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ELECTRON TRANSFER AND K-SHELL
EXCITATION IN SINGLE COLLISIONS OF 47-365 MeV Ca17+ WITH Ne, Ar, AND Kr
M. Clark, J. Tanis, K. Berkner, E. Bernstein, R. Dubois, W. Graham, R.
Mcfarland, T. Morgan, D. Mueller, A. Schlachter, et al.
To cite this version:
M. Clark, J. Tanis, K. Berkner, E. Bernstein, R. Dubois, et al.. ELECTRON TRANSFER AND K-SHELL EXCITATION IN SINGLE COLLISIONS OF 47-365 MeV Ca17+ WITH Ne, Ar, AND Kr. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-203-C9-206. �10.1051/jphyscol:1987930�.
�jpa-00227348�
Colloque C9, supplément au n°12. Tome 48, décembre 1987 C9-203
ELECTRON TRANSFER AND K-SHELL EXCITATION IN SINGLE COLLISIONS OF 47-365 MeV Ca1 7* WITH Ne, Ar, AND Kr
M.W. CLARK, J.A. TANIS, K.H. BERKNER*, E.M. BERNSTEIN, R.D. DUBOIS**, W.G. GRAHAM***, R.H. MCFARLAND** * *, T.J. MORGAN*, D.W. MUELLER**, A.S. SCHLACHTER*, K.R. STALDER***, J.W. STEARNS* a n d M . P . STOCKLI** * *
Western Michigan University, Kalamazoo, MI 49003, U.S.A.
"Lawrence Berkeley Laboratory, Berkeley, CA 94720, U.S.A.
"Pacific Northwest Laboratory, Richland, WA 99352, U.S.A.
***University of Ulster, GB-Coleraine, Northern Ireland, Great-Britain
""University of Missouri, Rolla, MO 65401, U.S.A.
*Wesleyan University, Middletown, CT 06457, U.S.A
**Louisiana State University, Baton Rouge, LA 70803, U.S.A.
*++SRI International, Menlo Park, CA 94025, U.S.A.
""Kansas State University, Manhattan, KS 66506, U.S.A.
ABSTRACT
Nous avons étudie la capture d'un électron et l ' e x c i t a t i o n de la couche K de la p r o j e c t i l e dans des c o l l i s i o n s uniques entre des ions de Ca de 47 à 365 HeV dans l e Ne, Ar, e t Er, par l a mesure des rayons x K en c o ï n c i d e n c e avec l a c a p t u r e d'un
"électron. On trouve que ce processus 'a deux é l e c t r o n s se passe surtout par capture et e x c i t a t i o n qui sont indépendantes; cela est le c o n t r a i r e de ce qu'on trouve dans une cible d'hélium, ou la capture et l ' e x c i t a t i o n viennient de l ' i n t e r a c t i o n entre les deux 'électrons.
The occurence of electron capture and p r o j e c t i l e K-shell e x c i t a t i o n in s i n g l e - c o l l i s i o n events has been investigated by measuring K-x-ray emission coincident with s i n g l e - e l e c t r o n c a p t u r e for 47-365 MeV Ca ions c o l l i d i n g w i t h Ne, Ar, and Kr.
The r e s u l t s i n d i c a t e t h a t t h i s t w o - e l e c t r o n p r o c e s s t a k e s p l a c e p r i m a r i l y due to ifidSELSSdent capture and e x c i t a t i o n events, in contrast to previous measurements for He t a r g e t s which show t h a t the c a p t u r e and e x c i t a t i o n a r i s e from the e l e c t r o n - e l e c t r o n i n t e r a c t i o n .
Recent s t u d i e s have shown t h a t e l e c t r o n transfer (capture) and p r o j e c t i l e 1—3 i n n e r - s h e l l e x c i t a t i o n can occur together in a single encounter between an ion and an atom giving r i s e to the formation of a doubly-excited intermediate s t a t e . This e x c i t e d s t a t e d s u b s e q u e n t l y decays by e i t h e r photon ( x - r a y ) e m i s s i o n or e l e c t r o n (Auger) e m i s s i o n . The combined t r a n s f e r and e x c i t a t i o n can r e s u l t from e i t h e r electron-nucleus i n t e r a c t i o n s or from e l e c t r o n - e l e c t r o n i n t e r a c t i o n s . In the former c a s e , independent p a r t i c l e i n t e r a c t i o n s give r i s e to electron capture ( p r o j e c t i l e nucleus i n t e r a c t i n g with t a r g e t electrons) and p r o j e c t i l e e x c i t a t i o n (target nucleus i n t e r a c t i n g w i t h p r o j e c t i l e e l e c t r o n s ) i n a s i n g l e e n c o u n t e r . In g e n e r a l , t h i s independent p a r t i c l e mechanism i s expected to dominate the f o r m a t i o n of the t r a n s f e r - e x c i t a t i o n p r o c e s s for e n e r g i e s where capture and e x c i t a t i o n occur with comparable p r o b a b i l i t i e s .
