Experimental and theoretical study of line mixing in methane spectra. IV. Influence of the temperature and of the band

Line-mixing effects are studied in infrared bands of CH4 perturbed by N-2 a t various pressures. The effects of temperature are investigated in the nu( 3) region whereas spectral shapes of the nu(2), nu(4), and nu(3) bands are compared at room temperature. The theoretical approach proposed in precedin g papers is used in order to model and analyze the influence of collisions on the spectral shape. All model parameters are now fixed to values determi ned in the previous studies. Comparisons between measurements and spectra c omputed with and without the inclusion of line mixing are made. They show t hat our approach satisfactorily accounts for the effects of temperature, pr essure, and of rotational quantum numbers on the absorption by the nu(3) ba nd. Furthermore, the effects of collisions on spectra in the nu(4) region a t room temperature are also correctly calculated. On the other hand, the pr oposed approach fails in modeling the evolution with increasing pressure of absorption in the spectral range containing the nu(2) band. This result is attributed to the Coriolis coupling between the nu(2) and nu(4) vibrationa l states and to a contribution whose physical origin remains unclear. The l atter, which is negligible when CH4-He mixtures are considered, behaves as collision-induced absorption. (C) 2000 American Institute of Physics. [S002 1-9606(00)00736-4].