Sub-Doppler Double-Resonance Spectroscopy of Methane Using a Frequency Comb Probe

We report the measurement of sub-Doppler double-resonance transitions in methane using a 3.3 $\mu$m continuous wave optical parametric oscillator to pump transitions in the fundamental ${\nu }_3$ band and a 1.67 $\mu$m frequency comb to probe ladder-type transitions in the 3${\nu }_3$ band over 200 cm${}^{-1}$ of bandwidth. We detected 36 ladder-type resonances for 9 pumped transitions with average center frequency accuracy of 1.7 MHz, limited by the pump frequency stability. The lines are assigned using the TheoReTS line list, and the intensity ratios for different relative pump/probe polarizations. This method provides accurate assignment of highly excited energy levels that cannot be done using high temperature spectra.