Using Ultrafast X-ray Spectroscopy To Address Questions in Ligand-Field Theory: The Excited State Spin and Structure of [Fe(dcpp)₂]²⁺

Abstract

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Photophysical Investigation of Iron(II) Complexes Bearing Bidentate Annulated Isomeric Pyridine-NHC Ligands

“…31 In other words, an increased stiffness is likely to create an excited state barrier leading to an increased ESL. This was realized already by the latest work of the McCusker group, 51,52 and it explains the difference between our bidentate and tridentate complexes. As in the former, the Fe-N bond is more flexible, there is less energetic penalty for reaching the 3 MC-S0 crossing, and the effect of ligand substitutions does play a minor role for extending the ESL.…”

Resolving structures of transition metal complex reaction intermediates with femtosecond EXAFS

“…high-frequency fluctuations and compared to the maximum of transient difference (at B7.125 keV), resulting in an estimated signal-to-noise ratio (S/N) of B500. This is comparable to the one of previously published synchrotron EXAFS measurements using MHz repetition rate single photon counting TFY measurements, 30,31 but about one order of magnitude lower than what can be achieved with novel multi-MHz high-dynamic-range setups. 42…”

“…5 A further advantage of time-resolved EXAFS is the high accuracy -on the order of 1 picometer -in resolving the TM-ligand bond distances and the changes therein, 27 as well as the sensitivity in resolving asymmetries in the first coordination shell around the TM. [28][29][30][31][32] Extending EXAFS of weakly concentrated solvated samples to the femtosecond regime with beam slicing at synchrotrons 33 or tabletop plasma source 34 is not viable due to the low photon flux and/or limited photon energy tunability of these source. Measuring femtosecond resolved EXAFS at XFELs is challenging due several reasons: (i) the extremely large pulse-to-pulse fluctuations due to the Self-Amplified Spontaneous Emission (SASE) process require a systematic and rigorous normalization and (ii) scanning the incident X-ray energy over a broad range is technically challenging, as it either requires a very accurate scanning of the long (B100 m) undulator gaps, or, in the case of LCLS with fixed gap undulators, a continuous modulation of the accelerator electron energy.…”

Ultrafast X-ray Photochemistry at European XFEL: Capabilities of the Femtosecond X-ray Experiments (FXE) Instrument

“…The improving signal quality of these time-resolved X-ray experiments triggered new studies towards improved modeling and theoretical interpretation of the measurements. This was performed for both time-resolved XANES and EXAFS studies [8][9][10][11], while in parallel the structural interpretation of related time-resolved wide-angle X-ray scattering (WAXS) results [12][13][14][15] improved considerably. This allowed for comparison of both complementary structural tools on the same time-resolved experiment, where van der Veen et al [6] analyzed the recorded time-resolved EXAFS of a photoexcited Pt-POP compound, while M. Christensen et al [12] analyzed the pump-probe WAXS signals from the same molecule, yielding nearly identical results.…”

“…X-ray emission spectroscopy (XES) studies with 100 ps temporal resolution on photoexcited molecules in the solution phase were introduced by Vankó and colleagues at the Swiss Light Source [18], and were soon extended towards the first resonant inelastic X-ray scattering study with 100 ps time resolution [19]. This move opened the path for introducing more advanced X-ray spectroscopic techniques into the time domain with SR, and meanwhile Kα, Kβ and even valence-to-core XES studies were successful [10,11,20,21]. One new ingredient delivered by XES is the spin sensitivity around transition metal (TM) ions in photoactive TM complexes, and this property is now being used to unravel the intermediate (spin) states in the excited state manifold [22][23][24], which are extremely difficult to obtain with optical tools on the subnanosecond time scales.…”