Knowledge Bank

We demonstrate an application of two-color resonant four-wave mixing (TC-RFWM) spectroscopy to detect highly predissociative levels of the $C\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma +$ and $D\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$ states of CH in a hostile environment of an oxyacetylene flame. The 1-1 band of the $C-X$ transition is detected with the probe and the grating wavelengths in resonance with the $A\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Delta -X\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$ and $C\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma +-X\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$ transitions, respectively. Six branches of the $C-X$ system are spectrally resolved for the first time; in total 124 lines detected in this work correspond to excitation of the $C(v=1)$ state up to $N\prime =23$. Observed wave numbers are fitted to yield improved spectral parameters of the $C\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma +$ state. The D state was observed by using two grating beams in resonance with the $D\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi -B\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma -$ transition and the pump beam in resonance with a known $B\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma --X\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$ transition. A total of 86 lines associating with transitions to the $D(v=0)$ state with rotational quantum number N' up to 16 were detected; spectral parameters of the $D\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi (v=0)$ state were determined. Linewidth, broadened due to predissociation, up to $6cm-1$ were observed. Predissociation mechanisms of both the C and the D states are discussed.