Ultrafast Structural Dynamics Research Articles

Generation and Implementation of Continuum Infrared Pulses for Broadband Detection in 2D IR Spectroscopy.

In recent years, new methods of generating continuum mid-infrared pulses through filamentation in gases have been developed for ultrafast time-resolved infrared vibrational spectroscopy. The generated infrared pulses can have thousands of wavenumbers of bandwidth, spanning the entire mid-IR region while retaining pulse length below 100 fs. This technology has had a significant impact on problems involving ultrafast structural dynamics in congested spectra with broad features, such as those found in aqueous solutions and molecules with strong intermolecular interactions. This study describes the recent advances in generating and characterizing these pulses and the practical aspects of implementing these sources for broadband detection in transient absorption and 2D IR spectroscopy.

Multi-objective Bayesian active learning for MeV-ultrafast electron diffraction

Ultrafast electron diffraction using MeV energy beams(MeV-UED) has enabled unprecedented scientific opportunities in the study of ultrafast structural dynamics in a variety of gas, liquid and solid state systems. Broad scientific applications usually pose different requirements for electron probe properties. Due to the complex, nonlinear and correlated nature of accelerator systems, electron beam property optimization is a time-taking process and often relies on extensive hand-tuning by experienced human operators. Algorithm based efficient online tuning strategies are highly desired. Here, we demonstrate multi-objective Bayesian active learning for speeding up online beam tuning at the SLAC MeV-UED facility. The multi-objective Bayesian optimization algorithm was used for efficiently searching the parameter space and mapping out the Pareto Fronts which give the trade-offs between key beam properties. Such scheme enables an unprecedented overview of the global behavior of the experimental system and takes a significantly smaller number of measurements compared with traditional methods such as a grid scan. This methodology can be applied in other experimental scenarios that require simultaneously optimizing multiple objectives by explorations in high dimensional, nonlinear and correlated systems.

Deciphering Charge Transfer Processes in Transition Metal Complexes from the Perspective of Ultrafast Electronic and Nuclear Motions.

Chemical transformations in charge transfer states result from the interplay between electronic dynamics and nuclear reorganization along excited-state trajectories. Here, we investigate the ultrafast structural dynamics following photoinduced electron transfer from the metal-metal-to-ligand charge transfer state of an electron donor, a Pt dimer complex, to a covalently linked electron acceptor group using ultrafast time-resolved wide-angle X-ray scattering and optical transient absorption spectroscopy methods to disentangle the interdependence of the excited-state electronic and nuclear dynamics. Following photoexcitation, Pt-Pt bond formation and contraction takes up to 1 ps, much slower than the corresponding process in analogous complexes without electron acceptor groups. Because the Pt-Pt distance change is slow with respect to excited-state electron transfer, it can affect the rate of electron transfer. These results have potential impacts on controlling electron transfer rates via structural alterations to the electron donor group, tuning the charge transfer driving force.

Toward Direct Exploration of the Few-Femtosecond Dynamics of Electronic Coherence and Correlation in Quantum Materials Using Time- and Angle-Resolved Photoemission Spectroscopy

Over the last two decades, time- and angle-resolved photoemission spectroscopy (trARPES) has become a mature and established experimental technique for the study of ultrafast electronic and structural dynamics in materials. To date, most trARPES investigations have focused on the investigation of processes occurring on time scales of ≳30 fs, in particular, relaxation and thermalization, and have therefore been blind to the initial sub-10 fs dynamics related to electronic coherence and correlation effects. In this article, we illustrate how current trARPES setups reach their limits when it comes to addressing such extraordinarily short time scales and present an experimental configuration that provides the time, energy, and momentum resolutions required to monitor few-femtosecond dynamics on the relevant energy and momentum scales. We discuss the potential capabilities of such an experiment to study the electronic response of materials in the strong-field interaction regime at PHz frequencies and finally review a theoretical concept that may in the future even overcome the competing resolution limitations of trARPES experiments, as imposed by the time–bandwidth product of the probing laser pulse. Our roadmap for ultrafast trARPES indicates a path to break new experimental ground in quantum nonequilibrium electronic dynamics, from which new possibilities for ultrafast control of optical and electronic signals in quantum materials can be explored.

