Leiden University, Kamerlingh Onnes Laboratorium, P.O. Box 9504, NL-2300 RA Leiden, The Netherlands.
Ultramicroscopy. 2012 Apr;115:88-108. doi: 10.1016/j.ultramic.2011.11.005. Epub 2011 Nov 22.
We introduce an extended Contrast Transfer Function (CTF) approach for the calculation of image formation in low energy electron microscopy (LEEM) and photo electron emission microscopy (PEEM). This approach considers aberrations up to fifth order, appropriate for image formation in state-of-the-art aberration-corrected LEEM and PEEM. We derive Scherzer defocus values for both weak and strong phase objects, as well as for pure amplitude objects, in non-aberration-corrected and aberration-corrected LEEM. Using the extended CTF formalism, we calculate contrast and resolution of one-dimensional and two-dimensional pure phase, pure amplitude, and mixed phase and amplitude objects. PEEM imaging is treated by adapting this approach to the case of incoherent imaging. Based on these calculations, we show that the ultimate resolution in aberration-corrected LEEM is about 0.5 nm, and in aberration-corrected PEEM about 3.5 nm. The aperture sizes required to achieve these ultimate resolutions are precisely determined with the CTF method. The formalism discussed here is also relevant to imaging with high resolution transmission electron microscopy.
我们介绍了一种扩展的对比传递函数（CTF）方法，用于计算低能电子显微镜（LEEM）和光电发射显微镜（PEEM）中的成象。该方法考虑了高达五阶的像差，适用于最先进的具有像差校正功能的 LEEM 和 PEEM 的成象。我们为非像差校正和像差校正的 LEEM 中的弱相位和强相位物体以及纯振幅物体推导出了谢泽尔离焦值。使用扩展的 CTF 形式，我们计算了一维和二维纯相位、纯振幅以及混合相位和振幅物体的对比度和分辨率。通过将这种方法应用于非相干成像的情况，我们处理了 PEEM 成像。基于这些计算，我们表明，在具有像差校正功能的 LEEM 中的最终分辨率约为 0.5nm，在具有像差校正功能的 PEEM 中的最终分辨率约为 3.5nm。实现这些最终分辨率所需的孔径尺寸可以通过 CTF 方法精确确定。这里讨论的形式也与高分辨率透射电子显微镜的成像有关。
