The geometrical structure of the low-lying states of CO+2 has been calculated ab initio. The C̃ 2Σ+g/2A1 state is found to be slightly bent in its equilibrium geometry. A new assignment of the vibrational structure of the corresponding band in the photoelectron spectrum is suggested. State C̃ is predissociated by two competitive channels. One of them leads to O++CO, the other to CO++O. The mechanism of these predissociations involves a slow, rate-determining, intersystem crossing to a bent ã 4B1 state. The population of state ã has a choice between dissociating to O++CO fragments and undergoing a further, much faster, intersystem crossing to the ground state X̃ which dissociates to CO++O. Since radiationless transitions between X̃ and ã are relatively rapid, the state which is lower in energy (i.e., X̃) has a much larger population than the other (i.e., ã). Hence, the CO++O channel prevails as soon as it is energetically accessible. The rate-determining step of both processes is the intersystem crossing between states C̃ and ã. Its rate constant is estimated by a statistical method due to Zahr, Preston, and Miller, recasted in a simpler form. A value of about 4×107 s−1 is obtained.

The mass spectrometric study of 1-methylcyclopentene, methylenecyclopentane, cyclohexene, 1-hexyne and 2-hexyne enabled determination of the ionization potentials of these molecules together with the appearance potentials of their principal fragment ions. The heats of formation of these ions have been calculated and the values are discussed. There is evidence for hydrogen atom rearrangements occurring during the formation of some fragment ions. Adiabatic ionization energies of I-methylcycIopentene and methylenecyclopentane have been determined by photoionization.RBsumb-L'Btude par spectrometrie de masse du I-mCthylcyclopentiine, du methyliinecyclopentane, du cyclohexene, de I'hexyne-1 et de I'hexyne-2 nous a permis de mesurer les potentiels d'ionisation de ces substances ainsi que les potentiels d'apparition de leur principaux ions fragmentaires. Les chaleurs de formation de ces ions ont Ct B calcul6es et les valeurs trouvees sont discutkes. La structure de certains ions fragmentaires implique un rearrangement d'atomes d'hydrogkne au moment de leur formation. Les potentiels d'ionisation adiabatiques du 1-m&hylcyclopentene et du m6thyliinecyclopentane ont Bt6 determines par photoionisation.

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