Alkyl peroxy radical kinetics measured using near-infrared CW-cavity ring-down spectroscopy

A kinetic reactor system is described which couples pulsed laser photolytic production of radicals with continuous laser excitation cavity ring-down spectroscopic detection in the near-infrared (NIR). The atmospherically relevant alkyl peroxy radicals ethyl peroxy (C2H5O2) and methyl peroxy (CH3O2) were monitored via their structured absorbance spectra in the NIR near 1.3 mum. These peroxy radicals were then subjected to kinetic study as proof-of-principle for the new technique. Portions of the absorption spectra for the two radicals are reported which agree well with previously published spectra [Hunziker, H. E.; Wendt, H. R. J. Chem. Phys. 1976, 64, 3488. Pushkarsky, M. B.; Zalyubovsky, S. J.; Miller, T. A. J. Chem. Phys. 2000, 112, 10695]. The absorption cross sections were determined at selected wavelengths using the known self-reaction rate coefficients and observed kinetic data. The absorption cross sections determined are as follows: for two of the maxima in the origin band of C2H5O2, sigma(1317.01) (nm) = sigma(1316.40) (nm) = (3.0 +/- 1.5) x 10(-21) cm(2) molecule(-1), and for a maximum in a sequence band of CH3O2, sigma(1335.07) (nm) = (1.5 +/- 0.8) x 10(-20) cm(2) molecule(-1). Preliminary data for the prototypical peroxy radical cross-reaction between CH3O2 and C2H5O2 is presented. This data supports earlier work [Villenave, E.; Lesclaux, R. J. Phys. Chem. 1996, 100, 14372] which established a pressure independent value of k(CH3O2 + CH3CH2O2) = 2.0 x 10(-13) cm(3) molecule(-1) s(-1) at 298 K. As in most kinetic studies involving peroxy radicals, the accuracy of the reported rate coefficients is influenced by the details of the complex mechanisms used in the fitting. However, in the current studies, specific radical absorption(s) are used to follow each radical's decay, which should improve the precision of the determination.