Optical frequency comb double-resonance spectroscopy of the 9030–9175 cm−1 states of ethylene

We use optical–optical double-resonance (OODR) spectroscopy to measure for the first time hot-band transitions of ethylene between states in the 3000 and 9000 cm−1 energy ranges. A 3.2 μm continuous wave (CW) pump is used to populate selected states in the ν9 vibrational mode. The sub-Doppler OODR transitions are then probed with two different cavity-enhanced probes tunable around 1.7 μm: a frequency comb probe that allows for broadband measurements and simultaneous detection of many OODR lines and a CW probe that measures individual lines with higher signal-to-noise ratio and better frequency accuracy. We report center frequencies and relative intensities of 90 ladder-type hot-band transitions from three different states in the ν9 vibrational mode. We exploit combination differences and measurements of polarization-dependent intensity ratios to determine the final state rotational quantum numbers J. Comparison to theoretical predictions from ExoMol allows tentative assignments for 28 transitions. We also report improved line center frequencies for the three pump transitions in the ν9 band. Furthermore, we observe 18 sub-Doppler V-type transitions from the depleted ground state to the 6000 cm−1 region and assign 14 of them using ExoMol and the variational line list of Mraidi et al. [J. Quant. Spectrosc. Radiat. Transfer 310, 108734 (2023)].