Highly predissociative levels of CH B²Σ^- state detected with two-color resonant four-wave mixing spectroscopy

We demonstrate an application of two-color resonant four-wave mixing spectroscopy to detect highly predissociative levels of CH in the B²Σ^- state in a hostile environment of an oxyacetylene flame. The probe and grating wavelengths are in resonance with the A²Δ-X²Π and B²Σ^--X²Π systems, respectively. We measured 49 previously unobserved rovibronic lines of the 0-0 band and 38 additional lines of the 1-0 band of the B²Σ^--X²Π system to access rotational quantum number N^′ up to 21 and 13 of v^′=0 and 1 in the B²Σ^- state, respectively. Inclusion of additional line positions measured for the 0-0 and the 1-0 bands yields spectral parameters of the B²Σ^- state significantly improved over those obtained previously with only nonpredissociative lines; one additional centrifugal-distortion parameter L_v is evaluated significantly. Although power saturation is significant even at smallest practical laser energies, we estimated lifetimes through the power dependence of linewidths. The lifetimes of the highest measurable level of B²Σ^- (v^′=0 and 1) are 3±1 (N^′=20) and 8±2 (N^′=11) ps, respectively.