DELAYED COINCIDENCE AUGER ELECTRON LIFETIME MEASUREMENTS

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DELAYED COINCIDENCE AUGER ELECTRON LIFETIME MEASUREMENTS

S. Schumann, I. Sellin, R. Mann, H. Frischkorn, D. Rosich, Gy. Szabó, K.

Groeneveld

To cite this version:

S. Schumann, I. Sellin, R. Mann, H. Frischkorn, D. Rosich, et al.. DELAYED COINCIDENCE

AUGER ELECTRON LIFETIME MEASUREMENTS. Journal de Physique Colloques, 1979, 40 (C1),

pp.C1-221-C1-222. �10.1051/jphyscol:1979146�. �jpa-00218426�

JOURNAL

DE

Colloque C1, supplkment au n o 2, Tome 40, fkvrier 1979, page C1-221

S. Schumann,

A. S e l l i n ,

Mann, H. J. Frischkorn, D. Rosich, Gy. ~ z a b b , '

0 . Groeneveld I n s t i t u t f . Kernphysik d. U n i v e r s i t l t FrankfurtlM and GSI, Darmstadt, Germany

R6sumB. Les dur6es d e v i e d e s B t a t s m e t a s t a b l e s

e l e c t r o n s ( I s 2 s 2p)

s o n t determinees p a r une technique d e coincidence. Les s e c t i o n s e f f i c a c e s d e "quenching" de

e t d e Ne

d s d u i t e s d e s v a r i a t i o n s de l a duree de v i e en f o n c t i o n de l a p r e s s i o n de l a c i b l e . 2

A b s t r a c t . L i f e t i m e s of t h e ( 1 s 2 s 2p) 4~ s t a t e i n oxygen and i n neon have beeti measured with a pulsed beam delayed Auger e l e c t r o n coincidence apparatus. Quenching c r o s s s e c t i o n s have been e x t r a c t e d from t h e t a r g e t pressure-dependent l i f e t i m e s t ( p ) . L i f e t i m e s t(0,p-W)

(25.9

2) nsec and t(Ne,p+O) = (12.0

1 ) nsec have been determined.

INTRODUCTION

A m e t a s t a b l e c o n f i g u r a t i o n , which h a s a t t r a c t e d l a r g e i n t e r e s t i n r e c e n t y e a r s i s t h e lowest P s t a t e of t h e L i - l i k e i s o e l e c t r o n i c seq- 4 uence. One reason f o r t h i s i n t e r e s t i s t h a t i n t h e l s 2 s 2 p Pg12 s t a t e t h e m a t r i x elements f o r spin- 4 o r b i t i n t e r a c t i o n vanish [ I ] . T h i s a l l o w s us t o determine t h e decay r a t e f o r spin-spin i n t e r a c t i o n by measuring t h e l i f e t i m e of t h i s m e t a s t a b l e s t a t e . One f r e q u e n t l y a p p l i e d experimental method uses t h e beam-foil time-of-flight technique [2] where t h e decay of m e t a s t a b l e p r o j e c t i l e c o n f i g u r a t i o n s a r e s t u d i e d a s a f u n c t i o n of t h e time between t h e exci- t a t i o n of t h e beam when passing a t h i n t a r g e t f o i l and t h e o b s e r v a t i o n of t h e decay by e l e c t r o n o r x-ray emission.

For s e v e r a l y e a r s

has been known [ I ] t h a t experimental l i f e t i m e s o f t h i s m e t a s t a b l e l e v e l d e v i a t e s y s t e m a t i c a l l y by ca. 15% from theo- r e t i c a l v a l u e s [ 3 ] o v e r a wide r a n g e of Z ( 6 t o 18).

