Search

Photoacids show a strong increase in acidity in the first electronic excited state, enabling real time studies of proton transfer in acid base reactions, proton transport in energy storage devices and biomolecular sensor protein systems. Several explanations have been proposed for what determines photoacidity, ranging from variations in solvation free energy to changes in electronic structure occurring along the four stages of the Förster cycle. Here we use picosecond nitrogen K edge spectroscopy to monitor the electronic structure changes of the proton donating group in a protonated aromatic amine photoacid in solution upon photoexcitation and subsequent proton transfer dynamics. Probing core to valence transitions locally at the amine functional group and with orbital specificity, we clearly reveal pronounced electronic structure, dipole moment and energetic changes on the conjugate photobase side. This result paves the way for a detailed electronic structural characterization of the photoacidity phenomenon

The atomic specificity of X-ray spectroscopies provides a distinct perspective on molecular electronic structure. For 3 d metal coordination and organometallic complexes, the combination of metal- and ligand-specific X-ray spectroscopies directly interrogates metal–ligand covalency—the hybridization of metal and ligand electronic states. Resonant inelastic X-ray scattering (RIXS), the X-ray analog of resonance Raman scattering, provides access to all classes of valence excited states in transition-metal complexes, making it a particularly powerful means of characterizing the valence electronic structure of 3 d metal complexes. Recent advances in X-ray free-electron laser sources have enabled RIXS to be extended to the ultrafast time domain. We review RIXS studies of two archetypical photochemical processes: charge-transfer excitation in ferricyanide and ligand photodissociation in iron pentacarbonyl. These studies demonstratefemtosecond-resolution RIXS can directly characterize the time-evolving electronic structure, including the evolution of the metal–ligand covalency.

We determine how the transient electronic structure changes of imidazole base upon proton transfer from a naphthol photoacid in aqueous solution can be locally monitored with ultrafast nitrogen K-edge spectroscopy.

Using ultrafast X-ray absorption spectroscopy, we observe how a rhodium carbonyl catalyst is formed on femtosecond timescales and reveal the decisive orbital interactions which facilitate the efficient cleavage of an alkane C-H bond from solution.

Early excited state dynamics in the photodissociation of transition metal carbonyls determines the chemical nature of short-lived catalytically active reaction intermediates. However, time-resolved experiments have not yet revealed mechanistic details in the sub-picosecond regime. Hence, in this study the photoexcitation of ironpentacarbonyl Fe(CO)(5) is simulated with semi-classical excited state molecular dynamics. We find that the bright metal-to-ligand charge-transfer (MLCT) transition induces synchronous Fe-C oscillations in the trigonal bipyramidal complex leading to periodically reoccurring release of predominantly axial CO. Metaphorically the photoactivated Fe(CO)(5) acts as a CO geyser, as a result of dynamics in the potential energy landscape of the axial Fe-C distances and non-adiabatic transitions between manifolds of bound MLCT and dissociative metal-centered (MC) excited states. The predominant release of axial CO ligands and delayed release of equatorial CO ligands are explained in a unified mechanism based on the sigma*(Fe-C) anti-bonding character of the receiving orbital in the dissociative MC states. The photodissociation of transition metal carbonyls is involved in catalysis and synthetic processes. Here the authors, using semi-classical excited state molecular dynamics, observe details of the early stage dynamics in the photodissociation of Fe(CO)(5), including synchronous bursts of CO at periodic intervals of 90 femtoseconds.

The early excited state dynamics in the photodissociation of transition metal carbonyls determines the chemical nature of short-lived catalytically active reaction intermediates. However, time-resolved experiments have not yet revealed the mechanistic details in the sub-picosecond regime. Hence in this study, the photoexcitation of ironpentacarbonyl Fe(CO)5 has been simulated by semi-classical surface-hopping excited state molecular dynamics based on time-dependent density functional theory. We �find that the bright metal-to-ligand charge-transfer (MLCT) transition induces Fe-C oscillations in the trigonal bipyramidal complex leading to periodically reoccurring release of predominantly axial CO. Metaphorically the photoactivated Fe(CO)5 acts as a CO geyser, which we explain in terms of dynamics in the potential energy landscape of the axial Fe-C distances and non-adiabatic transitions between manifolds of bound MLCT and dissociative metal-centered(MC) excited states. The predominant release of axial CO ligands and a delayed release of equatorial CO ligands are explained in an unified mechanism based on the (sigma*)(Fe-C) anti-bonding character of the receiving orbital in the dissociative MC states both in the Franck-Condon region and at distorted geometries.

