Low-energy scattering by molecules adsorbed on metal surfaces

Abstract

Low-energy electron inelastic scattering by molecules adsorbed on metal surfaces is treated assuming the direct electron-induced dipole interaction. Previous calculations are improved by including the realistic reflectivity and phase shift for electron-metal surface scattering. A characteristic “gas-phase” term in the scattering amplitude, as well as three terms involving the electron-metal interaction, are analyzed. In the case of high reflectivity ( R 2 ≳ 0.1), the two forward-scattering terms dominate and the phase shifts factorize out, so in most experimental cases the semiclassical (“trajectory”) results are recovered. For lower reflectivities, the “gasphase” term also contributes. Numerical analysis shows that the conclusions of the perfect reflectivity ( R 2 = 1, δ = π/2) model remain qualitatively valid, though with reduced intensities. We also briefly comment on the question of the selection rule for vibrational excitation, and discuss the connection with other theories and available experimental data for CO adsorbed on high-reflectivity metals such as nickel.