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The deflection of charged particles is an intuitive way to visualize an electromagnetic oscillation of coherent light. Here, we present a real-time ultrafast oscilloscope for time-frozen visualization of a terahertz (THz) optical wave by probing light-driven motion of relativistic electrons. We found the unique condition of subwavelength metal slit waveguide for preserving the distortion-free optical waveform during its propagation. Momentary stamping of the wave, transversely travelling inside a metal slit, on an ultrashort wide electron bunch enables the single-shot recording of an ultrafast optical waveform. As a proof-of-concept experiment, we successfully demonstrated to capture the entire field oscillation of a THz pulse with a sampling rate of 75.7 TS/s. Owing to the use of transversely-wide and longitudinally-short electron bunch and transversely travelling wave, the proposed “single-shot oscilloscope” will open up new avenue for developing the real-time petahertz (PHz) metrology., A travelling wave inside a metal slit can reveal its own waveform by probing deflecting motions of charged particles. Here, a real-time THz oscilloscope was demonstrated by utilizing the relativistic electrons and the subwavelength slit waveguide.

We have developed a one-and-half -cell S-band radio-frequency (RF) photoelectron gun (photogun) fed by a coaxial coupler. The RF photogun is dedicated to ultrafast-electron-diffraction experiments by generating electron bunches of 3-MeV energy and a-few-pC charge, which is not strict condition compared to those for X-ray free-electron lasers. Brazing of RF cavities is welldeveloped process for making RF guns or RF accelerators. Sometimes, however, a failure occurs in the brazing process, causing the entire electron gun or accelerating cavity to spoil. Axial-symmetric design of the RF photogun permits indium sealing for cavity cells, a photocathode plate, and a coupling RF part. We firstly report that the indium-sealed RF photogun successfully meets the required performance and long-term stability for ultrafast electron diffraction experiments. We have stably operated the RF photogun for more than three years with the electron beam conditions of ~ 3-MeV energy, up-to-10-pC charge, and a repetition rate of 50 Hz. The quantum efficiency of the copper photocathode had improved from 10−6 to 10−5 depending on vacuum condition from 10−8 to 5 × 10−10 Torr, respectively. Measured emittance and energy spread of the generated electron beam showed 0.3 mm-mrad and less than 0.25%, respectively, for a bunch charge of ~ 2 pC, which agree well with those obtained by ASTRA simulation.

Nature / Physical science 582(7813), 520 - 524 (2020). doi:10.1038/s41586-020-2417-3, Fundamental studies of chemical reactions often consider the molecular dynamics along a reaction coordinate using a calculated or suggested potential energy surface1,2,3,4,5. But fully mapping such dynamics experimentally, by following all nuclear motions in a time-resolved manner���that is, the motions of wavepackets���is challenging and has not yet been realized even for the simple stereotypical bimolecular reaction6,7,8: A���B + C ��� A + B���C. Here we track the trajectories of these vibrational wavepackets during photoinduced bond formation of the gold trimer complex [Au(CN)2���]3 in an aqueous monomer solution, using femtosecond X-ray liquidography9,10,11,12 with X-ray free-electron lasers13,14. In the complex, which forms when three monomers A, B and C cluster together through non-covalent interactions15,16, the distance between A and B is shorter than that between B and C. Tracking the wavepacket in three-dimensional nuclear coordinates reveals that within the first 60 femtoseconds after photoexcitation, a covalent bond forms between A and B to give A���B + C. The second covalent bond, between B and C, subsequently forms within 360 femtoseconds to give a linear and covalently bonded trimer complex A���B���C. The trimer exhibits harmonic vibrations that we map and unambiguously assign to specific normal modes using only the experimental data. In principle, more intense X-rays could visualize the motion not only of highly scattering atoms such as gold but also of lighter atoms such as carbon and nitrogen, which will open the door to the direct tracking of the atomic motions involved in many chemical reactions., Published by Macmillan28177, London

With the recent successful development of X-ray free electron lasers (X-FELs), it became possible to explore sub-nano structure dynamics of materials with 100-fs temporal accuracy. Ultrafast electron diffraction (UED) can achieve similar performance at a much lower cost and on a smaller scale by using ultrashort and low-energy electron beams. The UEDs are suitable for studying thin films, surfaces, and gas samples that are difficult to study with the X-FELs. Starting from non-relativistic UEDs using low-energy electron beams of less than 100 keV, it led to the development of relativistic UEDs using a-few-MeV electron beams. These efforts have contributed to the identification of the unexplored mechanism of matter by observing the dynamics of atoms with higher temporal accuracy. Electron beam is easier to handle than X-rays, and various technologies are being developed to improve the performance of UED. We review UEDs historically based on the development of core technologies. And application researches with the UEDs will be outlined in this paper.

