Calcium Valence-to-Core X-ray Emission Spectroscopy: A Sensitive Probe of Oxo Protonation in Structural Models of the Oxygen-Evolving Complex

Calcium is an abundant, nontoxic metal that finds many roles in synthetic and biological systems including the oxygen-evolving complex (OEC) of photosystem II. Characterization methods for calcium centers, however, are underdeveloped compared to those available for transition metals. Valence-to-core X-ray emission spectroscopy (VtC XES) selectively probes the electronic structure of an element’s chemical environment, providing insight that complements the geometric information available from other techniques. Here, the utility of calcium VtC XES is established using an in-house dispersive spectrometer in combination with density functional theory. Spectral trends are rationalized within a molecular orbital framework, and Kβ_(2,5) transitions, derived from molecular orbitals with primarily ligand p character, are found to be a promising probe of the calcium coordination environment. In particular, it is shown that calcium VtC XES is sensitive to the electronic structure changes that accompany oxo protonation in Mn₃CaO₄-based molecular mimics of the OEC. Through correlation to calculations, the potential of calcium VtC XES to address unresolved questions regarding the mechanism of biological water oxidation is highlighted

Photon shot-noise limited transient absorption soft X-ray spectroscopy at the European XFEL

Femtosecond transient soft X-ray Absorption Spectroscopy (XAS) is a very promising technique that can be employed at X-ray Free Electron Lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here we present a dedicated setup for soft X-rays available at the Spectroscopy & Coherent Scattering (SCS) instrument at the European X-ray Free Electron Laser (EuXFEL). It consists of a beam-splitting off-axis zone plate (BOZ) used in transmission to create three copies of the incoming beam, which are used to measure the transmitted intensity through the excited and unexcited sample, as well as to monitor the incoming intensity. Since these three intensity signals are detected shot-by-shot and simultaneously, this setup allows normalized shot-by-shot analysis of the transmission. For photon detection, the DSSC imaging detector, which is capable of recording up to 800 images at 4.5 MHz frame rate during the FEL burst, is employed and allows approaching the photon shot-noise limit. We review the setup and its capabilities, as well as the online and offline analysis tools provided to users.Comment: 11 figure

Symmetry-dependent ultrafast manipulation of nanoscale magnetic domains

Femtosecond optical pumping of magnetic materials has been used to achieve ultrafast switching and recently to nucleate symmetry-broken magnetic states. However, when the magnetic order parameter already presents a broken-symmetry state, such as a domain pattern, the dynamics are poorly understood and consensus remains elusive. Here, we resolve the controversies in the literature by studying the ultrafast response of magnetic domain patterns with varying degrees of translation symmetry with ultrafast x-ray resonant scattering. A data analysis technique is introduced to disentangle the isotropic and anisotropic components of the x-ray scattering. We find that the scattered intensity exhibits a radial shift restricted to the isotropic component, indicating that the far-from-equilibrium magnetization dynamics are intrinsically related to the spatial features of the domain pattern. Our results suggest alternative pathways for the spatiotemporal manipulation of magnetism via far-from-equilibrium dynamics and by carefully tuning the ground-state magnetic textures

Nonequilibrium sub–10 nm spin-wave soliton formation in FePt nanoparticles

Magnetic nanoparticles such as FePt in the L1 0 phase are the bedrock of our current data storage technology. As the grains become smaller to keep up with technological demands, the superparamagnetic limit calls for materials with higher magnetocrystalline anisotropy. This, in turn, reduces the magnetic exchange length to just a few nanometers, enabling magnetic structures to be induced within the nanoparticles. Here, we describe the existence of spin-wave solitons, dynamic localized bound states of spin-wave excitations, in FePt nanoparticles. We show with time-resolved x-ray diffraction and micromagnetic modeling that spin-wave solitons of sub–10 nm sizes form out of the demagnetized state following femtosecond laser excitation. The measured soliton spin precession frequency of 0.1 THz positions this system as a platform to develop novel miniature devices

Factors Associated with Revision Surgery after Internal Fixation of Hip Fractures

Background: Femoral neck fractures are associated with high rates of revision surgery after management with internal fixation. Using data from the Fixation using Alternative Implants for the Treatment of Hip fractures (FAITH) trial evaluating methods of internal fixation in patients with femoral neck fractures, we investigated associations between baseline and surgical factors and the need for revision surgery to promote healing, relieve pain, treat infection or improve function over 24 months postsurgery. Additionally, we investigated factors associated with (1) hardware removal and (2) implant exchange from cancellous screws (CS) or sliding hip screw (SHS) to total hip arthroplasty, hemiarthroplasty, or another internal fixation device. Methods: We identified 15 potential factors a priori that may be associated with revision surgery, 7 with hardware removal, and 14 with implant exchange. We used multivariable Cox proportional hazards analyses in our investigation. Results: Factors associated with increased risk of revision surgery included: female sex, [hazard ratio (HR) 1.79, 95% confidence interval (CI) 1.25-2.50; P = 0.001], higher body mass index (fo

Probing the Electronic Structure of a Photoexcited Solar Cell Dye with Transient X-ray Absorption Spectroscopy

This study uses transient X-ray absorption (XA) spectroscopy and timedependent density functional theory (TD-DFT) to directly visualize the charge density around the metal atom and the surrounding ligands following an ultrafast metal-to-ligand charge-transfer (MLCT) process in the widely used RuII solar cell dye, Ru(dcbpy)2(NCS)2 (termed N3). We measure the Ru L-edge XA spectra of the singlet ground (1A1) and the transient triplet (3MLCT) excited state of N34 and perform TD-DFT calculations of 2p core-level excitations, which identify a unique spectral signature of the electron density on the NCS ligands. We find that the Ru 2p, Ru eg, and NCS orbitals are stabilized by 2.0, 1.0, and 0.6 eV, respectively, in the transient 3MLCT state of the dye. These results highlight the role of the NCS ligands in governing the oxidation state of the Ru center

