Atom scattering studies of liquid structure and dynamics: Collisions of Xe with a model of squalane

Molecular dynamics (MD) computer simulations are carried out for scattering of high-energy Xe atoms off liquid squalane, and the results are compared with those of molecular-beam scattering experiments. A crude model for squalane is adopted, describing the hydrocarbon chain molecule as a sphere, and ignoring the role of internal modes. Good overall agreement is found between the results of the simulations and experiment, both for angular distributions and for trends in energy transfer properties. In particular, excellent agreement is obtained for the dependence of the energy transfer on the deflection angle for in-plane scattering. Theory predicts less trapping events than found experimentally, probably due to the crude model adopted for the squalane molecules. The partial success of the model in predicting some properties and not others is discussed. The other main conclusions of the study are (1) The instantaneous local structure of the liquid surface is highly corrugated, giving rise to a broad angular distribution and to extensive out-of-plane scattering. (2) High-energy atoms undergo both a trapping desorption and also direct inelastic scattering, the latter yielding information on liquid structure. (3) The angular distribution of atoms at a selected final velocity is sensitive to the local structure and dynamics of the surface. (4) The direct scattering can be conveniently interpreted in terms of contributions from single, double, and multiple collision events, these being roughly equal in relative weight. Forward scattering at grazing angle is dominated by single collisions, while double and multiple collisions have higher contribution at other directions. The double collision contribution in particular contains structural information. (5) There is a substantial yield per collision for sputtering of the squalane-like soft spheres. These results provide insight into the dynamics of gas-liquid collisions, and indicate the usefulness of beam scattering as a tool for studying liquid structure and dynamics.