In a d d i t i o n , the e l e c t r o n - e l e c t r o n i n t e r a c t i o n can give r i s e to e l e c t r o n t r a n s f e r and e x c i t a t i o n v i a the i n v e r s e of an Auger t r a n s i t i o n . In t h i s case the (weakly) bound t a r g e t e l e c t r o n s a r e c o n s i d e r e d to be free r e l a t i v e to the f a s t moving ion t h e r e b y p r o v i d i n g a "beam'' of e l e c t r o n s in the p r o j e c t i l e r e s t frame.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987930
C9-204 JOURNAL
DE
PHYSIQUEHence, t h i s l a t t e r p r o c e s s i s r e s o n a n t f o r r e l a t i v e v e l o c i t i e s b e t w e e n t h e p r o - j e c t i l e a n d t a r g e t w h i c h a r e e q u a l t o t h e v e l o c i t i e s o f o u t g o i n g e l e c t r o n s i n t h e c o r r e s p o n d i n g Auger t r a n s i t i o n s . T h i s c o r r e l a t e d e l e c t r o n mechanism c a n g i v e l i s e t o t h e same i n t e r m e d i a t e and f i n a l s t a t e s a s t h e independent p a r t i c l e i n t e r a c t i o n s , so t h e two mechanisms, i n g e n e r a l , compete w i t h one a n o t h e r . The combined t r a n s f e r and e x c i t a t i o n mechanism due t o t h e e l e c t r o n - e l e c t r o n i n t e r a c t i o n i s c a l l e d r e s o n a n t t r a n s f e r and e x c i t a t i o n (RTE) w h i l e t h e independent p a r t i c l e p r o c e s s i s c a l l e d non- r e s o n a n t t r a n s f e r and e x c i t a t i o n o r mE.
The e x i s t e n c e o f RTE a n d NTE h a v e b e e n c l e a r l y e s t a b l i s h e d i n s e v e r a l s h o w i n g t h a t t h e s e p r o c e s s e s c a n b e d i s t i n g u i s h e d a n d i n v e s t i g a t e d s e p a r a t e l y . F u r t h e r m o r e , RTE and NTE a r e shown4 t o have n e a r l y e q u a l p r o b a b i l i t i e s f o r some c o l l i s i o n s y s t e m s , s u c h a s 1 6 ~ 1 3 +
+
He. A t f i r s t g l a n c e i t m i g h t b e expected t h a t RTE s h o u l d be a second o r d e r p r o c e s s , b u t when one c o n s i d e r s t h a t t h e Auger e f f e c t i s a s t r i k i n g e x a m p l e o f e l e c t r o n - c o r r e l a t i o n e f f e c t s i n i s o l a t e d , e x c i t e d atoms, i t i s , p e r h a p s , n o t s u r p r i s i n g t h a t t h e s e same c o r r e l a t i o n e f f e c t s m a n i f e s t t h e m s e l v e s i n dynamic s y s t e m s a s w e l l .I n a d d i t i o n t o e s t a b l i s h i n g t h e e x i s t e n c e g f RTE and NTE, t h e dependence o f RTE on p r o j e c t i l e a t o m i c number5 and charge s t a t e have been measured. L i t t l e s y s t e - m a t i c i n f o r m a t i o n o n t h e s e p a r a m e t e r s i s c u r r e n t l y a v a i l a b l e f o r NTE, h o w e v e r . F u r t h e r m o r e , t h e t a r g e t Z d e p e n d e n c e o f n e i t h e r RTE n o r NTE h a v e b e e n p r e v i o u s l y i n v e s t i g a t e d .
I n a n e f f o r t t o s t u d y RTE f o r heavy t a r g e t s , a s w e l l a s t h e r e l a t i v e importance of RTE and NTE i n r a t h e r symmetric heavy-ion c o l l i s i o n s and t h e i r r e s p e c t i v e c o n t r i - b u t ~ o n s t o K-vacancy p r o d u c t i o n , we have measured K x-ray e m i s s i o n ( f o l l o w i n g decay o f t h e i n t e r m e d i a t e e x c i t e d s t a t e ) c o i n c i d e n t w i t h s i n g l e - e l e c t r o n c a p t u r e f o r 47- 365 MeV c a 1 7 + + Ne, A r , and Kr t a r g e t s . R e s u l t s a r e c o m p a r e d w i t h o u r p r e v i o u s measurements1 f o r ca17+ + He c o l l i s i o n s .