The early excited state dynamics in the photodissociation of transition metal carbonyls determines the chemical nature of short-lived catalytically active reaction intermediates. However, time-resolved experiments have not yet revealed the mechanistic details in the sub-picosecond regime. Hence in this study, the photoexcitation of ironpentacarbonyl Fe(CO)5 has been simulated by semi-classical surface-hopping excited state molecular dynamics based on time-dependent density functional theory. We �find that the bright metal-to-ligand charge-transfer (MLCT) transition induces Fe-C oscillations in the trigonal bipyramidal complex leading to periodically reoccurring release of predominantly axial CO. Metaphorically the photoactivated Fe(CO)5 acts as a CO geyser, which we explain in terms of dynamics in the potential energy landscape of the axial Fe-C distances and non-adiabatic transitions between manifolds of bound MLCT and dissociative metal-centered(MC) excited states. The predominant release of axial CO ligands and a delayed release of equatorial CO ligands are explained in an unified mechanism based on the (sigma*)(Fe-C) anti-bonding character of the receiving orbital in the dissociative MC states both in the Franck-Condon region and at distorted geometries.

The metal centres in metalloenzymes and molecular catalysts are responsible for the rearrangement of atoms and electrons during complex chemical reactions, and they enable selective pathways of charge and spin transfer, bond breaking/making and the formation of new molecules. Mapping the electronic structural changes at the metal sites during the reactions gives a unique mechanistic insight that has been difficult to obtain to date. The development of X-ray free-electron lasers (XFELs) enables powerful new probes of electronic structure dynamics to advance our understanding of metalloenzymes. The ultrashort, intense and tunable XFEL pulses enable X-ray spectroscopic studies of metalloenzymes, molecular catalysts and chemical reactions, under functional conditions and in real time. In this Technical Review, we describe the current state of the art of X-ray spectroscopy studies at XFELs and highlight some new techniques currently under development. With more XFEL facilities starting operation and more in the planning or construction phase, new capabilities are expected, including high repetition rate, better XFEL pulse control and advanced instrumentation. For the first time, it will be possible to make real-time molecular movies of metalloenzymes and catalysts in solution, while chemical reactions are taking place. X-ray lasers offer unprecedented capabilities, with their tunable, intense and short X-ray pulses. This Technical Review discusses the current and future use of X-ray lasers for probing molecular catalysts and metalloenzymes and their chemical reactions in real time and under functional conditions.

We demonstrate for the case of photoexcited [Ru 2,2 amp; 8242; bipyridine 3]2 how femtosecond resonant inelastic X ray scattering RIXS at the ligand K edge allows one to uniquely probe changes in the valence electronic structure following a metal to ligand charge transfer MLCT excitation. Metal ligand hybridization is probed by nitrogen 1s resonances providing information on both the electron accepting ligand in the MLCT state and the hole density of the metal center. By comparing to spectrum calculations based on density functional theory, we are able to distinguish the electronic structure of the electron accepting ligand and the other ligands and determine a temporal upper limit of 250 40 fs for electron localization following the charge transfer excitation. The spin of the localized electron is deduced from the selection rules of the RIXS process establishing new experimental capabilities for probing transient charge and spin densities

We locally probe with picosecond N K-edge spectroscopy electronic structure changes along all stages of the Förster photocycle of a prototypical photoacid, and determine how photoacid behaviour is driven by the conjugate photobase side.

