A MOLECULAR-DYNAMICS SIMULATION OF THE DIFFUSION OF SODIUM ON A CU(001) SURFACE

A molecular dynamics simulation of the diffusion of sodium adatoms on a Cu(001) surface has been performed. A pairwise Morse potential, fitted to the adatom adsorption geometry, binding energy, vibrational frequencies and activation energy for diffusion is used to model the sodium-copper interaction. Good agreement is obtained with quasielastic helium scattering data for the system, confirming the interpretation that sodium diffuses by jumps from one four-fold hollow site to another with an effective activation energy of 51 meV and a jump attempt frequency of 0.53 THz. The measured jump rate is lower than that predicted by transition state theory. The MD simulation suggests that this discrepancy arises because the rate of energy transfer between the adatom and the substrate is so slow that it controls the attempt rate rather than the vibrational frequencies of the adatom as transition state theory would predict.