It was noted that the motion of molecules on solid surfaces was of interest in technological applications, and also a theoretical challenge at a fundamental level. It was pointed out that the concept of an activation barrier was very convenient for the interpretation of experiments and as an input to Monte Carlo simulations, but could be inadequate when mismatch with the substrate and molecular vibrations were considered. A study was made here of simple objects (adatom, dimer) diffusing on a substrate at a finite temperature, T, by using the Langevin approach. In the driven case, the characteristic curves were analyzed; comparing the motion for different values of the intramolecular spacing, for both T = 0 and T ≠ 0. The mobility of the dimer was higher than that of the monomer when the drift velocity was less than the natural stretching frequency. The role of intramolecular excitations was crucial in this respect. In the undriven case, the diffusive dynamics was considered as a function of temperature. Contrary to atomic diffusion, for the dimer it was not possible to define a single, temperature independent, activation barrier. The results suggested that vibrations could account for drastic variations of the activation barrier. This revealed a complex behaviour determined by the interplay between vibrations and a temperature dependent intramolecular equilibrium length.

Microscopic Mechanisms of Thermal and Driven Diffusion of Non-Rigid Molecules on Surfaces. C.Fusco, A.Fasolino: Thin Solid Films, 2003, 428[1-2], 34-9