Franck Thibault

Pressure induced shifts of CO2 lines: Measurements in the 0003–0000 band and theoretical analysis

Self‐broadened and N2, Ar, He‐broadened halfwidth and pressure shift coefficients of the rotational transitions in the 0003–0000 band of 12C16O2 have been measured from laboratory absorption recorded at room temperature with a Fourier transform spectrometer. Comparison is made with the results of a theoretical calculation based on a semiclassical model. Good agreement is obtained for all the IR vibrational bands for which data are available. It has been shown that the shifts mainly originate from vibrational dephasing. Infrared line shifts have been compared with results obtained from stimulated Raman spectroscopy and we show that they are not consistent. A number of possible explanations have been discussed to account for this discrepancy.

Measurements and empirical modeling of pure CO 2 absorption in the 2.3-μm region at room temperature: far wings, allowed and collision-induced bands

Measurements of pure CO(2) absorption in the 2.3-μm region are presented. The 3800-4700-cm(-1) range has been investigated at room temperature for pressures in the 10-50-atm range by using long optical paths. Phenomena that contribute to absorption are listed and analyzed, including the contribution of far line wings as well as those of the central region of both allowed and collision-induced absorption bands. The presence of simultaneous transitions is also discussed. Simple and practical approaches are proposed for the modeling of absorption, which include a line-shape correction factor χ that extends to approximately 600 cm(-1) from line centers.

Shifting and broadening in the fundamental band of CO highly diluted in He and AR: A comparison with theory

We present measurements of the shifts and widths of the rovibrational lines of the fundamental band of CO highly diluted in He and Ar at 296 K. The shifts are decomposed into parts odd and even in the line number, m. These are then compared with close coupled calculations carried out with the best known interaction potentials. There is general agreement between the calculated and measured values of the broadening and shifting. Furthermore, the results illustrate that the decomposition of the shifts into parts, odd and even in m, is a powerful tool for separating out the relative contributions of the isotropic and anisotropic part of the interaction to the shifts and which part needs to be corrected if there is a discrepancy. Thus, shift measurements can be added to the list of experiments that may be used to determine reliable interaction potentials. The results also show, given a potential, that close coupled calculations are accurate and could be used to confirm or establish empirical models of the temperature dependence of the broadening or shifting, etc. Such modeling is important at atmospheric physics.

Spectroscopic, collisional, and thermodynamic properties of the He–CO2 complex from an ab initio potential: Theoretical predictions and confrontation with the experimental data

Symmetry-adapted perturbation theory has been applied to compute the intermolecular potential energy surface of the He–CO2 complex. The ab initio potential has a global minimum of em=−50.38 cm−1 at Rm=5.81 bohr for the “T”-shaped geometry of the complex, and a local one of em=−28.94 cm−1 at Rm=8.03 bohr for the linear He⋅⋅⋅O=C=O structure. The computed potential energy surface has been analytically fitted and used in converged variational calculations to generate bound rovibrational states of the He–CO2 complex and the infrared spectrum corresponding to the simultaneous excitation of the ν3 vibration and internal rotation in the CO2 subunit within the complex. The complex was shown to be a semirigid asymmetric top and the rovibrational energy levels could be classified with the asymmetric top quantum numbers. The computed frequencies of the infrared transitions in the ν4 band of the spectrum are in very good agreement with the high resolution experimental data of Weida et al. [J. Chem. Phys. 101, 8351 (19...

Inelastic collisions in para-H2: Translation-rotation state-to-state rate coefficients and cross sections at low temperature and energy

We report an experimental determination of the k(00-->02) rate coefficient for inelastic H(2):H(2) collisions in the temperature range from 2 to 110 K based on Raman spectroscopy data in supersonic expansions of para-H(2). For this purpose a more accurate method for inverting the master equation of rotational populations is presented. The procedure permits us to reduce the measured k(00-->02) rate coefficient to the corresponding sigma(00-->02) cross section in the range of precollisional energy from 360 to 600 cm(-1). Numerical calculations of sigma(00-->02) carried out in the frame of the coupled channel method are also reported for different intermolecular potentials of H(2). A good agreement is found between the experimental cross section and the numerical one derived from Diep and Johnsons potential [J. Chem. Phys. 112, 4465 (2000)].

