ACCURATE LASER MEASUREMENTS OF THE WATER VAPOR SELF-CONTINUUM ABSORPTION IN FOUR NEAR INFRARED ATMOSPHERIC WINDOWS. A TEST OF THE MT_CKD MODEL.

The semi empirical MT_CKD model of the absorption continuum of water vapor is widely used in atmospheric radiative transfer codes of the atmosphere of Earth and exoplanets but lacks of experimental validation in the atmospheric windows. Recent laboratory measurements by Fourier transform Spectroscopy have led to self-continuum cross-sections much larger than the MT_CKD values in the near infrared transparency windows. _x000d_ In the present work, we report on accurate water vapor absorption continuum measurements by Cavity Ring Down Spectroscopy (CRDS) and Optical-Feedback-Cavity Enhanced Laser Spectroscopy (OF-CEAS) at selected spectral points of the transparency windows centered around 4.0, 2.1 and 1.25 $mu$m. The temperature dependence of the absorption continuum at 4.38 $mu$m and 3.32 $mu$m is measured in the 23-39 $^{circ}$C range. The self-continuum water vapor absorption is derived either from the baseline variation of spectra recorded for a series of pressure values over a small spectral interval or from baseline monitoring at fixed laser frequency, during pressure ramps. In order to avoid possible bias approaching the water saturation pressure, the maximum pressure value was limited to about 16 Torr, corresponding to a 75% humidity rate._x000d_ After subtraction of the local water monomer lines contribution, self-continuum cross-sections, C$_{S}$, were determined with a few % accuracy from the pressure squared dependence of the spectra base line level. _x000d_ Together with our previous CRDS and OF-CEAS measurements in the 2.1 and 1.6 $mu$m windows, the derived water vapor self-continuum provides a unique set of water vapor self-continuum cross-sections for a test of the MT_CKD model in four transparency windows. Although showing some important deviations of the absolute values (up to a factor of 4 at the center of the 2.1 $mu$m window), our accurate measurements validate the overall frequency dependence of the MT_CKD2.8 model. _x000d

COMB-ASSISTED CAVITY RING DOWN SPECTROSCOPY OF 17O ENRICHED WATER BETWEEN 7443 AND 7921 CM−1

The room temperature absorption spectrum of water vapour highly enriched in chem{^{17}O} has been recorded by Cavity Ring Down Spectroscopy (CRDS) between 7443 and 7921 wn. 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, $alpha_{min}sim10^{-11}-10^{-10}$ wn). _x000d_ 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 (chem{H_2^{16}O}, chem{H_2^{17}O}, chem{H_2^{18}O}, chem{HD^{16}O}, chem{HD^{17}O} and chem{HD^{18}O}). Their intensities span six orders of magnitude from 10$^{-28}$ to 10$^{-22}$ cm/molecule. Most of the new results concern the chem{H_2^{17}O} and chem{HD^{17}O} 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 chem{H_2^{17}O} and chem{HD^{17}O} were previously known in the region, respectively._x000d_ By comparison to highly accurate chem{H_2^{16}O} line positions available in the literature, the average accuracy on our line centers is checked to be on the order of 3 MHz (10$^{-4}$ wn) or better for unblended lines. This small uncertainty represents a significant improvement of the line center determination of many chem{H_2^{16}O} lines in the considered region

