High-resolution infrared flash kinetic spectroscopy of OH radicals

A high-resolution infrared flash kinetic spectrometer is used for time- and frequency-resolved studies of the OH radical. OH is produced by 193-nm excimer laser photolysis of HNO₃/buffer gas mixtures in a 100-cm flow tube and is probed via weak fractional absorption of light from a widely tunable (2.35-3.59 μm) single-mode (Δv ≤ 2 MHz) color center laser. The IR absorption technique allows fast (≤ 10^-6 s), sensitive (<10⁹ radicals/cm³ per quantum state) detection of OH and is designed to permit determination of absolute OH number densities. The spectrometer is used to measure rate constants for the reactions of OH with ethane (k₁), propane (k₂), n-butane (k₃), and isobutane (k₄). The reliability of these measurements is tested on a variety of rotational, spin-orbit, and λ-doublet states, with several buffer gases, and over more than an order of magnitude of alkane concentrations. The resulting rate constants are, in units of 10^-12 cm³ molecule^-1 s^-1, k₁ = 0.243 ± 0.012, k₂ = 1.02 ± 0.05, k₃ = 2.35 ± 0.08, and k₄ = 2.11 ± 0.09. The rate constants for the ethane, n-butane, and isobutane reactions agree with some previous determinations but are found to be between 10% and 25% lower than values currently used in atmospheric modeling; it is recommended that these values be revised to reflect the lower rates from this study. Current models of atmospheric air flow based on these rate constants, as well as those of previously accepted values, are found to be inconsistent with daily changes in observed atmospheric alkane concentrations.