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Spectroscopic studies of C2, CH and OH radicals in low pressure acetylene+oxygen flames

Temperatures and concentrations of OH(X 2II and A 2∑+), CH(X 2II and A 2∆) and C2(X3IIu and A 3IIg) have been further studied in low-pressure C2H2 + O2 flames. For any given state (with the possible but unlikely exception of upper state OH), vibrational and rotational temperatures are in close agreement. The upper state temperature of a given species always exceeds the corresponding lower state temperature. The highest upper state temperatures normally recorded are 7000 K, for C2(A 3IIg) and OH(A 2∑+), but, quite exceptionally, values up to 9000 K are found for OH(A 2∑+) at the very earliest stages of combustion. No lower state temperature exceeds 3000 K. The temperature of CH(2II) is quite normal, equalling within 100 K the measured translational flame temperatures. The temperatures of CH(2∆), OH(X 2II) and C2(X 3IIu) are all similar, lying in the range 2500 to 3000 K. C2(A 3IIg) and OH(A 2∑+) temperatures are broadly similar too, falling between 5000 and 7000 K. Firm experimental evidence in support of Gaydon’s reaction C2(X 2II0) + OH(X 2II) → CH(A 2∆) + CO(X 1∑+) Is provided for the first time; its rate constant at 2200 K is estimated to be 8 ± 4 x 10-12 ml molecules-1 s-1. This reaction is not, however, responsible for formation of CH(X 2H). Neither C2(X 3IIu) nor C2(A3IIg) are formed in the reaction CH + CH → C2 + H2. At the pressures of these experiments, OH(A 2Ʃ -) is strongly quenehed, but both CH(A 2∆) and C2(A 3IIz) decay predominantly through radiative emission.