Temperature dependent collisional quenching rates of CH(A) by methanol, acetone, methane, oxygen, and nitrogen

Abstract

Laser-induced fluorescence is a widely used technique for measuring the concentrations of gaseous species in reactive environments. To determine absolute number densities from laser-induced fluorescence signals, the collisional quenching rate of the excited state molecule needs to be known. The methylidyne (CH) radical is an important species in combustion, catalysis, and plasma applications, the latter two of which require laser-induced fluorescence measurements at lower temperatures. Quantitative detection of CH is also needed for photofragmentation laser-induced fluorescence measurements, where CH is produced by photolysis of a larger molecule, such as the methyl radical (CH\(_{3}\)), by a pump laser, and then is excited by a probe laser to induce fluorescence. We have measured the collisional quenching rates of CH(A) by methanol, methane, oxygen, nitrogen, and acetone at temperatures between 300 and 600 K. The CH(A) quenching rate by methanol, which is highly relevant in catalysis, has not previously been studied. The quenching rates for acetone, which is used as a precursor to photolytically produce methyl, and methane have been studied but not at elevated temperatures. We find that methanol and acetone both have high quenching rate coefficients of \(2.2\cdot10^{-10}\) to \(2.5\cdot10^{-10}\) cm\(^3\)/s with only a small temperature dependence. We also find that the quenching rate of methane has a significant temperature dependence ranging from \(2.5\cdot10^{-11}\) cm\(^3\)/s at 300 K to \(5.0\cdot10^{-11}\) cm\(^3\)/s at 600 K. The quenching rates determined in this work are important for laser-induced fluorescence studies of catalysis, plasmas, and combustion processes.