Adsorption of H₂ on amorphous solid water studied with molecular dynamics simulations

Area of Science:

• Astrochemistry
• Interstellar Medium Physics
• Surface Science

Background:

• Dense interstellar clouds contain abundant hydrogen molecules (H2) and amorphous solid water (ASW) ice.
• Understanding H2 adsorption on ASW is crucial for modeling interstellar chemistry and ice formation.

Purpose of the Study:

• To develop a general framework for studying the adsorption dynamics of light species on interstellar ices.
• To quantify binding energies, sticking probabilities, and thermal sticking coefficients for H2 on ASW.

Main Methods:

• Developed a computational framework to simulate H2 collisions with ASW surfaces.
• Calculated binding energies and their distributions.
• Determined sticking probabilities for incident energies from 1 meV to 60 meV.
• Calculated thermal sticking coefficients for surface temperatures from 10 K to 110 K.

Main Results:

• Sticking probability is highly dependent on adsorbate kinetic energy and surface temperature.
• Sticking probability shows minimal dependence on the angle of incidence.
• Observed significant sticking probabilities even above the thermal desorption temperature.
• Adsorption and thermal desorption occur on distinct timescales.

Conclusions:

• The developed framework provides a comprehensive understanding of H2 adsorption dynamics on interstellar ices.
• Results bridge a gap in experimental data and extend the range of gas temperatures considered in astrochemical models.
• The method can be extended to study other adsorbates, including radicals and charged species.