SELECTIVE ELECTRON CAPTURE EFFECTS AT HIGHLY IONIZED Ne TARGET ATOM AFTER 1.4 MeV/amu HEAVY ION IMPACT

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SELECTIVE ELECTRON CAPTURE EFFECTS AT HIGHLY IONIZED Ne TARGET ATOM AFTER 1.4

MeV/amu HEAVY ION IMPACT

R. Mann, F. Folkmann

To cite this version:

R. Mann, F. Folkmann. SELECTIVE ELECTRON CAPTURE EFFECTS AT HIGHLY IONIZED

Ne TARGET ATOM AFTER 1.4 MeV/amu HEAVY ION IMPACT. Journal de Physique Colloques,

1979, 40 (C1), pp.C1-236-C1-238. �10.1051/jphyscol:1979148�. �jpa-00218428�

JOURNAL DE PHYSIQUE Colloque C1, supplkment au n o 2, Tome 40, fkvrier 1979, page C1-236

SELECTIVE ELECTRON CAPTURE EFFECTS AT HIGHLY IONIZED Ne TARGET ATOM AFTER 1.4 MeV/amu HEAVY ION IMPACT

R. Mann and F. Folkmann

GSI, Postfach 541, D6100 Darmstadt, W-Germany

RESUME. On a observe une f o r t e influence de l a pression de l a c i b l e gazeuse s u r l e s i n t e n s i t e s de r a i e s Auger dues 1 des t r a n s i t i o n s dans l e s couches externes d'atomes de neon fortement ionises par l'impact d'ions de Xenon ( ~ e ~ ~ ' ) e t d'argon (Ar12+) acc6leres ii 1,4 MeV par nucleon. Les i n t e n s i t e s des r a i e s dues 1 des t r a n s i t i o n s dans l e s courbes externes de systemes 1 3 ~ l e c t r o n s ( l s Z s , p n l ) (n=4,5,6 ; 1=0,1,2,3) dependent d'une fagon systematique du potentiel d ' i o n i s a t i o n de d i f f e r e n t s qaz ajoutes 1 l a c i b l e gazeuse. Cette observation indique que l a capture, par des atomes neutres environnants,,d1&lec- trons d'atomes fortement ionises de l a c i b l e e s t un processus important dans l ' e x c i t a t i o n par impact d ' i o n s lourds de haute energie d'atomes l 6 a e r s .

ABSTRACT.

-

I n t e n s i t i e s of Auger lines from outer s h e l l configurations of Lithium l i k e Neon atoms induced by xeZ4+ and Arl" (1.4 MeV/u) impact show, t h a t these configurations a r e not populated following a n-3 r u l e . A s e l e c t i v e change of l i n e i n t e n s i t i e s from t r a n s i t i o n s of outer s h e l l s of 3-electron systems (lsZs, p nl)(n=4,5,6; 1=0,1,2,3), depending systematically on the ionization potential of d i f f e r e n t gases mixed

t o the t a r g e t gas, i s observed. This indicates t h a t electron capture of highly ionized t a r g e t atoms from the surrounding neutral species i s an important process of the excitation of l i g h t t a r g e t atoms by high energy heavy ion impact.

3. RESULTS AMD DISCUSSION.

-

Figure 1 e x h i b i t s the 1. INTRODUCTION.

-

Auger electron spectra from Neon

Auger spectra of Ne atoms induced by xeZ4+ projec- gas t a r g e t s a f t e r highly energetic heavy ion impact

t i l e s f o r pure Ne gas ( a ) and a mixture with CH4;

were recently measured and investigated by several _I

authors /1,2,3/. I t was found

/a

t h a t i n the Neon

I I I I I

_

~ e ~ ~ + - Ne

atom a f t e r bombardment with 1.4 MeV/amu xe31f ions

•

mostly t r a n s i t i o n s from three electron configura- L4 mTorr

tions were excited. An unexpectedly strong popula- ^I

T4

tion of the n = 4,5,6 s h e l l s compared t o n = 3 was

I 11 1 + ^I

observed. I t was not considered so f a r , whether these s t a t e s were d i r e c t l y populated during t h e pro- j e c t i l e t a r g e t atom c o l l i s i o n or secondarily by e.g.

electron capture processes i n the t a r g e t gas. An

investigation of such electron capture processes i s 5 the subject of t h i s experiment.

