Lineshapes in collision-induced absorption

Abstract

We present the classical and quantum versions of collision induced absorption spectra for three exactly solvable 1-D models, and obtain the asymptotic behaviour at high frequency. It is found that the quantum spectra fall off more slowly than the classical, and for realistic systems, they both fall off more slowly than exponential. An improved version of the Landau-Lifshitz WKB method is suggested as a numerically viable method for calculating the high frequency wing in quantum spectra and is tested on the exactly solvable models. The error is found to be negligible in two models and tolerable in the third model, even for the least classical system. Our discussion of an extension of the method using exact wavefunctions increases its scope beyond the collision induced absorption problem.