Electron-energy-loss spectroscopy and ordered adsorbate layers on the Ni(100) surface

We have studied the lattice dynamics of ordered overlayers of adsorbate atoms on the (100) surface of an fcc crystal, with emphasis on contact with recent electron-energy-loss data on the $p(2\mathrm{}\times 2)$ and $c(2\mathrm{}\times 2)$ overlayers of oxygen on the Ni(100) surface. This is done within a model that fits the bulk phonon spectrum of Ni with nearest-neighbor centralforce couplings, and extracts adsorbate-substrate force constants from ab initio calculations of potential-energy curves carried out by Upton and Goddard. We obtain a very good fit to the data, but only if we assume the $c(2\mathrm{}\times 2)$ layer to be much closer to the surface than the $p(2\mathrm{}\times 2)$, as suggested by Upton and Goddard. We have also applied the model to ordered overlayers of sulfur on the Ni(100) surface, and we provide theoretical energy-loss spectra for this case. Throughout the discussion an emphasis is placed on features in the energy-loss spectrum which lie below the maximum phonon frequency of the substrate.