Inner-valence excited and multiply excited states of molecular oxygen around the double-ionization potential as probed by a pair of fluorescence photons

The cross sections for the generation of a pair of fluorescence photons in the photoexcitation of O2 differential with respect to solid angles for the emission of the photon pair have been measured as a function of incident photon energy in the range 23?47 eV using the photon?photon coincidence method, the (?, 2?) method, to investigate the inner-valence excited states and multiply excited states of O2 as superexcited states. Four superexcited states of O2 with the 3??u or 3?u symmetry have been newly found around 29, 36, 38 and 44 eV in the measured cross section curve free from ionization. It turns out that they are inner-valence excited states and multiply excited states. Two remarkable points are shown: (1) there exist superexcited states of O2 dissociating into neutral fragments even in the energy range above the double-ionization potential of O2 at 36.13 eV and (2) the values of the doubly differential cross sections decrease to a large extent around the double-ionization potential with increasing incident photon energy, both of which have been compared with the results of N2 by our previous (?, 2?) experiment. The origin of (2) is discussed in detail.