Electron Densities and Electron Correlations by X-Ray Scattering

在化学系化学结构、动力学和机制A （CSDM-A）项目的支持下，布朗大学的Peter M. Weber教授正在使用x射线散射技术来观察电子如何围绕分子的原子核运行。电子轨道是电子优先定位的地方，并且有不同的形状，这取决于原子核和电子能量的排列。在有多个电子的分子中，电子之间的斥力会影响轨道形状。简而言之，一个电子的运动取决于所有其他电子的运动，从而产生电子运动中的相关性，这种相关性会深刻地影响所有分子性质。例如，当分子吸收光时，电子被提升到更高的能态，其分布和相关性发生变化，从而导致大量光化学现象。然而，考虑到电子运动的速度，观察这些变化是很困难的。韦伯教授和他的学生将使用时间分辨x射线散射方法来精确测量激发态分子结构、电子密度分布和电子-电子对相关性，这是分子性质的核心。他们的发现可能会带来新的基准，帮助量子化学计算机程序的持续发展。由于量子化学计算在分子科学中广泛使用，该项目可能在许多相关领域产生深远的影响和影响。该项目将训练学生掌握先进的实验方法和复杂的计算机模拟。时间分辨x射线散射实验将在SLAC国家加速器实验室的直线加速器相干光源x射线自由电子激光器上进行。飞秒激光脉冲将被用来将分子激发到激发态。经过短暂的延时后，自由电子激光发出的超短x射线脉冲从分子中散射出去。将测量到的散射信号与计算得到的电子密度分布和相关性的理论散射信号进行了比较。该项目将扩展用于确定分子结构和基态电子密度到激发态的x射线方法，并包括电子-电子相关性。研究将集中于选择具有不同p电子特征和电子相关性的模型系统。直接从计算方法和迭代改进过程模拟散射模式的代码已经开发出来，并将得到应用。实验测量的密度、相关性和结构与高级计算化学方法的结果进行了比较，并作为量子化学计算的基准。这些研究将提高我们对分子结构和化学反应性的理解，这反过来决定了许多化学和材料的性质。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

With support from the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program in the Division of Chemistry, Professor Peter M. Weber of Brown University is using x-ray scattering techniques to observe how electrons orbit the atomic nuclei of molecules. Electron orbitals are the places where electrons are preferentially located and have distinct shapes that depend upon the arrangement of the nuclei and the electron energy. In molecules with more than one electron, repulsions between the electrons affect the orbital shape. In short, the motion of an electron depends upon the motion of all the other electrons, giving rise to correlations in electron motion that profoundly affect all molecular properties. For example, when molecules absorb light, an electron is promoted to a higher energy state and the distributions and correlations change, resulting in a multitude of photochemical phenomena. Observing these changes are difficult though, given the speed with which electrons move. Professor Weber and his students will use time-resolved x-ray scattering methods to measure precise excited state molecular structures, electron density distributions and electron-electron pair correlations that are central to molecular properties. Their discoveries could lead to new benchmarks that aid the continued development of quantum chemistry computer programs. Since quantum chemistry calculations are used throughout the molecular sciences, the project could have far-reaching ramifications and impact in many related fields. The project will train students in advanced experimental methodologies and complex computer simulations.Time-resolved X-ray scattering experiments will be conducted at the Linac Coherent Light Source X-Ray Free Electron Laser of SLAC National Accelerator Laboratory. Femtosecond laser pulses will be used to excite molecules to excited electronic states. After a short time-delay, an ultrashort x-ray pulse from the free electron laser is scattered off the molecules. The measured scattering signals are compared to theoretical ones derived from computed electron density distributions and correlations. The project will extend the x-ray methods used to determine molecular structures and electron densities of ground states to the excited state and include electron-electron correlations. The studies will focus on a selection of model systems with different p-electron characters and electron correlations. Codes for simulating scattering patterns directly from computational methods and an iterative refinement process have been developed and will be applied. The experimentally measured densities, correlations and structures are compared to the results of high-level computational chemistry methods and serve as benchmarks for the quantum chemistry calculations. The studies will improve our understanding of molecular structures and chemical reactivity, which in turn determines many chemical and material properties.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.