Time-resolved infrared-ultraviolet double resonance (IR−UV DR) spectroscopy is used to prepare acetylene molecules (C 2 H 2 ) in specific rovibrational states of the 12 700 cm -1 4ν CH manifold of the electronic ground-state Χ, monitoring their direct excitation and collision-induced state-to-state energy transfer, by probing at ∼299 or ∼296 nm with laser-induced fluorescence via the Α electronic state. The 4ν CH manifold derives much of its IR brightness from the (ν 1 + 3ν 3 ) combination band, such that many of the rotational levels J monitored by IR−UV DR are derived from the (1 0 3 0 0) 0 vibrational state. The 4ν CH manifold of C 2 H 2 is congested and affected by anharmonic, l -resonance, and Coriolis couplings that cause other IR-dark, UV-bright rovibrational levels to attain appreciable IR−UV DR intensity and to add to the complexity of intramolecular dynamics in that manifold. Consequently, collision-induced rovibrational satellites observed by IR−UV DR comprise not only regular even-Δ J features but also supposedly forbidden odd-Δ J features, of which the energy-transfer channel from J = 12 to J = 1 is particularly efficient. This paper focuses on low- J rovibrational levels of the 4ν CH manifold, particularly those with J = 0 and J = 1 in view of their anomalously large Stark effects that are likely to make them susceptible to collision-induced rovibrational mixing. Three complementary forms of IR−UV DR experiment are reported: IR-scanned, UV-scanned, and kinetic. These indicate that strong IR−UV DR signals observed by probing the (1 0 3 0 0) 0 J = 0 rovibrational level are complicated by underlying IR-dark, UV-bright states, making J = 0 unsuitable for systematic IR−UV DR studies. The (1 0 3 0 0) 0 J = 1 rovibrational level is more amenable to unambiguous characterization and yields insight concerning even- and odd-Δ J collision-induced rovibrational energy transfer and associated mechanisms.
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Payne et al. (2003) studied this question.