  Benzonitrile (c-C6H5CN) has been recently detected in cold and dense regions
of the interstellar medium (ISM), where it has been used as a signpost of a
rich aromatic organic chemistry that might lead to the production of polycyclic
aromatic hydrocarbons (PAHs). One possible origin of this benzonitrile is
interstellar ice chemistry involving benzene (c-C6H6) and nitrile molecules
(organic molecules containing the -CN 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 Lyman-alpha 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.
