ELECTRONS FROM INTRAATOMIC AND INTERATOMIC AUGER PROCESSES IN LOW-ENERGY HE-2+ COLLISIONS WITH PARTIALLY ALKALATED W(110) SURFACES

Electron energy spectra from He-2+ (100 eV) ions colliding under grazing incidence with partially alkali(Na, Cs)-covered W(110) surfaces are reported as a function of the alkali coverage theta (0 less-than-or-equal-to theta less-than-or-equal-to 1 ML). For clean W(110) the sequence of electronic transitions during the collision is similar as reported previously for other clean metals. For theta > 0.4 ML He-2+ is preferentially neutralized into He** via He+*(n = 3) states by two-step resonant capture of two electrons. Electrons are mainly emitted by Auger de-excitation of He+*(n = 3 --> 2) and autoionization of He** to He+ at this stage. In a second step the He+ ions produced in the course of the neutralization process again capture resonantly one or two electrons. Auger de-excitation of He* and autodetachment of He-*(2S) then primarily give rise to electron emission whereby He neutrals in the ground state are formed. It is furthermore concluded that in a major portion of the collision events three electrons are emitted.