Ultrafast Structural Dynamics Research Articles

Ultrafast vibrational wave packet dynamics of the aqueous tyrosyl radical anion induced by photodetachment.

The ultrafast dynamics triggered by the photodetachment of the tyrosinate dianion in aqueous environment shed light on the elementary processes that accompany the interaction of ionizing radiation with biological matter. Photodetachment of the tryosinate dianion yields the tyrosyl radical anion, an important intermediate in biological redox reactions, although the study of its ultrafast dynamics is limited. Here, we utilize femtosecond optical pump-probe spectroscopy to investigate the ultrafast structural reorganization dynamics that follow the photodetachment of the tyrosinate dianion in aqueous solution. Photodetachment of the tyrosinate dianion leads to vibrational wave packet motion along seven vibrational modes that are coupled to the photodetachment process. The vibrational modes are assigned with the aid of density functional theory (DFT) calculations. Our results offer a glimpse of the elementary dynamics of ionized biomolecules and suggest the possibility of extending this approach to investigate the ionization-induced structural rearrangement of other aromatic amino acids and larger biomolecules.

Ultrafast structural dynamics of in-cage isomerization of diiodomethane in solution.

Despite extensive studies on the isomer species formed by photodissociation of haloalkanes in solution, the molecular structure of the precursor of the isomer, which is often assumed to be a vibrationally hot isomer formed from the radical pair, and its in-cage isomerization mechanism remain elusive. Here, the structural dynamics of CH2I2 upon 267 nm photoexcitation in methanol were probed with femtosecond X-ray solution scattering at an X-ray free-electron laser. The determined molecular structure of the transiently formed species that converts to the CH2I–I isomer has the I–I distance of 4.17 Å, which is longer than that of the isomer (3.15 Å) by more than 1.0 Å and the mean-squared displacement of 0.45 Å2, which is about 100 times larger than those of typical regular chemical bonds. These unusual structural characteristics are consistent with either a vibrationally hot form of the CH2I–I isomer or the loosely-bound radical pair (CH2I˙⋯I˙).

Ultrafast strain dynamics in SrCoO<sub>2.5</sub> thin films

<sec>In order to understand the relationship between the structure of materials and its function, it is necessary to investigate the changes of the transient structure of materials over time. Laser-based plasma X-ray sources are currently widely used in the study of ultrafast structure dynamics in condensed matter due to their miniaturization and ultrahigh spatial-temporal resolution. Strongly correlated transition-metal oxides have attracted enormous attention due to their peculiar properties, among them Co-based oxides has now become one of the most promising candidates for renewable energy applications. With the variation of the oxygen stoichiometry, the physical properties of SrCoO<sub>3–<i>x</i></sub>, ferromagnetic metal perovskite SrCoO<sub>3</sub> and antiferromagnetic insulator brownmillerite SrCoO<sub>2.5</sub> can be reversibly transferred. Besides, the various complex physical properties make SrCoO<sub>2.5</sub> quite popular for fundamental research, the development of solid oxide fuel cells, etc. However, the research of its dynamic behavior under transient photo-excitation is still limited. Therefore, it is necessary to study the strain fields of SrCoO<sub>2.5</sub> films with different thickness. </sec><sec>This report focuses on the structural dynamics of SrCoO<sub>2.5</sub> films induced by ultrashort laser pulses. The ultrafast X-ray diffraction simulations exhibit transient changes of Bragg peak positions of the SrCoO<sub>2.5</sub> excited by laser. By studying the 40 nm- and 60 nm-thick samples, we observe a continuous shift of the Bragg peak towards lower angels at first and then a backshift until it reaches a new equilibrium. In contrast, the 100 nm-thick SrCoO<sub>2.5</sub> film exhibits a transient splitting of Bragg peak into two distinct parts until the initial peak disappears. For further research, we use Thomsen model to simulate the generation and evolution of acoustic deformation of SCO<sub>2.5</sub> thin film on a substrate supporting the LaAlO<sub>3</sub> film. In the case of the thicker film, we find that an inhomogeneity of temperature distribution will lead its thermal stress characteristics to change, and result in the transient splitting of Bragg peak. We believe that this work is important for analyzing the laser excited ultrafast dynamics of cobalt-based perovskite materials.</sec>

THz-Enhanced DC Ultrafast Electron Diffractometer

Terahertz- (THz-) based electron manipulation has recently been shown to hold tremendous promise as a technology for manipulating and driving the next generation of compact ultrafast electron sources. Here, we demonstrate an ultrafast electron diffractometer with THz-driven pulse compression. The electron bunches from a conventional DC gun are compressed by a factor of 10 and reach a duration of ~180 fs (FWHM) with 10,000 electrons/pulse at a 1 kHz repetition rate. The resulting ultrafast electron source is used in a proof-of-principle experiment to probe the photoinduced dynamics of single-crystal silicon. The THz-compressed electron beams produce high-quality diffraction patterns and enable the observation of the ultrafast structural dynamics with improved time resolution. These results validate the maturity of THz-driven ultrafast electron sources for use in precision applications.

