Statistical mechanics of dense ionized matter. VIII. Dynamical properties of binary ionic mixtures

Molecular-dynamics calculations are reported for binary mixtures of ions of like charge in a uniform neutralizing background. For a system simulating the mixture ${\mathrm{H}}^{+}$-${\mathrm{He}}^{2+}$ the calculations show that the spectrum of charge fluctuations is dominated by a propagating plasmon mode with a lifetime which is finite even in the long-wavelength limit. At high charge densities there is an additional low-frequency contribution to the spectrum; this is shown to arise from a coupling between charge fluctuations and fluctuations in mass density. The results are in satisfactory agreement with a simple hydrodynamic calculation, though it appears that in practice the hydrodynamic limit is never fully attained; it is also found that a memory-function approach incorporating the coupling of mass and charge gives a good description of the collective dynamical modes at high charge densities.