高串电荷低能电子条纹衍射

Chiwon Lee, Günther H. Kassier, R. J. Dwayne Miller
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摘要

对于光激发时不可逆结构动态的时间分辨衍射研究,由于热效应和累积应变,扰动周期的数量受到限制,从而影响晶体的有序程度和空间分辨率。对于更容易发生紊乱和形成缺陷的表面研究来说,这一问题更加严重。采用传统的频闪泵探针方案对这些系统进行超快电子衍射研究,需要对精心制备的衍射样品进行重复测量,以获取并平均高于背景的信号,动态范围从几十皮秒到几百皮秒不等。在这里,我们提出了超快条纹低能电子衍射(LEED),原则上每个标称数据采集时间段只需要一次激发。通过利用条纹法的时空相关特性和透射几何中的高电荷 2 keV 电子束,我们证明与传统的时间扫描测量相比,激发周期和总电子剂量的累积数量减少了约一个数量级,模型拟合残差的均方根误差减少了 48%。我们相信,我们的研究结果证明了一种可行的替代方法,这种方法的灵敏度比基于纳米芯片的超快 LEED 研究更高,因为后者每次激发只需几个电子,就能获得所有类别的结构动态,从而提供表面过程的原子级视图。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
High bunch charge low-energy electron streak diffraction
For time-resolved diffraction studies of irreversible structural dynamics upon photoexcitation, there are constraints on the number of perturbation cycles due to thermal effects and accumulated strain, which impact the degree of crystal order and spatial resolution. This problem is exasperated for surface studies that are more prone to disordering and defect formation. Ultrafast electron diffraction studies of these systems, with the conventional stroboscopic pump–probe protocol, require repetitive measurements on well-prepared diffraction samples to acquire and average signals above background in the dynamic range of interest from few tens to hundreds of picoseconds. Here, we present ultrafast streaked low-energy electron diffraction (LEED) that demands, in principle, only a single excitation per nominal data acquisition timeframe. By exploiting the space–time correlation characteristics of the streaking method and high-charge 2 keV electron bunches in the transmission geometry, we demonstrate about one order of magnitude reduction in the accumulated number of the excitation cycles and total electron dose, and 48% decrease in the root mean square error of the model fit residual compared to the conventional time-scanning measurement. We believe that our results demonstrate a viable alternative method with higher sensitivity to that of nanotip-based ultrafast LEED studies relying on a few electrons per a single excitation, to access to all classes of structural dynamics to provide an atomic level view of surface processes.
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