DYNAMICS OF INCLUSIVE MULTI-ELECTRON EMISSION IN CHARGE TRANSFER DOMINATED HIGHLY CHARGED ION ATOM COLLISIONS

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DYNAMICS OF INCLUSIVE MULTI-ELECTRON

EMISSION IN CHARGE TRANSFER DOMINATED

HIGHLY CHARGED ION ATOM COLLISIONS

B. Kraessig, A. Gonzalez, R. Koch, T. Quinteros, A. Skutlartz, S. Hagmann

To cite this version:

DYNAMICS OF INCLUSIVE MULTI-ELECTRON EMISSION IN CHARGE TRANSFER DOMINATED HIGHLY CHARGED ION ATOM COLLISIONS"'

B. KRAESSIG*,",

A.

GONZALEZ*,***

,

R. K O C H * , * * * *

,

T.

QUINTEROS*

,

A. SKUTLARTZ*,**** and S. HAGMANN*

*J.R. Macdonald Laboratory, Kansas State University, Manhattan, KS-66506, U.S.A.

* * ~ a k u l t l t

fiir

Physik, Universitdt Freiburg, F.R.G. ***CONICET, Buenos Aires, Argentina

* * * *

Institut ffft Kernphysik, Universitdt Frankfurt, D-6000 Frankfq.t/Main, F. R. G.

Abstract

We have measured scattering angle dependent recoil ion production cross sections for

well resolved final charge state of the projectile and derived inclusive multi-electron

emission probabilities for 0.53 MeV/u F ~ +

+

Ne, that is in a region where quasi-resonant charge transfer is a dominant reaction channel. We observe that for all scattering angles high multiplicities of electron emission are coupled to high numbers of electrons captured;

this phenomenon and the observed individual scattering angle dependencies of the emission

probabilities are not understood quantitatively

Introduction

In the past years considerable attention has turned to the mechanisms of electron

transfer processes in those ion atom collision which cannot be treated perturbatively. A

particularly interesting area is the study of near symmetric collisions of swift highly

charged ions in high charge states ( i . e . far above the equilibrium charge state of those

ions after foil stripping) and velocities still high however when considering outer shells

electrons of the target but slow with respect to inner shell electrons. In such collision

systems, e.g. 0.53 MeV/u F ~ * ~ +

+

Ne, one has observed very large electron capture cross sections into the projectile, even exceeding the geometric cross section, very large

electron loss cross sections for the target and also large 6-electron production cross

sections. l

In a recent series of experiments we have establisi~ed that emission of continuous

energy electrons (- 6-electrons) into the continuum cannot be explained by present most

advanced ab inito theories like SCA. In particular the decrease of 6-electron emission

p p -

")~hia work supported by the Division of Chemical Sciences. U.S. Department or Energy.

Cl-160

JOURNAL

DE

PHYSIQUE

probability towards small impact parameters at emission angles around 90' and its steep

increase in the same impact parameter range at 0" emission angle remain une~plained.~

For this reason we want to clarify the interaction of the strong capture channel with

the ionization channel.

As a first step we investigated the scattering angle dependence of inclusive multi-

electron emission by measuring recoil charge state distributions in coincidence with the

deflection angle of the scattered projectile in well defined final charge states (see Fig.

1). A highly collimated (typ. 0.1 X 0. lmm over 7m) beam of 0.53 MeV/u F ~ + traverses a Ne gas jet and scattered projectiles are magnetically charge state selected and detected in a

16 ring parallel plate avalanche detector, in coincidence with recoil ions extracted from

the interaction volume using standard TOF techniques for recoil charge state identification.

I I I aperlurs

Zmm S

4 - iow m o p e l r 4-IOW 4 1""

rllla X - y sills slits 8 W r ~ e r i c o l 127' F-cup

rlelleclw seclor

onolyzer sector p o s l l m sensll~rs

onolyzsr w # l # c l e dcleclor x y defleaor

Fig. 1 Experimental Set-up.

In Fig. 2 a typical time of

0 53

MeV/u

F9+ + Ne

--->

F5+

+

Nen+

250 , A F - , - -_. -

--

--

-

flight spectrum is shown for Charge S t a t e Spectrum o f Ne Recoil Ions

I

R +

recoiling Ne ions in coincidence

A

_-

with F ~ + ions, that is those who

captured 4 electrons in a collisions. 50

The dominance of the 8-c recoil charge

I00

state as well as,the higher intensity

for 9+ than for 7+ is clearly seen. 50

It is interesting to note that charge

states 4+ to 6+ which are associated O

with no, single or double additional Fig. 2 TOF spectrum of recoiling Ne ions

ionization of the target are very weak coincident with F ~ + scattered and that quadruple capture can only projectile

outgoing electron.

