Line-Mixing Effects in Q Branches of CO₂: I. Influence of Parity in Δ ↔ Π Bands

A theoretical model based on the energy corrected sudden (ECS) approximation is used in order to account for line- mixing effects in Δ ↔ Π infrared Q branches of ¹²C¹⁶O₂. Its quality is demonstrated by comparisons with numerous laboratory spectra of CO₂-He and CO₂-N₂ mixtures: three Q branches in the 4 and 17 μm regions are investigated at room temperature in a wide pressure range. The influence of mixing between Q(J) lines associated with odd and even values of the rotational quantum number J is demonstrated and analyzed in detail. It is shown that, in contrast to available fitting law approaches, the ECS model correctly predicts the influence of the parity of the rotational quantum numbers J and J′ on coupling between the Q(J) and Q(J′) lines. Comparisons between the effects of collisions of CO₂ with N₂ and He are made and analyzed. They show that these two systems involve different line couplings within the Q branch.