SUPPRESSION OF ELECTRON-INDUCED POSITIVE-ION EMISSION BY A MOLECULAR OVERLAYER - ION-MOLECULE CHARGE-EXCHANGE AT A SURFACE

We report suppression of electron-stimulated desorption of positive ions (O+ and F+) from a TiO2(110) surface caused by adsorption of a fractional monolayer of molecular NH3. A linear decrease of O+ and F+ emission with NH3 coverage is observed. This system allows us to distinguish between neutralization of desorbing ions via interaction with the substrate or 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) (x10(-15) cm(2)) have been determined for O+ and F+ ions, respectively.