LOW-ENERGY (LESS-THAN-5 EV) F- ION TRANSMISSION THROUGH CONDENSED LAYERS OF WATER( AND F)

We report on the transmission of F+ and F- ions through ultrathin film s of condensed water at 20 K, and compare the experimental results wit h theoretical calculations. The F+ and F- ions are produced by ESD (el ectron 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 simila r 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 e mission, and (b) 1 ML of H2O increases the emission of F- ions and cau ses a dramatic change in the ion angular distribution. The striking ch anges in the angular distribution of F- ions transmitted through conde nsed H2O films indicate that elastic scattering is an important proces s in determining the attenuation of F- by H2O. No direct evidence for any kind of ion-molecule chemical reaction or collision induced dissoc iation reaction has been found. The strong attenuation of F+ without s ubstantial changes in angular distribution suggests that charge transf er processes are important in limiting the transmission of F+ ions. Ou r quantum mechanical calculations indicate that the increase in F- emi ssion upon adsorption of less than or equal to 1 ML of H2O is mainly d ue to a decrease in the neutralization probability of F- with the subs trate, by a dielectric screening mechanism. The calculations also show that the increase in the F- survival probability saturates after a wa ter bilayer (1 ML H2O) is formed, which is in excellent agreement with the experiment. Our measurements show no evidence for diffusion of H2 O on the PF3/Ru(0001) surface between 20 and 60 K; the lack of diffusi on, together with exponential attenuation of F+ and F- with H2O thickn ess, indicates that H2O vapor condensed on the PF3/Ru(0001) surface at 20 K grows statistically. (C) 1997 American Institute of Physics.