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A first-principle theoretical approach to study the process of radiativeelectron attachment is developed and applied to the negative molecular ionsCN$^-$, C$_4$H$^-$, and C$_2$H$^-$. Among these anions, the first two havealready been observed in the interstellar space. Cross sections and ratecoefficients for formation of these ions by radiative electron attachment tothe corresponding neutral radicals are calculated. For completeness of thetheoretical approach, two pathways for the process have been considered: (i) Adirect pathway, in which the electron in collision with the moleculespontaneously emits a photon and forms a negative ion in one of the lowestvibrational levels, and (ii) an indirect, or two-step pathway, in which theelectron is initially captured through non-Born-Oppenheimer coupling into avibrationally resonant excited state of the anion, which then stabilizes byradiative decay. We develop a general model to describe the second pathway andshow that its contribution to the formation of cosmic anions is small incomparison to the direct mechanism. The obtained rate coefficients at 30~K are$7\times 10^{-16}$cm$^3$/s for CN$^-$, $7\times 10^{-17}$cm$^3$/s forC$_2$H$^-$, and $2\times 10^{-16}$cm$^3$/s for C$_4$H$^-$. These rates weaklydepend on temperature between 10K and 100 K. The validity of our calculationsis verified by comparing the present theoretical results with data from recentphotodetachment experiments.

Theoretical study of radiative electron attachment to CN, C2H, and C4H radicals