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 (C 24 H 12 ). 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 σ add = 2.7 +2.7 −0.9 × 10 −2 Å 2 is derived from the experimental hydrogenation data.

Studies of pentacene hydrogenation reveal a preferential hydrogenation sequence. The guiding concepts apply more broadly to aromatic carbonaceous systems and may be crucial in catalytic formation of molecular hydrogen in interstellar space.

Aims. We investigate the interaction of a linear, catacondensed polycyclic aromatic hydrocarbon (PAH), pentacene (C 22 H 14 ), and its oxygen-functionalised form 6, 13 -pentacenequinone (C 22 H 12 O 2 ) with atomic hydrogen (H) under interstellar conditions. We compare their reaction cross-sections and reaction products to elucidate the possible role played by oxygen-functionalised PAHs in the formation of small oxygen-carrying molecules in the interstellar medium. Methods. We present temperature-programmed desorption measurements in combination with mass spectrometry. The evolution of the mass distribution of the desorbed species with increasing H-atom fluence and their peak desorption temperatures give insight into the reaction products.Results. The experiments reveal reaction cross-sections that are significantly larger for the oxygen-functionalised species compared to pentacene. For both pentacene and 6, 13 -pentacenequinone, hydrogenated species with an even number of excess H-atoms dominate over hydrogenated species with an odd number of H-atoms. The end product, after exposure to large H-atom fluences, for both pentacene and PQ is fully superhydrogenated pentacene (C 22 H 36 ), with little evidence for any remaining oxygen-containing species. This suggests the release of small molecules such as OH and/or H 2 O by the abstraction of oxygen atoms during hydrogenation, indicating that oxygen-functionalised PAHs can enable the formation of small oxygen-bearing molecules under interstellar conditions.

Experimental data showing superhydrogenation of neutral polycyclic aromatic hydrocarbons (PAHs) coronene, pentacene and pentacenequinone is presented. PAH monolayers were prepared on a highly oriented pyrolytic graphite surface and subsequently exposed to a beam of atomic hydrogen. The superhydrogenated PAH species were examined via temperature programmed desorption measurements. Stable intermediate superhydrogenation degrees as well as fully superhydrogenated species are observed and the initial reaction cross section for coronene has been determined.

Experimental evidence for the formation of hydrogenated fullerene molecules is presented. Films of C60 were grown on a highly oriented pyrolytic graphite (substrate) and exposed to a beam of deuterium atoms. Thermal desorption combined with mass spectrometry was used to determine the deuterated fullerene products formed, revealing a maximum degree of deuteration corresponding to C60D36. Release of D2 from the deuterated C60 film occurs at a much higher temperature than for D-saturated graphite.