Ultraviolet photochemical reaction of [Fe(III)(C2O4)3](3-) in aqueous solutions studied by femtosecond time-resolved X-ray absorption spectroscopy using an X-ray free electron laser

Synergistic LMCT and Ni Catalysis for Methylative Cross-Coupling Using tert-Butanol: Modulating Radical Pathways via Selective Bond Homolysis

Ligand-to-metal charge transfer (LMCT) excitation has emerged as a potent strategy for the selective generation of heteroatom-centered radicals, yet its full potential in modulating open-shell radical pathways remains underexplored. Here, we present a photocatalytic methylative cross-coupling reaction that capitalizes on the synergistic interplay between LMCT and Ni catalysis, enabling the use of tert-butanol as an efficient and benign methylating reagent. The electron-deficient ligand 2,6-ditrifluoromethyl benzoate facilitates Ce(IV)-mediated bond scission of tert-butanol, generating a methyl radical that is subsequently captured by the Ni catalytic cycle to form C-CH3 bonds. Under mild reaction conditions, this strategy affords efficient methylation of sp3 carbons adjacent to carbonyls and sp2 centers, demonstrating broad functional group tolerance and applicability in late-stage functionalization of bioactive molecules. Additionally, trideuteromethylative coupling can be facilely achieved using commercial tert-butanol-d10. This approach circumvents the need for traditional tert-butoxy radical precursors, such as peroxides, while strategically modulating the radical pathway to favor β-scission and suppress unwanted tert-butoxy radical formation in solution. Mechanistic studies reveal that the benzoate ligand plays a crucial role in enabling LMCT excitation and facilitating methyl radical generation, supporting a concerted Ce-OR and β-C-C bond homolysis mechanism, further evidenced by the modulation of regioselectivity in alkoxy radical-mediated β-scission.

Effect of alkali metal cations on dehydrogenative coupling of formate anions to oxalate

Introduction

With the growing global concern over CO2 emissions, reducing CO2 output has become an urgent requirement. The iron production industry is among those with the highest CO2 emissions, primarily due to the use of coke as a reductant and the use of a heat source at approximately 2,000°C. To address this issue, various alternative reductants, including CO, H2, and lignite, have been explored. Building on these efforts, we recently reported a novel ironmaking system using oxalic acid (HOOC-COOH) as the reductant. Formate salts, hydrogenated forms of CO2, are promising precursors for oxalate salts; however, their behavior during dimerization remains poorly understood. Herein, we investigate the influence of group 1 and 2 metal cations on the base-promoted dehydrogenative coupling of formate to form oxalate.

Methods

First, dehydrogenative coupling of sodium formate was executed by using various types of groups 1 and 2 metal carbonates. Second, the base was replaced from metal carbonates to metal hydroxides to check the reactivity. Finally, a countercation of sodium formate was replaced to various types of groups 1 and 2 metals. To elucidate the reaction mechanism, DFT calculation was executed.

Results and discussion

Treatment of sodium formate with various bases (group 1 and 2 metal carbonates or hydroxides) revealed that group 1 metal hydroxides are more effective than metal carbonates for oxalate formation, with cesium hydroxide (CsOH) exhibiting high reactivity. Density functional theory (DFT) calculations suggest that this kinetic advantage arises not only from increased basicity but also from intermediate destabilization in the Na/Cs mixed-cation system. Additionally, both experimental and theoretical investigations reveal that oxalate yield is influenced by the thermodynamic stability of intermediates and products (oxalate salts), highlighting the crucial role of cations in the reaction.

Transient X-Ray Absorption Near Edge Structure Spectroscopy Using Broadband Free-Electron Laser Pulses

A new method for time-resolved X-ray absorption near edge structure (XANES) spectroscopy that enables faster data acquisition and requires smaller sample quantities for high-quality data, thus allowing the analysis of more samples in a shorter time is introduced. The method uses large bandwidth free electron laser pulses to measure laser-excited XANES spectra in transmission mode. A beam-splitting grating configuration allows simultaneous measurements of the spectra of the incoming X-ray Free Electron Laser (XFEL) pulses and transmission XANES, which is crucial for compensating the pulse-dependent intensity and spectrum fluctuations due to the self-amplified spontaneous emission operation. The implementation of this new methodology is applied on a liquid solution of ammonium iron(III) oxalate jet and is compared to previous results, showing great improvements in the speed of acquisition and spectral resolution, and the ability to measure a large 2-D spectral-time map quickly.

