Ultrafast X-ray Absorption Research Articles

Extended X-ray absorption spectroscopy using an ultrashort pulse laboratory-scale laser-plasma accelerator

Laser-driven compact particle accelerators can provide ultrashort pulses of broadband X-rays, well suited for undertaking X-ray absorption spectroscopy measurements on a femtosecond timescale. Here the Extended X-ray Absorption Fine Structure (EXAFS) features of the K-edge of a copper sample have been observed over a 250 eV window in a single shot using a laser wakefield accelerator, providing information on both the electronic and ionic structure simultaneously. This capability will allow the investigation of ultrafast processes, and in particular, probing high-energy-density matter and physics far-from-equilibrium where the sample refresh rate is slow and shot number is limited. For example, states that replicate the tremendous pressures and temperatures of planetary bodies or the conditions inside nuclear fusion reactions. Using high-power lasers to pump these samples also has the advantage of being inherently synchronised to the laser-driven X-ray probe. A perspective on the additional strengths of a laboratory-based ultrafast X-ray absorption source is presented.

The high power laser facility at beamline ID24-ED at the ESRF

ABSTRACT This paper describes technical details and new scientific opportunities offered by the new High Power Laser Facility at the ESRF, opened to users since 2022. This facility enables laser-induced dynamic compression experiments probed by ultra-fast X-ray Absorption Spectroscopy at beamline ID24-ED. This capability allows to reach and probe more extreme pressure and temperature states and higher strain rates with respect to static compression, addressing open questions in fundamental, planetary and material science.

Ultrafast X-ray Absorption Spectroscopy Reveals Excited-State Dynamics of B12 Coenzymes Controlled by the Axial Base.

Polarized time-resolved X-ray absorption spectroscopy at the Co K-edge is used to probe the excited-state dynamics and photolysis of base-off methylcobalamin and the excited-state structure of base-off adenosylcobalamin. For both molecules, the final excited-state minimum shows evidence for an expansion of the cavity around the Co ion by ca. 0.04 to 0.05 Å. The 5-coordinate base-off cob(II)alamin that is formed following photodissociation has a structure similar to that of the 5-coordinate base-on cob(II)alamin, with a ring expansion of 0.03 to 0.04 Å and a contraction of the lower axial bond length relative to that in the 6-coordinate ground state. These data provide insights into the role of the lower axial ligand in modulating the reactivity of B12 coenzymes.

Advances in Diffraction Studies of Light-Induced Transient Species in Molecular Crystals and Selected Complementary Techniques

The review provides a summary of the current methods of tracing photoexcitation processes and structural dynamics in the solid state, putting major emphasis on the X-ray diffraction techniques (time-resolved Laue diffraction on synchrotron sources and time-resolved serial femtosecond crystallography on X-ray free-electron lasers). The recent developments and nowadays experimental possibilities in the field are discussed along with the data processing and analysis approaches, and illustrated with some striking literature examples of the respective successful studies. Selected complementary methods, such as ultrafast electron diffraction or time-resolved X-ray absorption spectroscopy, are briefly presented.

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.

Tracking the Local Structure Change during the Photoabsorption Processes of Photocatalysts by the Ultrafast Pump-Probe XAFS Method

The birth of synchrotron radiation (SR) facilities and X-ray free electron lasers (XFELs) has led to the development of new characterization tools that use X-rays and opened frontiers in science and technology. Ultrafast X-ray absorption fine structure (XAFS) spectroscopy for photocatalysts is one such significant research technique. Although carrier behavior in photocatalysts has been discussed in terms of the band theory and their energy levels in reciprocal space (k-space) based on optical spectroscopic results, it has rarely been discussed where photocarriers are located in real-space (r-space) based on direct observation of the excited states. XAFS provides information on the local electronic and geometrical structures around an X-ray-absorbing atom and can address photocarrier dynamics in the r-space observed from the X-ray-absorbing atom. In this article, we discuss the time dependent structure change of tungsten trioxide (WO3) and bismuth vanadate (BiVO4) photocatalysts studied by the ultrafast pump-probe XAFS method in the femtosecond to nanosecond time scale with the Photon Factory Advanced Ring (PF-AR) and the SPring-8 Angstrom Compact free-electron LAser (SACLA). WO3 shows a femtosecond decay process of photoexcited electrons followed by a structural change to a metastable state with a hundred picosecond speed, which is relaxed to the ground-state structure with a nanosecond time constant. The Bi L3 edge of BiVO4 shows little contribution of the Bi 6s electron to the photoabsorption process; however, it is sensitive to the structural change induced by the photoexcited electron. Time-resolved XAFS measurements in a wide range time domain and with varied wavelengths of the excitation pump laser facilitate understanding of the overall details regarding the photocarrier dynamics that have a significant influence on the photocatalytic performance.

