Non-thermal Interactions on Low Temperature Ice and Aqueous Interfaces

Abstract

Electron-impact ionization of low-temperature water ice leads to H+, H2+, and H+(H2O)n=1-8 desorption. The threshold energy for ESD of H2+ from CI and H3O+ from PASW and ASW is 22 ± 3 eV. There is also a H2+ yield increase at 40 ± 3 eV and a 70 ± 3 eV threshold for ESD of H+(H2O)n=2-8 from PASW and ASW. H2+ production and desorption involves direct molecular elimination and reactive scattering of an energetic proton. Both of these channels likely involve localized two-hole one-electron and/or two-hole final states containing 4a1, 3a1 and/or 2a1 character. The 70 eV cluster ion threshold implicates either an initial (2a1-2) state localized on a monomer or the presence of at least two neighboring water molecules each containing a single hole. The resulting correlated two-hole or two-hole, one-electron configurations are localized within a complex and result in an intermolecular Coulomb repulsion and cluster ion ejection. The changes in the yields with phase and temperature are associated with structural and physical changes in the adsorbed water and longer lifetimes of excited state configurations containing a1 character. The dependence of the ESD cation yields on the local potential has been utilized to examine the details of HCl interactions on low temperature ice surfaces. The addition of HCl increases cluster ion yields from pure ice while decreasing H+ and H2+ yields. These changes reflect the changes in the local electronic potential due to the changing bond lengths at the surface of the ice as HCl ionizes and the surrounding water molecules reorient to solvate the ions. This work has been extended to ionic solutions at higher temperatures using a liquid jet and ultraviolet photoionization to interrogate the surface of aqueous ionic interfaces. Desorption of protonated water clusters and solvated sodium ion clusters were measured over a range of concentrations from NaCl, NaBr, and NaI solutions. The flux dependence indicated a multiple photon process and the proposed mechanism involves a Coulomb explosion resulting from the repulsion of nearby ions. The surface is investigated with regard to its importance in heterogeneous atmospheric chemistry.