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High-sensitivity spectra of the main ozone isotopologue were recorded using a cavity ring-down spectrometer in the region 7920–8670 cm−1, which covers the range up to the dissociation threshold and higher. A sensitivity on the order of 2 × 10−11 cm−1 has been attained, which makes it possible to detect high-energy combination rovibrational bands up to ten vibrational quanta in the electronic ground state. Line positions and intensities are measured for these bands. Hot vibronic bands of the 16O3 isotopologue formed by transitions from the (100) and (020) electronic ground state levels to the excited 3A2 triplet state were recorded for the first time providing new information about the dependence of predissociation broadening on rotational quantum numbers.

The absorption spectrum of the O-18(3) isotopologue of ozone was recorded by cavity-ring-down spectroscopy between 7920 and 7985 cm(-1) with a routine noise equivalent absorption, alpha(min), on the order of a few 10(-11) cm(-1). This permitted to detect and analyze an extremely weak vibrational band assigned as 7 nu(1) + nu(3) following predictions from the ab initio potential energy surface reported in J Chem Phys 2013;139:134307. This is the most excited rovibrational band of the O-18(3) isotopologue detected so far. In total, 259 ro-vibrational transitions were measured and modeled using an effective Hamiltonian model. The vibrational dipole transition moment parameter of the band deduced from the analysis and the fit to 177 experimental line intensities was evaluated as 2.6 x 10(-6) Debye. The absorption coefficient of the recorded pure O-18(3) spectrum was found significantly smaller compared to that of the O-16(3) in the same range. The set of 171 upper state energies experimentally determined cover the range from 92.6% (at J = 1) to 97% (at J = 26) of the first dissociation threshold D-0. The comparison with ab initio predictions for the band origin and nu-dependent rotational constants is discussed. An absorption line list containing 477 transitions is provided as Supplementary material for the 7 nu(1) + nu(3) band. (C) 2021 Elsevier Ltd. All rights reserved.

International audience; Measurements of the water vapor absorption cross-sections at two spectral points of the 2.1 μm and 4.0 μm transparency windows are performed by optical feedback cavity enhanced absorption spectroscopy (OFCEAS). The self-continuum cross-section, C S , is measured for temperature values of 30 and 47 °C (303 and 320 K) at the 2853 cm −1 spectral point, corresponding to the lowest opacity region of the 4.0 μm transparency window. The C S values derived from the pressure squared dependence of the self-continuum, are found consistent with previous CEAS measurements in the considered window but significantly smaller than measurements by Fourier transform spectroscopy (FTS). The C S temperature dependence is discussed in relation with FTS measurements at high temperature. Foreign-continuum cross-sections, C F , are newly obtained from OFCEAS spectra of moist air in flow regime at the 4302 cm −1 spectral point of the low energy edge of the 2.1 μm window. After subtraction of the monomer and self-continuum contributions, C F values are derived from the linear variation of the foreign-continuum absorption with the product of the water vapor and air partial pressures. The measurements were performed for temperature values of 34 and 47 °C (307 and 320 K) and no significant temperature dependency was observed. The present C F value at 4302 cm −1 is gathered with previous CEAS measurements at seven spectral points of the 2.1 μm window. This consistent set of C F values is used to derive from a polynomial fit, the empirical frequency dependence of C F (ν) over the 4250-50 0 0 cm −1 range. Overall, the semi-empirical MT_CKD_3.5 values of C F are significantly underestimated in the centre of the considered window.

The weak low frequency tail of the ozone Wulf band is newly studied by high sensitivity cavity ring down spectroscopy (CRDS) in the 7920–8700 cm−1 range. Although strongly obscured by impurity lines, the ozone absorption signature shows up in the recorded spectra as two mostly unresolved absorption bands. These two bands with maximum near 8154 and 8450 cm−1 are hot bands from the (020) and (100) ground state vibrational levels reaching the same predissociated 3A2(000) upper state of 16O3. The observed bands include a mostly structureless continuum and lines exhibiting a clear broadening due to the predissociation of the 3A2(000) upper state. On the basis of assignments of the rotational structure available in the literature for the 3A2(000)-X1A1(000) cold band centered near 9553 cm−1, seventy-four relatively narrow lines could be rotationally assigned to the 3A2(000)-X1A1(020) and 3A2(000)-X1A1(100) hot bands. A line shape analysis, performed using a Voigt profile, provided the width of the Lorentzian component. Additional recordings performed at various pressures for 18 lines allowed to disentangle the pressure and predissociation contributions to the fitted Lorentzian widths. As a result, the newly derived pressure broadening coefficients of the considered rovibronic transitions are found to be similar to those of the rovibrational infrared bands. All the lines studied are affected by significant predissociation due the spin-orbit coupling between the 3A2(000) level and high lying dissociative vibrational levels of the X1A1 electronic ground state. The resulting set of predissociation widths for sixty lines ranges between 9 × 10−3 and 0.2 cm−1 corresponding to upper state lifetime of 560 and 25 ps, respectively. The reported results are discussed in relation with previous experimental and theoretical studies.

The MERLIN mission (MEthane Remote LIdar missioN) is a space mission dedicated to the methane atmospheric retrieval based on absorption LIDAR measurements at nadir. The methane columns will be obtained by difference between the LIDAR signal at the on-line wavelength corresponding to the R(6) manifold of the 2ν3 band of methane at 6077 cm−1 and the off-line signal at a wavenumber about 1 cm−1 below. As atmospheric water vapor lines may affect the methane retrievals, the present study is dedicated to a detailed laboratory characterization of the interfering water vapor absorption in the targeted region. To this aim, high sensitivity absorption spectra of pure water vapor and water vapor in air are recorded at room temperature using a cavity ring down spectrometer (CRDS) combined with a self-referenced frequency comb. The CRDS recordings cover the 6071.4–6080.0 cm−1 interval with a noise equivalent absorption, αmin, on the order of 5 × 10−11 cm−1 for a 10−3 cm−1 sampling of the spectra. The measured pure and air-broadened spectra are analyzed using the Nelkin-Ghatak (NG) or the speed-dependent Nelkin-Ghatak (sdNG) profile. A multi-spectrum fitting procedure in which series of spectra recorded at different pressures (between 0.7 and 12 Torr for pure water vapor and between 100 and 700 Torr for H2O in air) are simultaneously adjusted, is used to retrieve the spectroscopic parameters. Accurate line parameters including self- and air-pressure shifts, self- and air-broadenings, Dicke narrowing and the speed-dependent coefficients are determined for most of the lines with intensity larger than 10−28 cm/molecule at 296 K. The list is complemented with literature data (in particular from the HITRAN database) to be complete down to a 10−29 cm/molecule intensity cut-off. The obtained spectroscopic results are discussed in comparison with literature data.

