2022A&A...663A.136J

Aims. We investigate the interaction of a linear, catacondensed polycyclic aromatic hydrocarbon (PAH), pentacene (C₂₂H₁₄), and its oxygen-functionalised form 6, 13 pentacenequinone (C₂₂H₁₂O₂) 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 (C₂₂H₃₆), with little evidence for any remaining oxygen-containing species. This suggests the release of small molecules such as OH and/or H₂O 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.