Vibron and lattice frequency shifts in the Raman spectra of solid α-N₂ and γ-N₂ and librational force constants of diatomic molecular crystals

Abstract

The observed pressure dependence of shifts and splittings of the Raman active vibrational modes in α-N₂ and γ-N₂ has been successfully reproduced in a calculation using the recent vibron model of Zumofen and Dressler. Different forms of dispersive and repulsive interactions as well as the variations of the potential parameters with bond length are discussed. At the phase transition, the observed drop in the frequency is correctly predicted by softer multiparametric exponential repulsive potential of Raich and Gillis. The observed negative D shift at 1 bar resulting mainly from the large isotropic dispersive interaction indicates that crystal forces acting on the molecule are stretching and not compressive as deduced from most intermolecular potentials. The origin of the large contribution to the librational force constants by these forces is examined and is related to the proper functional dependence of the potential parameters with bond length, generally neglected in lattice dynamic calculations. The observed Gruneisen parameters of librational modes demonstrate the necessity of including quadrupole interactions.