Energy transfer in quasibinary and collective scattering events at a Ag(001) surface

We have measured the in-plane scattered distributions of K+ ions incident on a Ag 001 surface with energies from 9 to 100 eV. Energyand angle-resolved spectra of the scattered flux show multiple peaks distinctly separated in energy corresponding to specific ion trajectories at the surface. Using a calculated interaction potential combined with the classical scattering simulation SAFARI we are able to model these trajectories and detail their energy-loss characteristics. At energies below approximately 24 eV we find that the ions scatter with a larger energy than one would expect from quasibinary collisions. Our trajectory analysis reveals this is a transition from the quasibinary behavior to a collective surface response more commonly associated with thermal energy atom scattering.