Energy distributions of ions produced by electron-stimulated desorption

We have measured the initial kinetic energy distributions of ions produced by electron bombardment of various oxides and halides. The instrument used allows ions directly ejected from the sample surface to be distinguished from ions formed by electron impact in the gas phase. Singly and multiply charged positive ions of species present in the matrix as anions and cations were desorbed by high energy ( 11 keV) electron impact. Directly desorbed positive halogen ions show a narrow, low energy peak, consistent with conventional models of electron stimulated desorption (ESD). In addition, some of the cation species exhibited similar narrow energy spectra. Charge states up to +6 were observed for the halides; with the exception of F²⁺ and Cl²⁺, multiple charge states were due to electron impact ionization of desorbed neutrals. Charge states up to +4 were seen for silicon from electron-bombarded SiO₂; energy distributions of Si⁺, Si²⁺ and Si³⁺ showed that these species were desorbed directly from the surface. The energy distributions of O⁺ and O²⁺ ions ejected from SiO₂ are relatively wide, compared to the energy distribution of Si⁺ ions. In contrast, O⁺ ions ejected from TiO₂ have a much narrower energy distribution, like those observed for the halogen ions.