Mechanisms for correlated surface diffusion of weakly bonded dimers

We propose a model for the potential energy of a dimer on a surface that allows to identify the relevant reaction paths for diffusion and assess the favored path as the result of a competition between gain of potential energy due to the atom surface interaction and loss of elastic energy due to stretching of the dimer bond. We show that atoms forming weakly bonded dimers tend to diffuse via individual jumps although they remain bound to each other during the process, giving a rationale for the correlated piecewise diffusion observed on Si(001). For dimer equilibrium lengths smaller than the surface periodicity, the model predicts the not yet considered diffusion path that we call pinched diffusion with a contraction of the dimer bond length. Determination of the few parameters of the model requires only a study of the interaction of a single atom with the surface and of an isolated dimer instead of the complex potential energy of a dimer on a surface.