International audience; The 1.27 μm O2 band is increasingly called upon for air-mass determination from ground-based and space-borne atmospheric spectra because of its spectral proximity to CO2, and CH4 bands at 1.6 μm, in contrast to the more distant A-band at 0.76 μm. For this purpose, it is important to well characterize not only the narrow absorption lines but also the strong underlying broad collision-induced absorption (CIA) structure and its temperature dependence. Spectra of pure O2, and of O2 in N2, were recorded by cavity ring down spectroscopy (CRDS) at 271 K and 332 K, with the help of a newly developed temperature regulated cell. The quality of the spectra allowed determining the small variations (at the few percent level) of the urn:x-wiley:2169897X:media:jgrd57131:jgrd57131-math-0001, urn:x-wiley:2169897X:media:jgrd57131:jgrd57131-math-0002, and urn:x-wiley:2169897X:media:jgrd57131:jgrd57131-math-0003 binary coefficients with temperature. Together with the binary coefficients at 297 K reported previously, they allow characterizing the temperature dependence of the O2 CIA at 1.27 µm in atmospheric conditions. The obtained results are compared with previous measurements by Fourier transform spectroscopy (FTS) and recent theoretical calculations. In the wings of the O2-O2 CIA, an increase with temperature is observed in agreement with calculations but contrary to these calculations, a decrease is observed near the maximum of the CIA band. The temperature dependence is found in good agreement with the theoretical urn:x-wiley:2169897X:media:jgrd57131:jgrd57131-math-0004values and the experimental urn:x-wiley:2169897X:media:jgrd57131:jgrd57131-math-0005 values derived from the FTS measurements. Binary coefficients integrated over the entire band show almost no variation with temperature for O2-N2 and a small increase for pure O2, in agreement with the theoretical predictions.

The recent detections of electric-quadrupole (E2) transitions in water vapor and magnetic-dipole (M1) transitions in carbon dioxide have opened a new field in molecular spectroscopy. While in their present status, the spectroscopic databases provide only electric-dipole (E1) transitions for polyatomic molecules (H$_2$O, CO$_2$, N$_2$O, CH$_4$, O$_3$…), the possible impact of weak E2 and M1 bands to the modeling of the Earth and planetary atmospheres has to be addressed. This is especially important in the case of carbon dioxide for which E2 and M1 bands may be located in spectral windows of weak E1 absorption. In the present work, a high sensitivity absorption spectrum of CO$_2$ is recorded by Optical-Feedback-Cavity Enhanced Absorption Spectroscopy (OFCEAS) in the 3.3 µm transparency window of carbon dioxide. The studied spectral interval corresponds to the region where M1 transitions of the $ν_{2}+ν_{3}$band of carbon dioxide were recently identified in the spectrum of the Martian atmosphere. Here, both M1 and E2 transitions of the $ν_{2}+ν_{3}$ band are detected by OFCEAS. Using recent ab initio calculations of the E2 spectrum of $^{12}$C$^{16}$O$_2$, intensity measurements of five M1 lines and three E2 lines allow us to disentangle the M1 and E2 contributions. Indeed, E2 intensity values (on the order of a few 10$^{–29}$ cm/molecule) are found in reasonable agreement with ab initio calculations while the intensity of the M1 lines (including an E2 contribution) agree very well with recent very long path measurements by Fourier Transform spectroscopy. We thus conclude that both E2 and M1 transitions should be systematically incorporated in the CO$_2$ line list provided by spectroscopic databases.

Spectra of CO2 + H2 mixtures have been recorded at room temperature by using the cavity ring down technique, for four spectral intervals in the 2.12–2.35 µm CO2 transparency window which is within the (1-0) band of H2. The binary coefficients B C O 2 − H 2 + B H 2 − C O 2 have been retrieved from the spectra recorded at different pressures after subtraction of the CO2 monomer contribution and of the H2-H2 and CO2-CO2 collision induced absorptions (CIAs). In order to reduce the uncertainties, new measurement of the pure H2 CIA, the main subtracted contribution, at the percent level are also reported. The new set of experimental binary coefficients is compared to values provided by semi-empirical calculations made with the assumption of an isotropic CO2-H2 interaction potential and neglecting the short-range interaction-induced electric dipole. This comparison shows the limits of using such a model, which is the only one available, in that spectral region.

Ozone formation and depletion play a key role in various physical and chemical atmospheric processes which remain to be understood in more detail. The modeling of such phenomena based on ozone dynamics calculations requires a detailed knowledge of quantum states, transition probabilities, and ozone potential energy surface (PES) near the dissociation threshold ${D}_{0}$. Knowledge of highly excited rovibrational states with energies approaching ${D}_{0}$ is necessary for a reliable interpretation of satellite measurements of the ozone absorption or emission in the upper atmosphere in nonlocal thermodynamic equilibrium conditions. In addition, these states provide an ideal probe of PESs computed by ab initio methods but their detection by absorption spectroscopy is particularly challenging due to the sharp decrease of the intensity with the energy of the corresponding vibrational combination and overtone bands. In this work, two very high-energy vibrational bands are detected by high sensitivity cavity ring-down spectroscopy providing a noise equivalent absorption ${\ensuremath{\alpha}}_{\mathrm{min}}$ on the order of a few ${10}^{\ensuremath{-}11}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$. The corresponding cold bands, assigned as ${\ensuremath{\nu}}_{1}+6{\ensuremath{\nu}}_{2}+3{\ensuremath{\nu}}_{3}$ and $6{\ensuremath{\nu}}_{1}+{\ensuremath{\nu}}_{2}+{\ensuremath{\nu}}_{3}$ centered at 7969 and $7993\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, respectively, are about nine orders of magnitude less intense than the well-known ${\ensuremath{\nu}}_{3}$ fundamental band near 10 \textmu{}m. The spectrum analysis allowed for the experimental determination of 240 rovibrational energy levels of the upper vibrational states. The corresponding rotational patterns, located between 93.1% and 96.7% of the dissociation threshold, are the most excited ones measured so far by absorption spectroscopy. The values of the band centers and rotational constants derived from the spectrum analysis provide further confirmation of the absence of an activation barrier on the minimum-energy path towards the dissociation threshold [Tyuterev et al., Phys. Rev. Lett. 113, 143002 (2014)]. The vibrational dynamics is discussed in the context of the radiative transition probabilities and related Einstein coefficients for the observed bands. Another important question in the context of the nuclear dynamics concerns the interaction among the three identical potential wells of the ozone molecule in the ground electronic state, which appears to be due to the Jahn-Teller effect. Recent theoretical results accounting for the three potential wells in a full symmetry approach predicted that vibration states should exhibit deviations from the conventional one-potential-well ab initio calculations, near the dissociation threshold. In the present work, we observe a better agreement of the experimentally determined energy value with the three-wells prediction for the (1,6,3) vibrational state involving a large simultaneous excitation of both bending (${\ensuremath{\upsilon}}_{2}=6$) and asymmetric stretch (${\ensuremath{\upsilon}}_{3}=3$) vibrations pointing towards a nearby well. This could be tentatively interpreted as a manifestation of the three-wells effect.

Series of spectra of the S(0) and Q(1)-Q(4) transitions of the (2-0) band of D2 near 5860 cm−1 are recorded by comb-referenced cavity ring down spectroscopy, at four pressures up to 200 Torr. After averaging, high resolution (30 MHz step) and low noise (noise equivalent absorption, αmin≈ 1.5 × 10−11 cm−1) spectra are obtained for each pressure value. This leads to signal to noise ratios of several hundred for the considered very weak electric quadrupole transitions with intensities ranging between 4 × 10−29 and 4 × 10−28 cm/molecule. The Q(3) and Q(4) transitions are newly detected. A multi-spectrum treatment of the D2 line profiles at different pressures performed using a quadratic speed-dependent Nelkin-Ghatak profile leads to residuals close to the noise level. Considering statistical fit errors and possible biases, total uncertainties on the frequencies extrapolated at zero pressure are estimated to be between 0.30 MHz and 1.83 MHz. The derived experimental frequencies and intensities of (2-0) transitions are gathered with literature values and compared to ab initio values reported in J. Komasa, M. Puchalski, P. Czachorowski, G. Łach, K. Pachucki, Phys. Rev. A 100, 032519 (2019). An agreement within the combined uncertainties is achieved for the S(0), Q(1), Q(2) and Q(3) transitions. Line intensities are also discussed in relation with literature values.

