Ultrafast electron diffraction (UED) has been developed to study transient structures in complex chemical reactions initiated with femtosecond laser pulses. This direct imaging of was achieved using our third-generation apparatus equipped an pulse (1.07 ± 0.27 picoseconds) source, a charge-coupled device camera, and mass spectrometer. Two prototypical gas-phase were studied: the nonconcerted elimination reaction haloethane, wherein structure intermediate determined, ring opening cyclic...

We report direct determination of the structures and dynamics interfacial water on a hydrophilic surface with atomic-scale resolution using ultrafast electron crystallography. On nanometer scale, we observed coexistence ordered crystallite-like ice structures, evident in superposition Bragg spots Debye-Scherrer rings. The were determined to be dominantly cubic, but each undergoes different after substrate temperature jump. From changes local bond distances (OH.O O.O) time, elucidated...

Using optical, TEM, and ultrafast electron diffraction experiments we find that single crystal ${\mathrm{VO}}_{2}$ microbeams gently placed on insulating substrates or metal grids exhibit different behaviors, with structural metal-insulator transitions occurring at the same temperature for substrates, while a new monoclinic phase lies between metallic rutile phase. The electronic in these are strongly first order discuss their origins context of current understanding multiorbital splitting,...

Characterizing and understanding the emergence of multiple macroscopically ordered electronic phases through subtle tuning temperature, pressure, chemical doping has been a long-standing central issue for complex materials research. We report first comprehensive studies optical doping-induced stable metastable hidden visualized in situ by femtosecond electron crystallography. The phase transitions are triggered infrared pulses, temperature-optical density diagram is constructed substantiated...

Abstract Nonequilibrium phase transitions play a pivotal role in broad physical contexts, from condensed matter to cosmology. Tracking the formation of nonequilibrium phases requires resolution long-range cooperativity on ultra-short timescales. Here, we study spontaneous transformation charge-density wave CeTe 3 stripe order into bi-directional state inaccessible thermodynamically but is induced by intense laser pulses. With ≈100 fs coherent electron diffraction, capture entire course this...

We use ultrafast electron crystallography to study structural changes induced in graphite by a femtosecond laser pulse. At moderate fluences of < or =21 mJ/cm2, lattice vibrations are observed thermalize on time scale approximately 8 ps. higher approaching the damage threshold, vibration amplitudes saturate. Following marked initial contraction, is driven nonthermally into transient state with sp3-like character, forming interlayer bonds. Using ab initio density functional calculations, we...

The static structure of macromolecular assemblies can be mapped out with atomic-scale resolution by using electron diffraction and microscopy crystals. For transient nonequilibrium structures, which are critical to the understanding dynamics mechanisms, both spatial temporal resolutions required; shortest scales length (0.1–1 nm) time (10 –13 10 –12 s) represent quantum limit, nonstatistical regime rates. Here, we report development ultrafast crystallography for direct determination...

Understanding space charge effects is central for the development of high-brightness ultrafast electron diffraction and microscopy techniques imaging material transformation with atomic scale detail at fs to ps timescales. We present methods results direct photoelectron beam characterization employing a shadow projection technique investigate generation ultrafast, non-uniform, intense pulses in dc photo-gun geometry. Combined N-particle simulations an analytical Gaussian model, we elucidate...

Abstract Photoinduced threshold switching processes that lead to bistability and the formation of metastable phases in photoinduced phase transition VO 2 are elucidated through ultrafast electron diffraction diffusive scattering techniques with varying excitation wavelengths. We uncover two distinct regimes dynamical change: a nearly instantaneous crossover into an intermediate state its decay led by lattice instabilities over 10 ps timescales. The structure this is identified be monoclinic,...

Studies of molecular structures at or near their equilibrium configurations have long provided information on geometry in terms bond distances and angles. Far-from-equilibrium are relatively unknown—especially for complex systems—and generally, neither dynamics nor average geometries can be extrapolated from values. For such nonequilibrium structures, vibrational amplitudes play a central role phenomena as energy redistribution chemical reactivity. Ultrafast electron diffraction, which was...

We report the studies of ultrafast electron nanocrystallography on size-selected Au nanoparticles (2-20 nm) supported a molecular interface. Reversible surface melting, and recrystallization were investigated with dynamical full-profile radial distribution functions determined sub-picosecond picometer accuracies. In an photoinduced are driven to non-equilibrium transformation, characterized by initial lattice deformations, nonequilibrium electron-phonon coupling, upon collective bonding...

