Understanding surface core-level shifts using the Auger parameter: A study of Pd atoms adsorbed on ultrathin SiO 2 films

Abstract

Auger parameter (Î"α) 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 (Î"BEs) of 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 (1.1 eV towards EF), coupled with decreased final-state relaxation contributions (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 Î"BE noted by XPS. © 2014 American Physical Society.