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FEW-ELECTRON STATES OF Ne PRODUCED BY 1.4 MeV/amu HEAVY ION IMPACT
H. Beyer, F. Folkmann, K.-H. Schartner
To cite this version:
H. Beyer, F. Folkmann, K.-H. Schartner. FEW-ELECTRON STATES OF Ne PRODUCED BY 1.4 MeV/amu HEAVY ION IMPACT. Journal de Physique Colloques, 1979, 40 (C1), pp.C1-17-C1-19.
�10.1051/jphyscol:1979105�. �jpa-00218384�
JOURNAL DE PHYSIQUE Colloque C1, suppl6ment au n
"
2, Tome 40, fivrier 1979, page C1-17FEW-ELECTRON STATES OF Ne PRODUCED BY 1.4 MeV/amu HEAVY ICN IMPACT*
t x
+
H.F. Beyer
,
F. Fol kmann and K.-H. schartnerX ' 6 ~ 1 , Postfach 110541, 0-6100 Darmstadt, 11, W-GermanyX I . Physikalisches I n s t i t u t der Justus Liebig.Universitat D-6300 Giessen, W-Germany
RESUME.
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Pour gtudier l e s @ t a t s atomiques fortement ionis6s nous avons bombard6 une c i b l e gazeuse de Ne avec des ions t r e s lourds d t ~ r 1 2 + , ~ i ' ~ + , W16', ~ r ' ~ + , ~ e e t ~~ ~b accgl6r6s +~ ~ +d 1,4.MeV/u.m.a. par l ' a c c @ l @ r a t e u r URILAC de GSI. En u t i l i s a n t un spectromstre avec un c r i s t a l courb6 nous avons mesur@ l e s rayons X , qui e t a i e n t h i s par des ions Ne reculant lentement. Cela permet de peupler l e s memes @ t a t s atomiyues qui sont observgs en beam f o i l spectroscopy e t dans l ' e x c i t a t i o n des plasmas mais en ne l l & t u - ciiant cjue s u r une c o l l i s i o n 6 l a f o i s . Les spectres montrent surtout des r a i es correspondant 6 des confi- gurations 6 t r o i s , deux ou u n seul 6lectron du ?de avec un degr6 d'ionisation croissant pour l e s ions plus 1 ourcis.ABSTRACT.
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To study highly ionized atomic s t a t e s , we have bombarded a Ne gas t a r g e t with very heavy ions,&2+ Ti14+
-
16+,
,
rJi,
~ r l ~ + , XeZ4' and ~b~~~ of s p e c i f i c energy 1.4 BeV/amu from the' UNILAC a t GSI, and measured the K x-rays emitted from the slowly recoiling neon ions with a curved crystal spectrometer. This represents a well defined way of populating the same atomic s t a t e s , which a r e observed in beam f o i l spec- troscopy and plasma e x c i t a t i o n , but under near t o s i n g l e c o l l i s i o n conditions. The spectra exhibit mainly l i n e s from t h r e e , two and one electron configurations of Ne with an increasing degree of ionization f o r the heavier projecti 1 es.1. INTRODUCTICN.
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In atomic c o l l i s i o n s creation of few electron s t a t e s has mainly been studied i n the f a s t of l i g h t ions excited by a t h i n f o i l . In a recent l e t t e r /1/ we have shown t h a t a beam of very heavy f a s t ions available nowadays from modern accelerators i s capable of producing these highly ionized s t a t e s a l s o in a l i g h t t a r g e t gas l i k e neon.These highly charged and very slow t a r g e t neon ions are the subject of the present investigation, where the K x-ray spectra emitted from these ions are studied.
