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Accurate assignments of highly excited molecular ro-vibrational states are needed for the verification of theoretical predictions of high-temperature spectra observed e.g. in astrophysics. Optical-optical double-resonance (OODR) spectroscopy using a continuous wave (CW) pump and a cavity-enhanced comb probe is a new tool for broadband, sensitive and selective detection and assignment of sub-Doppler hot-band molecular transitions [1]. It allows determination of term values and rotational assignment of highly excited molecular states, providing unique reference data for verification of theoretical predictions. We have previously used this methods for detection and assignment of transitions in the 3ν3 ← ν3 range of methane, reaching levels with rotational quantum numbers, J, up to 4 in the underexplored 9000 cm-1 range of the P6 (triacontad) polyad [1]. Testing the predictions for higher rotational levels is important, because i) these states dominate the spectra at high temperatures, and ii) variational calculations, on which the state-of-the-art databases for astrophysical applications are based, may suffer from a lack of convergence for these levels.

Unambiguous quantum number assignment for observed states is a key aspect of spectroscopic analysis, but it becomes especially challenging for the crowded spectra of hot-band molecular transitions. These transitions can be resolved with sub-MHz precision over a wide bandwidth using optical-optical double-resonance (OODR) spectroscopy based on a continuous wave pump and an optical frequency comb probe [1]. In OODR, the change in total angular momentum quantum numbers, ΔJ, for probe transitions can be assigned from the ratios of line intensities from two consecutive measurements with the pump linearly polarized parallel and perpendicular to the probe's linear polarization [1,2]. This is because at the OODR transitions the molecular response is different along and perpendicular to the pump polarization axis, and depends on the J-values of the three states involved in the OODR transition [3].

We present an optical-optical double-resonance (OODR) spectrometer based on a 3.3 µm continuous wave pump and two cavity-enhanced probes: a frequency comb tunable in the 1.64–1.8 µm range, and a comb-referenced continuous wave (CW) laser tunable in the 1.6–1.75 µm range. The comb probe provides broad spectral coverage (bandwidth up to 7 THz) for simultaneous detection of many sub-Doppler OODR transitions with sub-MHz line position accuracy, while the CW probe allows targeting individual transitions with kHz accuracy and a higher signal-to-noise ratio in shorter time. Using the pump stabilized to the frequency of the R(0) transition in the v₃ band of methane and the comb probe covering the 5550 to 6070 cm⁻¹ interval, we detect 37 ladder-type transitions in the 3v₃ ← v₃ band region and 6 V-type transitions in the 2v₃ band region and assign them using available theoretical predictions. Using the CW probe, we measure selected ladder- and V-type transitions with much higher precision. We also detect Lamb dips in the R(0) – R(3) transitions of the 2v₃ band and report their center frequencies with kHz-level accuracy. The synergy effects of the comb- and CW-OODR open new possibilities in precision spectroscopy of levels that cannot be reached from the ground state.

We use optical-optical double-resonance spectroscopy with a continuous wave (CW) pump and a cavity-enhanced frequency comb probe to measure the energy levels of methane in the upper part of the triacontad polyad (P6) with higher rotational quantum numbers than previously assigned. A high-power CW optical parametric oscillator, tunable around 3000 cm-1, is consecutively locked to the P(7, A2), Q(7, A2), R(7, A2), and Q(6, F2) transitions in the ν3 band, and a comb covering the 5800-6100 cm-1 range probes sub-Doppler ladder-type transitions from the pumped levels with J' = 6 to 8, respectively. We report 118 probe transitions in the 3ν3 ← ν3 spectral range with uncertainties down to 300 kHz (1 × 10-5 cm-1), reaching 84 unique final states in the 9070-9370 cm-1 range with rotational quantum numbers J between 5 and 9. We assign these states using combination differences and by comparison with theoretical predictions from a new ab initio-based effective Hamiltonian and dipole moment operator. This is the first line-by-line experimental verification of theoretical predictions for these hot-band transitions, and we find a better agreement of transition wavenumbers with the new calculations compared to the TheoReTS/HITEMP and ExoMol databases. We also compare the relative intensities and find an overall good agreement with all three sets of predictions. Finally, we report the wavenumbers of 27 transitions in the 2ν3 spectral range, observed as V-type transitions from the ground state, and compare them to the new Hamiltonian, HITRAN2020, ExoMol, and the WKMLC line lists.

A procedure for automated low uncertainty assessment of empty cavity mode frequencies in Fabry-Pérot cavity based refractometry that does not require access to laser frequency measuring instrumentation is presented. It requires a previously well-characterized system regarding mirror phase shifts, Gouy phase, and mode number, and is based on the fact that the assessed refractivity should not change when mode jumps take place. It is demonstrated that the procedure is capable of assessing mode frequencies with an uncertainty of 30 MHz, which, when assessing pressure of nitrogen, corresponds to an uncertainty of 0.3 mPa.

We use optical-optical double-resonance spectroscopy with a high-power continuous wave pump and a cavity-enhanced comb probe to expand sub-Doppler measurements of the 3v3 ← v3 band of CH4 to higher rotational levels. We assign the final states using combination differences, i.e., by reaching the same state using different pump/probe combinations.

We use optical-optical double-resonance spectroscopy with a high-power continuous wave pump and a cavity-enhanced comb probe to expand sub-Doppler measurements of the 3ν3←ν3 band of CH4 to higher rotational levels. We assign the final states using combination differences, i.e., by reaching the same state using different pump/probe combinations.