This work presents an experimental picture of molecular ballistic diffusion on a surface, a process that is difficult to pinpoint because it generally occurs on 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 polyaromatic 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 polyaromatic hydrocarbons adsorbed on graphite.
Calvo-Almazán, I., Sacchi, M., Tamtögl, A., Bahn, E., Koza, M. M., Miret-Artés, S., & Fouquet, P. (2016). Ballistic Diffusion in Polyaromatic Hydrocarbons on Graphite. The journal of physical chemistry letters , 7(0), 5285-5290. https://doi.org/10.1021/acs.jpclett.6b02305