Line-mixing effects have been studied in the 3 band of CH 4 perturbed by N 2 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 Ϫ1 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 pressure and of rotational quantum numbers on the spectral shape under conditions where modifications introduced by line mixing are important. For high rotational quantum number lines, the main features induced by collisions are predicted but some discrepancies remain; the latter may be due to improper line-coupling elements but there is strong evidence that the use of inaccurate line broadening parameters also contributes to errors in calculated spectra.

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[1] A careful comparison of four sets of ozone line intensities measured independently in the 10 mm region has been achieved. It is demonstrated that three of the experimental sets agree to better than 2% whereas the last one is consistently 4.4% higher. This is also the case for the ozone line intensities given in the HITRAN database, which are about 4% higher than the values given in the three experimental sets in agreement. New and more accurate transition moment constants for the n 1 and n 3 bands of 16 O 3 were derived and used to generate new line positions and intensities. These new spectroscopic parameters allow one to simulate atmospheric spectra recorded by the Atmospheric Trace Molecule Spectroscopy Experiment better than the existing HITRAN spectroscopic parameters, showing that on a relative basis the new spectral parameters are of better quality.

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