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

A first-principle theoretical approach to study the process of radiative electron attachment is developed and applied to the negative molecular ions CN⁻, C₄H⁻, and C₂H⁻. Among these anions, the first two have already been observed in the interstellar space. Cross sections and rate coefficients for formation of these ions by direct radiative electron attachment to the corresponding neutral radicals are calculated. For the CN molecule, we also considered the indirect pathway, in which the electron is initially captured through non-Born-Oppenheimer coupling into a vibrationally resonant excited state of the anion, which then stabilizes by radiative decay. We have shown that the contribution of the indirect pathway to the formation of CN⁻ is negligible in comparison to the direct mechanism. The obtained rate coefficients for the direct mechanism at 30 K are 7 × 10⁻¹⁶ cm³/s for CN⁻, 7 × 10⁻¹⁷ cm³/s for C₂H⁻, and 2 × 10⁻¹⁶ cm³/s for C₄H⁻. These rates weakly depend on temperature between 10 K and 100 K. The validity of our calculations is verified by comparing the present theoretical results with data from recent photodetachment experiments