Production of H and O(3P) Atoms in the Reaction of CH₂ with O₂

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Abstract

The reaction CH(2) + O(2) → products has been studied by using atomic resonance absorption spectrometry (ARAS) of H and O((3)P) atoms behind reflected shock waves over 1850-2050 K and 2.1-1.7 atm. Measurements of [H] and [O] are conducted using mixtures of highly diluted CH(2)I(2) (0.2 and 0.4 ppm) with excess O(2) (100 and 262 ppm) in Ar; comparative measurement of [H] in the 0.2 and 0.4 ppm CH(2)I(2) + 300 ppm H(2) mixture has been conducted simultaneously to confirm the initial concentration of CH(2). The apparent reaction rate of CH(2) + O(2) (k(2)'), including the contributions of (3)CH(2) + O(2) → products (2) and (1)CH(2) + O(2) → products (3), has been measured from the evolutions of [H] and [O] and summarized as k(2)'/cm(3) molecule(-1) s(-1) = (1.90 ± 0.31) × 10(-11). The contribution of (1)CH(2) + O(2) reaction on the measured k(2)' has been evaluated as 0.15 ± 0.04, with an assumption that k(3) is independent of temperature and given by the result measured at room temperature [Langford, A. O.; Petek, H.; Moore, C. B. J. Chem. Phys. 1983, 78, 6650-6659]. The net rate for the (3)CH(2) + O(2) reaction is given as k(2)/cm(3) molecule(-1) s(-1) = (1.69 ± 0.31) × 10(-11). The result on k(2) in this study is found to be consistent with the extrapolation of the previous work at lower temperature range of 295-600 K. [Vinckler, C.; Debruyn, W. J. Phys. Chem. 1979, 83, 2057-2062]; these results can be summarized as k(2)/cm(3)molecule(-1) s(-1) = 2.74 × 10(-11) exp(-874/T), (T = 295-2050 K). The apparent production yields of H and O atoms for the reaction channels (1,3)CH(2) + O(2) → H + products (2a, 3a) and (1,3)CH(2) + O(2) → O((3)P) + products (2b, 3b) have been evaluated as φ(2a)' = 0.59 ± 0.06 and φ(2b)' = 0.23 ± 0.06, respectively. The contributions of (1)CH(2) + O(2) reaction on measured φ(2a)' and φ(2b)' are indicated to be minor; the net branching fractions for the (3)CH(2)+O(2) reaction are estimated as φ(2a) = 0.58 ± 0.06 and φ(2b) = 0.25 ± 0.06. No obvious temperature dependence is indicated in the measured rate constant nor in the branching fractions of H and O atoms. The mechanism of the reaction of CH(2) with O(2) is discussed based on the result of the present study together with those of the previous theoretical/experimental studies.

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