Nonequilibrium vibrational distribution and kinetics of OH chemiluminescence in the hydrogen–oxygen reaction

Abstract

This paper describes a spectroscopic approach for studying the kinetics of the reaction H + O + M → OH(A2Σ+) + M (R1). Vibrational distributions of OH(A) produced during high-temperature reactions of H2/O2 mixtures diluted with Ar have been measured by observing the emission spectra of the OH A2Σ+–X2Π transition. The experiments were performed behind reflected shock waves at temperatures of 1600–2200 K and pressures of 1.4–5.2 bar in a high-repetition-rate shock tube. The observed emissions are attributed to the chemiluminescence of OH(A) formed in (R1). The recorded spectra were fitted with the simulated ones to determine the vibrational distributions of OH(A) for vibrational levels of υ = 0–4. The distributions were significantly nonthermal due to competition between vibrational energy transfer and other involved processes. Then, in order to model the kinetics of (R1), a master equation for the vibrational states of OH(A) is constructed based on the obtained vibrational distributions, as well as available data on predissociation, collisional quenching, spontaneous emission, and vibrational energy transfer of OH(A). The rate constants for (R1) predicted for certain mixtures are found to be fairly consistent with those reported previously based on OH(A) concentration measurements. Meanwhile, substantially smaller rate constants are predicted for highly diluted mixtures. This result is supported by the variation seen in the relative emission rates observed for different mixtures. It is suggested that the rate constant for (R1) depends significantly on third-body molecules due to the dependency of the vibrational energy transfer rate on collision partners.