Theoretical study of the far infrared absorption spectrum of dense nitrogen

The far infrared absorption spectrum of dense nitrogen is calculated by assuming that the induction mechanism is quadrupolar and that the intermolecular potential is predominantly isotropic. The profile is then described as the convolution of a translational spectrum and a rotational spectrum. The latter expresses the free rotations of the molecules, whereas the former, calculated from the Zwanzig–Mori theory of Brownian motion, is characterized by two translational modes, a diffusive low frequency mode and an oscillatory high frequency mode. The translational spectrum is found to be responsible for the density dependence of the absorption spectrum, contributing significantly to the profile at high densities and for all frequencies. The parameters of the theory are analytically given and the influence on the profile of a small anisotropy in the intermolecular potential is estimated by a perturbation approach.