A new era of LMCT: leveraging ligand-to-metal charge transfer excited states for photochemical reactions

Ligand-to-metal charge transfer (LMCT) excited states are capable of undergoing a wide array of photochemical reactions, yet receive minimal attention compared to other charge transfer excited states. This work provides general criteria for designing transition metal complexes that exhibit low energy LMCT excited states and routes to drive photochemistry from these excited states. General design principles regarding metal identity, oxidation state, geometry, and ligand sets are summarized. Fundamental photoreactions from these states including visible light-induced homolysis, excited state electron transfer, and other photoinduced chemical transformations are discussed and key design principles for enabling these photochemical reactions are further highlighted. Guided by these fundamentals, this review outlines critical considerations for the future design and application of coordination complexes with LMCT excited states.

Light-Induced Intramolecular Electron Transfer in a 1D [CuFe] Coordination Polymer Containing the [Fe(C2O4)3]3- Core

A one-dimensional (1D) ladder-like coordination polymer {NH4[{Cu(bpy)}2(C2O4)Fe(C2O4)3]·H2O}n (1; bpy = 2,2'-bipyridine) containing [Cu(bpy)(μ-C2O4)Cu(bpy)]2+ cationic units linked by oxalate groups of [Fe(C2O4)3]3- building blocks was investigated as a new type of photoactive solid-state system. It exhibits a photocoloration effect when exposed to direct sunlight or UV/vis irradiation. The photochromic properties and mechanism were studied by powder and single-crystal X-ray diffraction, UV/vis diffuse reflectance, IR and electron paramagnetic resonance spectroscopy, magnetization and impedance measurements, and density functional theory calculations. The process of photochromism involves simultaneous intramolecular electron transfers from the oxalate ligand to Fe(III) and to [CuII(bpy)(μ-C2O4)CuII(bpy)]2+, leading to the reduction of the metal centers to the electronic states Fe(II) and Cu(I), accompanied by the release of gaseous CO2.

Exploring Ligand-to-Metal Charge-Transfer States in the Photo-Ferrioxalate System Using Excited-State Specific Optimization

The photo-ferrioxalate system (PFS), [Fe(III)(C2O4)]3-, more than an exact chemical actinometer, has been extensively applied in wastewater and environment treatment. Despite many experimental efforts to improve clarity, important aspects of the mechanism of ferrioxalate photolysis are still under debate. In this paper, we employ the recently developed WΓ-CASSCF to investigate the ligand-to-metal charge-transfer states that are key to ferrioxalate photolysis. This investigation provides a qualitative picture of these states and key potential energy surface features related to the photolysis. Our theoretical results are consistent with the prompt charge-transfer picture seen in recent experiments and clarify some features that are not visible in experiments. Two ligand-to-metal charge-transfer states contribute to the photolysis of ferrioxalate, and the avoided crossing barrier between them is low compared with the initial photoexcitation energy. Our data also clarify that one Fe-O bond cleaves first, followed by the C-C bond and the other Fe-O bond.

