Ion Molecule Reactions and Energetic Processing on Interstellar Icy Grains 13 views

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.

PHD (Doctor of Philosophy)

Astrochemistry; Interstellar ices; Ion–molecule reactions; Amorphous solid water; Radiation chemistry; Density functional theory

English

All rights reserved by the author (no additional license for public reuse)

2026-07-31