Abnormal Excitation of OH in H2/O2/N2 Flames

Abstract

The emission from the first four vibrational levels of OH2Σ+ and the concentrations of ground state OH2II have been measured in a series of rich H2/O2/N2 flames held on flat porous burners as a function of distance from the burner. The intensity of emission of each of the bands is proportional to the cube of the concentration of ground state OH, but the constant of proportionality depends on the band and the flame conditions. This dependence, as well as absolute intensity measurements, establish the fact that the radiation is nonthermal. The dependence of the emission intensity on an integral power of OH strongly indicates that the excited OH is formed as a result of one or more of a set of possible radical recombination reactions. It is shown that in general it is not possible to obtain a unique determination of the excitation mechanism for any band. The only exception to this is that definite evidence for the preassociation reaction, O+H→OH2Σ, v=2, has been obtained. The vibrational distributions of excited OH are shown to be consistent with ``temperatures'' from 3000°K to 5000°K (compared with maximum flame gas temperatures of 1600°K), which is probably to be expected from a reaction with the exothermicity of H+OH+OH→H2+OH*. It is concluded from this and from the general behavior of the data, that this last reaction is most likely responsible for the nonthermal emission of all the bands, with the exception of that specifically known to be due to the preassociation reaction.