The accurate knowledge of the water vapor absorption in the 10 mu m atmospheric window is of strong importance because this spectral range coincides with the maximum black body emission of Earth. Presently, the water vapor self-continuum is measured at the 1185 cm -1 (8.45 mu m) spectral point, located in the most transparent interval of the 10 mu m window. Measurements are performed at four temperatures ranging from 296 to 308 K using a newly developed Optical Feedback Cavity Ring Down Spectrometer (OF-CRDS). Self-continuum cross-sections, CS, are derived from the pressure dependence of the absorption during pressure ramps of pure water vapor up to 18 mbar. From the quadratic pressure dependence observed for the absorption coefficient at each temperature, we derived the value of the cross-section (CS = 0.996(12) x 10-22 cm2 molecule- 1atm- 1 at 296 K) and its temperature dependence. These results are discussed in relation with previous literature measurements available in the 10 mu m window. Our cross-section is found about 20% smaller than the MT_CKD_4.1 value and the available experimental works seem to indicate that the MT_CKD_4.1 temperature dependence is overestimated.

The weak binary collision -induced absorption (CIA) of molecular hydrogen is measured at room temperature in the first overtone region near 1.20 mu m. Binary absorption coefficients are derived by cavity ring down spectroscopy (CRDS) at 28 selected spectral points sampling the (2-0) band between 7974 and 8650 cm -1. While all previous studies used high density samples, the sensitivity of the CRDS method allowed deriving accurate CIA by using pressure ramps of pure H2 limited to a maximum pressure of 1 atm. After subtraction of the Rayleigh contribution, a purely quadratic pressure dependence is obtained for the absorption coefficient at each measurement point and the CIA binary coefficients are derived with a 1.5 % accuracy. The comparison to theoretical values widely used for astronomical applications shows deviations values between 5 and 25 %.

The hydrogen dimer, (H2)2, is among the most weakly bound van der Waals complexes and a prototype species for first principles ab initio studies. The detection of the (H2)2 infrared absorption spectrum was reported more than sixty years ago at a temperature of 20 K. Due to the sharp decrease of the (H2)2 abundance with temperature, detection at room temperature was generally considered hardly achievable. Here we report the first room temperature detection of partly resolved rotational structures of (H2)2 by cavity ring down spectroscopy at sub-atmospheric pressures, in the region of the first overtone band of H2 near 1.2 μm. The quantitative analysis of the absorption features observed around ten allowed or forbidden transition frequencies of the monomer provides insight on the structure of this elusive species and a benchmark for future theoretical studies.

Three electric quadrupole transitions in the second overtone band of D-2 are newly measured by comb-referenced cavity ring down spectroscopy around 1.18 mu m. These extremely weak transitions (line intensities smaller than 10(-29) cm/molecule) are the first to be detected in the (3-0) band of D-2. The spectra of the O(3), O(2), and Q(2) lines near 8321, 8446, and 8607 cm(-1), respectively, are recorded at room temperature for pressure values ranging between 100 and 600 Torr. Accurate transition frequencies and line intensities of the three D-2 transitions are determined from a line fitting procedure using beyond-Voigt profiles, including strong Dicke narrowing. Considering statistical fit errors and possible biases due to the interference with water lines (which are six orders of magnitude stronger than the studied D-2 lines), total uncertainties on the frequencies extrapolated at zero pressure are estimated below 14 MHz (similar to 4.7 x 10(-4) cm(-1)). The derived experimental frequencies and intensities are compared to ab initio values. An overall agreement is achieved, confirming the positional accuracy of the most advanced theoretical calculations.

