Understanding surface core-level shifts using the Auger parameter: A study of Pd atoms adsorbed on ultrathin SiO2 films

Auger parameter (Delta alpha) measurements have been employed to determine the extent to which initial-and final-state effects govern surface core-level shifts in x-ray photoelectron spectroscopy (XPS) measurements of Pd atoms confined between a bilayer SiO2 film and its Ru(0001) support. For atoms bound in this manner, we note negative binding energy shifts (Delta BEs) of similar to 0.3 eV, relative to the Pd 3d peak position in the bulk, and attribute these shifts to large variations in the initial-state orbital energies of the supported atoms (similar to 1.1 eV towards E-F), coupled with decreased final-state relaxation contributions (similar to 0.8 eV). Theoretical calculations reveal that, despite small partial positive charges and decreased final-state screening, the decreased 4d-5sp hybridization of the undercoordinated Pd atoms results in large enough upward 3d orbital-energy shifts to yield the net-negative Delta BE noted by XPS.