Atomic geometry of the PbS(100) surface

The surface structure of PbS(100), in the form of the mineral galena, was studied with the X-ray standing wave technique using surface-sensitive Auger electron yield detection and normal incidence geometry. We find that neither S nor Pb surface atoms are displaced from their respective bulk site positions. This lack of surface relaxation differs from the recent suggestion based on theory that a sizable contraction is needed to explain the experimentally observed lack of a surface core-level shift in Pb 5d photoemission data. To shed more light on this issue, we have also measured high-resolution Pb 4f, Pb 5d, and S 2p photoemission spectra for PbS(100) and confirmed that the surface core-level shifts are absent for cations as well as for anions.