Abstract
Correlation functions and spectral densities associated with the collision induced absorption of dense rare gas mixtures are generated by the Zwanzig-Mori theory. Two modes are theoretically predicted. A diffusive mode, due to the mutual diffusion of rare gas atoms, creates a low frequency dip in the line shape. An oscillatory mode, due to the oscillations of atoms in the fluid mixture, generates a caging spike in the wing of the spectrum.