Rovibrational energy transfer in the 4v_CH 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 4v_CH manifold of the electronic groundstate X~, 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 à electronic state. The 4v_CH manifold derives much of its IR brightness from the (v₁ + 3v₃) 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 4v_CH 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-?J features but also supposedly forbidden odd-?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 4v_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-?J collision-induced rovibrational energy transfer and associated mechanisms.