Electron Densities and Electron Correlations by X-Ray Scattering

Professor Peter M. Weber at Brown University is supported to develop advanced X-ray scattering experiments at the SLAC National Accelerator Laboratory Linac Coherent Light Source (LCLS) to measure electron density distributions and electron correlations in excited-state molecular systems. Electron density distributions and correlations between electrons are fundamental quantum mechanical properties that determine molecular structure, chemical reactivity, and energy flow during chemical transformations. Although modern quantum chemistry calculations can approximate these properties, especially for ground states, accurate experimental measurements for excited-state systems remain limited. Professor Weber and his students will record highly precise X-ray scattering signals from molecular model systems exhibiting varying degrees of electron conjugation, charge transfer, and correlation effects. These measurements will establish new experimental benchmarks for quantum chemistry that support the continued development of computational methods used broadly across chemistry, materials science, biology, and related molecular sciences. The project will also provide interdisciplinary training for students in advanced experimental methods, ultrafast X-ray science, and computational modeling. Electron correlations are complex quantum mechanical interactions that strongly influence chemical bonding, molecular structure, and the dynamics of chemical reactions. These effects are particularly challenging to calculate for excited-state molecules, and experimental methods capable of directly probing them have only recently become feasible. Professor Weber and his team will use the LCLS X-ray free-electron laser, one of the world’s most advanced X-ray sources, to perform exceptionally high-quality scattering measurements that reveal detailed information about electronic structure. The experiments will measure quantitatively accurate elastic and total scattering signals, providing access to one-electron and two-electron density distributions, respectively. Ghost imaging algorithms will be advanced to delineate between these distinct electron densities. Studies will examine ground-state and excited-state densities in model systems with localized, conjugated, and aromatic molecular bonds, as well as charge-transfer and transient dynamical processes. Molecular models include 1,3-cyclohexadiene, 1,3-cyclopentadiene, N,N’-dimethylpiperazine, and N,N’-dimethylhomopiperazine,. By comparing experimentally measured scattering signals with theoretical predictions, the project will create new benchmarks for quantum chemistry and improve our understanding of molecular electronic structure, chemical reactivity, and ultrafast reaction dynamics. 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.