Since the discovery of electron-wave duality, electron scattering instrumentation has developed into a powerful array of techniques for revealing the atomic structure of matter. Beyond detecting local lattice variations in equilibrium structures with the highest possible spatial resolution, recent research efforts have been directed toward the long-sought-after dream of visualizing the dynamic evolution of matter in real time. The atomic behavior at ultrafast timescales carries critical information on phase transition and chemical reaction dynamics, the coupling of electronic and nuclear degrees of freedom in materials and molecules, and the correlation among structure, function, and previously hidden metastable or nonequilibrium states of matter. Ultrafast electron pulses play an essential role in this scientific endeavor, and their generation has been facilitated by rapid technical advances in both ultrafast laser and particle accelerator technologies. This review presents a summary of the noteworthy developments in this field in the last few decades. The physics and technology of ultrafast electron beams is presented with an emphasis on the figures of merit most relevant for ultrafast electron diffraction experiments. Recent developments in the generation, manipulation, and characterization of ultrashort electron beams aimed at improving the combined spatiotemporal resolution of these measurements are discussed. The fundamentals of electron scattering from atomic matter and the theoretical frameworks for retrieving dynamic structural information from solid-state and gas-phase samples is described. Essential experimental techniques and several landmark works that have applied these approaches are also highlighted to demonstrate the widening applicability of these methods. Ultrafast electron probes with ever-improving capabilities, combined with other complementary photon-based or spectroscopic approaches, hold tremendous potential for revolutionizing our ability to observe and understand energy and matter at atomic scales.

中文翻译：

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超快电子衍射：可视化物质的动态状态

自从发现电子波二象性以来，电子散射仪器已发展成为揭示物质原子结构的一系列强大技术。除了以尽可能高的空间分辨率检测平衡结构中的局部晶格变化之外，最近的研究工作还致力于实现长期追求的梦想，即实时可视化物质的动态演化。超快时间尺度的原子行为携带了有关相变和化学反应动力学、材料和分子中电子和核自由度的耦合以及结构、功能和先前隐藏的亚稳态或非平衡态物质之间的相关性的关键信息。超快电子脉冲在这项科学事业中发挥着至关重要的作用，超快激光和粒子加速器技术的快速进步促进了超快电子脉冲的产生。这篇评论总结了过去几十年来该领域值得注意的发展。介绍了超快电子束的物理和技术，重点是与超快电子衍射实验最相关的品质因数。讨论了超短电子束的生成、操纵和表征方面的最新进展，旨在提高这些测量的组合时空分辨率。描述了原子物质电子散射的基本原理以及从固态和气相样品中检索动态结构信息的理论框架。还强调了基本的实验技术和应用这些方法的几项里程碑式的工作，以证明这些方法的广泛适用性。具有不断改进能力的超快电子探针与其他互补的基于光子或光谱的方法相结合，具有巨大的潜力，可以彻底改变我们在原子尺度上观察和理解能量和物质的能力。