This chapter illustrates how resonant inelastic x-ray scattering (RIXS) is used to address questions in chemistry, with special focus on the electronic structure and catalytic activity of first row transition metals. RIXS is a two-photon process that is the x-ray equivalent of resonance Raman spectroscopy. The final states correspond to vibrational, valence electronic or even core excitations. In addition to the advantages of a local element-selective x-ray spectroscopic probe, RIXS gives new information compared to single-photon x-ray absorption and x-ray emission experiments. Metal L-edge RIXS shows intense metal-centered ligand- field transitions, even in cases where they are spin or parity forbidden in optical absorption spectroscopy. By selecting different resonances by appropriately tuning the incident energy, it is possible to isolate different ligand-field and charge-transfer transitions. The observation of a large number of electronic states that can be properly assigned, sometimes with the help of theoretical methods, gives novel opportunities to quantify metal-ligand interactions and their contributions to reactivity. RIXS in the K pre-edge can be used to obtain L- and M-edge like spectra including insight into charge-transfer excitations all with the advantages of a hard x-ray probe. Finally, it is shown how time-resolved RIXS down to the femtosecond timescale probes the orbitals of transient reaction intermediates. The usefulness of RIXS in chemistry is shown for a diverse set of systems, including coordination complexes, metal enzymes, and nanoparticles.

We use N K edge absorption spectroscopy to explore the electronic structure of the amine group, one of the most prototypical chemical functionalities playing a key role in acid amp; 8722;base chemistry, electron donor amp; 8722; acceptor interactions, and nucleophilic substitution reactions. In this study, we focus on aliphatic amines and make use of the nitrogen 1s core electron excitations to elucidate the roles of N amp; 8722;H amp; 963; and N amp; 8722;C amp; 963; contributions in the unoccupied orbitals. We have measured N K edge absorption spectra of the ethylamine bases EtxNH3 amp; 8722;x x 0...3; Et amp; 8722; C2H5 amp; 8722; and the conjugate positively charged ethylammonium cation acids EtyNH4 amp; 8722;y y 0...4; Et amp; 8722; C2H5 amp; 8722; dissolved in the protic solvents ethanol and water. Upon consecutive exchange of N amp; 8722;H for ethyl groups, we observe a spectral shift, a systematic decrease of the N K edge pre edge peak, and a major contribution in the postedge region for the ethylamine series. Instead, for the ethylammonium ions, the consecutive exchange of N amp; 8722;H for ethyl groups leads to an apparent reduction of pre edge and post edge intensities relative to the main edge band, without significant frequency shifts. Building on findings from our previously reported study on aqueous ammonia and ammonium ions, we can rationalize these observations by comparing calculated N K edge absorption spectra of free and hydrogen bonded clusters. Hydrogen bonding interactions lead only to minor spectral effects in the ethylamine series, but have a large impact in the ethylammonium ion series. Visualization of the unoccupied molecular orbitals shows the consecutive change in molecular orbital character from N amp; 8722;H amp; 963; to N amp; 8722;C amp; 963; in these alkylamine alkylammonium ion series. This can act as a benchmark for future studies on chemically more involved amine compounds

The journal of physical chemistry letters 9(12), 3538 - 3543 (2018). doi:10.1021/acs.jpclett.8b01429, Soft X-ray spectroscopies are ideal probes of the local valence electronic structure of photocatalytically active metal sites. Here, we apply the selectivity of time-resolved resonant inelastic X-ray scattering at the iron L-edge to the transient charge distribution of an optically excited charge-transfer state in aqueous ferricyanide. Through comparison to steady-state spectra and quantum chemical calculations, the coupled effects of valence-shell closing and ligand-hole creation are experimentally and theoretically disentangled and described in terms of orbital occupancy, metal–ligand covalency, and ligand field splitting, thereby extending established steady-state concepts to the excited-state domain. π-Back-donation is found to be mainly determined by the metal site occupation, whereas the ligand hole instead influences σ-donation. Our results demonstrate how ultrafast resonant inelastic X-ray scattering can help characterize local charge distributions around catalytic metal centers in short-lived charge-transfer excited states, as a step toward future rationalization and tailoring of photocatalytic capabilities of transition-metal complexes., Published by ACS, Washington, DC

