Sensitivity of electron-energy-loss spectra to adsorption site: An ordered overlayer on the Ni(111) surface

For a model of the ($\sqrt{3}\times \sqrt{3}$) overlayer of atoms adsorbed on the Ni(111) surface, we calculate the electron-energy-loss spectrum for small-angle scattering by the adsorbate vibrations. A central issue is the sensitivity of the calculated spectra to the nature of the adsorption site. For frequencies below the maximum phonon frequency of the substrate, we find distinctly different spectra for the two inequivalent threefold sites. We also explore the spectrum when the adsorbate occupies a twofold bridge site. We obtain a good account of the data of Ibach and Bruchmann on the oxygen overlayer on Ni(111) if we place the adsorbate layer on the threefold hollow site (fcc site).