Far-infrared absorption in nitrogen-rare gas compressed mixtures

Abstract

The far-infrared absorption spectra of N₂-Xe and N₂-He mixtures, respectively, have been measured at room temperature and for different densities and concentrations. By considering that the quadrupolar induction mechanism dominates the long range dipole, whereas the isotropic overlap mechanism dominates the short range part, the absorption spectrum is expressed in terms of many-body translational correlation functions modulated, in the case of the quadrupolar induction, by the free rotation of the nitrogen molecules. In N₂-Xe mixtures, the high polarizability of the Xe atom ensures that the quadrupolar induction is mostly responsible for the absorption intensity. Moreover, a line shape analysis based on the Zwanzig-Mori theory predicts strong cancellation in the translational spectrum when the density increases. In N₂-He mixtures it is shown that the overlap induction governs the overall spectral shape when the concentration of N₂ is sufficiently small (∼1%). In such mixtures, the spectral density is characterized by a low frequency dip quite similar to the one observed in rare gas mixtures and in the fundamental vibration band of H₂-He mixtures. According to a recent theory [10], this dip is interpreted as the result of a dynamical cancellation effect between 2-, 3- and 4-body time correlation functions associated with overlap induced dipoles.