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We use sub-Doppler optical-optical double-resonance (OODR) spectroscopy with a 3.3 µm single-frequency pump and a cavity-enhanced 1.65 µm comb probe to measure 33 ladder-type (3ν3 ← ν3) and 8 V-type (2ν3) transitions in the 5880–6090 cm-1 range of methane, reaching states with rovibrational E symmetry in the region of the P 6 and P 4 polyads, respectively. We assign the ladder-type transitions using new Hamiltonian predictions and the ExoMol line list, and the V-type transitions using the new Hamiltonian, ExoMol, HITRAN2020, and the WKLMC line lists. While 7 of the states in the 3ν3 range have been previously observed either in earlier OODR work (without cavity enhancement) with 1.5 MHz accuracy or in FTIR measurements of cold bands with 150 MHz resolution, the states reported here have uncertainties down to 150 kHz (5 × 10–6 cm-1). The E-symmetry states exhibit first-order Stark splitting, which will be reported in our future work.

Optical-optical double-resonance (OODR) spectroscopy using a narrow-linewidth pump and a frequency comb probe has previously been used to measure and assign sub-Doppler transitions in the 3ν3 ← ν3 spectral region [Hjältén et al., J. Chem. Phys. 161, 124311 (2024)] when pumping from the J = (7, A2) ground state. Doppler-broadened double-resonance transitions were also observed in those OODR spectra. In this paper, 68 of these Doppler-broadened transitions are assigned to four-level double-resonance transitions involving collisional transfer from the pumped A1 symmetry state to other A1 and A2 symmetry (I = 2 meta nuclear spin) levels of the ν3 fundamental state. Assignments are made using combination differences and comparison with the term values and intensities of lines predicted by a new effective Hamiltonian, the accuracy of which has been validated by the sub-Doppler transitions. No collisional OODR transitions were observed to known final states starting from states in the ν1 fundamental band, nor from other symmetries of the ν3 fundamental band.

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.

When Fabry-Perot (FP) refractometry is used to assess the refractivity of gases, it has so far been assumed that the Gouy phase is independent of the presence of gas in the cavity. Here we show, by both theory and experiments, that this is only correct for a non-deformable cavity. For a deformable one, the pressure can affect the radius of curvature of the mirrors. This gives the Gouy phase a component that is proportional to gas pressure. Although being a small effect (1.6 nrad/Pa), since it affects the Gouy phase only when the cavity contains gas, it affects the refractivity on a 10−6 level.

Fabry–Pérot-based refractometry has demonstrated the ability to assess gas pressure with high accuracy and has been prophesized to be able to realize the SI unit for pressure, the pascal, based on quantum calculations of the molar polarizabilities of gases. So far, the technology has mostly been limited to well-controlled laboratories. However, recently, an easy-to-use transportable refractometer has been constructed. Although its performance has previously been assessed under well-controlled laboratory conditions, to assess its ability to serve as an actually transportable system, a ring-type comparison addressing various well-characterized pressure balances in the 10–90 kPa range at several European national metrology institutes is presented in this work. It was found that the transportable refractometer is capable of being transported and swiftly set up to be operational with retained performance in a variety of environments. The system could also verify that the pressure balances used within the ring-type comparison agree with each other. These results constitute an important step toward broadening the application areas of FP-based refractometry technology and bringing it within reach of various types of stakeholders, not least within industry.

This work models and experimentally assesses the influence of absorption of laser light in mirrors in Fabry-Pérot based refractometers used for realization of pressure. Model parameters are assessed by experimental characterizations. Characterizations of two refractometers agree well with the predictions of the model. It is shown that, when pressures are assessed in the viscous region, the absorption of laser light in mirrors will give rise to a small alteration in the proportional response and a pressure-independent offset, where the latter is significant for He but considerably smaller for Ar and N₂.

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.

We use optical frequency comb Fourier transform spectroscopy to record high-resolution, low-pressure, room-temperature spectra of formaldehyde (H₂¹²C¹⁶O) in the range of 1250 to 1390 cm⁻¹. Through line-by-line fitting, we retrieve line positions and intensities of 747 rovibrational transitions: 558 from the ν₆ band, 129 from the ν₄ band, and 14 from the ν₃ band, as well as 46 from four different hot bands. We incorporate the accurate and precise line positions (0.4 MHz median uncertainty) into the MARVEL (measured active vibration-rotation energy levels) analysis of the H₂CO spectrum. This increases the number of MARVEL-predicted energy levels by 82 and of rovibrational transitions by 5382, and substantially reduces uncertainties of MARVEL-derived H₂CO energy levels over a large range: from pure rotational levels below 200 cm⁻¹ up to multiply excited vibrational levels at 6000 cm⁻¹. This work is an important step toward filling the gaps in formaldehyde data in the HITRAN database.