Dissociative Double Ionization of O2 Below the Adiabatic Double Ionization Potential P. Bolognesi #, D.B. Thompson #^2, L. Avaldi ^n, M.C.A. Lopes #", M.A. MacDonald*, D.R. Cooper # and G.C. King # # Schuster Laboratory, Physics Department, The University of # Manchester, Manchester M13 9PL, UK ^n IMAI del CNR, Area della Ricerca di Roma, CP10-00016 Monterotondo, Italy * Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK Mechanisms for O2 fragmentation leading to the formation of the O+- O+ ion pairs, in the photon energy region 32-41 eV, have been investigated using a threshold-photoelectron-photoelectron coincidence (TPEPECO) technique. The measurements show that fast dissociation of a singly-charged ion, followed by atomic autoionization, is the dominant decay route in this energy region.