Model calculations for the anomalous M4,5VV Auger spectrum for dilute palladium in silver.

Abstract

A model Hamiltonian is introduced for an impurity in a filled band host, and where the intra-atomic electron interaction U on the impurity site is included. The Auger spectrum ending in two holes in the valence band (VV) is solved exactly for the impurity. For a certain range of values of U there is besides the normal Cini-Sawatzky quasiatomic peak a new, discrete two-hole state at much lower binding energy. For a one-dimensional system the two-hole problem is treated exactly and the solution is used to explain the nature of the new two-hole bound state. Also, a self-consistent mean-field type of treatment is applied to elucidate the origin of the two bound states. Extensions of the model to include the intra-atomic Coulomb interaction on the host atoms show also that interatomic quasiatomic Auger processes contribute to the decay of the initial impurity core hole. In connection with this, a detailed comparison between the Cini and Sawatzky expressions for the spectral distribution function is performed. The solution for the model Hamiltonian is used to explain the strong anomaly of the Pd ${M}_{4}$,5VV Auger spectrum for dilute palladium alloys with silver. By a schematization of the palladium local density of states, a direct comparison between the calculated and the experimental spectrum is made and a good agreement is found. By variations of the local density of states and the intra-atomic Coulomb repulsion, the conditions for the appearance of the anomalous peak are investigated.