Transition metals in inorganic systems and metalloproteins can occur in different oxidation states, which makes them ideal redox-active catalysts. To gain a mechanistic understanding of the catalytic reactions, knowledge of the oxidation state of the active metals, ideally in operando, is therefore critical. L-edge X-ray absorption spectroscopy (XAS) is a powerful technique that is frequently used to infer the oxidation state via a distinct blue shift of L-edge absorption energies with increasing oxidation state. A unified description accounting for quantum-chemical notions whereupon oxidation does not occur locally on the metal but on the whole molecule and the basic understanding that L-edge XAS probes the electronic structure locally at the metal has been missing to date. Here we quantify how charge and spin densities change at the metal and throughout the molecule for both redox and core-excitation processes. We explain the origin of the L-edge XAS shift between the high-spin complexes Mn-II(acac)(2) and Mn-III(acac)(3) as representative model systems and use ab initio theory to uncouple effects of oxidation-state changes from geometric effects. The shift reflects an increased electron affinity of Mn-III in the core-excited states compared to the ground state due to a contraction of the Mn 3d shell upon core-excitation with accompanied changes in the classical Coulomb interactions. This new picture quantifies how the metal-centered core hole probes changes in formal oxidation state and encloses and substantiates earlier explanations. The approach is broadly applicable to mechanistic studies of redox-catalytic reactions in molecular systems where charge and spin localization/delocalization determine reaction pathways., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe, 656

© 2018 the Owner Societies. X-ray induced sample damage can impede electronic and structural investigations of radiation-sensitive samples studied with X-rays. Here we quantify dose-dependent sample damage to the prototypical MnIII(acac)3complex in solution and at room temperature for the soft X-ray range, using X-ray absorption spectroscopy at the Mn L-edge. We observe the appearance of a reduced MnIIspecies as the X-ray dose is increased. We find a half-damage dose of 1.6 MGy and quantify a spectroscopically tolerable dose on the order of 0.3 MGy (1 Gy = 1 J kg-1), where 90% of MnIII(acac)3are intact. Our dose-limit is around one order of magnitude lower than the Henderson limit (half-damage dose of 20 MGy) which is commonly employed for protein crystallography with hard X-rays. It is comparable, however, to the dose-limits obtained for collecting un-damaged Mn K-edge spectra of the photosystem II protein, using hard X-rays. The dose-dependent reduction of MnIIIobserved here for solution samples occurs at a dose limit that is two to four orders of magnitude smaller than the dose limits previously reported for soft X-ray spectroscopy of iron samples in the solid phase. We compare our measured to calculated spectra from ab initio restricted active space (RAS) theory and discuss possible mechanisms for the observed dose-dependent damage of MnIII(acac)3in solution. On the basis of our results, we assess the influence of sample damage in other experimental studies with soft X-rays from storage-ring synchrotron radiation sources and X-ray free-electron lasers.

© 2018 American Chemical Society. The 3d transition metals play a pivotal role in many charge transfer processes in catalysis and biology. X-ray absorption spectroscopy at the L-edge of metal sites probes metal 2p-3d excitations, providing key access to their valence electronic structure, which is crucial for understanding these processes. We report L-edge absorption spectra of MnII(acac)2and MnIII(acac)3complexes in solution, utilizing a liquid flatjet for X-ray absorption spectroscopy in transmission mode. With this, we derive absolute absorption cross-sections for the L-edge transitions with peak magnitudes as large as 12 and 9 Mb for MnII(acac)2and MnIII(acac)3, respectively. We provide insight into the electronic structure with ab initio restricted active space calculations of these L-edge transitions, reproducing the experimental spectra with excellent agreement in terms of shapes, relative energies, and relative intensities for the two complexes. Crystal field multiplet theory is used to assign spectral features in terms of the electronic structure. Comparison to charge transfer multiplet calculations reveals the importance of charge transfer in the core-excited final states. On the basis of our experimental observations, we extrapolate the feasibility of 3d transition metal L-edge absorption spectroscopy using the liquid flatjet approach in probing highly dilute biological solution samples and possible extensions to table-top soft X-ray sources.