Unusual photoinduced crystal structure dynamics in TaTe2 with double zigzag chain superstructure

Transition metal dichalcogenides with superperiodic lattice distortions have been widely investigated as the platform of ultrafast structural phase manipulations. Here we performed ultrafast electron diffraction on RT TaTe2, which exhibits a peculiar double zigzag chain pattern of Ta atoms. From the time-dependent electron diffraction pattern, we revealed a photoinduced change in the crystal structure occurring within <0.5 ps, although there is no corresponding high-temperature equilibrium phase. We further clarified the slower response (∼1.5 ps) reflecting the lattice thermalization. Our result suggests the unusual ultrafast crystal structure dynamics specific to the non-equilibrium transient process in TaTe2.

Femtosecond Electron Diffraction Reveals Local Disorder and Local Anharmonicity in Thermoelectric SnSe.

In addition to long-range periodicity, local disorder, with local structures deviating from the average lattice structure, dominates the physical properties of phonons, electrons, and spin subsystems in crystalline functional materials. Experimentally characterizing the 3D atomic configuration of such a local disorder and correlating it with advanced functions remains challenging. Using a combination of femtosecond electron diffraction, structure factor calculations, and time-dependent density functional theory molecular dynamics simulations, the static local disorder and its local anharmonicity in thermoelectric SnSe are identified exclusively. The ultrafast structural dynamics reveal that the crystalline SnSe is composed of multiple locally correlated configurations dominated by the static off-symmetry displacements of Sn (≈0.4 Å) and such a set of locally correlated structures is termed local disorder. Moreover, the anharmonicity of this local disorder induces an ultrafast atomic displacement within 100 fs, indicating the signature of probable THz Einstein oscillators. The identified local disorder and local anharmonicity suggest a glass-like thermal transport channel, which updates the fundamental insight into the long-debated ultralow thermal conductivity of SnSe. The method of revealing the 3D local disorder and the locally correlated interactions by ultrafast structural dynamics will inspire broad interest in the construction of structure-property relationships in material science.

Influence of pump laser fluence on ultrafast myoglobin structural dynamics.

High-intensity femtosecond pulses from an X-ray free-electron laser enable pump-probe experiments for the investigation of electronic and nuclear changes during light-induced reactions. On timescales ranging from femtoseconds to milliseconds and for a variety of biological systems, time-resolved serial femtosecond crystallography (TR-SFX) has provided detailed structural data for light-induced isomerization, breakage or formation of chemical bonds and electron transfer1,2. However, all ultrafast TR-SFX studies to date have employed such high pump laser energies that nominally several photons were absorbed per chromophore3-17. As multiphoton absorption may force the protein response into non-physiological pathways, it is of great concern18,19 whether this experimental approach20 allows valid conclusions to be drawn vis-à-vis biologically relevant single-photon-induced reactions18,19. Here we describe ultrafast pump-probe SFX experiments on the photodissociation of carboxymyoglobin, showing that different pump laser fluences yield markedly different results. In particular, the dynamics of structural changes and observed indicators of the mechanistically important coherent oscillations of the Fe-CO bond distance (predicted by recent quantum wavepacket dynamics21) are seen to depend strongly on pump laser energy, in line with quantum chemical analysis. Our results confirm both the feasibility and necessity of performing ultrafast TR-SFX pump-probe experiments in the linear photoexcitation regime. We consider this to be a starting point for reassessing both the design and theinterpretation of ultrafast TR-SFX pump-probe experiments20 such that mechanistically relevant insight emerges.

Anion Recognition in Solution: Insights from Thermodynamics and Ultrafast Structural Dynamics.

Anion recognition through noncovalent interactions stands as an emerging field in supramolecular chemistry, exerting a profound influence on the regulation of biological functions. Herein, the thermodynamics of complexation between sodium cyanate (NaOCN) and calix[4]pyrrole was systematically investigated by linear and nonlinear IR spectroscopy, highlighting enthalpy changes as the dominant driving force. The overall orientational relaxation of bound anion can be described by an Arrhenius-type activated process, yielding an activation energy of 15.0 ± 1.0 kJ mol-1. The structural dynamics of contact ion pairs (CIPs) formed between Na+ and OCN- in solution showed a negligible temperature effect, suggesting entropy changes as the principal governing factor. Further analysis revealed that anion recognition in solution is mediated by conformational changes of the receptor and collective rearrangement of hydrogen bond dynamics. This study, framed within the paradigms of thermodynamics and ultrafast structural dynamics, substantially advances our comprehension of the microscopic mechanisms underlying anion recognition in the realm of supramolecular chemistry.

