Non Franck–Condon effects in the photoionization of molecular oxygen to the O2+ X 2Πg state in the 19–31 eV photon energy region

Photoionization to the O2+ X 2Πg state is studied using photoelectron spectroscopy. Experimentally, vibrational branching ratios for the v=0 to 3 levels are obtained in the 19–31 eV region which show strong non Franck–Condon effects. Using ab initio many body perturbation theory branching ratios, vibrationally resolved partial cross-sections and total cross-section for ionization to the X 2Πg state are calculated. Molecular states which autoionize to the O2+ X 2Πg state continuum are discussed. Photoionization of the O2 πg electron is not fully explained by channel coupling effects and autoionization from known Rydberg series and valence states.