2. EXPERIMENT.
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The heavy ions ~ r l " , ~ i ' ~ + , i1il6', Kr18+, ~ e and ~~ ~b were accelerated by the +~ ~ + UNILAC a t GSI t o a s p e c i f i c energy of 1.4 MeV/amu and were focussed i n t o a d i f f e r e n t i a l l y pumped gas t a r g e t t h e pressure of which was about G . l mbar.The high resolution (FWHM 5 5 eV) x-ray spectra were measured with a Johannson type curved crystal spectrometer with the 5" Rowland c i r c l e oriented a t r i g h t angle w i t h respect t o the ion beam d i - rection. A RbAP crystal was used f o r analyzing t h e neon K x-rays. The photon detector was an Ar.CH4 flow mode proportional counter with a thin ( < lum) polypropylene window.
3. RESULTS AND DISCUSSION.
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In Fig. 1 a r e shown the Ne K x-ray spectra obtained f o r the impact of~ i l ~ + , ~ r ' ~ + a n d ~ b ~ ~ + . The 1 ines o r i g i n a t e from i n i - t i a l configurations as indicated i n the figure. For l i n e i d e n t i f i c a t i o n t r a n s i t i o n energies have been calculated using a mu1 ticonfigurational Dirac-Fock program /2/. The heavy p r o j e c t i l e s leave i n nearly a l l the cases two o r l e s s electrons on t h e t a r g e t L shell when the K s h e l l gets ionized. This i s con- cluded from the l i n e s originating from t h r e e , two and one electron configurations. Contributions from four electron s t a t e s a r e very weak and get reduced by an order of magnitude when going from ~ r l " t o
Kr18+ impact. They a r e completely l o s t in t h e ~ e ~ ~ + and ~ b induced spectra. The degree of ionization ~ ~ +
i s strongly increased when the p r o j e c t i l e ' s atomic
number and charge i s increased going from ~ rt o ~ ~ +
~ b ~ ~ + . The contribution from H-like Ne i s raised from 6% up t o 20%.
In p a r t i c u l a r the following t r a n s i t i o n s a r e ob- served: ( l s 2 p ) np -+ I s 2p 2
( l s 2 s ) np -+ 1s 2s 2 ( 1 s ) np -+ I s 2
np -+ 1s with n=2
...
6Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979105
JOURNAL DE PHYSIQUE
ENERGY / keV
0 90 1 W 1 10 1 2 0 1 3 0
I I I I I I I l l 1
F i g . 1: Ne K x-ray spectra normalized t o t o t a l i n - t e g r a l i n t e n s i t y
I n t a b l e I t h e x-ray i n t e n s i t y r a t i o o f t h e P t o 4 'P s t a t e s , belonging t o t h e lsSs2p and l s 2 p c o n f i - 2
gurations, i s considered. From t h e o r e t i c a l f l u o - r e s a n c e . y i e l d c a l c u l a t i o n s /3,4,5/ and t h e
assumption o f a s t a t i s t i c a l p o p u l a t i o n o f mu1 t i p l e t s t a t e s w i t h i n a g i v e n c o n f i g u r a t i o n a 4 ~ / 2 ~ r a t i o o f 0.88 i s d b t a i n e d f o r t h e ls2s2p c o n f i g u r a t i o n and 0.13 f o r t h e l s 2 p c o n f i g u r a t i o n . For a d i s - 2
t r i b u t i o n between ls2s2p and 1szpL according t o t h e i r s t a t i s t i c a l weights a mean value o f 0.38 i s expected which i s f a r a p a r t from t h e experimental observations which a r e between 0.9 and 2.0. Matt- hews e t a l . /6/ have measured t h e neon s a t e l l i t e spectrum f o r 1.3 MeV/amu cl13++ Ne c o l l i s i o n s which
agrees w i t h o u r argon induced spectrum. They 4 2
measured a P/ P r a t i o o f 0.90 and claimed a com- p l e t e underpopulation o f t h e l s 2 p c o n f i g u r a t i o n 2
b u t a s t a t i s t i c a l p o p u l a t i o n o f t h e m u l t i p l e t s t a t e s w i t h i n t h e ls2s2p c o n f i g u r a t i o n which would per- f e c t l y agree w i t h t h e e x p e c t a t i o n o f 0.88.
