000001088 001__ 1088
000001088 005__ 20181219105857.0
000001088 02470 $$anbn:de:gbv:18-83431$$2urn
000001088 037__ $$aTHESIS-2017-001 000001088 041__$$aeng
000001088 088__ $$aXFEL.EU THESIS-2017-001 000001088 245__$$aTracking Chemical Reactions with Ultrafast X-ray Spectroscopic Techniques
000001088 260__ $$hGermany$$c2017$$bEuropean X-Ray Free-Electron Laser Facility GmbH$$aSchenefeld
000001088 269__ $$a2017 000001088 336__$$aTheses
000001088 502__ $$bPh.D. Thesis$$cUniversität Hamburg$$d2016$$i2012-02-01$$j2016-12-16 000001088 520__$$aChemical 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.
000001088 546__ $$aEnglish 000001088 6531_$$aferrocyanide
000001088 6531_ $$apicosecond 000001088 6531_$$aligand-dissociation
000001088 6531_ $$apentacordinated 000001088 6531_$$aphotoaquated
000001088 655__ $$aFemtochemistry and solid- and liquid-state chemistry 000001088 690__$$aExternal experiment
000001088 7001_ $$aAssefa, Tadesse Abebaw 000001088 790__$$aEuXFEL guest
000001088 85641 $$uhttp://ediss.sub.uni-hamburg.de/volltexte/2017/8343 000001088 8560_$$fkurt.ament@xfel.eu
000001088 8564_ $$uhttps://xfel.tind.io/record/1088/files/Cover.pdf$$s12320
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000001088 900__ $$aInstrument FXE 000001088 980__$$aTHESIS