T. Gabard

Methane line parameters in HITRAN

Abstract Two editions of the methane line parameters (line positions, intensities and broadening coefficients) available from HITRAN in 2000 and 2001 are described. In both versions, the spectral interval covered was the same (from 0.01 to 6184.5 cm −1 ), but the database increased from 48,033 transitions in 2000 to 211,465 lines in 2001 because weaker transitions of 12 CH 4 and new bands of 13 CH 4 and CH3D were included. The newer list became available in 2001 in the “Update” section of HITRAN. The sources of information are described, and the prospects for future improvements are discussed.

Experimental and theoretical study of line mixing in methane spectra. I. The N2-broadened ν3 band at room temperature

Line-mixing effects have been studied in the ν3 band of CH4 perturbed by N2 at room temperature. New measurements have been made and a model is proposed which is not, for the first time, strictly empirical. Three different experimental set ups have been used in order to measure absorption in the 2800–3200 cm−1 spectral region for total pressures in the 0.25–2 and 25–80 atm ranges. Analysis of the spectra demonstrates the significant influence of line mixing on the shape of the Q branch and of the P and R manifolds. A model is proposed which is based on state-to-state collisional transfer rates calculated from the intermolecular potential surface with a semiclassical approach. The line-coupling relaxation matrix is constructed from these data and two additional parameters which are fitted on measured absorption. Comparisons between measurements and spectra computed accounting for and neglecting line mixing are made. They prove the quality of the approach which satisfactory accounts for the effects of press...

Experimental and theoretical study of line mixing in methane spectra. II. Influence of the collision partner (He and Ar) in the v3 IR band

Line mixing effects are studied in the v3 band of CH4 perturbed by Ar and He at room temperature. Experiments have been made in the 2800–3200 cm−1 spectral region using four different setups. They cover a wide range of total densities, including low (0.25–2 atm), medium (25–100 atm), and high (200–1000 atm) pressure conditions. Analysis of the spectra demonstrates that the spectral shapes (of the band, the Q branch, the P and R manifolds,…) are significantly influenced by line mixing. The theoretical approach proposed in the preceding paper is used in order to model and analyze these effects. As done previously, semiclassical state-to-state rates are used together with a few empirical constants. Comparisons between measurements and spectra computed with and without the inclusion of line mixing are made. They prove the quality of the approach which satisfactorily accounts for the effects of pressure and of rotational quantum numbers on the spectral shape. It is shown that collisions with He and Ar lead to ...

Experimental and theoretical study of line mixing in methane spectra. IV. Influence of the temperature and of the band

Line-mixing effects are studied in infrared bands of CH4 perturbed by N2 at various pressures. The effects of temperature are investigated in the ν3 region whereas spectral shapes of the ν2, ν4, and ν3 bands are compared at room temperature. The theoretical approach proposed in preceding papers is used in order to model and analyze the influence of collisions on the spectral shape. All model parameters are now fixed to values determined in the previous studies. Comparisons between measurements and spectra computed with and without the inclusion of line mixing are made. They show that our approach satisfactorily accounts for the effects of temperature, pressure, and of rotational quantum numbers on the absorption by the ν3 band. Furthermore, the effects of collisions on spectra in the ν4 region at room temperature are also correctly calculated. On the other hand, the proposed approach fails in modeling the evolution with increasing pressure of absorption in the spectral range containing the ν2 band. This r...

Argon-broadened line parameters in the v3 band of 12CH4

Abstract Prompted by improved measurements of collisional line shapes in the v3 band P, Q and R branches of 12CH4, we have performed semi-classical line broadening calculations for methane perturbed by argon. We have used the theoretical approach developed by Robert and Bonamy as an extension of the well-known Anderson-Tsao-Curnutte theory. The semi-classical theory as reformulated here is shown to fully account for the tetrahedral symmetry of methane type molecules. The variation of argon-broadened linewidth coefficients in the v3 band of 12CH4 with the branch, J, symmetry and energy level fine structure is discussed.

