Collisionally induced population transfer effect in infrared absorption spectra. II. The wing of the Ar-broadened ν3 band of CO2

Abstract

The absorption beyond the ν3-band head of CO2 broadened by argon has been measured at room temperature. The absorption exhibits a strong sub-Lorentzian behavior (several orders of magnitude) resulting from collisionally induced line interferences which transfer intensity from this wing region to the ν3-band center. This wing absorption region implies detuning frequencies from resonances much larger than the reciprocal duration of collision. Consequently, finite duration of collisions in rotational energy transfers and initial correlations must be included in absorption calculation. A line-by-line coupling theory accounting for both these effects has been recently proposed [J. Chem. Phys. 89, 625 (1988)] and is applied here to a detailed study of the CO2–Ar collisional system. A convenient generalized detailed balance correction is introduced in this theory to overcome the limitation of the assumed resonant character of the energy transfer in the short time limit with respect to the thermal time ( βℏ)−1. The calculated absorption is in quantitative agreement with experiment. The origin and the nature of the empirical correcting factor currently used in similar studies are clearly established on a firm physical basis.