Shape resonance and electronic autoionization in vibrationally resolved partial photoionization cross sections of O2

Abstract Vibrationally resolved partial photoionization cross sections of O 2 leading to the b 4 Σ − g and B 2 Σ −1 g (3σ −1 g ) tonic states have been measured at high resolution in a wide photon energy range from 0.5 eV above threshold to 24.5 eV. The σ u shape resonance is observed around 21.5 eV, in fairly good agreement with “one-electron” theoretical predictions. The series of resonances from 19 to 20 5 eV result from an autoionization process with a strong vibrational selectivity, explained by the similar geometries of the resonance states and the b ionic state. The large resonance widths originate from a strong Rydberg-valence orbital mixing Calculations using multichannel quantum defect theory reproduce the most important features.