Formation of ${\mathrm{CN}}^{-}$, ${\mathrm{C}}_3{\mathrm{N}}^{-}$, and ${\mathrm{C}}_5{\mathrm{N}}^{-}$ Molecules by Radiative Electron Attachment and their Destruction by Photodetachment

The existence of negative ions in interstellar clouds has been associated for several decades with the process of radiative electron attachment. In this Letter, we report compelling evidence supporting the fact that the radiative attachment of a low-energy electron is inefficient to form the carbon chain anions ${\mathrm{CN}}^{-}$, ${\mathrm{C}}_3{\mathrm{N}}^{-}$, and ${\mathrm{C}}_5{\mathrm{N}}^{-}$ detected in interstellar clouds. The validity of the approach is confirmed by good agreement with experimental data obtained for the inverse photodetachment process, which represents the major cause of anion destruction in interstellar space. As a consequence, we suggest alternative models that could explain the formation of anions.

Figure 1

Theoretical and experimental photodetachment cross sections for the ground vibrational level of ${\mathrm{CN}}^{-}$. The cross section obtained in this study (solid red line) is compared with available experimental data (green triangle) [10] and with the previous theoretical calculation (dot-dashed line) [12] performed using the complex Kohn method.

Figure 2

${\mathrm{C}}_3{\mathrm{N}}^{-}$ and ${\mathrm{C}}_5{\mathrm{N}}^{-}$ photodetachment cross sections obtained for a fixed geometry (the solid lines) and for the process where the final vibrational level of the molecule is the same as the initial one, $v_i=v_n=0$ (the dashed lines), accounting for the Franck-Condon factor of Eq. (3). Available experimental data for ${\mathrm{C}}_3{\mathrm{N}}^{-}$ [10], also shown in the figure, should be compared with the cross section obtained for a fixed geometry (red solid line).

Figure 3

REA cross sections for CN, ${\mathrm{C}}_3\mathrm{N}$, and ${\mathrm{C}}_5\mathrm{N}$ molecules. The ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_5\mathrm{N}$ cross sections are calculated with the Franck-Condon factor for the $v_n=0\to v_i=0$ process.