The diffusion of Na adatoms on a Cu(001) surface was studied by means of quasi-elastic He atom scattering. A jump mechanism was found with an activation energy of 0.051eV, a jump length of 0.256nm, and a jump attempt frequency of 0.53THz. The adatom vibrational frequency parallel to the surface was found to be 1.23THz. The data were explained by using a realistic molecular dynamics simulation. This revealed that the attempt frequency was governed by the rate of energy exchange between adatoms and the substrate and not, as transition-state theory predicted, by the vibrational frequencies of the adatom.

J.Ellis, J.P.Toennies: Physical Review Letters, 1993, 70[14], 2118-21