Table 1. 4~ / 'P i n t e n s i t y r a t i o o f t h e L i - l i k e s a t e l l i t e s
- -- -
S t a t i s t i c a l expectation Experimental observation
l s 2 ~ 2 p 1 s Mean ~ ~ ~c113+ ~ r~ ~1 N i l 6 + Kr18+ xeZ4+ ~~ ~+ '
0.88 0 . 1 3 0 . 3 8 0 . 9 0 ~ 0 . 9 3 1.07 1.02 1.45 1.91 1.99 1 . 0 5 ~ 0.43'
areference 6
bwith ~ n c l u s i o n o f an = 0 t r a n s i t i o n s
As o u r experimental f i n d i n g s exceed t h i s v a l u e by up t o a f a c t o r o f 2.2 t h e above e x p l a n a t i o n cannot h o l d a t l e a s t n o t f o r t h e impact o f t h e h e a v i e r pro- j e c t i l e s . I n c l u s i o n o f t h e l s z p 2 4~ + ls2s2p 4 P
cascading t r a n s i t i o n s /5,7,8/ w i l l enhance t h e s t a - 4 2
t i s t i c a l e x p e c t a t i o n o f t h e P/ P r a t i o by o n l y 15%
(see t a b l e I).
The h i g h photon energy p a r t o f t h e s p e c t r a con- t a i n s an appreciable amount o f i n t e n s i t y from con- f i g u r a t i o n s ( l s 2 g ) n p w i t h n 2 3. The c o n f i g u r a t i o n s ( l s 2 t ) n l have a l s o t h e c a p a b i l i t y o f decaying by L x-ray emission and thereby f e e d i n g t h e L i - l i k e sate1 1 i t e s
.
From r e c e n t fluorescence y i e l d c a l cu- l a t i o n s /9/ i t i s concluded t h a t about 35% o f t h e decay o f t h e (ls2;)31 s t a t e s occurs v i a L x-ray emission and t h a t n e a r l y a l l o f t h e L x-ray i n t e n - s i t y i s f e d i n t o t h e q u a r t e t s t a t e s o f t h e ls2s2p and l s 2 p c o n f i g u r a t i o n s . Assuming s i m i l a r con- 2 d i t i o n s t o h o l d f o r t h e o t h e r L i - l i k e s t a t e s w i t h n > 3 t h e cascading L x-ray i n t e n s i t y i s estimated t o be i n t h e same order as t h e K x-ray i n t e n s i t y observed i n t h e present experiment. That would4 2
completely e x p l a i n t h e b o o s t i n g o f t h e P/ P r a t i o . W i t h i n t h e L i - l i k e l i n e s e r i e s a v e r y h i g h i n - t e n s i t y from n=4 s t a t e s i s observed which has l e d us t o propose second c o l l i s i o n processes t o be r e s - p o n s i b l e f o r p r e f e r e n t i a l l y c r e a t i n g t h e n=4 s t a t e s /I/. Another o b s e r v a t i o n i s an e x t r a o r d i n a r i l y h i g h i n t e n s i t y from t h e 6p + 1s t r a n s i t i o n w i t h i n t h e
hydrogen-like s e r i e s which can be seen i n t h e Pb- induced spectrum o f F i g . 1. Both f a c t s s t r o n g l y suggest t h a t these s t a t e s a r e n o t d i r e c t l y popu- l a t e d b u t t h a t t h e r e e x i s t l o n g l i v e d metastable c o l l e c t o r s which may capture an e l e c t r o n from a surrounding t a r g e t atom i n a second c o l l i s i o n . Bare n u c l e i , t h e l s 2 s metastable core and even t h e l s 2 p 3 ~ 0 , 2 s t a t e s w i t h l i f e t i m e s o f r o u g h l y sec w i l l a c t as such c o l l e c t o r s . Though l i t t l e theore- t i c a l work i s r e l a t e d t o these e x o t i c c o l l i s i o n systems o f extremely slow and h i g h l y i o n i z e d ex- c i t e d i o n s some basic ideas may be obtained from t h e o r e t i c a l i n v e s t i g a t i o n s performed a t h i g h e r im- p a c t v e l o c i t i e s /10,11/. I t i s expected t h a t e l e c - t r o n s a r e captured s e l e c t i v e l y i n t o h i g h l y i n g atomic she1 1s w i t h cross s e c t i o n s exceeding even 1 0 - l ~ cm2.
