Ballistic Diffusion in Poly-aromatic Hydrocarbons on Graphite

This work presents an experimental picture of molecular ballistic diffusion on a surface, a process which is difficult to pinpoint since it generally occurs at very short length scales. By combining neutron-time-of-flight data, with molecular dynamics simulations and density functional theory calculations, we provide a complete description of the ballistic translations and rotations of a poly-aromatic hydrocarbon (PAH) adsorbed on the basal plane of graphite. Pyrene, C16H10, adsorbed on graphite is a unique system where at relative surface coverages of about 10-20 %, its mean free path matches the experimentally accessible time/space scale of neutron time-of-flight spectroscopy (IN6 at the Institut Laue-Langevin). The comparison between the diffusive behavior of large and small PAHs such as pyrene and benzene adsorbed on graphite, brings a strong experimental indication that the interaction between molecules is the dominating mechanism in the surface diffusion of poly-aromatic hydrocarbons adsorbed on graphite