Dissociation of Vibrational State-Selected O₂⁺ Ions in the B²Σ_g^¯ State Using Threshold Photoelectron–Photoion Coincidence Velocity Imaging

Abstract

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)Σ(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)Σ(g)(¯) state were obtained directly from the three-dimensional time-sliced TPEPICO velocity images. Two dissociation channels, O(+)((4)S) + O((3)P) and O(+)((4)S) + O((1)D), 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)Σ(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)Σ(g)(¯) predissociate on a time scale that is much slower than that of molecular rotation.

Following coherent multichannel nuclear wave packets in pump-probe studies of O2 with ultrashort laser pulses

“…Clearly, further experiment is desirable in order to shed light into this problem. We also remark that the smallness of the recently measured β asymmetry parameter by Tang et al [33] strongly suggests that the lifetime of B 2 Σ − g state is larger than the rotational period. However, an earlier measurement by Evans et al [34] suggests a lifetime of only about 0.4 − 2 ps, whereas Richard-Viard et al [35] found a rather long lifetime of about 70 ns.…”

Direct Experimental Evidence for Dissociative Photoionization of Oxygen Molecule via 2Σu– Ionic “Optical Dark” State

“…Therefore, the real image needed to be handled prior to extracting speed and angular distributions through a multistep data reduction scheme including subtracting the false coincident events and deconvolution. Its details have been described in refs − . Thus, the modified image of O + dissociated from the O 2 + ( 2 Σ u – ,v + =0) ion at 20.353 eV is shown in Figure b.…”

“…The β values for the 2 Σ u – state are kept at ∼0.4 for the lowest dissociation channel, except the v + = 5 state of 2 Σ u – , which is seriously influenced by the quasi-resonant v + = 4 state of B 2 Σ g – . According to the fact that the β values of dissociation of the B 2 Σ g – state are near 0.25, predissociation of the 2 Σ u – state along the lowest channel is faster than that of the B 2 Σ g – state.…”

Advances in threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion coincidence (TPEPICO)

“…The translational energy release in the dissociation of small ions by ion imaging has been used to great advantage mostly by the group of Zhou. 113,117,118 The analysis of the ion images was greatly simplified by the application of time sliced imaging in which only the ions ejected perpendicular to the extraction axis are collected by a multichannel plate detector followed by a phosphor imaging system. Consider the dissociation of a diatomic ion to a single dissociation channel with a single translational energy.…”