The formation and dissociation of the ozone dication

Electron-impact double ionization of the ozone molecule has been investigated using ion-ion coincidence spectroscopy coupled with time-of-flight mass spectrometry. The coincidence spectra show that O-3(2+) dissociates to form O-2(+) and O+. Using the kinetic energy release determined for this fragmentation, we find that an excitation of O-3 by 34.3 eV is required to populate the lowest energy state of O-3(2+) which dissociates to O-2(+) and O+. This excitation energy is corroborated by a determination of the variation of the ion yield of this dissociation process as a function of the ionizing electron energy. Comparison with theoretical calculations indicates that the ground electronic state of O-3(2+) is principally responsible for this decay channel. Hence, the determination of the excitation energy of this state provides the first experimental value of the vertical double ionization potential of O-3. NO O-3(2-) ions that are stable on a mass spectrometric (microsecond) time scale were detected. Hence, the vast proportion of the Franck-Condon zone for the population of the ground state of O-3(2+) from O-3 appears to lie in unbound regions of the dication's potential energy surface. We estimate, at 150 eV electron energy, that the ionization branching ratio for the formation of any long-lived states of O-3(2+) is less than 0.003