Identification of stable configurations in the superhydrogenation sequence of polycyclic aromatic hydrocarbon molecules

Superhydrogenated polycyclic aromatic hydrocarbon (PAH) molecules have been demonstrated to act as catalysts for molecular hydrogen formation under interstellar conditions. Here we present combined thermal desorption mass spectrometry measurements and density functional theory calculations that reveal the most stable configurations in the superhydrogenation sequence of the PAH molecule coronene (C24H12). Specifically, the experiments demonstrate the presence of stable configurations of superhydrogenated coronene at specific hydrogenation levels of 2, 10, 14, 18, and 24 extra hydrogen atoms. Density functional theory calculations of binding energies and barrier heights explain why these configurations are particularly stable and provide new insights into the superhydrogenation process of PAH molecules under interstellar conditions. Furthermore, an experimental cross-section for the first hydrogen atom addition to the neutral coronene molecule of σadd = 2.7+2.7-0.9 × 10-2 A2 is derived from the experimental hydrogenation data.