  A calculation of the inelastic scattering rate of Xe atoms on Cu(111) is
presented. We focus in the regimes of low and intermediate velocities, where
the energy loss is mainly associated to the excitation electron-hole pairs in
the substrate. We consider trajectories parallel to the surface and restrict
ourselves to the Van der Waals contribution. The decay rate is calculated
within a self-energy formulation. The effect of the response function of the
substrate is studied by comparing the results obtained with two different
approaches: the Specular Reflection Model and the Random Phase Approximation.
In the latter, the surface is described by a finite slab and the wave functions
are obtained from a one-dimensional model potential that describes the main
features of the surface electronic structure while correctly retains the
image-like asymptotic behaviour. We have also studied the influence of the
surface state on the calculation, finding that it represents around 50% of the
total probability of electron-hole pairs excitation.
