The energy and angular distributions of low-energy He+ scattered from Al(111)

Measurements of 500 eV(4)He(+) scattered from Al(111) surfaces provide deta iled information about the scattering process and surface crystallography. We report various observations concerning collision inelasticity, multiple scattering, and shadow-cone focusing effects. A large inelastic energy loss of the scattered He+ is observed, which causes multiple-scattering events at oblique incidence angles to have energies below the elastic single-scatt ering energy. Inelastic He+ scattering from clean, ordered Al(111) shows in teresting intensity variations as a function of azimuthal orientation and i ncidence angle. At a critical incidence angle, where the top edge of a shad ow cone intersects a nearest-neighbor Al surface atom, the scattered ion in tensity exhibits 60 degrees azimuthal periodicity. At a more glancing angle of incidence, a double-peaked structure appears. The scattered He+ intensi ty varies as a function of incidence angle, as expected from classical shad ow cone calculations.