L-SUBSHELL IONIZATION PROBALITIES IN FAST ASYMMETRIC ION-ATOM COLLISIONS

(1)

HAL Id: jpa-00227346

https://hal.archives-ouvertes.fr/jpa-00227346

Submitted on 1 Jan 1987

HAL

is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire

HAL, est

destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

L-SUBSHELL IONIZATION PROBALITIES IN FAST ASYMMETRIC ION-ATOM COLLISIONS

H. Schmidt-Böcking, R. Dörner, A. Skutlartz, C. Kelbch, J. Ullrich, S.

Kelbch, S. Hagmann

To cite this version:

H. Schmidt-Böcking, R. Dörner, A. Skutlartz, C. Kelbch, J. Ullrich, et al.. L-SUBSHELL IONIZA-

TION PROBALITIES IN FAST ASYMMETRIC ION-ATOM COLLISIONS. Journal de Physique

Colloques, 1987, 48 (C9), pp.C9-191-C9-197. �10.1051/jphyscol:1987928�. �jpa-00227346�

(2)

L-SUBSHELL IONIZATION PROBALITIES I N FAST ASYMMETRIC ION-ATOM COLLISIONS( )

H. SCHMIDT-BOCKING, R. DORNER, A. SKUTLARTZ*'(~', C. KELBCH, J. ULLRICH, S. KELBCH and S. HAGMANN*

Institut fiir Kernphysik, Universitdt Frankfurt, 0-6000 Frankfurt-am-Main 90, F.R.G.

' ~ a n s a s State University, J. Macdonald Laboratory, Manhattan, KS 66506, U.S.A.

Abstract

It has been established that K-shell ionization probabilities in very asymmetric fast ion-atom collisions can be described well by first order perturbation theory. For the L-substates, however, impact parameter dependent ionization probabilities show severe discrepancies with these theoretical approaches. From the L-subshell ioniza- tion probabilities, the measured impact parameter dependent L I I I -

alignment and 6-electron emission probabilities we can deduce that the dynamical behaviour of the electron wave function during the collision has a strong influence on the measured transition probabilities.

Introduction

Systematic investigations [I-31 of total K- and L-shell ionization in fast asymmetric ion-atom collisions give the impression that cross sections as well as impact parameter (b-) dependent ionization probabilities P(b) [I] are fairly well described by first order perturbation theory 14-71 if correction terms are included. It is assumed that the remaining "small" discrepancies are due to non perfect approximations and there are no indications for a severe misunderstanding of these processes implying that important aspects of the ionization mechanism have not been considered.

Nearly only such data have been measured so far which give informa- tions on systems where the initial target states have spherical symmetry, e.g. total shell cross sections, K-shell [I-31 or total L- shell ionization probabilities [8]. The question arises, how accurate first order perturbation theory describes the ionization process, if spherically asymmetric substates such as 2p electrons are ionized and highly differential cross sections are measured. Only very few measurements have been performed recently, where highly differential ionization cross sections (e.g. b dependent L-subshell ionization probabilities) have been investigated [9-131. Since spherically asymmetric states can be oriented with respect to a quantization axis they are interesting objects to study the influence of possible dynamical effects (e.g. rotation of the internuclear axis during the collision) on the ionization probability.

(l)~upported by Bundesministerium fur Porschung und Technologic BMRT (060f/73) F.R.G.. and by the Division of Chemical Science. Department of Energy.

(2)~resent address : Institut fur Kernphysik, Universitat Frankfurt, D-6000 Frankfurt-am-Main 90. F.R.G.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987928

(3)

C9-192 JOURNAL DE PHYSIQUE

Therefore we have measured the L I - and LIII-subshell ionization probability and furthermore the impact parameter dependence of the alignment of the L I I I magnetic substates for several collision systems [11-141. The experimental data show partially nice agreement with the first order perturbation theory (SCA) if the probability is very low and spherical symmetric wave functions are investigated.

With increasing perturbation, however, severe discreapancies between theoretical and experimental b-dependent L-subshell ionization probabilities are observed. Furthermore, even for collision systems, where the probabilities are low (small perturbation) deviations between theory and experiment occur, indicating that present theories are not able to treat the dynamical behaviour of electron wave functions precisely during the collision. This is supported by recent impact parameter dependent investigations of 6-electron emission probabilities from the Ne L-shell measured at angles of ie = 9Q0 and ie = Q0 with respect to the beam axis [15-181. These emission probabilities show very dramatic differences in their b dependence, proving that the unisotropy of the 6-electron emission varies strongly with b. These results are also not explained at present by theoretical approaches. The analysis of these data indicate that the approximation of stationary target states is obviously not adequate to describe important mechanisms of the ionization process and extensive coupled channel calculations are necessary.

