Time-resolved X-ray Absorption Spectroscopy Research Articles

MagneDyn: the beamline for magneto dynamics studies at FERMI.

The future Magneto Dynamics (MagneDyn) beamline will be devoted to study the electronic states and the local magnetic properties of excited and transient states of complex systems by means of the time-resolved X-ray absorption spectroscopy technique. The beamline will use FERMI's high-energy source covering the wavelength range from 60 nm down to 1.3 nm. An on-line photon energy spectrometer will allow spectra to be measured with high resolution while delivering most of the beam to the end-stations. Downstream the beam will be possibly split and delayed, by means of a delay line, and then focused with a set of active Kirkpatrick-Baez mirrors. These mirrors will be able to focus the radiation in one of the two MagneDyn experimental chambers: the electromagnet end-station and the resonant inelastic X-ray scattering end-station. After an introduction of the MagneDyn scientific case, the layout will be discussed showing the expected performances of the beamline.

Beyond structure: ultrafast X-ray absorption spectroscopy as a probe of non-adiabatic wavepacket dynamics.

The excited state non-adiabatic dynamics of polyatomic molecules, leading to the coupling of structural and electronic dynamics, is a fundamentally important yet challenging problem for both experiment and theory. Ongoing developments in ultrafast extreme vacuum ultraviolet (XUV) and soft X-ray sources present new probes of coupled electronic-structural dynamics because of their novel and desirable characteristics. As one example, inner-shell spectroscopy offers localized, atom-specific probes of evolving electronic structure and bonding (via chemical shifts). In this work, we present the first on-the-fly ultrafast X-ray time-resolved absorption spectrum simulations of excited state wavepacket dynamics: photo-excited ethylene. This was achieved by coupling the ab initio multiple spawning (AIMS) method, employing on-the-fly dynamics simulations, with high-level algebraic diagrammatic construction (ADC) X-ray absorption cross-section calculations. Using the excited state dynamics of ethylene as a test case, we assessed the ability of X-ray absorption spectroscopy to project out the electronic character of complex wavepacket dynamics, and evaluated the sensitivity of the calculated spectra to large amplitude nuclear motion. In particular, we demonstrate the pronounced sensitivity of the pre-edge region of the X-ray absorption spectrum to the electronic and structural evolution of the excited-state wavepacket. We conclude that ultrafast time-resolved X-ray absorption spectroscopy may become a powerful tool in the interrogation of excited state non-adiabatic molecular dynamics.

Direct Observation of Photoinduced Charge Separation in Ruthenium Complex/Ni(OH)2 Nanoparticle Hybrid.

Ni(OH)2 have emerged as important functional materials for solar fuel conversion because of their potential as cost-effective bifunctional catalysts for both hydrogen and oxygen evolution reactions. However, their roles as photocatalysts in the photoinduced charge separation (CS) reactions remain unexplored. In this paper, we investigate the CS dynamics of a newly designed hybrid catalyst by integrating a Ru complex with Ni(OH)2 nanoparticles (NPs). Using time resolved X-ray absorption spectroscopy (XTA), we directly observed the formation of the reduced Ni metal site (~60 ps), unambiguously demonstrating CS process in the hybrid through ultrafast electron transfer from Ru complex to Ni(OH)2 NPs. Compared to the ultrafast CS process, the charge recombination in the hybrid is ultraslow (≫50 ns). These results not only suggest the possibility of developing Ni(OH)2 as solar fuel catalysts, but also represent the first time direct observation of efficient CS in a hybrid catalyst using XTA.

Depth-Resolved X-ray Absorption Spectroscopy by Means of Grazing Emission X-ray Fluorescence.

Grazing emission X-ray fluorescence (GEXRF) is well suited for nondestructive elemental-sensitive depth-profiling measurements on samples with nanometer-sized features. By varying the grazing emission angle under which the X-ray fluorescence signal is detected, the probed depth range can be tuned from a few to several hundred nanometers. The dependence of the XRF intensity on the grazing emission angle can be assessed in a sequence of measurements or in a scanning-free approach using a position-sensitive area detector. Hereafter, we will show that the combination of scanning-free GEXRF and fluorescence detected X-ray absorption spectroscopy (XAS) allows for depth-resolved chemical speciation measurements with nanometer-scale accuracy. While the conventional grazing emission geometry is advantageous to minimize self-absorption effects, the use of a scanning-free setup makes the sequential scanning of the grazing emission angles obsolete and paves the way toward time-resolved depth-sensitive XAS measurements. The presented experimental approach was applied to study the surface oxidation of an Fe layer on the top of bulk Si and of a Ge bulk sample. Thanks to the penetrating properties and the insensitivity toward the electric conduction properties of the incident and emitted X-rays, the presented experimental approach is well suited for in situ sample surface studies in the nanometer regime.

