CW-Cavity Ring Research Articles

Measurement of line strengths in the à 2A’ ← [formula omitted]2A” transition of HO2 and DO2

The absorption spectrum of the à 2A’ ← X˜2A” 000–000 band of HO2 and DO2 radicals has been measured in the range 6941–7077 cm−1 with a resolution of around 0.005 cm−1. HO2 and DO2 radicals were generated from the reaction of Cl-atoms with CH3OH and CD3OD, respectively, whereby Cl-atoms were generated by pulsed photolysis of (COCl)2 at 248 nm. One time-resolved absorption curve was measured by cw-Cavity Ring Down Spectroscopy (cw-CRDS) for each wavelength. The relative absorption coefficient for each wavelength was then obtained from the ring down time τ, extrapolated to t = 0 s with respect to the photolysis pulse, and the ring down time τ0 before the photolysis pulse. Absolute absorption coefficients were quantified for several selected lines relative to a well characterized absorption line of HO2 at 6638.21 cm−1 by a second cw-CRDS absorption path. The maximum absorption cross section at 50 Torr Helium was for HO2 the transition N Ka Kc J = 11 0 11 11.5 ← 11 1 11 11.5 with σ7000.28 cm−1 = 2.12 × 10−19 cm2 and for DO2 the doublet transition N Ka Kc J = 18 1 18 18.5 ← 18 0 18 18.5 and 18 1 18 17.5 ← 18 0 18 17.5 with σ7019.83 cm−1 = 2.97 × 10−19 cm2. The full spectrum has been very well reproduced by employing spectroscopic data from earlier works.

Rate Constant of the Reaction between CH3O2 Radicals and OH Radicals Revisited.

The reaction between CH3O2 and OH radicals has been studied in a laser photolysis cell using the reaction of F atoms with CH4 and H2O for the simultaneous generation of both radicals, with F atoms generated through 248 nm photolysis of XeF2. An experimental setup combining cw-Cavity Ring Down Spectroscopy (cw-CRDS) and high repetition rate laser-induced fluorescence (LIF) to a laser photolysis cell has been used. The absolute concentration of CH3O2 was measured by cw-CRDS, while the relative concentration of OH(v = 0) radicals was determined by LIF. To remove dubiety from the quantification of CH3O2 by cw-CRDS in the near-infrared, its absorption cross section has been determined at 7489.16 cm-1 using two different methods. A rate constant of k1 = (1.60 ± 0.4) × 10-10 cm3 s-1 has been determined at 295 K, nearly a factor of 2 lower than an earlier determination from our group ((2.8 ± 1.4) × 10-10 cm3 s-1) using CH3I photolysis as a precursor. Quenching of electronically excited I atoms (from CH3I photolysis) in collision with OH(v = 0) is suspected to be responsible for a bias in the earlier, fast rate constant.

Quantification of HO2 and other products of dimethyl ether oxidation (H2O2, H2O, and CH2O) in a jet-stirred reactor at elevated temperatures by low-pressure sampling and continuous-wave cavity ring-down spectroscopy

Measuring the formation of HO2 and H2O2 from the oxidation of fuels is challenging but extremely important for determining their tendency to follow chain-termination pathways from R + O2 compared to chain-branching leading to the production of OH radicals. Furthermore, such data are vital for improving existing detailed chemical kinetics models. Dimethyl ether (DME), a clean renewable fuel, is the simplest ether exhibiting cool flame oxidation chemistry, a key-process for auto-ignition in internal combustion engines. Although the oxidation of DME has been studied in the past, little information is available for the production of HO2 and H2O2. The oxidation of dimethyl ether was performed in a jet-stirred reactor at atmospheric pressure, over a range of temperatures (∼540–850 K) and equivalence ratios (φ = 0.5–2) and, for the first time, the concentrations of HO2 and H2O2 were measured using a newly developed experimental setup involving low-pressure sampling and near-infrared cw-cavity ring down spectroscopy. Concentrations of H2O and CH2O were also measured. These new experimental results extend the available kinetic database for the oxidation of dimethyl ether which is needed to assess the validity of combustion kinetics models, as shown here.