Experiment

The data presented were measured at the 2.5 and 7 MV Van de Graaff accelerator located at the Institut fur Kernphysik, University Frankfurt, the 6 MV tandem Van de Graaff accelerators at Kansas State University, ManhattadKansas and at the Max-Planck-Institut fur Kernphysik, Heidelberg. The impact parameter dependence of L-subshell ionization probabilities and of the corresponding alignment components was obtained applying standart x-ray

-

particle coincidence techniques. The different well collimated ion beams hit thin solid targets. The scattered particles were detected by a position sensi- tive parallel plate avalanche detector (PPAD) in coincidence with x- rays detected by Si(Li) detectors. Information on the L I - and the LIII-subshell ionization probabilities was derived from the measured L respectively the L x-ray emission probability. The L I I I s&b$&ell alignment data $ere deduced from the polar anisotropy of L x-ray emission probabilities.

T A ~

6-electron emission probabilities were obtained by employing standard electron-particle coincidence techniques bombarding thin gas targets. For the particle detection the same PPAD, for the electron detection at @e = 9Q0 and m e = Q0 electrostatic spherical analyzeres in scanning mode were used. The absolute normalization of the 6- electron emission probabilities dZ P (Ee , ie .b) /dEe die was achieved by normalising the simultaneously measured K-Auger yields to previously measured probabilities [16].

Results

First order perturbation theory [19,20] predicts an interesting intersection of LI and LIII-subshell ionization probabilities at small impact parameters and very low projectile velocities, inspite of the fact that the LIII-electron is less thightly bound. Due to the different radial distribution of the L-substates the probability of LI-electrons with high momentum close to the target nucleus is larger than the corresponding one of the L I I I electrons. Since at

(4)

electon momentum "contributes" to the necessary minimum momentum transfer by scattering on the projectile nucleus, the LI subshell ionization probability can exceed the one of L I I I . This intersection does not occur at high projectile velocities, where the minimum momentum transfer to the electron is easily provided by the momentum of the fast projectile only.

In Fig. 1 the theoretical prediction and the measured data are presented for 3.6 MeV a on Pt showing nice agreement at small impact parameters [ I l l . It is to mention that the theoretical calculation includes three-body kinematics correctly. The perturbated electron wave function is approximated by the relativistic Dirac wave function

1

^Fig.^1:L-subshell ionization proba-

bilities I1Lt ,b) as function

lo-? of the impact parameter b in

comwarison with SCA calcula- -

tions [7] (solid line: united atom Dirac wave functions; dashed line: separa- ted atom Dirac wave func-

- "

^lo-$

d

Y

10-5

b l f m l

of the united-atom system ( Z u a = ZP+ZT = 78+2; ZP , Z T : projectile and target nuclear charge) centered in the target nucleus. The quantisa- tion axis is taken along the beam direction and thus does not follow the internuclear axis which rotates during the collision.

Furthermore, any coupling between the L-substates due to the projectile Coulomb force acting during the collision is not taken into account.

From the good agreement for LI and L I I I between experiment and first order theory at small b one could derive that even the ionization of different subshells is quite well described by these theoretical approaches. However, the observed agreement turns out to be somewhat "accidental" as will be seen below. In this collision system the absolute probability is rather low, thus indeed higher order effects as coupling between the L-subshells can be neglected. In addition very small b are considered, i.e. small distances from the target nucleus, where only electrons with high momentum contribute considerably to the ionisation probability. The angular distribution of these high momentum electrons, however, is again more spherical symmetric. Considering L I I I at large b, where the spatial distribution of each substate is spherically asymmetric, experiment and theory deviate. Since the calculation is based on united atom wave functions the SCA prediction should even underestimate the measured values. Therefore, the disagreement observed here is severe and clearly shows that the SCA approach in its present form is based on too crude approximations.

I(L11

-

:

- I\)

-

100 1 OM)

tions)

.

