Ab initio molecular dynamics simulations show that the reaction of energy released by the H2 formation by H + H reaction is quickly absorbed in part by the ice surface, the remaining energy being used to promote H2 desorption.

Molecular hydrogen is the most abundant molecular species in the Universe. While no doubts exist that it is mainly formed on the interstellar dust grain surfaces, many details of this process remain poorly known. In this work, we focus on the fate of the energy released by the H2 formation on the dust icy mantles, how it is partitioned between the substrate and the newly formed H2, a process that has a profound impact on the interstellar medium. We carried out state-of-art ab-initio molecular dynamics simulations of H2 formation on periodic crystalline and amorphous ice surface models. Our calculations show that up to two thirds of the energy liberated in the reaction (∼300 kJ/mol ∼3.1 eV) is absorbed by the ice in less than 1 ps. The remaining energy (∼140 kJ/mol ∼1.5 eV) is kept by the newly born H2. Since it is ten times larger than the H2 binding energy on the ice, the new H2 molecule will eventually be released into the gas-phase. The ice water molecules within ∼4 Å from the reaction site acquire enough energy, between 3 and 14 kJ/mol (360-1560 K), to potentially liberate other frozen H2 and, perhaps, frozen CO molecules. If confirmed, the latter process would solve the long standing conundrum of the presence of gaseous CO in molecular clouds. Finally, the vibrational state of the newly formed H2 drops from highly excited states (ν=6) to low (ν≤2) vibrational levels in a timescale of the order of ps.

This work is published in The Astrophysical Journal.

Link to the article in ApJ: https://iopscience.iop.org/article/10.3847/1538-4357/ac0142/meta

Link to the article as green open access: https://arxiv.org/abs/2105.06843