Temperature, pressure, and perturber dependencies of line-mixing effects in CO₂ infrared spectra. I. Σ←Π Q branches

Experimental and theoretical results on the influence of line mixing on the shape of infrared CO₂ Q branches of importance for atmospheric applications are presented. Two Q branches of Σ←Π symmetry, which lie near 618 and 720 cm^-1 and belong to the 10⁰0_II←01¹0_I and 10⁰0_I←01¹0_I bands, have been studied for many conditions of temperature (200-300 K), total pressure (0.5-10 atm), and mixture (with He, Ar, O₂, and N₂). The theoretical approach used is based on the Energy Corrected Sudden approximation; its parameters have been deduced from both line-broadening data and measured absorption by the Q branches. Comparisons between experimental and computed spectra demonstrate the quality of the model, regardless of the conditions. Detailed analysis of the influences of the Q-lines spectral spacing, temperature, total pressure, and collision partner are presented. They show that significantly larger line-mixing effects are obtained when CO₂-He is considered with respect to CO₂-(Ar,O₂,N₂). This is analyzed in terms of the relative contributions of the short- and midrange interaction forces and of propensity rules resulting from the coupling of angular momenta.