Collective dynamics in binary liquids: spectra dependence on mass ratio

We discuss the spectrum of longitudinal propagating collective excitations in a liquid metallic alloy Li4Pb and a Lennard-Jones binary mixture obtained by an eight-variable approach of generalized collective modes. Reported wavenumber-dependent amplitudes of contributions from high- and low-frequency propagating excitations to dynamical structure factors in Li4Pb permit a new viewpoint on the 'fast sound' phenomenon. A three-variable analytical model for slow and fast mass-concentration fluctuations is used for explanation of the high-frequency branch in the long-wavelength region. An additional analysis of longitudinal dynamics in liquid binary mixtures with different mass ratio of components permits us to establish a tendency in frequency and damping coefficients of the high-frequency propagating excitations