Hydrogenation of PAH cations: a first step towards H2 formation

TL;DR

This study experimentally investigates how coronene cations hydrogenate in the presence of atomic hydrogen, revealing barriers to H attachment and suggesting PAH cations' significant role in interstellar H2 formation.

Contribution

First experimental measurement of hydrogenation barriers on PAH cations, confirming theoretical predictions and highlighting their potential in interstellar H2 formation.

Findings

Odd hydrogenation states dominate up to 11 H atoms

Barrier heights of 72 meV and 40 meV for second and fourth H attachments

PAH cations can become highly hydrogenated, contributing to H2 formation

Abstract

Molecular hydrogen is the most abundant molecule in the universe. A large fraction of H2 forms by association of hydrogen atoms adsorbed on polycyclic aromatic hydrocarbons (PAHs), where formation rates depend crucially on the H sticking probability. We have experimentally studied PAH hydrogenation by exposing coronene cations, confined in a radiofrequency ion trap, to gas phase atomic hydrogen. A systematic increase of the number of H atoms adsorbed on the coronene with the time of exposure is observed. Odd coronene hydrogenation states dominate the mass spectrum up to 11 H atoms attached. This indicates the presence of a barrier preventing H attachment to these molecular systems. For the second and fourth hydrogenation, barrier heights of 72 +- 6 meV and 40 +- 10 meV, respectively are found which is in good agreement with theoretical predictions for the hydrogenation of neutral PAHs. Our experiments however prove that the barrier does not vanish for higher hydrogenation states. These results imply that PAH cations, as their neutral counterparts, exist in highly hydrogenated forms in the interstellar medium. Due to this catalytic activity, PAH cations and neutrals seem to contribute similarly to the formation of H2.