Dissociative photoionization of O2 is studied by an energy–mass analyzing system and synchrotron radiation. The kinetic energy distribution spectra of the O+ ion fragments and the dissociative photoionization efﬁciency curves for the O+ fragments with zero and 0.8 eV kinetic energies are observed at photon energies from 18.5 to 24.7 eV. Two vibrational progressions are observed in the zero kinetic energy O+ spectrum and identiﬁed as Rydberg series nsơg (n = 4 and 5) converging to the 32IIu ionic state of O2. The spectroscopic constants for these Rydberg states are obtained. Post dissociative autoionizations are observed for the O*(2P)ndn(n = 3~5) atomic states in the zero kinetic energy O+ spectrum. A shape resonance structure for the B2S− G ionic state is observed in the 0.8 eV kinetic energy O+ spectrum. The related dissociation mechanisms are discussed.
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