Spence J C H
Department of Physics, Arizona State University, Tempe, AZ 85287-1504, USA.
Acta Crystallogr A. 2009 Jan;65(Pt 1):28-38. doi: 10.1107/S0108767308032728. Epub 2008 Dec 2.
A method is described for reconstructing the two-dimensional real-space charge density of an isolated object from measurement of the soft X-ray transmission diffraction pattern when it is affected by strong multiple scattering. The Bloch-wave scattering-matrix approach is used to show that the diffracted amplitude depends only on a simple product of X-ray wavelength and sample thickness (unlike the case of relativistic electron diffraction) under reasonable approximations. The multislice formulation then gives the effect of a small change in wavelength, which involves only single scattering. Dynamical diffraction patterns are recorded at two adjacent wavelengths, phased by iterative methods, transformed to real space and divided to give a single-scattering wavefunction. This can then be used to produce a charge-density map. The extension of the method to tomography is discussed. Consideration is first also given to the possibility that absorption due to the photoelectric effect may be so severe for soft X-rays that multiple elastic scattering becomes so much less probable than photoelectric absorption that it may be neglected entirely. A discussion of signs in soft X-ray, positron and electron multiple-scattering theory is given.
本文描述了一种方法，用于在受强多重散射影响时，通过测量软X射线透射衍射图样来重建孤立物体的二维实空间电荷密度。在合理近似下，采用布洛赫波散射矩阵方法表明，衍射振幅仅取决于X射线波长与样品厚度的简单乘积（这与相对论电子衍射情况不同）。然后，多层公式给出了波长微小变化的影响，这仅涉及单次散射。在两个相邻波长下记录动态衍射图样，通过迭代方法进行相位调整，转换到实空间并相除，得到单次散射波函数。然后可利用此波函数生成电荷密度图。文中讨论了该方法在断层扫描中的扩展。首先还考虑了由于光电效应导致的吸收对于软X射线可能非常严重，以至于多重弹性散射比光电吸收的可能性小得多，以至于可能完全被忽略的可能性。文中还对软X射线、正电子和电子多重散射理论中的符号进行了讨论。