AN EMPIRICAL SPECTROSCOPIC DATABASE FOR ACETYLENE IN THE REGIONS OF 5850-9415 CM−1

Six studies have been recently devoted to a systematic analysis of the high-resolution near infrared absorption spectrum of acetylene recorded by Cavity Ring Down spectroscopy (CRDS) in Grenoble and by Fourier-transform spectroscopy (FTS) in Brussels and Hefei. On the basis of these works, in the present contribution, we construct an empirical database for acetylene in the 5850 - 9415 $wn$ region excluding the 6341-7000 $wn$ interval corresponding to the very strong nub{1}+ nub{3} manifold. The database gathers and extends information included in our CRDS and FTS studies. In particular, the intensities of about 1700 lines measured by CRDS in the 7244-7920 $wn$ are reported for the first time together with those of several bands of $^{12}$C$^{13}$CH$_{2}$ present in natural isotopic abundance in the acetylene sample. _x000d_ The Herman-Wallis coefficients of most of the bands are derived from a fit of the measured intensity values. A recommended line list is provided with positions calculated using empirical spectroscopic parameters of the lower and upper energy vibrational levels and intensities calculated using the derived Herman-Wallis coefficients. This approach allows completing the experimental list by adding missing lines and improving poorly determined positions and intensities. As a result the constructed line list includes a total of 10973 lines belonging to 146 bands of $^{12}$C$_{2}$H$_{2}$ and 29 bands of $^{12}$C$^{13}$CH$_{2}$. For comparison the HITRAN2012 database in the same region includes 869 lines of 14 bands, all belonging to $^{12}$C$_{2}$H$_{2}$. Our weakest lines have an intensity on the order of 10$^{-29}$ cm/molecule,about three orders of magnitude smaller than the HITRAN intensity cut off. Line profile parameters are added to the line list which is provided in HITRAN format._x000d_ The comparison to the HITRAN2012 line list or to results obtained using the global effective operator approach is discussed in terms of completeness and accuracy._x000d

First observation of electric-quadrupole infrared transitions in water vapour

Molecular absorption of infrared radiation is generally due to ro-vibrational electric-dipole transitions. Electric-quadrupole transitions may still occur, but they are typically a million times weaker than electric-dipole transitions, rendering their observation extremely challenging. In polyatomic or polar diatomic molecules, ro-vibrational quadrupole transitions have never been observed. Here, we report the first direct detection of quadrupole transitions in water vapor. The detected quadrupole lines have intensity largely above the standard dipole intensity cut-off of spectroscopic databases and thus are important for accurate atmospheric and astronomical remote sensing

Electric quadrupole transitions in carbon dioxide

Recent advances in the high sensitivity spectroscopy have made it possible, in combination with accurate theoretical predictions, to observe for the first time very weak electric quadrupole transitions in a polar polyatomic molecule of water. Here we present accurate theoretical predictions of the complete quadrupole ro-vibrational spectrum of a non-polar molecule CO$_2$, important in atmospheric and astrophysical applications. Our predictions are validated by recent cavity enhanced absorption spectroscopy measurements and are used to assign few weak features in the recent ExoMars ACS MIR spectroscopic observations of the martian atmosphere. Predicted quadrupole transitions appear in some of the mid-infrared CO$_2$ and water vapor transparency regions, making them important for detection and characterization of the minor absorbers in water- and CO$_2$-rich environments, such as present in the atmospheres of Earth, Venus and Mars

Communication: Saturated CO 2

International audienc

A Database of Water Transitions from Experiment and Theory (IUPAC Technical Report)

The report and results of an IUPAC Task Group (TG) formed in 2004 on A Database of Water Transitions from Experiment and Theory (Project No. 2004-035-1-100) are presented. Energy levels and recommended labels involving exact and approximate quantum numbers for the main isotopologues of water in the gas phase, H216O, H2180, H217O, HD16O, HD18O, HD17O, D216O, D218O, and D217O, are determined from measured transition frequencies. The transition frequencies and energy levels are validated using first-principles nuclear motion computations and the MARVEL (measured active rotational-vibrational energy levels) approach. The extensive data including lines and levels are required for analysis and synthesis of spectra, thermochemical applications, the construction of theoretical models, and the removal of spectral contamination by ubiquitous water lines. These datasets can also be used to assess where measurements are lacking for each isotopologue and to provide accurate frequencies for many yet-to-be measured transitions. The lack of high-quality frequency calibration standards in the near infrared is identified as an issue that has hindered the determination of high-accuracy energy levels at higher frequencies. The generation of spectra using the MARVEL energy levels combined with transition intensities computed using high accuracy ab initio dipole moment surfaces are discussed. A recommendation of the TG is for further work to identify a single, suitable model to represent pressure- (and temperature-) dependent line profiles more accurately than Voigt profiles