Nonequilibrium concentrations of the vibrationally excited OH radical in a methane flame measured by cavity ring-down spectroscopy

Strong deviation from the Maxwell–Boltzmann equilibrium for the populations of the v″=0 and v″=1 vibrational levels of OH(X 2Πi) radicals was observed in the preflame zone of the methane/oxygen/nitrogen flame. The OH concentrations for the v″=0 and v″=1 levels were measured using cavity ring-down spectroscopy in the 30 Torr flat flame. In that flame the population ratio for v″=1/v″=0 is ∼20 times the equilibrium value at locations ∼6 mm below the luminescent zone. Above the flame front this ratio was found to be close to the equilibrium value.