Theory of the Anomalous Rayleigh Dispersion at H/W(110) Surfaces

Abstract
The nature of the anomalies observed in the helium-atom scattering (HAS) and electron energy-loss (EELS) spectra of hydrogenated W(110) surfaces is investigated by calculating the full dispersion of the Rayleigh mode, using density-functional perturbation theory. We find excellent agreement between our calculations and the upper anomalous branch which is observed with both HAS and EELS. The lower branch—which is only detected by HAS—is instead not predicted. A hitherto unrecognized mechanism possibly responsible for the appearance of the second branch is proposed.