Calculation of Pressure Broadened Spectral Line Shapes Including Collisional Transfer of Intensity

Abstract

Spectral line shapes, including widths and shifts of isolated lines and collisional transfer of intensity among overlapping lines, can be described within the impact approximation by generalized (kinetic theory) collision cross sections. Theoretical determination of line shapes thus requires accurate molecular scattering calculations to obtain collisional S-matrices and this, in turn, requires detailed knowledge of the intermolecular forces. Advances in computational abilities now permit rather accurate calculations, at least for some simple systems. This is illustrated by a review of recent studies of broadening and shifting of isolated lines of CO and D2 in He and of HCl in Ar. Line-coupling cross sections, which describe collisional transfer of intensity, have received much less attention although they are quite important for several practical applications and are no more difficult to compute. Recent work for CO in He demonstrates the ability to obtain an accurate theoretical description for both “micro-windows” in the infrared fundamental band and for the Raman Q-branch.