Hydrocarbons are partly responsible for the opacity of warm Jupiters' atmospheres in the infrared. Laboratory high-resolution spectroscopic data, including hot band rovibrational transitions, are crucial to model and interpret telescope observations. In this work, a set of six hot bands and 11 cold bands of ethylene (12C2H4) is observed using cavity ringdown spectroscopy between 5880 and 6200 cm-1. The ethylene sample is preheated to 650 and 850 K before being expanded through a Laval nozzle to produce a high Mach number expansion. The rotational temperature drops to ∼12-13 K in the jet, while the vibrational population accumulates in the first excited vibrational state ν10, from which all the observed hot bands originate. The observed transitions are assigned using the lower state combination difference approach; a set of A, B, and C rotational constants, along with the energy of the upper state, is determined using PGOPHER software for the 17 observed vibrational bands. The TheoReTS (Theoretical Reims-Tomsk Spectral data) model, employed to identify the upper vibrational states, will benefit from these newly identified transitions.

Quantitative spectroscopic detection of dibromomethane, CH_2Br_2, 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 CH_2Br_2 in the 1180-1210 cm^-1 region measured using optical frequency comb Fourier transform spectroscopy. This region is dominated by the strong CH_2 wagging (ν_8) 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 CH_2^79Br^81Br, CH_2^79Br_2, and CH_2^81Br_2, and we assign rovibrational transitions of the ν_8 fundamental and the overlapping ν_4+ν_8-ν_4 hot bands using two methods. First, an empirical non-linear least square 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 K_a = 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 CH_2Br_2 in the 8 μm spectral region.

Nitrogen-broadening coefficients for ethylene absorption lines in the v(7) band are computed theoretically to fill in the enormous gaps existing in available experimental data. A semi-empirical method relying on the Anderson theory is employed for this purpose, with the model parameters adjusted on some rare room-temperature measurements and used further for calculations in extended temperature ranges. The spectroscopically active molecule C(2)H4 is rigorously treated as an asymmetric top, and its energy levels and wavefunctions are accurately computed by a quantum-chemistry methodology based on ab initio potential energy surfaces. Theoretical broadening values are obtained for the temperature range 100-1000 K relevant to studies of planetary and exoplanetary atmospheres. These values are further analyzed by the traditional power law to get the associated temperature-dependence exponents. Comparisons of the computed theoretical results with previous measurements and calculations are provided at a low temperature of 173 K to validate the theoretical method chosen. Since the power law describes perfectly the considered temperature range, the sets of reference-temperature broadening coefficients and their temperature exponents can be safely used for higher temperatures where semi-classical approaches applicability only grows. These data are provided for the quantum numbers 0 <= J <= 47, K <= 26 and the three P-, Q-, R-branches of the v(7) band, i.e. represent the most comprehensive up-to-date description of N-2-broadened ethylene linewidths suitable for integration into spectroscopic databases and high-temperature simulations of (exo)planetary atmospheres.

Accurate models of hot-band methane spectra are needed in astrophysics. Previous measurements of methane hot-band transitions in the P6←P2 polyad range have been limited to final rotational numbers of J ≤ 9, with theoretical predictions at higher J remaining unvalidated. We use optical-optical double resonance spectroscopy (OODR) with a 3.3 µm narrow linewidth pump to excite the ν3 P(12, A1(2)) transition (P2←P0) and a cavity-enhanced frequency comb centered around 1.68 µm to probe sub-Doppler ladder-type (P6←P2) and V-type (P4←P0) transitions, as well as Doppler-broadened collision-induced four-level transitions (P6←P2). 49 ladder-type transitions with final rotational states J = 10-12 in the range of 9510 to 9810 cm−1 (i.e., P6 polyad) were assigned to effective Hamiltonian predictions and the ExoMol database, of which 6 reached vibrational states that had not been observed experimentally before. 19 sub-Doppler V-type transitions with final states J = 11-13 in the range of 6590 to 6900 cm−1 (i.e., P4 polyad) were observed and assigned to the Hamiltonian and ExoMol, while only 2 of these V-type transitions could be unambiguously assigned to the WKLMC and HITRAN line lists. 170 Doppler-broadened four-level double-resonance (4LDR) lines were observed, 7 of which were newly observed lines compared with our previous work when pumping transitions starting from the J = 7 level in the ground state [Lehmann et al., J. Chem. Phys. 163, 144304 (2025)]. We could not assign these lines as they did not form combination differences with other observed 4LDR transitions.

We report room-temperature spectra of CH 2 Br 2 (1180–1210 cm −1 ) measured using comb-based Fourier transform spectroscopy and provide spectral models of the ν 8 and ν 4 + ν 8 − ν 4 bands using empirical fits, benchmarked with ab initio -based simulations.

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.

We use sub-Doppler optical-optical double-resonance (OODR) spectroscopy with a 3.3 mu m single-frequency pump and a cavity-enhanced 1.65 mu m comb probe to measure 33 ladder-type (31)3 <- 1)3) and 8 V-type (21)3) transitions in the 5880-6090 cm-1 range of methane, reaching states with rovibrational E symmetry in the region of the P6 and P4 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 31)3 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 x 10-6 cm-1). The E-symmetry states exhibit first-order Stark splitting, which will be reported in our future work.

