Electronic interactions in bimetallic systems: Core-level binding energy shifts

Abstract

The electronic properties of Cu, Ni, and Pd films supported on single-crystal metal surfaces have been examined by means of x-ray photoelectron spectroscopy (XPS). The shifts in core-level binding energies indicate that adatoms in a monolayer of Ni, Cu, or Pd are electronically perturbed with respect to surface atoms of Ni(100), Cu(100), or Pd(100). The present results show a correlation between the shifts in XPS surface core-level binding energies and variations in the desorption temperature of CO from metal adlayers. The shifts in XPS binding energies and CO desorption temperatures can be explained in terms of (1) variations that occur in the admetal–admetal interactions when the admetal adopts the lattice parameters of the substrate and (2) effects of the admetal–substrate interaction. Chemisorption of CO induces a large decrease in the electron density of the metal adlayers. This is a consequence of (1) charge transfer from the metal adlayer to the substrate (induced by a repulsive interaction between the admetal σ charge and electrons in the 5σ orbital of CO) and (2) transfer of electrons from the admetal into the 2π orbitals of CO (π backbonding).