Pressure dependence of the phenylperoxy radical (C6H5O2) formation in the reaction of phenyl radical (C6H5) with molecular oxygen (O2) has been investigated with the cavity ring-down method by detecting the C6H5O2 at 504 nm. The rate constant of the C6H5 + O2 reaction was determined to be k = (1.2 ± 0.1) × 10−11 cm3 molecule−1 s−1 by the least-square analysis of several sets of the C6H5O2 rise profiles at 30 Torr and at 296 K. This value is almost identical to those previously obtained at 40 Torr (Yu and Lin, J Am Chem Soc 1994, 98, 9571–9576) and 760 Torr (Tonokura et al., J Phys Chem A 2002, 106, 5908–5917), indicating that the rate constant has no pressure dependence in the pressure range of 30–760 Torr. The signal intensity ratio of C6H5O2 to C6H5 was obtained in the pressure range of 5–120 Torr. Varying the third body carrier gas (N2) had a significant effect on the production of the C6H5O2. To validate the k determination by C6H5O2 rise detection and determine the branching ratio of C6H5O2 in the C6H5 + O2 reaction system, theoretical calculations were also performed. The rate constant obtained with Rice–Ramsperger–Kassel–Marcus/master equation calculations is in good agreement with that measured in this experiment. The branching ratio forming C6H5O2 obtained by the theoretical calculation also exhibited pressure dependence, which is consistent with the experimental results. The branching ratio increases with increasing pressure and reaches approximately 0.85 at 120 Torr. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 44: 41–50, 2012
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