The vibrational frequency of nitrogen near the fluid–solid transition in the pure substance and in mixtures

Abstract

At high densities intramolecular vibrations are strongly dependent on the interactions with the surrounding molecules. In this paper a study is made of the consequences of these interactions on the Raman Q-branch of nitrogen. In particular the difference between a disordered and an ordered surrounding is surveyed. For this purpose, high-resolution Raman spectroscopy has been performed at room temperature on pure nitrogen as well as on a dilute mixture of nitrogen in argon, around the fluid–solid phase transition of these systems, which occur at ≈2.5 GPa and at ≈ 1.3 GPa, respectively. Going from the liquid to the solid phase, a positive jump in the line shift and a dramatical drop in the linewidth are seen in both systems at the transition pressure. For a better understanding of the underlying mechanisms, molecular dynamical simulations have been performed on corresponding model systems. The results of these calculations are in fair agreement with the experimental data and reveal the reasons for the discontinuities. Although the average distance of the nearest neighbor molecules around the nitrogen molecule increases, the distance to the nearest neighbor molecules in line with the molecular axis of the nitrogen decrease at the phase transition. This results in a positive jump in the frequency. Further, the time-autocorrelation function of the vibration frequency has a long persisting positive tail in the fluid phase. This behavior is absent in the solid phase. Even more important is that this function has negative values during a substantial time interval in the solid phase. As a result, the correlation time is greatly reduced at the phase transition, which results in an important reduction of the linewidth as well. Finally, it is proven that also in the solid phase the nitrogen is really dissolved in argon.