Time-resolved X-ray diffraction provides direct information on three-dimensional structures of reacting molecules and thus can be used to elucidate structural dynamics of chemical and biological reactions. In this review, we discuss time-resolved X-ray diffraction on small molecules and proteins with particular emphasis on its application to crystalline (crystallography) and liquid-solution (liquidography) samples. Time-resolved X-ray diffraction has been used to study picosecond and slower dynamics at synchrotrons and can now access even femtosecond dynamics with the recent arrival of X-ray free-electron lasers.

The experimental observation of femtosecond dynamics in atoms and molecules by stroboscopic technologies utilizing x ray or electron flashes has attracted much attention and has rapidly developed. We propose a feasible ultrafast electron diffraction (UED) technology with high brightness and a sub-10 fs temporal resolution. We previously demonstrated a UED system with an overall temporal resolution of 31 fs by using an RF photoelectron gun and a 90° achromatic bending structure. This UED structure enabled a bunch duration of 25 fs and a low timing jitter of less than 10 fs while maintaining a high bunch charge of 0.6 pC. In this paper, we demonstrate a simple way to further compress the electron bunch duration to sub-10 fs based on installing an energy filter in the dispersion section of the achromatic bend. The energy filter removes the electrons belonging to nonlinear parts of the phase space. Through numerical simulations, we demonstrate that the electron bunches can be compressed, at the sample position, to a 6.2 fs (rms) duration for a 100 fC charge. This result suggests that the energy filtering approach is more viable and effective than complicated beam-shaping techniques that commonly handle the nonlinear distribution of the electron beam. Furthermore, a gas-filled hollow core fiber compressor and a Ti:sapphire amplifier are used to implement pump laser pulses of less than 5 fs (rms). Thus, we could present the full simulation results of a sub-10 fs UED, and we believe that it will be one of the technical prototypes to challenge the sub-fs time resolution.

Determination of the optimum kinetic model is an essential prerequisite for characterizing dynamics and mechanism of a reaction. Here, we propose a simple method, termed as singular value decomposition-aided pseudo principal-component analysis (SAPPA), to facilitate determination of the optimum kinetic model from time-resolved data by bypassing any need to examine candidate kinetic models. We demonstrate the wide applicability of SAPPA by examining three different sets of experimental time-resolved data and show that SAPPA can efficiently determine the optimum kinetic model. In addition, the results of SAPPA for both time-resolved X-ray solution scattering (TRXSS) and transient absorption (TA) data of the same protein reveal that global structural changes of protein, which is probed by TRXSS, may occur more slowly than local structural changes around the chromophore, which is probed by TA spectroscopy. (C) 2017 Author(s).

Here we report sub-100-ps structural dynamics of horse heart myoglobin revealed by time-resolved X-ray solution scattering. By applying the time-slicing scheme to the measurement and subsequent deconvolution, we investigate the protein structural dynamics that occur faster than the X-ray temporal pulse width of synchrotrons (∼100 ps). The singular value decomposition analysis of the experimental data suggests that two structurally distinguishable intermediates are formed within 100 ps. In particular, the global structural change occurring on the time scale of 70 ps is identified.

Conformational substates of proteins are generally considered to play important roles in regulating protein functions, but an understanding of how they influence the structural dynamics and functions of the proteins has been elusive. Here, we investigate the structural dynamics of sperm whale myoglobin associated with the conformational substates using picosecond X-ray solution scattering. By applying kinetic analysis considering all of the plausible candidate models, we establish a kinetic model for the entire cycle of the protein transition in a wide time range from 100 ps to 10 ms. Four structurally distinct intermediates are formed during the cycle, and most importantly, the transition from the first intermediate to the second one (B → C) occurs biphasically. We attribute the biphasic kinetics to the involvement of two conformational substates of the first intermediate, which are generated by the interplay between the distal histidine and the photodissociated CO.