High Dose‐Rate MeV Electron Beam from a Tightly‐Focused Femtosecond IR Laser in Ambient Air

AbstractUltrashort electron beams with femtosecond to picosecond bunch durations offer unique opportunities to explore active research areas ranging from ultrafast structural dynamics to ultra‐high dose‐rate radiobiological studies. It presents a straightforward method to generate relativistic electron beams in ambient air via the tight focusing of a few‐cycle, mJ‐class femtosecond infrared laser. It demonstrates experimentally that electrons can reach up to 1.4 MeV at a dose‐rate of 0.15 Gy/s, providing enough dose rate for radiation therapy applications. 3D Particle‐In‐Cell simulations confirm that the acceleration mechanism is based on the relativistic ponderomotive force and show theoretical agreement with the measured electron energies and divergence. Relativistic peak intensities up to 1019 Wcm−2 are reached in ambient air due to a very low B‐integral accumulation during focusing, which prevents intensity clamping. Furthermore, it discusses the scalability of this method with the continuing development of mJ‐class high average power lasers, and providing a promising approach for FLASH radiation therapy.

Capturing the non-equilibrium state in light–matter–free-electron interactions through ultrafast transmission electron microscopy

Ultrafast transmission electron microscope (UTEM) with the multimodality of time-resolved diffraction, imaging, and spectroscopy provides a unique platform to reveal the fundamental features associated with the interaction between free electrons and matter. In this review, we summarize the principles, instrumentation, and recent developments of the UTEM and its applications in capturing dynamic processes and non-equilibrium transient states. The combination of the transmission electron microscope with a femtosecond laser via the pump–probe method guarantees the high spatiotemporal resolution, allowing the investigation of the transient process in real, reciprocal and energy spaces. Ultrafast structural dynamics can be studied by diffraction and imaging methods, revealing the coherent acoustic phonon generation and photo-induced phase transition process. In the energy dimension, time-resolved electron energy-loss spectroscopy enables the examination of the intrinsic electronic dynamics of materials, while the photon-induced near-field electron microscopy extends the application of the UTEM to the imaging of optical near fields with high real-space resolution. It is noted that light–free-electron interactions have the ability to shape electron wave packets in both longitudinal and transverse directions, showing the potential application in the generation of attosecond electron pulses and vortex electron beams.

Probing ultrafast structural and electronic dynamics in chemical and materials research using X-ray free-electron lasers

Probing ultrafast structural and electronic dynamics in chemical and materials research using X-ray free-electron lasers

Optical control of ultrafast structural dynamics in a fluorescent protein

The photoisomerization reaction of a fluorescent protein chromophore occurs on the ultrafast timescale. The structural dynamics that result from femtosecond optical excitation have contributions from vibrational and electronic processes and from reaction dynamics that involve the crossing through a conical intersection. The creation and progression of the ultrafast structural dynamics strongly depends on optical and molecular parameters. When using X-ray crystallography as a probe of ultrafast dynamics, the origin of the observed nuclear motions is not known. Now, high-resolution pump–probe X-ray crystallography reveals complex sub-ångström, ultrafast motions and hydrogen-bonding rearrangements in the active site of a fluorescent protein. However, we demonstrate that the measured motions are not part of the photoisomerization reaction but instead arise from impulsively driven coherent vibrational processes in the electronic ground state. A coherent-control experiment using a two-colour and two-pulse optical excitation strongly amplifies the X-ray crystallographic difference density, while it fully depletes the photoisomerization process. A coherent control mechanism was tested and confirmed the wave packets assignment.

Why Does Single-Atom Photocatalysis Work Better Than Conventional Photocatalysis? A Study on Ultrafast Excited Carrier and Structure Dynamics

With the introduction of single atoms in photocatalysis, a small change in the electronic and geometric structure of the substrate can result in higher energy conversion efficiency, whereas the underlying microscopic dynamics are rarely illustrated. Here, employing real-time time-dependent density functional theory, we explore the ultrafast electronic and structural dynamics of single-atom photocatalysts (SAPCs) in water splitting at the microscopic scale. The results demonstrate that a single-atom Pt loaded on graphitic carbon nitride greatly promotes photogenerated carriers compared to traditional photocatalysts, and effectively separates the excited electrons from holes, prolonging the lifetime of the excited carriers. The flexible oxidation state (Pt2+, Pt0, or Pt3+) renders the single atom as an active site to adsorb the reactant and to catalyze the reactions as a charge transfer bridge at different stages during the photoreaction process. Our results offer deep insights into the single-atom photocatalytic reactions and benefit the design of high-efficiency SAPCs.

