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

Abstract

Development of a triplasmatron ion source for the generation of SF 5 + and F − primary ion beams on an ion microscope secondary ion mass spectrometry instrumentWe report on the transmission of F ϩ and F Ϫ ions through ultrathin films of condensed water at 20 K, and compare the experimental results with theoretical calculations. The F ϩ and F Ϫ ions are produced by ESD ͑electron stimulated desorption͒ of a PF 3 monolayer adsorbed on a Ru͑0001͒ surface ͑PF 3 /Ru͑0001͒ surface͒. We find two surprising results: ͑a͒ the off-normal F ϩ signal is attenuated to ϳ1% by only ϳ2.5 monolayer ͑ML͒ of H 2 O, while a much thicker layer, ϳ10 ML of H 2 O, is necessary for equivalent attenuation of the F Ϫ ion emission, and ͑b͒ 1 ML of H 2 O increases the emission of F Ϫ ions and causes a dramatic change in the ion angular distribution. The striking changes in the angular distribution of F Ϫ ions transmitted through condensed H 2 O films indicate that elastic scattering is an important process in determining the attenuation of F Ϫ by H 2 O. No direct evidence for any kind of ion-molecule chemical reaction or collision induced dissociation reaction has been found. The strong attenuation of F ϩ without substantial changes in angular distribution suggests that charge transfer processes are important in limiting the transmission of F ϩ ions. Our quantum mechanical calculations indicate that the increase in F Ϫ emission upon adsorption of р1 ML of H 2 O is mainly due to a decrease in the neutralization probability of F Ϫ with the substrate, by a dielectric screening mechanism. The calculations also show that the increase in the F Ϫ survival probability saturates after a water bilayer ͑1 ML H 2 O͒ is formed, which is in excellent agreement with the experiment. Our measurements show no evidence for diffusion of H 2 O on the PF 3 /Ru͑0001͒ surface between 20 and 60 K; the lack of diffusion, together with exponential attenuation of F ϩ and F Ϫ with H 2 O thickness, indicates that H 2 O vapor condensed on the PF 3 /Ru͑0001͒ surface at 20 K grows statistically.

Negative-Ion Enhancements in Electron-Stimulated Desorption ofCF2Cl2Coadsorbed with Nonpolar and Polar Gases on Ru(0001)

“…First, we note that neither the dielectric screening effect [5] nor the R-matrix model [7] can account for the observed strongly CF 2 Cl 2 -precoveragedependent enhancements of F 2 and Cl 2 ; molecular precoverage is not an input parameter in these model calculations [5,7]. Moreover, the dielectric screening effect would predict greater anionic enhancements for H 2 O than for NH 3 coadsorption, as the dielectric constant of H 2 O is greater than that of NH 3 [13].…”

“…The F 2 yield from electronstimulated desorption (ESD) of ϳ1 monolayer (ML) of PF 3 adsorbed on a Ru(0001) is enhanced by 1.5-4 when a ϳ1 ML rare gas (RG) (Xe, Kr) or water film is deposited on the PF 3 monolayer [4]. This enhancement was attributed to a dielectric screening effect [5]: The dielectric layer on the surface induces a potential barrier that increases the ion survival probability. Moreover, in ESD of fractional-monolayer molecules (ABs) absorbed on top of thick RG films, negative-ion enhancements due to the formation of anionic excitons (RG ‫2ء‬ ) were observed [6].…”

Electron-driven chemistry of halogenated compounds in condensed phases: Effects of solvent matrices on reaction dynamics and kinetics

“…Similarly, the ESD anion enhancement experiments [30,33] have been interpreted in terms of an increase in DEA cross-section, associated with trapped pre-solvated electrons at the H 2 O surface (see section 5.1.3). This, also, may not be a complete explanation: the enhanced anion desorption yield may also be influenced by several other effects [28,30,119] due to the presence of the H 2 O co-adsorbate; for example, changes in anion neutralization rates (electron hopping times) that increase ion lifetimes [120].…”

Low energy charged particles interacting with amorphous solid water layers

“…For 5 and 25 eV there are available electronic bands associated with the anti-bonding 2a 1 and 4a 1 bands; 36 thus, the solvated electrons are more strongly affected by the field strength. Yet another possibility is that 25 eV electrons are more energetic than the threshold energy for ionization of ASW, 27,28 suggesting that the charge transport mechanism through the bulk in pure ASW and ice can also originate from Bjerrum (L and D are hydrogen displacements) and ionic defects that develop in the hydrogen bonded matrix. [37][38][39] The activation barrier for formation and separation of D and L hydrogen dislocations (D -for two facing hydrogen atoms, L -for the two atoms directed away from each other) are about 0.7 eV and 1.0 eV, respectively, in the case of ionic (H 3 O + , OH − ) defects.…”