The coherent dynamic structure factor $(Q, co) of liquid Cs has been measured by inelastic neutron scattering near the melting point at 308 K. Using triple-axis spectrometers at the Institut Laue-Langevin in Grenoble and at the Forschungs-Reaktor Munchen the scattering law was determined for energy transfers %co from -2 to 10 meV and for momentum transfers AQ between 0.2 and 2.55 A . The measurement has been corrected for all significant effects, including multiple and incoherent scattering as well as resolution broadening. In this paper we present mainly experimental results including a table of the measured scattering law. The analysis of the dispersion relation and the full width at half maximum of the longitudinal current correlation function J,(Q,co) reveals an anomalous dispersion due to shear relaxation in the liquid. In the vicinity of the structure-factor maximum the measured half-width of the coherent central peak of $(Q, co) confirms recent theoretical assumptions of a collective-diffusion-like structural relaxation process in dense liquids near the melting point. PACS number(s): 62.15.+i, 67.40.Fd

This paper gives a survey of the particle dynamics in the liquid alkali metals observed with inelastic x-ray and neutron scattering experiments. Liquid rubidium and sodium are chosen as model fluids to represent the behaviour of this group of fluids. In the dense metallic monatomic melt the microscopic dynamics is characterized by collective excitations similar to those in the corresponding solids. The collective particle behaviour is appropriately described using a memory function formalism with two relaxation channels for the density correlation. A similar behaviour is found for the single particle motion where again two relaxation mechanisms are needed to accurately reproduce the experimental findings. Special emphasis is given to the density dependence of the particle dynamics. An interesting issue in liquid metals is the metal to non-metal transition, which is observed if the fluid is sufficiently expanded with increasing temperature and pressure. This causes distinct variations in the interparticle interactions, which feed back onto the motional behaviour. The associated variations in structure and dynamics are reflected in the shape of the scattering laws. The experimentally observed features are discussed and compared with simple models and with the results from computer simulations.

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Although the collective dynamics of liquid alkali metals are rather well understood near the melting point, there is not much research on these dynamics at temperatures higher than the melting temperature. We carried out a set of neutron scattering experiments on liquid rubidium, an alkali metal, to study the effects of temperature on the collective dynamics. In particular, we determined the dispersion relations at different temperatures from the current correlation spectra functions and from the evolution in the momentum transfer, Q, of the longitudinal phase velocity. We also performed a set of molecular dynamics simulations for each temperature sampled in the experimental data. These numerical results confirm the neutron scattering findings. The most striking feature is a frequency change at a momentum transfer of about Q Ϸ 0.45 Å −1 with rising temperature. The amount of change exceeds predictions from generalized hydrodynamics based on global density changes. The frequency softening with increasing temperature might be related to local structural changes on length scales of this momentum transfer.