Development Of X Ray Holography Methods For Structure Determination

Download or read book Development of X-ray Holography Methods for Structure Determination written by Yuhao Wang and published by . This book was released on 2012 with total page 190 pages. Available in PDF, EPUB and Kindle. Book excerpt: Holographic methods show much promise to enable direct determination of atomic structure with minimal assumptions and approximations. The approach can, in principle, provide three dimensional information on atomic positions. However, significant developments in experimental techniques, instrumentation and in data collection and analysis are needed. A review of the holography method is given with a focus on X-ray fluorescence holography. Methods for analysis of X-ray holographic data are also reviewed. An overview of the detectors relevant to X-ray measurements is also presented. An experimental apparatus for rapid acquisition of X-ray holographs using novel X-ray detectors has been developed. The integration of high speed detectors and the utilization of rapid sampling methods to produce high quality holograms form the core of this work. A new method for direct extraction of the electron charge density based on expansion of the hologram with respect to a spherical harmonic basis is developed. This approach attacks the problem of obtaining the electron density from the hologram by the introduction of periodic constraints (fixed unit cells) while maintaining flexibility by making no assumptions about the positions of atoms within the unit cells. Problems with local or long range distortions can be solved by utilizing cells of the appropriate size. The method makes no other assumptions. Model charge densities derived from this approach are shown to match quite well with the input model crystal structures with no need for heavy filtering typical of the Barton Transform. The algorithm can be fully automated and hence falls into the class of "Direct Methods". This new approach may move the method of X-ray holography from the developmental stage to a powerful and routine tool for the solution of single crystal structures relevant to inorganic materials and organic systems.