2024ApJ...972...42S

While polycyclic aromatic hydrocarbons (PAHs) are now accepted to be abundant in interstellar space, the abundance and influence of superhydrogenated PAHs (HPAHs) in the interstellar medium (ISM) are still under investigation. HPAHs may act as catalysts for or reactants in small-molecule formation via hydrogen abstraction reactions, H₂ evaporation, and carbon skeleton fragmentation. Here, we present a gas-phase infrared (IR) action spectroscopy study of the HPAH 4, 5, 9, 10-tetrahydropyrene (THP; C₁₆H₁₄), performed at the Free Electron Lasers for Infrared eXperiments facility. IR action spectroscopy was performed on the THP cation, protonated THP, and their fragments produced by collision-induced dissociation in the range from 600 to 1800 cm^–1. Calculated IR spectra, at the density functional theory level, agree with experimental IR spectra to a high degree and were utilized to determine molecular structures of the HPAH fragments. Molecular dynamics simulations compared with experimental mass spectra reveal favorable HPAH fragmentation pathways. Molecular hydrogen (H₂) is observed to be a primary fragment of [THP+H]⁺ with superhydrogenated duo groups. This contrasts the notion that HPAHs typically undergo carbon skeleton fragmentation leading to C_xH_y formation. These observations show that lowered symmetry and duo or trio aliphatic groups on HPAHs uniquely change their IR spectra, stability, and fragmentation patterns. As a result, these species may contribute to H₂ formation in the ISM.