Resonances in Electron-Impact Electron Detachment of $\mathrm{C}_2{}^{-}$

Molecular $R$-matrix with pseudostate calculations are reported for the electron-impact ionization cross section of the carbon dimer anion. A ${}^1\Sigma _g^{+}$ resonance is found near the detachment threshold and two further resonances, of ${}^3\Pi _g$ and ${}^1\Pi _g$ symmetry, are found near 10 eV close to the structures observed experimentally. These unusual shape resonances are a result of the competition between the repulsive Coulomb interaction and the large, attractive polarizability of $\mathrm{C}_2{}^{-}$. Use of the Born approximation to allow for higher partial waves gives a total cross section close to that observed experimentally.

Figure 1

Electron-impact ionization cross section for $\mathrm{C}_2{}^{-}$ calculated using different pseudostate basis sets.

Figure 2

Total ionization cross section. Electron-impact ionization cross section due to Pedersen et al. [3] with (squares) and without (circles) dissociative channels; dashed curve molecular RMPS calculation with ${\alpha }_0=0.17$, $\beta =1.3$, solid curve molecular RMPS calculation with Born correction. The arrow indicates the ionization threshold, below which the cross sections should be zero.