As preliminary results we deduced inclusive multi-electron emission probabilities from

scattering angle dependent recoil ion production probabilities. For pure target ionization

processes (i.e. qin

-

qout

-

9+ for the projectile) up to quadruple capture (i.e. qin

-

9+, qout

-

5+) and simultaneous multiple target ionization we measured absolute recoil ion production probabilities.

In Fig. 3 the scattering angle

dependent probability for double

capture and target ionization is

displayed for recoil charge states

between n-5 and n-9, i.e. between

3 and 7 electrons have been ejected into the continuum simultaneously

with the capture of two electrons

into the projectile. The average

probability slightly increases from _{8 [mrod]}

n-5 to n-8 to drop sharply for n-9. Fig. 3 Scattering angle dependence of This increases in probability with recoil ion production probabilities

increasing number of ejected for a F'+ out-going projectile.

electrons is even much more

pronounced for triple and quadruple capture. This observation may be explained by a

depletion of the L-shell with n--8, for n-9+ a vacancy in the target K-shell must be created, most probably in a K-shell to K-shell charge t r a n ~ f e r . ~ It is not clear, however, at this

time whether the peaking observed in the probabilities for n-7, 8 is associated with an

increased single KK charge transfer probability observed previously in the impact parameter

dependence of KK charge transfer.l A further clarification can only be given using triple coincidence electron-recoil projectile data. It is interesting toTcompare various

projectile final charge state (qf) recoil final charge state (n) combinations leading to the

same number m of electrons emitted into the continuum. This is done in Fig. 4 for m-2 and

Fig. 5 for m-5.

Evidently, it is much more likely to emit two electrons .into the continuum where no

Cl-162 JOURNAL DE PHYSIQUE

l

INCLUSIVE MULTIELECTRON EMISSION

PROBABILITY m= 2

PROJECTILE SCATTERING ANGLE 8 lmrad]

Fig. 4

INCLUSIVE MULTIELECTRON EMISSION

PROBABILITY m = 5

1

PROJECTILE SCATTERING ANGLE 0 [mrod]

Fig. 5

Fig. 4 Inclusive multielectron emission probability for an electron multiplicity m-2.

Fig. 5 Inclusive multielectron emission probability for an electron multiplicity m-5.

projectile. This difference is very pronounced at small scattering angles and less so but

still visible at the largest scattering angles covered in this experiment corresponding to

impact parameters well inside the K-shell radius.

For the case where m-5 electrons are emitted into the continuum the situation has

drastically changed as compared to m-2; the scattering angle averaged probability rises

slightly from q f - 9 , i.e. no capture, to q f 6 , i.e. 3-fold capture. Except for the pure five fold ionization of the target where bad statistics prohibits deriving more detailed

information of the variation of the probability with the scattering angle, the other

combinations display a maximum around 0.15 mrad corresponding to roughly twice the diameter

of the target K-shell as impact parameter. At such impact parameters the quasi-resonant

charge transfer is known to peak.1 It is thus suggested that a significant fraction of high

multiplicity electron emission events is due to rearrangement in the widest sense during and

after K-shell to K-shell charge transfer events, be it "shake off" during the collision due

to the sudden change of charges seen (indistinguishable from "true" ionization), be it

radiationless de-excitation (-autoionization) of highly excited target and projectile ions

the continuum the (qf, qR) combination contributing most shifts to higher capture

multiplicity as well; this is true for all scattering angles observed.

A quantitative interpretation of this observation that high multiplicity of electron

emission into the continuum is closely coupled to high capture multiplicities can only be

done when we also can determine the energy distribution of these electrons in the continuum,

and separate true continuous energy electrons from ionization during the collision from

autoionization and Auger electrons emitted during rearrangement of the separated collision

partners.

References

[l] S. Hagmann et al., in XI11 ICPEAC, Berlin 1983, "Electronic and Atomic Collisions",

eds. J. Eichler et al., book of invited talks, (North Holland, Amsterdam) 1983.

[ 2 ] A . Skutlartz, S. Hagmann, Phys. Rev A 28 (1983) 3268.

[ 3 ] H. Schmidt-Bocking, A. Skutlartz, S. Hagmann, Phys. Lett. 122 (1987) 421.