CRDS detection and modelling of vibrational bands of 18O3 approaching the dissociation threshold (7400–7920 cm−1)

The absorption spectrum of the 18O3 isotopologue of ozone is recorded by CW-Cavity Ring Down Spectroscopy between 7400 and 7920cm−1. The typical noise equivalent absorption of the recordings is αmin≈1×10−10cm−1 allowing for the detection of extremely weak vibrational bands located only a few % below the dissociation limit. The absorption spectrum also includes the weak 3A2(000)–X(110) hot vibronic band at 7877.53cm−1 affected by predissociation broadening. Overall, about 1000 lines were rovibrationnally assigned to six vibrational bands. Their line positions were modelled using effective Hamiltonian models covering three vibrational interacting systems and involving several “dark” states perturbing observed transitions. Line positions could be reproduced with rms deviations on the order of 0.01cm−1. The dipole transition moment parameters of the six bands were determined from the retrieved line intensities. The obtained set of effective parameters and the experimentally determined energy levels were used to generate a list of 1461 transitions provided as Supplementary materials.

Detection and analysis of three highly excited vibrational bands of 16O3 by CW-CRDS near the dissociation threshold

The present contribution is devoted to the analysis of three extremely weak A-type bands of 16O3 recorded near 7686, 7739 and 7860cm−1i.e. only a few % below the dissociation limit, D0, at about 8560cm−1. They correspond to the most excited vibration–rotation states of ozone observed so far via high-resolution absorption spectroscopy. They were detected by high sensitivity CW-cavity ring down spectroscopy with a typical noise equivalent absorption, αmin, on the order of 2×10−10cm−1 and modelled using the effective operator approach. The derived band centres and rotational constants show a very good agreement with recent theoretical predictions based on a new ab initio potential energy surface.

Three new bands of 18O16O18O by CW-CRDS between 6340 and 6800 cm−1

Three very weak bands of the 18O16O18O isotopologue of ozone have been detected by high sensitivity CW-Cavity Ring Down Spectroscopy between 6340 and 6800cm−1. They are vibrationally assigned as 2ν1+5ν3, ν1+4ν2+3ν3 and 3ν2+5ν3 and correspond to the highest frequency bands of this isotopologue detected so far. A total of 464, 318 and 194 transitions were rovibrationally assigned, respectively. The good agreement with theoretical values achieved for the derived band centres and rotational constants confirms the accuracy of the potential energy surface recently obtained via extensive ab initio calculations. A set of line intensities was measured and fitted to derive the first transition moment parameter of the three bands. The determined sets of effective Hamiltonian parameter and transition moment operators, as well as the experimental energy levels, were used to generate a complete list of 1526 transitions, provided as Supplementary materials. The calculated line-list allows generating a synthetic spectrum which reproduces satisfactorily the experimental spectrum.

Electric quadrupole transitions and collision-induced absorption in the region of the first overtone band of H2 near 1.25 μm

The Q(1)–Q(4) electric quadrupole transitions of the first overtone band of H2 have been recorded for six pressure values up to 640Torr, by CW-Cavity Ring Down Spectroscopy near 8000cm−1. The noise equivalent absorption of the spectra is on the order of αmin≈5×10−12cm−1. Line intensities derived from a profile fit accounting for Dicke narrowing effects, range from 1.0×10−29 to 2.6×10−27cm/molecule for the Q(4) and Q(1) line, respectively. The claimed absolute uncertainty on the derived line positions and on the line strengths are of the order of 0.001cm−1 and 1%, respectively. The pressure line shifts of the four lines were derived allowing for an accurate determination of the position at zero pressure limit. The obtained positions and intensities agree within the experimental uncertainty with the most recent theoretical calculations including non-adiabatic, relativistic and quantum electrodynamical effects. The present frequency determinations confirm the high accuracy of these calculations. From the pressure dependence of the baseline of the CRDS spectra, the self continuum cross section of the collision induced absorption band of H2 is determined in the vicinity of the Q(1) line. The derived cross section is found in good agreement with recent theoretical values.

