  We present density-functional theory calculations of Ru 3d and O 1s surface
core-level shifts (SCLSs) at an oxygen-rich Ru(0001) surface, namely for the
O(1x1)/Ru(0001) chemisorption phase and for two surface terminations of fully
oxidized RuO_2(110). Including final-state effects, the computed SCLSs can be
employed for the analysis of experimental X-ray photoelectron spectroscopy
(XPS) data enabling a detailed study of the oxidation behaviour of the Ru(0001)
surface. We show that certain peaks can be used as a fingerprint for the
existence of the various phases and propose that the long disputed satellite
peak in RuO_2(110) XPS data originates from a hitherto unaccounted surface
termination.
