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X-rays are used to probe molecules with element-specificity and atomic locality owing to the core-level electrons that they excite. This makes X-rays extremely attractive for the study of chemical systems that have a few unique atoms, like many transition metal catalysts, as it allows one to probe the catalytic site directly. X-rays have been used in this capacity for decades, with a gamut of single-photon or linear techniques ranging from scattering to absorption to fluorescence. Most linear methods are poorly sensitive to interesting chemical properties like bond type, which has motivated the study of weaker, more sensitive signals like valence to core emission at XFELs and synchrotrons. Non-linear X-ray methods, or multi-photon methods, are a different avenue uniquely enabled by the short pulse duration of XFELs to obtaining improved chemical sensitivity. Multi-photon interactions probe a much wider space of electronic configurations than single-photon interactions, which both expands the information they can reveal and makes them more complex to study. Following the creation of a single core-hole by the first photon interaction, the system can relax via a cascade of inner-shell transitions and/or auger decay, which will ultimately alter the chemical configuration of the material. When X-ray interactions are nearly simultaneous the explosion of intermediate states is limited, and the initial state of matter can be non-trivially related to subsequent photon interactions. It is in this ultrafast, early time period of X-ray non-linear interactions where chemical systems study stands to benefit from multi-photon interactions.Despite the difficulty associated with even seeing non-linear X-ray effects, it is possible at LCLS and other XFEL facilities. In this proposal we aim to develop multidimensional spectroscopy to study ultrafast multi-photon processes in chemical systems using natural fluctuations of the SASE source. Using SASE fluctuations are central to the project, because at present we have few other reliable pulse control options. We focus on double core-hole ionization as a tractable multi-photon signal that offers improved chemical contrast. Double core holes are created by two photon interactions and can reside on either a single atom or two separate atoms. The challenge that double core-hole (DCH) signals present, in common with other non-linear signals, is that they are difficult to disentangle from signals produced by other processes, namely single core hole fluorescence. Inferring the signal’s dependence on the exciting field in multiple dimensions (multidimensional spectroscopy) is our strategy to isolate the DCH signal, segregating it from single core-hole signals by its unique dependence on both photon energies.The majority of this project is centered around development of Gaussian Process based regression tools to extract the multi-dimensional signals. The code for our projects are hosted on github:RIXS Spectroscopyhttps://github.com/fullerf/stochastic_spectroscopyNonlinear Spectroscopyhttps://github.com/fullerf/complex_spectroscopyPublications of Note:Communications Chemistry (2021) https://doi.org/10.1038/s42004-021-00512-3
Menlo Park, CA,USA
X-ray crystallography at X-ray free-electron laser sources is a powerful method for studying macromolecules at biologically relevant temperatures. Moreover, when combined with complementary techniques like X-ray emission spectroscopy, both global structures and chemical properties of metalloenzymes can be obtained concurrently, providing insights into the interplay between the protein structure and dynamics and the chemistry at an active site. The implementation of such a multimodal approach can be compromised by conflicting requirements to optimize each individual method. In particular, the method used for sample delivery greatly affects the data quality. This project is centered around a hybrid combination of "fixed target" and "continuous sample delivery" in order to deliver controlled sample amounts on demand. Presently, we use acoustic droplet ejection coupled with a conveyor belt drive that is optimized for crystallography and spectroscopy measurements of photochemical and chemical reactions over a wide range of time scales. We have studied a variety of systems with this approach ranging from photosystem II, phytochromes, ribonucleotide reductace, hydrogenases, and other enzymes which use oxygen as a substrate. The approach is best used to study reactions on the millisecond to second scale, but permits high rep-rate acquisition even for second-scale delays after activation.Publications of note:Nature Methods (2017). https://doi.org/10.1038/nmeth.4195Nature (2018). https://doi.org/10.1038/s41586-018-0681-2PNAS (2019) https://doi.org/10.1073/pnas.1912041116