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 H-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 exp --1 along the Gamma H (\001\) direction. The anomalous behavior extends into the neighborhood of this high-symmetry direction.