Surface core-level shifts of clean and oxygen-covered Ru(0001)

We have performed high resolution x-ray photoemission spectroscopy experiments on the Ru(0001) surface, both clean and covered with well-defined amounts of oxygen up to 1 monolayer coverage. For the clean surface we detected two distinct components in the Ru 3d(5/2) core-level spectra, fur which a definite assignment was made using the high resolution angle-scan photoelectron diffraction approach. For the p(2 X 2), p(2 X 1), (2 X 2)-3O, and (1 X 1)-O oxygen structures we found Ru 3d(5/2) core-level peaks that are shifted to higher binding energies by up to 1 eV. Very good agreement with density functional theory calculations of these surface core-level shifts (SCLS's) is reported. The overriding parameter for the resulting Ru SCLS's turns out to be the number of directly coordinated O atoms. Since the calculations permit the separation of initial and final state effects, our results give valuable information for the understanding of bonding and screening at the surface that is not accessible in measurements of the core-level energies alone.