论文部分内容阅读
与X射线晶体学中存在的相位问题类似,在电子衍射中也存在相位问题:在电子衍射实验中只能收集到衍射强度而丢失了相位。最近,衍射重构成像方法(diffractive imaging) ,即直接从衍射重构出晶体结构的方法,从理论和实验都有了重大进展。从理论上,人们提出和发展许多有效的相位解析方法。从实验上,高强度的X射线源,场发射电子枪以及高灵敏度的记录媒介的发展都对此有贡献。直接从衍射重构出晶体结构有许多的优点:首先在重构像中,物镜球差的影响很小。这是由于物镜传递函数对衍射强度的影响远远小于对相位的影响;其次,从同一晶体收集的电子衍射有更多的高阶衍射斑,使得衍射重构能得到较高的分辨率(小于0.1 nm) ;同时,在同样辐射条件下晶体的电子衍射比其高分辨像具有更高的信噪比。这对于用电镜解析对辐射损伤敏感的有机物和生物蛋白晶体是有用的。本文叙述了一个解决电子衍射相位的新方法。在本文的程序中,同时使用了Oszl偄nyi和S櫣to提出的正负交替反转法(charge-flipping algorithm)和Fienup的重构方法(hybrid input-outputalgorithm)。作者用模拟数据来验证该方法的有效性。在程序中输入计算的运动学电子衍射强度,模拟晶体的二维静电势场分布能被重构出来。使用归一化结构因子可以提高正空间的重构像衬度;这对解决相位问题是有利的。使用Fienup的重构方法可以有效地解决由局域最小值而引起程序停滞问题。在正负交替反转法中通常会有停滞问题而不能找到全局最小值。正负交替反转法会逐步地在正空间中产生较大的零值电势区域,从而减小了正空间中未知数的数目。当未知数数目小于或等于从傅立叶变换建立起来的等式数目时,晶体的相位就可以解决了。
Similar to the phase problem existing in X-ray crystallography, there is also a phase problem in electron diffraction. In electron diffraction experiments, only the diffraction intensity is collected and the phase is lost. Recently, diffractive imaging, a method of reconstructing crystal structure directly from diffraction, has made great progress both theoretically and experimentally. In theory, many effective methods of phase analysis have been proposed and developed. Experimentally, high-intensity X-ray sources, field-emission electron guns, and the development of highly sensitive recording media contribute to this. Reconstruction of the crystal structure directly from diffraction has many advantages: First, in the reconstructed image, the influence of the spherical aberration of the objective is very small. This is due to the fact that the effect of the objective-lens transfer function on the diffraction intensity is much less than the effect on the phase. Secondly, there are more higher-order diffraction spots on the electron diffraction collected from the same crystal, which results in higher resolution (less than 0.1 nm). Meanwhile, the electron diffraction of crystal under the same radiation has a higher signal-to-noise ratio than its high-resolution image. This is useful for electron microscopic analysis of organic and biological protein crystals that are sensitive to radiation damage. This article describes a new method to solve the electron diffraction phase. In the procedure of this paper, both the charge-flipping algorithm and the hybrid input-output algorithm of Fienup are used at the same time. The authors use simulation data to verify the effectiveness of the method. Entering the calculated kinematic electron diffraction intensities in the program, the 2D electrostatic potential distribution of the simulated crystal can be reconstructed. Using normalized structure factors can improve the reconstructed image contrast in positive space; this is advantageous for solving the phase problem. Using Fienup’s reconstruction method can effectively solve the problem of program stall caused by the local minimum. In the alternating positive and negative alternating law usually there will be stagnation problems can not find the global minimum. The alternating positive and negative method will gradually generate a larger area of zero potential in positive space, thus reducing the number of unknowns in positive space. When the number of unknowns is less than or equal to the number of equations built from the Fourier transform, the phase of the crystal can be resolved.