Formation of H2 on polycyclic aromatic hydrocarbons under conditions of the ISM: an ab initio molecular dynamics study

Abstract

ABSTRACT Understanding how the H2 molecule is formed under the chemical conditions of the interstellar medium (ISM) is critical to the whole chemistry of it. Formation of H2 in the ISM requires a third body acting as a reservoir of energy. Polycyclic aromatic hydrocarbons (PAHs) are excellent candidates to play that role. In this work, we simulated the collisions of hydrogen atoms with coronene to form H2 via the Eley–Rideal mechanism. To do so, we used Born–Oppenheimer (ab initio) molecular dynamics simulations. Our results show that adsorption of H atoms and subsequent release of H2 readily happen on coronene for H atoms with kinetic energy as large as 1 eV. Special attention is paid to dissipation and partition of the energy released in the reactions. The capacity of coronene to dissipate collision and reaction energies varies with the reaction site. Inner sites dissipate energy easier and faster than edge sites, thus evidencing an interplay between the potential energy surface around the reaction centre and its ability to cool the projectile. As for the recombination of H atoms and the subsequent formation of H2, it is observed that $\sim 15~{{\ \rm per\ cent}}$ of the energy is dissipated by the coronene molecule as vibrational energy and the remaining energy is carried by H2. The H2 molecules desorb from coronene with an excited vibrational state (υ ≥ 3), a large amount of translational kinetic energy (≥0.4 eV), and with a small activation of the rotational degree of freedom.