The irradiation of water ice by C$^+$ ions in the cosmic environment

TL;DR

This study uses first principles molecular dynamics to explore how low-energy C+ ions interact with amorphous water ice at 30 K, revealing reaction pathways and products relevant to cosmic environments.

Contribution

It provides detailed mechanistic insights into C+ ion interactions with water ice, highlighting reaction pathways and product formation at the molecular level.

Findings

COH and CO are dominant reaction products.

Proton transfer to form hydronium occurs during reactions.

Direct impact mechanism is more prominent at higher energies.

Abstract

We present a first principles molecular dynamics (FPMD) study of the interaction of low energy, positively charged, carbon (C+) projectiles with amorphous solid water clusters at 30 K. Reactions involving the carbon ion at an initial energy of 11 eV and 1.7 eV with 30-molecule clusters have been investigated. Simulations indicate that the neutral isoformyl radical, COH, and carbon monoxide, CO, are the dominant products of these reactions. All these reactions are accompanied by the transfer of a proton from the reacting water molecule to the ice, where it forms a hydronium ion. We find that COH is formed either via a direct, "knock-out", mechanism following the impact of the C+ projectile upon a water molecule or by creation of a COH_2^+ intermediate. The direct mechanism is more prominent at higher energies. CO is generally produced following the dissociation of COH. More frequent production of the formyl radical, HCO, is observed here than in gas phase calculations. A less commonly occurring product is the dihydroxymethyl, CH(OH)_2, radical. Although a minor result, its existence gives an indication of the increasing chemical complexity which is possible in such heterogeneous environments.