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

Time-resolved X-ray methods are widely used for monitoring transient intermediates over the course of photochemical reactions. Ultrafast X-ray absorption and emission spectroscopies as well as elastic X-ray scattering deliver detailed electronic and structural information on chemical dynamics in the solution phase. In this work, we describe the opportunities at the Femtosecond X-ray Experiments (FXE) instrument of European XFEL. Guided by the idea of combining spectroscopic and scattering techniques in one experiment, the FXE instrument has completed the initial commissioning phase for most of its components and performed first successful experiments within the baseline capabilities. This is demonstrated by its currently 115 fs (FWHM) temporal resolution to acquire ultrafast X-ray emission spectra by simultaneously recording iron Kα and Kβ lines, next to wide angle X-ray scattering patterns on a photoexcited aqueous solution of [Fe(bpy)3]2+, a transition metal model compound.

Direct observation of the electronic states of photoexcited hematite with ultrafast 2p3d X-ray absorption spectroscopy and resonant inelastic X-ray scattering.

Hematite, α-Fe2O3, is an important semiconductor for photoelectrochemical water splitting. Its low charge carrier mobility and the presence of midgap states provide favourable conditions for electron-hole recombination, hence affecting the semiconductor's photoelectrochemical efficiency. The nature of the excited state and charge carrier transport in hematite is strongly debated. In order to further understand the fundamental properties of the hematite photoexcited state, we conducted femtosecond 2p (L3) X-ray absorption (XAS) and 2p3d resonant inelastic scattering (RIXS) measurements on hematite thin-films at the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL). The observed spectral changes and kinetic processes are in agreement with previous 3p XAS reports. The potential additional information that could be acquired from 2p3d RIXS experiments is also discussed.

Ultrafast nonadiabatic dynamics probed by nitrogen K-edge absorption spectroscopy.

The emergence of X-ray free electron lasers (X-FELs) has made it possible to probe structural dynamics on the femtosecond timescale. This extension of experimental capabilities also calls for a simultaneous development in theory to help interpret the underlying structure and dynamics encoded within the experimental observable. In the ultrafast regime this often requires a time-dependent theoretical treatment that describes nuclear dynamics beyond the Born-Oppenheimer approximation. In this work, we perform quantum dynamics simulations based upon time-evolving Gaussian basis functions (GBFs) and simulate the ultrafast X-ray Absorption Near-Edge Structure (XANES) spectra of photoexcited pyrazine including two strongly coupled electronically excited states and four normal mode degrees of freedom. Two methods to simulate the excited state XANES spectra are applied, the first is based upon the multi-configurational second order perturbation theory restricted active space (RASPT2) method and the second uses a combination of the maximum overlap method (MOM) and time-dependent density functional theory (TDDFT). We demonstrate that despite the simplicity of the MOM/TDDFT method, it captures several qualitative features of the RASPT2 simulations at much reduced computational effort. However, features such as the conical intersection are a particular exception as they require a multi-configurational treatment. For the nuclear dynamics, we demonstrate that even a small number of GBFs can provide reasonable description of the spectroscopic observable. This work provides perspectives for computationally efficient approaches important for addressing larger systems.

Tumbling with a limp: local asymmetry in water's hydrogen bond network and its consequences.

Ab initio molecular dynamics simulations of liquid water under equilibrium ambient conditions, together with a novel energy decomposition analysis, have recently shown that a substantial fraction of water molecules exhibit a significant asymmetry between the strengths of the two donor and/or the two acceptor interactions. We refer to this recently unraveled aspect as the "local asymmetry in the hydrogen bond network". We discuss how this novel aspect was first revealed, and provide metrics that can be consistently employed on simulated water trajectories to quantify this local heterogeneity in the hydrogen bond network and its dynamics. We then discuss the static aspects of the asymmetry, pertaining to the frozen geometry of liquid water at any given instant of time and the distribution of hydrogen bond strengths therein, and also its dynamic characteristics pertaining to how fast this asymmetry decays and the kinds of molecular motions responsible for this decay. Following this we discuss the spectroscopic manifestations of this asymmetry, from ultrafast X-ray absorption spectra to infrared spectroscopy and down to the much slower terahertz regime. Finally, we discuss the implications of these findings in a broad context and their relation to the current notions about the structure and dynamics of liquid water.

Hole dynamics in a photovoltaic donor-acceptor couple revealed by simulated time-resolved X-ray absorption spectroscopy.

