SE-INDUCED 3D CORE-LEVEL SHIFTS OF GAAS(110)

We present ab initio pseudopotential calculations of 3d core-level shi fts of the surface atoms for clean and Se-deposited GaAs(110) surfaces . We are able to explain the experimental findings for the clean GaAs( 110) surface in the picture of the initial-stale model. If relaxation effects are accounted for we find a distinct overestimation of the sur face core-level shifts (SCLS) both for As and Ga atoms with respect to experimental results. We conclude that final-state effects play only a minor role in the dynamics of the photoemission process for the GaAs (110) surface. In order to clarify the surface chemistry of the Se/GaA s(110) system we determine the SCLS of the species involved for a seri es of structural models. Only one of these models gives rise to a reas onable agreement between calculated and measured SCLS. Therefore we st rongly support a geometry where each surface As atom is substituted by Se and one further Se binds to the surface Ga atom.