The [Au(CN)2 (-)]3 trimer in water experiences a strong van der Waals interaction between the d(10) gold atoms due to large relativistic effect and can serve as an excellent model system to study the bond formation process in real time. The trimer in the ground state (S0) exists as a bent structure without the covalent bond between the gold atoms, and upon the laser excitation, one electron in the antibonding orbital goes to the bonding orbital, thereby inducing the formation of a covalent bond between gold atoms. This process has been studied by various time-resolved techniques, and most of the interpretation on the structure and dynamics converge except that the structure of the first intermediate (S1) has been debated due to different interpretations between femtosecond optical spectroscopy and femtosecond X-ray solution scattering. Recently, the excitation wavelength of 267 nm employed in our previous scattering experiment was suggested as the culprit for misinterpretation. Here, we revisited this issue by performing femtosecond X-ray solution scattering with 310 nm excitation and compared the results with our previous study employing 267 nm excitation. The data show that a linear S1 structure is formed within 500 fs regardless of excitation wavelength and the structural dynamics observed at both excitation wavelengths are identical to each other within experimental errors.

Characterization of transient molecular structures formed during chemical and biological processes is essential for understanding their mechanisms and functions. Over the last decade, time-resolved X-ray liquidography (TRXL) and time-resolved X-ray absorption spectroscopy (TRXAS) have emerged as powerful techniques for molecular and electronic structural analysis of photoinduced reactions in the solution phase. Both techniques make use of a pump–probe scheme that consists of (1) an optical pump pulse to initiate a photoinduced process and (2) an X-ray probe pulse to monitor changes in the molecular structure as a function of time delay between pump and probe pulses. TRXL is sensitive to changes in the global molecular structure and therefore can be used to elucidate structural changes of reacting solute molecules as well as the collective response of solvent molecules. On the other hand, TRXAS can be used to probe changes in both local geometrical and electronic structures of specific X-ray-absorbing atoms due to the element-specific nature of core-level transitions. These techniques are complementary to each other and a combination of the two methods will enhance the capability of accurately obtaining structural changes induced by photoexcitation. Here we review the principles of TRXL and TRXAS and present recent application examples of the two methods for studying chemical and biological processes in solution. Furthermore, we briefly discuss the prospect of using X-ray free electron lasers for the two techniques, which will allow us to keep track of structural dynamics on femtosecond time scales in various solution-phase molecular reactions.

Proteins serve as molecular machines in performing their biological functions, but the detailed structural transitions are difficult to observe in their native aqueous environments in real time. For example, despite extensive studies, the solution-phase structures of the intermediates along the allosteric pathways for the transitions between the relaxed (R) and tense (T) forms have been elusive. In this work, we employed picosecond X-ray solution scattering and novel structural analysis to track the details of the structural dynamics of wild-type homodimeric hemoglobin (HbI) from the clam Scapharca inaequivalvis and its F97Y mutant over a wide time range from 100 ps to 56.2 ms. From kinetic analysis of the measured time-resolved X-ray solution scattering data, we identified three structurally distinct intermediates (I(1), I(2), and I(3)) and their kinetic pathways common for both the wild type and the mutant. The data revealed that the singly liganded and unliganded forms of each intermediate share the same structure, providing direct evidence that the ligand photolysis of only a single subunit induces the same structural change as the complete photolysis of both subunits does. In addition, by applying novel structural analysis to the scattering data, we elucidated the detailed structural changes in the protein, including changes in the heme-heme distance, the quaternary rotation angle of subunits, and interfacial water gain/loss. The earliest, R-like I(1) intermediate is generated within 100 ps and transforms to the R-like I(2) intermediate with a time constant of 3.2 ± 0.2 ns. Subsequently, the late, T-like I(3) intermediate is formed via subunit rotation, a decrease in the heme-heme distance, and substantial gain of interfacial water and exhibits ligation-dependent formation kinetics with time constants of 730 ± 120 ns for the fully photolyzed form and 5.6 ± 0.8 μs for the partially photolyzed form. For the mutant, the overall kinetics are accelerated, and the formation of the T-like I(3) intermediate involves interfacial water loss (instead of water entry) and lacks the contraction of the heme-heme distance, thus underscoring the dramatic effect of the F97Y mutation. The ability to keep track of the detailed movements of the protein in aqueous solution in real time provides new insights into the protein structural dynamics.

