Curtis Wyatt A, Willis Simon A, Flannigan David J
Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, MN 55455, USA.
Minnesota Institute for Ultrafast Science, University of Minnesota, Minneapolis, MN 55455, USA.
Phys Chem Chem Phys. 2022 Jun 8;24(22):14044-14054. doi: 10.1039/d2cp01250b.
In femtosecond (fs) 4D ultrafast electron microscopy (UEM), a tradeoff is made between photoelectrons per packet and time resolution. One consequence of this can be longer-than-desirable acquisition times for low-density packets, and particularly for low repetition rates when complete photothermal dissipation is required. Thus, gaining an understanding of photoelectron trajectories in the gun region is important for identifying factors that limit collection efficiency (CE; fraction of photoelectrons that enter the illumination system). Here, we continue our work on the systematic study of photoelectron trajectories in the gun region of a Thermo Fisher/FEI Tecnai Femto UEM, focusing specifically on CE in the single-electron regime. Using General Particle Tracer, calculated field maps, and the exact architecture of the Tecnai Femto UEM, we simulated the effects of fs laser parameters and key gun elements on CE. The results indicate CE strongly depends upon the laser spot size on the source, the (unbiased) Wehnelt aperture diameter, and the incident photon energy. The CE dispersion with laser spot size is found to be strongly dependent on aperture diameter, being nearly dispersionless for the largest apertures. A gun crossover is also observed, with the beam-waist position being dependent on the aperture diameter, further illustrating that the Wehnelt aperture acts as a simple, fixed electrostatic lens in UEM mode. This work provides further insights into the operational aspects of fs 4D UEM.
在飞秒（fs）四维超快电子显微镜（UEM）中，每个数据包中的光电子与时间分辨率之间存在权衡。由此产生的一个后果是，对于低密度数据包，尤其是在需要完全光热耗散的低重复率情况下，采集时间可能会比预期的更长。因此，了解枪区中的光电子轨迹对于识别限制收集效率（CE；进入照明系统的光电子比例）的因素非常重要。在这里，我们继续对赛默飞世尔/飞立Tecnai Femto UEM枪区的光电子轨迹进行系统研究，特别关注单电子状态下的CE。使用通用粒子追踪器、计算得到的场图以及Tecnai Femto UEM的精确结构，我们模拟了飞秒激光参数和关键枪元件对CE的影响。结果表明，CE强烈依赖于源上的激光光斑尺寸、（无偏）韦内尔特孔径直径和入射光子能量。发现CE随激光光斑尺寸的色散强烈依赖于孔径直径，对于最大孔径几乎无色散。还观察到枪交叉现象，束腰位置取决于孔径直径，进一步说明韦内尔特孔径在UEM模式下充当简单的固定静电透镜。这项工作为飞秒四维UEM的操作方面提供了进一步的见解。
