Direct identification of the C2H(X(2)Sigma(+))+O(P-3)->CH(A(2)Delta)+CO reaction as the source of the CH(A(2)Delta->X(2)Pi) chemiluminescence in C2H2/O/H atomic flames

In this work, the intensity of the CH(A(2) Delta) chemiluminescence, I(CH*), as well as the concentrations of ground state C2H radicals and O atoms were measured as a function of the reaction time in a variety of helium-diluted C2H2/O/H mixtures in an isothermal flow reactor at temperatures of 290, 410, 520, 590, 675, and 925 K and at a total pressure of 2 Torr. The species concentrations [O] and [C2H] were measured using molecular beam sampling-threshold ionization mass spectrometry (MB-TIMS). At each temperature, the intensity I(CH*) was found to be directly proportional to the [C2H][O] concentration product, over a range of two decades, irrespective of the initial mixture composition or the reaction time. Using the NO + O --> NO2* chemiluminescence as a calibration standard, CH* formation rates were derived from the measured 1(CH*), and the values of the rate coefficient k(2a) of the CH*-forming reaction channel C2H + O --> CH(A(2) Delta) + CO (r2a) were thus derived from the slopes of the I(CH*) versus [C2H][O] plots. The results, for 290 K < T < 925 K, can be represented by the Arrhenius expression k(2a) = 2.4 x 10(-11) exp[-230/T(K)] cm(3) molecule(-1) s(-1); the possible systematic error is a factor of 2, due to the uncertainty of the C2H calibration factor. The value at 290 K, 1.1 x 10(-11), is in fair agreement with our recent result obtained in an independent pulse laser photolysis/chemiluminescence experiment. The addition of methane was found to suppress I(CH*) in quantitative agreement with the C2H formation mechanism in C2H2/O/H systems elucidated earlier by us. It is argued that the fast reaction r2a is a major if not the dominant CH* source also in hot hydrocarbon flames.