Electron Donor-Electron Acceptor Interactions in Bimetallic Surfaces: Theory and XPS Studies

The electronic interactions in Pd/Mo( 1 IO), Pd/Ru(0001), and Cu/Mo( 110) 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 Cu or Pd are electronically perturbed with respect to surface atoms of Cu(100) or Pd(100). By comparison of these results with those previously presented in the literature for adlayers of Pd or Cu, a simple theory is developed that explains the nature of electron donor-electron acceptor interactions in metal overlayers: formation of surface metal-metal bonds leads to a gain in electrons by the element initially having the larger fraction of empty states in its valence band. This behavior is contrary to that predicted by bulk electronegativities and observed in bulk alloys, indicating that the electronegativities of the surface atoms are substantially different from those of the bulk.