One p o s s i b l e o r i g i n i s t h e n e g l e c t of c o n f i g u r a t i o n i n t e r a c t i o n i n t h e t h e o r e t i c a l c a l c u l a t i o n . Another i s long-lived cascades from n

1 2 a s - s u g g e s t e d r e c e n t l y i n [ 4 ] . To h e l p d i s t i n g u i s h between t h e s e p o s s i b i l i t i e s - n o t o n l y i n t h e c a s e of t h r e e - e l e c t - r o n systems, b u t more g e n e r a l l y - i t i s h e l p f u l t o u s e t h e independent delayed c o i n c i d e n c e l i f e t i m e measurement (DCLM) method r e c e n t l y developed by u s

[IT. We have a p p l i e d a v a r i a t i o n of t h e DCLM t o t h e d e t e r m i n a t i o n o f Auger e l e c t r o n emission l i f e t i m e s of l e v e l s of highly-ionized and highly-excited t a r g e t atoms c r e a t e d by pulsed, v e r y highly-ionized f a s t beams from t h e UNILAC.

EXPERIMENT

Fig. 1 shows t h e experimental s e t u p .

Every 37 n s t h e UNILAC beam h a s a micropulse s t r u c - t u r e of 1 n s l e n g t h : t h e r e is a time i n t e r v a l of about 36 n s t o study t h e d e e x c i t a t i o n of m e t a s t a b l e s t a t e s of t a r g e t atoms o r molecules e x c i t e d d u r i n g t h e beam bunch. The t a r g e t atoms o r molecules of i n t e r e s t a r e e x c i t e d under s i n g l e c o l l i s i o n condi- t i o n s by a Cu beam of 1.4 MeV per mass u n i t . The d e e x c i t a t i o n

s t u d i e d ' b y measuring t h e a u t o i o n i - z a t i o n e l e c t r o n s w i t h a n e l e c t r o s t a t i c spectrometer, which c o n s i s t s of a , r e t a r d i n g f i e l d followed by a s p h e r i c a l a n a l y z e r [5].

To process t h e e l e c t r o n i c s i g n a l s , s t a n d a r d e l e c t r o n i c modules a r e employed. The s t a r t p u l s e s f o r t h e time-to-pulse-height c o n v e r t e r a r e d e r i v e d from t h e a r r i v a l of a n e l e c t r o n a t t h e spectrometer.

The s t o p p u l s e s a r e o b t a i n e d from a c a p a c i t i v e p l i a s e probe i n s t a l l e d i n t h e beam l i n e 161. The o v e r a l l tlme r e s o l u t i o n amounts t o 4 ns. I n F i g . 1 i s included a schematic time spectrum of t h e decay of a m e t a s t a b l e state. The measured time s p e c t r a have a p e r i o d i c s t r u c t u r e w i t h a maximum corresponding t o every beam bunch. The time i n t e r v a l which d i r e c t l y g i v e s t h e decay curve of t h e c o n f i g u r a t i o n i s i n d i - c a t e d by v e r t i c a l dashed l i n e s .

RESULTS

L i f e t i m e s of t h e ( I s 2s 2p) 4 P s t a t e i n .oxygen and neon have been determined. I n b o t h c a s e s

t h e r e i s a l i n e a r dependence of t h e measured l i f e - time

on t h e t a r g e t p r e s s u r e

T h e t r u e l i f e t i m e of t h e . s t a t e s i s c a l c u l a t e d by e x t r a p o l a t i n g t o p

0. The t(p+O)-values a g r e e w e l l w i t h r e s u l t s from beam f o i l measurements [ I ] o r Dirac-Fock c a l - c u l a t i o n s 133. (Table I) .

The r e a s o n f o r t h e t a r g e t p r e s s u r e depend- ence t ( p ) i s t h e " c o l l i s i o n a l quenching" of meta-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979146

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4

Fig.

