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The use of pump-probe experiments to study complex transient events has been an area of significant interest in materials science, biology, and chemistry. While the emphasis has been on laser pump with laser probe and laser pump with x-ray probe experiments, there is a significant and growing interest in using electrons as probes. Early experiments used electrons for gas-phase diffraction of photostimulated chemical reactions. More recently, scientists are beginning to explore phenomena in the solid state such as phase transformations, twinning, solid-state chemical reactions, radiation damage, and shock propagation. This review focuses on the emerging area of ultrafast electron microscopy (UEM), which comprises ultrafast electron diffraction (UED) and dynamic transmission electron microscopy (DTEM). The topics that are treated include the following: (1) The physics of electrons as an ultrafast probe. This encompasses the propagation dynamics of the electrons (space-charge effect, Child’s law, Boersch effect) and extends to relativistic effects. (2) The anatomy of UED and DTEM instruments. This includes discussions of the photoactivated electron gun (also known as photogun or photoelectron gun) at conventional energies (60–200 keV) and extends to MeV beams generated by rf guns. Another critical aspect of the systems is the electron detector. Charge-coupled device cameras and microchannel-plate-based cameras are compared and contrasted. The effect of various physical phenomena on detective quantum efficiency is discussed. (3) Practical aspects of operation. This includes determination of time zero, measurement of pulse-length, and strategies for pulse compression. (4) Current and potential applications in materials science, biology, and chemistry. UEM has the potential to make a significant impact in future science and technology. Understanding of reaction pathways of complex transient phenomena in materials science, biology, and chemistry will provide fundamental knowledge for discovery-class science.

The photochemically induced electrocyclic ring-opening reaction of 1,3-cyclohexadiene to 1,3,5-hexatriene serves as a prototype for many important reactions in chemistry and in biological systems. Based on experimental and computational studies, a detailed picture of the reaction has now emerged: Excitation to the Franck-Condon region places the molecule on a steeply repulsive part of the 1B potential energy surface, which propels the molecule in exactly the conrotatory direction that conforms to the Woodward-Hoffmann rules of orbital symmetry. Bypassing a cusp in a symmetry-breaking direction, the wave packet enters the 2A state within 55 fs. It continues to move directly toward the 2A/1A conical intersection, where it crosses in approximately 80 fs to the ground state. This article summarizes the published experimental and theoretical work to describe the current understanding of the reaction while pointing to important questions that remain to be addressed.

The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D synthesis. Here, we present direct and unambiguous observation of the ring-opening reaction path on

An rf photocathode electron gun is used as an electron source for ultrafast time-resolved pump-probe electron diffraction. We observed single-shot diffraction patterns from a 160 nm Al foil using the 5.4 MeV electron beam from the Gun Test Facility at the Stanford Linear Accelerator. Excellent agreement with simulations suggests that single-shot diffraction experiments with a time resolution approaching 100 fs are possible. SLAC-PUB-12162 Submitted to Applied Physics Letters 2Our understanding about dynamical processes in chemistry, materials science and biology on the picosecond and sub-picosecond time scale stems almost exclusively from time-resolved spectroscopy. Structural changes, on atomic length scales, can only be inferred indirectly from the analysis of spectra. Both x-ray and electron diffraction share the goal of 'imaging' molecular structures with a time resolution that captures the motions as systems evolve, whether they be solids, liquids or gases. Lab scale experiments in both electron diffraction 1,2 and x-ray scattering 3 have produced impressive results. Recently, in anticipation of the construction of the Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center (SLAC), an experiment using the electron bunch from the SLAC Linac to produce spontaneous undulator radiation 4 has shown the possibilities for ultrafast x-ray scattering from condensed systems with 100 fs time resolution. 5 This has encouraged us to approach ultrafast electron diffraction (UED) using experimental techniques based on electron sources developed for particle accelerators, with the aim of obtaining single-shot diffraction patterns on a 100 fs time scale.Electron diffraction is complementary to x-ray scattering, but features much larger cross sections that allow the study of surface phenomena, the bulk structures of thin foils and membranes, as well as molecular structures of gas phase samples. 6 As with linac based x-ray sources there has been significant development of electron sources for UED based on the use of photocathodes. 7 Unfortunately, the space-charge interactions of the electrons within a pulse, and the initial kinetic energy distribution with which the electrons are generated, have made it difficult to obtain pulses much shorter than 1 ps 8,9,10 ,in 'conventional' UED experiments using ≈30 keV electron beams. To improve the time resolution one could use fewer electrons per pulse, but that requires longer data acquisition times to obtain the necessary signal-to-noise ratio. 11 Alternatively, it is possible to increase the electric field inside the electron gun, while reducing the flight distance between the gun and the target. 12 Both tend to reduce the time of flight of the electron pulse, thereby giving the electron pulse less time to spread. Even so, this 3 approach is limited because the maximum DC and pulsed electric fields are 12 MV/m and 25 MV/m, respectively. 13,14 In the present work we take a fresh approach to ultrafast time-resolved pump-probe diffraction by using MeV electron be...

