Rotational Temperatures of OH in Methane-Air Flames

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Abstract
Rotational line intensity distributions in emission and absorption of the 0↔0 band of the 2Σ↔2Π transition of OH have been measured in methane‐air flames. The number of electronically excited OH radicals in the inner cone greatly exceeds the equilibrium concentration, and the intensity distribution of the high quantum numbers gives a temperature of 5200°K. However, OH radicals in the ground state were in thermal equilibrium at 2000°K. A break in the intensity distribution indicates a two stage combustion reaction. It is suggested that the excessive amount of excited OH is due to its formation in the intermediate reaction steps. Line reversal temperatures using Na, Hg, and OH have been measured in the same flame for purposes of comparison. Also the rotational temperature of excited OH in a methane‐oxygen flame was measured.