Molecular investigation of the multi-phase photochemistry of Fe(III)-citrate in aqueous solution

Iron (Fe) is ubiquitous in nature and found as Fe^II or Fe^III in minerals or as dissolved ions Fe²⁺ or Fe³⁺ in aqueous systems. The interactions of soluble Fe have important implications for fresh water and marine biogeochemical cycles, which have impacts on global terrestrial and atmospheric environments. Upon dissolution of Fe^III into natural aquatic systems, organic carboxylic acids efficiently chelate Fe^III to form [Fe^III-carboxylate]²⁺ complexes that undergo a wide range of photochemistry-induced radical reactions. The chemical composition and photochemical transformations of these mixtures are largely unknown, making it challenging to estimate their environmental impact. To investigate the photochemical process of Fe^III-carboxylates at the molecular level, we conduct a comprehensive experimental study employing UV-visible spectroscopy, liquid chromatography coupled to photodiode array and high-resolution mass spectrometry detection, and oil immersion flow microscopy. In this study, aqueous solutions of Fe^III-citrate were photolyzed under 365 nm light in an experimental setup with an apparent quantum yield of (φ) ∼0.02, followed by chemical analyses of reacted mixtures withdrawn at increment time intervals of the experiment. The apparent photochemical reaction kinetics of Fe³⁺-citrates (aq) were expressed as two generalized consecutive reactions of with the experimental rate constants of j₁ ∼ 0.12 min^-1 and j₂ ∼ 0.05 min^-1, respectively. Molecular characterization results indicate that R and I consist of both water-soluble organic and Fe-organic species, while P compounds are a mixture of water-soluble and colloidal materials. The latter were identified as Fe-carbonaceous colloids formed at long photolysis times. The carbonaceous content of these colloids was identified as unsaturated organic species with low oxygen content and carbon with a reduced oxidation state, indicative of their plausible radical recombination mechanism under oxygen-deprived conditions typical for the extensively photolyzed mixtures. Based on the molecular characterization results, we discuss the comprehensive reaction mechanism of Fe^III-citrate photochemistry and report on the formation of previously unexplored colloidal reaction products, which may contribute to atmospheric and terrestrial light-absorbing materials in aquatic environments.

Femtosecond X-ray Spectroscopy Directly Quantifies Transient Excited-State Mixed Valency

Quantifying charge delocalization associated with short-lived photoexcited states of molecular complexes in solution remains experimentally challenging, requiring local element specific femtosecond experimental probes of time-evolving electron transfer. In this study, we quantify the evolving valence hole charge distribution in the photoexcited charge transfer state of a prototypical mixed valence bimetallic iron-ruthenium complex, [(CN)₅Fe^IICNRu^III(NH₃)₅]^-, in water by combining femtosecond X-ray spectroscopy measurements with time-dependent density functional theory calculations of the excited-state dynamics. We estimate the valence hole charge that accumulated at the Fe atom to be 0.6 ± 0.2, resulting from excited-state metal-to-metal charge transfer, on an ∼60 fs time scale. Our combined experimental and computational approach provides a spectroscopic ruler for quantifying excited-state valency in solvated complexes.

Ligand-Field Effects in a Ruthenium(II) Polypyridyl Complex Probed by Femtosecond X-ray Absorption Spectroscopy

We employ femtosecond X-ray absorption spectroscopy of [Ru(m-bpy)₃]²⁺ (m-bpy = 6-methyl-2,2'-bipyridine) to elucidate the time evolution of the spin and charge density upon metal-to-ligand charge-transfer (MLCT) excitation. The core-level transitions at the Ru L₃-edge reveal a very short MLCT lifetime of 0.9 ps and relaxation to the lowest triplet metal-centered state (³MC) which exhibits a lifetime of about 300 ps. Time-dependent density functional theory relates ligand methylation to a lower ligand field strength that stabilizes the ³MC state. A quarter of the ³MLCT population appears to be trapped which may be attributed to intramolecular vibrational relaxation or further electron transfer to the solvent. Our results demonstrate that small changes in the ligand field allow control of the photophysical properties. Moreover, this study underscores the high information content of femtosecond L-edge spectroscopy as a probe of valence charge density and spin-state in 4d transition metals.

