Anisotropic diffusion at a melting surface studied with He-atom scattering

Abstract

Two-dimensional self-diffusion processes at surfaces can be studied on an atomic scale with quasielastic scattering of low-energy He atoms. The analytical strength of this new application of He-atom scattering is demonstrated for the Pb(110) surface at temperatures close to the bulk melting point, $T_m^{\mathrm{Pb}=600.7\mathrm{}}$ K. The width of the quasielastic-scattering energy distribution of diffusely scattered He atoms is a direct measure of the lateral atomic mobilities at the surface. The results show that at T≥(3/4$T_m^{\mathrm{Pb}}$ the atoms of this surface have noticeable lateral diffusivities. Above ∼535 K the surface mobilities exceed the bulk-liquid value. Measurements of the quasielastic energy broadening as a function of parallel momentum transfer provide direct information on the surface diffusion mechanisms. The results exhibit a strong directional anisotropy. The diffusion can be described in terms of jumps along the [11¯0] and [001] directions. Jump lengths along the close-packed [11¯0] direction seem to be continuously distributed around an average jump length of ∼4.4 Å. Along [001] the diffusion proceeds in jumps over single lattice spacings.