The measurements r e p o r t e d h e r e were conducted a t Lawrence B e r k e l e y L a b o r a t o r y u s i n g t h e SuperHILAC f a c i l i t y . P r o j e c t i l e s i n a g i v e n c h a r g e s t a t e p a s s t h r o u g h a d i f f e r e n t i a l l y pumped g a s c e l l . X r a y s produced i n c o l l i s i o n s w i t h t h e t a r g e t gas a r e d e t e c t e d w i t h a S i ( L i ) d e t e c t o r m o u n t e d a t 90' t o t h e beam a x i s . The beam, a f t e r e m e r g i n g f r o m t h e g a s c e l l , i s m a g n e t i c a l l y a n a l y z e d i n t o i t s c h a r g e - s t a t e components. I o n s which undergo e l e c t r o n c a p t u r e i n t h e t a r g e t g a s a r e d e t e c t e d w i t h a s o l i d - s t a t e d e t e c t o r . The non-charge-changed component of t h e emerging beam i s c o l l e c t e d i n a F a r a d a y cup. c o i n c i d e n c e s between K x r a y s and p r o j e c t i l e i o n s which c a p t u r e a n e l e c t r o n a r e m e a s u r e d u s i n g a t i m e - t o - a m p l i t u d e c o n v e r t e r . The coin- c i d e n c e y i e l d s a r e m e a s u r e d a s a f u n c t i o n o f g a s p r e s s u r e t o o b t a i n t h e d e s i r e d c r o s s s e c t i o n s and t o e n s u r e t h a t s i n g l e - c o l l i s i o n c o n d i t i o n s p r e v a i l .
The r e s u l t s o f t h e c o i n c i d e n c e measur ments a r e shown i n F i g . 1 f o r ca17+ + Ar a l o n g w ~ t h o u r p r e v i o u s r e s u l t s f o l C a f 7 +
+
Be. I t i s n o t e d e t o n c e t h a t t h e c o i n c i d e n c e c r o s s s e c t i o n s f o r t h e Ar t a r g e t a r e f o u n d t o b e more t h a n 1 0 t i m e s l a r g e r t h a n t h o s e f o r t h e He t a r g e t . R e s u l t s o b t a i n e d f o r t h e Ne a n d K r t a r g e t s w e r e s i m i i a r . A l s o shown i n t h e f i g u r e a r e t h e c a l c u l a t e d RTE c r o s s s e c t i o n s f o r e a c h o f t h e t a r g e t s . S i n c e t h e RTE t h e o r y a s s u m e s t h e p r o j e c t i l e v e l o c i t y t o b e much g r e a t e r t h a n t h e bound t a r g e t e l e c t r o n v e l o c i t y , o n l y t h o s e t a r g e t e l e c t r o n s s a t i s f y i n g t h i s c o n d i t i o n were i n c l u d e d i n t h e c a l c u l a t i o n s , namely t h e L- and M- s h e l l e l e c t r o n s f o r Ar.I n t h e c a s e o f NTE, c a l c u l a t i o n s 7 f o r c a 1 7 +
+
He i n d i c a t e a maximum c o n t r i - b u t i o n t o t h e c o i n c i d e n c e c r o s s s e c t i o n o f about 5% from t h i s p r o c e s s n e a r 100 MeV w i t h t h e NTE p r o b a b i l i t y f a l l i n g o f f r a p i d l y f o r h i g h e r e n e z g i e s . No NTE c a l c u l a - t i o n s f o r t a r g e t s h e a v i e r t h a n He a r e c u r r e n t l y a v a i l a b l e . Q u a l i t a t i v e l y , however, i t i s e x p e c t e d t h a t t h e maximum NTE c o n t r i b u t i o n f o r heavy t a r g e t s w i l l occur f o r beam e n e r g i e s h i g h e r t h a n f o r t h e He t a r g e t . T h i s i s b e c a u s e t h e p r o j e c t i l e e x c i t a - t i o n g e n e r a l . 1 ~ h a s t h e same e n e r g y dependence f o r i n c r e a s i n g t a r g e t Z, b u t a l a r g e r nlagnitude, w h i l e t h e c a p t u r e c r o s s s e c t i o n does n o t d e c r e a s e s o s t e e p l y f o r t a r g e t s h e a v i e r t h a n He. Then, s i n c e t h e NTE c r o s s s e c t i o n i s r o u g h l y p r o p o r t i o n a l t o t h e p r o d u c t o f t h e independent c a p t u r e a n d e x c i t a t i o n p r o b a b i l i t i e s , t h e maximum NTE c o n t r i b u t ~ o n i s e x p e c t e d f o r h i g h e r beam e n e r g i e s f o r i n c r e a s i n g t a r g e t Z.FIG.