International audience

The very weak absorption spectrum of natural CO2 near 1.74 µm (5702–5879 cm−1) is studied at high sensitivity. The investigated region corresponds to a transparency window of very weak opacity which is of particular interest for Venus. Very weak lines with intensity value as low as 10−30 cm/molecule at 296 K are detected by Cavity Ring Down Spectroscopy. On the basis of the predictions of effective Hamiltonian models, 1135 lines of six carbon dioxide isotopologues - 12C16O2, 13C16O2, 16O12C18O, 16O12C17O, 16O13C18O and 16O13C17O - were rovibrationnally assigned to 26 bands. The accurate spectroscopic parameters of 16 bands are determined from standard band-by-band analysis (typical rms deviations of the line positions are 8 × 10−4 cm−1). These newly observed bands include perturbed bands, weak hot bands and bands of minor isotopologues (in particular 16O12C18O in natural abundance) and provide critical validation tests for the most recent spectroscopic databases. The comparison to the Carbon Dioxide Spectroscopic Databank (CDSD), HITRAN2016 database and recent ab initio line lists is presented. Deficiencies are evidenced for some weak perpendicular bands of the HITRAN2016 list and identified as due to inaccurate CDSD intensities which were preferred to ab initio intensities. While Ames and UCL ab initio intensities are believed to be accurate within a few % for the strong unperturbed bands, the reported measurements allow testing important (>50%) differences between ab initio values of some weak perturbed bands.

The room temperature absorption spectrum of 13CH4 is recorded by high sensitivity cavity ring down spectroscopy (CRDS) near 1.58 µm. The investigated region (6147–6653 cm−1) corresponds to a spectral region of weak absorption located between the strong bands of the tetradecad and icosad. The sensitivity of the recordings allowed for the measurement of more than 13000 lines for which line centers and line intensities are derived using a multiline fitting program. The obtained list is the first extensive list of 13C enriched methane in the region. It is believed to include a significant number of lines of 13CH3D. In addition, 185 lines were measured at 80 K by direct absorption spectroscopy (DAS) near 6600 cm−1. The line intensity detectivity threshold is on the order of a few 10−29 cm/molecule for the CRDS recordings at 296 K and 10−26 cm/molecule for the DAS recordings at 80 K. From the intensity ratio of the lines in common in the 296 K and 80 K empirical line lists near 6600 cm−1, 438 empirical values of the lower state energy level (Eemp) were derived. An additional set of 200 Eemp values were obtained by combining the present CRDS line intensities to DAS literature data near 6150 cm−1. The comparison to the TheoReTS ab initio line list shows an overall very good agreement for the line intensities. Significant deviations on the order of 0.1 cm−1 are noted for line positions. The obtained experimental data will be valuable to tune ab initio line positions to empirical values in future versions of the theoretical line lists. Finally, a first set of 900 lines belonging to the 5ν4 and ν2 + 4ν4 bands of the icosad are rovibrationally assigned on the basis of an effective Hamiltonian model. The energy values of the involved vibrational sublevels are reported.

Water vapour foreign-continuum absorption cross-sections, CF, are measured for the first time at room temperature in the 1.6 µm transparency window, of importance for atmospheric applications. The measurements are performed by cavity ring down spectroscopy (CRDS) at 15 selected spectral data points. These data, covering the 5700–6640 cm−1 spectral range, are derived from the variation of the absorption signal during pressure ramps of humidified air up to 1 atm with a typical 1% water vapour relative concentration. The foreign-continuum absorption was obtained as the excess of the measured loss rate compared to the sum of the loss rate measured with dry air, the local water monomer contribution and the self-continuum absorption. CF values were derived from the linear dependence of the foreign-continuum absorption with the product of the partial pressures of water vapour and air. The semi-empirical MT_CKD CF values are found significantly underestimated in the centre of the window.

The atmospheric band of O2 near 1.27 µm plays an important role in determining the sounded air-mass from ground or space borne atmospheric spectra. This band consists of narrow absorption lines of the a 1 Δ g − X 3 Σ g − ( 0 − 0 ) transitions superimposed to a much broader collision-induced absorption structure. The present contribution is part of a long standing project aiming to improve different aspects of the spectroscopy of this band by highly sensitive cavity ring down spectroscopy (CRDS). In the present contribution, low pressure (5 and 10 Torr) spectra of pure O2 were recorded with unprecedented sensitivity in the 7920–8085 cm−1 interval (noise equivalent absorption, αmin, on the order of 10−12 cm−1) using an external cavity diode laser. About 170 lines including electric quadrupole transitions were accurately measured. The weakest lines have intensity on the order of 10−30 cm/molecule. The coupling of the CRDS spectrometer with a self-referenced frequency comb allows for an important gain on the accuracy of the line center determination. Detailed line profile analysis using the quadratic speed-dependent Nelkin-Ghatak profile was performed for a series of twelve lines recorded for pressures up to 150 Torr. In particular, the very weak self-pressure shifts (on the order of 10−3 cm−1/atm) could be determined for the first time. Line intensities with uncertainty of 1% are reported for lines with intensity larger than 10−28 cm/molecule. Accurate spectroscopic parameters of the a1Δg(v = 0) upper level were fitted to the zero-pressure line centers. An rms value of 108 kHz (3.6 × 10−6 cm−1) is achieved for the (meas.-calc.) differences of the 16O2 a1Δg(v = 0) upper level (Jmax= 37). Significant deviations compared to the HITRAN database are discussed.

The high-resolution absorption spectrum of acetylene has been recorded at room temperature by high sensitivity cavity ring down spectroscopy (CRDS) in the 5693–5882 cm–1 region. The noise level of the recordings corresponds to a typical minimum detectable absorption, αmin, below 10-10 cm−1. The positions and intensities of about 4000 lines were determined and analyzed together with about 3900 lines left unassigned in a previous CRDS study in the 5850–6340 cm−1 region. In total, about 2620 12C2H2 and 250 12C13CH2 absorption lines are assigned in the present work, most of them being newly reported. The 12C2H2 lines belong to 53 bands and include 24 bands newly identified or improved in the region of the previous study. Altogether, 107 bands in the entire 5693–6340 cm−1 while the HITRAN database provides line parameters of only nine 12C2H2 bands in the same region. The lines of 12C13CH2 in normal abundance (2.2%) belong to eight bands whose intensities are reported for the first time. Spectroscopic parameters of the upper vibrational levels were derived from standard band-by-band fits of the line positions (typical rms values of the (obs.-calc.) deviations are better than 0.002 cm–1). The vibrational transition dipole moment squared and Herman–Wallis coefficients of each band were derived from a fit of the measured intensity values. A recommended line list is provided for the bands newly assigned with positions calculated using empirical spectroscopic parameters of the lower and upper energy vibrational levels and intensities computed using the derived Herman–Wallis coefficients. As a result, the constructed line list includes a total of 4471 transitions belonging to 53 bands of 12C2H2 and 8 bands of 12C13CH2. This list of acetylene transitions above 5693 cm−1 extends to lower energy and completes our empirical spectroscopic database for acetylene in the 5850–9415 cm−1 range.