We describe how inversion symmetry separation of electronic state manifolds in resonant inelastic soft X-ray scattering (RIXS) can be applied to probe excited-state dynamics with compelling selectivity. In a case study of Fe L-3-edge RIXS in the ferricyanide complex Fe(CN)(6)(3-), we demonstrate with multi-configurational restricted active space spectrum simulations how the information content of RIXS spectral fingerprints can be used to unambiguously separate species of different electronic configurations, spin multiplicities, and structures, with possible involvement in the decay dynamics of photo-excited ligand-to-metal charge-transfer. Specifically, we propose that this could be applied to confirm or reject the presence of a hitherto elusive transient Quartet species. Thus, RIXS offers a particular possibility to settle a recent controversy regarding the decay pathway, and we expect the technique to be similarly applicable in other model systems of photo-induced dynamics., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe, 779

The Q2 prototypical photoinduced dissociation of Fe CO 5 in the gas phase is used to test time resolved x ray photoelectron spectroscopy for studying photochemical reactions. Upon one photon excitation at 266 nm, Fe CO 5 successively dissociates to Fe CO 4 and Fe CO 3 along a pathway where both fragments retain the singlet multiplicity of Fe CO 5. The x ray free electron laser FLASH is used to probe the reaction intermediates Fe CO 4 and Fe CO 3 with time resolved valence and core level photoelectron spectroscopy, and experimental results are interpreted with ab initio quantum chemical calculations. Changes in the valence photoelectron spectra are shown to reflect changes in the valenceorbital interactions upon Fe CO dissociation, thereby validating fundamental theoretical concepts in Fe CO bonding. Chemical shifts of CO 3 inner valence and Fe 3p core level binding energies are shown to correlate with changes in the coordination number of the Fe center. We interpret this with coordination dependent charge localization and core hole screening based on calculated changes in electron densities upon core hole creation in the final ionic states. This extends the established capabilities of steady state electron spectroscopy for chemical analysis to time resolved investigations. It could also serve as a benchmark for howcharge and spin density changes in molecular dissociation and excited state dynamics are expressed in valence and core level photoelectron spectroscopy

While ubiquitous, the making and breaking of hydrogen bonds in solution is notoriously difficult to study due to the associated complex changes of nuclear and electronic structures. With the aim to reduce the according uncertainty in correlating experimental observables and hydrogen-bond configurations, we combine the information from proximate methods to study the N-H center dot center dot center dot O hydrogen bond in solution. We investigate hydrogen-bonding of the N-H group of N-methylaniline with oxygen from liquid DMSO and acetone with infrared spectra in the N-H stretching region and X-ray absorption spectra at the N K-edge. We experimentally observe blue shifts of the infrared stretching band and an X-ray absorption pre-edge peak when going from DMSO to acetone. With ab initio molecular dynamics simulations and calculated spectra, we qualitatively reproduce the experimental observables but we do not reach quantitative agreement with experiment. The infrared spectra support the notion of weakening the N-H center dot center dot center dot O hydrogen bond from DMSO to acetone. However, we fail to theoretically reproduce the measured shift of the X-ray absorption pre-edge peak. We discuss possible shortcomings of the simulation models and spectrum calculations. Common features and distinct differences with the O-H center dot center dot center dot O hydrogen bond are highlighted, and the implications for monitoring hydrogen-bond breaking in solution are discussed.

We present a setup combining a liquid flatjet sample delivery and a MHz laser system for time-resolved soft X-ray absorption measurements of liquid samples at the high brilliance undulator beamline UE52-SGM at Bessy II yielding unprecedented statistics in this spectral range. We demonstrate that the efficient detection of transient absorption changes in transmission mode enables the identification of photoexcited species in dilute samples. With iron(II)-trisbipyridine in aqueous solution as a benchmark system, we present absorption measurements at various edges in the soft X-ray regime. In combination with the wavelength tunability of the laser system, the set-up opens up opportunities to study the photochemistry of many systems at low concentrations, relevant to materials sciences, chemistry, and biology., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe, 780

In a multifaceted investigation combining local soft X-ray and vibrational spectroscopic probes with ab initio molecular dynamics simulations, hydrogen-bonding interactions of two key principal amine compounds in aqueous solution, ammonia (NH3) and ammonium ion (NH4+), are quantitatively assessed in terms of electronic structure, solvation structure, and dynamics. From the X-ray measurements and complementary determination of the IR-active hydrogen stretching and bending modes of NH3 and NH4+ in aqueous solution, the picture emerges of a comparatively strongly hydrogen-bonded NH4+ ion via N–H donating interactions, whereas NH3 has a strongly accepting hydrogen bond with one water molecule at the nitrogen lone pair but only weakly N–H donating hydrogen bonds. In contrast to the case of hydrogen bonding among solvent water molecules, we find that energy mismatch between occupied orbitals of both the solutes NH3 and NH4+ and the surrounding water prevents strong mixing between orbitals upon hydrogen bonding ...