Here we report structural dynamics of equine myoglobin (Mb) in response to the CO photodissociation visualized by picosecond time-resolved X-ray solution scattering. The data clearly reveals new structural dynamics that occurs in the timescale of ~360 picoseconds (ps) and ~9 nanoseconds (ns), which have not been clearly detected in previous studies.

Time-resolved X-ray solution scattering is used to visualize and probe the dynamics of the individual steps in the formation of a gold trimer complex, including covalent bond formation, with a time resolution of ∼500 femtoseconds. The making and breaking of atomic bonds are essential processes in chemical reactions. Although the ultrafast dynamics of bond-breaking have been studied intensively using time-resolved techniques, it is very difficult to study the structural dynamics of bond-making, mainly because of its bimolecular nature. In this manuscript, the authors used femtosecond (fs) time-resolved X-ray solution scattering to visualize the formation of a gold trimer complex, . Their experiments probe the dynamics of individual reaction steps, including covalent bond formation, bent-to-linear transition, bond contraction and tetramer formation with the time resolution of ∼500 fs. They also determined the three-dimensional structures of reaction intermediates with sub-angstrom spatial resolution. This work represents the first example of following the entire process of bond formation both visually and quantitatively with femtosecond time resolution. The making and breaking of atomic bonds are essential processes in chemical reactions. Although the ultrafast dynamics of bond breaking have been studied intensively using time-resolved techniques1,2,3, it is very difficult to study the structural dynamics of bond making, mainly because of its bimolecular nature. It is especially difficult to initiate and follow diffusion-limited bond formation in solution with ultrahigh time resolution. Here we use femtosecond time-resolved X-ray solution scattering to visualize the formation of a gold trimer complex, in real time without the limitation imposed by slow diffusion. This photoexcited gold trimer, which has weakly bound gold atoms in the ground state4,5,6, undergoes a sequence of structural changes, and our experiments probe the dynamics of individual reaction steps, including covalent bond formation, the bent-to-linear transition, bond contraction and tetramer formation with a time resolution of ∼500 femtoseconds. We also determined the three-dimensional structures of reaction intermediates with sub-angstrom spatial resolution. This work demonstrates that it is possible to track in detail and in real time the structural changes that occur during a chemical reaction in solution using X-ray free-electron lasers7 and advanced analysis of time-resolved solution scattering data.

Homodimeric hemoglobin (HbI) consisting of two subunits is a good model system for investigating the allosteric structural transition as it exhibits cooperativity in ligand binding. In this work, as an effort to extend our previous study on wild-type and F97Y mutant HbI, we investigate structural dynamics of a mutant HbI in solution to examine the role of well-organized interfacial water cluster, which has been known to mediate intersubunit communication in HbI. In the T72V mutant of HbI, the interfacial water cluster in the T state is perturbed due to the lack of Thr72, resulting in two less interfacial water molecules than in wild-type HbI. By performing picosecond time-resolved X-ray solution scattering experiment and kinetic analysis on the T72V mutant, we identify three structurally distinct intermediates (I1, I2, and I3) and show that the kinetics of the T72V mutant are well described by the same kinetic model used for wild-type and F97Y HbI, which involves biphasic kinetics, geminate recombination, and bimolecular CO recombination. The optimized kinetic model shows that the R-T transition and bimolecular CO recombination are faster in the T72V mutant than in the wild type. From structural analysis using species-associated difference scattering curves for the intermediates, we find that the T-like deoxy I3 intermediate in solution has a different structure from deoxy HbI in crystal. In addition, we extract detailed structural parameters of the intermediates such as E-F distance, intersubunit rotation angle, and heme-heme distance. By comparing the structures of protein intermediates in wild-type HbI and the T72V mutant, we reveal how the perturbation in the interfacial water cluster affects the kinetics and structures of reaction intermediates of HbI.