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

Broadening of the ν2 Raman Band of CH4 by C3H8 and C4H10

Raman spectroscopy is a promising method for the analysis of natural gas. It is necessary to account for the broadening effects on spectral lines to improve measurement accuracy. In this study, the broadening coefficients for methane lines in the region of the ν₂ band perturbed by propane, n-butane, and isobutane at room temperature were measured. We estimated the measurement errors of the concentration of oxygen and carbon dioxide in the case of neglecting the broadening effects on the methane spectrum by the pressure of C₂-C₆ alkanes. The obtained data are suited for the correct simulation of the methane spectrum in the hydrocarbon-bearing gases and can be used to improve the accuracy of the analysis of natural gas by Raman spectroscopy.

Line-mixing in absorption bands of linear molecules diluted in high-density rare gases: measurements and modeling for OCS-He

Absorption coefficients in the bending ν2 and stretching 2ν3 bands of OCS perturbed by He at high pressures up to 300 atm have been measured and analyzed in the impact-approximation region by two theoretical models accounting for the line mixing. In the first approach, the frequency-independent relaxation operator is treated semi-classically [N. N. Filippov and M. V. Tonkov, J. Quant. Spectrosc. Radiat. Transf. 50, 111 (1993)] with adiabatic corrections. To characterize the strength of collisions and the line mixing efficiency a single parameter G is used and its value is estimated from the intermolecular interaction potential. The adiabaticity of collisions and the corresponding correction to the collisional cross sections amplitude are taken into account by two other parameters ν(corr) and σ deduced from fitting to experimental line widths. In the second approach, the symmetrized non-Markovian relaxation operator of Energy-Corrected Sudden type developed for rototranslational Raman spectra of linear rotators [J. Buldyreva and L. Bonamy, Phys. Rev. A 60, 370 (1999)] and satisfying all the basic properties (in particular, the detailed balance relation and the double-sided sum rules) is adapted to absorption spectra for the first time. Its off-diagonal elements are determined via the common adiabaticity factor and the basic transition rates whose parameters are adjusted on experimental values of isolated line widths. Both models provide a very consistent description of OCS-He 2ν3 and ν2 band intensities up to the maximal experimentally studied pressure, with a slightly more realistic picturing of the inter-branch exchange by the second approach.

Potential energy surface, bound states, and rotational inelastic cross sections of the He-CH[sub 4] system: A theoretical investigation

We determined two potential energy surfaces (PES) for the He-CH(4) system by means of MP4 and Valence Bond (VB) calculations. The MP4 potential is similar to the one commonly adopted for this system [U. Buck, K. H. Kohl, A. Kolhase, M. Faubel, and U. Staemmler, Mol. Phys. 55, 1255 (1985)], while the VB PES is slightly more attractive. To evaluate the reliability of these potentials, we investigated the scattering properties by performing close coupling calculations, and concluded that: (i) the available experimental data do not permit the ranking among the PES considered; (ii) some theoretical predictions differ considerably from the experimental data, and these discrepancies cannot entirely be ascribed to the inaccuracy of the ab initio calculations; (iii) the scattering properties at low energy might discriminate between the MP4 and VB potentials.

Experimental and theoretical study of line mixing in NH3 spectra. II. Effect of the perturber in infrared parallel bands

In a previous paper [J. Chem. Phys. 116, 7544 (2002) (Paper I)] a model, based on the energy corrected sudden approximation, was proposed for the construction of the line-mixing relaxation matrix. It was successfully tested by comparisons with measured infrared spectra of ammonia-helium mixtures. The present paper extends this preliminary study by considering mixtures of NH3 with H2 and Ar. Measurements have been made at room temperature in the regions of the nu2 and nu1 bands for pressures up to several hundred atmospheres. As in Paper I, the relaxation operator is constructed, within the impact approximation, using the ECS approximation. The data required are dynamical factors (which can be predicted from the NH3-X potential energy surface) and a scaling length (adjusted using line broadening data). Comparisons between measured and calculated absorptions demonstrate the quality of the model which satisfactory corrects for the large deviations with respect to the purely Lorentzian behavior. Line-mixing effects for NH3-Ar and NH3-H2 are qualitatively similar to those observed for NH3-He but quantitative differences exist, particularly when intra- and interbranch couplings are considered. Finally, the proposed model leads to very satisfactory results in the wings of both the purely rotational and nu2 bands of NH3 diluted in H2, opening promising perspectives for the remote sensing study of planetary atmospheres.