Rovibrational Energy Transfer in the 4νCH Manifold of Acetylene, Viewed by IR−UV Double Resonance Spectroscopy. 1. Foundation Studies at Low J

Time-resolved infrared-ultraviolet double resonance (IR−UV DR) spectroscopy is used to prepare acetylene molecules (C2H2) in specific rovibrational states of the 12 700 cm-1 4νCH 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 4νCH manifold derives much of its IR brightness from the (ν1 + 3ν3) 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 4νCH manifold of C2H2 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-ΔJ features but also supposedly forbidden odd-ΔJ 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 4νCH 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-ΔJ collision-induced rovibrational energy transfer and associated mechanisms.