Ultrafast photoelectron spectroscopy of photoexcited aqueous ferrioxalate

The photochemistry of metal-organic compounds in solution is determined by both intra- and inter-molecular relaxation processes after photoexcitation. Understanding its prime mechanisms is crucial to optimise the reactive paths and control their outcome. Here we investigate the photoinduced dynamics of aqueous ferrioxalate ([Fe^III(C₂O₄)₃]^3-) upon 263 nm excitation using ultrafast liquid phase photoelectron spectroscopy (PES). The initial step is found to be a ligand-to-metal electron transfer, occuring on a time scale faster than our time resolution (≲30 fs). Furthermore, we observe that about 25% of the initially formed ferrous species population are lost in ∼2 ps. Cast in the contest of previous ultrafast infrared and X-ray spectroscopic studies, we suggest that upon prompt photoreduction of the metal centre, the excited molecules dissociate in <140 fs into the pair of CO₂ and [(CO₂)Fe^II(C₂O₄)₂]^3- fragments, with unity quantum yield. About 25% of these pairs geminately recombine in ∼2 ps, due to interaction with the solvent molecules, reforming the ground state of the parent ferric molecule.

Practical Chemical Actinometry-A Review

Actinometers are physical or chemical systems that can be employed to determine photon fluxes. Chemical actinometers are photochemical systems with known quantum yields that can be employed to determine accurate photon fluxes for specific photochemical reactions. This review explores in detail several practical chemical actinometers (ferrioxalate, iodide-iodate, uranyl oxalate, nitrate, uridine, hydrogen peroxide and several actinometers for the vacuum ultraviolet). Each actinometer is described with recommended conditions for its use.

Ultrafast x-ray pump x-ray probe transient absorption spectroscopy: A computational study and proposed experiment probing core-valence electronic correlations in solvated complexes

Femtosecond x-ray pump-x-ray probe experiments are currently possible at free electron lasers such as the linac coherent light source, which opens new opportunities for studying solvated transition metal complexes. In order to make the most effective use of these kinds of experiments, it is necessary to determine which chemical properties an x-ray probe pulse will measure. We have combined electron cascade calculations and excited-state time-dependent density functional theory calculations to predict the initial state prepared by an x-ray pump and the subsequent x-ray probe spectra at the Fe K-edge in the solvated model transition metal complex, K₄Fe^II(CN)₆. We find several key spectral features that report on the ligand-field splitting and the 3p and 3d electron interactions. We then show how these features could be measured in an experiment.

Spiers Memorial Lecture: Introduction to ultrafast spectroscopy and imaging of photochemical reactions

A brief overview is presented on ultrafast spectroscopy and imaging of photochemical reactions by highlighting several experimental studies reported in the last five years. A particular focus is placed on new experiments performed using high-order harmonic generation, X-ray free electron lasers, and relativistic electron beams. Exploration of fundamental chemical reaction dynamics using these advanced experimental methodologies is in an early stage, and exciting new research opportunities await in this rapidly expanding and advancing research field. At the same time, there is no experimental methodology that provides all aspects of the electronic and structural dynamics in a single experiment, and investigations using different methodologies with various perspectives need to be considered in a comprehensive manner.

Ultrafast time-resolved x-ray absorption spectroscopy of ionized urea and its dimer through ab initio nonadiabatic dynamics

Investigating the early dynamics of chemical systems following ionization is essential for our understanding of radiation damage. However, experimental as well as theoretical investigations are very challenging due to the complex nature of these processes. Time-resolved x-ray absorption spectroscopy on a femtosecond timescale, in combination with appropriate simulations, is able to provide crucial insights into the ultrafast processes that occur upon ionization due to its element-specific probing nature. In this theoretical study, we investigate the ultrafast dynamics of valence-ionized states of urea and its dimer employing Tully's fewest switches surface hopping approach using Koopmans' theorem to describe the ionized system. We demonstrate that following valence ionization through a pump pulse, the time-resolved x-ray absorption spectra at the carbon, nitrogen, and oxygen K-edges reveal rich insights into the dynamics. Excited states of the ionized system give rise to time-delayed blueshifts in the x-ray absorption spectra as a result of electronic relaxation dynamics through nonadiabatic transitions. Moreover, our statistical analysis reveals specific structural dynamics in the molecule that induce time-dependent changes in the spectra. For the urea monomer, we elucidate the possibility to trace effects of specific molecular vibrations in the time-resolved x-ray absorption spectra. For the urea dimer, where ionization triggers a proton transfer reaction, we show how the x-ray absorption spectra can reveal specific details on the progress of proton transfer.

