Momentum distribution of charged light particles after grazing scattering from metal surfaces

We have studied the components of the translational energy of the beam projected along the surface normal and that projected parallel to the surface (denoted by Eperpendicularto and Eparallel, respectively) for the case of positron and electron beams scattered off metal surfaces. The study has been made in terms of the Eperpendicularto, Eparallel and total energy (Eperpendicularto+Eparallel) distributions. The energy range considered is from a few tens of eV up to the order of a keV. The total energy distribution exhibits a Poisson distribution associated with the surface plasmon excitations, but the Eperpendicularto and Eparallel distributions do not. The coupling between the surface parallel and normal momentum components of the beam is discussed. (orig.)