Semi‐Classical Line‐shape Theory

This chapter focuses on pressure-broadening theories that can be applied to the collision systems found in many atmospheres: CO 2 -CO 2 , H 2 O-N 2 , CO 2 -H 2 O, CH 4 -O 2 , and so on. ACT theory was used in many early studies that were determining half-widths for collision systems important in the planetary atmospheres and other collision systems. Semi-classical designates that the internal structure of the colliding molecules is treated via quantum mechanics and the dynamics of the collision process are determined via classical mechanics. The early ATC calculations used straight line trajectories to determine the resonance functions. The CRBM formalism is capable of determining the temperature and speed dependence of the half-width and line shift with small uncertainties. It is well understood that the shifts of spectral lines are strongly dependent on the vibrational states involved in the transition. The approximations made in the past increase the uncertainty beyond the needs of the spectroscopic community.