Radiative electron attachment to molecules of astrophysical interest. Benchmark study of CN$^-$.*

We develop a first-principles approach to study the process of radiative electron attachment (REA) to linear molecules of astrophysical interest $Mol +e^- \to Mol^- + \hbar\omega$. ($Mol^-$ = C$_n$H$^-$, C$_n$N$^-$). The approach is based on accurate ab initio calculations of electronic bound and continuum states of the negative ion. The electronic continuum states are obtained with the complex-Kohn variational method. A preliminary calculation for the formation of the simplest observed ion, CN$^-$, by REA gave a low rate coefficient. We will present also a preliminary result for the C$_4$H$^-$ formation by REA. For this molecule, the REA rate coefficient is expected to be somewhat larger due to the Renner-Teller non-adiabatic coupling that should enhance electron capture. The goal of this study is to answer the question if negative molecular ions C$_n$H$^-$ and C$_n$N$^-$ recently observed in the interstellar space could indeed be formed by REA as previously suggested.