Electron Densities and Electron Correlations by X-Ray Scattering

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.