Forward electron emission in grazing 15-110 keV H+ and He+ collisions with crystalline surfaces

We have measured energy and angular distributions of electrons emitted during grazing collisions of H+ and He+ at intermediate energies (15-110 keV) with Al(111), GaAs(110), Si(111), Si(100), Cu(111) and Cu(100) surfaces. We present the results as a function of the projectile energy, observation angle, and elevation and azimuthal angles of incidence. The energy distributions are dominated by a structure with its maximum at an energy E(m) higher than the convoy electron energy E(ce). The behavior of the energy shift Delta E(c) = E(m) - E(ce) as a function of the ion energy E(p) is strongly dependent on the surface type, in particular for H+ impinging on Al(111) Delta E(c) reaches a maximum at E(p) similar to 60 keV. We observe that the structure at E(m) dominates the electron spectra even at projectile velocities lower than the Fermi velocity of the targets (0.5 vf < upsilon(p) < 1 vf). The measurements as a function of the crystallographic orientation of the sample show that the position and shape of this structure are almost independent of the azimuthal angle of incidence, but its intensity increases in surface channeling conditions. From this observation we conclude that, for grazing proton bombardment at E(p) similar to 60 keV, the close-encounter ion-atom collisions do not play an important role in the forward electron emission around E(m).