Coverage dependence of the microscopic diffusion of Na atoms on the Cu(001) surface: A combined helium atom scattering experiment and molecular dynamics study

Previous quasielastic helium atom scattering studies of the low-coverage diffusion of sodium $({\Theta }_{\mathrm{Na}}<~0.05)$ on the Cu(001) surface have been extended up to coverages of ${\Theta }_{\mathrm{Na}}=\mathrm{0.18.}\mathrm{}$ These measurements, at surface temperatures between $T_s=180\mathrm{}$ and 390 K and parallel wave vector transfers up to $\Delta K=3.34\mathrm{}{\AA }^{-1},$ are compared with molecular dynamics simulations that include the dipole-dipole interactions between the Na atoms. Whereas the simulations can explain the increase in the observed activation energy and the change in the shape of the peak width with momentum transfer, they fail to reproduce the marked overall increases in peak widths observed as the coverage is increased. Several possible explanations are discussed and are shown to be unable to explain the results.