Femtosecond Optical Laser System with Spatiotemporal Stabilization for Pump-Probe Experiments at SACLA

We constructed a synchronized femtosecond optical laser system with spatiotemporal stabilization for pump-probe experiments at SPring-8 Angstrom Compact Free Electron Laser (SACLA). Stabilization of output power and pointing has been achieved with a small fluctuation level of a few percent by controlling conditions of temperature and air-flow in the optical paths. A feedback system using a balanced optical-microwave phase detector (BOMPD) has been successfully realized to reduce jitter down to 50 fs. We demonstrated the temporal stability with a time-resolved X-ray diffraction measurement and observed the coherent phonon oscillation of the photo-excited Bi without the post-processing using the timing monitor.

Predicting the redshift on the ultraviolet spectrum using the peak area method

The ultraviolet (UV) absorption peaks of NaCl, NaOH, and $\beta $β-phenylethylamine (PEA) in an aqueous solution move toward redshift. We proposed the peak area method for the quantitative analysis of PEA, NaCl, and NaOH. First, we obtained the predictable regularities of the redshift of the single component sample. Then, we obtained the regularities of the redshift of the UV spectrum for the mixture by the peak height and peak area methods. Finally, the BP-ANN algorithm was applied to determine the concentration of the mixture using the peak height and peak area method. The results of the testing set showed that correlation coefficients (${{\rm R}^2}$R²) of 0.992, 0.993, and 0.992 were obtained by peak height method and 0.997, 0.998, and 0.998 were obtained by peak area method for NaCl, NaOH, and PEA, respectively. Meanwhile, the relative errors of the prediction of NaCl, NaOH, and PEA obtained by peak area method were less than 3%, whereas the REP of NaCl, NaOH, and PEA obtained by peak height method were more than 5%. Compared with the results of the peak height method, the results showed that the peak area was more accurate than the peak height in predicting the redshift of the UV spectrum.

Vibrations tell the tale. A time-resolved mid-infrared perspective of the photochemistry of iron complexes

Time-resolved infrared spectroscopies are used to elucidate multiscalar photochemical processes of iron complexes.

Core-level nonlinear spectroscopy triggered by stochastic X-ray pulses

Stochastic processes are highly relevant in research fields as different as neuroscience, economy, ecology, chemistry, and fundamental physics. However, due to their intrinsic unpredictability, stochastic mechanisms are very challenging for any kind of investigations and practical applications. Here we report the deliberate use of stochastic X-ray pulses in two-dimensional spectroscopy to the simultaneous mapping of unoccupied and occupied electronic states of atoms in a regime where the opacity and transparency properties of matter are subject to the incident intensity and photon energy. A readily transferable matrix formalism is presented to extract the electronic states from a dataset measured with the monitored input from a stochastic excitation source. The presented formalism enables investigations of the response of the electronic structure to irradiation with intense X-ray pulses while the time structure of the incident pulses is preserved.

A versatile experimental system for tracking ultrafast chemical reactions with X-ray free-electron lasers

An experimental system, SPINETT (SACLA Pump-probe INstrumEnt for Tracking Transient dynamics), dedicated for ultrafast pump-probe experiments using X-ray free-electron lasers has been developed. SPINETT consists of a chamber operated under 1 atm helium pressure, two Von Hamos spectrometers, and a large two-dimensional detector having a short work distance. This platform covers complementary X-ray techniques; one can perform time-resolved X-ray absorption spectroscopy, time-resolved X-ray emission spectroscopy, and time-resolved X-ray diffuse scattering. Two types of liquid injectors have been prepared for low-viscosity chemical solutions and for protein microcrystals embedded in a matrix. We performed a test experiment at SPring-8 Angstrom Compact free-electron LAser and demonstrated the capability of SPINETT to obtain the local electronic structure and geometrical information simultaneously.