2. EXPERIMENT.

-

Ne gas t a r g e t s were bombarded with 1.4 MeV/amu and xeZ4* ions from the UNILAC. A

4

hemispheric e l e c t r o s t a t i c electron energy analyzer

5 .d _I!

( 2 E = 1.0 eV and QE = 2.0 eV FWHM) measured the Au- ^O

I 11 11 1 I

ger electrons emitted from the t a r g e t atoms under observation angles of 90' < o < 156' with respect t o the p r o j e c t i l e direction. The pressures i n the t a r g e t region were 185 mTorr, 46 mTorr, and 15 mTorr.

Other gases with d i f f e r e n t ionization p o t e n t i a l s a s CH4 and d were mixed i n t o the t a r g e t gas Ne.

-

Electron Energy lev1

Fig. 1: Ne-Auger spectra induced by xeZ4+(1 . 4 M ~ V / U ) impact on pure Ne gas ( a ) and Ne

+

^CH4 mixture ( b ) .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979148

Most of the l i n e s a r e a t t r i b u t e d t o the decay of three electron configurations with one K-shell electron, an 2s o r 2p electron and an electron in a n = 2,3,. ..or 7 o r b i t a l (denoted as nx), which agree with calculated t r a n s i t i o n s energies of these configurations using a mu1 ticonfigurational Dirac-Fock program /4/. While in the pure Ne case ( a ) the

n i

l i n e i n t e n s i t y from (ls2p 4p,d,f) ^-t 1s 2

+

e- t r a n s i t i o n s prevail in the upper part of the spectrum t h e n6-intensity from (ls2s6p,d,f) + 1s 2

+

e- t r a n s i t i o n s increases by a f a c t o r 3,5 (becom- ing dominant) and the n4 and n5-intensities de- crease by f a c t o r s of 1,9 and 1,5, i f CH4 gas i s mixed t o Ne ( b ) . This may be explained by electron capture from neutral t a r g e t atoms or molecules in- t o outer s h e l l s of highly stripped He-like t a r g e t atoms, which have an extremely slow recoil energy Er(z 10 eV) /5/ induced by the f a s t heavy ion col- l i s i o n . A capture cross section 0, = 1.4 x 10-l5 cm 2 r e s u l t s from a 40 mTorr t a r g e t pressure and a 0.5 cm f r e e path length

x

of the highly stripped t a r g e t atoms in the t a r g e t region accepted from tile spectrometer. Here only the s e t a s t a b l e core s t a t e ( l s 2 s ) 3 S (Lifetime T = 91Hs) survives a f r e e f l i g h t time of about 1 0 - ~ s , given by the experi- mental E r and

x

values. Fig. 2 shows Ne Auger spec- t r a induced by impact on pure Ne gas ( c ) and on mixtures with He a t a high t a r g e t pressure ( d ) . In contrast t o Fig. 1 the spectra excited with Ar12+ p r o j e c t i l e s exhibit higher r e l a t i v e i n t e n s i - t i e s of the K-LL t r a n s i t i o n s a t 652 eV t o 720 eV and of the n3 1 ines compared t o

n i ,

^n5, n6 l i n e s because a lower ionization of Ne by ~ r l " impact and therefore a higher d i r e c t population of Li-li ke and partly Be-1 i ke configurations/Z/. A1 though a strong increase of r e l a t i v e i n t e n s i t i e s (normalized t o n3) of the n4 l i n e a t 850 eV and a l i n e Be a t 717 eV from a ( 1 ~ 2 . ~ 2 ~ ~ ) ^-t ( l s 2 2 s )

+

e- t r a n s i t i o n f o r mixtures with He demonstrates a s e l e c t i v e elec- tron capture e f f e c t . As the ionization p o t e n t i a l s I of the neutral t a r g e t gases (CH4, Ne, He) in- crease (Tab. I ) , the electrons a r e captured syste- P matical l y t o "innerl'shell s (n6.. .n4) of higher binding energies Eb(Fig. l b , a , Fig. 2d). In con- t r a s t t o the n4, n5, n6 l i n e s which a r e a t t r i b u t e d t o s t a t e s with the metastable ( 1 ~ 2 s ) ~ ~ core confi- gurations,the

n;i

l i n e , strongly excited i n pure We gas, agrees with calculated t r a n s i t i o n energies of s t a t e s having t h e core configuration ( l s 2 p ) of a l i f e t i m e T ^Q 9 ns. Because of the slow recoil ve-

