Dissociation of Vibrational State-Selected O2+ Ions in the B2Σg State Using Threshold Photoelectron-Photoion Coincidence Velocity Imaging

Using the recently developed threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging mass spectrometer (Tang et al. Rev. Sri. Instrum. 2009, 8Q, 113101), dissociation of vibrational state-selected O-2(+) (B-2 Sigma(g), nu(+) = 0-6) ions was investigated. Both the speed and angular distributions of the O+ fragments dissociated from individually vibronic levels of the B-2 Sigma(g) state were obtained directly from the three-dimensional time-sliced TPEPICO velocity images. Two dissociation channels, O+(S-4) + O(P-3) and O+(S-4) + O(D-1), were respectively observed, and their branching ratios were found to be heavily dependent on the vibrational states. A new intersection mechanism was suggested for the predissociation of O2+(B-2 Sigma(g)) ions, especially for dissociation at the energy of the nu(+) = 4 level. In addition, the anisotropic parameters for O+ fragments from different dissociative pathways were determined to be close to zero, indicating that the nu(+) = 0-6 levels of B-2 Sigma(g) predissociate on a time scale that is much slower than that of molecular rotation.