VV AND CV CORRELATION-EFFECTS IN CVV AUGER-ELECTRON AND APPEARANCE-POTENTIAL SPECTROSCOPY - EXACT RESULTS FOR LIMITING CASES

We investigate the influence of correlations among the valence-band el ectrons (VV correlations) and correlations between the valence-band an d core electrons (CV correlations) in CVV Auger-electron spectroSCopy (AES) and appearance-potential spectroscopy (APS). The AES and APS int ensities are given by properly defined three-particle spectral densiti es, which are exactly determined for the limiting cases of the complet ely filled and empty valence bands. We solve the equations of motion f or the corresponding three-particle Green functions within the framewo rk of the single-band Hubbard model, which is extended to include, in addition to the on-site Coulomb interaction U among the valence-band e lectrons, the on-site Coulomb interaction U(c) between valence-band an d core electrons as well. For AES the calculation can be done analytic ally, yielding the same result as in the Cini-Sawatzky model except fo r an additional energetic shift of the spectrum by 2U(c). For APS the calculation has to be performed numerically. The role of the core-hole potential turns out to be qualitatively different from that for AES. The APS Spectrum may exhibit up to three different features, which are ascribed to effects of final-state correlations: the band-like part o f the spectrum corresponds to final states in which both valence-band electrons are moving independently through the lattice. In the case of strong correlations two satellites are additionally observed. The fir st one corresponds to two-electron bound states that are more or less localized at the site where the transition takes place. It has a small width and takes almost the whole spectral weight as soon as it is spl it off. The second one has a width equal to the width of the free Bloc h band and quite a small spectral weight. It is interpreted as belongi ng to final states in which one electron is localized in the core-hole potential while the other one is moving through the lattice. Apart fr om this rather weak satellite feature, the APS line shape is qualitati vely well described within the Cini-Sawatzky model, provided that the coupling parameter U is replaced by an effective coupling U(eff) = U - 2U(c).