X-ray optics for advanced ultrafast pump-probe X-ray experiments at SACLA

X-ray optics were implemented for advanced ultrafast X-ray experiments with different techniques at the hard X-ray beamline BL3 of SPring-8 Ångstrom Compact free-electron LAser. A double channel-cut crystal monochromator (DCCM) and compound refractive lenses (CRLs) were installed to tailor the beam conditions. These X-ray optics can work simultaneously with an arrival-timing monitor that compensates for timing jitter and drift. Inner-walls of channel-cut crystals (CCs) in the DCCM were processed by plasma chemical vaporization machining to remove crystallographic damage. Four-bounced reflection profiles of the CCs were investigated and excellent diffraction qualities were achieved. The use of CRLs enabled two-dimensional X-ray focusing with a spot size of ∼1.5 µm × 1.5 µm full width at half-maximum, while keeping reasonable throughputs for a wide photon energy range of 5-15 keV.

Beyond Koopmans' theorem: electron binding energies in disordered materials

The topical review focuses on calculating ionization energies (IE), or electronic polarons in quasi-particle terminology, in large disordered systems, e.g. for a solute dissolved in a molecular solvent. The simplest estimate of the ionization energy is provided by one-electron energies in the Hartree-Fock theory, but the calculated quantities are not accurate. Density functional theory as many-body theory provides a principal opportunity for calculating one-electron energies including correlation and relaxation effects, i.e. the true energies of electronic polarons. We argue that such a principal possibility materializes within the concept of optimally tuned range-separated hybrid functionals (OT-RSH). We describe various schemes for optimal tuning. Importantly, the OT-RSH scheme is investigated for systems capped with dielectric continuum, providing a consistent picture on the QM/dielectric boundary. Finally, some limitations and open issues of the OT-RSH approach are addressed.

Droplet-based in situ X-ray absorption spectroscopy cell for studying crystallization processes at the tender X-ray energy range

The understanding of nucleation and crystallization is fundamental in science and technology. In solution, these processes are complex involving multiple transformations from ions and ion pairs through amorphous intermediates to multiple crystalline phases. X-ray absorption spectroscopy (XAS), which is sensitive to liquid, amorphous and crystalline phases offers prospects of demystifying these processes. However, for low Z elements the use of in situ X-ray absorption spectroscopy requires the tender X-ray range, which is often limited by vacuum requirements thereby complicating these measurements. To overcome these challenges, we developed a versatile and user-friendly droplet-based in situ X-ray absorption spectroscopy cell for studying crystallization processes. Time-resolved in situ experiments under ambient conditions are carried out in the cell whilst the cell is mounted in the vacuum chamber of a tender X-ray beamline. By following changes in the Ca K-edge X-ray absorption near edge structure (XANES), we captured in situ the intermediate phases involved during calcium carbonate crystallization from aqueous solutions. In addition, through linear combination fitting it was possible to qualitatively observe the evolution of each phase during the reaction demonstrating the potential of the cell in studying complex multiphase chemical processes.

We introduce a new in situ cell for time-resolved reactions involving aerosols/droplets using tender X-ray absorption spectroscopy and related methods.

Time-resolved Fourier-transform infrared spectroscopy reveals the hidden bimolecular process of the ferrioxalate actinometer

Step-scan FTIR-spectroscopy reveals the bimolecular reaction in the ferrioxalate photochemistry, which builds the molecular-level foundation of the Hatchard–Parker actinometer.

Femtosecond infrared spectroscopy reveals the primary events of the ferrioxalate actinometer

Ultrafast vibrational spectroscopy unravels the primary processes of Hatchard and Parker's ferrioxalate actinometer – a widely used tool in the photochemistry community.

