Electron emission in slow collisions of He projectiles (He*, He+, He++) with Li- and LiF-surfaces

We report the spectra of the electrons emitted in slow collisions of He+ (50 eV) and He++ (100 eV) ions with a Li covered W(110) surface and with LiF films grown on Li/W(110). The preparation of the surfaces was made under in situ control of the metastable impact electron spectroscopy (MIES). The interpretation of the spectra is made on the basis of simulated electron spectra: For collisions with Li/W(110) the ion induced electron spectra are mainly due to Auger processes of the projectiles after their resonant capture of electrons from the surface. Both Auger deexcitation and intra-atomic Auger processes, such as the autodetachment of He--* (1s2s(2)) and the autoionization of the ''hollow atoms'' He-** (2/2l') (l,l'=0, 1), are observed. For collisions with LiF films the neutralization of the projectiles takes place mostly by Auger capture processes involving two electrons from the surface. The resonant capture of an electron by the He++ into He-+* (n=2) state still occurs, but is weak. The formation of ''hollow atoms'' does not take place in front of dense LiF films. There are some indications that the quasi-resonant electron transfer involving F 2s electrons plays also a role in the interaction of He++ with the surface.