It was shown that the quasi-elastic scattering of low-energy He atoms could be used to study 2-dimensional diffusion on surfaces at the atomic scale. Measurements were made of self-diffusion along the [1¯10] azimuth on the (110) surface of Pb at 521K. Other experiments had shown that this surface was partially disordered at that temperature, due to the onset of surface melting. From the dependence of the width of the quasi-elastic energy distribution of scattered He atoms, upon the initial and final angles, both the diffusion mechanism and the diffusion coefficient were deduced. The results showed that the surface diffusion coefficient, along [1¯10] at 521K, exceeded the value for bulk liquid Pb at the melting point of 600.7K. The diffusive motion could be described in terms of a continuous distribution of jump lengths with an average value of about 0.44nm.

J.W.M.Frenken, B.J.Hinch, J.P.Toennies: Surface Science, 1989, 211-212, 21-30