Observing atomic motions as they occur is the dream goal of ultrafast electron microscopy (UEM). Great progress has been made so far thanks to the efforts of many scientists in developing the photoemission sources and beam blankers needed to create short pulses of electrons for the UEM experiments. While details on these setups have typically been reported, a systematic overview of methods used to obtain a pulsed beam and a comparison of relevant source parameters have not yet been conducted. In this report, we outline the basic requirements and parameters that are important for UEM. Different types of imaging modes in UEM are analyzed and summarized. After reviewing and analyzing the different kinds of photoemission sources and beam blankers that have been reported in the literature, we estimate the reduced brightness for all the photoemission sources reviewed and compare this to the brightness in the continuous and blanked beams. As for the problem of pulse broadening caused by the repulsive forces between electrons, four main methods available to mitigate the dispersion are summarized. We anticipate that the analysis and conclusions provided in this manuscript will be instructive for designing an UEM setup and could thus push the further development of UEM.

中文翻译：

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用于超快电子显微镜的光发射源和光束消隐器。

观察原子运动的发生是超快电子显微镜（UEM）的梦想目标。迄今为止，由于许多科学家在开发光发射源和产生电子束短脉冲所需的电子束消隐器方面的努力，取得了巨大进展，这些电子脉冲用于UEM实验。虽然通常已经报告了有关这些设置的详细信息，但尚未进行用于获得脉冲束的方法的系统概述以及相关光源参数的比较。在此报告中，我们概述了对于UEM至关重要的基本要求和参数。对UEM中不同类型的成像模式进行了分析和总结。在回顾并分析了文献中报道的不同种类的光发射源和光束消隐器之后，我们估算所有审查的光发射源的降低的亮度，并将其与连续光束和消隐光束的亮度进行比较。关于由电子之间的排斥力引起的脉冲展宽的问题，总结了可用于减轻色散的四种主要方法。我们希望本手稿中提供的分析和结论对设计UEM装置具有指导意义，因此可以推动UEM的进一步发展。