Short-wavelength free-electron laser sources and science: a review

This review is focused on free-electron lasers (FELs) in the hard to soft x-ray regime. The aim is to provide newcomers to the area with insights into: the basic physics of FELs, the qualities of the radiation they produce, the challenges of transmitting that radiation to end users and the diversity of current scientific applications. Initial consideration is given to FEL theory in order to provide the foundation for discussion of FEL output properties and the technical challenges of short-wavelength FELs. This is followed by an overview of existing x-ray FEL facilities, future facilities and FEL frontiers. To provide a context for information in the above sections, a detailed comparison of the photon pulse characteristics of FEL sources with those of other sources of high brightness x-rays is made. A brief summary of FEL beamline design and photon diagnostics then precedes an overview of FEL scientific applications. Recent highlights are covered in sections on structural biology, atomic and molecular physics, photochemistry, non-linear spectroscopy, shock physics, solid density plasmas. A short industrial perspective is also included to emphasise potential in this area.

Perspective: Opportunities for ultrafast science at SwissFEL

We present the main specifications of the newly constructed Swiss Free Electron Laser, SwissFEL, and explore its potential impact on ultrafast science. In light of recent achievements at current X-ray free electron lasers, we discuss the potential territory for new scientific breakthroughs offered by SwissFEL in Chemistry, Biology, and Materials Science, as well as nonlinear X-ray science.

Beating Darwin-Bragg losses in lab-based ultrafast x-ray experiments

The use of low temperature thermal detectors for avoiding Darwin-Bragg losses in lab-based ultrafast experiments has begun. An outline of the background of this new development is offered, showing the relevant history and initiative taken by this work.

Femtosecond time-resolved X-ray absorption spectroscopy of anatase TiO2 nanoparticles using XFEL

The charge-carrier dynamics of anatase TiO₂ nanoparticles in an aqueous solution were studied by femtosecond time-resolved X-ray absorption spectroscopy using an X-ray free electron laser in combination with a synchronized ultraviolet femtosecond laser (268 nm). Using an arrival time monitor for the X-ray pulses, we obtained a temporal resolution of 170 fs. The transient X-ray absorption spectra revealed an ultrafast Ti K-edge shift and a subsequent growth of a pre-edge structure. The edge shift occurred in ca. 100 fs and is ascribed to reduction of Ti by localization of generated conduction band electrons into shallow traps of self-trapped polarons or deep traps at penta-coordinate Ti sites. Growth of the pre-edge feature and reduction of the above-edge peak intensity occur with similar time constants of 300-400 fs, which we assign to the structural distortion dynamics near the surface.

Communication: Direct evidence for sequential dissociation of gas-phase Fe(CO)5 via a singlet pathway upon excitation at 266 nm

We prove the hitherto hypothesized sequential dissociation of Fe(CO)5 in the gas phase upon photoexcitation at 266 nm via a singlet pathway with time-resolved valence and core-level photoelectron spectroscopy with an x-ray free-electron laser. Valence photoelectron spectra are used to identify free CO molecules and to determine the time constants of stepwise dissociation to Fe(CO)4 within the temporal resolution of the experiment and further to Fe(CO)3 within 3 ps. Fe 3p core-level photoelectron spectra directly reflect the singlet spin state of the Fe center in Fe(CO)5, Fe(CO)4, and Fe(CO)3 showing that the dissociation exclusively occurs along a singlet pathway without triplet-state contribution. Our results are important for assessing intra- and intermolecular relaxation processes in the photodissociation dynamics of the prototypical Fe(CO)5 complex in the gas phase and in solution, and they establish time-resolved core-level photoelectron spectroscopy as a powerful tool for determining the multiplicity of transition metals in photochemical reactions of coordination complexes.

Ultrafast Time-Resolved X-ray Absorption Spectroscopy of Ferrioxalate Photolysis with a Laser Plasma X-ray Source and Microcalorimeter Array

