DEPTH OF ORIGIN OF IONS IN DESORPTION INDUCED BY ELECTRONIC-TRANSITIONS (DIET) - ION TRANSMISSION THROUGH ULTRATHIN FILMS

Measurements to address a fundamental issue in transport of low energy (< 10 eV) ions through the surface layers of a solid are described. Our goal is to identify the dominant energy-transfer and charge-transfer processes that limit the survival probability of ions excited below the surface. Our approach is to study the interaction of low energy positive and negative ions (O+, F+, F-) with ultrathin films of condensed gases (Ar, Kr, Xe, H2O, NH3) ranging from fractional monolayer to six monolayers in thickness. The ions are produced by electron stimulated desorption from an appropriate substrate (e.g., oxidized W(100) for O+, PF3 on Ru(0001) for F+, F-). The ions desorb from the surface with well defined energy (< 10 eV) and angular distributions, and their yield, mass/energy and desorption angle are measured using a digital, time-of-flight ESDIAD detector (electron stimulated desorption ion angular distribution). The gases are condensed at < 25 K onto the crystal substrate, and their film thickness is determined by means of thermal desorption spectroscopy. We find that 10% of the O+ ESD signal can be transmitted through 1.6 atomic monolayers (ML) of Ar, 2.9 ML of Kr and 4.0 ML of Xe. In contrast, the O+ signal is attenuated to < 1% by 0.5 ML of H2O. We attribute the attenuation of O+ in rare gas films mainly to elastic backscattering, whereas the attenuation of O+ by H2O and NH3 films is dominated by charge transfer neutralization. F+ ions are almost completely attenuated by 1 monolayer of Xe, while F- ions experience a four-fold increase in yield when the substrate is covered by 1 monolayer of Xe. We discuss these results in terms of charge and energy transfer models, and draw conclusions about the depth of origin of ions produced in DIET (desorption induced by electronic transitions) processes.