The estimation of adatom vibrational mode frequencies in periodic adatom overlayers: Oxygen and carbon monoxide on Ni(100) as examples

We describe a simple method for estimating adatom vibrational frequencies when the adatoms are arranged in a periodic overlayer on the substrate crystal. The frequencies obtained are a function of the wavevector parallel to the crystal, and thus their dispersion can be determined. Estimated frequencies are compared with exact frequencies, obtained with Green function calculations, for p(2×2) and c(2×2) overlayers of oxygen on Ni(100) and a c(2×2) overlayer of CO on Ni(100).