Transmission of low-energyO+ions through ultrathin films of Ar, Kr, and Xe

We present a systematic study of the transmission of low-energy (<10 eV) ${\mathrm{O}}^{+}$ ions through ultrathin films of Ar, Kr, and Xe. The ions are produced by electron-stimulated desorption from an oxidized W(100) crystal; they desorb from the surface in directions close to the surface normal with a peak kinetic energy of ∼7 eV and their yield, mass/energy, and angle are measured with a digital electron-stimulated desorption ion angular distribution (ESDIAD) detector. Rare gases are condensed at ∼25 K onto the oxidized W(100) crystal and their film thickness is determined by means of thermal-desorption spectroscopy. The ${\mathrm{O}}^{+}$ ions desorbed in the presence of a rare-gas film have to pass through the film before reaching the detector. We find that 10% of ${\mathrm{O}}^{+}$ can be transmitted through 1.6 atomic layers of Ar, 2.9 ML of Kr, and 4.0 ML of Xe. From the ${\mathrm{O}}^{+}$ signal attenuation by films thicker than 2 ML we derive attenuation cross sections of 6.0×${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^2$ for Ar, 2.2×${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^2$ for Kr, and 1.5×${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^2$ for Xe. For Xe, we observe indications that the angular distribution of the ions changes due to large-angle scattering, and for Kr (and previously for Xe) we measure a shift in the energy distribution towards lower energies; we interpret this to be due to elastic forward scattering of the oxygen ions by the Xe atoms.