Linewidths of C2H2 perturbed by H2: experiments and calculations from an ab initio potential

In this work we present a theoretical and experimental study of the acetylene-hydrogen system. A potential surface considering rigid monomers has been obtained by ab initio quantum chemistry methods. This 4-dimensional potential is further employed to compute, using the close-coupling approach and the coupled-states approximation, pressure broadening coefficients of C2H2 isotropic Raman Q lines over a temperature range of 77 to 2000 K. Experimental data for the acetylene nu2 Raman lines broadened by molecular hydrogen are obtained using stimulated Raman spectroscopy. The comparison of theoretical values with experimental data at 143 K is promising. Approximations to increase the computational efficiency are proposed.

Line‐by‐line measurements of interference parameters for the 0–1 and 0–2 bands of CO in He, and comparison with coupled‐states calculations

Here we present the first line‐by‐line measurements of interference parameters Y0k describing line‐mixing effects in the weak overlapping regime for He‐broadened CO lines in the 0–1 and 0–2 bands at 296 K. A detailed analysis of the line shape at intermediate perturber pressures (up to about 10 atm) has been performed, starting from previous theoretical calculations, which has demonstrated the possibility of an individual measurement of Yk parameters. The method is based on the existence of a component of the line shift, quadratic with the perturber pressure (density) and proportional to Y0k. Comparison of our measurements with results predicted from coupled‐states calculations shows good overall agreement. Linear pressure shifts have also been simultaneously measured. The uncertainty on the data is important since linear shifts are very small; however, some features can be considered as significant although we have no clear understanding of them. Possible explanations are discussed.

Raman and infrared linewidths of CO in Ar

We present measurements of Raman linewidths in the fundamental Q branch of CO for mixtures with Ar at temperatures of 77, 195, and 300 K, recorded using an inverse Raman spectrometer. Starting from a recent ab initio potential energy surface, theoretical values of Ar broadening coefficients for CO infrared and Raman lines (isotropic and anisotropic components) at temperatures in the range 77 to 1100 K are calculated via quantum-mechanical methods. The relative merits of the close coupling theoretical results over the coupled states results are underlined. Finally, a comparison of the calculated pressure broadening coefficients is made with the present experimental data as well as with recently available infrared data. There is general agreement between the calculated and measured values of the broadenings for all the temperatures probed. We conclude that the temperature dependence of the infrared and Raman broadening coefficients have been correctly determined theoretically and may be used to test a commo...

Experimental and theoretical study of line mixing in NH3 spectra. I. Scaling analysis of parallel bands perturbed by He

Line mixing effects have been studied in the ν2 and ν1 parallel bands of NH3 perturbed by He at room temperature. Experiments have been made with a Fourier transform spectrometer covering a wide range of total pressures up to about 400 atm. Analysis of the spectra demonstrates, for the first time, that the spectral shapes of entire ammonia bands can be significantly influenced by line mixing. A theoretical approach based on the energy corrected sudden approximation (ECS) is used to predict and analyze these effects. The model parameters include dynamical factors directly computed from an NH3–He potential energy surface and a scaling length which has been determined from a fit of line-broadening data. Comparisons with measurements show that the ECS model leads to surprisingly satisfactory predictions when considering the large spacing between rotational levels. The large effects of line mixing within the Q branches and in the far wing of the absorption bands are analyzed. It is shown that purely Lorentzian...

Experimental and theoretical CO2–Ar pressure-broadening cross sections and their temperature dependence

Line broadening coefficients of the CO2–Ar fundamental ν3 band are obtained experimentally and theoretically for temperatures from 120 to 765 K. The experimental results are obtained by Fourier-transform infrared spectroscopy. Theoretical values are provided ia quantum-mechanical (close coupling and coupled states) approaches as well as semiclassical (improved Smith–Giraud–Cooper and Robert–Bonamy) methods. The most up-to-date CO2–Ar ab initio intermolecular potential (J. M. Hutson, A. Ernesti, M. M. Law, C. F. Roche and R. J. Wheatley, J. Chem. Phys., 1996, 105, 9130) is employed for all calculations. For all the temperatures probed, theoretical values are found to be in a rather good agreement with experiment. In addition, the Raman Q(j) line broadening coefficients and the application of the random phase approximation are presented.