Enhanced reactivity of oxygen-functionalised PAHs with atomic hydrogen – A route to the formation of small oxygen-carrying molecules

¹ Center for Interstellar Catalysis (InterCat), Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, Aarhus C, 8000 Denmark
² Interdisciplinary Nano-Science Centre (iNano), Aarhus University, Gustav Wieds Vej 14, Aarhus C, 8000 Denmark
e-mail: liv@phys.au.dk

Received: 11 February 2022
Accepted: 4 June 2022

Abstract

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.