Coherent infrared–ultraviolet double-resonance spectroscopy of CH3

Two-color polarization spectroscopy (TC-PS) and two-color resonant four-wave mixing spectroscopy (TC-RFWM) are used to detect photolytically produced CH3 radicals. An infrared laser pumps individual lines in the ν3 fundamental of the X̃2A2″ state, and an ultraviolet laser probes the pumped levels to reveal rotationally resolved spectra of transitions to the predissociated B̃2A1′ state. The spectra are fit with a complex Lorentzian lineshape and yield an updated value of 46 239.4±1.2 cm−1 for T0 of the B̃ state. A detection limit of 2×1013 CH3 molecules per cm3 per quantum state is observed for these coherent double-resonance techniques.