Phase Determination of Bragg Reflections by Wavelength-Modulated Diffraction

x射线衍射法作为一种测定结构的方法一直被使用，但存在一个严重的问题，即“相位问题”。也就是说，虽然可以从测量的强度中知道Bragg反射的振幅，但不可能得到Bragg反射相位的信息。利用同步辐射的高亮度和波长可调性，提出了一种波长调制衍射测相的新方法。在立命馆大学SR中心的BL-1光束线上构建了最适合这一目的的系统，并开发了处理测量数据的软件。在该系统中，双晶单色仪反复旋转，使入射辐射的波长在晶体中所含原子的吸收边缘附近的范围内连续变化，同时样品晶体同步旋转以反射不同波长的辐射。衍射图样被记录在成像板上，其中布拉格反射被看作是细长的斑点，其强度分布在吸收边缘显示正或负梯度。通过求解以测量强度梯度为常数项的线性方程组，可以推导出相位。以铁原子为异常散射体的二茂铁衍生物晶体进行了可行性实验，并取得了成功的结果。

X-ray diffraction method has long been used as a method of structure determination, but there exists a serious problem called "phase problem". That is, although it is possible to know the amplitude of Bragg reflections from measured intensity, it is not possible to get information on the phase of Bragg reflections. Taking advantage of high brightness and wavelength tunability of synchrotron radiation, this research was performed to develop a new method of phase determination by wavelength modulated diffraction. System most suitable to this purpose was constructed at the beamline BL-1 in the SR Center at Ritsumeikan University and software to handle measured data was developed. In the system, the double-crystal monochromator is repeatedly rotated so that the wavelength of incident radiation is continuously changed over the range in the vicinity of the absorption edge of an atom contained in a crystal, while the sample crystal is rotated synchronously to reflect the radiation of varying wavelength. Diffraction pattern is recorded on an imaging plate, in which Bragg reflections are seen as elongated spots with an intensity profile showing positive or negative gradients at the absorption edge. By solving simultaneous linear equations with measured intensity gradients as constant terms, the phase can be derived. Feasibility experiments using a ferrocene derivative crystal with the iron atoms as anomalous scatters was carried out with successful results.