Binding energies and sticking coefficients of H₂ on crystalline and amorphous CO ice

¹ Institute for Theoretical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
e-mail: molpeceres@theochem.uni-stuttgart.de
² Institute of Low Temperature Science, Hokkaido University, N19W8, Kita-ku, Sapporo, Hokkaido, Japan

Received: 30 November 2020
Accepted: 7 March 2021

Abstract

Context. Molecular hydrogen (H₂) is the most abundant interstellar molecule and plays an important role in the chemistry and physics of the interstellar medium. The interaction of H₂ with interstellar ices is relevant for several processes (e.g., nuclear spin conversion and chemical reactions on the surface of the ice). To model surface processes, quantities such as binding energies and sticking coefficients are required.

Aims. We provide sticking coefficients and binding energies for the H₂/CO system. These data are absent in the literature so far and could help modelers and experimentalists to draw conclusions on the H₂/CO interaction in cold molecular clouds.

Methods. Ab initio molecular dynamics simulations, in combination with neural network potentials, were employed in our simulations. Atomistic neural networks were trained against density functional theory calculations on model systems. We sampled a wide range of H₂ internal energies and three surface temperatures.

Results. Our results show that the binding energy for the H₂/CO system is low on average, − 157 K for amorphous CO and −266 K for crystalline CO. This carries several implications for the rest of the work. H₂ binding to crystalline CO is stronger by 109 K than to amorphous CO, while amorphous CO shows a wider H₂ binding energy distribution. Sticking coefficients are never unity and vary strongly with surface temperature, but less so with ice phase, with values between 0.95 and 0.17. With the values of this study, between 17 and 25% of a beam of H₂ molecules at room temperature would stick to the surface, depending on the temperature of the surface and the ice phase. Residence times vary by several orders of magnitude between crystalline and amorphous CO, with the latter showing residence times on the order of seconds at 5 K. H₂ may diffuse before desorption in amorphous ices, which might help to accommodate it in deeper binding sites.

Conclusions. Based on our results, a significant fraction of H₂ molecules will stick on CO ice under experimental conditions, even more so under the harsh conditions of prestellar cores. However, with the low H₂–CO binding energies, residence times of H₂ on CO ice before desorption are too short to consider a significant population of H₂ molecules on pure CO ices. Diffusion is possible in a time window before desorption, which might help accommodate H₂ on deeper binding sites, which would increase residence times on the surface.