Electron-Energy-Loss Spectroscopy and Oxygen Overlayers on the Ni(100) Surface

We present an analysis of the electron-energy-loss spectra produced by dipole scattering from lattice vibrations of the $p(2\mathrm{}\times 2)$ and $c(2\mathrm{}\times 2)$ oxygen overlayer on the Ni(100) surface. We conclude that the distance between the oxygen layer and the Ni surface is very much smaller for the $c(2\mathrm{}\times 2)$ overlayer than for the $p(2\mathrm{}\times 2)$, in accord with a recent suggestion by Upton and Goddard.