Radiative electronic attachment to molecules of astrophysical interest*

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, obtained through the complex Kohn variational method. We present our benchmark calculation for the formation of the simplest observed ion CN$^-$ by REA, as well as our preliminary results for the formation of C$_3$N$^-$ and C$_4$H$^-$ by REA. We calculated a low rate of $10^{-15}$cm$^3/$s at 30 K for CN$^-$ and rates about 20 time larger for C$_3$N$^-$ and C$_4$H$^-$ due to larger transition dipole moments. These two latter ions possess dipole states and we have also considered their potential role in the process of radiative electronic attachment. Finally, our results suggest that the negative molecular ions, recently observed in the interstellar medium, can hardly be formed by the process of radiative electron attachment.