Dynamics of atomic adsorbates: S on Cu(1 0 0)

Dissociated H2S on Cu(1 0 0) at 300 K has been studied by a combination of Auger electron spectroscopy, and infrared reflection absorption spectroscopy (IRAS) in the 400–2200 cm−1 frequency range using synchrotron radiation. The absorption of ir radiation across a broad range of frequencies upon S adsorption can be explained by diffusive scattering of substrate electrons from the adsorbates. S coverages up to and including the p(2×2) overlayer (0.25 monolayers (ML)) induce a change in reflectivity ΔR/R at high frequency (2200 cm−1) which changes monotonically with increasing coverage up to 0.044 ML, and then remains constant. Persson and Volokitin have proposed a model where the linear change in ΔR/R versus coverage is proportional to a scattering cross-section σ of the adatoms for the substrate free electrons. The observed behavior suggests a coverage-dependent σ for the S adsorbates. S at very low coverages, adsorbed at defect sites and step edges, has a σ∼34 Å2. At higher coverages S adatoms occupy fourfold hollow terrace sites, where either local structural affects related to the bonding site or coherent scattering from locally ordered adsorbates can alter the S scattering cross-section. Surprisingly, σ for these additional adatoms is less than 10% of σ for the S adatoms occupying the defect sites and step edges.