Simple semi-classical model of pressure-broadened infrared/microwave linewidths in the temperature range 200–3000 K

There is a need for line broadening parameters for molecules found in exoplanetary atmospheres for a variety of broadeners and a range of temperatures. The use of an easily handled semi-classical theoretical expression is suggested for the calculation of pressure-broadened linewidths for (vib)rotational transitions over a large temperature range (200–3000 K) starting from a minimal set of input parameters: kinetic molecular properties and the character of the leading term in the intermolecular interaction potential. Applications to NO and OH colliding with rare-gas atoms and non-polar molecules demonstrate good consistency with available measurements over the full indicated temperature range. The procedure therefore can be expected to provide realistic estimates for line broadening of ‘exotic’ molecules and molecular ions present in hot planetary atmospheres

Calculation of ozone line shifting induced by N2 and O2 pressure

International audienceOzone line shifts by nitrogen and oxygen pressure are computed for the ν1+ν3, 2ν1 and 2ν3 bands of the 5μm spectral region by a semiempirical approach. The calculated values agree with measurements better than 0.001 cm-1atm-1 for 98% of O3-N2 lines and 87% of O3-O2 lines. In contrast with the water molecule case, the polarization components of the interaction potential are shown to contribute to the line shift more efficiently than the electrostatic interactions. As intermediate results, the mean dipole polarizability and the components of the polarizability tensor for the vibrational states (101), (200), and (002) of ozone molecule are determined by least-squares fitting of theoretical shifts to some experimental values. The temperature exponents for the ν1+ν3 band lines are also estimated

Analysis of self-broadened pure rotational and rovibrational lines of methyl chloride at room temperature

International audienceRovibrational absorption spectra of methyl chloride in the spectral region between 2800 and 3200 cm 1 were recorded with a high-resolution Fourier spectrometer. A multispectrum fitting procedure was used to analyze 527 transitions of the ν1 band and to retrieve the self-broadening coefficients for various J- and K-values with an estimated accuracy around 8%. Pure rotational transitions of CH3Cl in the submillimeter/terahertz region (0.2-1.4 THz) were also investigated using two complementary techniques of frequency-multiplication and continuous-wave photomixing. 43 pure rotational self-broadening coefficients were extracted with the accuracy between 3 and 5%. The whole set of measured values was used to model the J- and K-rotational dependences of the self-broadening coefficients by second-order polynomials. In addition, semi-classical calculations were performed, based on the real symmetric-top geometry of the active molecule, an intermolecular potential model including not only the dominant electrostatic but also the short-range forces, as well as on an exact classical treatment of the relative translational motion of the colliding partners. Comparison of all experimental and theoretical results shows similar rotational dependences and no significant vibrational dependence, so that extrapolations to other spectral regions should be straightforward

Recommended isolated-line profile for representing high-resolution spectroscopic transitions (IUPAC Technical Report)

The report of an IUPAC Task Group, formed in 2011 on "Intensities and line shapes in high-resolution spectra of water isotopologues from experiment and theory" (Project No. 2011-022-2-100), on line profiles of isolated high-resolution rotational-vibrational transitions perturbed by neutral gas-phase molecules is presented. The well-documented inadequacies of the Voigt profile (VP), used almost universally by databases and radiative-transfer codes, to represent pressure effects and Doppler broadening in isolated vibrational-rotational and pure rotational transitions of the water molecule have resulted in the development of a variety of alternative line-profile models. These models capture more of the physics of the influence of pressure on line shapes but, in general, at the price of greater complexity. The Task Group recommends that the partially Correlated quadratic-Speed-Dependent Hard-Collision profile should be adopted as the appropriate model for high-resolution spectroscopy. For simplicity this should be called the Hartmann--Tran profile (HTP). The HTP is sophisticated enough to capture the various collisional contributions to the isolated line shape, can be computed in a straightforward and rapid manner, and reduces to simpler profiles, including the Voigt profile, under certain simplifying assumptions.Comment: Accepted for publication in Pure and Applied Chemistr

