MULTIPLE IONIZATION OF XENON BY PROTON IMPACT

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MULTIPLE IONIZATION OF XENON BY PROTON IMPACT

S. Manson, R. Dubois

To cite this version:

S. Manson, R. Dubois. MULTIPLE IONIZATION OF XENON BY PROTON IMPACT. Journal de

Physique Colloques, 1987, 48 (C9), pp.C9-263-C9-266. �10.1051/jphyscol:1987945�. �jpa-00227363�

supplbment au n012, Tome 48, d6cembre 1987

MULTIPLE IONIZATION OF XENON BY PROTON IMPACT

S.T. MANSON and R.D. DUBOIS'

Department of Physics and Astronomy, Georgia State University, Atlanta, GA-30303, U.S.A.

"pacific Northwest Laboratory, Richland, WA 99352, U.S.A.

@s-

-

U?e Qtude ,exp6rimentale et thhrique de 1' ionisation multiple du xenon a 6te effectuee par impact protonique (0.2

-

^{2.0 MeV)}

.

Les sections efficaces ,absolues,de production d'ions xenon de charge ccmprise entre +1 et +3 ont,bte mesyrees, et des calculs de sections efficaces par sous-couche ont 6te effectues. -rience et thhrie sont en bon accord et indiquent que l'ionisation multiple du xenon par protons rapides met en jeu 1' ionisation de couches internes, contrairement aux gaz rares plus lggers pour lesquels 1' ionisation multiple directe des couches externes peut &re pr6dominante.

Abstract

-

An experimental and theoretical study of multiple ionization of xenon for 0.2

-

2.0 MeV proton impact has been made. Absolute cross sections for producing xenon ions with charges from +1 to +3 have been measured, and calculations of subshell cross sections have been performed.

Experiment and theory are consistent and indicate that multiple ionization of xenon by fast protons occurs via inner shell ionization. This is in contrast to the lighter noble gases where direct multiple outer shell ionization can be predominant.

Multiple ionization in ion-atom collisions is, at best, a p r l y understood process owing to the multiplicity of ionization pathways available leading to a particular stage of ionization. For example, multiple ionization can result form direct multiple outer-shell ionization, single or double outer-shell charge transfer accompanied by simultaneous ionization, or inner-shell ionization followed by Auger relaxations; either the ionization or the relaxation process can be accompanied by the emission of further electrons. While multiple ionization only accounts for about 1% of the total ionization for H+

+

^He

collisions, the percentage increases for heavier targets; multiple ionization dominates in H+

+

Kr collisions I: 1 1

.

In previous work, multiple ionization in proton collisions with He, Ne, Ar and Kr was investigated [l]

.

In this paper,

H+ + Xe ionizing collisions are investigated.

Experimentally, the cross sections for single and multiple ionization of xenon were obtained'by measuring recoil target ion

-

projectile ion coincidences.

Since the apparatus and procedure are described elsewhere f 2 ]

,

ollly a brief description will be given here.

A week proton beam ( 10-l5 A) was passed through a gas cell containing xenon gas. After leaving the cell, the beam was electrostatically charge state analyzed and the various components detected by a channel electron multiplier.

Xenon ions, created by ionizing collisions, were extracted perpendicular to the ion beam by an electric field. These ions passed through a field free drift region and were detected by a channel electron multiplier.

Since the recoil ion flight times depend on their ionization charge states, coincidences between the recoil ions and projectile ions provide information about the relative cross sections for multiple ionization of xenon. These relative cross sections are measured for direct target ionization (H+

-

^recoil

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987945

C9-264 JOURNAL DE PHYSIQUE

TABLE I. Table showing how single, double, and triple (q=l-3) ionization of xenon are related to initial vacancy production cross sections and to measurable final target ionization state cross sections. The relationship between these cross sections is to be read as a matrix as illustrated.

q +1

+2

4-3 i 1 1 1 1 1 1 1 1 1 1 1

i

l l

.

I l l l i I

l j I

0

l 0 -1 1 0 1 0 1 0 -1 1 0 -1 1 0 1 0

1

0

Initial vacancy

0

02 4d 4p

03

4d0 4d 4p

4p 3s

Subsguent processes

4d00 Auger

4p00 Auger

4d00 Auger

4d000 Auger

4p4d0 Coster-Kronig plus 4d00 Auger or 4p000 Auger

4s4p0 Coster-Kronig plus 4p00 Auger or 4s4d0 Coster-Kronig plus 4d00 Auger, or 4s000 Auger

