Broadening of Vibrational Raman Spectra by Concentration Fluctuations: A Molecular Dynamics Survey

Molecular dynamics calculations have been performed on model systems for nitrogen and mixtures of nitrogen with other compounds at high pressure and at ambient temperatures. From these simulations the line shape of the Raman Q-branch was calculated. A short description of the applied methods is given. Our aim is to investigate whether line broadening, experimentally observed in mixtures, may be at least partially caused by critical fluctuations, as appeared to be the case in nitrogen helium mixtures. For this purpose it is necessary to investigate first the behavior of a noncritical mixture. We analyzed extensively the results of the mixture of nitrogen in neon at 2.4 GPa and 296 K, which is far away from a critical state. In this system a maximum in the linewidth is seen at equal volume fractions, experimentally as well as in simulations. The details revealed by the simulations allow a comparison with existing models, such as that given by Knapp and Fischer. It is seen that assumptions made in that theory are in clear contradiction with the findings of the simulations.