New analysis of lone-pair binding-energy shifts in photoemission from adsorbed molecules: CO andNH3on Cu(100)

Abstract

The origin of the differential ionization potential shifts of the CO 5σ and ${\mathrm{NH}}_3$ 3$a_1$ lone pairs observed in the photoemission spectra of these molecules chemisorbed on transition-metal surfaces is identified. These shifts are unambiguously shown to arise largely from electrostatic, environmental-potential effects. The consequences of interunit charge transfer, donation and back donation, and of intraunit polarization for these differential ionization-potential shifts have been determined using the constrained-space-orbital-variation method. They are found to be smaller than the electrostatic shift due to the superposition of the unperturbed charge distributions of the separated, metal and ligand units. Thus, these differential shifts do not indicate the nature of the adsorbate-metal chemical bond; they may, however, indicate the adsorption geometry. Ab initio Hartree-Fock cluster-model studies of chemisorption at an on-top site for CO/Cu(100) and ${\mathrm{NH}}_3$/Cu(100) have been made, but the results are expected to apply to metal surfaces in general.