Chernyshov Dmitry, van Beek Wouter, Emerich Hermann, Milanesio Marco, Urakawa Atsushi, Viterbo Davide, Palin Luca, Caliandro Rocco
Swiss-Norwegian Beamlines at ESRF, Grenoble, France.
Acta Crystallogr A. 2011 Jul;67(Pt 4):327-35. doi: 10.1107/S0108767311010695. Epub 2011 May 10.
A theory is developed to describe the kinematic diffraction response of a crystal when it is subjected to a periodically varying external perturbation. It is shown that if a part of the local electron density varies linearly with an external stimulus, the diffracted signal is not only a function of the stimulation frequency Ω, but also of its double 2Ω. These frequency components can provide, under certain conditions, selective access to partial diffraction contributions that are normally summed up in the interference pattern. A phasing process applied to partial diffraction terms would allow recovery of the substructure actively responding to the stimulus. Two ways of frequency filtering are discussed (demodulation and correlation) with respect to extracting information from such an experiment. Also considered is the effect of the variation of different structural parameters on the diffraction intensity that have to be accounted for while planning modulation-enhanced experiments. Finally, the advantages and limitations of the proposed concept are discussed, together with possible experiments.
本文提出了一种理论，用于描述晶体在受到周期性变化的外部扰动时的运动学衍射响应。结果表明，如果局部电子密度的一部分随外部刺激呈线性变化，衍射信号不仅是刺激频率Ω的函数，也是其两倍频率2Ω的函数。在某些条件下，这些频率成分可以提供对通常在干涉图样中叠加的部分衍射贡献的选择性访问。应用于部分衍射项的相位处理过程将允许恢复对刺激有积极响应的子结构。讨论了关于从此类实验中提取信息的两种频率滤波方法（解调与相关）。同时还考虑了在规划调制增强实验时必须考虑的不同结构参数变化对衍射强度的影响。最后，讨论了所提出概念的优点和局限性以及可能的实验。
