Collisionally induced population transfer effect in infrared absorption spectra. I. A line-by-line coupling theory from resonances to the far wings

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Abstract
The line-by-line coupling for a pressure broadened rovibrational band is formulated in the far-wing limit. The present quasistatic theory assumes that the wave frequency is displaced from the line centers by an amount that is large compared with both the reciprocal duration of a typcial binary collision and frequency separation between strongly coupled lines. This theory generalizes that of Rosenkranz [J. Chem. Phys. 83, 6139 (1985)] where the Fano’s relaxation operator was reduced to a scalar parameter through a band average. The present approach permits computation of far-wing absorption more specifically tailored to individual lines. Such a line-by-line approach is needed for rovibrational bands where some far lines contribute significantly to the total absorption. In order to obtain a qualitative picture of the line coupling as a function of the frequency displacement, calculations for collisions of CO2 with Ar have been performed for some lines. The results are compared with the corresponding variation of the diagonal relaxation matrix element.