Charge transfer driven by ultrafast spin transition in a CoFe Prussian blue analogue.

Photoinduced charge-transfer is an important process in nature and technology and is responsible for the emergence of exotic functionalities, such as magnetic order for cyanide-bridged bimetallic coordination networks. Despite its broad interest and intensive developments in chemistry and material sciences, the atomic-scale description of the initial photoinduced process, which couples intermetallic charge-transfer and spin transition, has been debated for decades; it has been beyond reach due to its extreme speed. Here we study this process in a prototype cyanide-bridged CoFe system by femtosecond X-ray and optical absorption spectroscopies, enabling the disentanglement of ultrafast electronic and structural dynamics. Our results demonstrate that it is the spin transition that occurs first on the Co site within ~50 fs, and it is this that drives the subsequent Fe-to-Co charge-transfer within ~200 fs. This study represents a step towards understanding and controlling charge-transfer-based functions using light.

Ultrafast strain engineering and coherent structural dynamics from resonantly driven optical phonons in LaAlO3

Strain engineering has been extended recently to the picosecond timescales, driving ultrafast metal–insulator phase transitions and the propagation of ultrasonic demagnetization fronts. However, the nonlinear lattice dynamics underpinning interfacial optoelectronic phase switching have not yet been addressed. Here we perform time-resolved all-optical pump-probe experiments to study ultrafast lattice dynamics initiated by impulsive light excitation tuned in resonance with a polar lattice vibration in LaAlO3 single crystals, one of the most widely utilized substrates for oxide electronics. We show that ionic Raman scattering drives coherent rotations of the oxygen octahedra around a high-symmetry crystal axis. By means of DFT calculations we identify the underlying nonlinear phonon–phonon coupling channel. Resonant lattice excitation is also shown to generate longitudinal and transverse acoustic wave packets, enabled by anisotropic optically induced strain. Importantly, shear strain wave packets are found to be generated with high efficiency at the phonon resonance, opening exciting perspectives for ultrafast material control.

Preface to the special issue: Selected papers from the 5th International Conference on Ultrafast Structural Dynamics

Preface to the special issue: Selected papers from the 5th International Conference on Ultrafast Structural Dynamics

Phonon transport probed at carbon nanotube yarn/sheet boundaries by ultrafast structural dynamics

Modern integrated devices and electrical circuits have often been designed with carbon nanostructures, such as carbon nanotubes (CNTs) and graphene due to their high thermal and electrical transport properties. These transport properties are strongly correlated to their acoustic phonon and carrier dynamics. Thus, understanding the phonon and carrier dynamics of carbon nanostructures in extremely small regions will lead to their further practical applications. Here, we demonstrate ultrafast time-resolved electron diffraction and ultrafast transient spectroscopy to characterize the phonon and carrier dynamics at the boundary of quasi-one-dimensional CNTs before and after Joule annealing. The results from ultrafast time-resolved electron diffraction show that the CNTs after Joule annealing reach the phonon equilibrium state extremely fast with a timescale of 10 ps, which indicates that thermal transport in CNTs improves following Joule annealing. The methodology described in this study connects conventional macroscopic thermo- and electrodynamics to those at the nanometer scale. Realistic timescale kinetic simulations were performed to further elaborate on the phenomena that occur in CNTs during Joule annealing. The insights obtained in this study are expected to pave the way to parameterize the unexplored thermal and electrical properties of carbon materials at the nanometer scale.

Ultrafast structural relaxation dynamics of laser-excited graphene: Ab initio molecular dynamics simulations including electron-phonon interactions

The time evolution of the lattice structure in graphene after ultrashort laser excitation results in a complex dynamics of electrons and ions. In particular, a femtosecond laser pulse heats up the electrons, which then couple strongly to particular optical phonon modes, called SCOPs (strongly coupled optical phonons), located around the $\mathrm{\ensuremath{\Gamma}}$ and K (K') points of the Brillouin zone. In this paper, a fully ab initio description of the ultrafast structural response of graphene to femtosecond laser excitation is presented. Our atomistic simulations show that upon intense ultrafast laser excitation, a biexponential decay of the Bragg peaks takes place, in agreement with experiments. The calculated time-dependent phonon energies show that SCOPs equilibrate considerably faster with phonons having frequencies above 10 THz than with lower frequency phonon modes.

