Theoretical study of collision-induced far-infrared absorption of dense rare-gas mixtures

A theory investigating collision-induced absorption in dense rare-gas mixtures is presented. The basic assumption of the theory is that the collision-induced dipole moment of a pair of dissimilar atoms is proportional to the force acting between them. It is then shown that the relevant dynamical variable is the interdiffusion current fluctuation, $\overrightarrow{\mathrm{J}}=(1-x)\mathrm{}\Sigma i^{N_A}{\overrightarrow{\mathrm{v}}}_i-x\Sigma {i^{\prime }}^{N_B}{\overrightarrow{\mathrm{v}}}_{i^{\prime }}$, in the fluid mixture. The correlation function and absorption spectrum are generated by the Zwanzig-Mori theory of Brownian motion. Two modes are theoretically predicted. A diffusive mode, due to the mutual diffusion of rare-gas atoms, produces a low-frequency dip in the line shape whereas an oscillatory mode, due to the oscillations of atoms in the local structure of the fluid, generates a caging spike in the wing of the spectrum. Spectral behavior is shown to depend on density, concentration, and atomic masses of the two components of the mixture and is discussed in detail.