CW-Cavity Ring Down Spectroscopy of deuterated water in the 1.58 μm atmospheric transparency window

The spectrum of water vapour enriched in deuterium has been recorded by highly sensitive CW-Cavity Ring Down Spectroscopy in the 5855–6802cm−1 spectral region. The studied region includes the 1.58µm atmospheric transparency window of particular interest for remote sensing. More than 8000 absorption lines belonging to eight water isotopologues – H216O, H218O HD16O, D216O, HD18O, D218O, HD17O and D217O – were identified.The spectrum was assigned using both the IUPAC database of experimental transitions and energy levels, and accurate variational calculations. Overall, 1396 and 1277 experimental energy levels belonging to 18 and 16 upper vibrational states were retrieved for the HD16O and D216O species, respectively. 773 energy levels are newly derived. New experimental information concerns the high J (up to 20) and high Ka (up to 12) energy levels of the (101), (021), (040), (210), (120) states of HD16O and of the (111), (031), (210), (012), (130) states of D216O. While only a few energy levels were available, the rotational structure of the (130) and (220) states of HD16O could be analysed in detail. Rotational sublevels of the (140) state of D216O are reported for the first time. A detailed comparison of the derived energy levels with the values recommended by an IUPAC task group is presented. In particular, a significant improvement is evidenced for a number of quasi degenerate energy levels of HD16O. The obtained results are also discussed in relation with several recent studies.

Rate constant of the reaction between CH3O2 and OH radicals

The rate constant of the reaction(R1)OH+CH3O2→productshas been measured at 294K by simultaneous coupling of Laser Induced Fluorescence (LIF) and cw-Cavity Ring Down Spectroscopy (cw-CDRS) to laser photolysis. OH radicals were generated by 248nm photolysis of either H2O2 or O3 in the presence of H2O, CH3O2 radicals were generated simultaneously by photolysis of CH3I and their absolute concentrations were obtained by cw-CRDS. OH decays were measured under excess CH3O2 concentrations and a very fast rate constant of k1=(2.8±1.4)×10−10cm3s−1 was found independent of pressure at 50 and 100Torr helium.

High sensitivity Cavity Ring Down spectroscopy of 18O enriched carbon dioxide between 5850 and 7000 cm−1: Part III–Analysis and theoretical modeling of the 12C17O2, 16O12C17O, 17O12C18O, 16O13C17O and 17O13C18O spectra

More than 19,700 transitions belonging to 11 isotopologues of carbon dioxide have been assigned in the room temperature absorption spectrum of highly 18O enriched carbon dioxide recorded by very high sensitivity CW-Cavity Ring Down spectroscopy between 5851 and 6990cm−1 (1.71–1.43μm). This third and last report is devoted to the analysis of the bands of five 17O containing isotopologues present at very low concentration in the studied spectra: 16O12C17O, 17O12C18O, 16O13C17O, 17O13C18O and 12C17O2 (627, 728, 637, 738 and 727 in short hand notation). On the basis of the predictions of effective Hamiltonian models, a total of 1759, 1786, 335, 273 and 551 transitions belonging to 24, 24, 5, 4 and 7 bands were rovibrationally assigned for 627, 728, 637, 738 and 727, respectively. For comparison, only five bands were previously measured in the region for the 728 species. All the identified bands belong to the ΔP=8 and 9 series of transitions, where P=2V1+V2+3V3 is the polyad number (Vi are vibrational quantum numbers). The band-by-band analysis has allowed deriving accurate spectroscopic parameters of 61 bands from a fit of the measured line positions. Two interpolyad resonance perturbations were identified.Using the newly measured line positions and those collected from the literature, the global modeling of the line positions within the effective Hamiltonian approach was performed and a new set of Hamiltonian parameters was obtained for the 17O12C18O, 16O13C17O and 17O13C18O isotopologues. For the five studied isotopologues, the effective dipole moment parameters of the ΔP=8 and 9 series were derived from a global fit of the measured line intensities. The obtained results will be used for the improvement of the quality of the line positions and intensities in the most currently used spectroscopic databases of carbon dioxide.

