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Most of the all-quantum-mechanical calculations of line shape relaxation cross sections that have been previously published were based on the “impact approximation” allowing us to express off-diagonal elements in terms of collision theory S-matrices. Two consequences of that approximation are pointed out. The first is that impact cross sections do not obey detailed balance except in limited cases. The second, which is shown of more severe consequence for line coupling applications, is the loss of a sum rule relating the off-diagonal elements (coupling a given line to all the others) to the diagonal one. Finally, it appears clearly that off-the-energy shell calculations with realistic potential surfaces are absolutely necessary for ab initio computing line shapes of overlapping lines.

Quantum coupled states (CS) and semiclassical Robert–Bonamy (RB) calculations of pure H 2 broadening and shifting coefficients of Q ( j ) rovibrational lines have been performed using a simple atom–atom potential energy surface (PES). The main interest is to test the validity of the semiclassical method versus the CS calculation. This is the first comparison of the two methods for a diatom–diatom system for which rotation–rotation (R–R) energy transfers contribute to the broadening and shifting coefficients. Agreement between the two methods is found acceptable on the total result, but is excellent when resonant or quasi-resonant rotational energy transfer do not occur. Comparison with experiment confirms the empirical laws used to model the temperature dependence of the broadening and shifting coefficients. It also permits us to analyze through the semiclassical model, the weakness and advantages of the atom–atom PES used.

International audience; In order to improve the accuracy of H2 CARS thermometry, H2 Q-branch CARS spectra have been recorded for various H2-N2 mixtures in a high-pressure cell at different pressures and temperatures (up to 40 bar and 875 K). Due to the low spectral resolution of broadband CARS experiments, the relevant spectral lineshape factor is the linewidth ratio G(Q(3))/G(Q(1)), since Q(1) and Q(3) are the most intense lines of the Q-branch spectrum in this temperature range. For the first time, the speed-inhomogeneous effects are accounted for in the simulation of the CARS profiles. The evaluated temperatures are in good agreement with reference values obtained by thermocouples. The specific role on the accuracy of H2 CARS thermometry of the speed inhomogeneity is carefully analyzed, in connection with the influence of the nitrogen concentration.

The test of an interaction potential for HF-argon recently determined from the spectroscopy of the Van der Waals complex requires accurate experimental widths and shifts cross sections. Since inconsistencies existed between previous measurements, collisional lineshapes were studied at T = 296 K by high-resolution Fourier transform spectroscopy for the 0-0, 0-1, and 0-2 bands. The accurate close-coupling molecular scattering calculations of Green and Hutson (1994. J. Chem. Phys. 100, 891) were extended to a complementary set of kinetic energies in order to check the thermal averaging procedure. Agreement between measured and computed room temperature cross sections is excellent for the three vibrational bands. This confirms the accuracy of the HF-argon surface and particularly its dependence on the HF vibrational level, which is sensitively probed both by the widths and by the shifts cross sections at high j(i) values. Copyright 1999 Academic Press.