Chemistry of nitrile anions in the interstellar medium

Despite the extreme conditions of temperature (down to 10K) and density (down to 100 molecules/cm³), the giant molecular clouds and the circumstellar envelopes present a rich and complex chemistry. To date, more than 180 molecules have been detected in the InterStellar Medium (ISM) with a large abundance of nitriles (RC≡N). In addition, several anions have been recently observed in this medium: C₄H^¯, C₆H^¯, C₈H^¯, CN^¯, C₃N^¯ and C₅N^¯. These last species should play a key role in the molecular growth towards complexity. To explore this hypothesis, their reactivity must be studied in the laboratory. The FALP-MS and the CRESU experimental apparatuses of the Rennes University are able to measure absolute rate coefficient of various chemical reactions, including the ion – molecule reactions, in gas phase at low temperature (from 300K for the FALP-MS down to 15K for the CRESU). Therefore, these experimental tools are particularly adapted to the kinetic studies of reactions potentially involved in the Interstellar Medium. One of the difficulties encountered in experiments with anions is their generation. We describe here the formation of the CN^¯ and C₃N^¯ anions by dissociative electron attachment on the molecular precursors BrCN and BrC₃N