Spectra calculations in central and wing regions of CO₂ IR bands between 10 and 20μm. I: Model and laboratory measurements

Temperature (200-300 K) and pressure (70-200 atm) dependent laboratory measurements of infrared transmission by CO₂-N₂ mixtures have been made. From these experiments the absorption coefficient is reconstructed, over a range of several orders of magnitude, between 600 and 1000 cm^-1. The elevated densities used in the experiments (up to 200 atm) magnify the contribution of the wings of the ν₂ band lines. In order to analyze the spectra, a theoretical model based on the energy corrected sudden approximation is proposed which accounts for line-mixing effects within the impact approximation. This approach uses the model and associated parameters built previously to model Q branches (JQSRT 1999;61:153) but extends it by now including all P, Q, and R lines. No adjustable parameters are used and fundamental properties of the collisional relaxation operator are verified by using a renormalization procedure. Comparisons between measured and calculated spectra confirm that neglecting line-mixing (Lorentzian model) leads to an overestimation of absorption by up to three orders of magnitude in the far wings. On the other hand, the proposed approach leads to satisfactory results both in regions dominated by contributions of local lines and in the wing: measured spectra are correctly modeled over a range where absorption varies by more than four orders of magnitude. The largest discrepancies, which appear about 150 cm^-1 from the ν₂ center, can be due to finite duration of collisions effects or to uncertainties in the experimental determination of very weak absorption.