Semiclassical calculations of half-widths and line shifts for transitions in the 30012←00001 and 30013←00001 bands of CO ₂. III: Self collisions

This paper is the third in a series devoted to accurate semi-empirical calculations of pressure-broadened half-widths, pressure-induced line shifts, and the temperature dependence of the half-widths of carbon dioxide. In this work complex Robert-Bonamy (CRB) calculations were made for transitions in two of the Fermi-tetrad bands for self-collisions, i.e. the CO ₂-CO ₂ system. The intermolecular potential (IP) was adjusted to match measurements of the half-width, its temperature dependence, and the line shift. It is shown that small changes in the parameters describing the IP lead to noticeable changes in the line shape parameters and that it is possible to find a set of IP parameters, which, when used in the CRB formalism, yield half-widths, their temperature dependence, and line shifts in excellent agreement with measurement. This work demonstrates that this agreement can be obtained if the atom-atom potential is expanded to high order and rank (here 20 4 4), the real and imaginary (S ₁ and Im(S ₂)) components are retained, and the determination of the trajectories is made by solving Hamilton's equations. It was found that the temperature dependence of the half-width is sensitive to the range of temperatures used in the fit and that the vibrational dependence of the line shape parameters for these two bands is very small. Databases of the half-width, its temperature dependence, and the line shift for the atmospheres of Venus (296-700K fit range for the temperature exponents of the half-widths) and Mars (125-296K fit range for the temperature exponents of the half-widths) are provided. The calculations are compared with the measured data for the bands under study.