Kinetic behaviour of OH X2Π and A2σ+ using molecular resonance fluorescence spectrometry

Abstract

A method for the investigation of OH X²Π(v″= 0) radical kinetics in a discharge-flow system at 300 K, with detection of OH by molecular resonance fluorescence, using the OH A²Σ⁺–X²Π(0, 0) transition, is described. Quenching cross sections for OH A²Σ⁺(by H₂O, N₂, Ar and He) were determined from steady-state kinetic analysis, and the effects of electronic quenching on the kinetic measurements were shown to be unimportant. The intensity of resonance fluorescence, under the conditions used, varied in direct proportion to [OH, X²Π, v″= 0] up to concentrations of 5 × 10¹⁴ cm^–3.

The kinetics of decay of OH radicals near 120 N m^–2 total pressure include second order (2) and first order wall (w) reactions, OH + OH [graphic omitted] H₂O + O (2), OH [graphic omitted] products. (w) Two independent methods of determination of k₂ are described, allowing for k_w, and based on fitting of experimental data to computed concentration profiles derived from a reaction scheme incorporating the major elementary reactions which can affect [OH]. These methods gave values for k₂(300 K, cm³ molecule^–1 s^–1) of (1.27 ± 0.21)× 10^–12 and (1.50 ± 0.34)× 10^–12, leading to a mean value from our results, k₂=(1.4 ± 0.2)× 10^–12. Consideration of the present and previous data for k₂ leads to a proposed overall value k₂=(1.7 ± 0.6)× 10^–12 cm³ molecule^–1 s^–1 at 300 K.