The exemplary case of the photochemically activated prototypical metal complex iron pentacarbonyl Fe(CO)5 in solution is used to illustrate how the time-dependent changes in resonant inelastic X-ray scattering (RIXS), the X-ray analogue of resonant Raman scattering, reflect changes in the frontier-orbital interactions and populations in the system. It is shown how probing frontier-orbital interactions with atomic specificity and with a temporal resolution in the femtosecond range with ultrashort soft X-ray pulses from an X-ray free-electron laser enables correlation of metal–ligand coordination with orbital symmetry, spin multiplicity and reactivity. Tuneable, intense and ultrashort soft X-ray pulses for spectroscopy could enable predicting photochemical processes in model systems, explaining photochemical catalysis of real functional systems and understanding the local chemistry or the local chemical interactions in metalloproteins as they function. Upcoming high-repetition rate X-ray free-electron lasers will extend the potential of femtosecond time-resolved soft X-ray spectroscopy.

X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn ∼ 6–15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn4CaO5) in Photosystem II (Mn

Hydration shells around ions are crucial for many fundamental biological and chemical processes. Their local physicochemical properties are quite different from those of bulk water and hard to probe experimentally. We address this problem by combining soft X-ray spectroscopy using a liquid jet and molecular dynamics (MD) simulations together with ab initio electronic structure calculations to elucidate the water–ion interaction in a MgCl2 solution at the molecular level. Our results reveal that salt ions mainly affect the electronic properties of water molecules in close vicinity and that the oxygen K-edge X-ray emission spectrum of water molecules in the first solvation shell differs significantly from that of bulk water. Ion-specific effects are identified by fingerprint features in the water X-ray emission spectra. While Mg2+ ions cause a bathochromic shift of the water lone pair orbital, the 3p orbital of the Cl– ions causes an additional peak in the water emission spectrum at around 528 eV. peerReviewed

Fe L-edge X-ray absorption spectra of gas-phase iron pentacarbonyl and ferrocene measured in total-ion yield mode are reported. Comparison to previously published spectra of free iron atoms and gaseous iron chloride demonstrates how the interplay of local atomic multiplet effects and orbital interactions in the metal-ligand bonds varies for the different systems. We find changes in the degree of metal-ligand covalency to be reflected in the measured 2p absorption onset. Orbital- or state-specific fragmentation is furthermore investigated in iron pentacarbonyl and ferrocene by analyzing the partial-ion-yield spectra at the Fe L-edge. Strong dependence of the yields of different fragments is observed in ferrocene in contrast to iron pentacarbonyl. This difference is attributed to the different degrees to which the 2p core hole is screened in the two systems and to which charge is rearranged in the Auger final states. We provide experimental benchmark spectra for new ab initio approaches for calculating metal L-edge absorption spectra of metal complexes.

© 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. Light-induced oxidation of water by photosystem II (PS II) in plants, algae and cyanobacteria has generated most of the dioxygen in the atmosphere. PS II, a membrane-bound multi-subunit pigment protein complex, couples the one-electron photochemistry at the reaction centre with the four-electron redox chemistry of water oxidation at the Mn 4 CaO 5 cluster in the oxygen-evolving complex (OEC). Under illumination, the OEC cycles through five intermediate S-states (S 0 to S 4), in which S 1 is the dark-stable state and S 3 is the last semi-stable state before O-O bond formation and O 2 evolution. A detailed understanding of the O-O bond formation mechanism remains a challenge, and will require elucidation of both the structures of the OEC in the different S-states and the binding of the two substrate waters to the catalytic site. Here we report the use of femtosecond pulses from an X-ray free electron laser (XFEL) to obtain damage-free, room temperature structures of dark-adapted (S 1), two-flash illuminated (2F; S 3 -enriched), and ammonia-bound two-flash illuminated (2F-NH 3; S 3 -enriched) PS II. Although the recent 1.95 Å resolution structure of PS II at cryogenic temperature using an XFEL provided a damage-free view of the S 1 state, measurements at room temperature are required to study the structural landscape of proteins under functional conditions, and also for in situ advancement of the S-states. To investigate the water-binding site(s), ammonia, a water analogue, has been used as a marker, as it binds to the Mn 4 CaO 5 cluster in the S 2 and S 3 states. Since the ammonia-bound OEC is active, the ammonia-binding Mn site is not a substrate water site. This approach, together with a comparison of the native dark and 2F states, is used to discriminate between proposed O-O bond formation mechanisms.

