The rate constant of the reaction OH + CH 3 O 2 → products (R1). has been measured at 294K by simultaneous coupling of Laser Induced Fluorescence (LIF) and cw-Cavity Ring Down Spectroscopy (cw-CDRS) to laser photolysis. OH radicals were generated by 248nm photolysis of either O 3 in the presence of H 2 O or H 2 O 2 , CH 3 O 2 radicalswere generated simultaneously by photolysis of CH 3 I and their absolute concentrations were obtained by cw-CRDS. OH decays were measured under excess CH 3 O 2 concentrations and a very fast rate constant of k 1 = (2.8±1.4)×10 -10 cm 3 s -1 was found independent of pressure at 50 and 100 Torr helium.

The absorption spectrum of CH3O2 radicals and CH3I molecules has been measured in the range 7473-7497 cm(-1). CH3O2 radicals have been generated by 248 nm laser photolysis of CH3I in the presence of O2, and the relative absorption has been measured by time-resolved continuous-wave cavity ring-down spectroscopy (cw-CRDS). Calibration of the relative absorption spectrum has been carried out on three distinct wavelengths by carefully measuring CH3O2 decays under different experimental conditions and extracting the initial radical concentration (and with this the absolute absorption cross sections) by using the well-known rate constant for the CH3O2 self-reaction. The following, pressure-independent absorption cross sections were determined: 3.41 × 10(-20), 3.40 × 10(-20), and 2.11 × 10(-20) cm(2) at 7748.18, 7489.16, and 7493.33 cm(-1). These values are 2-3 times higher than previous determinations ( Pushkarsky, M. B.; Zalyubovsky, S. J.; Miller, T. A. J. Chem. Phys. 2000, 112 (24), 10695 - 10698 and Atkinson, D. B.; Spillman, J. L. J. Phys. Chem. A 2002, 106 (38), 8891 - 8902). The absorption spectrum of the stable precursor CH3I has also been determined and three characteristic sharp absorption lines with absorption cross sections up to 2 × 10(-21) cm(2) have been observed in this wavelength range.