Cross-Examination of Ultrafast Structural, Interfacial, and Carrier Dynamics of Supported Monolayer MoS2.

In this Letter, the ultrafast structural, interfacial, and carrier dynamics of monolayer MoS2 supported on sapphire are cross-examined by the combination of ultrafast electron diffraction (UED) and transient reflectivity techniques. The out-of-plane motions directly probed by reflection UED suggest a limited anisotropy in the atomic motions of monolayer MoS2, which is distinct from that of related materials such as graphene and WSe2. Besides thermal diffusion, the MoS2-sapphire interface exhibits structural dynamics trailing those of the overlaying MoS2 and are in stark contrast with the sapphire bulk, which is consistent with the limited thermal boundary conductance. These structural dynamics provide justification for the determination of carriers being trapped by defects in ∼600 fs and releasing energy within a few picoseconds. The rich findings attest to the strength of combining techniques with real-time optical and direct structure probes for a detailed understanding of dynamical processes in functional materials.

A Compact Ultrafast Electron Diffractometer with Relativistic Femtosecond Electron Pulses

We have developed a compact relativistic femtosecond electron diffractometer with a radio-frequency photocathode electron gun and an electron lens system. The electron gun generated 2.5-MeV-energy electron pulses with a duration of 55 ± 5 fs containing 6.3 × 104 electrons per pulse. Using these pulses, we successfully detected high-contrast electron diffraction images of single crystalline, polycrystalline, and amorphous materials. An excellent spatial resolution of diffraction images was obtained as 0.027 ± 0.001 Å−1. In the time-resolved electron diffraction measurement, a laser-excited ultrafast electronically driven phase transition in single-crystalline silicon was observed with a temporal resolution of 100 fs. The results demonstrate the advantages of the compact relativistic femtosecond electron diffractometer, including access to high-order Bragg reflections, single shot imaging with the relativistic femtosecond electron pulse, and the feasibility of time-resolved electron diffraction to study ultrafast structural dynamics.

Ultrafast lattice and electronic dynamics in single-walled carbon nanotubes.

Understanding the photoinduced ultrafast structural transitions and electronic dynamics in single-walled carbon nanotubes (SWCNTs) is important for the development of SWCNT-based optoelectronic devices. In this study, we conducted femtosecond-resolved electron diffraction and electron energy-loss spectroscopy (EELS) measurements on SWCNTs using ultrafast transmission electron microscopy. The experimental results demonstrated that dominant time constants of the dynamic processes were ∼1.4 ps for electron-driven lattice expansion, ∼17.4 ps for thermal phonon-driven lattice expansion associated with electron–phonon coupling. The time-resolved EELS measurements clearly revealed a notable red shift of plasmon peaks by ∼100 meV upon femtosecond laser excitation. Different features of charge carrier excitation and relaxation were carefully discussed in correlation with the lattice dynamics and photoinduced absorption signals of SWCNTs. Our results provide a comprehensive understanding of the ultrafast dynamics in SWCNTs and powerful techniques to characterize the dynamics of low-dimensional structures.

Design of a femtosecond electron diffractometer with adjustable gaps

One of the grand challenges in ultrafast science is real-time visualization of the microscopic structural evolution on atomic time and length scales. A promising pump-probe technique using a femtosecond laser pulse to initiate the ultrafast dynamics and another ultrashort electron pulse to probe the resulting changes has been developed and widely used to study ultrafast structural dynamics in chemical reactions, phase transitions, charge density waves, and even biological functions. In the past three decades, a number of different ultrafast electron guns have been developed to generate ultashort electron sources, mainly including hybrid electron gun with radio-frequency (RF) cavities for compressing the pulse broadening, relativistic electron gun for suppressing the coulomb interaction, single-electron pulses without space charge effect and compact direct current (DC) electron gun for minimizing the electron propagation distance. At present, these developments with different final electron energy and available total charge have improved the time response of ultrafast electron diffraction (UED) setups to a new frontier approaching to 100 fs regime. Although enormous efforts have been made, the superior capabilities and potentials of ultrafast electron diffraction (UED) are still hindered by space-charge induced pulse broadening. Besides, the penetration depth of electrons increases with the electron energy, while the scattering probability of electrons has the opposite consequence. Thus, in addition to the temporal resolution enhancement, it is also important that the electron energy should be tunable in a wide range to meet the requirements for samples with different thickness. Here in this work, we design a novel ultra-compact electron gun which combines a well-designed cathode profile, thereby providing a uniform field and a movable anode configuration to achieve a temporal resolution on the order of 100 fs over an accelerating voltage range from 10 kV to 125 kV. By optimizing the design of the high-voltage electrode profile, the field enhancement factor on the axis and along the cathode surface are both less than ~4% at different cathode-anode spacings, and thus the maximum on-axis field strength of ~10 MV/m is achieved under various accelerating voltages. This effectively suppresses the space charge broadening effect of the electron pulse. Furthermore, the anode aperture is designed as a stepped hole in which the dense sample grid can be placed, and the sample under study is directly supported by the grid and located at the anode, which reduces the cathode-to-sample distance, thus minimizing the electron pulse broadening from the cathode to sample. Moreover, the defocusing effect caused by the anode hole on the electron beam can be effectively reduced, therefore improving the lateral focusing performance of the electron beam.