l o c i t y of the t a r g e t atoms, a molecular orbital pic- t u r e could be applicable t o describe the capture process, i n which additionally t o the electron cap- t u r e i n t o an outer shell the 2s core electron may be transferred t o the 2p s t a t e f o r c e r t a i n c o l l i - sion systems. A similar description of electron capture processes f o r the c o l l i s i o n system 08+ -+ H i s given by Hare1 and Salin /6/ who predicts a se- l e c t i v e popwlation of the Oxygen outer shell orbi- t a l ( n 4 ) , where the binding energy i s greater than the ionization potential of H .

-

Electron Energy IeVl Fig. 2: Ne-Augerspectra a f t e r impact of Ar 12+

(1,4 YeV/u) ions on Ne ( c ) and Ne

+

He mixture ( d ) . The population of the i n i t i a l s t a t e of the Be-line of Fig. 2d a r i s i n g a t high t a r g e t pressure i s ex- plained by an.electron capture process (ls2s2p) 4 P

+

e- -t (ls2s2p 2 ), which quenches the metastable ( l s 2 ~ 2 ~ ) ~ ~ - + l s 2

+

e- t r a n s i t i o n a t 656,2 eV. A quenching of the P-state was f i r s t observed from 4 S e l l i n e t a l . /7/ who measured the lifetime of the

C1-238 JOURNAL DE PHYSIQUE

4 ~ 5 / 2 s t a t e and who found a decreasing l i f e t i m e ( T % 10ns) w i t h i n c r e a s i n g Ne t a r g e t pressure, b u t t h e quenching process was n o t understood i n d e t a i l . From a r e c o i l energy Er = 10 eV, T c Ions, p 2, 185 mTorr a capture c r o s s s e c t i o n o f % 1.5 x 10-14cm2 i s c a l c u l a t e d w i t h some u n c e r t a i n t i e s i n Er and i n p which was measured w i t h an i o n gauge. However, i t i s n o t c l a r i f i e d y e t whether t h e e l e c t r o n i s cap- t u r e d d i r e c t l y i n t o the 2p s h e l l o r whether i t cas- cades t o 2p from an o u t e r she1 1 i n which i t was p r i m a r i l y captured.

T h i s experiment demonstrates t h e importance o f a secondary e l e c t r o n capture from t h e surrounding t a r g e t gas, producing s p e c i f i c s p e c t r a l l i n e s o f h i g h l y i o n i z e d t a r g e t atoms by heavy i o n impact.

The procedure o f adding atoms w i t h s e l e c t e d i o n i - z a t i o n p o t e n t i a l s t o h i g h l y s t r i p p e d slow t a r g e t atoms, combined w i t h t h e conventional pressure v a r i a t i o n can be used as a t o o l f o r o b t a i n i n g more d e t a i l e d i n f o r m a t i o n on e l e c t r o n capture processes and f u r t h e r t o d i s t i n g u i s h between a d i r e c t and a secondary p r o d u c t i o n o f s p e c t r a l 1 i nes

.

Table 1: I o n i z a t i o n p o t e n t i a l s I o f d i f f e r e n t gases and b i n d i n g energies Eb o f t h e n x - s h e l l e l e c - P t r o n o f Ne w i t h a c o n f i g u r a t i o n C.

I,(eV) E,,(eV) n, gas C

REFERENCES:

111 Mann, R., Folkmann, F., i n d Groeneveld, K.O., Phys. Rev. L e t t .

37

1674 (1976)

/2/ S t o - l t e r f o h t , N. Schneider, D., Mann, R., and Fol kmann, F., J . Phys. L281 (1977) /3/ Schneider, D., Moore, C.F., and Johnson, B.M.,

J. Phys. L153 (1976)

/4/ Desclaux, J.P., Comput. Phys. Commun.

9

^{31 (B75)}

/5/ Mann, R., Folkmann, F., Peterson, R.S., Szabo, Gy., Groeneveld, K.O., J. Phys. B

/6/ Harel, C., and S a l i n , A.J. Phys.

m,

3511 0977) /7/ Schumann, S., S e l l i n I .A., Mann, R., Frischkorn,

H.J., Rosich, D., Szabo, Gy., and Groeneveld, K.O. p u b l i s h e d i n t h i s volume.