HIGH-RESOLUTION INNER-VALENCE UV PHOTOELECTRON-SPECTRA OF THE O2 MOLECULE AND CONFIGURATION-INTERACTION CALCULATIONS OF 2-PI-U STATES BETWEEN 20 AND 26 EV

High-resolution He I and He II excited inner-valence photoelectron spectra of the oxygen molecule have been recorded between 20 and 26 eV. In this range three photoelectron bands are clearly observed that are associated with transitions to the B (2)SIGMA(g)-, (2)PI(u) and c (4)SIGMA(u)- cationic states. Vibrational structure is observed in all photoelectron bands, and the vibrational constants have been determined. The rotational profile of the vibrational lines is resolved for the B state, and an analysis is made in terms of transitions involving DELTA-N = 0 and +/- 2. In addition to the three strong bands, a fourth, weak vibrational progression has been observed in the range of the B (2)SIGMA(g)- state. The state of 2PI(u) symmetry observed around 24 eV shows a long vibrational progression with spacings that decrease successively towards higher electron binding energies. The progression converges at 23.83 eV, slightly below the position where the band has the highest intensity. This (2)PI(u) state is thus shown to be bound with a dissociation energy D0 of 2.5 eV. The assignments are confirmed by ab initio calculations, which also provide a vibrational analysis and potential curves that agree very well with the experimental results. These calculations show that the potential curves follow the electron configurations rather than the adiabatic curves in the inner-valence region.