X-ray free electron lasers (XFELs) enable unprecedented new ways to study the electronic structure and dynamics of transition metal systems. L-edge absorption spectroscopy is a powerful technique for such studies and the feasibility of this method at XFELs for solutions and solids has been demonstrated. However, the required x-ray bandwidth is an order of magnitude narrower than that of self-amplified spontaneous emission (SASE), and additional monochromatization is needed. Here we compare L-edge x-ray absorption spectroscopy (XAS) of a prototypical transition metal system based on monochromatizing the SASE radiation of the linac coherent light source (LCLS) with a new technique based on self-seeding of LCLS. We demonstrate how L-edge XAS can be performed using the self-seeding scheme without the need of an additional beam line monochromator. We show how the spectral shape and pulse energy depend on the undulator setup and how this affects the x-ray spectroscopy measurements. (C) 2016 Optical Society of America

Ultrafast electronic and structural dynamics of matter govern rate and selectivity of chemical reactions, as well as phase transitions and efficient switching in functional materials. Since x-rays determine electronic and structural properties with elemental, chemical, orbital and magnetic selectivity, short pulse x-ray sources have become central enablers of ultrafast science. Despite of these strengths, ultrafast x-rays have been poor at picking up excited state moieties from the unexcited ones. With time-resolved anti-Stokes resonant x-ray Raman scattering (AS-RXRS) performed at the LCLS, and ab initio theory we establish background free excited state selectivity in addition to the elemental, chemical, orbital and magnetic selectivity of x-rays. This unparalleled selectivity extracts low concentration excited state species along the pathway of photo induced ligand exchange of Fe(CO)5 in ethanol. Conceptually a full theoretical treatment of all accessible insights to excited state dynamics with AS-RXRS with transform-limited x-ray pulses is given—which will be covered experimentally by upcoming transform-limited x-ray sources. peerReviewed

The valence excited states of ferric and ferrous hexacyanide ions in aqueous solution were mapped by resonant inelastic X ray scattering RIXS at the Fe L2,3 and N K edges. Probing of both the central Fe and the ligand N atoms enabled identification of the metal and ligand centered excited states, as well as ligand to metal and metal to ligand charge transfer excited states. Ab initio calculations utilizing the RASPT2 method were used to simulate the Fe L2,3 edge RIXS spectra and enabled quantification of the covalencies of both occupied and empty orbitals of amp; 960; and amp; 963; symmetry. We found that amp; 960; back donation in the ferric complex is smaller than that in the ferrous complex. This is evidenced by the relative amounts of Fe 3d character in the nominally 2 amp; 960; CN amp; 8722; molecular orbital of 7 and 9 in ferric and ferrous hexacyanide, respectively. Utilizing the direct sensitivity of Fe L3 edge RIXS to the Fe 3d character in the occupied molecular orbitals, we also found that the donation interactions are dominated by amp; 963; bonding. The latter was found to be stronger in the ferric complex, with an Fe 3d contribution to the nominally 5 amp; 963; CN amp; 8722; molecular orbitals of 29 compared to 20 in the ferrous complex. These results are consistent with the notion that a higher charge at the central metal atom increases donation and decreases back donation

Here we present a review of X-ray absorption spectroscopy and X-ray Raman scattering with the perspective to understand the spectra of water including changes with temperature, mass of the water mo ...

