The Spectrum of Density Fluctuations of Noble Gases Probed by THz Neutron and X-ray Spectroscopy

Approximately 50 years of inelastic scattering studies of noble gases are reviewed to illustrate the main advances achieved in the understanding of the THz dynamics of simple systems. The gradual departure of the spectral shape from the hydrodynamic regime is discussed with an emphasis on the phenomenology of fast (sub-ps) relaxation processes. This review shows that relaxation phenomena in noble gases have an essentially collisional origin, which is also revealed by the parallelism between their characteristic timescale and the interatomic collision time. Additionally, recent THz spectroscopy results on noble gases at extreme thermodynamic conditions are discussed to illustrate the need for a revision of our current understanding of the supercritical phase.