Element-specific characterization of transient electronic structure of solvated Fe(II) complexes with time-resolved soft X-ray absorption spectroscopy.

Polypyridyl transition-metal complexes are an intriguing class of compounds due to the relatively facile chemical designs and variations in ligand-field strengths that allow for spin-state changes and hence electronic configurations in response to external perturbations such as pressure and light. Light-activated spin-conversion complexes have possible applications in a variety of molecular-based devices, and ultrafast excited-state evolution in these complexes is of fundamental interest for understanding of the origins of spin-state conversion in metal complexes. Knowledge of the interplay of structure and valence charge distributions is important to understand which degrees of freedom drive spin-conversion and which respond in a favorable (or unfavorable) manner. To track the response of the constituent components, various types of time-resolved X-ray probe methods have been utilized for a broad range of chemical and biological systems relevant to catalysis, solar energy conversions, and functional molecular devices. In particular, transient soft X-ray spectroscopy of solvated molecules can offer complementary information on the detailed electronic structures and valence charge distributions of photoinduced intermediate species: First-row transition-metal L-edges consist of 2p-3d transitions, which directly probe the unoccupied valence density of states and feature lifetime broadening in the range of 100 meV, making them sensitive spectral probes of metal-ligand interactions. In this Account, we present some of our recent progress in employing picosecond and femtosecond soft X-ray pulses from synchrotron sources to investigate element specific valence charge distributions and spin-state evolutions in Fe(II) polypyridyl complexes via core-level transitions. Our results on transient L-edge spectroscopy of Fe(II) complexes clearly show that the reduction in σ-donation is compensated by significant attenuation of π-backbonding upon spin-crossover. This underscores the important information contained in transient metal L-edge spectroscopy on changes in the 3d orbitals including oxidation states, orbital symmetries, and covalency, which largely define the chemistry of these complexes. In addition, ligand K-edge spectroscopy reveals the "ligand view" of the valence charge density by probing 1s-2p core-level transitions at the K-edge of light elements such as nitrogen, carbon, and oxygen. In the case of Fe(II) spin-conversion complexes, additional details of the metal-ligand interactions can be obtained by this type of X-ray spectroscopy. With new initiatives in and construction of X-ray free-electron laser sources, we expect time-resolved soft X-ray spectroscopy to pave a new way to study electronic and molecular dynamics of functional materials, thereby answering many interesting scientific questions in inorganic chemistry and material science.

Microsecond X-ray Absorption Spectroscopy Identification of Co(I) Intermediates in Cobaloxime-Catalyzed Hydrogen Evolution.

Rational development of efficient photocatalytic systems for hydrogen production requires understanding the catalytic mechanism and detailed information about the structure of intermediates in the catalytic cycle. We demonstrate how time-resolved X-ray absorption spectroscopy in the microsecond time range can be used to identify such intermediates and to determine their local geometric structure. This method was used to obtain the solution structure of the Co(I) intermediate of cobaloxime, which is a non-noble metal catalyst for solar hydrogen production from water. Distances between cobalt and the nearest ligands including two solvent molecules and displacement of the cobalt atom out of plane formed by the planar ligands have been determined. Combining in situ X-ray absorption and UV/Vis data, we demonstrate how slight modification of the catalyst structure can lead to the formation of a catalytically inactive Co(I) state under similar conditions. Possible deactivation mechanisms are discussed.

Reversibility of Ferri-/Ferrocyanide Redox during Operando Soft X-ray Spectroscopy

The ferri-/ferrocyanide redox couple is ubiquitous in many fields of physical chemistry. We studied its photochemical response to intense synchrotron radiation by in situ X-ray absorption spectroscopy (XAS). For photon flux densities equal to and above 2 × 1011 s–1 mm–2, precipitation of ferric (hydr)oxide from both ferricyanide and ferrocyanide solutions was clearly detectable, despite flowing fast enough to replace the solution in the flow cell every 0.4 s (flow rate 1.5 mL/min). During cyclic voltammetry, precipitation of ferric (hydr)oxide was promoted at reducing voltages and observed below 1011 s–1 mm–2. This was accompanied by inhibition of the ferri-/ferrocyanide redox, which we probed by time-resolved operando XAS. Our study highlights the importance of considering both electrochemical and spectroscopic conditions when designing in situ experiments.

