2018ApJ...854...61Q

Polycyclic aromatic hydrocarbons (PAHs), comprised of fused benzene (C₆H₆) rings, emit infrared radiation (3-12 µm) due to the vibrational transitions of the C-H bonds of the aromatic rings. The 3.3 µm aromatic band is generally accompanied by the band at 3.4 µm assigned to the vibration of aliphatic C-H bonds of compounds such as PAHs with an excess of peripheral H atoms (H_n-PAHs). Herein we study the stability of fully hydrogenated benzene (or cyclohexane, C₆H₁₂) under the impact of stellar radiation in the photodissociation region (PDR) of NGC 7027. Using synchrotron radiation and time-of-flight mass spectrometry, we investigated the ionization and dissociation processes at energy ranges of UV (10-200 eV) and soft X-rays (280-310 eV). Density Functional Theory (DFT) calculations were used to determine the most stable structures and the relevant low-lying isomers of singly charged C₆H₁₂ ions. Partial Ion Yield (PIY) analysis gives evidence of the higher tendency toward dissociation of cyclohexane in comparison to benzene. However, because of the high photoabsorption cross-section of benzene at the C1s resonance edge, its photodissociation and photoionization cross-sections are enhanced, leading to a higher efficiency of dissociation of benzene in the PDR of NGC 7027. We suggest that a similar effect is experienced by PAHs in X-ray photon-rich environments, which ultimately acts as an auxiliary protection mechanism of super-hydrogenated polycyclic hydrocarbons. Finally, we propose that the single photoionization of cyclohexane could enhance the abundance of branched molecules in interstellar and circumstellar media.