Abstract More than two decades of inelastic X-ray scattering (IXS) studies on noble gases and alkali metals are reviewed to illustrate the advances they prompted in our understanding of the terahertz dynamics of simplest systems. The various literature results outline a remarkably coherent picture of common and distinctive behaviours of liquids and their crystalline counterparts. Furthermore, they draw a consistent and comprehensive picture of the evolution of collective modes at the crossover between the hydrodynamic and the single particle regime, their coupling with fast (sub-ps) relaxation processes and their gradual disappearance upon approaching microscopic scales. The gradual transition of the spectrum towards the single particle limit along with its coupling with collisional relaxations will be discussed in some detail. Finally, less understood emerging topics will be discussed as the occurrence of polyamorphic crossovers, the onset of non-hydrodynamic modes and quantum effects on the spectrum, as well as recent IXS results challenging our vision of the supercritical phase as an intrinsically homogeneous thermodynamic domain.

中文翻译：

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通过非弹性 X 射线散射探测的最简单流体的太赫兹动力学

摘要 回顾了二十多年对惰性气体和碱金属的非弹性 X 射线散射 (IXS) 研究，以说明它们在我们理解最简单系统的太赫兹动力学方面取得的进展。各种文献结果概述了液体及其结晶对应物的常见和独特行为的非常连贯的图片。此外，他们描绘了在流体动力学和单粒子状态之间的交叉处集体模式的演变、它们与快速（亚 ps）弛豫过程的耦合以及它们在接近微观尺度时逐渐消失的一致和全面的图景。将详细讨论光谱向单粒子极限的逐渐转变及其与碰撞弛豫的耦合。最后，