Reversibility of Ferri-Ferrocyanide Redox during in-Situ Soft X-Ray Spectroscopy

Ferri-ferrocyanide is a common facile redox couple in electrochemistry and related fields; select applications include charge storage in flow batteries, prevention of electrolyte decomposition in batteries, redox shuttling in dye-sensitized solar cells and photoanodes, as well as measuring charge transfer on catalytic surfaces. Ferri-ferrocyanide is also frequently employed as a standard to establish new steady-state and time-resolved in-situ spectroscopy methods. Despite the ubiquity of the ferri-ferrocyanide redox couple in diverse fields, discussion exists as to whether its kinetics may depend on the composition of both the electrode and electrolyte. Furthermore, ferri-ferrocyanide decomposes when exposed to ionizing radiation such as soft X-rays at a synchrotron, which may lead to electrodeposition on the electrode. We studied the photochemical response of ferri-ferrocyanide to intense synchrotron radiation by in-situ X-ray absorption spectroscopy at the iron L edge. For photon flux densities above a defined threshold, precipitation of ferric (hydr)oxide from both ferricyanide and ferrocyanide solutions was clearly detectable. The formation of the precipitate was accelerated during cyclic voltammetry, which we probed by time-resolved in-operando X-ray absorption spectroscopy. The iron redox was completely inhibited in the probed volume after only two electrochemical cycles (70 min). Our study highlights the importance of considering both electrochemical and spectroscopic conditions when designing in-situ experiments that can shed light on charge transfer and chemistry of active sites in energy and health research.

Operando spatially and time-resolved X-ray absorption spectroscopy and infrared thermography during oscillatory CO oxidation

A comprehensive approach was applied to investigate oscillatory CO oxidation over a Pt/Al2O3-based diesel oxidation catalyst with small Pt particles (about 1.5nm diameter) in a fixed-bed microreactor under relevant reaction conditions by combining spatially and time-resolved operando X-ray absorption spectroscopy, infrared thermography, and online mass spectrometry. The catalyst-bed zone responsible for the oscillatory behavior and the emerging hot spot was identified by means of IR thermography. Oscillations of the Pt oxidation state and the hot spot region evolved simultaneously and moved from the end toward the beginning of the catalyst bed with increasing reaction temperature. The changes in CO oxidation activity during oscillations can be unambiguously correlated with dynamic structural changes of the Pt particles. The applied operando approach is complementary to surface science studies and also studies on model Pt particles. Surface oxidation of small Pt nanoparticles leads to a fast deactivation of the catalyst, which is regenerated in a slow reduction step. The presence of metallic Pt is required for high activity of the catalyst.

Thermal Annealing Effect on Real Time Atomic Relocation of Iron-Cobalt Alloys Prepared by Electro-Deposition

Atomic structures of fabricated Fe-Co alloys were investigated real time by time resolved x-ray absorption spectroscopy (TRXAS) at annealing temperature of 400° C. The various compositions of Fe-Co alloy thin films were prepared by electro-deposition on indium-tin oxide glass substrate with iron sulfate and cobalt chloride electrolyte solutions. The current density of 80 A/m2was used to deposit Fe-Co thin film with varying the ratio of Fe and Co ion concentrations. The Fe-Co alloys with boron nitride powders were mixed, grinded and pressed in the pallet form in order to insert into environmental controlled sample holder for TRXAS measurement. The samples were annealed under 20% oxygen and 80% nitrogen atmospheric by increasing the temperature up to 400° C and then remaining for an hour. The influence of annealing temperature can modify the atomic structures of Fe-Co alloys as indicated in the variation of extended x-ray absorption fine structure (EXAFS) peaks. Most of the EXAFS peaks followed the temperature profile i.e. increasing or decreasing during the temperature rising and then maintaining about constant values during holding the temperature at 400° C. By transforming the EXAFS peaks into scattering radius space, the modification of relative scattering amplitude from each nearest neighbor position indicated the atomic relocation during annealing the samples. Some of scattering patterns from Fe and Co oxides can be found and their amounts changed during rising the annealing temperature. In addition, the atomic relocations extracted from the Co absorption edge was very much smaller than those of Fe edge probably due to relative more stable of Co atomic positions for this range of annealing temperature.

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.

