Rovibrational energy transfer in the 4vCH 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) 4nu(CH) manifold of the electronic groundstate (X) over tilde, monitoring their direct excitation and collision-induced state-to-state energy transfer, by probing at similar to299 or similar to296 nm with laser-induced fluorescence via the (A) over tilde electronic state. The 4nu(CH) manifold derives much of its IR brightness from the (nu(1) + 3nu(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)degrees vibrational state. The 4nu(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-DeltaJ features but also supposedly forbidden odd-DeltaJ 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 4nu(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)degrees 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)degrees J = 1 rovibrational level is more amenable to unambiguous characterization and yields insight concerning even- and odd-DeltaJ collision-induced rovibrational energy transfer and associated mechanisms.