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PHOTOIONIZATION QF XENON WITH SOFT X-RAYS
U. Becker, H. Kerkhoff, M. Kupsch, B. Langer, D. Szostak, R. Wehlitz
To cite this version:
U. Becker, H. Kerkhoff, M. Kupsch, B. Langer, D. Szostak, et al.. PHOTOIONIZATION QF XENON WITH SOFT X-RAYS. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-497-C9-500.
�10.1051/jphyscol:1987980�. �jpa-00227401�
JOURNAL DE PHYSIQUE
Colloque C9, supplement au n012, Tome 48, decembre 1987
PHOTOIONIZATION OF XENON WITH SOFT X-RAYS
U. BECKER"), H.G. KERKHOFF, M. KUPSCH, B. LANGER, D. SZOSTAK and R. WEHLITZ
Institut fiir Strahlungs- und Kernphysik, Technische Universitdt Berlin, Sekr,
PN3-2, Hardenbergstrasse 36, 1)-1000 Berlin
12, F.R.G.Abstract: P a r t i a l cross sections and angular-distribution asymmetry parameters were measured f o r Xe 4d and 3d subshell photoionization using photon energies of 250 t o 1000 eV. E f f e c t s o f interchannel coupling are observed a t the onset o f the 3d cross section. Outside this energy region our results agree qualitatively w i t h the predictions o f the Hartree-Fock theory i f one takes i n t o account a l l photoelectrons associated w i t h a given subshell including rnultielectron processes.
Regarding inner shell processes xenon is i n many aspects unique, i t was and s t i l l is the testing ground f o r many theoretical models and deve10~ments.l Numerous experimental and theoretical studies have been published on the xenon case, many of them concerning subshell photoionization. However, i n the inner shell region beyond the 4d shape resonance few experimental data exist. Spectra a t high photon energy exhibit pronounced correlation e f f e c t s especially in the Xe 4p subshell, where the 4p hole s t a t e strongly couples w i t h double hole states of the 4d subshell especially 4d-24f. B u t there exists no comprehensive set o f subshell photoionization cross sections 0, and angular distribution asymmetry parameters Be covering the photon energy range between 150 eV and 1000 eV. There is even less quantitative information on the contribution o f multielectron processes such as simultaneous ionization and excitation and double ionization t o the t o t a l photoionization cross section. The t o t a l
photoionization cross section o f xenon a t high photon energy is dominated by contributions
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D-8700 WUrzburg, F.R.GArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987980
JOURNAL DE PHYSIQUE
f r o m subshells w i t h Q = 2. The characteristic feature o f the xenon photoabsorption spectrum is the pronounced modulation o f the 4d cross section due t o a centrifugal barrier- shape resonance and a Cooper m i n i m ~ m . ~ , ~ The question arises, how w e l l does theory describe this complex situation also w i t h respect t o the p a r t i a l cross sections and asymmetry parameters D ?
The energy region above 200 eV was the subject o f the present study.
Special emphasis was focussed on the onset of the 3d p a r t i a l cross section and i t s e f f e c t on the other already open photoionization channels. Here we present the f i r s t measurements o f Xe 4d and 3d p a r t i a l cross sections and asymmetry parameters between 250 eV and 1000 eV. We observed interchannel coupling between the 3d subshell and most other subshells a t t h e 3d threshold exhibited most dramatically i n rapidly changing angular distributions. Our results show that the sum o f one-
0
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.
[. . .
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(eV) Fig. 1: Time o f f l i g h t photoelectron spectra o f X e taken a t photonenergies of 350, 800, and 860 eV.electron processes and multielectron processes associated w i t h the ionization o f a given subshell resembles the theoretical Hartree-Fock (HF) cross sections reasonably well.