New crystallography using relativistic femtosecond electron pulses

Ultrafast electron microscopy (UEM) with femtosecond temporal resolution has long been a cherished dream tool for scientists wishing to study ultrafast structural dynamics in materials, appealing to researchers from across a wide range of speciality areas. Associate Professor Jinfeng Yang, from the Institute of Scientific and Industrial Research, at Osaka University in Japan, leads a team working on ultrafast electron diffraction (UED) and ultrafast electron microscopy (UEM) development. 'Through the study of ultrafast phenomena with the UEM, we hope to gain a deeper understanding of materials and their physical properties and achieve a novel breakthrough in materials science,' he highlights. 'We fully expect to facilitate new knowledge and discoveries as a result of our work.' The team's work on relativistic UEM has led to the creation of unprecedented innovative technology that enables femtosecond atomic-scale imaging using just a single shot measurement. This will pave the way for the study of dynamics of irreversible processes within materials sciences. Not only does the group's work represent a huge step forward in innovative technology for researchers working across a number of scientific fields, but it is also progress in developing a very compact, ultra-high voltage electron microscopy. It can also be used in a variety of settings such as general research institutions and laboratories. In addition, through its provision of a solution to the problem of femtosecond temporal resolution our technology is breaking new ground in electronic microscopy developments,' says Yang.

Tracking Photoinduced Au-Au Bond Formation through Transient Terahertz Vibrations Observed by Femtosecond Time-Domain Raman Spectroscopy.

Real-time observation of chemical bond formation and subsequent nuclear rearrangements is an ultimate goal of chemical science. Yet, such attempts have been hampered by the technical difficulty of triggering bond formation at well-defined, desired timing. The trimer of dicyanoaurate complex ([Au(CN)2-]3) is an ideal system for achieving this aim because the tight covalent Au-Au bonds are formed upon photoexcitation. Despite the apparent simplicity of the system, however, recent time-resolved studies failed to construct a consistent picture of its ultrafast dynamics. Here, we report femtosecond time-domain Raman tracking of ultrafast structural dynamics of the [Au(CN)2-] trimer upon photoinduced Au-Au bond formation. The obtained Raman data reveal that the Au-Au breathing vibration at ∼90 cm-1 exhibits a gradual frequency upshift in a few picoseconds, demonstrating a continuous bent-to-linear structural change on the triplet-state potential energy surface upon the Au-Au bond formation. The comprehensive ultrafast spectroscopic study settles the controversy on this prototypical molecular assembly.

Ultrafast intramolecular proton transfer reactions and solvation dynamics of DMSO.

Ultrafast intramolecular proton transfers of 1,2-dihydroxyanthraquinone (alizarin-h2) and its deuterated product (alizarin-d2) in dimethyl sulfoxide (DMSO) have been investigated by femtosecond stimulated Raman spectroscopy. The population dynamics in the solute vibrational mode of νC=O and the coherent oscillations observed in all of the skeletal vibrational modes νC=O and νC=C clearly showed the ultrafast excited-state intramolecular proton transfer dynamics of 110 and 170 fs for alizarin-h2 and alizarin-d2, respectively. Interestingly, we have observed that the solvent vibrational modes νS=O and νCSC may also represent ultrafast structural dynamics at the frequencies for its “free” or “aggregated” species. From the kinetic analysis of the νS=O and νCSC modes of DMSO, the ultrafast changes in the solvation or intermolecular interactions between DMSO molecules initiated by the structural changes of solute molecules have been thoroughly investigated. We propose that the solvent vibrational modes νS=O and νCSC of DMSO can be used as a “sensor” for ultrafast chemical reactions accompanying the structural changes and subsequent solute-solvent interactions.

