The use of quasielastic helium atom scattering to study correlated motion in adsorbate overlayers

Quasielastic helium atom scattering (QHAS) has been shown to be a powerful probe of surface diffusion on atomic length- and time-scales. So far, however, its application has been limited by data analysis techniques which rely on the assumption that there are no interactions between diffusing particles. It has therefore only been used for systems composed of low coverages of independent adsorbed particles. In this paper it is shown how molecular dynamics simulations can be used to overcome this limitation and realise the technique's potential for studying correlated motion in systems of strongly interacting particles. The simulations are applied to experimental data on the diffusion of Na on Cu(001), where it is shown that correlated adatom motions are the cause of the change observed in the momentum-transfer dependence of the quasielastic broadening as the coverage is increased, and that these correlated motions are sensitive to the form of the adatom-adatom interaction potential. The applicability of QHAS to studies of two-dimensional liquids and the two-dimensional melting phase transition is demonstrated with simulations of a monolayer of Xe adsorbed on a flat surface. The momentum dependence of the quasielastic broadening shows an abrupt change in behaviour at the melting transition, in contrast to the more continuous changes observed in the quantities usually measured such as diffusion coefficient, diffraction peak shape and position.