Use of generalized Langevin theory to describe far infrared absorptions in non-dipolar liquids

The Mori continued fraction representation of the Kubo response function, truncated at first order, generates a spectral function which is successful in describing absorptions of non-dipolar liquids in the high microwave and far infrared regions (1–250 cm–1). There is some evidence that the equilibrium averages [Ko(0) and K1(0)] inherent in this representation are both intermolecular properties, in contrast to the case of pure dipolar absorption, where Ko(0) is a single molecule property. The correlation function of the derived spectral function is compared and contrasted with that of the extended diffusion model of Gordon.The experimental measurements are also compared with a gas phase model of bimolecular collision-induced absorption, and a “cell” model of the liquid state due to Litovitz and co-workers, both of which are less satisfactory than the generalised Langevin equation.