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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.

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)].

Quantitative spectroscopic detection of dibromomethane, CH2Br2, for environmental monitoring, workplace safety, and exoplanetary studies is limited by the lack of accurate absorption cross-section data and rigorous spectroscopic models. We report the first high-resolution (6.3 MHz point spacing) absorption cross-section of CH2Br2 in the 1180–1210 cm−1 region, measured using optical frequency comb Fourier transform spectroscopy. This spectral region is dominated by the strong CH2 wagging (ν8) fundamental vibration, which is about 50 times stronger than the fundamental C–H stretch around 3077 cm−1. The measurements resolve isotopologue-specific rovibrational features of CH279Br81Br, CH279Br2, and CH281Br2, and we assign rovibrational transitions of the ν8 fundamental and the overlapping ν4 + ν8 − ν4 hot bands using two methods. First, an empirical non-linear least squares fit implemented in PGOPHER provides high-precision line assignment and spectroscopic constants, including accurate band origins, rotational constants, and quartic centrifugal distortion parameters, for the three isotopologues, covering rotational levels up to Ka = 25 and J = 144, with an average RMS residual of 0.00037 cm−1 (11.1 MHz). Compared with previously reported band parameters retrieved from a fit to narrowband (1.78 cm−1) supersonically cooled spectra (B. E. Brumfield et al., J. Mol. Spectrosc., 2011, 266, 57–62), our fit provides much improved global agreement between measured and simulated spectra. In parallel, an ab initio-based effective Hamiltonian approach was used to model the complete rovibrational polyads, including weak hot-band transitions and polyad interactions inaccessible to purely empirical fits, and provided the first ab initio-based line intensities of CH2Br2 in the 8 µm spectral region.

The HITRAN database is a curated compilation of validated molecular spectroscopic parameters, established in the early 1970s. It is used by various computer codes to predict and simulate the transmission and emission of light in gaseous media (with an emphasis on terrestrial and planetary atmospheres). The HITRAN compilation is composed of six major components. These components include the line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, experimentally derived absorption cross-sections (for molecules where it is not yet feasible for representation in a line-by-line form), collision-induced absorption data, aerosol indices of refraction, and general tables (including partition sums) that apply globally to the data. Responding to community requests, HITRAN2024 also incorporates — for the first time — a water-vapor continuum model.

This paper describes the details of the choices of data and their compilation for the 2024 quadrennial edition of HITRAN. The HITRAN2024 edition takes advantage of recent experimental and theoretical data that were meticulously validated, in particular, against laboratory and atmospheric spectra. The new edition replaces the previous HITRAN edition of 2020 (including various updates during the intervening years).

The extent of the updates of the line-by-line section in the HITRAN2024 edition ranges from updating a few lines of specific molecules/isotopologues to complete replacements of the lists, and also the introduction of additional isotopologues and six new (to HITRAN) molecules: H3+, CH3, S2, COFCl, HONO, ClNO2. Many new vibrational bands were added, extending the spectral coverage and completeness of the line lists. In addition, the accuracy of the parameters for major atmospheric absorbers has been increased substantially, often bringing the uncertainties down to unprecedented levels below 0.1%.

The HITRAN2024 edition is available through www.hitran.org as well as the HITRAN Application Programming Interface (HAPI). The functionality of the tools to work with the HITRAN data has been extended for the new edition.

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.

The 15N-site preference of N2O, δ15NSP = [14N15NO]/[15N14NO] - 1 ≈ δ15Nα - δ15Nβ = SP, quantifies the relative enrichment of 15N at the central (α) versus terminal (β) position in nitrous oxide and serves as a robust isotopic fingerprint for tracing N2O sources and formation pathways. One such pathway involves dimerization of nitroxyl (HNO), which can occur directly or enzymatically. The direct dimerization of HNO in aqueous solution has been suggested to proceed via acid-base equilibria, forming cis-hyponitrite or cis-hyponitrous acid intermediates that decompose to N2O. Measuring δ15NSP(N2O) from HNO dimerization would not only test the postulated formation pathway but also offer an easily reproducible chemical reference standard for isotopic studies. Using high-precision mid-infrared frequency comb spectroscopy, we determine the 15N-site preference by analyzing the absorption ratio of multiple rovibrational line pairs of the α and β isotopomers. At pH = 0.62, δ15NSP(N2O) decreases with synthesis temperature from 36.6‰ at T = 278 K to 23.4‰ at T = 336 K, in very good agreement with prediction based on a kinetic equilibrium model of the strongly pH-dependent cis-hyponitrous acid/cis-hyponitrite acid-base system. These results confirm N2O formation via the cis pathway is dominated─at the low synthesis pH─by the decomposition of the neutral cis-hyponitrous acid. Alternative formation of N2O from trans-hyponitrite, predicted to yield δ15NSP ≈ -7‰, can be excluded. Our work, combining high-precision spectroscopic measurements with first-principles ab initio and transition state theory calculations, is the first step toward establishing the chemical synthesis of N2O from HNO dimerization under acidic conditions as an absolute δ15NSP reference.

Nitrous oxide is a long-lived greenhouse gas. Its isotopic composition provides valuable insights into sources and sinks, and about the mechanisms of formation. A major challenge in the spectroscopic analysis of the isotopocule compositions is the availability of accurate spectroscopic parameters, particularly for the minor 15N isotopocules. In this work, we introduce high-resolution spectroscopic measurements of four isotopocules of nitrous oxide: ¹⁴N₂O, ¹⁴N¹⁵NO, ¹⁵N¹⁴NO, and ¹⁵N₂O in the mid-infrared range of 3300 – 3550 cm^–1 using a frequency comb-based Fourier transform spectrometer. The nitrous oxide samples were obtained from a chemical synthesis involving acid-catalyzed amine-borane reduction of equimolar amounts of ¹⁵N isotopically enriched sodium nitrite and ¹⁴N sodium nitrite. The high-resolution spectra, measured in a temperature-controlled single-pass absorption cell, were used to retrieve line center frequencies and relative intensities for a total of 426 rovibrational transitions of the ν1 + ν3 band of the four isotopocules, and of the one order of magnitude weaker 2ν2 + ν3 and ν1 + ν2 + ν3 – ν2 bands in the same spectral region. We compare the determined line center frequencies and relative intensities with spectroscopic parameters available in high-resolution molecular databases. For ¹⁴N₂O, ¹⁴N¹⁵NO and ¹⁵N¹⁴NO we find good agreement with the HITRAN database. The ¹⁵N₂O isotopocule is missing in HITRAN, and we find that its line center frequencies in the GEISA database, the Institute of Atmospheric Optics database, as well as the Ames-1 line list deviate severely from the comb measurements.