THE GIANT SURFACE PHONON ANOMALY ON HYDROGEN SATURATED W(110) AND MO(110)

Surface phonon dispersion curves for clean and hydrogen covered W(110) and Mo(110) surfaces have been measured with inelastic helium atom scattering. Whilst the clean surfaces exhibit regular behavior, a sharp dip in the transverse surface phonon branch along the [0011 direction at a critical wavevector Q(c) is evident after hydrogen saturation. In addition, in the case of H/W(110) weak and broad elastic intensity is found in the He diffraction pattern at Q(c), indicative of critical scattering, which appears to be independent of the surface temperature in the range between 75 and 300 K. The origin of the phonon anomaly and its connection, if any, to an earlier reported ''top layer shift'' reconstruction observed with LEED has not yet been established.