Yu Lei, Wan Weishi, Yu Ka Man, Altman Michael, Tang Wen-Xin
College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China.
ShanghaiTech University, Shanghai, 200031, China.
Ultramicroscopy. 2021 Jun;225:113284. doi: 10.1016/j.ultramic.2021.113284. Epub 2021 Apr 15.
We present experimental observations of high order phase contrast in aberration corrected low energy electron microscopy (AC-LEEM). Phase contrast produced by atomic steps on a Ag (111) surface exhibits prominent high order interference fringes, which have not been reported before. These phase contrast features depend upon defocus and incident electron energy, similar to the prominent first order fringes observed previously and in agreement with Fourier optics (FO) model predictions. The comparison of experimental results and FO model simulations demonstrates that fringe amplitudes are strongly affected at large defocus by the source divergence. This effect is exploited to quantitatively determine the divergence, 0.055 ± 0.005 mrad, of the field emission source in AC-LEEM under the imaging conditions used. Although the divergence determines the spatial coherence of the illumination in microscopy, it has not been possible to characterize this key instrumental parameter in LEEM before.
我们展示了在像差校正低能电子显微镜（AC-LEEM）中高阶相衬的实验观察结果。Ag(111)表面原子台阶产生的相衬呈现出显著的高阶干涉条纹，这是之前未曾报道过的。这些相衬特征取决于离焦和入射电子能量，类似于之前观察到的显著一阶条纹，并且与傅里叶光学（FO）模型预测相符。实验结果与FO模型模拟的比较表明，在大离焦时条纹幅度受源发散的强烈影响。利用这一效应定量确定了在所用成像条件下AC-LEEM中场发射源的发散度为0.055±0.005毫弧度。尽管发散度决定了显微镜中照明的空间相干性，但在此之前在LEEM中一直无法对这一关键仪器参数进行表征。
