Excited Electronic States of O2−

Excited electronic states of the $\mathrm{O}_{2}^{}{}_{}{}^{\ensuremath{-}}$ molecule have been calculated with configuration-interaction (CI) variational trial functions that assure formally correct asymptotic behavior as well as the single-configuration self-consistent-field (SCF) approximation. CI results were obtained by both multiconfiguration self-consistent-field (MC-SCF) and pseudonatural orbital (PNO) techniques. The MC-SCF results are most accurate and are used to analyze the energy curves and wave functions of these states for internuclear separations larger than 3 a. u. All the excited states are found to have equilibrium-internuclear separations at least 1 a. u. larger than the ground state. The two lowest energy states, the $^{4}\ensuremath{\Sigma}_{u}^{\ensuremath{-}}$ and $^{2}\ensuremath{\Pi}_{u}$, are characterized, respectively, as shape and valence Feshbach resonances. They are sufficiently bound to make it likely they play a role in low-energy-electron scattering by oxygen.