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Collective vibrations of the clean and hydrogen-saturated W (110) surface have been investigated with high-resolution inelastic helium-atom scattering. In contrast to the clean W (110) surface which exhibits ``normal'' surface phonon dispersion, the hydrogen-saturated surface shows a pronounced surface phonon anomaly in terms of a sharply defined dip in the surface phonon dispersion at Q=0. 9 A^-1 along the H (001) direction. The anomalous behavior extends into the neighborhood of this high-symmetry direction.
Hulpke et al. (Mon,) studied this question.