Collisional line broadening in the atmosphere of light particles: problems and solutions in the framework of semiclassical treatment

For the case of perturbation by light particles, the computation of spectral line parameters by traditional semiclassical approaches reveals the problem of translational energy conservation. The rototranslational energy balance is, however, easily maintained within classical molecular dynamics. The line broadening of two 'problematic' systems, C2H2-He and CO2-He, is addressed from these two different points of view. In the framework of a traditional semiclassical approach, the non-validity of the translational energy conservation hypothesis leads to a significant overestimation of the linebroadening for both systems. This overestimation is, however, substantially reduced for the ensemble Of C2H-He and CO2-He linewidths when a non-perturbative completely classical method is employed. Used with caution, this latter approach therefore seems thus to be more appropriate for linewidth computations in the case of molecular systems of the type 'heavy radiator + light perturber'. Copyright (C) 2005 John Wiley Sons, Ltd.