中文摘要：

以原子级时空分辨监测物质的动力学行为并从最根本层面理解自然界中的微观基本过程一直是飞秒物理、飞秒化学以及飞秒生物学研究的目标.时间分辨电子衍射巧妙地结合了抽运-探测技术和电子衍射技术,可实现直接＂观察＂和＂冻结＂类似的超快过程.然而,目前常用的超快电子探针的横向相干性仍受到电子源的初始尺寸、有效温度、能量弥散以及电子间固有库仑排斥的限制,还很难分辨化学和生物相关的复杂有机分子.近年来大量研究工作都集中在发展高相干的超短电子源,其不仅对时间分辨电子衍射研究起到推动作用,也将极大地促进超快电子显微镜、相干衍射成像以及叠层成像等的发展.本综述以相干性为主线,介绍了几种常用平面阴极光电发射源的研究进展,并从产生机理、独有特性和实验成果方面讨论了尖端发射源和冷原子电子源这两类新型的高相干超快电子源,最后对衍射技术的相干性发展和未来应用进行了展望.

英文摘要：

Microscopic dynamic process of material structure which determines the inherent property of substance takes place on a molecular and atomic scale. Understanding the underlying mechanisms of the various fundamental processes has always been the goal of chemistry, physics, biology and materials science. With Ahmed Zewail＇s pioneering work in the field of femtoscience, the time-resolved electron diffraction, combining the pump-probe and electron diffraction technique, has become an excellent tool with sufficient temporal precision to directly deliver insights into ultrafast phenomena on an atomic level. Central to this method is the ultrashort electron pulses generated from a metal photocathode. However, up to now, owing to the initial size, effective temperature, energy dispersion and inherent coulomb repulsion of electron source, the state-of-the-art transverse coherence of conventional planar cathode photoemission source is still insufficient to resolve the complex chemical and biological organic molecules. Hence, in recent years, many efforts have focused on developing high-coherence ultrashort electron sources. The main methods include minimizing the initial beam size, weakening the space charge, reducing the effective temperature, and matching the photon energy of laser with the work function of cathode material. In this review, we firstly summarize the history and advantages of the electron probe, secondly sketch out the figure of merit of the electron source. And then taking coherence as the main line, we review recent progress in common planar photoemission sources, and discuss the latest development of tip-based electron sources and cold atom electron sources in terms of their generation mechanisms, unique properties and research progress. Finally, the development and future applications of the diffraction technique are prospected. In general, the high-coherence length of photoelectric surface source is often at the expense of the current. The needle source can obtain the highest coherence length, but