DETECTION OF HIGHLY PREDISSOCIATIVE LEVELS OF CH $B^{2}\Sigma^{-}$ STATE WITH TWO-COLOR RESONANT FOUR-WAVE MIXING SPECTROSCOPY

Author Institution: Department of Chemistry, National Tsing Hua UniversityWe demonstrate an application of two-color resonant four-wave mixing (TC-RFWM) spectroscopy to detect highly predissociative levels of the $B ^{2}\Sigma^{-}$ state of CH in a hostile environment of an oxyacetylene flame. The probe and the grating wavelengths are in resonance with the $A^{2}\Delta-X^{2}\Pi$ and $B^{2}\Sigma^{-}$ transitions, respectively. We measured 42 previously unobserved rovibronic lines of the 0-0 band and additional 30 lines of the 1-0 band of the $B^{2}\Sigma^{-}$ system to access the rotational quantum number N' up to 20 and 12 of =0 and 1 of the $B^{2}\Sigma^{-}$ state, respectively. Inclusion of measured additional line positions of the 0-0 and the 1-0 bands yields spectral parameters of the $B^{2}\Sigma^{-}$ state significantly improved over those obtained previously with only non-predissociative lines; one additional rotational constant $L_{v}$ is determined. Although power saturation is significant even at lowest applicable laser energies, we estimated lifetimes with studies of power dependence of line widths. The lifetimes of the highest detectable level of $B^{2}\Sigma^{-} v' = 0$ and 1 are $3\pm 1$ ps $(N' = 20)$ and $8\pm 2$ ps $(N' = 11)$, respectively. We will also present preliminary data of TC-RFWM of CH in supersonic jet; saturation dip is readily observable even at low laser energies