2022ApJ...940..113M

Benzonitrile (c-C₆H₅CN) has been recently detected in cold and dense regions of the interstellar medium, where it has been used as a signpost of a rich aromatic organic chemistry that might lead to the production of polycyclic aromatic hydrocarbons. One possible origin of this benzonitrile is interstellar ice chemistry involving benzene (c-C₆H₆) and nitrile molecules (organic molecules containing the -C≡N group). We have addressed the plausibility of this c-C₆H₅CN formation pathway through laboratory experiments using our new setup SPACE TIGER. The SPACE TIGER experimental setup is designed to explore the physics and chemistry of interstellar ice mantles using laser-based ice processing and product detection methods. We have found that c-C₆H₅CN is formed upon irradiation of c-C₆H₆:CH₃CN binary ice mixtures with 2 keV electrons and Lyα photons at low temperatures (4-10 K). Formation of c-C₆H₅CN was also observed when c-C₆H₆ and CH₃CN were embedded in a CO ice matrix, but it was efficiently quenched in a H₂O ice matrix. The results presented in this work imply that interstellar ice chemistry involving benzene and nitrile molecules could contribute to the formation of the observed benzonitrile only if these species are present on top of the ice mantles or embedded in the CO-rich ice layer, instead of being mixed into the H₂O-rich ice layer.