Far infrared spectroscopy and empirical intermolecular potential for α- and γ-N2 under pressure

The far infrared spectrum of solid N2 has been investigated under pressure in both the α and γ phases. Measurements of pressure shifts and integrated intensities have been performed and the Grüneisen parameters are derived. From experiments carried out on a thick crystal sample it is deduced firstly that librational modes have no measurable intensities and, secondly, that the Tu translational mode has a structure. A series of semiempirical calculations has been performed. Emphasis has been placed on the derivation of a unique intermolecular potential valid both in the α and γ phases which allows calculating with a sufficient accuracy a large number of physical quantities, i.e., translational and librational frequencies, Grüneisen parameters, cohesive energy, elastic constants, dispersion curves, and frequency shifts under pressure.

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