Trifluoromethane (CHF3) is a pollutant gas with a large global warming potential. Although CHF3 emissions are actively monitored using ground- and space-based spectrometers, this molecule is still absent from the reference spectroscopic databases. CHF3 exhibits complex resonance interactions among its ro-vibrational energy levels, resulting in a highly congested infrared spectrum with irregular patterns. To support atmospheric retrieval procedures for CHF3, accurate spectral predictions are thus required, particularly for line intensities. We present the first global prediction of the ro-vibrational spectrum of CHF3 using an effective model based on full-dimensional ab initio potential energy and dipole moment surfaces. The non-empirical effective model presented here can be considered as an efficient alternative to full variational calculations for polyatomic molecules. This model includes the most relevant cold and hot transitions at room temperature in the region of the fundamental bands (0-3100 cm-1). To this end, 670 vibrational sub-states corresponding to a polyad number of Pmax = 22 (≈ 4500 cm-1) were considered, together with rotational angular momentum values up to Jmax = 99, leading to the construction of comprehensive line lists for 12CHF3, 13CHF3, and 12CDF3, composed of 89662832, 33068581 and 23265983 lines, respectively. The final theoretical spectrum of CHF3 was successfully validated against cross sections measured by the Pacific Northwest National Laboratory. This work clearly demonstrates the advantages of ab initio calculations for predicting ro-vibrational spectra of heavy polyatomic molecules, such as those containing F, Cl or Br.

Four spectra of methane in natural isotopic abundance in the 900-1050 cm-1 region were recorded using a Fourier transform spectrometer in Tomsk, Russia, with long optical paths 167 m and 1058 m at temperatures 28 and 51 degrees C. Line positions and intensities were retrieved by non-linear least-squares curve-fitting procedures and analyzed using effective Hamiltonian and effective dipole moment models. The enhanced absorption at long paths permitted to measure new transitions: in cold bands up to J max = 28 and in hot bands up to J max = 20. The new experimental line list contains line positions and intensities for 2570 features. Quantum assignments were made for 1246 lines of the main isotopologue 12 CH 4 . Comparisons of the theoretical absorption simulations with experimental spectra revealed a considerable improvement compared to the HITRAN2020 database. All assigned 1246 line positions were fitted to RMS standard deviations of 0.00065 cm-1 .

The SMAUG device is used to thermally populate the vibrational states of the methane molecule and depopulate its rotational states through its supersonic expansion in argon. The methane molecule is probed during expansion by cavity ringdown spectroscopy (CRDS) between 5880 and 6220 cm-1. The laser beam passes through both the isentropic core of the flow, characterized by a very low rotational temperature of 40.4 f 0.8 K, and the hotter boundary layers surrounding it, which are characterized by an average rotational temperature of 364.3 f 7.7 K. Analysis of the spectrum reveals that the vibrational population does not follow a Boltzmann distribution. A special procedure is developed to assign an effective vibrational temperature to each vibrational state, ranging from 532.4 f 32.2 to 1112.0 f 81.7 K for molecules in the isentropic core, and from 811.6 f 43.1 to 851.8 f 45.6 K for those in the shear layer. Only the first vibrational states of each methane polyad remain significantly populated leading to detectable transitions. A total of 2525 transitions from the cold gas of the isentropic core and from the warmer gas of the boundary layers are assigned from a new ab initio effective model and 1000 are confirmed by lower state combination differences (LSCD). Among these transitions, 718 originates from cold bands and 1807 from hot bands starting from vibrational states nu 4, nu 2 (dyad), 2 nu 4 (pentad) and 3 nu 4 (octad).

We report the construction of comprehensive line lists for the three stable isotopologues of silicon tetrafluoride (28SiF4, 29SiF4, and 30SiF4) using a new effective Hamiltonian and dipole moment model built from accurate ab initio potential energy and dipole moment surfaces developed in this work. The vibrational energy levels were grouped into a series of polyads up to Pmax = 19, while the ro-vibrational energy levels were computed up to Jmax = 99. Each line list covers the spectral range 0–2500 cm−1 and contains almost 500 million transitions at T = 296 K, with each being generated from 685 vibrational states and sub-states. Most of the cold and hot band transitions computed in this work were not available in the literature beforehand. The absorption cross-sections computed from the produced line lists were successfully validated by direct comparison with the experimental data measured by Pacific Northwest National Laboratory at room temperature. Most of the ro-vibrational band structures observed in the experimental spectra can now be elucidated using the line lists proposed in this 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.

Potential energy surfaces (PES) of methane which are constructed using ten symmetrised combinations of four bond length and six interbond angles are referred to as (10S) PESs. At high energy ranges, it was found that (10S) PESs permitted improving by factor of five the fit quality of ab initio electronic energies of methane versus standard nine symmetric coordinates (9S) PES representations both in internal, orthogonal or normal-mode coordinates. We extend predictions of vibrational band origins to spectral range above 10,000 cm- 1 to help future analyses of experimental spectra. The accuracy at high energies is increased without notable deterioration at low-E levels (rms deviation of 0.26 cm- 1). Based on a comparison between various variants of (10S) PESs we expect that new theoretical band origins should have the accuracy not worse than 1-3 cm- 1 up to 13,400 cm- 1.

