Shape resonance behavior in 1π_g photoionization of O_2

We report calculations of vibrationally resolved cross sections and photoelectron angular distributions for photoionization of O_2 leading to the X^ 2 Π_g (ν^+ =0–4) states of O^+_2 using Hartree–Fock continuum photoelectron orbitals. These studies were motivated by recent results which show that a σ_u shape resonance plays a dominant role in producing non‐Franck–Condon vibrational distributions in resonant multiphoton ionization of O_2 via the C ^3Π_g (1π_g3sσ_g) Rydberg state. In the present study, we investigate how this shape resonance influences photoionization dynamics in single‐photon ionization. Below 21 eV photon energy, we find significant non‐Franck–Condon effects in the vibrational branching ratios as well as in the vibrationally resolved photoelectron angular distributions. Substantial autoionization hinders a direct comparison between theory and experiment