High-repetition-rate seeded free-electron laser enhanced by self-modulation

The spectroscopic methods for the ultrafast electronic and structural dynamics of materials require fully coherent extreme ultraviolet and soft X-ray radiation with high-average brightness. Seeded free-electron lasers (FELs) are ideal sources for delivering fully coherent soft X-ray pulses. However, due to state-of-the-art laser system limitations, it is challenging to meet the ultraviolet seed laser’s requirements of sufficient energy modulation and high repetition rates simultaneously. The self-modulation scheme has been proposed and recently demonstrated in a seeded FEL to relax the seed laser requirements. Using numerical simulations, we show that the required seed laser intensity in the self-modulation is ~3 orders of magnitude lower than that in the standard high-gain harmonic generation (HGHG). The harmonic self-modulation can launch a single-stage HGHG FEL lasing at the 30th harmonic of the seed laser. Moreover, the proof-of-principle experimental results confirm that the harmonic self-modulation can still amplify the laser-induced energy modulation. These achievements reveal that the self-modulation can not only remarkably reduce the requirements of the seed laser but also improve the harmonic upconversion efficiency, which paves the way for realizing high-repetition-rate and fully coherent soft X-ray FELs.

Investigating ultrafast structural dynamics using high repetition rate x-ray FEL radiation at European XFEL

European XFEL is an international facility providing hard and soft x-ray free-electron laser radiation for user experiments with a wide range of scientific applications. Its superconducting linear accelerator enables high repetition rate experiments with a broad range of x-ray pulse delivery patterns. The combination of time-resolved experiments, providing access to the time-domain from sub-femtoseconds to milliseconds, with atomic resolution x-ray geometric and electronic structure determination methods is responsible for the bulk of scientific applications of European XFEL. In addition, the extreme x-ray intensities and coherence properties open new methods for studying matter out of equilibrium. After start of operation in 2017, the facility now harvests scientific applications with impact to the challenge areas climate and energy, health, environment and sustainability, and digitalization. Extensions of European XFEL aim to increase performance and capabilities for new scientific applications. An upgrade of the facility in the early 2030s will increase the applicability of European XFEL to solid materials and provide dedicated instruments for improved conditions in specific research fields.

Sensitivity of Kβ mainline X-ray emission to structural dynamics in iron photosensitizer.

Photochemistry and photophysics processes involve structures far from equilibrium. In these reactions, there is often strong coupling between nuclear and electronic degrees of freedom. For first-row transition metals, Kβ X-ray emission spectroscopy (XES) is a sensitive probe of electronic structure due to the direct overlap between the valence orbitals and the 3p hole in the final state. Here the sensitivity of Kβ mainline (Kβ1,3) XES to structural dynamics is analyzed by simulating spectral changes along the excited state dynamics of an iron photosensitizer [FeII(bmip)2]2+ [bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine], using both restricted active space (RAS) multiconfigurational wavefunction theory and a one-electron orbital-energy approach in density-functional theory (1-DFT). Both methods predict a spectral blue-shift with increasing metal-ligand distance, which changes the emission intensity for any given detection energy. These results support the suggestion that the [FeII(bmip)2]2+ femtosecond Kβ XES signal shows oscillations due to coherent wavepacket dynamics. Based on the RAS results, the sensitivity to structural dynamics is twice as high for Kβ compared to Kα, with the drawback of a lower signal-to-noise ratio. Kβ sensitivity is favored by a larger spectral blue-shift with increasing metal-ligand distance and larger changes in spectral shape. Comparing the two simulations methods, 1-DFT predicts smaller energy shifts and lower sensitivity, likely due to missing final-state effects. The simulations can be used to design and interpret XES probes of non-equilibrium structures to gain mechanistic insights in photocatalysis.

Exploring fingerprints of ultrafast structural dynamics in molecular solutions with an X-ray laser.

