Production of H and O(P-3) Atoms in the Reaction of CH2 with O-2

Abstract

The reaction CH2 + O-2 -> 4 products has been studied by using atomic resonance absorption spectrometry (ARAS) of H and O(P-3) 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 CH2I2 (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 CH2I2 + 300 ppm H-2 mixture has been conducted simultaneously to confirm the initial concentration of CH2. The apparent reaction rate of CH2 + O-2 (k(2)'), including the contributions of (CH2)-C-3 + O-2 -> products (2) and (CH2)-C-1 + 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) x 10(-11). The contribution of (CH2)-C-1 + 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 (CH2)-C-3 + O-2 reaction is given as k(2)/cm(3) molecule(-1) s(-1) = (1.69 +/- 0.31) x 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 x 10(-11) exp(-874/T), (T = 295-2050 K). The apparent production yields of H and O atoms for the reaction channels (CH2)-C-1,3 + O-2 -> H + products (2a, 3a) and (1)'(CH2)-C-3 + O-2 -> O(P-3) + products (2h, 3b) have been evaluated as phi(2a)' = 0.59 +/- 0.06 and phi(2b)' = 0.23 +/- 0.06, respectively. The contributions of (CH2)-C-1 + O-2 reaction on measured phi(2a)' and phi(2b)' are indicated to be minor; the net branching fractions for the (CH2)-C-3+O-2 reaction are estimated as phi(2a) = 0.58 +/- 0.06 and phi(2b) = 0.25 +/- 0.06. No obvious temperature dependence is indicated in the measured rate constant nor in the branching fractions of H and 0 atoms. The mechanism of the reaction of CH2 with O-2 is discussed based on the result of the present study together with those of the previous theoretical/experimental studies.