Raman spectrum of solid nitrogen at high pressures and low temperatures

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Abstract
The Raman spectrum of solid nitrogen has been studied at high pressures and low temperatures using a method of sample preparation that allows the separation of effects due to change of molar volume and of temperature, respectively. Two lines have been observed in the lattice region of the γ phase which are identified as Eg and B1g librational modes on the basis of frequency and relative intensity calculations. An asymmetrical line has been observed in the stretching region of this phase. In the α phase, the measured Grüneisen parameters indicate that neither the quadrupolar nor the 6–12 atom–atom interaction potential has the correct volume dependence. The temperature dependence of the frequency and linewidth of the Eg librational mode is proposed to be due to libron–phonon interactions. Two very broad Raman lines are observed in the lattice region of the β phase. The low- and high-frequency lines are identified with translational and librational modes, respectively. The observations are consistent with a precessing molecule model for the β phase.