Adsorption of S on the (1Â 1Â 1) surfaces of the noble metals Ag and Au studied by direct recoiling spectroscopy

In this work we present a study of the adsorption of S on the surfaces of the noble metals Au(1 1 1) and Ag(1 1 1) by ion scattering performed in the forward direction. Low energy electron diffraction patterns taken at specific exposures were used to identify the S phases. Photoemission and Auger electron spectroscopies were used to determine the coverage dependence on dose. All the techniques indicate that the adsorption process is different in both surfaces since very low coverages. Analysis of the scattering peak intensity and shape provides evidence about the coexistence of Ag substrate atoms together with the S adsorbate in the top layer for all the coverages investigated. For Au we discard this effect for coverages up to ∼0.3 ML and measure a small Au recoil peak coming from the top layer for the high coverage regime (∼0.5 ML). At intermediate coverages the shape of the scattering spectra presents a strong dependence on azimuth that is attributed to S-S shadowing and consistent with the diffraction symmetry. The variations on azimuth are analyzed with a simple shadowing calculation that uses the crystallographic model proposed by Yu et al. (2007) [1].