Microscopic dynamics of molecular liquids and glasses: Role of orientations and translation-rotation coupling

We investigate the dynamics of a fluid of dipolar hard spheres in its liquid and glassy phases, with emphasis on the microscopic time or frequency regime. This system shows rather different glass transition scenarios related to its rich equilibrium behavior, which ranges from a simple hard sphere fluid to long range ferroelectric orientational order. In the liquid phase close to the ideal glass transition line and in the glassy regime a medium range orientational order occurs leading to a softening of an orientational mode. To investigate the role of this mode we use the molecular mode-coupling equations to calculate the spectra φₗₘ^''(q,ω) and χₗₘ^''(q,ω). In the center of mass spectra φ₀₀^''(q,ω) and χ₀₀^''(q,ω) we found, besides a high frequency peak at ωhf, a peak at ωₒₚ, about one decade below ωhf. ωₒₚ has almost no q dependence and exhibits an ``isotope'' effect ωₒₚ∝I^-1/2, with I the moment of inertia. We give evidence that the existence of this peak is related to the occurrence of medium range orientational order. It is shown that some of these features also exist for schematic mode coupling models.