Dissociation of vibrational state-selected O_2~+ ions in the B~2∑_g~- state using threshold photoelectron-photoion coincidence velocity imaging

摘要

Using the recently developed threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging mass spectrometer (Tang et al. Rev. Sci. Instrum.2009, 80, 113101), dissociation of vibrational state-selected O _2~+(B~2∑_g~-, v ~+ = 0-6) ions was investigated. Both the speed and angular distributions of the O~+ fragments dissociated from individually vibronic levels of the B~2∑_g~- state were obtained directly from the three-dimensional time-sliced TPEPICO velocity images. Two dissociation channels, O~+(~4S) + O(~3P) and O~+(~4S) + O(~1D), 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 O_2~+(B~2∑ _g~-) ions, especially for dissociation at the energy of the v~+ = 4 level. In addition, the anisotropic parameters for O ~+ fragments from different dissociative pathways were determined to be close to zero, indicating that the v~+ = 0-6 levels of B ~2∑_g~- predissociate on a time scale that is much slower than that of molecular rotation.