Detection of CH in an oxyacetylene flame using two-color resonant four-wave mixing technique

Abstract

CH free radicals in an oxyacetylene flame at atmospheric pressure were detected by means of the two-color resonant four-wave mixing technique. Ground-state grating schemes with ω1=ω2 (pump) and ω3 (probe)=ω4 (signal) were used; ω1(=ω2) and ω3 are in resonance with distinct rovibronic transitions of B 2Σ−−X 2Π and A 2Δ−X 2Π of CH, respectively. Varying the relative polarization of the pump and probe beams significantly affects the ratio of signal to noise. Two schemes with cross polarization pairs (ω1⊥ω2) and (ω3⊥ω4) produced results best suited to detect trace species in the luminous, dense medium; the relative J dependence observed for these schemes agrees with theoretical predictions. The double-resonance feature of this technique, advantageous to identify congested lines, is also demonstrated. When an A–X transition line is employed as a probe, previously unobserved predissociative levels of B 2Σ− are detected; the R(12) line of B←X (1,0) transion has a halfwidth greater than 10 cm−1, corresponding to a lifetime of 0.5 ps.