(5)

C9-194 JOURNAL DE PHYSIQUE

In Fig. 2 the measured LI and LIII-subshell ionization probabilities are presented in comparison with the SCA calculations 1121. The disagreement between experiment and theory is now striking. With increasing absolute probability ( g inreasing perturbation) the expri-

Ne3'+ Pt 0.9 MeVlu

n

^{0.9 MeVlu}

Fig. 2: L-subshell ionization probabilities I(Li,b) as function of the impact parameter b in comparison with SCA predictions [7]

(solid line). The crosses and dashed line represent the corresponding values for the total L-shell.

mental LI and L I I I - I (b) disagree dramatically with SCA predictions and finally kbth show the same relative impact parameter dependence.

.

The created L-subshell vacancies are thus statistically shared between the substates with the ratios I /IL /I = 6/2/2, indicating that the coupling between tkklf.,-subktakks is very strong. In Fig. 2 the total L-shell ionization probabilities IL (b) are presented too in comparison with the SCA prediction (dashed line). These experimental data are fairly well described over the whole b-range by the SCA approach using united atom wave functions. It is to note that the total L-shell is again a spherically symmetric wave function and but more important, vacancy transfer between the L-substates does not affect the total L-shell ionization probability.

As an example the I /I ratio is shown for 4 MeV p on Sm in Fig. 3. The solid likk r&&&sents the SCA prediction, the dashed line the ratio expected for pure statistical distribution. This result clearly demonstrates that already for p on Sm representing a

"perfect" collision system for the perturbation approach, coupled channel calculations are necessary to describe the L-subshell ionization process. Such coupled channel calculations now available [21-241 yield much better agreement with the experimental data, but still do not give fully satisfactory results.

(6)

I (LI I I ,b) (solid line:

SCA calculation [7], dashed line: complete statistical distribu-

For the L I I I magnetic substates the impact parameter dependence of the AZE -alignment tensor has been measured for 4 MeV p on Sm [13]

and very recently for 1 MeV p on Sm [25]. The SCA calculations [7]

for 4 MeV show only qualitative agreement with the data. A maximum in the experimental Aze (b) can be observed at b about two times smaller than predicted by SCA. Even recent coupled channel calculations [21,22] cannot reproduce this maximum at small impact parameters and are quite close to the SCA calculations [7]. For 1 MeV p on Sm the deviation between experiment and theory even increases [25] (Fig. 4).

Here the projectile velocity is about a factor of 3 smaller than the mean velocity of the Sm L electrons. Thus these electrons might adiabatically adjust to the rotation of the internuclear axis and the L I I I substates, oriented with respect to the internuclear axis R(t), vary adiabatically their orientation with R(t). Depending on b the angular velocity of this rotation changes, at large b where the axis rotates only slowly a coupled channel calculation using adiaba- tic molecular states should perfectly describe the ionization process, whereas at very small b the rotation is too fast so that stationary atomic states are well representing the electronic system.

Therefore appropriate coupled channel calculations should take into account this very complex dynamical behaviour of the electron wave functions during the collision. Then better agreement with experimental results might to be expected. A more detailed comparison of the experimental data with coupled channel calculations for the 1 MeV p on Sm systems in ref [25] shows that the ionization of the spherically symmetric LI-substate is again perfectly described within the experimental error bars 1s 10%) for the whole b range investigated. The impact parameter dependent ionization probabilities of the L I I I state, however, show severe systematic deviations at small and large b with coupled channel calculations 121,223 and the SCA 171.

The size and tendency of the deviations vary with the projectil energy, thus yielding for IL (b) in the collision system 4 MeV p on Sm accidentally a perfect Bbkeement with the SCA theory.

Fig. 4: LI I I subshell alignment tensor components Aze (b) as function of the impact parameter (solid line: SCA calculation

171 )

(7)

C9-196 JOURNAL DE PHYSIQUE

Besides these L-subshell ionization probabilities for 1 MeV a on Ne and 10 MeV F a + on Ne triple differential 6-electron emission probabilities d2P(b)/dQdE for Ne L-electrons as function of the impact parameter have been measured. Since the projectile velocity is 4.6 times faster than the mean Ne L-electron velocity, the SCA approach might be still applyable in this near symmetric F on Ne collision system. The SCA theory predicts a very smooth b dependence towards smaller b whereas the experimental data for F a + on Ne show strong exponential variations with b. Furthermore at an electron emission angle of Oe = 90O dZP(b)/dQdE is strongly decreasing towards small b, whereas at Oe = 0 O d2P (b) /dQdE exponentially increases (Fig. 5)

.

This dramatic anisotropy is not explained at present by theory.

This anisotropy might be due to the fact that the L-substates undergo a strong orientation during the collision induced by the projectile Coulomb force. Unfortunately coupled channel calculations for these triple differential cross sections are not available so far.

