| Summary: | 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.
|
|---|
