Formation of H-2 on polycyclic aromatic hydrocarbons under conditions of the ISM: an ab initio molecular dynamics study

Understanding how the H-2 molecule is formed under the chemical conditions of the interstellar medium (ISM) is critical to the whole chemistry of it. Formation of H-2 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 H-2 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 H-2 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 H-2, it is observed that similar to 15 of the energy is dissipated by the coronene molecule as vibrational energy and the remaining energy is carried by H-2. The H-2 molecules desorb from coronene with an excited vibrational state (nu = 3), a large amount of translational kinetic energy (>= 0.4 eV), and with a small activation of the rotational degree of freedom.

Automated summary

Understanding the formation of H-2 molecule in the interstellar medium (ISM) is crucial to its overall chemistry, and it requires a third body as an energy reservoir. In this study, we simulated the collisions of hydrogen atoms with coronene using molecular dynamics simulations. The results show that H atoms readily adsorb and form H-2 on coronene, and the energy released in the reactions is dissipated and partitioned differently depending on the reaction site.