Multiplet-specific shape resonant features in 3 sigma g photoionization of O2

We report multiplet-specific photoionization cross sections and photoelectron angular distributions for the 3σ_g orbital of O₂ leading to the b ⁴Σ^-_g and B ²Σ^-_g ion states obtained using Hartree-Fock photoelectron orbitals. These cross sections show significant nonstatistical behavior at low photoelectron energies which arises from the sensitivity of the kσ_u shape resonance to the exchange potentials of these two molecular ions. Specifically, the oscillator strength associated with the shape resonance in the B ²Σ^-_g cross section is shifted to lower energy compared to that of the b ⁴Σ^-_g cross section. This shift gives rise to a quartet to doublet cross section ratio of more than 15:1 near threshold. These effects are difficult to assess in the measured cross sections due to the presence of strong autoionization features. Significant multiplet dependence is also seen in the calculated photoelectron angular distributions.