Photochemical effects of KrF excimer excitation in laser-induced fluorescence measurements of OH in combustion environments

This paper presents experimental evidence that using the KrF excimer laser fur quantitative laser-induced fluorescence (LIF) studies of the OH A-X (3, 0) system is highly problematic if the effects of both photobleaching and p hotochemistry are not included for laser spectral irradiances greater than 20 MW/cm(2)cm(-1). Pump-probe and time-resolved measurements of the OH LTF signal in an atmospheric pressure, premixed CH4-air flame at low- and high- laser-spectral-irradiance conditions show that a. significant amount of OH is produced from photofragments resulting from the simultaneous 2-photon pr edissociation of H2O molecules in the (C) over tilde-(X) over tilde system. A 5+2-level rate-equation model that includes the effects of both photoble aching and photochemical OH production is shown to satisfactorily pre diet the data using a single adjustable parameter given by the effective, spectr ally integrated 2-photon cross-section of H2O near 248 nm. The time-integra ted OH LIF signal was found to depend on both the laser spectral irradiance and the local concentration of H2O. Additionally, use of the KrF excimer l aser for 2-line rotational thermometry can produce temperature errors as gr eat as +550 K at high laser-pulse energies.