LONGITUDINAL DYNAMICS IN A LENNARD-JONES BINARY LIQUID: CROSSOVER FROM HYDRODYNAMICS TO THE MOLECULAR REGIME

Longitudinal collective dynamics of an equimolar Lennard-Jones KrAr mixture is studied in detail in a wide range of spatial and time scales. Combining both the molecular dynamics simulations and analytical generalized collective mode approach, we calculated the spectrum of generalized collective excitations and analyzed the dominant dynamic processes that determine the main contributions to time correlation functions in different regions - starting from the hydrodynamic limit and up to the range of the so-called molecular regime. The origin of collective propagating modes as well as the specific features of their dispersion laws within and beyond the hydrodynamic region are established. It is shown that the structural relaxation and processes, connected with mutual diffusion of particles, determine mainly the central peak of total dynamic structure factor beyond the hydrodynamic region. The dispersion laws, obtained for the propagating modes in our analytical approach, are compared with the dispersion curves, estimated from the maxima positions of partial current spectral functions. The difference in these two sets of numerical results is discussed