Plasmon localization by adatoms in gold atomic wires on Si(775).

Self-organized gold chains on vicinal Si(111) surfaces represent prototype examples of quasi-one-dimensional objects that are stabilized by hybridization with Si surface states. Their plasmons contain important information about the unoccupied bandstructure close to the Fermi level. Using Si(775)-Au as an example, we report here the modifications of the plasmon dispersion by the simple atomic adatom species H and O. Using a combination of low energy electron diffraction and high-resolution electron energy loss spectroscopy, we study the interconnection between plasmonic excitation and the corresponding local surface structure. Both adsorbates do not destroy metallicity, but, similar to Si(553)-Au, atomic hydrogen enhances dimerization of the Au chains, which at small concentrations counteracts the disorder introduced by random adsorption. This effect, most likely caused by electron donation of H to the surface states, is missing in case of adsorbed oxygen, so that only the effect of disorder is observed. For both adsorbates increasing disorder as a function of adsorbate concentration finally results in plasmon localization and opening of a band gap.