The effect of infrared laser excitation on reaction dynamics: O+C2H4† and O+OCS†

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Abstract
A technique involving flash photolytic production and resonance fluorescence detection of O atoms coupled with cw production of vibrationally excited reactant molecules using a CO2 laser is described. The method relies on the high sensitivity and precision of the flash photolysis resonance fluorescence technique to compare reaction rates measured with the laser off with those measured with the laser on, thereby assessing any effect of reactant vibrational energy on the dynamics of the chemical reaction. The limiting value of the laser enhancement that can be discerned depends ultimately on two factors: (1) the equilibrium concentration of vibrationally excited reactant that can be experimentally realized (i.e., absorption of laser flux vs deactivation losses), and (2) the magnitude of the activation energy for the thermal reaction. This determines the extent of a purely thermal (heating) effect. We observed little or no effect of vibrational energy in C2H4 on the rate of reaction with O atoms. Because of a somewhat higher activation energy, it was considerably more difficult to measure an effect in the O+OCS reaction. Within the limits of detectability, all observations in this system could be attributed to heating effects. A factor of 1.5 increase in the rate constant for O+C2H4† over that for O+C2H4 and a factor of 3 for O+OCS† over O+OCS would have been detectable in these experiments. The results seem to indicate that there is little or no coupling of vibrational energy to the reaction coordinate leading to activated complexes in these two reaction systems.