THEORY OF THE ANOMALOUS RAYLEIGH DISPERSION AT H W(110) SURFACES/

The nature of the anomalies observed in the helium-atom scattering (HA S) and electron energy-loss (EELS) spectra of hydrogenated W(110) surf aces is investigated by calculating the full dispersion of the Rayleig h mode, using density-functional perturbation theory. We find excellen t agreement between our calculations and the upper anomalous branch wh ich is observed with both HAS and EELS. The lower branch-which is only detected by HAS-is instead not predicted. A hitherto unrecognized mec hanism possibly responsible for the appearance of the second branch is proposed.