Binding energies of small interstellar molecules on neutral and charged amorphous solid water surfaces

TL;DR

This study uses DFT calculations to determine how the binding energies of small interstellar molecules on amorphous solid water surfaces are affected by surface charge, providing insights into surface chemistry in the interstellar medium.

Contribution

It presents the first detailed computational analysis of how neutral and charged amorphous solid water surfaces influence the binding energies of key interstellar molecules.

Findings

CO binding energy increases with surface charge

CH4 binding energy remains unchanged by surface charge

NH3 binding energy decreases with surface charge

Abstract

The interstellar medium (ISM) is all but empty. To date, more than 300 molecules have already been discovered. Because of the extremely low temperature, the gas-phase chemistry is dominated by barrierless exothermic reactions of radicals and ions. However, several abundant molecules and organic molecules cannot be produced efficiently by gas-phase reactions. To explain the existence of such molecules in the ISM, gas-surface interactions between small molecules and dust particles covered with amorphous solid water (ASW) mantles must be considered. In general, surface processes such as adsorption, diffusion, desorption, and chemical reactions can be linked to the binding energy of molecules to the surface. Hence, a lot of studies have been performed to identify the binding energies of interstellar molecules on ASW surfaces. Cosmic radiation and free electrons may induce a negative charge on the dust particles, and the binding energies may be affected by this charge. In this study, we calculate the binding energies of CO, CH4, and NH3, on neutral and charged ASW surfaces using DFT calculations. Our results indicate that CO can interact with the surface charge, increasing its binding energy. In contrast, the binding energy of CH4 remains unchanged in the presence of surface charge, and that of NH3 typically decreases.