Fullerene C 60 is one of the most iconic forms of carbon found in the interstellar medium (ISM). The interstellar chemistry of carbon-rich components, including fullerenes, is driven by a variety of energetic processes including UV and X-ray irradiation, cosmic-ray (CR) bombardment, electron impact, and shock waves. These violent events strongly alter the particle phase and lead to the release of new molecular species in the gas phase. Only a few experimental studies on the shock processing of cosmic analogs have been conducted so far. We explored in the laboratory the destruction of buckminsterfullerene C 60 using a pressure-driven shock tube coupled with optical diagnostics. Our efforts were first devoted to probing in situ the shock-induced processing of C 60 at high temperatures (≤ 4500 K) by optical emission spectroscopy. The analysis of the spectra points to the massive production of C 2 units. A broad underlying continuum was observed as well and was attributed to the collective visible emission of carbon clusters, generated similarly in large amounts. This proposed assignment was performed with the help of calculated emission spectra of various carbon clusters. The competition between dissociation and radiative relaxation, determined by statistical analysis, alludes to a predominance of clusters with less than 40 carbon atoms. Our laboratory experiments, supported by molecular dynamics simulations performed in the canonical ensemble, suggest that C 60 is very stable, and that high-energy input is required to process it under interstellar low-density conditions and to produce C 2 units and an abundance of intermediate-sized carbon clusters. These results provide some insights into the life cycle of carbon in space. Our findings hint that only J-type shocks with velocities above ~100 km s −1 or C-type shocks with velocities above 9 km s −1 can lead to the destruction of fullerenes. Observational tracers of this process remain elusive, however. Our work confirms the potential of shock tubes for laboratory astrophysics.

•First water vapour foreign continuum measurements at room temperature in the 1.25 µm window.•CRDS loss rates were measured during pressure ramps of humidified nitrogen, humidified oxygen and humidified air.•The retrieved foreign continuum represents only a small part of the measured absorption.•Cross-sections are provided for selected spectral points between 8120 and 8500 cm−1.•The retrieved foreign cross-sections validate the semi-empirical MT_CKD values within the experimental error bars.

Accurate transition frequencies of six lines of the (2-0) vibrational band of H2 are reported near 1.2 μm, namely Q1-Q4, S0, and S1. These weak electric-quadrupole transitions were measured at room temperature by comb-referenced cavity ring-down spectroscopy. Accurate transition frequencies were determined by applying a multi-spectrum fit procedure with various profile models including speed-dependent collisional broadening and shifting phenomena. Although none of the considered profiles allows reproducing the shape of the strongest lines at the noise level, the zero-pressure line centers are found mostly independent of the used profile. The obtained values are the first H2 (2-0) transition frequencies referenced to an absolute frequency standard. As a result, a 1σ-accuracy better than 100 kHz was achieved for the Q1, S0, and S1 transition frequencies, improving by three orders of magnitude the accuracy of previous measurements. For the six measured transitions, the most recent calculated frequencies were found to be systematically underestimated by about 2.51 MHz, about twice their claimed uncertainties. The energy separation between J = 2 and J = 0 rotational levels of the vibrational ground state was derived from Q2 and S0 transition frequencies and found within the 110 kHz uncertainty of its theoretical value. The same level of agreement was achieved for the energy separation between the J = 3 and J = 1 rotational levels obtained by the difference of Q3 and S1 transition frequencies. The ab initio values of the intensity of the six transitions were validated within a few thousandths.

The 20 013-0 0 0 01 and 20 012-0 0 0 01 bands of 12C16O2 in the 2 mu m region are used by many instruments on-board satellites and in ground-based networks to monitor the CO2 column-averaged dry air mole frac-tion in the Earth's atmosphere. An accurate knowledge of the corresponding spectroscopic parameters and their temperature dependence is thus needed to map sources and sinks of carbon dioxide. In this work, we retrieve the line-shape parameters and their temperature dependence exponents/coefficients for the P(16) and P(28) transitions of the 20 012-0 0 0 01 band, and the R(24) and R(30) transitions of the 20 013-0 0 0 01 band. These parameters are obtained from a multi-spectrum fit procedure using a speed-dependent Nelkin-Ghatak profile including the line-mixing effect in its first-order approximation. High signal-to-noise spectra (quality factor between typically 30 0 0 and 60 0 0) are recorded at different condi-tions of pressure (from 50 to 750 Torr) and temperature (from 245 to 330 K) for mixtures of CO2 in air using a comb-assisted cavity ring down spectrometer with a temperature stabilized high finesse cavity. Air-broadening coefficients are measured with an estimated uncertainty better than 0.1% and absolute fre-quencies of the transitions are obtained with uncertainty better than 100 kHz. The determined line-shape parameters are consistent with previous values derived in the 1.6 mu m region confirming the absence of significant vibrational dependence (except for the air-pressure shift coefficient). The retrieved parameters agree well with literature values by Fourier transform spectroscopy and classical molecular dynamic sim-ulations. The parameters of the speed-dependent Voigt profiles provided by the HITRAN2020 database are mostly validated. The comparison to the CO2 line parameters adopted for the OCO missions is discussed.(c) 2023 Elsevier Ltd. All rights reserved.