The experiments were performed a t the electron storage r i n g BESSY using t h e high-energy toroidal grating monochromator (HE-TGM 1). The measurements were performed under single- bunch conditions w i t h t w o rotatable t i m e o f f l i g h t analyzers, thus providing information on both cross sections and photoelectron angular
distribution^.^
Fig. 1 shows a sequence o f spectra taken a t d i f f e r e n t photon energies. The spectrum a t h v = 350 eV displays qualitatively the strengths o f the multielectron processes associated w i t h the 4d and 114p11 subshellphotoionization. The t w o other spectra show the 3d photoemission main lines w i t h their associated satellite structure and Auger transitions.
The p a r t i a l cross sections o f 4d and 3d subshell photoionization are shown i n Fig. 2 together w i t h the corresponding asymrnetry parameters. The l o w energy p a r t o f the Xe 4d data includes values of Lindle e t aL5 taken around the Cooper minimum o f this subshell. I n the overlapping region both sets o f data are i n good agreement. The theoretical curve representing a Hartree- Fock calculation w i t h i n the velocity approximation HF(V) l y 6 describes the experimental data reasonably w e l l i n particular the cross section. The calculation p a r t l y underestimates the observed 4d cross sections which, however, include multielectron processes which contribute approximately 25 percent. There exist no theoretical B values a t higher photon energies beyond the strong B-oscillation due t o the shape resonance and the Cooper minimum, but extrapolating the trend shown by the calculations suggests qualitative agreement w i t h our data.
6 5
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1 0
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0.5
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0.0
-0.5
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200 400 600 800 1000 700 800 900 1000
Photon energy (eV)
Photon energy(eV)
Fig. 2: Xe 4d and 3d partial cross sections and angular distribution asymmetry parameters together w i t h theoretical curves l y 6 and t o t a l cross section data.3 The f i l l e d circles represent our 4d results ( 0 4 ~ and B4d)y the open circles are 4d data f r o m Lindle et a ~ . ~ Our 3d results are given separately f o r the t w o spin-orbit components 3 d 3 p and 3 d s p by f i l l e d and open circles.
The diamonds represent the sum o f the O(3dj/p) and o(3dSl2) which nearly coincide w i t h the MNN Auger intensities a t a l l energies where both components were measured.
C9-500 JOURNAL DE PHYSIQUE
The 4d cross section is only weakly affected by the onset o f the 3d subshell photoionization around 680 eV. This is i n marked contrast t o the behaviour o f the asymmetry parameter B4d which shows a strong modulation i n the photon energy region o f the 3d onset and a trend t o lower B values above 700 eV. A comparison of B4d w i t h the asymmetry parameter of the "4pft subshell shows a close similarity. This similar behavior supports the idea t h a t the "4p" peak must be related t o photoionization o f the 4d shell i n conjunction w i t h simultaneous excitation or ionization o f a further 4d electron than t o ionization o f the 4p s ~ b s h e l l . ~
Considering the 3d p a r t i a l cross sections and asymmetry parameters, we see t h a t the onset o f this channel is largely overestimated b y the HF calculation. This result agrees w i t h the conclusions already drawn from the absorption spectrum t h a t rearrangement o f the outer electrons upon the creation o f the 3d hole causes significant quenching o f the 3d cross section near threshold. Amusia and Ivanov took this e f f e c t i n t o account w i t h i n the generalized random-phase approximation w i t h exchange (RPAE), and their results agree reasonable w i t h the experimental data. I n contrast t o the failure of the HF-method i n quantitatively describing the 3d p a r t i a l cross sections there is b e t t e r agreement between experiment and this theory w i t h respect t o the asymmetry parameter B i n particular near threshold. However, the fact, t h a t the theoretical values are generally t o high a t higher energies and the difference i n !3 between the 3d512 and 3d312 spin-orbit components regardless o f the kinetic energy s h i f t shows t h a t a final quantative description o f the 3d subshell photoionization requires t h a t further effects, such as spin-orbit coupling, electron correlations and interchannel coupling must be taken into account.
The authors are indebted t o W.Braun and Professor A.M.Bradshaw for their help and cooperation w i t h the high-energy TGM. This work was supported by the Bundesminister fijr Forschung und Technologie (05 314 E X 82).
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