Complex systems in condensed phases involve a multidimensional energy landscape, and knowledge of transitional structures and separation of time scales for atomic movements is critical to understanding their dynamical behavior. Here, we report, using four-dimensional (4D) femtosecond electron diffraction, the visualization of transitional structures from the initial monoclinic to the final tetragonal phase in crystalline vanadium dioxide; the change was initiated by a near-infrared excitation. By revealing the spatiotemporal behavior from all observed Bragg diffractions in 3D, the femtosecond primary vanadium-vanadium bond dilation, the displacements of atoms in picoseconds, and the sound wave shear motion on hundreds of picoseconds were resolved, elucidating the nature of the structural pathways and the nonconcerted mechanism of the transformation.

We directly observed the hydration dynamics of an excess electron in the finite-sized water clusters of (H2O)n- with n = 15, 20, 25, 30, and 35. We initiated the solvent motion by exciting the hydrated electron in the cluster. By resolving the binding energy of the excess electron in real time with femtosecond resolution, we captured the ultrafast dynamics of the electron in the presolvated ("wet") and hydrated states and obtained, as a function of cluster size, the subsequent relaxation times. The solvation time (300 femtoseconds) after the internal conversion [140 femtoseconds for (H2O)35-] was similar to that of bulk water, indicating the dominant role of the local water structure in the dynamics of hydration. In contrast, the relaxation in other nuclear coordinates was on a much longer time scale (2 to 10 picoseconds) and depended critically on cluster size.

Nonequilibrium phase transitions, which are defined by the formation of macroscopic transient domains, are optically dark and cannot be observed through conventional temperature- or pressure-change studies. We have directly determined the structural dynamics of such a nonequilibrium phase transition in a cuprate superconductor. Ultrafast electron crystallography with the use of a tilted optical geometry technique afforded the necessary atomic-scale spatial and temporal resolutions. The observed transient behavior displays a notable "structural isosbestic" point and a threshold effect for the dependence of c-axis expansion (Deltac) on fluence (F), with Deltac/F = 0.02 angstrom/(millijoule per square centimeter). This threshold for photon doping occurs at approximately 0.12 photons per copper site, which is unexpectedly close to the density (per site) of chemically doped carriers needed to induce superconductivity.

Progress has been made in the development of four-dimensional ultrafast electron microscopy, which enables space-time imaging of structural dynamics in the condensed phase. In ultrafast electron microscopy, the electrons are accelerated, typically to 200 keV, and the microscope operates in the transmission mode. Here, we report the development of scanning ultrafast electron microscopy using a field-emission-source configuration. Scanning of pulses is made in the single-electron mode, for which the pulse contains at most one or a few electrons, thus achieving imaging without the spacecharge effect between electrons, and still in ten(s) of seconds. For imaging, the secondary electrons from surface structures are detected, as demonstrated here for material surfaces and biological specimens. By recording backscattered electrons, diffraction patterns from single crystals were also obtained. Scanning pulsedelectron microscopy with the acquired spatiotemporal resolutions, and its efficient heat-dissipation feature, is now poised to provide in situ 4D imaging and with environmental capability.biological imaging | Schottky emission source | structural dynamics | nanomaterials imaging T he development of ultrafast electron microscopy (UEM) has enabled imaging in both space and time with atomic-scale resolutions (1, 2). The central concept involved is that of singleelectron packets, which provide the high spatiotemporal resolutions due to the absence of the space-charge effect between electrons. Using femtosecond (fs) optical pulses, the electrons are generated from a LaB 6 photocathode and then accelerated typically to 200 keV. The time resolution is independent of the response of the video camera, as it is determined by the duration of the initial heating and electron pulses. With UEM, the different domains of electron microscopy were made possible: real-space imaging (3-5), diffraction (6-8), and electron-energy-loss spectroscopy (9, 10). Recent advances include 4D electron tomography (11), convergent-beam diffraction (12), and near-field electron microscopy (13).SEM provides the unique capability of obtaining 3D-like images for materials surfaces (14-16). Moreover, environmental microscopy (17) can easily be invoked. Significantly, the electron source in SEM, a field emitter with a tip dimension of tensto-hundreds of nanometers (nm), has higher brightness than that of the source in UEM (LaB 6 ), which has an active-area dimension of tens of micrometers (μm). Finally, the specimen is easier to handle; thick samples can be used and provide the means for heat dissipation, especially when the heating pulse is involved in dynamical studies.Introducing ultrashort time resolution in SEM was not possible before, as in the past time-resolved studies were made by "chopping" the electron beam through the technique of high-frequency (MHz or GHz) beam deflection and blanking (18,19). The temporal width of an electron pulse was hundreds of picoseconds (ps) and the overall resolution of the system was on the order of 10 nanoseconds (...