Vibrational frequency shifts in dense molecular fluids

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Abstract
A theory is presented for calculating the bond length and vibrational frequency shifts in dense molecular fluids. We combine equation-of-state calculations based on the effective spherical potentials of Shaw, Johnson, and co-workers with a simple model for the interactions of a molecule with the surrounding fluid. The repulsive interactions are approximated with the hard-sphere model of Schweizer and Chandler. We extend their model by including an improved treatment of the long-range attractive interactions and the centrifugal forces. We apply this model to fluid nitrogen and compare with static and shock wave experimental frequency shifts, as well as computer simulation data, over a wide range of pressures and temperatures. We find that while the results using this simple model are reasonably close to experiment in the region appropriate to shock conditions, agreement with data taken under static high-pressure conditions (at lower temperatures) is not as good. Inadequacies of the model and possible improvements are discussed.