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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.
Bungaro et al. (Mon,) studied this question.