Evidence for site-sensitive screening of core holes at the Si and Ge (001) surface

Typically surface core-level shifts (SCLS) of clean surfaces are explained in the initial-state model, thus ignoring the screening of the photon-induced hole. We will show that this approach is not valid for the (001) surfaces of Si and Ge. Using ab initio density-functional theory we calculate the SCLS from differences of total energies of slabs containing excited atoms at different positions at the surface and in the bulk. Comparison with initial-state results reveals an enhanced screening at the surface, which is even remarkably different for the two atoms forming the surface dimer.