High sensitivity Cavity Ring Down spectroscopy of 18O enriched carbon dioxide between 5850 and 7000 cm−1: Part II—Analysis and theoretical modeling of the 12C18O2, 13C18O2 and 16O13C18O spectra

More than 19000 transitions belonging to 11 isotopologues of carbon dioxide have been assigned in the room temperature absorption spectrum of highly 18O enriched carbon dioxide recorded by very high sensitivity CW-Cavity Ring Down spectroscopy between 5851 and 6990cm−1 (1.71–1.43μm). The line positions were determined with accuracy better than 1×10−3cm−1 while the absolute line intensities are reported with an uncertainty better than 10%. This second report is devoted to the analysis of the bands of three multiply substituted isotopologues: 12C18O2, 13C18O2 and 16O13C18O (828, 838 and 638 in short hand notation).On the basis of the predictions of effective Hamiltonian models, a total of 2870, 538 and 1375 transitions belonging to 59, 11 and 15 bands were rovibrationnally assigned for 828, 838 and 638, respectively. For comparison, only 11 bands were previously measured by Fourier Transform spectroscopy in the region, for the 828 species. All the identified bands correspond to a ΔP=9 variation of the polyad number (P=2V1+V2+3V3, where Vi are vibrational quantum numbers). The band-by-band analysis has allowed deriving accurate spectroscopic parameters of 81 bands from a fit of the measured line positions. A few resonance perturbations were identified. In particular, the 31113 and 51106 states of 638 belonging to different polyads (P=10 and P=11, respectively) are anharmonically coupled.Using the large set of newly measured line positions and those collected from the literature, the global modeling of the line positions within the effective Hamiltonian approach was performed and a new set of Hamiltonian parameters was obtained for each of the three considered isopotologues. Using a similar approach, the global fits of the obtained intensity values of the ΔP=9 series of transitions were used to derive the corresponding ΔP=9 effective dipole moment parameters of the three considered isotopologues. The obtained results will help to improve the quality of the spectral line parameters of the three considered minor isopotologues in the most currently used spectroscopic databases of carbon dioxide.

Photolysis of CF3CH2CHO in the Presence of O2 at 248 and 266 nm: Quantum Yields, Products, and Mechanism

Three different detection techniques, coupled to pulsed laser photolysis (PLP), have been employed to determine the quantum yields of CF3CH2CHO at 248 and 266 nm: CF3CH2CHO + hν → CF3CH2 + HCO (R1a), CF3CH2CHO + hν → CF3CH3 + CO (R1b), and CF3CH2CHO + hν → CF3CH2O + H (R1c). (a) In the presence of air, Fourier transform infrared (FTIR) spectroscopy was employed at a total pressure of 760 Torr to monitor and quantify the loss of CF3CH2CHO at both wavelengths as well as the build-up of formed products (CO, CF3CH3, CF3CHO, and CF3CH2OH) after various laser pulses. Cyclohexane was added as OH-scavenger in most experiments. CF3CH3 was observed and quantified at both wavelengths, confirming that channel R1b is occurring. Small amounts of HCOOH and COF2 were also detected. (b) Time-resolved cw-cavity ring down spectroscopy (cw-CRDS) at 40 Torr He coupled to photolysis at 248 nm was employed for the detection of HO2 radicals. Varying the O2 concentration allows distinguishing the origin of the HO2 radicals from either R1a or R1c. OH radicals were simultaneously detected by laser-induced fluorescence. (c) Time-resolved tunable diode laser absorption spectroscopy (TDLAS) at 30 Torr N2 coupled to photolysis at 266 nm was employed for the determination of the quantum yields of CO. By varying the O2 concentration, a distinction can be achieved between the yields of prompt CO R1b or decomposition of highly excited CF3CH2CO from R1c and HCO radicals R1a. Channel R1a has been identified as the major reaction path. The overall quantum yield, Φλ(CF3CH2CHO), at 248 nm was found as Φ248nm = (0.76 ± 0.14) and (0.73 ± 0.20) from cw-CRDS and FTIR experiments, respectively. At 266 nm, the overall quantum yield was found as Φ266nm = (0.55 ± 0.10) and (0.47 ± 0.10) from TDLAS and FTIR experiments, respectively.