In spite of being a greenhouse gas with a large global warming potential, the absorption spectrum of nitrous oxide in the near infrared is insufficiently characterized. In the present work, the spectral region near 1.74 µm (5696–5910 cm−1) is investigated by high sensitivity cavity ring down spectroscopy (CRDS). The noise level of the CRDS spectra corresponds to a typical minimum detectable absorption, αmin, below 10-10 cm−1. 3326 transitions are measured and rovibrationally assigned to 50 bands of five nitrous oxide isotopologues (14N216O, 14N15N16O, 15N14N16O, 14N218O and 14N217O) in natural isotopic abundance. The assigned weakest lines have an intensity below 10−28 cm/molecule. For comparison, only three 14N216O bands are included in the HITRAN database in the region, with a 2 × 10−25 cm/molecule intensity cut off. The rovibrational assignments were performed by comparison with predictions performed for each isotopologue in the frame of the effective operator approach. The overall quality of the predictions is satisfactory for line positions. Deviations larger than 0.1 cm−1 are nevertheless noted for 14N216O and 14N218O. The spectroscopic parameters of the upper level of the observed bands were derived from the standard band-by-band fit of the measured line positions. A significant number of bands were found to be perturbed by local rovibrational perturbations and in some cases, extra lines due to an intensity transfer could be assigned. The interaction mechanisms and the perturbers were univocally identified on the basis of the effective Hamiltonian model. In particular, interpolyad couplings were evidenced indicating that the polyad version of the effective Hamiltonian has to be extended to include Coriolis and interpolyad anharmonic interactions. No satisfactory modeling of the N2O line intensities is yet available in the region. The CRDS intensity values derived in this work provide a solid set of measurements for future semi-empirical intensity modeling in the region.

New measurements of the water vapour continuum in the 1.6 µm and 2.3 µm windows are obtained at room temperature using highly stable and sensitive cavity ring down (CRD) spectrometers. In the 1.6 µm window, self-continuum cross-sections, CS, are derived for 30 selected spectral points between 5700 and 6850 cm−1 using pressure ramps of pure water vapour (up to 15 Torr). Purely quadratic pressure dependence is obtained for each measurement point. Compared to our previous dataset (Mondelain et al., 2014), the retrieved Cs values are more accurate and show a general agreement. The spectral coverage is extended both on the high and low frequency edges of the 1.6 µm window. In addition, new CS values are derived in the 5120–5137 cm−1 interval extending to higher energy the coverage of the 2.3 µm window. The measurements near 5700 cm−1 and 5130 cm−1 are found lower than the MT_CKD_3.2 values by a factor of about 2 and 1.5, respectively. Foreign-continuum cross-sections, CF, are newly obtained in the 2.3 µm window from CRDS spectra of moist air in flow regime. Spectra were recorded for different water vapour partial pressures while maintaining the total pressure constant in the high finesse CRDS cavity. After subtraction of the monomer and self-continuum contributions, CF values were derived from the linear variation of the foreign-continuum absorption with the water vapour partial pressure. CF values determined at four spectral points between 4430 and 5000 cm−1 are larger by up a factor of five than the corresponding MT_CKD values. Considering these CRDS data at room temperature and literature values at 400 K the temperature dependence of the foreign-continuum is confirmed to be weak.

Highly sensitive cavity enhanced laser techniques are used for accurate measurements of the absorption of dinitrogen (N2) at two spectral points of the fundamental and first overtone bands. Spectra of pure N2 and of a N2/O2 mixture were recorded near 2491 and 4720 cm−1 by Optical Feedback Cavity Enhanced Absorption Spectroscopy (OF-CEAS) and Cavity Ring Down Spectroscopy (CRDS), respectively. The noise level of the OF-CEAS and CRDS spectra corresponds to a minimum detectable absorption, αmin, of about 10-9 and 10-11 cm−1, respectively. Measurements involve both the N2 collision-induced absorption (CIA) bands and the very weak 2–0 S(10) and S(11) electric quadrupolar transitions. Accurate CIA binary coefficients, B N 2 − N 2 and B N 2 − O 2 , are derived from the pressure-squared dependence of the baseline level of the spectra recorded at pressures below 1 atm. These CIA values are compared to previous determinations from high pressure spectra (up to 90 atm) and to recent classical molecular dynamics simulations (CMDS). An overall good agreement is obtained although a significant deviation is observed for the first overtone CIA near 4720 cm−1. The line position and intensity (on the order of a few 10−30 cm/molecule) of the 2–0 S(10) and S(11) electric quadrupolar transitions are determined experimentally for the first time and compared to theoretical values provided in the HITRAN2016 database.

Various spectroscopic data for absorption lines due to the magnetic dipole transitions of the a 1 Δ g − X 3 Σ g − ( 0 − 0 ) band of O2 centered at 1.27 µm are tested by comparison with high-resolution ground-based atmospheric measurements recorded by Fourier Transform Spectrometers at Park Falls and Caltech (USA). This band is of importance for atmospheric remote sensing since it will be used (together with the O2 A-band near 760 nm) by the passive short wave infrared spectrometer onboard the MicroCarb satellite mission (i.e. this band includes the B4 band of MicroCarb, from about 7800 cm−1 to 7912 cm−1) for the determination of surface pressure and atmospheric aerosols. Spectroscopic data of the HITRAN2016 and GEISA2015 databases as well as those from recent laboratory studies are here used in a radiative transfer code to simulate atmospheric transmissions under the conditions of the measurements. Comparisons are made for different solar zenith angles and for the whole B4 spectral range considered by MicroCarb. Spectroscopic data of water vapor are also tested by considering both relatively dry and humid atmospheric conditions. Averaging the “calculated-observed” residuals over numerous recordings made for close values of the solar zenith angle and humidity enables reduction of the uncertainties due to the radiometric noise of the instrument and to the imperfect description of the atmospheric state. This enables the detection of systematic differences in the spectral residuals caused by small changes in spectroscopic data. The results show that the spectroscopic parameters in the HITRAN2016 and GEISA2015 databases lead to large residuals while data of two recent laboratory studies, obtained from spectra measured with the cavity ring-down spectroscopy technique using the speed-dependent Nelkin-Ghatak profile lead to much better agreement with atmospheric measurements. Significant residuals are noted for water vapor absorption lines simulated using parameters provided by both the HITRAN and GEISA databases.

Spectroscopic catalogues, such as GEISA and HITRAN, do not yet include information on the water vapour continuum that pervades visible, infrared and microwave spectral regions. This is partly because, in some spectral regions, there are rather few laboratory measurements in conditions close to those in the Earth’s atmosphere; hence understanding of the characteristics of the continuum absorption is still emerging. This is particularly so in the near-infrared and visible, where there has been renewed interest and activity in recent years. In this paper we present a critical review focusing on recent laboratory measurements in two near-infrared window regions (centred on 4700 and 6300cm−1) and include reference to the window centred on 2600cm−1 where more measurements have been reported. The rather few available measurements, have used Fourier transform spectroscopy (FTS), cavity ring down spectroscopy, optical-feedback – cavity enhanced laser spectroscopy and, in very narrow regions, calorimetric interferometry. These systems have different advantages and disadvantages. Fourier Transform Spectroscopy can measure the continuum across both these and neighbouring windows; by contrast, the cavity laser techniques are limited to fewer wavenumbers, but have a much higher inherent sensitivity. The available results present a diverse view of the characteristics of continuum absorption, with differences in continuum strength exceeding a factor of 10 in the cores of these windows. In individual windows, the temperature dependence of the water vapour self-continuum differs significantly in the few sets of measurements that allow an analysis. The available data also indicate that the temperature dependence differs significantly between different near-infrared windows. These pioneering measurements provide an impetus for further measurements. Improvements and/or extensions in existing techniques would aid progress to a full characterisation of the continuum – as an example, we report pilot measurements of the water vapour self-continuum using a supercontinuum laser source coupled to an FTS. Such improvements, as well as additional measurements and analyses in other laboratories, would enable the inclusion of the water vapour continuum in future spectroscopic databases, and therefore allow for a more reliable forward modelling of the radiative properties of the atmosphere. It would also allow a more confident assessment of different theoretical descriptions of the underlying cause or causes of continuum absorption.

