Journal of Electron Spectroscopy and Related Phenomena 114–116 (2001) 679–682 www.elsevier.nl / locate / elspec
Quasi-atomic MVV Auger spectra of Pd metal: cascade processes A. de Siervo, R. Landers, M.F. Carazzolle, J. Morais, G.G. Kleiman* ´ Instituto de Fısica ‘ Gleb Wataghin’, Universidade Estadual de Campinas, Caixa Postal 6165,13081 -970 Campinas, SP, Brazil Received 8 August 2000; received in revised form 12 August 2000; accepted 12 August 2000
Abstract The study of Auger spectra of atoms affected by cascade processes has attacted considerable interest because of the availability of tunable photon energies of synchrotron radiation. We present results for the XAES MVV spectra of Pd and Ag measured, with synchrotron radiation, before and after ionization of the respective L 3 levels. The respective spectra change dramatically upon ionization and we attribute the extra structure produced after ionization as corresponding to the M 45 M 45 →M 45 VV transition, which is subsequent to the L 3 M 45 M 45 transition. The extra structure for Pd is markedly similar to that of Ag, indicating its quasi-atomic nature. These results allow us to interpret the extra spectral structure of Pd as quasi-atomic and to attribute its origin to a bound state of the two final-state holes in the full Pd d-band. We believe the filling of the band to be produced by d-valence electrons which screen the two initial M 45 holes. 2001 Elsevier Science B.V. All rights reserved. Keywords: Cascade; Auger; MVV; Screening; 4d metals
1. Introduction Cascade affected Auger spectra [1–3] have attracted interest because of the opportunity of using synchrotron radiation in studies of atoms [4–16] and, to a lesser extent, of solids [17–20]. The effect of such cascade processes on the well-studied M 45 N 45 N 45 (or MVV) valence band spectra of the 4d metals would be interesting to investigate. Our understanding of the MVV spectra of the 4d metals comes from the theories of Cini [21] and Sawatzky [22], which were formulated to explain the observation of the quasi-atomic MVV spectrum of *Corresponding author. Tel.: 155-19-788-5365; fax: 155-19-7885376. E-mail address:
[email protected] (G.G. Kleiman).
Ag [23]. When the holes’ effective Coulomb interaction (U ) (or Auger parameter) is much larger than the bandwidth (W ), bound states of the two valenceband holes are formed and the MVV spectrum is quasi-atomic, as in the case of Ag. When W4U, the spectrum is bandlike. When W¯U, the spectrum does not present atomic multiplet structure and it is strongly influenced by band effects, as in the case of Pd [24]. The systematic behavior of experimental energies of these spectra are explained by models exploiting the complete self-consistent screening of the core holes within the atomic cell [25–29]. Studies of the experimental Auger parameters [30–32] indicate that the screening of d-valence band holes is strikingly similar to that of core-holes, so that the explanation of the strength of U in terms of the nature of the
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00292-9
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screening electrons appears to be valid for d-valence band holes as well [24]. The complete screening model is normally assumed to be valid in analyses of spectra such as the MVV spectrum of Pd [24], but direct observation of spectra which are unambiguously associated with the corresponding fully relaxed states has not been reported. In principle, the Pd MVV spectrum would become quasi-atomic should the Pd d-band become full [21,22]. Quasi-atomic Pd MVV spectra have been observed in alloys [33], indicating a full Pd d-band and a large value of U. In the context of the complete screening model, it is interesting to study the effect of extra core holes produced by cascade processes on the MVV spectrum. In particular, measurements of the MVV spectra before and after ionizing the L 3 level should elucidate the situation. The final MM two-hole state of the L 3 M 45 M 45 transition can serve as the initial-state for an MVV transition. Would the resulting spectrum reflect the full d-band predicted by the complete screening model? This paper represents a preliminary report of results for the X-ray excited Auger (XAES) MVV spectra of Pd and Ag measured, with synchrotron radiation excitation, before and after ionization of their L 3 levels. The spectra change dramatically upon ionization and we attribute the extra structure produced after ionization as corresponding to the M 45 M 45 →M 45 VV transition. The extra structure for Pd is very similar to that of Ag, indicating its quasi-atomic nature. Results of preliminary atomic calculations for this rather complicated transition agree quantitatively with the experimental spectra of Pd and Ag. These results suggest that the extra spectral structure of Pd is quasi-atomic. We attribute the quasi-atomic form to a bound state of the two final-state holes in the full Pd d-band, which is filled by d-valence electrons which screen the two initial M 45 holes.
