Simulation and analysis of the motion of n-butane on Pt(111)

We have utilized molecular-dynamics simulations and static-energy calculations to study the dynamics and mechanisms of surface diffusion in a model of n-butane on Pt(111). The diffusion mechanism involves coupled translation and rotation in the surface plane. Motion perpendicular to the surface, as well as bond-angle bending and torsion, are negligible. The molecule remains in the trans conformation for 99% of the time at all temperatures and behaves essentially as rigid rod. We find that, at sufficiently low temperatures, the motion of the molecule can be interpreted in terms of the adspecies-hopping model with certain dynamical corrections. In particular, we find that the molecule exhibits a significant fraction of multiple-site jumps and correlated recrossings, which increases as the temperature increases. At temperatures approaching the desorption temperature, the adatom hopping description of diffusion is no longer applicable because adsorption becomes delocalized and the motion cannot be described in terms of hops.