471�
JOURNAL DE PHYSIQUE
Colloque C1, SupplQment au n 0 3 , Tome 49, Mars 1988
ULTRAVIOLET ABSORPTION OF REFRACTORY ELEMENTS B Y A DUAL LASER PLASMA METHOD
J.T. COSTELLO, W.G. LYNAM* and P.K. CARROLL*
School of Physical Sciences, NIHED., Dublin 9, Republic of Irel and
'physics Department, University College, Dublin 4, Republic of Irel and
A b s t r a c t
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The dual laser-produced plasma technique f o r t h e study o f i o n i c absorption spectra has been developed by t h e use o f two Q- switched ruby 1 asers t o enable independent generation o f t h e absorbing and b a c k - l i g h t i n g plasmas. O p t i c a l p u l s e handling i s used i n t h e c o u p l i n g c i c u i t s t o enable r e p r o d u c i b l e p u l s e delays from 250 nsec. t o 10 msec. t o be achieved. A t delay times > 700 nsec. spectra o f e s s e n t i a l l y pure n e u t r a l species a r e observed. The technique i s valuable, n o t o n l y f o r o b t a i n i n g t h e n e u t r a l spectra o f h i g h l y r e f r a c t o r y and/or c o r r o s i v e m a t e r i a l s b u t a l s o f o r studying behaviour o f i o n i c species as a f u n c t i o n o f time. Typical spectra a r e shown i n Fig. 1.I
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INTRODUCTIONThe f i r s t experiment i n which a laser-produced continuum was used t o study t h e absorption spectra o f i o n s generated i n a second laser-produced plasma was c a r q e d o u t by C a r r o l l and Kennedy ( 1 ) i n 2 1977 t o r e c o r d t h e spectrum of L i
.
I n t h i s work t h e a u t o i o n i z i n g I s-
2snp resonances ( t h e analogue o f the Madden-Cod1 i ng s e r i e s o f he1 i urn) were observed and t h e i r p o s i t i o n and p r o f i l e s compared w i t h theory ( 2 ) . The success o f t h e technique depended on the a v a i l a b i l i t y o f c l e a n laser-produced c o n t i n u a ( 3 ) and on t h e s u i t a b i l i t y o f t h e pulsed source f o r synchronized o r time r e s o l v e d s t u d i e s (4). I n t h e o r i g i n a l experiment t h e o u t p u t o f a s i n g l e ruby l a s e r was d i v i d e d by a beam s p l i t t e r t o produce b o t h b a c k l i g h t i n g and absorbing plasma. A l i m i t e d degree o f t i m e r e s o l u t i o n was achieved by v a r y i n g t h e o p t i c a l path l e n g t h s and some c o n t r o l o f t h e absorbing species, i.e. i o n stage, by a d j u s t i n g t h e focus o f t h e beam generating t h e absorbing plasma+. The technique has since been a p p l i e d s u c c e s s f u l l y t o t h e spectrum o f Be where by t h e use o f r e l a t i v e l y l o n g p a t h l e n g t h s time delays up t o 70 nsec. were a t t a i n e d (5).I 1
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TWO LASER SYSTEMI n s t e a d o f using a s i n g l e l a s e r and beam s p l i t t e r we have r e c e n t l y developed a two l a s e r system ( 6 ) which has t h e advantages o f ( a ) d e l i v e r i n g enhanced l a s e r power on each t a r g e t and ( b ) p r o v i d i n g a g r e a t e r range i n time r e s o l u t i o n . With the system i t i s p o s s i b l e n o t o n l y t o study v a r i o u s i o n stages b u t also, by u s i n g s u f i c i e n t l y l o n g time delays, t o observe absorption spectra o f n e u t r a l species f r e e from i o n i c c o n t a d n a t i o n . This i s p a r t i c u l a r l y u s e f y l i n studying r e f r a c t o r y o r c o r r o s i v e elements which are d i f f i c u l t o r impossible t o o b t a i n i n v a p o u r by more conventional methods. As an example we show i n Fig. 2 t h e 5 d - 5 , ~ f g i a n t resonance o f UI ( 7 ) together w i t h t h e t h e o r e t i c a l p r o f i l e as c a l c u l a t e d by Wendin (8).
S i m i l a r r e s u l t s were obtained f o r t h e analogous spectrum o f atomic t h o r i um which was observed i n absorption f o r t h e f i r s t time w i t h t h e new technique (6).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988151
Fig 1. TIME-RESOLVED XUV ABSORPTION SPECTRA OF BERYLLIUM.BORON,SILlCON AND STRONTIUM PLASMAS. 1s-2p,3p Transitions.
60 65
2s2np ('PIMA)
Bell ls22s (2S)-l~2~~p (2~)1I
(1) 2CW/cm1,750ns. (b) Boron. Time Delay:500ns. (2) 2CW/cn-?,4OOnsI
2p-ns,nd Transitions.1 100 110
I120
I
85 90
(c)Silicon. Time Delay:250ns.A(A)
(d) Strontium.Time Delay:425ns.MA)
The experimental l a s e r system i s shown schematically in Fig 3. The two ruby l a s e r s ( n o t i d e n t i c a l ) a r e triggered so t h a t the pumping pulses peak simultaneously, (they may be o f f s e t i f delays
>1000 nsec. a r e required).
