Interactions of adsorbates on copper(100) films investigated by surface resistivity measurements

Surface resistivity is demonstrated as a simple and non-invasive probe for investigating the electron dynamics in a thin metal film upon monolayer gas adsorption. The adsorbate-induced surface resistivity change---the increase in the resistivity of the thin metal film due to adsorbates---originates in the diffuse scattering of the conduction electrons by the perturbed potential introduced by adsorbates, corresponding to a complete randomization of the electron motion at the surface. The surface resistivity generally shows a linear coverage-dependence with adsorbates acting as independent, identical non-interacting scatterers. However, our studies concentrate on both an adsorbed system with a high density of surface defects, and a coadsorbed system with dissimilar adsorbates where the dependence of resistivity on coverage is not linear. Measurements of surface resistivity as a function of coverage for these complicated systems reveal the effects of adsorbate-defect or adsorbate-adsorbate interactions on the electron scattering.;The study of adsorbate-defect interaction effects has been carried out by measuring the contribution of CO adsorbed on different bonding sites to the resistivity of 50 nm epitaxial Cu(100) films. Terrace-bonded CO is found to make a much greater contribution to the surface resistivity than defect-bonded CO, which has a near-zero contribution corresponding to a near-zero net scattering cross section of electrons. The reduced scattering cross section of defect CO may be attributed to the higher local d-band energy at defects than at terraces. Another possible explanation involve strong scattering by the bare defect that is modestly reduced by the adsorption of CO.;Experiments on the resistivity-coverage relationship for oxygen adsorbed on sulfur-predosed Cu(100) films reveal the effects of adsorbate-adsorbate interactions. The surface resistivity change exhibits a linear coverage-dependence at low oxygen coverage while at high coverage the resistivity change due to added oxygen is almost completely suppressed by nearby sulfur atoms. Two types of interactions between sulfur and oxygen are considered: S-O repulsion and short-range suppression of the surface resistivity change. The repulsive interaction determines the occupation sequence of oxygen atoms, i.e. from sites far from the sulfur atoms, to sites close to sulfur. The resistivity suppression reduces the resistivity change essentially to zero when oxygen and sulfur atoms are adsorbed on adjacent sites. The apparently zero resistivity change results from zero net electron scattering, which may involve a reduction of oxyen's scattering cross section and an oxygen-induced reduction in scattering from the predosed sulfur.