Line-Mixing Effects in Q Branches of CO₂: II. the Particular Case of Parallel Bands

A theoretical model based on the energy corrected sudden (ECS) approximation is used to account for line-mixing effects in infrared Q branches of parallel bands of CO₂. Measurements have been made using a tunable difference frequency spectrometer system in a Π-Π transition: spectra of the 11¹I_Π ← 01¹0₁ Q branch near 3580.3 cm^-1 have been recorded at room temperature for CO₂-He and CO₂-N₂ mixtures in the 0.1-1 atm total pressure range. Comparisons of experimental and computed spectra prove the quality of the ECS approach. It is shown that the influence of line mixing on the shape of parallel Q branches is very weak and much smaller than in perpendicular transitions. This result is analyzed in detail and explained in terms of the effects of the vibrational angular momenta on the dipole transition moments and vector coupling coefficients involved in the relaxation operator elements. The effects of collisions of CO2 with N₂ and He are compared and discussed.