Time-resolved X-ray absorption spectroscopy was performed for aqueous ammonium iron(III) oxalate trihydrate solutions using an X-ray free electron laser and a synchronized ultraviolet laser. The spectral and time resolutions of the experiment were 1.3 eV and 200 fs, respectively. A femtosecond 268 nm pulse was employed to excite [Fe(III)(C2O4)3]3− in solution from the high-spin ground electronic state to ligand-to-metal charge transfer state(s), and the subsequent dynamics were studied by observing the time-evolution of the X-ray absorption spectrum near the Fe K-edge. Upon 268 nm photoexcitation, the Fe K-edge underwent a red-shift by more than 4 eV within 140 fs; however, the magnitude of the redshift subsequently diminished within 3 ps. The Fe K-edge of the photoproduct remained lower in energy than that of [Fe(III)(C2O4)3]3−. The observed red-shift of the Fe K-edge and the spectral feature of the product indicate that Fe(III) is upon excitation immediately photoreduced to Fe(II), followed by ligand dissociation from Fe(II). Based on a comparison of the X-ray absorption spectra with density functional theory calculations, we propose that the dissociation proceeds in two steps, forming first [(CO2•)Fe(II)(C2O4)2]3− and subsequently [Fe(II)(C2O4)2]2−.

Time resolved X-ray absorption spectroscopy in condensed matter: A road map to the future

Nowadays cutting edge femtosecond EUV and soft X-rays radiation sources are the driving force of groundbreaking time resolved X-ray spectroscopies. These new light sources are allowing pioneering experiments in the field of ultrafast phenomena and disclosing new insights about the physics of the out-of-equilibrium matter. Here we report an introductory and concise outlook about some possible perspectives in this field.

Rate and mechanism of the photoreduction of birnessite (MnO2) nanosheets

The photoreductive dissolution of Mn(IV) oxide minerals in sunlit aquatic environments couples the Mn cycle to the oxidation of organic matter and fate of trace elements associated with Mn oxides, but the intrinsic rate and mechanism of mineral dissolution in the absence of organic electron donors is unknown. We investigated the photoreduction of δ-MnO2 nanosheets at pH 6.5 with Na or Ca as the interlayer cation under 400-nm light irradiation and quantified the yield and timescales of Mn(III) production. Our study of transient intermediate states using time-resolved optical and X-ray absorption spectroscopy showed key roles for chemically distinct Mn(III) species. The reaction pathway involves (i) formation of Jahn-Teller distorted Mn(III) sites in the octahedral sheet within 0.6 ps of photoexcitation; (ii) Mn(III) migration into the interlayer within 600 ps; and (iii) increased nanosheet stacking. We propose that irreversible Mn reduction is coupled to hole-scavenging by surface water molecules or hydroxyl groups, with associated radical formation. This work demonstrates the importance of direct MnO2 photoreduction in environmental processes and provides a framework to test new hypotheses regarding the role of organic molecules and metal species in photochemical reactions with Mn oxide phases. The timescales for the production and evolution of Mn(III) species and a catalytic role for interlayer Ca(2+) identified here from spectroscopic measurements can also guide the design of efficient Mn-based catalysts for water oxidation.

Catalysis seen in action.

Synchrotron radiation techniques are widely applied in materials research and heterogeneous catalysis. In homogeneous catalysis, its use so far is rather limited despite its high potential. Here, insights in the strengths and limitations of X-ray spectroscopy technique in the field of homogeneous catalysis are given, including new technique developments. A relevant homogeneous catalyst, used in the industrially important selective oligomerization of ethene, is taken as a worked-out example. Emphasis is placed on time-resolved operando X-ray absorption spectroscopy with outlooks to novel high energy resolution and emission techniques. All experiments described have been or can be done at the Diamond Light Source Ltd (Didcot, UK).

Hard disk drive based microsecond X-ray chopper for characterization of ionization chambers and photodiodes.

A fast X-ray chopper capable of producing ms long X-ray pulses with a typical rise time of few μs was realized. It is ideally suited to investigate the temporal response of X-ray detectors with response times of the order of μs to ms, in particular, any kind of ionization chambers and large area photo diodes. The drive mechanism consists of a brushless DC motor and driver electronics from a common hard disk drive, keeping the cost at an absolute minimum. Due to its simple construction and small dimensions, this chopper operates at home lab based X-ray tubes and synchrotron radiation sources as well. The dynamics of the most important detectors used in time resolved X-ray absorption spectroscopy, namely, ionization chambers and Passivated Implanted Planar Silicon photodiodes, were investigated in detail. The results emphasize the applicability of this X-ray chopper.

