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Tracking Chemical Reactions with Ultrafast X-ray Spectroscopic Techniques
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Instrument FXE
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Chemical reactions in transition metal complexes can be triggered with light, which results in structural and electronic changes. These changes can be visualised by measuring the geometric and electronic structure of a complex in real-time during a chemical reaction pathway. Time-resolved (TR) X-ray Absorption and Emission Spectroscopy (XAS and XES) delivers information about the geometric (via XANES and EXAFS) and electronic (via XES) transient changes when used in synchronisation with an ultrafast laser in a pump-probe scheme. Element specificity and sensitivity to the local structure of X-ray spectroscopic tools around the absorbing atom allows us to look at the photoexcited changes of transition metal complexes. We studied aqueous ferrocyanide ([$Fe^{II}(CN)_{6}]^{4-}$) following photoexcitation with UV laserlight which results in two photoproducts namely [$Fe^{II}(CN)_{5}H_{2}O]^{3-}$ and [$Fe^{III}(CN)_{6}]^{3-}$. Picosecond-resolved XANES measurements after 266 nm excitation show both photoproducts and the ratio of extracted excited state fractions is consistent with quantum yield measurements reported by Shirom et al. TR XES was used to look at the spin state and ligand environment changes with the core-to-core ($Kβ_{1,3}$) and valance-to-core (vtc) (or $Kβ_{2,5}$) emission lines of the central Fe atom. Furthermore, we investigated the ultrafast ligand dissociation of aqueous ferrocyanide ions upon irradiation of 355 nm laser light. Based on a comparison of the simulated pre-edge peaks of 1s->3d transition and the experimental data, we concluded that the reaction pathway commences via ligand detachment followed by the formation of the long-lived photoaquated complex. TR XES reveals the spin state of the intermediate complex. Combining these finding we interpret that the aquation process happens from a non-singlet potential energy surface and takes about 13 ps. Also, we characterised the molecular structure of photoexcited [$Fe^{II}(terpy)_{2}]^{2+}$ molecule via TR EXAFS. The data analysis in energy space used two structural model expansions which are the representations of DFT predicted $^{5}E$ and $^{5}B_{2}$ quintet high spin states. After statistical evaluation of the two models, $^{5}E$ high spin state model is in better agreement with experimental data. The transient EXAFS fitting implemented here can be used to characterise molecular structures of other spin crossover complexes.
Schenefeld, Germany, European X-Ray Free-Electron Laser Facility GmbH, 2017
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urn: nbn:de:gbv:18-83431
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Degree type:
Ph.D. Thesis
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Universität Hamburg


 Record created 2017-03-30, last modified 2018-12-19

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