Effects of potential energy on spattering of F+ from the CaF2(111) surfaceby noble-gas ion bombardment

The mechanism of ion- and electron-stimulated desorption of F+ from the CaF 2(111) surface has been investigated. By He+ bombardment, a considerable am ount of the F+ ions are sputtered due to the potential energy of He+. The y ield of F+ via the potential sputtering from the polycrystalline CaF2 film is three times as large as that From the CaF2(111) surface, suggesting that the fluorine at the fewer coordination sites or the defect sites preferent ially desorbs as F+. The probability for the potential sputtering increases relative to that for the kinetic sputtering with increase of the primary e nergy of He+, whereas most of the F+ ions by Ar+ are kinetically sputtered. On the basis of these experiments as well as ab initio molecular-orbital c alculations, it is concluded that desorption of fluorine is initiated by cr eation of the F 2s hole as a result of the nonadiabatic charge exchange wit h the primary ion. The ionization of the core-excited fluorine occurs on th e way out from the surface due to the intra-atomic Auger decay. The fluorin e can be ionized positively during collision with He backscattered from the solid due to the electron promotion mechanism, that is responsible for the kinetic sputtering of F+. [S0163-1829(99)09343-1].