Benzene diffusion on graphite described by a rough hard disk model

New insight into the nature of diffusion and the origin of friction of a prototype system for weak physisorption - benzene molecules, C6H6, adsorbed on the basal plane (0 0 0 1) of graphite - has been obtained with quasi-elastic neutron scattering (QENS). Spectra were measured at relative adsorbate coverages between 0.1 and 1.0 monolayers (ML) and at sample temperatures between 60 K and 140 K. Our experimental observations require a substantial modification of the model of interaction between benzene molecules and graphite surfaces: in contrast to recent studies we find only weak kinetic surface friction, but a substantial dissipative interaction during inter-molecular collisions. At coverages of up to 0.5 ML the molecular dynamics are successfully modeled by a rough hard disk model, which we derive from a three-dimensional rough hard sphere model. At the full monolayer, three body and higher order collisions are dominant and the rough hard disk model breaks down, as expected.