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. (C) 1999 Elsevier Science B.V. All rights reserved.