Abstrart-Molecular
dynamics calculations are used to investigate the behavior of overlayers of ethylene molecules physisorbed on graphite. The ethylene intermolecular potential, which is taken from the literature, is shown to yield a reasonable structure for the bulk crystal. The well depth of the ethylene/ graphite potential, which is also derived from atom-atom potentials, is fitted to the isosteric heat of adsorption at zero coverage on the assumption that the graphite surface is planar. Solid monolayers are shown to exist in which the ethylene C=C axes are either parallel (LD phase) or perpendicular (HD phase) to the graphite basal plane. At low temperatures and under the constraint of zero spreading pressure, both solid phases order into herringbone structures. On heating, these solids transform to rotator phases before melting. Calculated translational and rotational diffusion constants for the disordered phases are compared with experimental values derived from quasielastic neutron scattering data. The density of states for collective translational and librational motions in the solid phases are presented along with the spectrum for motion of the H atoms. The latter is compared with the results of incoherent inelastic neutron scattering experiments. Despite the simplified nature of the molecule-surface interaction potential, the molecular dynamics calculations provide detailed interpretations of the above mentioned experimental results. The research exemplifies how simulation techniques offer a powerful complement to experimental investigations.