A femtosecond plasma imaging modality based on a new development of ultrafast electron microscope is introduced. We investigated the laser-induced formation high-temperature microplasmas and their subsequent non-equilibrium evolution. Based straightforward field principle, we directly retrieve detailed information about dynamics, including wave structures, particle densities, temperatures. discover that subjected to strong magnetic field, photo-generated manifest in novel transient cyclotron...

The electron-phonon mechanism that gives rise to various charge-ordered systems is often controversial because of the cooperative nature transformation, and structural aspect transformation generally poorly understood. Using femtosecond electron crystallography, we reveal a two-step ($\ensuremath{\approx}$400 fs 3.3 ps) suppression order parameter two-dimensional charge-density wave (CDW) clearly decouples from its electronic counterpart following optical quenching. Through atomic...

A frontier challenge in implementing femtosecond electron microscopy is to gain precise optical control of intense beams mitigate collective space charge effects for significantly improving the throughput. Here, we explore flexible uses an RF cavity as a longitudinal lens high-intensity beam column condensing both temporally and spectrally, relevant design ultrafast microscopy. Through introduction novel atomic grating approach characterization bunch phase optics, elucidate principles...

The structure of two metal phthalocyanines, MPc where M=Sn and Mg, have been determined by gas-phase electron diffraction for the first time; furthermore, ZnPc established earlier same technique has reanalyzed. focus these studies is position ion relative to 16-membered C8N8 ring. Tin(II)phthalocyanine we find be nonplanar with Sn(II) 1.0(1) Å above molecular plane in accordance its crystal earlier; other molecules were found planar. Slight nonplanarity previous study now removed a new...

We present an electron projection imaging method to study the ultrafast evolution of photoelectron density distribution and transient fields near surface. The dynamical profile photoelectrons from graphite reveals origin a thermionic emission, followed by adiabatic process leading acceleration cooling before freely expanding cloud is established. hot emission found couple with surface charge dipole layer formation, sheet several orders magnitude higher than that vacuum emitted cloud.

Radiationless transitions in molecules are ubiquitous photophysical, chemical, and biological systems. Because such usually ultrafast nature, the determination of transient structures is extremely difficult, particularly for complex with many degrees freedom. Here, we report direct molecular structure during ultrafast, nonradiative transition aromatic pyridine, excited above so-called channel three threshold, which marks onset an anomalous decay. The approach invoked electron diffraction...

A formidable contender to X-ray diffraction is ultrafast electron crystallography. Whereas the former more suited investigate bulk of substrate, time, length, and sensitivity scales crystallography provide powerful complementary information on atomic-scale structural dynamics at surface (see image GaAs crystal).

We present a method to measure transient photovoltage at nanointerfaces using ultrafast electron diffraction. In particular, we report our results on the photoexcitation of carriers and their ensuing relaxations in hydroxyl-terminated silicon surface, which is standard substrate for fabricating molecular electronics interfaces. The surface voltage determined by observing Coulomb refraction changes induced modified space-charge barrier within selectively probed volume femtosecond pulses. are...

In low-dimensional electronic materials, the charge or spin ordering can be subtly controlled by specific mode modes, giving rise to functioning states such as charge- and spin-density waves, Mott insulators, superconductors. The coupling between electrons atomic lattice effectively investigated ultrafast optical, photoemission, electron diffraction techniques providing detailed description of microscopic collective state evolutions in separate subsystems. However, phononic relaxation time...

Using a multilevel fast multipole method, coupled with the shadow imaging of femtosecond photoelectron pulses for validation, we quantitatively elucidate photocathode, space charge, and virtual cathode physics, which fundamentally limit spatiotemporal spectroscopic resolution throughput ultrafast electron microscope (UEM) systems. We present simple microscopic description to capture nonlinear beam dynamics based on two-fluid picture an unexpected dominant role image potential pinning in...

In this communication, we report our first study of self-assembled adsorbates on metal surfaces. Specifically, studied single-crystal clean surfaces Au(111) with and without a monolayer reaction involving the assembly 2-mercaptoacetic acid from 2,2'-dithiodiacetic acid. We also monolayers iron hemes. With ultrafast electron crystallography, are able to observe isolate structural dynamics substrate (gold) adsorbate(s) following an temperature jump.