The line position of the very weak S(2) transition of deuterium in the 2–0 band has been measured with a Comb-Assisted Cavity Ring Down spectrometer. The high sensitivity spectra were recorded at 5 and 10 mbar with a Noise Equivalent Absorption, α min , of 8 × 10 –11 cm −1 . The line positions at 5 and 10 mbar were measured with sub-MHz accuracy (460 and 260 kHz, respectively). After correction of the line pressure-shift, the frequency at zero pressure of the S(2) transition of the first overtone band was determined to be 187 104 299.51 ± 0.50 MHz. This value agrees within 1.7 MHz with the frequency obtained from the best available ab initio calculations and corresponds to only 15% of the claimed theoretical uncertainty.

The room temperature absorption spectrum of water vapor highly enriched in 17O has been recorded by Cavity Ring Down Spectroscopy (CRDS) between 5850 and 6671 cm−1. Two series of recordings were performed with pressure values of 1.0 and 12.0 Torr. The investigated spectral region corresponds to the important 1.55 μm transparency window of the atmosphere where water absorption is very weak. The high sensitivity of the recordings (αmin ~ 5×10−11 cm−1) allows detecting lines with intensity spanning six orders of magnitude (1.4×10−30–3.6×10−24 cm/molecule at room temperature). The experimental list includes more than 10,300 lines. The assignments of water lines were performed using known experimental energy levels as well as calculated line lists based on the results of Partridge and Schwenke. More than 8500 lines were assigned to 9619 transitions of six water isotopologues (H216O, H217O, H218O, HD16O, HD17O and HD18O). All but four transitions of the 16O and 18O isotopologues were assigned using known experimental energy levels. More than half of the assigned H217O and HD17O transitions correspond to new (or corrected) upper energy levels. About 1000 new H217O transitions associated with upper states of the second triad and of the first hexad were identified. Most of the newly assigned HD17O transitions belong to the ν1+ν3 and 2ν2+ν3 bands. The assigned transitions allowed to newly determine or correct 20 highly excited rotational levels of the vibrational ground state of this isotopologue. Overall 791 and 266 energy levels are newly determined for H217O and HD17O, respectively. A few additional levels were corrected compared to literature values. The obtained experimental results are compared to the spectroscopic parameters provided by the HITRAN database and to the empirical energy levels recommended by an IUPAC task group.

The absorption spectrum of the 13CH4 methane isotopologue has been recently recorded by Differential Absorption Spectroscopy (DAS) at 80 K in the 5853–6201 cm−1 spectral range. An empirical list of 3717 lines was constructed for this spectral range corresponding to the upper part of the Tetradecad dominated by the 2ν3 band near 5987 cm−1. In this work, we present rovibrational analyses of these spectra obtained via two theoretical approaches. Assignments of strong and medium lines were achieved with variational calculations using ab initio potential energy (PES) and dipole moment surfaces. For further analysis a non-empirical effective Hamiltonian (EH) of the methane polyads constructed by high-order Contact Transformations (CT) from an ab initio PES was employed. Initially predicted values of EH parameters were empirically optimized using 2898 assigned line positions fitted with an rms deviation of 5×10−3 cm−1. More than 1860 measured line intensities were modeled using the effective dipole transition moments approach with the rms deviation of about 10%. These new data were used for the simultaneous fit of the 13CH4 Hamiltonian parameters of the {Ground state/Dyad/Pentad/Octad/Tetradecad} system and the dipole moment parameters of the {Ground state-Tetradecad} system. Overall, 10 vibrational states and 28 vibration sublevels of the 13CH4 Tetradecad are determined. The comparison of their energy values with corresponding theoretical calculations is discussed.

The high-resolution absorption spectrum of acetylene has been recorded at room temperature by high sensitivity cavity ring down spectroscopy (CRDS) in the 6627–7065 cm–1 region. The studied spectral range corresponds to a spectral interval of very weak absorption (an acetylene transparency window). The positions and intensities of more than 6500 lines are determined in the considered interval where previous intensity information was mostly missing. On the basis of previous studies and on effective Hamiltonian predictions, 3062 12C2H2, 462 12C13CH2 and 104 12C2HD absorption lines belonging to a total of 123 vibrational bands are assigned. For comparison, the HITRAN2016 database provides line parameters of only ten 12C2H2 bands in the same region. Spectroscopic parameters of 113 upper vibrational levels were derived from standard band-by-band fits of the line positions (typical rms values of the (obs.-calc.) deviations are better than 0.003 cm–1). Many bands are found to be affected by perturbations. The vibrational transition dipole moment squared and Herman-Wallis coefficients of 47 bands were derived from a fit of the measured intensity values. In order to generate a recommended line list in the region, the derived spectroscopic parameters and Herman-Wallis coefficients were used to compute the line parameters for these 47 bands while experimental position and intensity values were kept for the other bands. Overall, the obtained recommended list including a total of 5260 transitions will help to fill a spectral gap around 1.47 µm where very scarce spectroscopic information is provided in the current spectroscopic databases. When compared to the HITRAN list in the region, some important deviations concerning both line positions and line intensities are evidenced as a result of inaccurate high J extrapolations.

The room temperature absorption spectrum of 17O enriched carbon dioxide is investigated by high sensitivity cavity ring down spectroscopy (CRDS) between 5695 and 5850 cm−1. About 1100 lines are measured and assigned to 26 bands of the six 17O containing carbon dioxide isotopologues: 16O12C17O, 16O13C17O, 17O12C18O, 17O13C18O, 12C17O2 and 13C17O2. The set of observed lines are rovibrationnally assigned on the basis of the predictions of the effective Hamiltonian (EH) model. The observed bands belong to the ΔP = 8 and 9 series of transitions where P = 2 V1 + V2 + 3 V3 is the polyad number (Vi being the vibrational quantum numbers). They consist in 12 previously unobserved bands and 14 already-known bands, for which additional high J lines are assigned. Bands of the 12C17O2 and 13C17O2 isotopologues (two for each) are observed for the first time in this spectral interval. The spectroscopic parameters of the upper vibrational levels are determined from a band-by-band fit of the line positions (typical rms deviations are less than 0.001 cm−1). The comparison to the most recent carbon dioxide spectroscopic databases is presented. The reported data will be used to improve the modeling of the line positions of 17O containing carbon dioxide isotopologues.

An accurate empirical list of 57,995 transitions is constructed for natural water in the 5690–8340 cm−1 near infrared region (1.76–1.20 µm). The new list represents an updated version of the list reported in Mikhailenko et al. (2016). The spectral range is extended to lower energy and the transition frequencies benefited from a series of recent measurements by comb-assisted cavity ring down spectroscopy (CA-CRDS). The line list construction uses as starting point the variational line lists computed on the basis of the results of Schwenke and Partridge for the six most abundant water isotopologues (H2 16O, H2 18O, H2 17O, HD16O, HD18O, HD17O). Variational line positions are replaced by position values calculated from empirical rotation-vibration energy levels. The set of required empirical energy levels is improved in accuracy and enlarged, in particular for the minor isotopologues. A large number of energy levels and thus transition frequencies, relying on spectra recorded by CA-CRDS, have accuracy on the order of 10−4 cm−1. All the transitions are provided with unique vibrational labeling supported by effective Hamiltonian calculations in case of ambiguity. A detailed comparison is presented with the list of natural water included in the HITRAN2016 database and with the very recent H2 16O and H2 18O lists reported in Conway et al. (2020). The advantage of our list in terms of line position accuracy is demonstrated and illustrated by direct comparison with CRDS recordings at disposal. Intensity comparison shows a general agreement but a number of weaknesses of the most recent intensity calculations are evidenced. All the considered theoretical line lists include a few bands (e.g. 4ν2, 5ν2, ν1+2ν2 and ν1+3ν2) with intensities deviating significantly from the observations. © 2020 Elsevier Ltd E.K. Conway (Center for Astrophysics, Cambridge and University College London) is acknowledged for providing us the submitted version of Ref. [44] together with the corresponding H216O and H218O lists, prior publication. The support of the University Grenoble Alpes and CNRS (France) in the frame of International Research Project SAMIA is acknowledged. SNM activity was also partly supported in the frame of the Russian Science Foundation, grant no. 18-11-00024.