We apply ultrashort X-ray laser pulses to track optically excited structural dynamics of [Ir2(dimen)4]2+ molecules in solution. In our exploratory study we determine angular correlations in the scattered X-rays, which comprise a complex fingerprint of the ultrafast dynamics. Model-assisted analysis of the experimental correlation data allows us to elucidate various aspects of the photoinduced changes in the excited molecular ensembles. We unambiguously identify that in our experiment the photoinduced transition dipole moments in [Ir2(dimen)4]2+ molecules are oriented perpendicular to the Ir-Ir bond. The analysis also shows that the ground state conformer of [Ir2(dimen)4]2+ with a larger Ir-Ir distance is mostly responsible for the formation of the excited state. We also reveal that the ensemble of solute molecules can be characterized with a substantial structural heterogeneity due to solvent influence. The proposed X-ray correlation approach offers an alternative path for studies of ultrafast structural dynamics of molecular ensembles in the liquid and gas phases.

Disentangling interferences in the photoelectron momentum distribution from strong-field ionization

Using the semiclassical two-step model for strong-field ionization, we theoretically investigate subcycle interference structures in the photoelectron momentum distribution. Specifically, we focus on the low-momentum fanlike interference structure. Employing a time-variable soft-core Coulomb potential, we demonstrate that the low-momentum interference arises from the interference between drifted and undrifted electrons from opposite direct quarter cycles. We also find that the main scattering in the nuclear Coulomb potential occurs just after ionization. Our findings suggest that the low-momentum region of the photoelectron spectrum is particularly sensitive to the ion potential and thereby offers another path to probe ultrafast electronic structure dynamics.

(Invited) Simulating Dynamic Excitons Via Quantum Molecular Dynamics: A Case Study in Lead Halide Perovskites

The ultrafast electronic and structural dynamics invoked by photoexcitation, i.e., “dynamic exciton” phenomena, not only are important in the context of practical applications such as solar cells, but also raise many questions from the viewpoint of fundamental science. Experimental measurement, computational simulations, and theoretical interpretation will be the three pillars for deciphering the dynamic exciton phenomena.From the viewpoint of computational simulations, molecular dynamics (MD) techniques combined with quantum chemical calculations, i.e., quantum molecular dynamics (QMD), has been the popular tool to simulate the dynamic exciton phenomena. The quantum chemical calculations, which are typically conducted on the basis of the density-functional theory (DFT), require the large computational resources and time. These have been the limiting factors for the accessible spatial and time scales by the QMD simulations.To extend the coverage of simulations to more complex, large-scale systems, we have developed efficient excited-state QMD methods that can include nonadiabatic effects. Our method combines the density-functional tight binding (DFTB) method, which is an approximate DFT, and the surface hopping method, which is a theoretical framework to incorporate the nonadiabatic effects into the QMD simulations. The method was further improved to be suitable for condensed-phase simulations explicitly including the environment, i.e., solvent, by using a “divide-and-conquer” style quantum chemical calculation technique. These theoretical framework enables us to simulate the coupled electronic–structural dynamics in excited states of systems consisting of 102–103 atoms[1,2,3].In addition, using the developed method, we conducted the real-time simulations of the ultrafast processes invoked by photoexcitation of lead iodide perovskites, which are known as the key materials for perovskite solar cells. The dissociation of the exciton into the positive and negative charge carriers was observed. Moreover, the hot carrier cooling, where the charge carriers dissipate excess energy via the electron–phonon coupling and relax to the band edges, was also tracked. Finally, the direct evidence of the polaron formation, where the structural deformation is induced by the presence of charge carriers, was observed. These results highlight the importance of the coupling between electronic and structural degrees of freedom.In the talk, recent improvements in the methodology and future perspectives will also be presented[5].

Generation and Application of X−ray Pulses on a Terawatt Laser System

Currently, the Kurchatov Laser−Synchrotron Complex is conducting research on the interaction of powerful femtosecond laser radiation with matter. The purpose of these studies is to develop new techniques and approaches for the characterization of high−temperature plasma, X−rays and particle acceleration, which can be the basis for the work planned within the framework of the synchrotron−neutron project. Laser−driven X−ray production, electron acceleration, nuclear fusion, ultrafast structural dynamics of nanoparticles in intense laser fields remains an active research topic, aimed at generating advanced controllable sources.