Saturation spectroscopy has been used to determine the absolute frequencies of 107 ro-vibrational transitions of the two strongest 12CO2 bands of the 2 μm region. The considered 20012-00001 and 20013-00001 bands are of importance for the CO2 monitoring in our atmosphere. Lamb dips were measured using a cavity ring-down spectrometer linked to an optical frequency comb referenced to a GPS-disciplined Rb oscillator or to an ultra-stable optical frequency. The comb-coherence transfer (CCT) technique was applied to obtain a RF tunable narrow-line comb-disciplined laser source using an external cavity diode laser and a simple electro-optic modulator. This setup allows obtaining transition frequency measurements with kHz-level accuracy. The resulting accurate values of the energy levels of the 20012 and 20013 vibrational states are reproduced with a (1σ)-rms of about 1 kHz using the standard polynomial model. The two upper vibrational states appear thus to be highly isolated except for a local perturbation of the 20012 state leading to an energy shift of 15 kHz at J = 43. A recommended list of 145 transition frequencies with kHz accuracy is obtained providing secondary frequency standards across the 1.99-2.09 μm range. The reported frequencies will be valuable to constrain the zero-pressure frequencies of the considered transitions in 12CO2 retrieval from atmospheric spectra.

A series of spectra of the quadrupolar electric S(2) transition of H2 in the 1-0 band near 4917 cm-1 has been recorded at seven pressure values between 2 and 100 Torr. The comb-referenced cavity ring down spectroscopy (CR-CRDS) technique was used for the recording of this very weak transition. The accuracy of the spectrum frequency axis is achieved by linking the CRDS setup to an optical frequency comb referenced to a GPS-referenced 10 MHz rubidium clock. Applying a multi-spectrum fit procedure to the seven averaged spectra with a quadratic speed dependence Nelkin-Ghatak profile, the transition frequency is determined (ν0 = 147 408 142 357 kHz) with an uncertainty of 150 kHz (∼1 × 10-9 in relative). This represents the smallest uncertainty achieved so far for a transition in the fundamental band of H2. The experimental frequency reported in this work is 1.53 MHz higher than the best-to-date theoretical value. This difference represents 1.5 times the 1σ-uncertainty (about 1 MHz) of the calculated frequency. The measurements also allow for the determination of the absolute intensity value of the S(2) line which shows an agreement with the ab initio value at the per mil level. In addition, the cross section of the collision induced absorption (CIA) underlying the S(2) line is accurately retrieved from the quadratic pressure dependence of the baseline level of the recorded spectra.

Fifty nine high sensitivity spectra of the R(6) manifold of the 2 nu 3 band of methane in air, near 1.64 mu m, have been recorded in support of the MERLIN mission. For this purpose, a cavity ring down spectrometer (CRDS) with a spectrally narrowed and stable (sub-kHz) laser source was coupled to a temperature regulated high-finesse optical cavity. The frequency scale of each spectrum was accurately determined from measurements of the beat note between a part of the laser light and the closest tooth of a frequency comb referenced to a rubidium clock. Series of spectra were recorded between 243 and 313 K with a 10 K temperature step. For each series, total pressure values of 50, 100, 250, 500 and 750 Torr were adopted. A multi-spectrum fitting procedure with the Hartmann-Tran (HT) line profile, including the first-order line-mixing parameter, has been used to derive the spectroscopic parameters for each of the six R(6) components, along with the temperature dependence of the line-shape parameters. The results show that the fitted effective model is able to reproduce the experimental spectra with a relative precision better than 0.2% for the entire R(6) manifold spectral region and better than 0.05% at the ON-line position of the MERLIN mission for the 250, 500 and 750 Torr spectra. The relative precision increases to 0.3% and the residuals at the ON-line position to 0.1% when including the 50 and 100 Torr spectra. Comparisons with ground-based atmospheric measurements show that these data significantly improve the modeling of methane absorption in this spectral region. The complete line list of the methane spectrum in the region of the R(6) manifold allowing notable improvement of the modeling of the absorption cross-section at the ON-line position of the MERLIN mission is provided as Supplementary Material. (c) 2023 Elsevier Ltd. All rights reserved.