The 1.74 µm spectral region corresponds to a very weak absorption interval of carbon dioxide (or “transparency window”) of particular importance to sound the deep atmosphere and the surface of Venus. In the present work, we extend the characterization of this spectral interval by the study of the 13CO2 isotopologues by high sensitivity cavity ring down spectroscopy (CRDS) with a highly enriched 13C sample. Indeed, in spite of their low natural abundances, the 13C minor isotopologues (in particular 16O13C18O with natural abundance less than 4.5 × 10−5) contribute importantly to the small residual opacity. In the studied 5694–5851 cm−1 spectral interval about 980 lines with intensity values as low as a few 10−30 cm/molecule at 296 K were detected. On the basis of the predictions of effective Hamiltonian models, the measured CO2 transitions were assigned to six isotopologues: 12C16O2, 13C16O2, 16O12C18O, 16O13C18O, 16O13C17O and 13C18O2. A total of 20 bands were assigned to the 13C isotopologues. Six of them are newly reported while the set of observed rotational transitions is enlarged for the others. The spectroscopic parameters of nineteen 13CO2 bands are determined from standard band-by-band analysis (typical rms deviations of the line positions are 8 × 10−4 cm−1). The new data show an overall good agreement with the Carbon Dioxide Spectroscopic Databank (CDSD-296), HITRAN2016 database and ab initio CO2 line lists. The previously identified issue related to the mixing of line intensities with CDSD or ab initio origins in the HITRAN database is illustrated by the 41104-00001 band of 13C16O2. A general discussion of the bands affected by this issue in the HITRAN2016 list is included.

The weak 2ν1 + 3ν2 + ν3 absorption band of the nitrogen dioxide main isotopologue (14N16O2) is investigated near 6350 cm−1. The absorption spectrum was recorded by high sensitivity Cavity Ring Down Spectroscopy with a noise equivalent absorption of αmin ≈ 1 × 10−10 cm−1. In total, 1069 lines of the 2ν1 + 3ν2 + ν3 band were assigned with rotational quantum numbers N and Ka up to 48 and 8, respectively, that corresponds to 1276 vibration-rotation transitions. The derived set of line positions is reproduced with an effective Hamiltonian with an rms of 2.35 × 10−3 cm−1 for the (obs.-calc.) deviations. The effective Hamiltonian takes explicitly into account the Coriolis interactions between the spin rotational levels of the (2,3,1) upper vibrational state and those of two nearby dark states - (2,5,0) and (2,1,2) - together with the electron spin-rotation interactions. Using the corresponding eigenfunctions the principal effective dipole moment parameter responsible for the line intensities of the 2ν1 + 3ν2 + ν3 band is fitted to selected measured line intensities which are reproduced with an rms deviation of 6.2%.

The room temperature absorption spectrum of water vapour highly enriched in 17O is recorded by cavity ring down spectroscopy (CRDS) near 1.73 µm. Two series of recordings were performed with pressure values of about 0.12 and 8.5 Torr from 5693 to 5850 cm−1. The noise equivalent absorption (αmin) of the spectra is better than 10−10 cm−1 allowing for the measurements of water lines with intensity down to 10−29 cm/molecule. A total of 1132 lines were assigned to 1220 transitions of five water isotopologues (H216O, H217O, H218O, HD16O, and HD17O). Their intensities span near five orders of magnitude from 10−29 to 10−24 cm/molecule at 296 K. The assignments were performed using known experimental energy levels as well as calculated line lists based on the results of Schwenke and Partridge. Most of the new results concern the H217O isotopologue: 483 transitions were measured while only the 27 strongest were previously known. Overall 104 levels were newly determined for H217O (64 levels), H218O (16), HD17O (23) and HD16O (1) and a few additional levels were corrected compared to the literature. The obtained results are compared to the water line list in the HITRAN2016 database, to the ExoMol energy levels and to the empirical energy levels recommended by an IUPAC task group.

The very weak absorption spectrum of the minor isotopologues of carbon dioxide near 1.74 µm (5702–5879 cm−1) is studied by high sensitivity cavity ring down spectroscopy (CRDS) with an 18O enriched sample. The investigated spectral region corresponds to a transparency window of very weak opacity which is of particular interest for Venus. Very weak lines with intensity value as low as 10−30 cm/molecule at 296 K are measured. On the basis of the predictions of the effective Hamiltonian models, 3202 lines of nine carbon dioxide isotopologues - 12C16O2, 16O12C18O, 16O12C17O, 16O13C18O, 16O13C17O, 17O12C18O, 17O13C18O, 12C18O2 and 13C18O2 - were rovibrationnally assigned to 59 bands, 44 of which were observed for the first time. The accurate spectroscopic parameters of 53 bands are determined from standard band-by-band analysis (typical rms deviations of the line positions are 8 × 10−4 cm−1). Three bands are found to be strongly perturbed by resonance first order Coriolis interaction. The new measured line intensities allowed determining or improving the ΔP = 8 sets of the effective dipole moment (EDM) parameters for five non-symmetrical isotopologues - 16O12C18O, 16O12C17O, 16O13C18O, 16O13C17O and 17O12C18O - and ΔP = 9 sets for the 16O12C18O and 12C18O2 isotopologues (P = 2V1+ V2+ 3V3 is the polyad number, Vi being the vibrational quantum numbers). The set of EDM parameters available for eleven isotopologues is used to discuss their isotopic dependence. These newly observed bands provide critical validation tests for the most recent spectroscopic databases. The comparison to the Carbon Dioxide Spectroscopic Databank (CDSD-296), HITRAN2016 database and ab initio Ames line list is presented. Deficiency is evidenced for the weak 41104–00001 band of 16O12C18O in the HITRAN2016 list and identified as due to inaccurate CDSD intensities which were preferred to ab initio intensities.

The absorption spectrum of 13CH4 is recorded in the center of the tetradecad near 1.73 μm (5695–5850 cm−1) at 296 K and 80 K by direct absorption spectroscopy (DAS). The achieved noise equivalent absorption of the spectra is αmin ≈ 5 × 10−8 cm−1 leading to line intensity detectivity threshold on the order of 5 × 10−26 and 5 × 10−27 cm/molecule at 296 K and 80 K, respectively. As a result, two empirical line lists are constructed including about 3350 and 2070 lines at room temperature and liquid nitrogen temperature, respectively. For comparison, the HITRAN database provides in the region only the strongest 13CH4 lines (about 440 lines in total). From the intensity ratios of the lines measured at the two temperatures, 1529 empirical values of the lower state energy level (Eemp) were derived. Their accuracy is reflected by the clear propensity of the corresponding empirical values of the lower state rotational quantum number, Jemp, to be close to integer values. They provide accurate temperature dependence for 91% and 99% of the total 13CH4 absorption in the region at 296 K and 80 K, respectively. The comparison to the TheoReTS ab initio line list shows significant deviations for line positions. The obtained experimental data will be valuable to tune ab initio line positions to empirical values in future versions of the theoretical line lists.

