NON-FRANCK–CONDON EFFECTS IN THE PHOTOIONIZATION OF N₂ TO THE A ²Π_u STATE AND OF O₂ TO THE X ²Π_g STATE IN THE 19–34 eV PHOTON ENERGY REGION

Photoionization to the A ²Π_u state and to the X ²Π_g state is studied using photoelectron spectroscopy. The experimental vibrational branching ratios are obtained for the first time in the 19–34 eV region for the v′ = 0–3 levels, for both molecules. Ab initio many-body perturbation theory is used to calculate branching ratios for ionization to the A ²Π_u state and branching ratios, vibrationally resolved partial cross sections and total cross section for ionization to the X ²Π_g state. The Franck–Condon breakdown observed in the photoionization of the N₂ 1π_u electron is mainly explained by autoionization from Rydberg and valence states of N₂, whereas photoionization of the O₂π_g electron is not fully explained either by channel coupling effects or by autoionization from known Rydberg series and valence states.