FORWARD ELECTRON-EMISSION IN GRAZING 15-110 KEV H+ AND HE+ COLLISIONSWITH 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 ene rgy, observation angle, and elevation and azimuthal angles of incidenc e. The energy distributions are dominated by a structure with its maxi mum at an energy E(m) higher than the convoy electron energy E(ce). Th e 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) dominat es the electron spectra even at projectile velocities lower than the F ermi velocity of the targets (0.5 vf < upsilon(p) < 1 vf). The measure ments as a function of the crystallographic orientation of the sample show that the position and shape of this structure are almost independ ent of the azimuthal angle of incidence, but its intensity increases i n surface channeling conditions. From this observation we conclude tha t, 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 fo rward electron emission around E(m).