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

We have developed 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. The benchmark calculation for the formation of the simplest observed ion, CN-, by REA has produced a low rate coefficient, $5\times 10^{-17}$cm$^3/$s at 30 K. We will present also a preliminary result for the C$_4$H$^-$ formation by REA. For this molecule, the REA rate coefficient is expected is larger by about a factor of 10 due to a larger transition dipole moment. This study suggests that the negative molecular ions, recently observed in the interstellar medium, can hardly be formed by the process of radiative electron attachment.