First detection of transitions in the second quadrupole overtone band of nitrogen near 1.44 μm by CW-CRDS with 6 × 10−13 cm−1 sensitivity

The extremely weak S(4) and S(6) electric quadrupole transitions of the 3-0 band of nitrogen have been detected by very high sensitivity CW-Cavity Ring Down spectroscopy near 6950cm−1. A noise equivalent absorption on the order of αmin∼6×10−13cm−1 was achieved by averaging several thousand 0.5cm−1 wide spectra recorded successively over several days. The studied 14N2 lines are among the weakest ever measured in the laboratory (intensity on the order of 1.5×10−31cm/molecule). The positions determined with a 0.002cm−1 uncertainty and the line intensities are compared with theoretical values included in the HITRAN2012 database.

High sensitivity Cavity Ring Down spectroscopy of 18O enriched carbon dioxide between 5850 and 7000 cm−1: I. Analysis and theoretical modeling of the 16O12C18O spectrum

The room temperature absorption spectrum of highly 18O enriched carbon dioxide has been recorded by very high sensitivity CW-Cavity Ring Down spectroscopy between 5851 and 6990cm−1 (1.71–1.43μm). The achieved sensitivity (noise equivalent absorption αmin∼5×10−10−5×10–11cm−1) has allowed the detection of more than 19,000 transitions belonging to eleven isotopologues of carbon dioxide. This first report is devoted to the analysis of the 16O12C18O data. A total of 6452 transitions belonging to 71 bands were rovibrationnally assigned on the basis of the predictions of the effective Hamiltonian model. For comparison, only twelve 16O12C18O bands were previously measured by Fourier Transform spectroscopy in the region. Line intensities of the weakest transitions are on the order of 2×10–29cm/molecule. The studied spectral region is formed by ΔP=8, 9 and 10 series of transitions, where P=2V1+V2+3V3 is the polyad number (Vi are vibrational quantum numbers). The band-by-band analysis has allowed deriving accurate spectroscopic parameters of 69 bands from a fit of the measured line positions. A few resonance perturbations were identified. In particular, the 31112 (P=10) and 51105 (P=11) states belonging to different polyads are anharmonically coupled. The resulting intensity transfer leads to the observation of several extra lines of the 51105-01101 hot band which could be assigned.A major result of the present work is the measurement of the ΔP=8 bands of 16O12C18O near 5900cm−1 which are the main responsible of the absorption of carbon dioxide in natural isotopic abundance in the region but are absent in the current version of the CO2 spectroscopic databases (HITRAN, GEISA, and CDSD). These ΔP=8 bands may significantly impact the transmission of CO2 in the 1.70µm transparency window of importance for Venus spectra.Using the large set of newly measured line positions and those collected from the literature, the global modeling of the 16O12C18O line positions within the effective Hamiltonian approach has been performed and a new set of Hamiltonian parameters has been obtained. Using a similar approach, the global fits of the obtained intensity values of the ΔP=8, 9 and 10 series of transitions were used to derive the corresponding set of effective dipole moment parameters.The obtained results will help to improve importantly the quality of the 16O12C18O line positions and line intensities in the most currently used spectroscopic databases of carbon dioxide.

The CW-CRDS spectra of the 16O/18O isotopologues of ozone between 5930 and 6340 cm−1—Part 3: 16O18O18O and 18O16O18O

Our systematic investigation of the high sensitivity CW-Cavity Ring Down Spectra of 16O/18O ozone isotopologues at high vibrational excitation continues with the study of the 16O18O18O and 18O16O18O species. The first two papers of this series were devoted to the analysis of the same four bands of the 16O16O18O and 16O18O16O species in the 5930–6340cm−1domain. Here, after a brief reminder of relevant experiment and theory, we report the analyses of two bands of 16O18O18O, vibrationally assigned as 2ν2+5ν3 and 2ν1+2ν2+3ν3_2 and three bands of 18O16O18O, assigned to 2ν1+2ν2+3ν3_1, 5ν1+ν3 and 3ν1+ν2+3ν3.They correspond to the highest vibration excitations observed so far for the 16O18O18O and 18O16O18O isotopologues. Altogether for the two new bands of 16O18O18O, 1214 rovibrational transitions were assigned up to Jmax=29 and for the three new bands of 18O16O18O, 948 rovibrational transitions were assigned up to Jmax=27. Observed line positions were fitted with root-mean squares deviations ranging from 0.005 to 0.011cm−1, using effective Hamiltonian models accounting for dark state perturbations. The derived band centres and rotational constants are in good agreement with new theoretical calculations from the molecular potential function. The corresponding lists of 3365 lines are provided as Supplementary material.