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.

High-resolution Fourier transform infrared (FTIR) spectra of methyl fluoride (CH3F) were recorded in the mid- and far-infrared regions using the Bruker IFS 125HR spectrometers at GSMA (Reims, France) and at the SOLEIL synchrotron facility (Saint-Aubin, France). The measurements cover both the pure rotational transitions of the ground state (10–100 cm−1) and the vibrational triad region (1950–2450 cm−1), which includes the 2ν3, ν3+ν6, and 2ν6 bands. Spectra were recorded under various pressure conditions to optimize line visibility, with a high resolution. Line assignments were performed using predictions from the tensorial effective Hamiltonian implemented in the MIRS package, together with a newly developed automated assignment tool, SpectraMatcher, which facilitates line matching and discrimination of CH3F transitions from overlapping CO2 features. More than 5000 transitions (up to J=52 in the ground state and up to J=45 in the triad and K=19) were assigned and included in a global fit. The sixth-order tensorial effective Hamiltonian model yielded excellent agreement with experiment, with root mean square (RMS) deviations better than 7 × 10−4 cm−1 across all regions. This paper presents the first continuous rovibrational study of CH3F over both the triad and far-infrared ground state regions. The improved accuracy from previous studies stems from the improved set of effective Hamiltonian parameters which will also form a good basis from future applications in atmospheric modelling and spectroscopic databases.

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 $3\mathrm{v}_{3}\leftarrow \mathrm{v}_{3}$ 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.

In this paper, we use nested tensor-train contractions to compute vibrational and ro-vibrational energy levels of molecules with five and six atoms. At each step, we fully exploit symmetry by using symmetry adapted basis functions obtained from an irreducible tensor method. Contracted basis functions are determined by diagonalizing reduced dimensional Hamiltonian matrices. The size of matrices of eigenvectors, used to account for coupling between groups of coordinates, is reduced by discarding rows and columns. The size of the matrices that must be diagonalized is thus substantially reduced, making it possible to use direct eigensolvers, even for molecules with five and six atoms. The symmetry-adapted contracted vibrational basis functions have been used to compute J = 0 energy levels of the CH3CN (C3v) and J > 0 levels of CH4.

The methylene molecule (CH2) is a short-lived radical with lacking data on its spectral line intensities. Although the lifetime of CH2 is extremely short under Earth's conditions, it exists in a free form in interstellar media. CH2 is an important intermediate species in chemical reactions associated with the formation and destruction of complex hydrocarbons. We present the first rovibrational line lists of CH2 in its ground triplet and first excited singlet electronic state. To this end, our previously developed accurate ab initio potential energy surface (PES) was used for the ground electronic triplet state [Egorov et al. J. Comp. Chem. 2024. V. 45. (2). P. 83] while a new PES for the singlet state was constructed in this work using the single-reference coupled cluster approach [CCSD(T)] combined with the extrapolation to the complete basis set (CBS) limit based on the correlation-consistent orbital basis sets with the core-valence electron correlation effects [aug-cc-pCVXZ, X = T, Q, 5, and 6]. In addition, the contributions to the correlation energy from highly excited Slater determinants [CC(n), n = 3-5] were included as well as the scalar relativistic effects and DBOC. The most accurate description of the infrared band origins of singlet CH2 was thus achieved for the energy range where the impact of the nonadiabatic coupling due to the Renner-Teller effect can be neglected. To obtain the probabilities of the rovibrational transitions, new ab initio DMSs were constructed both for the triplet and singlet CH2 using the CCSD(T)/aug-cc-pCVQZ approach. Finally, the absorption spectra of triplet and singlet methylene were predicted from the variationally computed line lists.

The absorption spectrum of the (CH4)-C-13 methane isotopologue was recorded on a Bruker IFS-125HR Fourier transform spectrometer at 298 K in the 4970-6200 cm(-1) range. In this paper we report the results of assignment and modelling of the line positions and intensities of (CH4)-C-13 in the range of weaker absorption between 4970 and 5300 cm(-1), corresponding to the lower part of Tetradecad, dominated by the 4 nu(4) band system near 5180 cm(-1). The empirical list in this spectral range contains 1642 lines. Using an effective Hamiltonian initially derived from the analyses of cold Fourier transform (4970-5853 cm(-1)) and laser direct absorption (5853-6200 cm(-1)) spectra of the (CH4)-C-13, we assigned 1600 lines belonging to four bands of the Tetradecad up to J(max)=16. The 1548 line positions were fitted with an rms deviation of 1.7 x 10(-3) cm(-1). Measured line intensities were modeled for 674 transitions using the effective dipole transition moments to an rms deviation of about 7 %. The new data were used for the simultaneous global fit of the (CH4)-C-13 Hamiltonian parameters for the {Ground state / Dyad / Pentad / Octad / Tetradecad} system and the dipole moment parameters for the {Ground state - Tetradecad} system.