Part of the Q-switched output pulse of the l a s e r which generates the absorbing plasma i s passed through a variable time delay c i r c u i t and used to a c t i v a t e the Pockels c e l l u n i t of the second l a s e r . The output from both l a s e r s i s %1.5J i n 25 nsec. and the repetition r a t e of the system a s a whole i s limited by the performance of the slower l a s e r
(Q 1ppm). Inter- plasma delay times,
A t ,range from 250 nsec (our present minimum delay) t o 10 msec. For the l a r g e r
A tvalues
( >1000 nsec.) an appropriate delay i s introduced in the pumping pulses t o ensure t h a t maximum output i s available a t time of observation.
Theory F i g . 2. Absorption o f atomic
.
.
. . ..
. ..
U Atomic uranium i n the v i c i n i t yo f t h e 5d t h r e s h o l d .
Photon Energy (ev).
To improve the r e l i a b i l i t y of the original two l a s e r system ( 6 ) the timing electronics have been redesigned t o secure noise immunity. The current system uses a simp1 i f i e d trigger-pul s e delay generator, f i b r e optic coupling f o r the flash-tube t r i g g e r pulses and a high speed optical t r i g g e r u n i t t o a c t i v a t e the Pockels c e l l of the second l a s e r . As a r e s u l t a 100%
reproducibility in shot t o shot time delays has been achieved.
In Fig.
1we show spectra, some new, t o i l l u s t r a t e the c a p a b i l i t y of t h e system. In, t h e case of beryllium, a t a time delay of 400 nsec.
absorption of Be appears with our i n t e n s i t y comparable t o t h a t of the neutral [Fig. l a
(2,311whereas a t at
=750 nsec. [Fig. l a (111 i t i s absent except f o r a t r a c e of the 1s-2p t r a n s i t i o n which completely disappears a t s t i l l longer delays. In early work
(1)when only short time delays were available lower ion stages were favoured by defocusing the beam generating the .absorbing plasma but
w i t hthe new system t i g h t focusing [Fig.
1
(31, ~t
=400 nsec.1 may s t i l l be used; the crucial f a c t o r in reaching lower ion stages
i sthe a v a i l a b i l i t y of longer time delays.
F i g . 3 . Schematic diagram o f t h e t w i n - l a s e r system.
? MP-master t r i g g e r p u l s e ; G - t r i g g e r p u l s e d e l a y
FI[
..,B generator- T1 /T2 f l a s h t u b e t r i g g e r p u l s e s ; C1 /C2- R
1 - - ,
PI
p. ' h i g h v o l t a g e charge1Tri gger c i r c u i t s ; F1 /FZ-
flashlamps; Rl/RZ-ruby rods; Pl/P2-Pockels c e l l ;
PU1 /PU2-Pockel s c e l l d r i v e u n i t s ; '&beam s p l i t t e r ; L / A - f i b r e - o p t i c a l l y coupled t r i g g e r u n i t ; D-delay
C2 u n i t - 01 102-9-swi tched l a s e r p u l s e s .
In Fig. 1
( b )we show the 1s-2p,3p absorption of neutral boron, the 2s
absorption of which had previously been observed in the normal incidence
region
(9).Fig 1 ( c ) shows the Zp-ns,nd t r a n s i t i o n s of s i l i c o n . The
spectrum, here recorded f o r the f i r s t time, i s of some theoretical i n t e r e s t
because of the predictions of Daum and Kelly (10). Although analysis has
not y e t been carried out t h e dominant l i n e s a r e believed t o be due t o Si I
and 11. In Fig.
1( d l we show the spectrum of strontium a t a delay of
425nsec. The longer wavelength group of l i n e s corresponds t o the 3d-5p,6p
t r a n s i t i o n s previously observed by Mansfield and Connerade (11) and those a t
shorter wavelength comprise t h e 3d t r a n s i t i o n s of S r I I , which a r e
isoelectronic with the known 3d spectrum of
Rbl(12).
C1-246 JOURNAL
DE
PHYSIQUEI 1 1
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CONCLUSIONThe two l a s e r technique i s n o t o n l y e f f e c t i v e i n c o n t r o l l i n g t h e i o n stages observed i n dual plasma a b s o r p t i o n b u t a t s u f f i c i e n t l y l o n g t i m e delays enables pure n e u t r a l absorption t o be studied. I n p a r t i c u l a r r e f r a c t o r y and c o r r o s i v e m a t e r i a l s , whose atomic spectra have i n some cases n o t y e t been observed, are now r e a d i l y accessible t o study. It should a1 so prove u s e f u l i n the case o f o t h e r elements such as sulphur o r phosphorus whose vapour phase tends t o be dominated by molecular species.
This work was sllpported by NBST ( I r e l a n d ) Research Grant SC/131/86.
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2373, (1976) G.R. Daum and H. K e l l y , Phys. Rev. 13A, 715 (1976)M.W.D. M a n s f i e l d and J.P. C o n n e r a d e 3 r o c . Roy. Soc. A342, 421 (1975) M.W.D. M a n s f i e l d and J.P. Connerade, Proc. Roy. Soc.
