Density effects and relative diffusion in the far infrared absorption spectrum of compressed liquid nitrogen

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Abstract
New and extensive experimental data on the far infrared (F.I.R.) absorption spectrum of compressed liquid nitrogen are reported. By applying a hydrostatic pressure (1 bar ≲P ≲ 1·7 kbar) on the fluid it has been possible to investigate almost the complete liquid range, namely: 66K: 685 amagats; 87·3 K: 610, 631·5, 665, 689, 715·5, 742 amagats; 122K: 405, 481·5, 543·5, 605·5, 666, 729 amagats. The reduced spectral densities and also the three first even spectral moments were deduced from the absorption profiles. In order to compare these results with theoretical treatments available in the literature, we have evaluated the spectral moments M 0, M 2 and M 4 by assuming the decoupling between rotation and translation within the frame-work of the quadrupolar induction mechanism (Q.I.D.). Moreover, the density dependence was evaluated according to a lattice gas model while the quantum corrections were implemented into the calculation. The comparison between experimental and theoretical values shows that the decoupling approximation fails to reproduce quantitatively the density evolution of the spectral moments for densities higher than 650 amagats as long as the translational contributions are evaluated with an isotropic pair potential. On the other hand, at lower densities the model calculation gives a practical method to estimate the spectral moments. Following the suggestion made in a recent computer simulation by Steele [8] that the product approximation (or decoupling approximation) enables quantitative reproduction of the overall spectral shape, we have deconvoluted our experimental spectra with the help of spectral functions describing either free rotation or exhibiting hindered rotation (the latter has been used only for one state point). The resulting translational spectra have been treated in such a way as to evaluate the relative (or pair) diffusion coefficient D r as indicated by the theoretical work of Guillot and Birnbaum [2]. This method allows us to report in the present paper, and for the first time, experimental values for this quantity. These are compared to a recent computer investigation due to Hoheisel and Zeidler [12] on pair dynamics in atomic liquids.