Femtosecond gas-phase mega-electron-volt ultrafast electron diffraction.

The development of ultrafast gas electron diffraction with nonrelativistic electrons has enabled the determination of molecular structures with atomic spatial resolution. It has, however, been challenging to break the picosecond temporal resolution barrier and achieve the goal that has long been envisioned—making space- and-time resolved molecular movies of chemical reaction in the gas-phase. Recently, an ultrafast electron diffraction (UED) apparatus using mega-electron-volt (MeV) electrons was developed at the SLAC National Accelerator Laboratory for imaging ultrafast structural dynamics of molecules in the gas phase. The SLAC gas-phase MeV UED has achieved 65 fs root mean square temporal resolution, 0.63 Å spatial resolution, and 0.22 Å−1 reciprocal-space resolution. Such high spatial-temporal resolution has enabled the capturing of real-time molecular movies of fundamental photochemical mechanisms, such as chemical bond breaking, ring opening, and a nuclear wave packet crossing a conical intersection. In this paper, the design that enables the high spatial-temporal resolution of the SLAC gas phase MeV UED is presented. The compact design of the differential pump section of the SLAC gas phase MeV UED realized five orders-of-magnitude vacuum isolation between the electron source and gas sample chamber. The spatial resolution, temporal resolution, and long-term stability of the apparatus are systematically characterized.

Advances and opportunities in ultrafast X-ray crystallography and ultrafast structural optical crystallography of nuclear and electronic protein dynamics.

Both nuclear and electronic dynamics contribute to protein function and need multiple and complementary techniques to reveal their ultrafast structural dynamics response. Real-space information obtained from the measurement of electron density dynamics by X-ray crystallography provides aspects of both, while the molecular physics of coherence parameters and frequency-frequency correlation needs spectroscopy methods. Ultrafast pump-probe applications of protein dynamics in crystals provide real-space information through direct X-ray crystallographic structure analysis or through structural optical crystallographic analysis. A discussion of methods of analysis using ultrafast macromolecular X-ray crystallography and ultrafast nonlinear structural optical crystallography is presented. The current and future high repetition rate capabilities provided by X-ray free electron lasers for ultrafast diffraction studies provide opportunities for optical control and optical selection of nuclear coherence which may develop to access higher frequency dynamics through improvements of sensitivity and time resolution to reveal coherence directly. Specific selection of electronic coherence requires optical probes, which can provide real-space structural information through photoselection of oriented samples and specifically in birefringent crystals. Ultrafast structural optical crystallography of photosynthetic energy transfer has been demonstrated, and the theory of two-dimensional structural optical crystallography has shown a method for accessing the structural selection of electronic coherence.

Ultrafast Charge Transfer and Structural Dynamics Following Outer-Valence Ionization of a Halogen-Bonded Dimer.

We study the temporal evolution of the CH2O···ClF halogen-bonded dimer following vertical ionization out of outer-valence molecular orbitals on a femtosecond time scale, employing mixed quantum-classical molecular dynamics simulations. The charge density pattern in the ground state that is suitable for the formation of the ground-state halogen bond can be changed upon ionization. Depending on the molecular orbital that gets ionized, the change in the charge density transiently strengthens or weakens the halogen bond through altering the electrostatic interaction. A transient increase in the halogen bond strength is observed if the ionization enhances the positive charge on the halogen atom provided that the electron donor site possesses some negative charge. The evolution of the system following ionization is driven by energetic stabilization through transferring the electronic charge from the halogen bond acceptor/electron donor (CH2O) to the halogen bond donor/electron acceptor (ClF). The charge transfer oscillations are at the same time governed by the covalent bond vibrations.

Ultrafast structural rearrangement dynamics induced by the photodetachment of phenoxide in aqueous solution

The elementary processes that accompany the interaction of ionizing radiation with biologically relevant molecules are of fundamental importance. However, the ultrafast structural rearrangement dynamics induced by the ionization of biomolecules in aqueous solution remain hitherto unknown. Here, we employ femtosecond optical pump-probe spectroscopy to elucidate the vibrational wave packet dynamics that follow the photodetachment of phenoxide, a structural mimic of tyrosine, in aqueous solution. Photodetachment of phenoxide leads to wave packet dynamics of the phenoxyl radical along 12 different vibrational modes. Eight of the modes are totally symmetric and support structural rearrangement upon electron ejection. Comparison to a previous photodetachment study of phenoxide in the gas phase reveals the important role played by the solvent environment in driving ultrafast structural reorganization induced by ionizing radiation. This work provides insight into the ultrafast molecular dynamics that follow the interaction of ionizing radiation with molecules in aqueous solution.