A tubular furnace is combined with a diode laser spectrometer to record the high temperature near infrared absorption spectrum of methane at a pressure of 80 Torr. The investigated spectral range covers the 5693–6257 cm−1 (1.75–1.60 µm) range of the tetradecad dominated by the 2ν3 band near 6000 cm−1. Various determinations (thermocouples, Doppler line broadening, rotational temperature) provide a value of about 964 K for the gas temperature. Lines with intensity larger than 5 × 10−24 cm/molecule at 1000 K are detected. As a result of the high temperature, high-J rovibrational transitions up to J ≈ 20 and hot bands are identified in the spectrum. The recorded spectra provide tests of various methane line lists recently released in the literature: HITRAN2016, experimental list derived from emission spectra, the MeCaSDa empirical line list and the TheoReTs and ExoMol theoretical line lists. The agreement with the TheoReTs line list is found to be very good in the studied region.

A Cavity Ring Down Spectrometer has been developed for high sensitivity absorption spectroscopy in the near infrared at pressure up to 10 bar. In order to strictly avoid perturbations of the optical alignment by pressure forces, the pre-aligned CRDS cavity is inserted inside the high pressure cell. The stability of the spectra baseline has been validated by filling the CRDS cell with Ar and N2 up to 10 bar. We present here the first application of this CW-CRDS spectrometer to the study of the high pressure spectrum of CO2 at room temperature. The spectra were recorded between 5850 and 5960 cm−1 for a series of pressure values up to 6400 Torr. The studied spectral interval corresponds to the high energy range of the 1.75 µm transparency window of CO2 of particular interest for Venus. The CO2 absorption coefficient at a given pressure value was obtained as the increase of the CRDS loss rate from its value at zero pressure taking into account the small contribution due to Rayleigh scattering. The CO2 absorption spectrum includes the contribution of the self broadened local rovibrational lines together with a broad and weak continuum. The CO2 continuum was obtained as the difference of the CO2 absorption coefficient and of a local lines simulation using the CO2 HITRAN line list and a truncated Voigt profile. A quadratic pressure dependence of the absorption continuum was observed, with an average binary absorption coefficient increasing smoothly from 3.7 to 11.5×10−9 cm−1 amagat−2 between 5850 and 5960 cm−1. The derived continuum shows a spectral feature located in the region of a band of 16O12C18O (present in natural abundance) which dominates the spectrum in the region. This spectral feature was found to arise from collisional interferences between local lines and quantitatively accounted for using a theoretical approach based on the impact and Energy Corrected Sudden (ECS) approximations. The impact of the chosen far wings CO2 line shape model on the retrieved continuum absorption is discussed.

The absorption spectrum of water vapor in “natural” isotopic abundance has been recorded at room temperature by high sensitivity Continuous Wave Cavity Ring Down Spectroscopy (CW-CRDS) between 7911 and 8337 cm −1 . The investigated region covers most of the 1.25 µm transparency window of importance for atmospheric applications. The recordings were performed with sensitivity on the order of α min ~2×10 –11 cm −1 , more than two orders of magnitude better than previous investigations by Fourier Transform Spectroscopy (FTS). Measured line intensities cover a range of seven orders of magnitude (3×10 –30 –2×10 –23 cm/molecule at room temperature). The experimental line list provided as Supplementary Material includes more than 5000 transitions. As a result of the achieved sensitivity, more than 1150 lines of the experimental list were identified as being due to ammonia present as an impurity at the 5 ppm concentration level in the water sample. Although incomplete, the obtained ammonia line list seems to be the first one in the region. More than 3193 water lines were assigned to 3560 transitions of five water isotopologues (H 2 16 O, H 2 18 O, H 2 17 O, HD 16 O and HD 18 O). The assignments were performed using known experimental energy levels and calculated spectra based on variational calculations by Schwenke and Partridge. The obtained results are compared to the most relevant previous studies by Fourier Transform Spectroscopy in the region and to the exhaustive review of rovibrational line positions and levels performed recently by an IUPAC sponsored task group. Two-hundred and sixty-six levels are newly determined and 46 are corrected by more than 0.015 cm −1 compared to those recommended by the water IUPAC task group. The overall agreement between variational and measured intensities is satisfactory. A complete empirical list of 4473 transitions incorporating all the experimental information at disposal was constructed for water in the studied region. The intensity cut-off was fixed to 1×10 –29 cm/molecule at 296 K. A detailed comparison with the line list as provided by the HITRAN database illustrates the advantages of the new list.

The room temperature absorption spectrum of water vapour highly enriched in 17 O is studied by Comb Assisted-Cavity Ring Down Spectroscopy (CA-CRDS) near 1.4 µm. The investigated spectral region (6667–7443 cm −1 ) is a strong absorbing region corresponding to the first hexade of interacting vibrational states. As a consequence of the high sensitivity of the recordings (Noise Equivalent Absorption, α min ∼ 10 −10 cm −1 ), a series of spectra could be recorded at very low pressure (less than 0.1 Torr) allowing for accurate determination of line centres and the spectral resolution of highly blended multiplets. For instance, 85 lines of the main isotopologue located near much stronger lines are measured for the first time. The assignments were performed using known experimental energy levels as well as calculated line lists based on the results of Schwenke and Partridge. Overall, the experimental list includes 9144 water lines which were assigned to 9988 transitions of six isotopologues (H 2 16 O, H 2 17 O, H 2 18 O, HD 16 O, HD 17 O and HD 18 O). Their intensities span six orders of magnitude from 10 −27 to 10 −21 cm/molecule at 296 K. Most of the new results concern the H 2 17 O isotopologue: more than 2300 new lines were measured, 64 new levels were determined and seven levels were corrected. In the case of HD 17 O, 152 new transitions were assigned and ten levels were newly determined. The center values of non-blended lines are reported with an accuracy better than 3 MHz (10 −4 cm −1 ) which represents an improvement compared to previous determinations. The comparison to the water vapor line list provided by the recently released 2016 version of the HITRAN database reveals a number of issues.

The amplitude, the temperature dependence and the physical origin of the water vapor absorption continuum are a long standing issue in molecular spectroscopy with direct impact in atmospheric and planetary sciences. In the recent years, we have determined the self-continuum absorption of water vapor at different spectral points of the atmospheric windows at 4.0, 2.1, 1.6 and 1.25 μm, by highly sensitive cavity enhanced laser techniques. These accurate experimental constraints have been used to adjust the last version (V3.2) of the semi-empirical MT_CKD model (Mlawer–Tobin_Clough–Kneizys–Davies) widely incorporated in atmospheric radiative transfer codes. In the present work, the self-continuum cross-sections, CS, are newly determined at 3.3 μm (3007 cm−1) and 2.0 μm (5000 cm−1) by optical-feedback-cavity enhanced absorption spectroscopy (OFCEAS) and cavity ring-down spectroscopy (CRDS), respectively. These new data allow completing the spectral coverage of the 4.0 and 2.1 μm windows, respectively, and testing the recently released V3.2 version of the MT_CKD3 continuum. By complementing high temperature literature data to the present data, the temperature dependence of the self continuum is presented. 1 Institute of Engineering Univ. Grenoble Alpes

