Low energy (<5 eV) F+ and F- ion transmission through condensed layers of water

We report on the transmission of F+ and F- ions through ultrathin films of condensed water at 20 K, and compare the experimental results with theoretical calculations. The F+ and F- ions are produced by ESD (electron stimulated desorption) of a PF3 monolayer adsorbed on a Ru(0001) surface (PF3/Ru(0001) surface). We find two surprising results: (a) the off-normal F+ signal is attenuated to similar to 1% by only similar to 2.5 monolayer (ML) of H2O, while a much thicker layer, similar to 10 ML of H2O, is necessary for equivalent attenuation of the F- ion emission, and (b) 1 ML of H2O increases the emission of F- ions and causes a dramatic change in the ion angular distribution. The striking changes in the angular distribution of F- ions transmitted through condensed H2O films indicate that elastic scattering is an important process in determining the attenuation of F- by H2O. No direct evidence for any kind of ion-molecule chemical reaction or collision induced dissociation reaction has been found. The strong attenuation of F+ without substantial changes in angular distribution suggests that charge transfer processes are important in limiting the transmission of F+ ions. Our quantum mechanical calculations indicate that the increase in F- emission upon adsorption of less than or equal to 1 ML of H2O is mainly due to a decrease in the neutralization probability of F- with the substrate, by a dielectric screening mechanism. The calculations also show that the increase in the F- survival probability saturates after a water bilayer (1 ML H2O) is formed, which is in excellent agreement with the experiment. Our measurements show no evidence for diffusion of H2O on the PF3/Ru(0001) surface between 20 and 60 K; the lack of diffusion, together with exponential attenuation of F+ and F- with H2O thickness, indicates that H2O vapor condensed on the PF3/Ru(0001) surface at 20 K grows statistically. (C) 1997 American Institute of Physics.