Theoretical and experimental methodologies that can characterize electronic and nuclear dynamics, and the coupling between the two, are needed to understand photoinduced charge transfer in molecular building blocks used in organic photovoltaics. Ongoing developments in ultrafast pump-probe techniques such as time-resolved X-ray absorption spectroscopy, using an X-ray free electron laser in combination with an ultraviolet femtosecond laser, present desirable probes of coupled electronic and nuclear dynamics. In this work, we investigate the charge transfer dynamics of a donor-acceptor pair, which is widely used as a building block in low bandgap block copolymers for organic photovoltaics. We simulate the dynamics of the benzothiadiazole-thiophene molecule upon photoionization with a vacuum ultraviolet (VUV) pulse and study the potential of probing the subsequent charge dynamics using time-resolved X-ray absorption spectroscopy. The photoinduced dynamics are calculated using on-the-fly nonadiabatic molecular dynamics simulations based on Tully's Fewest Switches Surface Hopping approach. We calculate the X-ray absorption spectrum as a function of time after ionization at the Hartree-Fock level. The changes in the time-resolved X-ray absorption spectrum at the sulfur K-edge reveal the ultrafast charge carrier dynamics in the molecule occurring on a femtosecond time scale. These theoretical findings anticipate that ultrafast time-resolved X-ray absorption spectroscopy using an X-ray probe in combination with a VUV pump offers a new approach to investigate the detailed dynamics of organic photovoltaic materials.

High-flux x-ray photon emission by a superluminal hybrid electromagnetic mode of intense laser in a plasma waveguide

The feasibility of several novel ultrafast x-ray sources has been demonstrated through the interaction between laser and a micro-structured target. However, the resulting photon flux is still deficient for some applications. Here, we proposed a new method to yield high-flux synchrotron radiation by adopting compact hollow plasma waveguide. We verified the method theoretically and numerically. Linearly polarized laser pulses propagating in the waveguide are mainly converted into the electromagnetic mode HE11 with a superluminal phase speed of 1.02c when the pulse is coupled with the waveguide. The mode fields lead to a strong oscillating force and short oscillation period for energetic electrons, which are accelerated via the longitudinal acceleration mechanism. Then, the short x-ray beam is generated with high yield of 1011–1012. The corresponding photon flux reaches 3.5 × 1021 photons s−1 · 0.1% bandwidth. The high-flux source can be even used for single shot ultrafast imaging or the ultrafast x-ray diffraction and absorption studies.

Resolving the Ultrafast Changes of Chemically Inequivalent Metal–Ligand Bonds in Photoexcited Molecular Complexes with Transient X-ray Absorption Spectroscopy

Photoactive transition-metal complexes that incorporate heteroleptic ligands present a first coordination shell, which is asymmetric. Although it is generally expected that the metal–ligand bond lengths respond differently to photoexcitation, resolving these fine structural changes remains experimentally challenging, especially for flexible multidentate ligands. In this work, ultrafast X-ray absorption spectroscopy is employed to capture directly the asymmetric elongations of chemically inequivalent metal–ligand bonds in the photoexcited spin-switching FeII complex [FeII(tpen)]2+ solvated in acetonitrile, where tpen denotes N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,2-ethylenediamine. The possibility to correlate precisely the nature of the donor/acceptor coordinating atoms to specific photoinduced structural changes within a binding motif will provide advanced diagnostics for optimizing numerous photoactive chemical and biological building blocks.

Photochemical pathways in nucleobases measured with an X-ray FEL.

The conversion of light energy into other molecular energetic degrees of freedom is often dominated by ultrafast, non-adiabatic processes. Femtosecond spectroscopy with optical pulses has helped in shaping our understanding of crucial processes in molecular energy-conversion. The advent of new, ultrashort and bright X-ray free electron laser sources opens the possibility to use X-ray-typical element and site sensitivity for ultrafast molecular research. We present two types of spectroscopy, ultrafast Auger and ultrafast X-ray absorption spectroscopy, and discuss their sensitivity to molecular processes. While Auger spectroscopy is able to monitor bond distance changes in the vicinity of an X-ray created core hole, near-edge absorption spectroscopy can deliver high-fidelity information on non-adiabatic transitions involving lone-pair orbitals. We demonstrate these features on the example of the UV-excited nucleobase thymine, investigated at the oxygen K-edge. We find a C-O bond elongation in the Auger data in addition to ππ*/ nπ* non-adiabatic transition in X-ray near-edge absorption. We compare the results from both methods and draw a conclusive scenario of non-adiabatic molecular relaxation after UV excitation. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Tracing the 267 nm-Induced Radical Formation in Dimethyl Disulfide Using Time-Resolved X-ray Absorption Spectroscopy.

