Experimental and theoretical study of line mixing in methane spectra. I. The N-2-broadened nu(3) band at room temperature

Line-mixing effects have been studied in the nu(3) band of CH4 perturbed by N-2 at room temperature. New measurements have been made and a model is pr oposed which is not, for the first time, strictly empirical. Three differen t 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 influen ce 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 semiclas sical approach. The line-coupling relaxation matrix is constructed from the se data and two additional parameters which are fitted on measured absorpti on. Comparisons between measurements and spectra computed accounting for an d neglecting line mixing are made. They prove the quality of the approach w hich satisfactory accounts for the effects of pressure and of rotational qu antum numbers on the spectral shape under conditions where modifications in troduced by line mixing are important. For high rotational quantum number l ines, the main features induced by collisions are predicted but some discre pancies remain; the latter may be due to improper line-coupling elements bu t there is strong evidence that the use of inaccurate line broadening param eters also contributes to errors in calculated spectra. (C) 1999 American I nstitute of Physics. [S0021-9606(99)00116-6].