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 (γ, 2γ) method, to investigate the inner-valence excited states and multiply excited states of O₂ as superexcited states. Four superexcited states of O₂ with the ³Σ ^-_u or ³Π_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 (γ, 2γ) experiment. The origin of (2) is discussed in detail.