2020MNRAS.499.6066Q

Biphenyl (C₁₂H₁₀), or phenylbenzene, is an important building block of polycyclic aromatic hydrocarbons (PAHs), whose infrared spectral features are present in a variety of galactic and extragalactic sources. In this work, we use synchrotron radiation coupled with time-of-flight spectrometry to study the photoionization and photodissociation processes of biphenyl upon its interaction with soft X-ray photons at energies around the inner-shell C1s resonance. These results are compared with our previous studies with benzene (C₆H₆) and naphthalene (C₁₀H₈), and discussed in the context of four planetary nebulae featuring PAH infrared emission: BD+30^*3639, NGC 7027, NGC 5315, and NGC 40. We show that the mass spectrum of biphenyl before the C1s resonance energy is dominated by single photoionization processes leading to C₆H₅⁺, C₆H₄^+x⋅, and C₁₂H₁₀^+⋅, while after the resonance dissociation following multiple photoionization processes is dominant. The release of neutral C₆H₆ and C₆H₅^⋅ species accounts for one of the most relevant dissociation processes starting from the doubly ionized biphenyl, indicating that heterolytic charge separation of the two phenyl rings is also achieved. By using quantum chemical calculations, we show that the biphenylic structure is a high-lying isomer of the singly and doubly ionized C₁₂H₁₀ species, whose minimum energy geometries are related to the acenaphthene molecule, composed of a C₂-bridged naphthalene. Furthermore, we estimate the lifetime of biphenyl for 275 and 310 eV in photon-dominated regions of planetary nebulae. We discuss distinct processes that may enhance its lifetime and those of other small-sized PAHs in such astrophysical environments.