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

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.

arXiv comment30 pages, 10 figures, 2 tables

References in corpus (6)

• Observations of the Icy Universe
• Water, O2 and Ice in Molecular Clouds
• Surface chemistry in the interstellar medium - I - H2 formation by Langmuir-Hinshelwood and Eley-Rideal mechanisms
• Molecular dynamics study of photodissociation of water in crystalline and amorphous ice
• Molecular Hydrogen Formation on Ice Under Interstellar Conditions
• Sticking of molecules on non-porous amorphous water ice

Cited by in corpus (5)

• Adsorption energies of carbon, nitrogen, and oxygen atoms on the low-temperature amorphous water ice: A systematic estimation from quantum chemistry calculations
• Adsorption of H${}_2$ on Amorphous Solid Water Studied with Molecular Dynamics Simulations
• Quantifying the chemical desorption of H${}_2$S and PH${}_3$ from amorphous water ice surfaces
• Chemical Kinetics Simulations of Ice Chemistry on Porous Versus Non-Porous Dust Grains
• Interstellar Glycolaldehyde, Methyl Formate, and Acetic Acid. II. Chemical Modeling of the Bimodal Abundance Pattern in NGC 6334I