The energy corrected sudden approach is used in order to deduce collisional parameters and to model infrared quantities in Pi <-- Sigma bands of CO2-He and CO2-Ar mixtures in the 200-300 K temperature range. Measured line-broadening coefficients and absorption in the Q-branch of the nu(2) band at moderate pressure are first used for the determination (from a fit) of the time constant associated with the relaxation of the second order traceless tensor of the rotational angular momentum (all other collisional quantities have been determined previously). The results obtained are consistent with previous (calculated) temperature dependent values of the depolarized Rayleigh cross sections. The model is then successfully tested through computations of absorption in the nu(2) and (nu(1) + nu(2))(I) bands at elevated densities. Analysis of line-mixing effects is made, including study of the influence of interbranch transfers and of Coriolis coupling. Differences between the effects of collisions with He and Ar are pointed out and explained.