Investigation of OH dynamics in the argon sensitized pulse radiolysis of water vapor

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Abstract
Reactions of OH radical were studied in systems containing 650‐Torr Ar and from 0.5 to 9‐Torr H2O using the method of pulse radiolysis‐absorption spectroscopy. It was found that initial concentrations of OH radical increased and the half‐life for its loss decreased at higher water pressures. Dependence of OH half‐life on OH concentrations occurs because homogeneous OH loss processes are second order in reaction intermediates, whose concentration increases with added H2O. A contribution by water in chaperoning OH/OH and OH/H combination is also important. The kinetic scheme was examined with a Gear integrator. A good fit of the experimental rate data was obtained using literature values for rate constants of OH+OH, OH+H, and H+H reactions, when experimental zero‐time absorbance values were converted to OH concentrations based on an extinction coefficient of 6.37×104 l mol−1 cm−1 at 309.5 nm. Using the same extinction coefficient, an alternative empirical reduction of rate data gives k (apparent bimolecular) for loss of OH at 650‐Torr Ar pressure of 5.06×10−11 cm3 molec−1 s−1, consistent with literature rate constants within 10%. Dependence of OH yield on water concentration is interpreted on a two state model for argon precursor, involving short‐lived (resonance state) Ar* (1P1 and 3P1) and longer‐lived (metastable) Ar** (3P2 and 3P0). Adjustment of assumed initial Ar* and Ar** concentrations allowed prediction of measured OH concentrations within experimental error at all water pressures studied.