Schematic of delayed-coincidence l i f e t i m e apparatus.

s t a b l e s t a t e s : w i t h i n c r e a s i n g t a r g e t d e n s i t y ( i . e . t a r g e t p r e s s u r e p) t h e p r o b a b i l i t y i n c r e a s e s t h a t m e t a s t a b l e t a r g e t i o n s a r e t r a n s f e r r e d t o a d i f f e r -

e n t e x c i t e d s t a t e when c o l l i d i n g w i t h a n o t h e r t a r g e t p a r t i c l e , b e f o r e they decay from t h e i r m e t a s t a b l e s t a t e . This r e s u l t s i n t h e o b s e r v a t i o n of an a p p a r e n t s m a l l e r l i f e t i m e (Fig. 2) -

The c r o s s s e c t i o n f o r c o l l i s i o n a l q u e n c h i n g i s given by

With known t a r g e t d e n s i t y

T and t h e r e c o i l v e l o c i t y vT o f t h e e l e c t r o n emit-

t i n g i o n s , one can determine t h i s c r o s s s e c t i o n f r o m t h e s l o p e of t h e curves. The two curves

and Ne have d i f f e r e n t slopes, because t h e e l e c t r o n - e m i t t i n g 0 i o n s a r e approximately by a f a c t o r of 3 f a s t e r t h a n t h e Ne i o n s . T h i s i s caused by t h e "Coulomb explosion" of t h e 0 molecule

2

a f t e r t h e c o l l i s i o n [ 5 ] .

CONCLUSIONS

Advantages and disadvantages of t h e DCLM technique a r e summarized. 1. The maindisadvantage:

, . . . .

r [ 1 s 2 s 2p

'~5,~-

PROJECTILE : Cu (1.4 MeV IAMU

TARGET : O2 T(p+o)=(25.9 t l S I n 6

TARGET Ne

20 40 60 80

TARGET PRESSURE P [rel. u n l t ~ ]

Fig. 2. P r e s s u r e dependence of t h e l i f e t i m e T , O £ t h e ( 1 s 2s 2p) 4 PgI2 s t a t e i n

and Ne.

TABLE

L i f e t i m e

i n n s of t h e ( 1 s 2s 2p) P 4 s t a t e i n 512

0 and Ne e x t r a p o l a t e d t o p r e s s u r e p = 0.

2

Target 1 3

texp texp t t h e o r

O2 2 5 . 9 + 2 25 + 3 23.1

Ne 12.0

1 10.4 + 1 . 5 8.4

One only can s t u d y s t a t e s w i t h l i f e t i m e s l a r g e r t h a n approximately 1 n s and s m a l l e r than 36 n s . T h i s , of c o u r s e , depends on t h e p u l s e s t r u c t u r e of t h e a c c e l e r a t o r a v a i l a b l e . 2 . Important advan- tages: A t a r g e t gas i s much e a s i e r t o change than an a c c e l e r a t o r ion source; atoms bound i n d i f f e r e n t molecules can be s t u d i e d ; and, t h e times f o r measuring a complete decay c u r v e a r e v e r y s h o r t compared t o t h e beam-foil time-of-flight technique, e.g. where t h e t a r g e t h a s t o be moved f o r every p o i n t of t h e decay curve.

*supported by BMFT, NSF, ONR.

% m b o l d t awardee, Perm. Adr. Univ. of Tenn. and ORNL, USA.

'IAEA

Fellowship, Perm. Adr. ATOMKI, Debrecen, Hungary.

BIBLIOGRAPHY

[ l ] I. A. S e l l i n , i n BEAM FOIL SPECTROSCOPY, Ed: 141 H.

Dohmann, H. Pfeng, t o b e published i n S t . Bashkin, S p r i n g e r , Heidelberg 1978, p. 265.

Phys.

(1978).

[2]

0. Groeneveld,

Mann,

Nolte, S.Schumann, 151 R. Mann, F. Folkmann, K. 0 . Groeneveld, Phys.

B. Rricke, Phys. L e t t . a, 335 (1975) and Rev. L e t t . 37, 1964 (1976).

r e f e r e n c e s quoted t h e r e i n . [ 6 ] A. N i c l a s , P . S t r e h l , H. Williamson, GSI-

[3]

T. Cheng,

P. Lin, W.

Johnson, Physi Bericht-Pb-5-75-Mai (1976).

L e t t . e, 437 (1974).