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 io ns (O+ and F+) from a TiO2(110) surface caused by adsorption of a frac tional monolayer of molecular NH3. A linear decrease of O+ and F+ emis sion with NH3 coverage is observed. This system allows us to distingui sh between neutralization of desorbing ions via interaction with the s ubstrate or adsorbed molecules. We propose a novel charge exchange mec hanism where electron transfer from the occupied orbitals of the adsor bed molecules to the desorbing ions causes the decrease in detected io n 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.