LIMITATIONS OF THE TRAJECTORY APPROXIMATION IN ATOM-SURFACE SCATTERING

We have scattered low energy (100-400 eV) Na+ from a Cu(001) surface, and studied the peak in the energy spectra that corresponds to collisi ons in which the Na+ interacts primarily with a single Cu atom. This p eak is broadened by the thermal fluctuations in the momentum of the Cu atom. Its width has been measured as a function of surface temperatur e, scattering geometry, and incident energy. Our data agree well with a rigorous statistical analysis (within 10%) but poorly with the traje ctory approximation (TA), demonstrating that the TA can fail when reco il is substantial.