Transmission of low energy (<10 eV) 16O+ ions through condensed ammonia and water overlayers

Abstract

We have studied the transmission of low energy (<10 eV) 16O+ ions through ultrathin films of condensed molecular solids, NH3 and H218O, in order to address the fundamental scattering processes that occur in the desorption of ions from below the surface of solids. 16O+ ions with a peak energy of ∼7 eV and a narrow angular distribution [full-width at half-maximum (FWHM) ∼15°] are generated by means of electron stimulated desorption (ESD) from an 16O oxidized W(100) surface and their yield, energy and angular distribution are measured with a digital ESDIAD (ESD ion angular distribution) detector. Ultrathin NH3 and H218O films of known thickness are condensed on the oxidized surface at 25 K and changes in the ion yield, energy and angular distribution are observed as a function of coverage. We find that adsorption of only 0.5 monolayer of H218O is enough to suppress the 16O+ ion emission by a factor of 100, while three monolayers of NH3 are necessary for equivalent suppression of the 16O+ ion emission. The angular distribution of the ions increases slightly with increasing overlayer coverage. We also find that a small percentage of H218O dissociates upon adsorption. We suggest that one electron charge transfer between 16O+ and H218O, and between 16O+ and the dissociation product OH are the main reasons for the strong attenuation of 16O+ ions by only a fraction of a monolayer of H218O. Charge transfer is also believed to be the main process that causes suppression of 16O+ ions by ultrathin NH3 films. Other elastic and inelastic processes are not believed to contribute significantly to 16O+ attenuation in NH3 or H218O films.