Low-frequency vibrations of alkali atoms on Cu(001)

High-resolution He atom inelastic scattering measurements of the vibrations parallel to the surface for the isolated alkali atoms Na, K and Cs on Cu(001) show a strong fall off in frequencies from 5.6 meV (Na) to 0.65 meV (Cs) which cannot be explained in terms of the mass difference. A mean field theory is developed which reveals that the corresponding force constants depend on the adatom size; for small atoms the frequency is determined by the local substrate holding potential, whereas in the case of the larger adatom, Cs, the electrostatic adsorbate–adsorbate interaction is the main restoring force. Moreover, the theory provides an estimate of the diffusion barriers and the friction coefficients.