The detailed pathways of photoactivity on ultrafast time scales are a topic of contemporary interest. Using a tabletop apparatus based on a laser plasma X-ray source and an array of cryogenic microcalorimeter X-ray detectors, we measured a transient X-ray absorption spectrum during the ferrioxalate photoreduction reaction. With these high-efficiency detectors, we observe the Fe K edge move to lower energies and the amplitude of the extended X-ray absorption fine structure reduce, consistent with a photoreduction mechanism in which electron transfer precedes disassociation. These results are compared to previously published transient X-ray absorption measurements on the same reaction and found to be consistent with the results from Ogi et al. and inconsistent with the results of Chen et al. ( Ogi , Y. ; et al. Struct. Dyn. 2015 , 2 , 034901 ; Chen , J. ; Zhang , H. ; Tomov , I. V. ; Ding , X. ; Rentzepis , P. M. Chem. Phys. Lett. 2007 , 437 , 50 - 55 ). We provide quantitative limits on the Fe-O bond length change. Finally, we review potential improvements to our measurement technique, highlighting the future potential of tabletop X-ray science using microcalorimeter sensors.

Capturing local structure modulations of photoexcited BiVO4 by ultrafast transient XAFS

Femto- and picosecond transient XAFS revealed the photoexcitation dynamics of BiVO₄.

Imaging ultrafast excited state pathways in transition metal complexes by X-ray transient absorption and scattering using X-ray free electron laser source

This report will describe our recent studies of transition metal complex structural dynamics on the fs and ps time scales using an X-ray free electron laser source, Linac Coherent Light Source (LCLS). Ultrafast XANES spectra at the Ni K-edge of nickel(ii) tetramesitylporphyrin (NiTMP) were measured for optically excited states at a timescale from 100 fs to 50 ps, providing insight into its sub-ps electronic and structural relaxation processes. Importantly, a transient reduced state Ni(i) (π, 3dx2-y2) electronic state is captured through the interpretation of a short-lived excited state absorption on the low-energy shoulder of the edge, which is aided by the computation of X-ray transitions for postulated excited electronic states. The observed and computed inner shell to valence orbital transition energies demonstrate and quantify the influence of the electronic configuration on specific metal orbital energies. A strong influence of the valence orbital occupation on the inner shell orbital energies indicates that one should not use the transition energy from 1s to other orbitals to draw conclusions about the d-orbital energies. For photocatalysis, a transient electronic configuration could influence d-orbital energies up to a few eV and any attempt to steer the reaction pathway should account for this to ensure that external energies can be used optimally in driving desirable processes. NiTMP structural evolution and the influence of the porphyrin macrocycle conformation on relaxation kinetics can be likewise inferred from this study.

A beam branching method for timing and spectral characterization of hard X-ray free-electron lasers

We report a method for achieving advanced photon diagnostics of x-ray free-electron lasers (XFELs) under a quasi-noninvasive condition by using a beam-splitting scheme. Here, we used a transmission grating to generate multiple branches of x-ray beams. One of the two primary diffracted branches (+1st-order) is utilized for spectral measurement in a dispersive scheme, while the other (-1st-order) is dedicated for arrival timing diagnostics between the XFEL and the optical laser pulses. The transmitted x-ray beam (0th-order) is guided to an experimental station. To confirm the validity of this timing-monitoring scheme, we measured the correlation between the arrival timings of the -1st and 0th branches. The observed error was as small as 7.0 fs in root-mean-square. Our result showed the applicability of the beam branching scheme to advanced photon diagnostics, which will further enhance experimental capabilities of XFEL.

Time-resolved X-ray spectroscopies of chemical systems: New perspectives

The past 3-5 years have witnessed a dramatic increase in the number of time-resolved X-ray spectroscopic studies, mainly driven by novel technical and methodological developments. The latter include (i) the high repetition rate optical pump/X-ray probe studies, which have greatly boosted the signal-to-noise ratio for picosecond (ps) X-ray absorption spectroscopy studies, while enabling ps X-ray emission spectroscopy (XES) at synchrotrons; (ii) the X-ray free electron lasers (XFELs) are a game changer and have allowed the first femtosecond (fs) XES and resonant inelastic X-ray scattering experiments to be carried out; (iii) XFELs are also opening the road to the development of non-linear X-ray methods. In this perspective, I will mainly focus on the most recent technical developments and briefly address some examples of scientific questions that have been addressed thanks to them. I will look at the novel opportunities in the horizon.