A Comprehensive Study of Hydrogen Adsorbing to Amorphous Water-Ice: Defining Adsorption in Classical Molecular Dynamics

TL;DR

This paper uses classical molecular dynamics simulations to study hydrogen atom sticking probabilities on amorphous water-ice surfaces, highlighting the importance of defining sticking events accurately for astrophysical modeling.

Contribution

It introduces a new definition of sticking events in MD simulations and provides detailed probabilities and coefficients across various energies and temperatures.

Findings

Sticking probabilities range from 0.99 to 0.22 depending on conditions.

Sticking coefficients vary significantly with grain temperature and incident energy.

Accurate definition of sticking events impacts the computed probabilities.

Abstract

Gas-grain and gas-phase reactions dominate the formation of molecules in the interstellar medium (ISM). Gas-grain reactions require a substrate (e.g. a dust or ice grain) on which the reaction is able to occur. The formation of molecular hydrogen (H$_2$) in the ISM is the prototypical example of a gas-grain reaction. In these reactions, an atom of hydrogen will strike a surface, stick to it, and diffuse across it. When it encounters another adsorbed hydrogen atom, the two can react to form molecular hydrogen and then be ejected from the surface by the energy released in the reaction. We perform in-depth classical molecular dynamics (MD) simulations of hydrogen atoms interacting with an amorphous water-ice surface. This study focuses on the first step in the formation process; the sticking of the hydrogen atom to the substrate. We find that careful attention must be paid in dealing with the ambiguities in defining a sticking event. The technical definition of a sticking event will affect the computed sticking probabilities and coefficients. Here, using our new definition of a sticking event, we report sticking probabilities and sticking coefficients for nine different incident kinetic energies of hydrogen atoms [5 K - 400 K] across seven different temperatures of dust grains [10 K - 70 K]. We find that probabilities and coefficients vary both as a function of grain temperature and incident kinetic energy over the range of 0.99 - 0.22.