Band Effects on Stimulated Desorption of F+ from CaF2 and Fluorinated TiC(111) Surfaces

Abstract

The mechanism of stimulated F+ desorption has been investigated from fluorinated TiC(111) and CaF2 surfaces by He+, Ne+, Ar+, H+, and D+ irradiation. Upon He+ irradiation, the F+ ion is ejected from the surface of CaF2 via both kinetic and potential sputtering, whereas the potential sputtering of F+ is dominant from the fluorinated TiC(111) surface. The kinetically sputtered F+ ion from the metal surface undergoes efficient resonant neutralization due to the band effect. The probability for potential sputtering of F+ from the F/TiC(111) surface is dependent upon the primary ion species on the order of He+ > Ne+ ≫ Ar+. The F+ desorption is not simply related to the ionization potential of the projectile as evidenced by the fact that H+, though having the ionization potential smaller than Ar+ induces F+ emission with the yield about 70 times as large as that by Ar+ and almost one-fourth that by He+. The comparison between H+ and D+ irradiations clearly shows that the swifter ion leads to a higher F+ emission probability. From these facts, it is concluded that the F+ emission is initiated by formation of the F 2s core−hole state having the antibonding character as a consequence of the nonadiabatic transition of the primary ion's hole during collision. The ionization of the core-excited fluorine occurs due to the intra-atomic Auger decay (autoionization) after breakage of the chemisorptive bond, so that the resulting F+ ion can avoid efficient reneutralization even at metal surfaces.