SUB-DOPPLER DOUBLE-RESONANCE SPECTROSCOPY OF METHANE USING A FREQUENCY COMB PROBE

Methane is the first organic molecule detected in hot-Jupiter exoplanets [1] and the observed spectra carry information about the atmospheric conditions, photochemistry, and planetary formation. To extract this information, we need accurate theoretical models of the spectra, verified by high-precision laboratory measurements. However, the energy level structure of highly excited CH$_4$ is poorly understood; there exist limited line-by-line assignments of laboratory spectra above the Icosad polyad (~6,000 \wn). We perform double-resonance spectroscopy using a 3.3 $\mu$m continuous-wave pump and 1.67 $\mu$m frequency comb probe to measure sub-Doppler transitions to energy levels in the 3$\nu_{3}$ band (9000 \wn range) [2]. Using a single-pass cell, we detected 36 (J’=0-2) transitions, 26 for the first time, with 1.7 MHz frequency accuracy, limited by the stability of the pump. We assign the transitions using the intensity ratios measured with parallel and perpendicular pump/probe polarizations, and by comparison of transition frequencies and intensities to predictions from the TheoReTS database [3]. Our work is the first measurement of sub-Doppler molecular response using a frequency comb and the first verification of the accuracy of the theoretical prediction that start from highly vibrationally excited methane states. Currently, experiments are being performed using an enhancement for the comb probe. [1] M. R. Swain, G. Vasisht, and G. Tinetti, Nature 452, 329 (2008). [2] A. Foltynowicz et al., Arxiv: 2001.08781 (2020). [3] M. Rey, A. V. Nikitin, Y. L. Babikov, and V. G. Tyuterev, Journal of Molecular Spectroscopy 327, 138 (2016)