Disulfide bonds are pivotal for the structure, function, and stability of proteins, and understanding ultraviolet (UV)-induced S-S bond cleavage is highly relevant for elucidating the fundamental mechanisms underlying protein photochemistry. Here, the near-UV photodecomposition mechanisms in gas-phase dimethyl disulfide, a prototype system with a S-S bond, are probed by ultrafast transient X-ray absorption spectroscopy. The evolving electronic structure during and after the dissociation is simultaneously monitored at the sulfur L1,2,3-edges and the carbon K-edge with 100 fs (FWHM) temporal resolution using the broadband soft X-ray spectrum from a femtosecond high-order harmonics light source. Dissociation products are identified with the help of ADC and RASPT2 electronic-structure calculations. Rapid dissociation into two CH3S radicals within 120 ± 30 fs is identified as the major relaxation pathway after excitation with 267 nm radiation. Additionally, a 30 ± 10% contribution from asymmetric CH3S2 + CH3 dissociation is indicated by the appearance of CH3 radicals, which is, however, at least partly the result of multiphoton excitation.

Ultrafast x-ray absorption near edge spectroscopy of Fe3O4 using a laboratory based femtosecond x-ray source.

Ultrafast time-resolved x-ray absorption near edge spectroscopy (XANES) experiment was performed on a magnetite (Fe3O4) film using a femtosecond laser plasma x-ray source delivering Bremsstrahlung radiation. Ultrafast temporal evolution of the XANES of Fe3O4 following an excitation by an infra-red (IR) laser pulse was observed in a pump-probe scheme. The Fe K x-ray absorption edge shifts towards low energy upon IR excitation as much as 12 eV, which is mainly attributed to the charge transfer between the Fe ions. The shift in the absorption edge occurred within about 150 fs, typical time of non-thermal electronic redistribution. The charge transfer also causes an ultrafast increase in the IR transmission in the similar time scale.

Ultrafast X-ray Absorption Near Edge Structure Reveals Ballistic Excited State Structural Dynamics.

Polarized ultrafast time-resolved X-ray absorption near edge structure (XANES) allows characterization of excited state dynamics following excitation. Excitation of vitamin B12, cyanocobalamin (CNCbl), in the αβ-band at 550 nm and the γ-band at 365 nm was used to uniquely resolve axial and equatorial contributions to the excited state dynamics. The structural evolution of the excited molecule is best described by a coherent ballistic trajectory on the excited state potential energy surface. Prompt expansion of the Co cavity by ca. 0.03 Å is followed by significant elongation of the axial bonds (>0.25 Å) over the first 190 fs. Subsequent contraction of the Co cavity in both axial and equatorial directions results in the relaxed S1 excited state structure within 500 fs of excitation.

Metalloprotein entatic control of ligand-metal bonds quantified by ultrafast x-ray spectroscopy.

The multifunctional protein cytochrome c (cyt c) plays key roles in electron transport and apoptosis, switching function by modulating bonding between a heme iron and the sulfur in a methionine residue. This Fe-S(Met) bond is too weak to persist in the absence of protein constraints. We ruptured the bond in ferrous cyt c using an optical laser pulse and monitored the bond reformation within the protein active site using ultrafast x-ray pulses from an x-ray free-electron laser, determining that the Fe-S(Met) bond enthalpy is ~4 kcal/mol stronger than in the absence of protein constraints. The 4 kcal/mol is comparable with calculations of stabilization effects in other systems, demonstrating how biological systems use an entatic state for modest yet accessible energetics to modulate chemical function.

Optimization of a laser plasma x-ray source for ultrafast x-ray absorption spectroscopy

We present optimization of laser plasma x-ray experimental conditions for ultrafast x-ray absorption spectroscopy measurements on broad range of transition metal oxides. First, the x-ray flux generated from a laser plasma source was optimized with an emphasis on the Bremsstrahlung by investigating the influence of the angle of the incidence of laser beam on a Cu tape target. The x-ray flux emitted in both the front and transmitted side of the target was found to be optimal at the incident angle of 15 ~ 25 degrees. Moreover the manipulation of the Bremsstrahlung peak energy by the laser focus distribution was discussed. In addition to the source optimization, we present a scheme to find the time-delay zero position in a pump-probe experiment together with a normalization scheme for x-ray source fluctuations. As a feasibility check, we present the transmitted spectra of two materials, Ni and NiO.

Ultrafast X-ray measurements of the glass-like, high-frequency stiffness of aqueous solutions.

The ultrafast dynamics of the domains surrounding solutes in aqueous solution were measured using laser-generating GHz phonons inside of 30 mM ferrocyanide solutions and the resulting molecular motions of the solutes and their hydrogen-bonded solvation shells were detected using ultrafast X-ray absorption spectroscopy (UXAS). The measurements of the phonon dynamics around the solutes showed that the domains had a glass-like stiffness on a timescale of tens of picoseconds.