Surface dynamics and friction of K/Cu(001) characterized by helium-3 spin-echo and density functional theory

Helium-3 spin-echo measurements of K/Cu(001) are presented, the diffusional surface dynamics of the system at low coverages and on picosecond time scales. Langevin molecular-dynamics simulations are used, together with a potential-energy surface derived from density functional theory calculations, to provide further understanding of the experimental data. An anisotropic potential with a corrugation of 35-64 meV and a friction parameter of 1/4.5 ps-1 are found to give a good fit to the data for the lower coverages with the adsorbate interactions modeled with a dipole-dipole repulsion. Additionally, at the highest coverage, θ=0.084, a component of motion is observed perpendicular to the surface, analogous to that recently found for Na/Cu(001).