Dissociative attachment from the O2(a1Δg) state

The dissociative attachment cross section for production of O⁻ by electron impact on the metastable O₂(a¹Δ_g) state is studied. The cross section is found to be 4.6 ± 1.3 × 10⁻¹⁸ cm² at its maximum. From the measured energy dependence, we infer that the dissociative attachment process takes place through the ${\mathrm{O}}_2^{-}{(}^2{\Pi }_u)$ state as in the case of O⁻ production from the O₂ ground state. The information thus obtained is used to estimate the portion of the cross section for excitation of the a¹Δ_g state by electron impact which proceeds via the ${\mathrm{O}}_2^{-}{(}^2{\Pi }_u)$ state. This mechanism is shown to account for the location and approximate magnitude of the maximum in the excitation cross section. Finally, information is obtained concerning the cross section for positive ionization of the O₂(a¹Δ_g) state near threshold.