Weikai Cui
Interstellar icy grain mantles store and process molecular material in dense clouds, but astrochemical models generally do not treat direct ion–ice reactions, energetic ion impacts, mixed-ice effects, or condensed-phase ion formation in detail. This dissertation examines these processes using gas–grain chemical models, reactive molecular dynamics, electronic-structure calculations, electron-transport simulations, and ab initio molecular dynamics. First, thermal ion–ice chemistry is studied with a three-phase gas–grain model in which gas-phase ions, including C+ and HCO+, react directly with water-rich grain surfaces through an Eley–Rideal mechanism. Including these reactions changes the modeled abundances and formation pathways of several carbon- and oxygen-bearing species, including precursors to complex organic molecules. The results show that direct reactions between gas-phase ions and adsorbed molecules can contribute to low-temperature grain chemistry without requiring surface diffusion. Second, reactive molecular-dynamics simulations are used to examine few-eV C+ impacts on a 10 K amorphous solid water cluster containing 1000 water molecules. The simulations show local energy deposition, water dissociation, proton transfer, ion trapping, and the formation of small oxidized carbon-bearing products. Product distributions from the trajectory ensemble are converted into effective surface and bulk-mantle reaction channels and implemented in the MAGICKAL three-phase astrochemical model. This treatment allows reaction channels obtained on picosecond timescales to be tested in a chemical model covering dense-cloud evolution. Third, the impact calculations are extended to mixed H2O/CO2 ices to determine how composition and local molecular environment affect product branching. The molecular-dynamics products are reduced to composition-dependent branching fractions and interpolated for use in MAGICKAL. Particular attention is given to a C2O2-like product formed in the impact simulations. Density-functional-theory calculations are used to examine its spin-dependent relaxation and possible reactions with hydrogen atoms and hydroxyl radicals. Fourth, electron-stimulated desorption experiments on H2O and D2O films are combined with GEANT4 electron-transport simulations and PBEh-3c electronic-structure calculations to study the condensed-phase formation of gaseous H3+ and D3+. The transport calculations indicate that energy deposition by 500 eV electrons is concentrated near the ice/vacuum interface. Static cluster calculations are used to test whether weakly solvated H3+-like structures can remain distinct from protonated-water structures before desorption. Fifth, finite-water-cluster calculations are used to examine charge redistribution and structural relaxation in H2+ + H2 and H3+-like configurations. Charge-density-difference calculations show how the positive charge is distributed between the hydrogen species and the surrounding water molecules. Ab initio molecular-dynamics simulations are then used to test whether these configurations remain intact or undergo proton transfer and relaxation toward hydronium-like structures. The calculations show that their behavior depends strongly on the local coordination and hydrogen-bonding environment. Overall, the results show that direct ion processing can introduce reaction pathways that are absent from conventional diffusion-based grain chemistry, that mixed-ice composition changes the products of ion impacts, and that local water structure affects the stability of irradiation-generated cationic species. These processes should therefore be considered when modeling the chemical evolution of interstellar icy grains.