Self-consistent Green's-function technique for bulk and surface impurity calculations: Surface core-level shifts by complete screening.

Abstract

We have implemented an efficient self-consistent Green's-function technique, based on the tight-binding linear-muffin-tin-orbitals method, for calculating the electronic structure and total energy of a substitutional impurity located either in the bulk or at the surface. The technique makes use of the frozen-core and atomic-sphere approximations but, in addition, includes the dipole contribution to the intersphere potential. Within the concept of complete screening, we identify the surface core-level binding-energy shift with the surface segregation energy of a core-ionized atom and use the Green's-function impurity technique in a comprehensive study of the surface core-level shifts (SCLSs) of the 4d and 5d transition metals. In those cases, where observed data refer to single crystals, we obtain good agreement with experiment, whereas the calculations typically underestimate the measured shift obtained from a polycrystalline surface. Comparison is made with independent theoretical data for the surface core-level eigenvalue shift, and the much debated role of the so-called initial- and final-state contributions to the SCLS is discussed.