T

aq

1 1

1 1 1 a a b b 1 1 1 d d . d e e fc+g fc+g

hd+ic+j

hd+ic+j 11 a

q 1

1

a b 1

d

e fc+g

hd+ic+j

10 1-1 a a

q q

1

1

1

a b 1

1

d

d

e

fc+g

hd+ic+j

e.g., of = of + caJJQ + d a gQ + eaJJ + (fc + g) a £ + (hd + ic + j ) o £ a is the fraction of 4d vacancies undergoing 4d00 Auger delay

b is the fraction of 4p vacancies undergoing 4p00 (likely to be small) c is the fraction of 4d0 vacancies undergoing 4d00

d is the fraction of 4p0 vacancies undergoing 4p00 (likely to be small) e is the fraction of 4d vacancies undergoing 4d000

f is the fraction of 4p vacancies undergoing 4p4d0 Coster-Kronig g is the fraction of 4p vacancies undergoing 4p000 Auger

h is the fraction of 4s vacancies undergoing 4s4p0 Coster-Kronig i is the fraction of 4s vacancies undergoing 4s4d0

j is the fraction of 4s vacancies undergoing 4s000 Auger

absolute scale by appropriate s d l l g in order to yield the total electron emission yield which can then be mrmalized to the total ionization cross section 131 at each impact energy. Previous measurements of this type have indicated that the present individual cross sections are accurate to approximately '20%.

In addition, subshell cross sections were calculated within the framework of the first Born amroximation using methodology described in detail elsewhere _C41.

These subshell cross sections are, in general, only indirectly related to the multiple ionization. In Table I, the relationships between the fundamental processes, such as the subshell cross sections, to the various measurable cross sections, are shown for single, double and triple ionization of xenon by protons.

Note that the -scripts refer to initial and final projectile charge states, while subscripts relate to the target; the notation is discussed more fully in Ref. 1.

The experimental results for .:0 q = 1

-

³ (pure ionization, m charge transfer) are shown in Fig. 1. Note that, in the energy range covered, the charge transfer contributions, aFand a,'-: were found to be negligible. lhis differs from the situation for th; lighte; noble gases where charge transfer is important, particularly at the lawer end of the energy range shown [ll

.

^From

Table 1, it is clear that for single ionization, a? = ;:0

+

^;'0

.

In Fig. 1, the sum of the theoretical 5p and 5s cross sections is s kand lies parallel to

Fig. 1. Multiple ionization cross sections, ':0 (q = 1-31 for H+ + xe collisions. The points are exprimental while the full curves are Y

theoretical. The superscripts refer to the initial and final state of the incident proton, while the subscripts define the stage Of ionization of the xenon target atoms after the collision.

C9-266 JOURNAL DE PHYSIQUE

but a consistent 40% above the experimental of ; the source of this discrepancy is unclear.

In analogy to krypton [ 1 1

,

the dominant contribution to double ionization should be initial 4d ionization followed by a 4d00 Auger decay, at least at the higher energies. Since the branching ratio for 4 0 0 decay is approximately 0.7

[ 5 1, the measured u y should equal .7 0:; ; using the calculated 4d ionization cross section, this latter quantity is shown in Fig. 1 where excellent agreement with the measured o y is found over the entire energy range, even at the lower energies except 200

kev.

This shows that direct outer shell double ionization is not important, even as low as 350 keV, but is becoming important at 200 keV.

Comparing with the lighter noble gases

[U ,

we find that inner-shell contributions to double ionization are becoming more and more dominant, with increasing Z.

The situation for triple ionization is similar to double. Assuming the dominant contribution to be 4d ionization plus 4d000 double Auger decay with a measured branching ration of 0.3 c51 _{T ?}

,

o:i should equal . 3 11-

.

^{AS A e , the}

calculated a? is used and the resulting cross section,

show^-in

Fig. 1, is in excellent agreement with experiment, except at 200 kev wher@ direct triple ionization appears to contribute. 11

As a final note, the fall-off of '1 with energy is seen to be more rapid than

OF

^{or ;} further, the latter W curve are seen to be parallel. This indicates that double and triple ionization result from inner shell ionization.

Furthermore, the fact that they have the same energy dependence: indicates that both double and triple ionization arise from the same initial ionization, the 4d in this case.

This mrk was supported by the U. S. Army Research Office and the U. S.

Department of Energy, Office of Health and Environmental Research.

References

[I] DuBois, ^R. D., and Manson, S. T., Phys. Rev. A

35

(1987) 2007.

[2] DuBois, R. D., Phys. Rev Letters

52

(1984) 2348.

131 Rudd, M. E., Kim, Y. K., Madison, D. H., Gallagher, J. W., Rev. Mod. Phys.

57 (1985) 965.

-

C4] Manson, S. T., Phys. Rev. A

6

^{(1972) 10U.}

[51 Krause, M. 0, and Carlson, T. A., Phys. Rev.

149

(1965) 1057.