using a bending magnet beam line with the SXS double crystal monochromator (E /DE54000) at the Brazilian National Synchrotron Laboratory (LNLS) and an Omicron EA 125 / HR hemispherical analyzer with multi-detection operated in the FAT44 mode. In Fig. 1, we present the experimental MVV XAES spectra of pure Ag (L 3 binding energy 3351 eV) before and after ionizing the L 3 core level. The corresponding exciting photon energies are 3335 eV and 3364 eV. We have removed the background [34] from all the spectra in Fig. 1. The normal MVV regions of the Ag spectra before and after L 3 ionization are very similar and both are strongly quasi-atomic in nature. Upon L 3 ionization, a strong extra structure located at around 20 eV above the main peak appears. From the arguments given above, we associate this extra structure with the transition M 45 M 45 →M 45 VV and denote this feature as MVV2. We expect the final-state Ag valence band holes to be strongly bound in the presence of the extra M 45 hole. Within this interpre-
2. Experiment and theory
Fig. 1. MVV spectra of Ag before and after ionization of the L 3 level. The spectra before and after were, respectively, excited with photon energies of 3335.0 eV and 3364.0 eV. After ionization the spectra manifest strong extra structures (the ‘MVV2’ structure) at energies higher than that of the normal MVV transition. The spectrum of Ag shows quasi-atomic, atomic multiplet, form in the MVV spectrum region before and after ionization as well as in the MVV2 extra structure.
The samples were high purity thick foils polished to a mirror finish and then cleaned in situ through sputtering (with 2000 eV Ar ions) and annealing cycles until all traces of impurities were removed. The spectroscopic measurements were performed
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tation, the MVV2 spectrum reflects the atomic multiplet structure produced by the coupling of the three final-state holes (M 45 and two valence band holes). In Fig. 2, we present the corresponding spectra for Pd (L 3 binding energies 3173 eV). The corresponding exciting photon energies are 3164 eV and 3210 eV. Again the normal MVV regions in Fig. 2 are very similar and are not quasi-atomic, which reflects the weakly bound state of the two final-state valence band holes of the MVV spectra. Examination of the MVV2 region, however, indicates the presence of considerable structure. Indeed, the similarity with the Ag MVV2 region is striking. On the basis of this comparison, one would be led to conclude that the Pd MVV2 structure is truly quasi-atomic in nature, indicating a strongly bound state of the two finalstate valence band holes. Such a strongly bound state would seem possible only if the d-band were full. That is to say, the form of the structure presented seems possible because the screening electrons have filled the Pd d-band. We also performed preliminary calculations from
Fig. 2. MVV spectra Pd before and after ionization of the L 3 level. The spectra before and after were, respectively, excited with photon energies of 3164.0 eV and 3210.0 eV. In contrast to Ag (Fig. 1), Pd does not exhibit atomic multiplet structure in the MVV spectra before and after ionization. The similarity of the Pd MVV2 spectrum with that of Ag indicates its quasi-atomic nature.
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non-relativistic atomic theory [35–38] of the probabilities of the M 45 →M 45 VV and M 45 M 45 →M 45 VV transitions. Details of the calculation are complicated and we reserve them for discussion elsewhere [39]. In order to describe the total experimental spectrum, it was necessary to combine fluorescence [40–42], normal Auger, satellite contributions and normal Auger resulting from cascade processes. The parameters used were either experimental or calculational [39], without adjustable parameters. The atomic calculations are successful in quantitatively describing the MVV2 spectrum of Pd, but fail to agree with the MVV spectrum, in accordance with our discussion of the Pd MVV. Similar agreement was reached for both the MVV and MVV2 spectra of Ag.
3. Conclusions Ionization of the L 3 level using synchrotron radiation produces sizeable extra structure in the MVV spectra of Pd and Ag, associated with the transition from the M 45 M 45 two hole state (which is the final-state of the L 3 M 45 M 45 Auger transition) to the M 45 VV three hole state, denoted as the MVV2 transition. The forms of the MVV2 and MVV transitions of Ag after L 3 ionization are quasiatomic, in agreement with the complete screening model. Pd, on the other hand, presents a different behavior. The Pd MVV spectra before and after L 3 ionization have the same shape, which is associated with final-state valence band holes which are only weakly bound, so that the MVV spectra are not quasi-atomic. The MVV2 spectrum of Pd, however, is very similar in form to that of Ag, indicating its quasi-atomic nature. The results of preliminary atomic calculations agree reasonably well with the experimental Pd MVV2 spectrum, in shape as well as in relative intensity. The calculations, of course, do not describe the shape of the Pd MVV spectrum. We conclude that the MVV2 spectrum is truly quasiatomic in nature and is associated with a full initialstate d-band, filled by d-screening electrons. In short, the MVV2 transition can be considered as occurring between the fully relaxed initial-state of the system with two M 45 holes and the fully relaxed final-state with one M 45 and two valence band holes.
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Acknowledgements The authors would like to thank FAPESP, CAPES, CNPq, LNLS and the Pronex Program of Brasil for support.
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