Laser plasma x-ray source for ultrafast time-resolved x-ray absorption spectroscopy.

We describe a laser-driven x-ray plasma source designed for ultrafast x-ray absorption spectroscopy. The source is comprised of a 1 kHz, 20 W, femtosecond pulsed infrared laser and a water target. We present the x-ray spectra as a function of laser energy and pulse duration. Additionally, we investigate the plasma temperature and photon flux as we vary the laser energy. We obtain a 75 μm FWHM x-ray spot size, containing ∼106 photons/s, by focusing the produced x-rays with a polycapillary optic. Since the acquisition of x-ray absorption spectra requires the averaging of measurements from >107 laser pulses, we also present data on the source stability, including single pulse measurements of the x-ray yield and the x-ray spectral shape. In single pulse measurements, the x-ray flux has a measured standard deviation of 8%, where the laser pointing is the main cause of variability. Further, we show that the variability in x-ray spectral shape from single pulses is low, thus justifying the combining of x-rays obtained from different laser pulses into a single spectrum. Finally, we show a static x-ray absorption spectrum of a ferrioxalate solution as detected by a microcalorimeter array. Altogether, our results demonstrate that this water-jet based plasma source is a suitable candidate for laboratory-based time-resolved x-ray absorption spectroscopy experiments.

Identifying the major intermediate species by combining time-resolved X-ray solution scattering and X-ray absorption spectroscopy.

Identifying the intermediate species along a reaction pathway is a first step towards a complete understanding of the reaction mechanism, but often this task is not trivial. There has been a strong on-going debate: which of the three intermediates, the CHI2 radical, the CHI2-I isomer, and the CHI2(+) ion, is the dominant intermediate species formed in the photolysis of iodoform (CHI3)? Herein, by combining time-resolved X-ray liquidography (TRXL) and time-resolved X-ray absorption spectroscopy (TR-XAS), we present strong evidence that the CHI2 radical is dominantly formed from the photolysis of CHI3 in methanol at 267 nm within the available time resolution of the techniques (∼20 ps for TRXL and ∼100 ps for TR-XAS). The TRXL measurement, conducted using the time-slicing scheme, detected no CHI2-I isomer within our signal-to-noise ratio, indicating that, if formed, the CHI2-I isomer must be a minor intermediate. The TR-XAS transient spectra measured at the iodine L1 and L3 edges support the same conclusion. The present work demonstrates that the application of these two complementary time-resolved X-ray methods to the same system can provide a detailed understanding of the reaction mechanism.

Bis(μ-oxo) versus mono(μ-oxo)dicopper cores in a zeolite for converting methane to methanol: an in situ XAS and DFT investigation.

Dicopper species have been identified as the active sites in converting methane to methanol in Cu-zeolites. To understand the formation of these copper cores in mordenite, we used in situ time-resolved X-ray absorption spectroscopy during heat treatment. Significant dehydration enabled the reduction of the copper cores, after which molecular oxygen was cleaved. The activated oxygen bridged two copper atoms to make the reactive precursor for the activation of methane. Even though the active bridging oxygen was detected, the XAS data were unable to distinguish a bis(μ-oxo)dicopper core from a mono(μ-oxo)dicopper core since XAS measures the average structure of the total copper population and the sample contains a mixture of copper species. We therefore used DFT calculations to understand the energetics of the formation of the active copper species and found that if a copper dimer exists in a zeolite, the mono(μ-oxo)dicopper species is an energetically plausible structure. This is in contrast to molecular dicopper cores where the bis(μ-oxo)dicopper core is preferentially formed.

ChemInform Abstract: Recent Advances on Ultrafast X‐Ray Spectroscopy in the Chemical Sciences

AbstractReview: 141 refs.

Laser-driven switching dynamics in phase change materials investigated by time-resolved X-ray absorption spectroscopy

Time-dependent X-ray absorption spectroscopy at the Ge L3 edge is used to study the optical switching dynamics of epitaxial phase change materials grown on Si(111) membranes. Small differences between the static absorption spectra of the crystalline and laser-amorphized phase of GeTe are observed. Furthermore, a new approach for pump-probe studies on phase change materials is presented. However, no time-dependent change in the X-ray absorption > 0.1% has been detected so far. This is mainly attributed to sample deterioration at 3 kHz pump-probe rates.