The very weak absorption continuum of CO 2 is studied by Cavity Ring Down Spectroscopy in three 20 cm −1 wide spectral intervals near the centre of the 1.74 µm window (5693–5795 cm −1 ). For each spectral interval, a set of room temperature spectra is recorded at pressures between 0 and 10 bar thanks to a high pressure CRDS spectrometer. The absorption continuum is retrieved after subtraction of the contributions due to Rayleigh scattering and to local lines of CO 2 and water (present as an impurity in the sample) from the measured extinction. Due to some deficiencies of the CO 2 HITRAN2012 line list, a composite line list had to be built on the basis of the Ames calculated line list with line positions adjusted according to the Carbon Dioxide Spectroscopic Databank and self-broadening and pressure shift coefficients calculated with the Complex Robert Bonamy method. The local line contribution of the CO 2 monomer is calculated using this list and a Voigt profile truncated at ±25 cm −1 from the line centre. Line mixing effects were taken into account through the use of the impact and Energy Corrected Sudden approximations. The density dependence of the retrieved continuum absorption was found to be purely quadratic in the low frequency interval below 5710 cm −1 but a small significant linear contribution was required to reproduce the observations above this value. This linear increase is tentatively attributed to the foreign-continuum of water vapor present in CO 2 sample with a relative concentration of some tens ppm. The retrieved binary coefficient is observed to vary smoothly with the wavenumber with a minimum value of 6×10 −10 cm −1 amagat −2 . By gathering the present data with the results reported in Kassi et al. J Quant Spectrosc Radiat Transf 2015;167:97 , a recommended set of binary coefficients is provided for the 5700–5950 cm −1 region.

Accurate rovibrational analysis of the upper part of the Tetradecad 12 CH 4 spectrum in the 5855–6250 cm −1 region is reported for J ≤ 10. The analysis uses the WKLMC experimental line list previously obtained in Grenoble by Cavity Ring Down Spectroscopy and Differential laser Absorption Spectroscopy at T = 296 K. Based on a previously published ab initio potential energy surface, the full Hamiltonian of methane nuclear motion was reduced to an effective Hamiltonian using high-order Contact Transformation method with a subsequent empirical optimization of parameters. Overall, more than 3200 12 CH 4 transitions are assigned adding about 1800 new assignments to those provided in the HITRAN database for 12 CH 4 in the region. The 12 CH 4 experimental line positions were fitted with RMS standard deviations of 0.0015 cm −1 and 2381 line intensities were modeled using an effective dipole moment with the RMS deviation of 8%. Rovibrational energy levels of sixteen vibrational states of the upper part of the Tetradecad system were determined. For the first time all three sub-levels of the weak 4ν 2 band were assigned. As a result, a new version of the WKLMC list for natural methane at 296 K is provided including the new assignments in the 5855–6250 cm −1 region.

The room temperature absorption spectrum of water vapor highly enriched in 17O has been recorded by Cavity Ring Down Spectroscopy (CRDS) between 7443 and 7921 cm−1. Three series of recordings were performed with pressure values around 0.1, 1 and 10 Torr. The frequency calibration of the present spectra benefited of the combination of the CRDS spectrometer to a self-referenced frequency comb. The resulting CRD spectrometer combines excellent frequency accuracy over a broad spectral region with a high sensitivity (Noise Equivalent Absorption, αmin∼ 10–11–10−10 cm−1). The investigated spectral region corresponds to the high energy range of the first hexade. The assignments were performed using known experimental energy levels as well as calculated line lists based on the results of Partridge and Schwenke. Overall about 4150 lines were measured and assigned to 4670 transitions of six water isotopologues (H216O, H217O, H218O, HD16O, HD17O and HD18O). Their intensities span six orders of magnitude from 10–28 to 10–22 cm/molecule. Most of the new results concern the H217O and HD17O isotopologues for which about 1600 and 400 transitions were assigned leading to the determination of 329 and 207 new energy levels, respectively. For comparison only about 300 and four transitions of H217O and HD17O were previously known in the region, respectively. By comparison to highly accurate H216O line positions available in the literature, the average accuracy on our line centers is checked to be on the order of 3 MHz (10−4 cm−1) or better for not weak well isolated lines. This small uncertainty represents a significant improvement of the line center determination of many H216O lines included in the experimental list provided as Supplementary Material .

In a recent contribution [A. Campargue, S. Kassi, D. Mondelain, S. Vasilchenko, D. Romanini, Accurate laboratory determination of the near infrared water vapor self-continuum: A test of the MT_CKD model. J. Geophys. Res. Atmos., 121,13,180–13,203, doi:10.1002/2016JD025531], we reported accurate water vapor absorption continuum measurements by Cavity Ring-down Spectroscopy (CRDS) and Optical-Feedback-Cavity Enhanced Absorption Spectroscopy (OF-CEAS) at selected spectral points of 4 near infrared transparency windows. In the present work, the self-continuum cross-sections, CS, are determined for two new spectral points. The 2491 cm−1 spectral point in the region of maximum transparency of the 4.0 µm window was measured by OF-CEAS in the 23–52 °C temperature range. The 4435 cm−1 spectral point of the 2.1 µm window was measured by CRDS at room temperature. The self-continuum cross-sections were determined from the pressure squared dependence of the continuum absorption. Comparison to the literature shows a reasonable agreement with 1970 s and 1980 s measurements using a grating spectrograph in the 4.0 µm window and a very good consistency with our previous laser measurements in the 2.1 µm window. For both studied spectral points, our values are much smaller than previous room temperature measurements by Fourier Transform Spectroscopy. Significant deviations (up to about a factor 4) are noted compared to the widely used semi empirical MT_CKD model of the absorption continuum. The measured temperature dependence at 2491 cm−1 is consistent with previous high temperature measurements in the 4.0 µm window and follows an exp(D0/kT) law, D0 being the dissociation energy of the water dimer.

The weak absorption spectrum of water vapor in the important 2.2 μm transparency window is investigated with very high sensitivity. Overall, about 400 absorption lines were measured by Cavity Ring Down Spectroscopy (CRDS) and Optical-Feedback-Cavity Enhanced Laser Spectroscopy (OF-CEAS) in five spectral intervals: 4248.2–4257.3, 4298.4–4302.6, 4336.8.5-4367.5, 4422.4-4441.2 and 4514.6-4533.7 cm−1. The achieved sensitivity of the recordings (noise equivalent absorption, αmin, on the order of 2×10−10 cm−1) allowed detecting transitions with intensity values down to 1×10−28 cm/molecule, more than one order of magnitude better than previous studies by Fourier Transform spectroscopy. The rovibrational assignment was performed on the basis of variational calculations and of previously determined empirical energy values. Most of the newly assigned lines correspond to transitions of the ν1, ν3 and 3ν2 bands of H217O in natural isotopic abundance. Fourteen energy levels of H217O, H218O and HD18O are newly determined. An accurate and complete spectroscopic database is constructed for natural water in the 4190–4550 cm−1 region (2.39–2.20 µm). The list includes about 4500 transitions with intensity greater than 1×10−29 cm/molecule, for the six most abundant isotopologues in natural isotopic abundance. Line positions were obtained by difference of empirical energy values determined from literature data and complemented with the present CRDS results. The list is made mostly complete by including weak transitions not yet detected, with positions calculated from empirical levels and variational intensities. The variational intensities computed by a collaboration between the University College London and the Institute of Applied Physics in Nizhny Novgorod are found to improve significantly previous results by Schwenke and Partridge. Examples of comparison of the constructed line list to CRDS spectra and to simulations based on the HITRAN2012 list illustrate the advantages of the new list. The absence of many HDO lines is identified as most important deficiency of the HITRAN2012 list in the region.