Laboratory hydrogenation of the photo-fragments of PAH cations: Co-evolution interstellar chemistry

¹ Deep Space Exploration Laboratory/CAS Key Laboratory of Crust-Mantle Materials and Environment, University of Science and Technology of China, Hefei 230026, PR China
e-mail: jfzhen@ustc.edu.cn
² CAS Center for Excellence in Comparative Planetology, University of Science and Technology of China, Hefei 230026, PR China
³ CAS Center for Excellence in Quantum Information and Quantum Physics, Hefei National Laboratory for Physical Sciences at the Microscale, and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, PR China
⁴ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China

Received: 12 September 2022
Accepted: 16 November 2022

Abstract

To investigate co-evolution interstellar chemistry, we studied the gas-phase hydrogenation processes of possible photo-fragments of large polycyclic aromatic hydrocarbon (PAH) cations. Our experimental results show that hydrogenated photo-fragments of hexa-peri-hexabenzocoronene (HBC, C₄₂H₁₈) cations are efficiently formed. The predominance of even-mass fragments (C₄₂H_2n⁺, n = [0–9]) is observed in the photo-fragmentation experiments, while no even-odd hydrogenated mass patterns are observed in the hydrogenation experiments. We investigated the structure of these newly formed hydrogenated photo-fragments and the bonding energies for the reaction pathways with quantum chemistry calculations. We used a molecular kinetic reaction model to simulate the hydrogenation processes of the photo-fragments (e.g. C₄₂H₁₂⁺) as a function of the reaction time under the experimental conditions. We obtain the possible structure distribution of the newly formed hydrogenated fragments of C₄₂H₁₈⁺ and the infrared (IR) spectra of these possible molecules. We infer that the hydrogenation and photo-dehydrogenation channels are not reversible reaction channels. Hydrogenation tends to be more random and disorderly, with no restrictions or requirements for the carbon reaction sites of PAH species. As a result, under the co-evolution interstellar chemistry network, there is little chance that PAH compounds return to their initial state through hydrogenation processes after photo-dehydrogenation. Consequently, the hydrogenation states and forms of PAH compounds are intricate and complex in the interstellar medium (ISM).