Estimation of line parameters under line mixing effects: the ν3 band of CH4 in helium

Spectra of CH4-He mixtures were measured in the P and R branch regions of nu3 vibration-rotation band at lower pressures of 0.26-1.0 bar and for the whole band at higher pressures up to 90 bar. The line broadening coefficients were found from lower pressure data for the lines of the P branch. These coefficients were calculated in the framework of the Robert and Bonamy semi-classical approach. In general, they agree with the experimental ones, but a small J-dependent deviation was observed. Line mixing effects were observed at all pressures. At higher pressures they were interpreted in terms of the adjusted branch-coupling model. At lower pressures line mixing effects were found to be especially pronounced in the region of the relatively weak lines forming the clusters from R(16) to R(19).

Experimental and theoretical study of line mixing in methane spectra. III. The Q branch of the Raman ν1 band

The shape of the ν1 Raman Q branch of CH4 perturbed by Ar and He at room temperature has been studied. Stimulated Raman spectroscopy (SRS) experiments have been made in the 2915–2918 cm−1 spectral region for total pressures from 0.4 to 70 atm and mixtures of ≈5% CH4 with He and Ar. Analysis of the spectra demonstrates that the shape of the Q branch is significantly influenced by line mixing and much narrower than what is predicted by the addition of individual line profiles. For the first time, a model is proposed for the calculation and analysis of the effects of collisions on the considered spectra. In this approach, the rotational part of the relaxation matrix is constructed, with no adjustable parameter, starting from semiclassical state-to-state rates. Two empirical constants which account for the shift and broadening of the branch due to vibrational effects are introduced and their values are determined from fits of measured spectra. Comparisons between measurements and results computed with and wit...

Line parameters and shapes of high clusters: R branch of the ν3 band of CH4 in He mixtures

The IR absorption spectra of CH4 in pure gas and in mixture with helium were studied in the region of nu3 band at higher J line clusters R(17)-R(22). The frequencies and intensities of rotation-vibration lines were estimated from the experimental spectra at Doppler shape conditions. The line frequencies and intensities were calculated and used for the attribution of overlapped lines in clusters. The calculated line intensities are close to the experimental values. The calculated frequency structure of the higher J manifolds are somewhat wider than the observed one. The shapes of helium-broadened line clusters were compared with those calculated accounting for line mixing. The relaxation matrix W, which is necessary in shape calculations, was constructed using semiclassical collision rate constants. The calculated shapes are in satisfactory accordance with the measured ones.

Calculated helium-broadened line parameters in the ν4 band of 13CH4

Abstract Helium line-broadening coefficients for transitions in the ν 4 band of 13 CH 4 have been calculated using a semi-classical model. This model is analogous to the one that we have used in a previous study on argon line-broadening coefficients for transitions in the ν 3 band of 12 CH 4 . For the present study, over 270 room temperature measurements were considered. The J , symmetry and fine structure dependences of the data are satisfactorily reproduced, although deficiencies of the semi-classical model are evidenced.

Calculation of collision induced energy transfer rates in tetrahedral molecules. Application to 12CH4 perturbed by argon

Abstract The semi-classical calculation of collisional energy transfer rates in spherical top molecules has been investigated. Multipolar transition moments are expressed in fully symmetrized tensorial form to allow realistic eigenfunctions (corresponding to the polyad structure of such molecules) to be easily incorporated. The short range intermolecular potential has been chosen as an atom-atom Lennard-Jones potential, following earlier studies in the framework of the semi-classical line broadening theory of Robert and Bonamy. A preliminary application to the bending dyad of methane perturbed by argon is reported. A sixth order expansion of the intermolecular potential has been used. The parameters were adjusted to fit observed line width measurements in the v 4 band. The results obtained for line widths and state to state energy transfer rates are discussed.