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Abstract

Auger parameter (Δα) 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 (ΔBEs) 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 E_F), 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 ΔBE noted by XPS.