The CW-CRDS spectra of the 16O/18O isotopologues of ozone between 5930 and 6340 cm−1. Part 2: 16O18O16O

This work is part of a systematic investigation of the CW-Cavity Ring Down Spectra 16O/18O ozone isotopologues at high sensitivity (noise equivalent absorption αmin≈1×10−10cm−1). A first contribution [Mondelain D, Campargue A, Kassi S, Barbe A, Starikova E, De Backer-Barilly MR, et al. J Quant Spectrosc Radiat Transfer, 2013;116:49-66] was devoted to the analysis of four bands of the 16O16O18O species of CS symmetry between 5930 and 6340cm−1. Here we report the analysis of four bands of 16O18O16O of C2V symmetry, vibrationally assigned as 2ν1+2ν2+3ν3_1, 2ν2+5ν3, 5ν1+ν3 and 2ν1+2ν2+3ν3_2. They correspond to the highest vibration excitations observed so far for this isotopomer. Altogether, 1385 rovibrational transitions were assigned up to Jmax=31. Measured line positions were reproduced with rms deviations ranging from 0.005 to 0.013cm−1, using effective Hamiltonian models accounting for dark state resonance perturbations. The derived band centres and rotational constants are found in good agreement with new theoretical calculations from the molecular potential function. The corresponding lists of 2124 lines are provided as Supplementary Material.

High sensitivity CW-Cavity Ring Down Spectroscopy of N2O between 6950 and 7653 cm−1 (1.44–1.31 μm): II. Line intensities

The room temperature absorption spectra of nitrous oxide, N2O, have been recorded in the 6950–7653 cm−1 spectral region at 2 and 10Torr using a CW-CRDS spectrometer based on 24 fibered DFB lasers. The achieved sensitivity (noise equivalent absorption αmin≈5×10–11cm−1) allowed detecting lines with intensities as low as 1×10–29cm/molecule. In the preceding contribution (Lu Y, Mondelain D, Liu AW, Perevalov VI, Kassi S, Campargue A, J Quant Spectros and Radiat Transfer 2012;113:749–62), we reported the assignment of more than 7200 N2O lines in the region and the derivation of the corresponding spectroscopic parameters Gv, Bv, Dv and Hv. In the present work, more than 1300 14N216O absolute line intensities of cold and hot bands belonging to the ΔP=12, 13 and 14 series of transitions have been measured (P=2V1+V2+4V3 is the polyad number). The uncertainty of the obtained line intensity values varies from 4 to 7% for the majority of the lines. The obtained dataset extends importantly the set of measurements available in the literature, in particular for the ΔP =13 series for which previous data were very limited. The ΔP=12–14 effective dipole moment parameters were fitted to the intensity values measured in this work and available in the literature. The obtained sets of the dipole moment parameters allow reproducing the observed line intensities within their experimental uncertainties. The calculated intensities of the ΔP=12, 13 and 14 bands of 14N216O assigned by CRDS in the 6950–7653 cm−1 region are provided as supplementary material.

The CW-CRDS spectra of the 16O/18O isotopologues of ozone between 5930 and 6340 cm−1—Part 1: 16O16O18O