In the Earth's atmosphere, the foreign absorption continuum of water vapor is due to the interaction of water molecules with other atmospheric gases (mostly N 2 and O 2 ). Following our study of the selfcontinuum in the high energy edge of the 1.25 mu m transparency window (Korovela et al, J. Quant. Spectrosc. Radiat. Transfer 286 (2022) 108206), we report here the first room temperature measurements of water vapor foreign continuum in the same spectral region. Foreign continuum cross-sections, C F , are derived at several selected spectral points between 8120 and 8500 cm -1 for humidified nitrogen, humidified oxygen and humidified air (10 0 0 0 ppm of H 2 O). The absorption signal is measured by cavity ring-down spectroscopy (CRDS) using pressure ramps up to 750 Torr. While the measured total continuum absorption follows nicely the expected quadratic dependence versus the total pressure, the uncertainty on the retrieved weak foreign continuum is strongly affected by other contributions which have to be subtracted from the measured absorption (far wings of the resonance lines, O 2 collision induced absorption, selfcontinuum, Rayleigh scattering). The obtained H 2 O-air and H 2 O-N 2 C F cross-section values are found to be comparable while the H 2 O-O 2 C F values appear to be significantly smaller. Overall, the retrieved C F values for H 2 O-air mixture validate the MT_CKD model in the considered region.(c) 2022 Elsevier Ltd. All rights reserved.

The 1.25 mu m atmospheric transparency window is of importance for a number of atmospheric applica-tions. As a continuation of our previous works on the improvement of water vapor line parameters in the near infrared, the room temperature absorption spectrum of water vapor in natural isotopic abun-dance is recorded with unprecedented sensitivity between 8041 and 8633 cm -1, using comb-referenced cavity ring-down spectroscopy. The line positions and intensities of more than 5400 lines were retrieved. Their intensities range between 3.6 x 10 -30 and 1.5 x 10 -22 cm/molecule. The high sensitivity and low noise level of the recordings (alpha min approximate to 10-11 cm-1) allow for measuring more than 1600 new lines and determine their positions with an accuracy of about 10 -4 cm -1 in the case of isolated features. The rovi-brational assignments were performed using known experimental energy levels and calculated spectra based on variational calculations by Schwenke and Partridge. The final line list is assigned to more than 5400 transitions of the first six water isotopologues (H2 16O, H2 18O, H2 17O, HD16O, HD18O and HD17O). The measured line positions allow to determine the energy of 79 new levels of H2 16O, H2 18O, H2 17O, and HD16O, and to correct 139 previously reported term values. Although a good agreement is gener-ally observed, the comparison to the HITRAN2020 spectroscopic database and to the W2020 transition frequencies reveals a number of discrepancies both for line positions and line intensities. The lack of traceability of some HITRAN line parameters and some biases in the derivation procedure of the W2020 energy levels are confirmed in the studied range. Validation tests of the theoretical values of the line intensities against measured values show both band-by-band variations of the deviations on the order of a few % and line-by-line fluctuations within a given band.(c) 2023 Elsevier Ltd. All rights reserved.

