Direct Reconstruction of Surface Atomic Structures: Holography and Beyond

虽然电子衍射技术已经成功地揭示了许多复杂的和不寻常的原子结构存在于固体表面，他们是阻碍了他们最常见的方法，迄今为止，这是比较模拟衍射图案的大量模型结构与实验。 不幸的是，这限制了他们的能力，以确定复杂的未知结构，因为非常大量的结构参数，需要改变。 已知这种试错技术的计算机时间缩放随着优化参数的数量呈指数级缩放（NP完全问题）。 最近，全息技术，作为发展的光学，已被证明持有承诺，在克服一些这些困难，确定表面结构的电子衍射。 这项研究将使用唯一的方法，明确解决非线性物体波项，其不可忽略的性质是纯粹的全息算法准确确定结构的主要障碍。 该方法采用基于线性规划的算法迭代优化非线性问题的解的独特思想。 理论工作将被应用到低能电子衍射，俄歇和光电子衍射从清洁和吸附覆盖的表面的实验数据。 这项分析和计算研究将集中在一种新的方式来解释实验，确定表面的原子结构。 该方法类似于光学中使用的全息术。 在这种情况下，实验探针是电子而不是光。 该项目的成功完成将产生一种快速准确的方法来确定表面结构。 这种测定对于基础研究很重要，但在从催化到腐蚀的应用中尤其重要。 ***

9320275 Saldin Although electron diffraction techniques have been successful at revealing many of the complex and unusual atomic structures which exist at solid surfaces, they are hampered by their most common methodology to date, which is to compare simulated diffraction patterns for a large number of model structures with experiment. Unfortunately, this limits their ability to determine complex unknown structures because of the very large number of structural parameters which need to be varied. The computer time scaling of such trial and error techniques is known to scale exponentially with the number of optimized parameters (an NP-complete problem). Recently, holographic techniques, as developed for optics, have been shown to hold promise in overcoming some of these difficulties for determining surface structure by electron diffraction. This research will use the only method that explicitly addresses the nonlinear object wave terms whose non-negligible nature is the main barrier to accurate structure determination by purely holographic algorithms. This method employs the unique idea of using a linear programming based algorithm to iteratively optimize the solution to the nonlinear problem. The theoretical work will be applied to experimental data of low energy electron diffraction, Auger and photoelectron diffraction from both clean and adsorbate-covered surfaces. %%% This analytical and computational research will focus on a new way of interpreting experiments that determine the atomic structures of surfaces. The method is analogous to that of holography as used in optics. In this case the experimental probes are electrons instead of light. Successful completion of this project will yield a rapid and accurate method to determine surface structures. This determination is important for basic research, but is particularly important in applications ranging from catalysis to corrosion. ***