Besides t h e above mentioned observations d i r e c t evidence f o r s e l e c t i v e e l e c t r o n c a p t u r e i s g i v e n by a d d i t i o n a l experiments /12/ performed w i t h t h e same primary f a s t ions b u t w i t h t a r g e t gases c o n s i s t i n g o f m i x t u r e s o f neon and a v a r i e t y o f o t h e r gases as CH4, H2, Ar o r He, r e s p e c t i v e l y . By t h i s means a secondary c o l l i s i o n system i s o b t a i n e d c o n s i s t i n g o f t h e s l o w l y r e c o i l i n g i o n i c cores impinging on t h e surrounding atoms o r molecules. Depending on t h e admixtured gases w i t h defined i o n i z a t i o n poten- t i a l s d i f f e r e n t t r a n s i e n t molecules a r e formed i n t h e d i f f e r e n t experiments. I n F i g u r e 2 t h e r e s u l - t i n g e f f e c t i s demonstrated w i t h an i n t e n s e beam o f
~ ri o n s h i t t i n g a pure Ne t a r g e t and r e s p e c t i v e l y ~ ~ + a m i x t u r e o f equal p a r t s o f Ne and CH4. The l i n e
o r i g i n a t i n g from t h e ( l s 2 1 ) 4 p c o n f i g u r a t i o n i s d r a s t i c a l l y reduced when going from t h e Ne- t o t h e Ne-CH4 t a r g e t . The l o s t i n t e n s i t y from t h e n=4 s h e l l i s now found i n t h e n=5 s h e l l as t h e ( l s 2 1 ) 5 p i n t e n s i t y i s r a i s e d . A completely analogue obser- v a t i o n i s made i n t h e H - l i k e s e r i e s . The n=6 s h e l l i s populated i n case o f t h e Ne t a r g e t and t h e n=7
s h e l l i n case o f t h e Ne.CH4 t a r g e t . I n t h e ( ~ e - ~ e ) ~ ~ and ( ~ e - ~ e ) ' " system e l e c t r o n s a r e captured i n t o
t h e n=4 and n=6 s h e l l , r e s p e c t i v e l y whereas i n case o f ( P 4 e - c ~ ~ ) ~ ' and ( N ~ - C H ~ ) " + th e s h e l l s w i t h n-5 and n=7 a r e i n v o l v e d .
ENERGY / keV
,
1:s 1y 1: 1;o 1;s 1:o 17,
WAVrLENGTH / nm
F i g . 2: Ne s p e c t r a from Ne and Ne.CH4
*Part o f t h e s i s H.F. Beyer, U n i v e r s i t y o f Giessen
References :
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and Mokler, P.H., J. Phys. - B l l ( 1 9 7 8 ) L363.
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J. Phys.-
B11 (1978) L359.(1975) 959. /9/ Chen, M.H. Phys. Rev.
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(1977) 2318./4/ Bhalla, C.P., Phys. Rev. (1975) 122.
/ l o /
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,
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/11/ Olson, R.E. and Salop, A., Phys. Rev.A16
/6/ Matthews, D.L. and Fortner, R.J., Phys. Rev. (1977) 531 and references contained t h e r e i n . A14 (1976) 1561.
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