1. Cross sections for K X-ra emission coincident with single-electron capture for C+'la + He and Ar collisions:crosses, He target; squares, Ar target. The dashed curve and the solid curve shdw the calculated RTE cross sections for He and Ar, respectively.
There is seen to be a laige discrepancy (nearly a factor of 10) between the calculated RTE cross sections and the measured coincidence cross sections for the Ar target. (Again, the resuits for Ne and Kr targets are similar.) I n all of the studies to date for x rays coincident with single capture involving He or H2 tar- gets, good agreement (within
-
25%) between the measured coincidence cross sections and the RTE theory is found. Based on these previous results for light targets and the large discrepancy between experiment and RTE theory observed for the heavy targets studied here, we are led to the conclusion that electron transfer and projectile excitation in single collisions is dominated by the independent particle NTE process for the heavy targets.A
qualitative physical basis for this conclusion can be argued. Since the experimental requirement that a K-excitation event occurs implicitly selects close collisions, it follows that the independent capture event also must take place primarily at small internuclear distances. I n such collisions, electron capture from the inner, tightly bound target shells will be favored. (Even without the requirement of K-excitation, it is well-known that in high velocity collisions electron capture from the inner shells predominates.) Thus, it appears that the capture probability from the target inner shells is sufficiently large so that the two-step NTE process completely outweighs the one-step RTE process despite the anticipated increase in RTE due to the greater electron multiplicity in the outer shells of the heavy targets.In summary, we have shown that combined electron transfer and projectile exci- tation in single collisions occurs with high probability in collisions of highly charged ions with targets heavy compared to He. Comparison with expectations based on electron-correlation effects (RTE) indicates that the transfer and excitation result primarily from independent particle interactions. This is contrary to results obtained with light targets (He and Hz) for which the electron-electron interaction dominates the transfer-excitation process. The results point to the need for theoretical work in describing these two-electron interact ions in heavy, symmetric collisions.
This work was supported in part b y the
U.S.
Department of Energy, Office of Basic Energy Research, Division of Chemical Sciences, and the Science and Engineering Research Council, Great Britain.C9-206 30URNAL
DE
PHYSIQUE1. J . A . T a n i s , E . M . B e r n s t e i n , W.G.Graham, M.Clark, S . M . S h a f r o t h , B.M.Johnson, K . W . J o n e s , a n d M.Meron, P h y s . R e v . L e t t .
42,
1 3 2 5 (1982); J . A . T a n i s , E . M . B e r n s t e i n , W.G.Graham, M . P . S t o c k l i , M.Clark, R.E.McFarland. T.J.Morgan, K.H.Berkner, A . S . S c h l a c h t e r , and J . W S t e a r n s , Phys. Rev. L e t t .52,
2551 (1984).2. D.J.McLanghlin and Y.Hahn, Phys. L e t t . &8A_, 394 (1982).
3. D.Brandt, Phys. Rev. A21, 1314 (1983)
4. 3.A.Tanis. E . M . B e r n s t e i n . M.W.Clark, W.G.Graham, R.E.McFarland, T.J.Morgan, B.M.Johnson, K.W.Jones, and M.Meron, Phys. Rev. A3l, 4040 (1985).
5. J . A . T a n i s , E . M . B e r n s t e i n , C.S.Oglesby, W.G.Graham, M.Clark, R.H.McFarland, T.J.Morgan, M . P . S t o c k l i , K.H.Berkner, A . S . S c h l a c h t e r , J . W . S t e a r n s , B.M.Johnson, K.W.Jones, and M.Meron, Nncl. I n s t r u m . Meth. Phys. Res.
B1_O_L11_,
128 (1985).6. J.A.Tanis, E . M . B e r n s t e i n , M.W.Clark, W.G.Graham, R.H.McFarland, T.J.hlorgan, J . R . M o w a t , D . W . M u e P l e r , A . M u l l e r , M . P . S t o c k l i , K . H . B e r k n e r , P . G o h i 1 , R.J.McDonald, A . S . S c h l a c h t e r , and J . W . S t e a m s , Phys. Rev. A x , 2543 (1986).
7 . T.M.Reeves, J.M.Feagin, and E.Merzbacher. P u e _ e d _ i = g g o_f
14th
& t e r n a t i o n a _ &C_o_n_fege_n_c_e o_g P_h_~pjc_s o_f E l e _ ~ _ t g o _ n _ i c _ and A_to_mjc_ C _ o _ / l i s i o n ~ , A b s t r a c t s o f C o n t r i b u t e d P a p e r s , P a l o A l t o , C a l i f o r n i a , 1985, eds. M.J.Coggiola, D.L.Buestis, a n d R.P. S a x o n , p . 392.