The first investigation of the very sensitive CW-Cavity Ring Down Spectra (noise equivalent absorption αmin≈1×10−10cm−1) of 18O enriched ozone isotopologues is reported. The ozone generation from 18O enriched oxygen molecules resulted in six isotopic species. The contributions of the 16O16O18O/16O18O16O and of the 18O18O16O/18O16O18O isotopomers were separated using two partial pressures of 16O2/18O2 mixtures. In the 5930–6340cm−1 range, the four dominant bands of 16O16O18O were vibrationally assigned as 2ν1+2ν2+3ν3_1, 2ν2+5ν3, 5ν1+ν3 and 2ν1+2ν2+3ν3_2. They correspond to the highest vibration excitations observed so far in an asymmetric CS ozone isotopologue. The rovibrational spectra for 18O enriched CS ozone species show specific features related to the nuclear spin statistics and to simultaneous symmetry allowed Coriolis and anharmonic accidental resonances, which are discussed. Altogether for the four new bands, 2155 rovibrational transitions were assigned up to Jmax=32. Observed line positions were fitted with rms deviations ranging from 0.005 to 0.01cm−1 using effective Hamiltonian models accounting for dark state perturbations. The derived band centers and rotational constants are in good agreement with new theoretical calculations from the molecular potential function. The corresponding line lists are provided as Supplementary Material.

CRDS of water vapor at 0.1 Torr between 6886 and 7406 cm−1

The absorption spectrum of water vapor in “natural” isotopic abundance has been recorded by high sensitivity CW-Cavity Ring Down Spectroscopy (CW-CRDS) between 6885.79 and 7405.91cm−1. This strong absorbing region includes the first hexad of interacting vibrational bands which was previously studied by Fourier Transform Spectroscopy. The achieved sensitivity of the recordings varies from αmin∼2×10–11 to 2×10−10cm−1 allowing us to use a sample pressure of 0.1Torr, making pressure broadening of the line profile mostly negligible. Weak lines in the vicinity of much stronger lines could then be accurately measured. The weakest lines have intensity on the order of 5×10–28cm/molecule at 296K. A set of 4471 lines were assigned to 4916 transitions of five water isotopologues (H216O, H218O, H217O, HD16O and HD18O). A small number of new energy levels was determined mostly for the H217O isotopologue. The previous investigations and existing databases are critically evaluated. In particular, a number of corrections and new assignments are proposed for the water list provided by the HITRAN database in the considered region. As a result, a complete list of 12,700 transitions for water in “natural” isotopic abundance is provided as Supplementary Material for the 6885–7408cm−1 region.

Refinements of the WKMC empirical line lists (5852–7919 cm−1) for methane between 80 K and 296 K

During the last 4 years, empirical line lists for methane at room temperature and at 80K were constructed from spectra recorded by (i) differential absorption spectroscopy (DAS) in the high energy part of the tetradecad (5852−6195cm−1) and in the icosad (6717–7589cm−1) and (ii) high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58μm and 1.28μm transparency windows (6165–6750cm−1 and 7541–7919cm−1, respectively). We have recently constructed the global line lists for methane in “natural” isotopic abundance, covering the spectral region from 5854 to 7919cm−1 (Campargue A, Wang L, Kassi S, Mondelain D, Bézard B, Lellouch E, et al., An empirical line list for methane in the 1.26–1.71μm region for planetary investigations (T=80–300K). Application to Titan, Icarus 219 (2012) 110–128). These WKMC (Wang, Kassi, Mondelain, Campargue) empirical lists include about 43,000 and 46,420 lines at 80±3K and 296±3K, respectively. The “two temperature method” provided lower state energy values, Eemp, for about 24,000 transitions allowing us to account satisfactorily for the temperature dependence of the methane absorption over the considered region. The obtained lists have been already successfully applied in a large range of temperature conditions existing on Titan, Uranus, Pluto, Saturn and Jupiter.In the present contribution, we provide some improvements to our lists by using literature data to extend the set of lower state energy values and by correcting the distortion of the high Eemp values (J>10) due to the temperature gradient existing in the cryogenic cell used for the recordings. The proposed refinements are found to have an overall limited impact but they may be significant in some spectral intervals below 6500cm−1.The new version of our lists at 80K and 296K is provided as Supplementary Material: the WKMC@80K+ and WKMC@296K lists are adapted for planetary and atmospheric applications, respectively. The WKMC@80K+ list is made applicable over a wider range of temperatures and shows satisfactory extrapolation capabilities up to room temperature. It was obtained by transferring to the 80K list the 27,580 single lines present only in the 296K list, with corresponding lower state energy values chosen to make them below the detectivity limit at 80K.In the discussion, the different line lists and databases available for methane in the near infrared are compared and some suggestions are given.