THE VIBRATIONAL FREQUENCY OF NITROGEN NEAR THE FLUID-SOLID TRANSITIONIN THE PURE SUBSTANCE AND IN MIXTURES

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 e-bran ch of nitrogen. In particular the difference between a disordered and an ordered surroundings is surveyed. For this purpose, high-resolution Raman spectroscopy has been performed at room temperature on pure nit rogen as well as on a dilute mixture of nitrogen in argon, around the fluid-solid phase transition of these systems, which occur at approxim ate to 2.5 GPa and at approximate to 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 tra nsition pressure. For a better understanding of the underlying mechani sms, molecular dynamical simulations have been performed on correspond ing model systems. The results of these calculations are in fair agree ment with the experimental data and reveal, the reasons for the discon tinuities. Although the average distance of the nearest neighbor molec ules around the nitrogen molecule increases, the distance to the neare st neighbor molecules in line with the molecular axis of the nitrogen decrease at the phase transition. This results in a positive jump in t he frequency. Further, the time-autocorrelation function of the vibrat ion frequency has a long persisting positive tail in the fluid phase. This behavior is absent in the solid phase. Even more important is tha t this function has negative values during a substantial time interval in the solid phase. As a result, the correlation time is greatly redu ced at the phase transition, which results in an important reduction o f the linewidth as well. Finally, it is proven that also in the solid phase the nitrogen is really dissolved in argon. (C) 1998 American Ins titute of Physics.