It was pointed out that 2-dimensional self-diffusion processes at surfaces could be studied at the atomic scale by means of the quasi-elastic scattering of low-energy He atoms. The analytical power of this use of He atom scattering was demonstrated for the case of a (110) Pb surface at temperatures close to the bulk melting point of 600.7K. The width of the quasi-elastic scattering energy distribution of diffusely scattered He atoms was a direct measure of the lateral atomic mobilities at the surface. The results showed that, at homologous temperatures above 0.75, the atoms of the surface had appreciable lateral diffusivities. At temperatures above about 535K, the surface mobilities exceeded the value for the bulk liquid. Measurements of the quasi-elastic energy broadening, as a function of parallel momentum transfer, provided direct information on surface diffusion mechanisms. The results revealed the existence of a marked directional anisotropy. The diffusivity could be described in terms of jumps along the [1¯10] and [001] directions. The jump lengths along the close-packed [1¯10] direction seemed to be continuously distributed around an average jump length of about 0.44nm. Diffusion along the [001] direction proceeded via jumps over single lattice spacings.

J.W.M.Frenken, B.J.Hinch, J.P.Toennies, C.Wöll: Physical Review B, 1990, 41[2], 938-46