Simulating Ru L3-Edge X-ray Absorption Spectroscopy with Time-Dependent Density Functional Theory: Model Complexes and Electron Localization in Mixed-Valence Metal Dimers

Ruthenium L3-edge X-ray absorption (XA) spectroscopy probes unoccupied 4d orbitals of the metal atom and is increasingly being used to investigate the local electronic structure in ground and excited electronic states of Ru complexes. The simultaneous development of computational tools for simulating Ru L3-edge spectra is crucial for interpreting the spectral features at a molecular level. This study demonstrates that time-dependent density functional theory (TDDFT) is a viable and predictive tool for simulating ruthenium L3-edge XA spectroscopy. We systematically investigate the effects of exchange correlation functional and implicit and explicit solvent interactions on a series of RuII and RuIII complexes in their ground and electronic excited states. The TDDFT simulations reproduce all of the experimentally observed features in Ru L3-edge XA spectra within the experimental resolution (0.4 eV). Our simulations identify ligand-specific charge transfer features in complicated Ru L3-edge spectra of [Ru(CN)6]4- and RuII polypyridyl complexes illustrating the advantage of using TDDFT in complex systems. We conclude that the B3LYP functional most accurately predicts the transition energies of charge transfer features in these systems. We use our TDDFT approach to simulate experimental Ru L3-edge XA spectra of transition metal mixed-valence dimers of the form [(NC)5MII-CN-RuIII(NH3)5] (where M = Fe or Ru) dissolved in water. Our study determines the spectral signatures of electron delocalization in Ru L3-edge XA spectra. We find that the inclusion of explicit solvent molecules is necessary for reproducing the spectral features and the experimentally determined valencies in these mixed-valence complexes. This study validates the use of TDDFT for simulating Ru 2p excitations using popular quantum chemistry codes and providing a powerful interpretive tool for equilibrium and ultrafast Ru L3-edge XA spectroscopy

Interrogating the Electronic Structure and Photochemistry of Molecules with Transient X-ray Absorption Spectroscopy

Thesis (Ph.D.)--University of Washington, 2014A detailed understanding of photochemical processes in molecules and materials is necessary for understanding natural phenomena and engineering new technologies. In this thesis, light-induced changes in spin states, rearrangements of electron densities, and proton transfer events are considered. X-ray absorption (XA) spectroscopy is used to investigate the electronic structure of molecules in electronic ground and excited states to reveal fundamental insights on the nature of photochemical events. The photoinduced FeII spin crossover (SCO) reaction is investigated theoretically. Following photoexcitation, an FeII SCO molecule undergoes a conversion from a singlet ground state to a quintet excited state. Transition-potential density functional theory and time-dependent density functional theory (TDDFT) are used to simulate the transient Fe K-edge XA spectra. The spectral signature each spin state is identified in the near-edge and pre-edge regions of the XA spectra. Ruthenium L-edge spectroscopy is used to investigate the electronic structure and photochemistry of solar cell dye molecules and transition metal mixed-valence complexes. The metal-to-ligand charge transfer (MLCT) chemistry is studied in the widely used "N3" dye ([Ru(dcbpy)2(NCS)2¬]) molecule. Ru L-edge XA spectroscopy shows that the formation of the MLCT state corresponds to a light-induced oxidation of the Ru atom from RuII to RuIII. Moreover, charge transfer features in the show the important role of the NCS- ligands in the photochemistry of N3. TDDFT is used to simulate Ru L3-edge spectra of N3 and other Ru complexes. This validates its use as a predictive tool for simulating Ru L3-edge spectroscopy. Finally, the Ru L3-edge spectra of the mixed-valence metal complexes [(NC)5FeIICNRuIII(NH3)¬5]- and [(NC)5RuIICNRuIII(NH3)¬5]-. Quantum chemical simulation using explicit water molecules are required to reproduce experimental spectra highlighting the role of the hydrogen bonding solvent in determining the properties of these complexes. Finally, the photochemistry of 2-thiopyridine (2TP) is investigated. 2TP exists in solution in equilibrium with its tautomer 2-mercaptopyridine (2MP). The possibility of excited state proton transfer from 2TP to 2MP is considered. The transient S K-edge XA spectra are measured for 2TP in acetonitrile. The time-dependence of the transient absorption signal shows that multiple photoproducts are formed. This establishes transient S K-edge XA spectroscopy as a power for tool for studying photochemistry in organic and biological systems

Measuring Spin-Allowed and Spin-Forbidden d–d Excitations in Vanadium Complexes with 2p3d Resonant Inelastic X‑ray Scattering

Spectroscopic probes of the electronic structure of transition metal-containing materials are invaluable to the design of new molecular catalysts and magnetic systems. Herein, we show that 2p3d resonant inelastic X-ray scattering (RIXS) can be used to observe both spin-allowed and (in the V^III case) spin-forbidden d–d excitation energies in molecular vanadium complexes. The spin-allowed d–d excitation energies determined by 2p3d RIXS are in good agreement with available optical data. In V­(acac)₃, a previously undetected spin-forbidden singlet state has been observed. The presence of this feature provides a ligand-field independent signature of V^III. It is also shown that d–d excitations may be obtained for porphyrin complexes. This is generally prohibitive using optical approaches due to intense porphyrin π-to-π* transitions. In addition, the intensities of charge-transfer features in 2p3d RIXS spectroscopy are shown to be a clear indication of metal–ligand covalency. The utility of 2p3d RIXS for future studies of complex inorganic systems is highlighted