Previous quasi-elastic helium atom scattering studies of the low-coverage diffusion of sodium (ΘNa≤0.05) on the Cu(001) surface were extended up to coverages of ΘNa=0.18. These measurements, at surface temperatures between 180 and 390K and parallel wave vector transfers up to ΔK=3.34/Å, were compared with molecular dynamics simulations that included the dipole-dipole interactions between the Na atoms. Whereas the simulations could explain the increase in the observed activation energy and the change in the shape of the peak width with momentum transfer, they failed to reproduce the marked overall increases in peak widths observed as the coverage was increased.

Coverage Dependence of the Microscopic Diffusion of Na Atoms on the Cu(001) Surface: a Combined Helium Atom Scattering Experiment and Molecular Dynamics Study. Ellis, J., Graham, A.P., Hofmann, F., Toennies, J.P.: Physical Review B, 2001, 63[19], 195408