Conclusion

From the differential cross sections on L-subshell ionization in fast asymmetric ion atom collisions discussed here, we can deduce that even for very asymmetric collision systems, coupled channel calculations are needed to describe the ionization of different subshells.

These coupled channel calculations must use basis sets which can properly describe the dynamical behaviour of the different electronic states during the collision. The present SCA approach using stationary atomic states is not able to calculate the ionization of L- substates.

w -

Î _9,Î ₌_oOÎ

E,

, = 2 2 0 2 4 5 e F

9, = 90"

Eo= 611 +70eV

lo-=

I ^I ^I 0 0.1 0.2 0.3

Impact Parameter b [a,".]

Fig. 5: Triple differential 8-electron emission probabilities for 10 MeV Fa + on Ne as function of the impact parameter b (solid line: to guide the eyes only).

(8)

Justiniano, J. Konrad and S. Zehendner in which the 4 MeV p on Sm data were measured.

References:

[I] E. Laegsgaard, J.U. Andersen and M. Lund, Book of Invited Papers, 10th ICPEAC, ed., G. Watel (North-Holland, Amsterdam, 1978) p. 353

[21 H. Paul, Nucl. Instr. and Meth. m ( 1 9 8 2 ) 11

[3] R.S. Sokhi and D. Crumpton, Atomic Data Nuc. Data Tables 30 (1984) 49

[41

TM.

Hansteen, O.M. Johnson and L. Kocbach,Atomic Data Nuc.

Data Tables (1975) 305

[51 P.A. Amundsen, J.Phys.BlO (1977) 2177; J.Phys.Ba (1978) 3197 [6] D. Trautmann and F. Rosel, Nuc.Instrum.Methods 169 (1980) 259 [7] F. Rosel, D. Trautmann and G. Baur, Nuc.Instrum.Methods 192

(1982) 43

[81 E. Laegsgaard, J.U. Andersen and M. Lund, Phys-Fenn. 9 Suppl. SL (1974) 49

(91 K.E. Stiebing, H. Schmidt-Bocking, R. Schulh, K. Bethge and I. Tserruya, Phys.Rev. A= (1976) 146

[I01 J. Konrad, R. Schuch, R. Hoffmann and H. Schmidt-Bocking, Phys.Rev.Lett. 52 (1984) 188

[I11 J. Ullrich, V. Dangendorff, K. Dexheimer, K. Do, S. Hagmann, C. Kelbch, S. Kelbch, F. Rosel, W. Schadt, H. Schmidt-Bocking, K.E. Stiebing and D. Trautmann, Z.Phys. Da (1986) 137

[I21 K. Dexheimer,J. Ullrich, K.E. Stiebing, W. Schadt, S. Kelbch, C. Kelbch, R. Schuch, S . Zehendner and R. Schmidt-Bocking, J.Phys. B B (1986) 3083

[13] S. Zehendner, G.B. Baptista, R. Dorner, E. Justiniano, J. Konrad, H. Schmidt-Bocking and R. Schuch, Z.Phys. DA (1987) 243

[I41 H. Schmidt-Bocking, J. Ullrich, R. Dorner, K. Dexheimer, S. Kelbch, V. Dangendorf, R. Schuch, S. Zehendner, S. Hagmann and G.B. Baptista, Nuc.Instrum.Methods in Phys.Research B24/25

(1987) 64

1151 A. Skutlartz and S. Hagmann, Phys.Rev. A= (1983) 3268

[I61 A. Skutlartz, Ph.D. Dissertation, Kansas State University (1987) and to be published

[I71 H. Schmidt-Bocking, A. Skutlartz and S. Hagmann, Phys.Lett. A m (1987) 421

C. Kelbch, private communication and to be published

M. Pauli, F. Rosel and D. Trautmann, Phys.Lett. A 6 7 (1978) 28 0. Aashamar, P.A. Amundsen and L. Kocbach, Phys.Lett.Al (1978) 349

G. Mehler, T. de Reus, U. Muller, J. Reinhardt, B. Muller, W. Greiner and G. Soff, Nuc. Instrum-Methods A= (1985) 559 G. Mehler, T. de Reus, J. Reinhardt, G. Soff und U. ~ u l l e r , Z.Phys. A m (1985) 355

L. Sarkadi, private communication

P.A. Amundsen and D. Jakubassa-Amundsen, private communication R. DSrner, Diplomarbeit, Universitat Frankfurt (1987), and to be published