PHYSICAL REVIE% A VOLUME 5Q, NUMBER 1 JULY 1994 Atomic structure calculations of KLL Auger spectra from highly charged ion — solid-surface collisions S. Schippers, J. I imburg, 3. Das, ' R. Hoekstra, and R. Morgenstern Kernfysisch Versneller Instituut, Zernikelaan M, NL 97)-7AA Groningen, The Netherlands (Received 24 February 1994) Systematic atomic structure calculations of KLL Auger electron spectra from "hollow" atom configurations 1s2E 3E are performed for atomic numbers 6 ( Z ( 10. In comparison with the corresponding ls2E Li-like configurations it is shown that the additional Z — 3 M electrons, apart from taking part in LLM Coster-Kronig transitions, can be regarded basically as mere spectator electrons, neither severely in6uencing the peak structure due to the Li-like core nor affecting the KLL Auger transition rates. However, due to screening effects the spectra are shifted to higher energies by an amount scaling with gZ — 3 when adding the Z — 3 M electrons. The results facilitate the identification of distinct narrow peaks in projectile K Auger spectra observed in slow highly charged ion-surface interactions. It is shown that especially competing LLM Coster-Kronig transitions have to be considered in order to account for the measured relative peak intensities. A cascade model of the last deexcitation steps of a highly charged ion in front of the surface based on the atomic structure data provides detailed insight into the time scales involved. PACS number(s): 79.20.Rf, 31. 50.+w, 34.70.+e I. INTRODUCTION In the past few years K Auger electron spectroscopy has been intensively used as an experimental tool for studying the neutralization of highly charged ions at solid surfaces. Depending on the projectile's velocity "above" surface [1 — 4] and "below" surface [5 — 8) It Auger electron emission has been observed. In the latter case the pro- jectile's deexcitation is not yet complete when it hits the surface, so that part of the K vacancies are 6lled only after penetration of the projectile into the surface [9]. Hence below surface K Auger electron spectra are inHu- enced by elastic and inelastic scattering processes inside the solid [10] and generally appear as broad structures with a low energy tail due to energy losses of the emit- ted electrons inside the solid. Furthermore it has been found that below surface K Auger electron emission oc- curs at a rather late stage of the projectile deexcitation cascade where the projectile L shell already contains on the average five electrons [7,8]. In contrast to the broad below surface emission struc- ture above surface K Auger electron emission is char- acterized by sharp narrow peaks. Their Doppler shift, due to the emission from the moving projectile, clearly shows that these electrons are emitted on the incident part of the projectile trajectory [3]. According to a sce- nario which assumes stepwise deexcitation of the highly charged ions proceeding from outer to inner projectile shells, it has been argued that the observed above sur- face KLL Auger peaks stem from configurations with only two L electrons and many electrons in higher she11s. 'Present address: Max Planck-Institut fur Plasmaphysik, D-85748 Garching, Germany. Atoms in such transient configurations with highly in- verted populations of the principal shells have been termed "hollow" atoms [ll] and are a genuine feature of the highly charged ion-surface interaction. Recently we presented a comprehensive study using H-like and metastable He-like C, N, and 0 ions as pro- jectiles and a monocrystalline p-doped Si surface as a tar- get [12]. Several distinct above surface peaks have been observed and indentified with the aid of atomic struc- ture calculations. In particular they can be attributed to ALL Auger transitions from configurations ls2p23l+ ls2s2p3P s, and ls2s23E s with only two L electrons and as many M electrons as needed in order to neu- tralize the projectile of nuclear charge Z. Moreover we showed that competing Coster-Kronig transitions trans- forming the population f'rom ls2p23P s to ls2s2p3P configurations and from there tp 1828 3$ configura- tions have to be considered in order to account for the observed relative peak intensities. While our previous paper stresses the experimental findings, we here present the details of our atomic struc- ture calculations. It is noted that extensive atomic struc- ture calculations of K Auger electron energies from hol- low atoms have been performed previously [7,8, 13]. In those works configurations with different numbers of L electrons as appropriate for below surface studies have been considered. However, eKects originating &om difer- ent subshell populations have been disregarded. In this work we exclusively deal with configurations containing two L electrons causing the major part of above surface KLL Auger electron emission and we show that the elec- tron spectra are sensitive to diferent populations of the 2s and 2p L subshells. The organization of the paper is as follows. In Sec. II experimental KLL Auger spectra from 0 + and metastable 0 + ions colliding with a monocrystalline p- 1050-2947/94/50(1)/540(13)/$06. 00 50 1994 The American Physical Society