A high-sensitivity absorption spectrum of natural nitrous oxide has been recorded at 10Torr between 8321 and 8620 cm(-1) (1.20 - 1.16 mu m). The used cavity ring down spectrometer was referenced to a self-referenced frequency comb providing an accurate frequency scale of the spectra. The predictions of effective operator models were used for the line assignment. All identified bands correspond to the Delta P=14-16 series of transitions, where P= 2V(1)+V-2+4V(3) is the polyad number (Vi=1-3 are the vibrational quantum numbers). A total number of 3975 transitions belonging to the (N2O)-N-14-O-16, (NNO)-N-14-N-15-O-16, (NNO)-N-15-N-14-O-16, (N2O)-N-14-O-18, and (N2O)-N-15-O-16 isotopologues were measured with estimated accuracy better 1 x 10(-3) cm(-1) for most of the lines. Compared to the recent analysis of N2O spectra at 1 Torr recorded in the same region (Karlovets et al. JQSRT, 262 (2021) 107,508, doi: 10.1016/jjqsrt.2021.107508), the higher pressure of the recordings and improved quality of the predictions have allowed to: (i) newly assign twenty-two bands, including the 4v(3) band of the (N2O)-N-15-O-16 minor isotopologue, (ii) extend the assignments of previously known bands, (iii) determine the small self-induced pressure shifts of the two strongest bands (less than 10(-4) cm(-1)) by difference of the line positions at 1 and 10 Torr and use them to obtain the zero-pressure value of the line positions, (iv) derive new or improved spectroscopic constants of 69 bands from the standard bandby-band analysis, and (v) analyze six local resonance perturbations affecting three bands, including the identification of a few extra lines due to intensity transfer. Finally, the comparison of the line positions and intensities with their predicted values and with available databases is discussed. (C) 2021 Elsevier Ltd. All rights reserved.

The 12 CO 2 band at 1.6 mu m is used for carbon dioxide monitoring in the Earth atmosphere. The targeted accuracy of these measurements motivates important efforts to improve the quality of the spectroscopic parameters in atmospheric conditions. In the present work, the line shapes of the R(6), R(12), R(16), R(18) and R(20) transitions of the 30 013-0 0 0 01 band of 12 CO 2 in air are studied with a cavity ring down spectrometer (CRDS). For each transition, high signal-to-noise ratio spectra (between 20 0 0 and 20 0 0 0) are recorded at different temperatures (250, 274, 285, 295 and 320 K) and total pressures (50, 100, 250, 500 and 750 Torr). To this end, a spectrally narrowed and stable (sub-kHz) laser source is coupled into a temperature regulated high-finesse optical cavity. The frequency scale of each spectrum is accurately determined from measurements of the frequency of the beat note between a part of the laser light and the closest tooth of a frequency comb referenced to a rubidium clock. A multi-spectrum fit procedure with quadratic speed dependent Nelkin-Ghatak profiles, including line-mixing effects, has been used to derive for each transition, the different spectroscopic parameters and their temperature dependence. Results are discussed in comparison with previous experimental data, HITRAN2020 database and values obtained from requantized classical molecular dynamics simulations (rCMDS). (c) 2022 Elsevier Ltd. All rights reserved.

The HITRAN database is a compilation of molecular spectroscopic parameters. It was established in the early 1970s and 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 five major components: the line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, experimental infrared 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. This paper describes the contents of the 2020 quadrennial edition of HITRAN. The HITRAN2020 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 2016 (including its updates during the intervening years). All five components of HITRAN have undergone major updates. In particular, the extent of the updates in the HITRAN2020 edition range from updating a few lines of specific molecules to complete replacements of the lists, and also the introduction of additional isotopologues and new (to HITRAN) molecules: SO, CH3F, GeH4, CS2, CH3I and NF3. Many new vibrational bands were added, extending the spectral coverage and completeness of the line lists. Also, the accuracy of the parameters for major atmospheric absorbers has been increased substantially, often featuring sub-percent uncertainties. Broadening parameters associated with the ambient pressure of water vapor were introduced to HITRAN for the first time and are now available for several molecules. The HITRAN2020 edition continues to take advantage of the relational structure and efficient interface available at www.hitran.org and the HITRAN Application Programming Interface (HAPI). The functionality of both tools has been extended for the new edition. (C) 2021 The Author(s). Published by Elsevier Ltd.