Energy distribution in electronically excited CH and C2 in low pressure hydrocarbon flames

Abstract

A study has been made of the rotational energy distribution of electronically excited CH and of the vibrational energy distribution of electronically excited C2 radicals in low pressure (0.2 to 35 Torr) hydrocarbon flames. The rotational energy of the CH radical in the A2Δ (ν = 0) state has been found to be statistically distributed in the levels with quantum numbers 14 to 21. This distribution can be described by a temperature which has, however, no thermodynamic significance. In the methane–oxygen flame this temperature has been found to be independent of both the composition of the gas mixture and the pressure, whereas in the ethylene–oxygen system it varies with these parameters. Hydrogen–methane–oxygen flames behave as ethylene flames. Inert diluents such as nitrogen and carbon dioxide do not affect the temperature at very low pressures. This indicates that the energy distribution is not perturbed measurably by collisions before emission takes place. In order to explain these observations we have to accept that at least two reactions produce CH* in ethylene flames.The cross section for energy transfer out of the high rotational levels of CH(A2Δ) to flame gases is found to be smaller than 0.6 Å2 and that for transfer to CO2 is about 2 Å2.A statistical distribution also has been observed in the vibrational energy distribution of electronically excited C2. The corresponding temperature is about 6 500 °K and is independent of the composition of the gas mixture and of the fuel. It decreases very slowly with increasing pressure above 3 Torr. Inert diluents added to the gas mixture do not alter this temperature.The cross section for de-excitation of C2(A3πg) is found to be smaller than 2.5 Å2.