Ultrafast electron diffraction (UED) has been developed to study transient structures in complex chemical reactions initiated with femtosecond laser pulses. This direct imaging of reactions was achieved using our third-generation apparatus equipped with an electron pulse (1.07 +/- 0.27 picoseconds) source, a charge-coupled device camera, and a mass spectrometer. Two prototypical gas-phase reactions were studied: the nonconcerted elimination reaction of a haloethane, wherein the structure of the intermediate was determined, and the ring opening of a cyclic hydrocarbon containing no heavy atoms. These results demonstrate the vastly improved sensitivity, resolution, and versatility of UED for studying ultrafast structural dynamics in complex molecular systems.

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

With properly timed sequences of ultrafast electron pulses, it is now possible to image complex molecular structures in the four dimensions of space and time with resolutions of 0.01 Å and 1 ps, respectively. The new limits of ultrafast electron diffraction (UED) provide the means for the determination of transient molecular structures, including reactive intermediates and non‐equilibrium structures of complex energy landscapes. By freezing structures on the ultrafast timescale, we are able to develop concepts that correlate structure with dynamics. Examples include structure‐driven radiationless processes, dynamics‐driven reaction stereochemistry, pseudorotary transition‐state structures, and non‐equilibrium structures exhibiting negative temperature, bifurcation, or selective energy localization in bonds. These successes in the studies of complex molecular systems, even without heavy atoms, and the recent development of a new machine devoted to structures in the condensed phase, establish UED as a powerful method for mapping out temporally changing molecular structures in chemistry, and potentially, in biology. This review highlights the advances made at Caltech, with emphasis on the principles of UED, its evolution through four generations of instrumentation (UED‐1 to UED‐4) and its diverse applications.

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

Using femtosecond photodoping and crystallography to explore metastable and hidden quantum phases in tantalum disulfide.

We use ultrafast electron crystallography to study structural changes induced in graphite by a femtosecond laser pulse. At moderate fluences of ≤21 mJ/cm 2 , lattice vibrations are observed to thermalize on a time scale of ≈8 ps. At higher fluences approaching the damage threshold, lattice vibration amplitudes saturate. Following a marked initial contraction, graphite is driven nonthermally into a transient state with sp 3 -like character, forming interlayer bonds. Using ab initio density functional calculations, we trace the governing mechanism back to electronic structure changes following the photo-excitation.

The static structure of macromolecular assemblies can be mapped out with atomic-scale resolution by using electron diffraction and microscopy of crystals. For transient nonequilibrium structures, which are critical to the understanding of dynamics and mechanisms, both spatial and temporal resolutions are required; the shortest scales of length (0.1-1 nm) and time (10 ؊13 to 10 ؊12 s) represent the quantum limit, the nonstatistical regime of rates. Here, we report the development of ultrafast electron crystallography for direct determination of structures with submonolayer sensitivity. In these experiments, we use crystalline silicon as a template for different adsorbates: hydrogen, chlorine, and trifluoroiodomethane. We observe the coherent restructuring of the surface layers with subangstrom displacement of atoms after the ultrafast heat impulse. This nonequilibrium dynamics, which is monitored in steps of 2 ps (total change <10 ps), contrasts that of the nanometer substrate. The effect of adsorbates and the phase transition at higher fluences were also studied through the evolution of streaks of interferences, Bragg spots (and their rocking curves), and rings in the diffraction patterns. We compare these results with kinematical theory and those of x-ray diffraction developed to study bulk behaviors. The sensitivity achieved here, with the 6 orders of magnitude larger cross section than x-ray diffraction, and with the capabilities of combined spatial (Ϸ0.01 Å) and temporal (300 -600 fs) resolutions, promise diverse applications for this ultrafast electron crystallography tabletop methodology.E lectron and x-ray diffraction, if endowed with ultrafast temporal resolution, can provide the ability of atomic-scale determination of structure and dynamics. In this laboratory, the method of choice has been ultrafast electron diffraction (UED) for many reasons; for recent review see ref. 1 and references therein. The extension of UED to the condensed phase and surface dynamics represents a major challenge, and here, we report on this development. With the combined spatial and temporal resolutions, it is possible to study macromolecular systems (2) and reach the nonstatistical regime of energy localization and rates (3). The sensitivity to surfaces, nanometer crystals, and adsorbates offers a unique feature for comparison with x-ray bulk studies (4-9).The conceptual framework of the approach is illustrated in Fig. 1. On the crystal, with or without the adsorbate (surface terminated by a monolayer of atoms or molecules), an ultrashort packet of electrons of 30 keV (de Broglie wavelength Ϸ0.07 Å), impinges with a wave vector k ជ i at a grazing incidence angle i Ͻ 5°. For elastic scattering, s ជ ϭ k ជ Ϫ k ជ i ; k ជ being the momentum of the scattered electron, s ϭ 4 ͞ ⅐sin ͞2, and is the scattering angle between k ជ and k ជ i . Because electrons interact strongly, the diffraction patterns give characteristics of the surface structure defined by the substrate and adsorbate. A change in temperature of the substrate is introd...

Studies of molecular structures at or near their equilibrium configurations have long provided information on their geometry in terms of bond distances and angles. Far-from-equilibrium structures are relatively unknown-especially for complex systems-and generally, neither their dynamics nor their average geometries can be extrapolated from equilibrium values. For such nonequilibrium structures, vibrational amplitudes and bond distances play a central role in phenomena such as energy redistribution and chemical reactivity. Ultrafast electron diffraction, which was developed to study transient molecular structures, provides a direct method for probing the nature of complex molecules far from equilibrium. Here we present our ultrafast electron diffraction observations of transient structures for two cyclic hydrocarbons. At high internal energies of Ϸ4 eV, these molecules display markedly different behavior. For 1,3,5-cycloheptatriene, excitation results in the formation of hot ground-state structures with bond distances similar to those of the initial structure, but with nearly three times the average vibrational amplitude. Energy is redistributed within 5 ps, but with a negative temperature characterizing the nonequilibrium population. In contrast, the ring-opening reaction of 1,3-cyclohexadiene is shown to result in hot structures with a COC bond distance of over 1.7 Å, which is 0.2 Å away from any expected equilibrium value. Even up to 400 ps, energy remains trapped in large-amplitude motions comprised of torsion and asymmetric stretching. These studies promise a new direction for studying structural dynamics in nonequilibrium complex systems.T he premise of ultrafast electron diffraction (UED) is similar to ultrafast spectroscopies (for recent work from this laboratory, see refs. 1-5): a femtosecond laser pulse excites the molecules, and a second pulse, in this case a picosecond burst of electrons, probes the resulting structural evolution with the zero-of-time precisely determined in situ. With UED, the changing nuclear coordinates are directly recorded in time-dependent diffraction patterns. By timing the electron pulses to arrive before the light pulses, ground-state diffraction images are obtained at negative times. Time-resolved diffraction snapshots of the transient molecular structures are then recorded at positive times by varying the time delay between light and electron pulses. In the present work, we prepare vibrationally hot structures by radiationless transfer after the initial photon absorption and͞or as a result of a chemical reaction. Scheme 1 depicts the two reactions that are the subject of this contribution. Concepts of Equilibrium vs. Nonequilibrium Structures.Differences between diffraction patterns of structures at equilibrium and those far from equilibrium can be understood by first considering the case of a single bond (Fig. 1). The diffraction of structures far from equilibrium manifests itself as (i) increased damping of the oscillating molecular scattering signal; and (ii) apparent sh...