Saturated laser induced fluorescence is used for the sensitive detection of radicals in high pressure gases. The method and its application to a series of addition reactions of HO radicals in the high pressure regime are described. Experiments between 1 and 150 bar of the bath gas He allow for falloff extrapolations to the high pressure limit of the recombination reactions. Limiting rate constants (in cm3 molecule−1 s−1) of 2.2×10−11 for HO+HO→H2O2, of 3.3×10−11 for HO+NO→HONO, of 7.5×10−11 for HO+NO2→HONO2, and of 9.7×10−13 for HO+CO→HOCO (and H+CO2) are derived at 298 K.

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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 groundstate ˜, 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, UVbright 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 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-∆J collision-induced rovibrational energy transfer and associated mechanisms.

Collision-induced state-to-state molecular energy transfer between rovibrational states in the 12,700 cm(-1) 4nu(CH) manifold of the electronic ground state X of acetylene (C(2)H(2)) is monitored by time-resolved infrared-ultraviolet double resonance (IR-UV DR) spectroscopy. Rotational J-states associated with the (nu(1) + 3nu(3)) or (1 0 3 0 0)(0) vibrational combination level, initially prepared by an IR pulse, are probed at approximately 299, approximately 296, or approximately 323 nm with UV laser-induced fluorescence via the Alpha electronic state. The rovibrational J-states of interest belong to a congested manifold that is affected by anharmonic, l-resonance, and Coriolis couplings, yielding complex intramolecular dynamics. Consequently, collision-induced rovibrational satellites observed by IR-UV DR comprise not only regular even-DeltaJ features but also supposedly forbidden odd-DeltaJ features. A preceding paper (J. Phys. Chem. A 2003, 107, 10759) focused on low-J-value rovibrational levels of the 4nu(CH) manifold (particularly those with J = 0 and J = 1) whereas this paper examines locally perturbed states at higher values of J (particularly J = 17 and 18, which display anomalous doublet structure in IR-absorption spectra). Three complementary forms of IR-UV DR experiments (IR-scanned, UV-scanned, and kinetic) are used to address the extent to which intramolecular perturbations influence the efficiency of J-resolved collision-induced energy transfer with both even and odd DeltaJ.