Direct measurement of methyl radicals in a methane/air flame at atmospheric pressure by radar REMPI

We report the direct measurements of methyl radicals (CH3) in methane/air flames at atmospheric pressure by using coherent microwave Rayleigh scattering (Radar) from Resonance Enhanced Multi-Photon Ionization (REMPI), also known as the Radar REMPI technique. A tunable dye laser was used to selectively induce the (2 + 1) REMPI ionization of methyl radicals (CH3, 3 p(2) A ''(2)0(0)(0) band) in a near adiabatic and premixed laminar methane/air flame, generated by a Hencken burner. In situ measurements of the REMPI electrons were made by non-intrusively using a microwave homodyne transceiver detection system. The REMPI spectrum of the CH3 radical was obtained and a spatial distribution of the radicals limited by focused laser beam geometry, approximately 20 mu m normal to the flame front and 2.4 mm parallel to the flame, was determined. The measured CH3 was in good agreement with numerical simulations performed using the detailed kinetic mechanism of GRI-3.0. To the authors' knowledge, these experiments represent the first directly-measured spatially-resolved CH3 in a flame at atmospheric pressure. (C) 2011 Optical Society of America