Molecular dynamics simulations of energy dissipation and non-thermal diffusion on amorphous solid water

Area of Science:

• Astrochemistry
• Surface Science
• Computational Physics

Background:

• Interstellar molecules form through gas-phase and dust-grain surface reactions.
• Hydrogen-rich molecules are primarily synthesized via surface chemistry on interstellar grains.
• Understanding energy dissipation on icy surfaces is key to modeling molecule formation.

Purpose of the Study:

• To quantify the outcomes of energy dissipation for admolecules on amorphous solid water.
• To investigate the influence of binding energy, internal degrees of freedom, and molecular weight on surface reactions.
• To compare simulation results with previous studies on crystalline ice surfaces.

Main Methods:

• Thousands of molecular dynamics simulations were performed.
• Admolecules (CO2, H2O, CH4) were given translational energy (0.5–5 eV) on an amorphous solid water surface.
• Outcomes such as adsorption, diffusion, and desorption were analyzed.

Main Results:

• Admolecules bind at different locations, with water preferring the surface and methane filling nanopores.
• Molecules frequently travel tens of angstroms before stabilizing, enabling further reactions.
• Desorption probability depends on binding energy, translational energy, and binding site height.

Conclusions:

• The study provides quantitative data on admolecule behavior on amorphous water ice.
• Expressions are derived to predict grain surface formation and gas-phase return of molecules.
• Findings are crucial for refining astrochemical models of interstellar chemistry.