Supression of electron-induced positive ion emission by a molecular overlayer: Ion-molecule charge exchange at a surface

We report the suppression of electron-stimulated desorption of positive ions (${\mathrm{O}}^{+}$ and ${\mathrm{F}}^{+}$) from a ${\mathrm{TiO}}_2$ (110) surface caused by adsorption of a fractional monolayer of molecular ${\mathrm{NH}}_3$. A linear decrease of ${\mathrm{O}}^{+}$ and ${\mathrm{F}}^{+}$ emission with ${\mathrm{NH}}_3$ coverage is observed. This system allows us to distinguish between neutralization of desorbing ions via interaction with the substrate or with adsorbed molecules. We propose a novel charge exchange mechanism where electron transfer from the occupied orbitals of the adsorbed molecules to the desorbing ions causes the decrease in detected ion yield. Charge transfer cross sections of 2.8(±0.5) and 2.7(±1) (×${10}^{\mathrm{-}15}$) ${\mathrm{cm}}^2$ have been determined for ${\mathrm{O}}^{+}$ and ${\mathrm{F}}^{+}$ ions, respectively.