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 expla ined in the initial-state model, thus ignoring the screening of the ph oton-induced hole. We will show that this approach is not valid for th e (001) surfaces of Si and Ge. Using ab initio density-functional theo ry we calculate the SCLS from differences of total energies of slabs c ontaining excited atoms at different positions at the surface and in t he bulk. Comparison with initial-state results reveals an enhanced scr eening at the surface, which is even remarkably different for the two atoms forming the surface dimer.