Diffusion of hard disks and rodlike molecules on surfaces - art. no. 021204

We study the submonolayer diffusion of hard disks and rodlike molecules on smooth surfaces through numerical simulations and theoretical arguments. We concentrate on the behavior of the various diffusion coefficients as a fun ction of the two-dimensional (2D) number density rho in the case where ther e are no explicit surface-particle interactions. For the hard disk case, we find that while the tracer diffusion coefficient D-T(rho) decreases monoto nically up to the freezing transition, the collective diffusion coefficient D-C(rho) is wholly determined by the inverse compressibility which increas es rapidly on approaching freezing. We also study memory effects associated with tracer diffusion, and present theoretical estimates of D-T(rho) from the mode-mode coupling approximation. In the case of rigid rods with short- range repulsion and no orientational ordering, we find behavior very simila r to the case of disks with the same repulsive interaction. Both D-T(rho) a nd the angular diffusion coefficient D-R(rho) decrease with rho. Also in th is case D-C(rho) is determined by inverse compressibility and increases rap idly close to freezing. This is in contrast to the case of flexible chainli ke molecules in the lattice-gas limit, where D-C(rho) first increases and t hen decreases as a function of the density due to the interplay between com pressibility and mobility.