The relative roles of multiple electron scattering and in-molecule Fresnel diffraction in atomic-resolution transmission electron microscopy of small molecules are discussed. It is argued that, while multiple scattering tends to have only a moderate effect, the in-molecule Fresnel diffraction can be significant due to the shallow depth of focus in high-resolution electron microscopy. When the depth of focus is smaller than the size of molecule along the trajectory of the electrons, the projection approximation breaks down and pairs of images at orientations of the molecule rotated by 180 degrees are not the same. Put another way, it matters which way the electrons traverse the specimen. In such cases, diffraction tomography based on the first Born or first Rytov approximation represents a more suitable method for the reconstruction of the three-dimensional distribution of the electrostatic potential, compared to conventional computed tomography which intrinsically relies on the validity of the projection approximation. A simplified method for diffraction tomography based on the approximation of Fresnel diffraction by the transport of intensity equation is proposed and tested on numerically simulated examples.

中文翻译：

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多重散射和菲涅耳衍射在电子小分子成像中的相对作用

讨论了多电子散射和分子内菲涅耳衍射在小分子的原子分辨率透射电子显微镜中的相对作用。有人认为，虽然多次散射往往只产生中等影响，但由于高分辨率电子显微镜中的焦深较浅，分子内菲涅耳衍射可能很重要。当焦深小于沿电子轨迹的分子大小时，投影近似会失效，分子旋转 180 度方向上的图像对不相同。换句话说，电子以何种方式穿过样本很重要。在这种情况下，与本质上依赖于投影近似有效性的传统计算机断层扫描相比，基于第一波恩或第一 Rytov 近似的衍射断层扫描代表了一种更适合重建静电势三维分布的方法。提出了一种基于强度方程传输近似菲涅耳衍射的衍射层析成像的简化方法，并在数值模拟实例上进行了测试。