Hydrogen-induced phonon anomaly on the W(110) surface

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 {Γ}{} H{} ([001]) direction. The anomalous behavior extends into the neighborhood of this high-symmetry direction.