Line-mixing effects in the 3v₃ band of CO₂ perturbed by Ar

Measurements of absorption coefficients in the 3v₃ band of CO₂ at 1.44 μm perturbed by Ar up to 146 bar have been analyzed by using two line-mixing theoretical calculations within the impact approximation. In the first approach, the relaxation operator is treated semi-classically with adiabatic corrections. In the second, the relaxation operator is modelled with the Energy Corrected Sudden (ECS) approximation associated with a power fitting law providing the basic rotational state-to-state rates. Although the line-coupling spectroscopic cross-sections of the two models are significantly different, they both lead to satisfactory agreement with bandshapes at moderate densities (< 100 Amagat). Significant deviations between experimental and calculated spectra appear at higher densities. They are mainly attributed to the probable breakdown of the impact and binary collision approximations and to a number of reasons including an incorrect ECS calculation of the interbranch coupling, the nonlinear density dependence due to the finite volume of the molecules, and the neglect of the unknown imaginary part of the off-diagonal elements in the calculated relaxation matrix.