The EPR-type strangeness correlation in the K K system produced in the reaction pp ™ K K at rest has been tested using the CPLEAR detector. The strangeness was tagged via strong interaction with absorbers away from the creation point. The results are consistent with the QM non-separability of the wave function and exclude a spontaneous wave-function Ž . factorisation at creation CL ) 99.99% . q 1998 Elsevier Science B.V.

Changes in electron diffraction patterns are observed on ultrashort time scales upon irradiation of 1,3-cyclohexadiene with femtosecond laser pulses. 1,3-Cyclohexadiene is known to experience a ring opening reaction to hexatriene upon excitation to the 1 B 2 electronic state. Internal conversion brings the molecule to a saddle point, from where one pathway leads back to cyclohexadiene, while another path generates 1,3,5-hexatriene in one of its isomeric forms. Structural observations are made at picosecond time delays using an ultrashort electron pulse that is diffracted off the nascent product molecules. The diffraction images illustrate that structural observations of prototypical organic reactions can be made in real time, opening a new methodology to study chemical reaction dynamics.Molecular spectroscopy with femtosecond or picosecond time resolution has become tremendously successful in exploring energy relaxation processes and chemical reactions in real time. It is now possible to obtain spectra of molecules just as they cross a transition state during a chemical reaction. 1 Such spectroscopic studies have led to enormous insights about the flow of energy within and between molecules, allowing detailed inferences about the mechanisms of the reactions.Nonetheless, time-resolved spectroscopy is burdened by fundamental constraints. Most mechanistic chemistry is based on structural models, whereas spectroscopy reveals energy levels. Synthetic chemists describe reactions as transformations of molecular structures, with reaction channels that are determined by spatial distributions of functional groups, steric hindrances, or spatial electrostatic charge distributions. In contrast, spectroscopy can measure only energy levels and populations of molecules in energy levels. Thus, time-resolved spectroscopy, however useful, shows only the time dependence of energy level populations.Energy levels and structures are of course connected via potential energy surfaces and quantum mechanics. However, this link is conceptually difficult, and tremendous computational resources must applied to understand even simple chemical reactions from a quantum mechanical perspective. It therefore has been a long-standing goal of experimental physical chemists to observe time-dependent structures of molecules during chemical reactions. Such structural observations carry the promise of a much more direct connection to mechanistic organic chemistry. We report here the investigation of a prototypical organic reaction by time-resolved electron diffraction.The concept of probing time dependent molecular structures by diffraction has been well documented. 2 Provided one succeeds in generating short bursts of electrons or X-rays, both electron diffraction 3-37 and X-ray diffraction 38-49 can be adapted to the time domain. Indeed, developments of the recent past have shown that it is possible to generate pulses of even subpicosecond duration of both X-rays 49-51 and electrons. 29 In time-domain diffraction experiments a short laser pulse initia...