Quantitative detection of methyl radicals in non-equilibrium plasmas: a comparative study

Abstract

Tuneable infrared diode laser absorption spectroscopy at 16.5 µm and broadband ultraviolet absorption spectroscopy at 216 nm have both been used to measure the ground state concentrations of the methyl radical in two different types of non-equilibrium microwave plasmas (f = 2.45 GHz): (i) H2–Ar plasmas of a planar reactor with small admixtures of methane or methanol, at a pressure of 1.5 mbar, and (ii) H2–CH4 plasmas of a bell jar reactor, at pressures of 25 and 32 mbar under flowing conditions. For the first time, two different optical techniques have been directly compared to verify the available data about absorption cross sections and line strengths of the methyl radical. It was found that application of the CH3 absorption cross section of the transition at 216 nm, reported by Davidson et al (1995 J. Quant. Spectrosc. Radiat. Transfer 53 581) and of the line strength of the Q(8,8) line of the ν2 fundamental band near 16.44 µm, given by Wormhoudt et al (1989 Chem. Phys. Lett. 156 47), leads to satisfactory agreement.