COLLISIONAL LINE MIXING IN PARALLEL AND PERPENDICULAR BANDS OF LINEAR MOLECULES BY A NON-MARKOVIAN APPROACH

Author Institution: Institut UTINAM, UMR CNRS 6213, Universite de Franche-Comte, 16 route de Gray, 25030 Besancon cedex, FranceReliable modeling of radiative transfer in planetary atmospheres requires accounting for the collisional line mixing effects in the regions of closely spaced vibrotational lines as well as in the spectral wings. Because of too high CPU cost of calculations from ab initio potential energy surfaces (if available), the relaxation matrix describing the influence of collisions is usually built by dynamical scaling laws, such as Energy-Corrected Sudden law. Theoretical approaches currently used for calculation of absorption near the band center are based on the impact approximation (Markovian collisions without memory effects) and wings are modeled via introducing some empirical parameters [1,2]. Operating with the traditional non-symmetric metric in the Liouville space, these approaches need corrections of the ECS-modeled relaxation matrix elements ("relaxation times" and "renormalization procedure") in order to ensure the fundamental relations of detailed balance and sum rules.We present an extension to the infrared absorption case of the previously developed [3] for rototranslational Raman scattering spectra of linear molecules non-Markovian approach of ECS-type. Owing to the specific choice of symmetrized metric in the Liouville space, the relaxation matrix is corrected for initial bath-molecule correlations and satisfies non-Markovian sum rules and detailed balance. A few standard ECS parameters determined by fitting to experimental linewidths of the isotropic $Q$-branch enable i) retrieval of these isolated-line parameters for other spectroscopies (IR absorption and anisotropic Raman scattering); ii) reproducing of experimental intensities of these spectra. Besides including vibrational angular momenta in the IR bending shapes, Coriolis effects are also accounted for. The efficiency of the method is demonstrated on OCS-He and CO$_2$-CO$_2$ spectra up to 300 and 60 atm, respectively.\\ \ \\ $\lbrack$1$\rbrack$\ \ \ F.~Niro, C.~Boulet, and J.-M.~Hartmann, J. Quant. Spectrosc. Radiat. Transf. 88, 483 (2004).\\ $\lbrack$2$\rbrack$\ \ \ H.~Tran, C.~Boulet, S.~Stefani, M.~Snels, and G.~Piccioni, J. Quant. Spectrosc. Radiat. Transf. 112, 925 (2011).\\ $\lbrack$3$\rbrack$\ \ \ J.~Buldyreva and L.~Bonamy, Phys. Rev. A 60, 370-376 (1999)

A Theoretical Model For Wide-band Infrared-absorption Molecular Spectra At Any Pressure: Fiction Or Reality?

Various atmospheric applications require modeling of infrared absorption by the main atmospheric species in wide ranges of frequencies, pressures and temperatures. For different pressure regimes, different mechanisms are responsible for the observed intensities of vibration-rotation line manifolds, and the structure of the bands changes drastically when going from low to high densities. Therefore, no universal theoretical model exists presently to interpret simultaneously collACSed band-shapes observed at very high pressures and isolated-line shapes recorded in sub-atmospheric regimes. Using CO$_2$ absorption spectra as an example, we introduce some improvements in the non-Markovian Energy-Corrected Sudden model, developed for high-density spectra of arbitrary tensorial rank\footnote{J.V. Buldyreva and L. Bonamy, Phys. Rev. A 60(1), 370-376 (1999).} and generalized recently to parallel and perpendicular infrared absorption bands\footnote{J. Buldyreva and L. Daneshvar, J. Chem. Phys. 139, 164107 (2013).}, and test the applicability of this approach for the case of nearly Doppler pressure regime via comparisons with recently recorded experimental intensities\footnote{L. Daneshvar, T. F\"{o}ldes, J. Buldyreva, J. Vander Auwera, J. Quant. Spectrosc. Radiat. Transfer 2014 (to be submitted).}

Extension of the exact trajectory model to the case of asymmetric tops and its application to infrared nitrogen broadened linewidths of ethylene

International audienceThe model of exact trajectories involved in a semiclassical computation of (vib)rotational linewidths and shifts is extended to the case of asymmetric top colliders. General expressions for the second-order contributions to the scattering matrix are given which define the pressure broadening and shift of the line. This theoretical approach is tested on the particular case of the infrared ν7 band linewidths of C2H4 broadened by N2 which is frequently required for atmospheric applications. The computed linewidths compare favorably with available experimental data

Characterization of line mixing effects in the 11101←00001 band of carbon dioxide for pressures up to 19 atm

International audienceFourier-transform spectra of room-temperature carbon dioxide compressed to pressures up to 19 atm reported previously are re-analyzed to obtain weak line-mixing parameters and a representation of full line mixing for transitions observed in the P- and Q-branches of the 11101 ← 00001 (Π ← Σ) band centered at 2076.86 cm−1. Similar analyses for the R-branch transitions are prevented by strong Coriolis resonances that nearly completely transfer the intensities of R-branch transitions to the P-branch. The weak line mixing coefficients are retrieved experimentally and are modeled using the Exponential Power Gap scaling law. Another modeling is performed with an Energy-Corrected-Sudden type approach making use of a symmetric relaxation matrix satisfying all basic relations. Comparisons of experimental and calculated parameters with results available in the literature are presented and discussed

Energy‐corrected sudden modeling of the non‐Markovian rotational relaxation matrix for high‐pressure Raman spectra of pure nitrogen

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Measurements and calculations of ethylene line-broadening by argon in the ν₇ band at room temperature

International audienceArgon broadening coefficients are measured for 32 vibrotational lines in the ν7 band of ethylene at room temperature using a tunable diode-laser spectrometer. These lines with 3≤ J ≤ 19, 0≤ Ka ≤ 4, 2 ≤ Kc ≤ 19 in the P, Q and R branches are located in the spectral range 919-1023 cm-1. The fitting of experimental line shapes with Rautian profile provides the collisional widths slightly larger than those derived from Voigt profile. The independent theoretical estimation of these line widths is performed by a semiclassical approach of Robert-and-Bonamy type with exact isotropic trajectories generalized to asymmetric tops. Even with a rough atom-atom intermolecular potential model the calculated values show a quite good agreement with experimental results