We present polarization-controlled multiphoton two-color above-threshold ionization (TCATI) of molecules. The intensity modulations of valence photoelectron intensities of molecules arising from varying the relative orientation of the linear polarization vectors of femtosecond infrared (IR) and vacuum-ultraviolet (VUV) radiation in TCATI of the highest occupied molecular orbitals of ${\mathrm{H}}_{2}\mathrm{O}, {\mathrm{O}}_{2}$, and ${\mathrm{N}}_{2}$ are reported. The results on the molecular systems are compared to the $3p$ photoionization of atomic Ar, which serves as a reference system. Modeling the large differences of the modulation amplitudes within the soft-photon approximation enables us to extract the one-photon-ionization anisotropy parameter ${\ensuremath{\beta}}_{2}$. Accounting only for the first sideband due to two-photon TCATI by one VUV and one IR photon we find satisfactory agreement between experiment and simulation for ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{O}}_{2}$. However, the model fails for ${\mathrm{N}}_{2}$ and possible reasons are discussed. We discuss that the described approach may represent an alternative way of determining photoelectron angular distributions from valence shells of molecules and indicate future directions for modeling TCATI of molecules.

X-ray spectroscopy is a powerful tool to study the local charge distribution of chemical systems. Together with the liquid jet it becomes possible to probe chemical systems in their natural environment, the liquid phase. In this work, we present X-ray absorption (XA), X-ray emission (XE) and resonant inelastic X-ray scattering (RIXS) data of pure water and various salt solutions and show the possibilities these methods offer to elucidate the nature of ion-water interaction.

The structure of bulk liquid water was recently probed by x-ray scattering below the temperature limit of homogeneous nucleation (T-H) of similar to 232 K [J. A. Sellberg et al., Nature 510, 381-384 (2014)]. Here, we utilize a similar approach to study the structure of bulk liquid water below T-H using oxygen K-edge x-ray emission spectroscopy (XES). Based on previous XES experiments [T. Tokushima et al., Chem. Phys. Lett. 460, 387-400 (2008)] at higher temperatures, we expected the ratio of the 1b(1)' and 1b(1)" peaks associated with the lone-pair orbital in water to change strongly upon deep supercooling as the coordination of the hydrogen (H-) bonds becomes tetrahedral. In contrast, we observed only minor changes in the lone-pair spectral region, challenging an interpretation in terms of two interconverting species. A number of alternative hypotheses to explain the results are put forward and discussed. Although the spectra can be explained by various contributions from these hypotheses, we here emphasize the interpretation that the line shape of each component changes dramatically when approaching lower temperatures, where, in particular, the peak assigned to the proposed disordered component would become more symmetrical as vibrational interference becomes more important. (C) 2015 AIP Publishing LLC.

This review covers the outer core level photoionization of the free 3d metal atoms from Sc to Cu. The experimental 3p, 3s and 2p photoemission and photoabsorption spectra are discussed. A comparison emphasizes common features and distinct differences. The interpretation of the data based on ab initio calculations reveals the influence of multi-electron interactions in the 3d metal atoms. We focus on the fundamental effects and main interactions which govern the electronic structure of these open shell atoms. (Some figures in this article are in colour only in the electronic version)

More than 90 participants from Europe, the US and Japan gathered from April 27 to 29, 2005, in Zeuthen, near Berlin, to hold a lively international meeting on time-resolved soft X-ray science. The meeting continued the series of preceding workshops held in 2002 in Napa (California, USA) and in 2003 in Montreux (Switzerland). It was organized by the three synchrotron radiation sources BESSY (Berlin, Germany), the Swiss Light Source SLS (Villigen) and the French synchrotron radiation source SOLEIL (Orsay). The aim of the workshop was to bring together the existing ultrafast laser community and the emerging ultrafast X-ray community in order to discuss recent scientific highlights from both fields and to outline new directions for the application of ultrafast X-rays.

Direct evidence of orbital mixing between water and solvated transition-metal ions: An oxygen 1s XAS and DFT study of aqueous systems

In the present work, different varied line space (VLS) and reflection zone plate (RZP) gratings are analyzed for their suitability in low-signal femtosecond soft X-ray spectroscopy. The need for high efficiency suggests a straightened focal line whose sharpness and residual curvature will determine the quality. One- and two-dimensional VLS structures feature an attractive trade-off between a sufficient optical performance and a strongly relaxed fabrication, due to moderate line densities which are easily accessible by e-beam lithography. Based on fanned-out RZP arrays, their continuous limit version is identified to generate an almost perfect focal line however, with an aberration level three orders of magnitude better than for the VLS gratings and well below the diffraction limit over large acceptance angles.