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

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 B²Σ ^--X²Π and A²Δ-X²Π 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 B²Σ ^- 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.