Wavelength dependent photodissociation of CH3OOH: Quantum yields for CH3O and OH, and measurement of the OH + CH3OOH rate coefficient

Methyl hydroperoxide, CH3OOH, has been synthesised with >99.5% purity, confirmed using UV absorption spectroscopy and high-pressure liquid chromatography (HPLC) followed by post-column derivatisation. The UV absorption cross-section for CH3OOH was measured and for <325 nm was in good agreement with the literature. Laser-flash photolysis combined with laser-induced fluorescence (LIF) spectroscopy has been used to measure both OH and CH3O photofragments following the photolysis of CH3OOH in the wavelength range 223–355 nm. Using the previously measured unity quantum yield for OH at 248 nm as a reference, the LIF signals immediately following photolysis were used to measure wavelength dependent quantum yields for OH and CH3O, taking into account changes in laser pulse energy and absorption cross-section. The quantum yields for both species were unity within experimental error. The rate coefficient for the reaction of OH with CH3OOH (R1a) to generate CH3O2 + H2O products was measured at 295 K to be k(R1a) = (9.0 ± 0.2) × 10−12 cm3 molecule−1 s−1, considerably higher (by about a factor of two) than previous values measured by Vaghjiani and Ravishankara [G.L. Vaghjiani, A.R. Ravishankara, J. Phys. Chem. 93 (1989) 1948–1959] and Niki et al. [H. Niki, P.D. Maker, C.M. Savage, L.P. Breitenbach, J. Phys. Chem. 87 (1983) 2190–2193].