Direct Identification of the C₂H(X²Σ⁺) + O(³P) → CH(A²Δ) + CO Reaction as the Source of the CH(A²Δ→X²Π) Chemiluminescence in C₂H₂/O/H Atomic Flames

In this work, the intensity of the CH(A 2 Δ) chemiluminescence, I (CH*), as well as the concentrations of ground state C 2 H radicals and O atoms were measured as a function of the reaction time in a variety of helium-diluted C 2 H 2 /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 [C 2 H] 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 [C 2 H][O] concentration product, over a range of two decades, irrespective of the initial mixture composition or the reaction time. Using the NO + O → NO 2 * chemiluminescence as a calibration standard, CH* formation rates were derived from the measured I (CH*), and the values of the rate coefficient k 2a of the CH*-forming reaction channel C 2 H + O → CH(A 2 Δ) + CO (r2a) were thus derived from the slopes of the I (CH*) versus [C 2 H][O] plots. The results, for 290 K < T < 925 K, can be represented by the Arrhenius expression k 2a = 2.4 × 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 C 2 H calibration factor. The value at 290 K, 1.1 × 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 C 2 H formation mechanism in C 2 H 2 /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.