OBSERVATION OF JUMP DIFFUSION OF ISOLATED SODIUM ATOMS ON A CU(001) SURFACE BY HELIUM ATOM SCATTERING

The diffusion of sodium adatoms on a Cu(001) surface has been studied with quasielastic helium atom scattering. A jump mechanism was found with activation energy 51 meV, jump length 2.56 angstrom, and jump attempt frequency nu0 0.53 THz. The adatom vibrational frequency parallel to the surface is measured to be 1.23 THz. The data are interpreted with the aid of a realistic molecular-dynamics simulation which reveals that nu0 is governed by the rate of energy exchange between the adatoms and the substrate, and not, as transition state theory would predict, by the vibrational frequencies of the adatom.