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.

https://science.sciencemag.org/content/sci/291/5503/458.full.pdf

Direct imaging of transient molecular structures with ultrafast diffraction.

In this review, we highlight the progress made in the development of 4D ultrafast electron diffraction (UED), crystallography (UEC), and microscopy (UEM) with a focus on concepts, methodologies, and prototypical applications. The joint atomic-scale resolutions in space and time, and sensitivity reached, make it possible to determine complex transient structures and assemblies in different phases. These applications include studies of isolated chemical reactions (molecular beams), interfaces, surfaces and nanocrystals, self-assembly, and 2D crystalline fatty-acid bilayers. In 4D UEM, we are now able, using timed, single-electron packets, to image nano-to-micro scale structures of materials and biological cells. Future applications of these methods are foreseen across areas of physics, chemistry, and biology.

/pdf/4d-ultrafast-electron-diffraction-crystallography-and-d9ufb2mj8f.pdf

4D ultrafast electron diffraction, crystallography, and microscopy.

One of the great dream experiments in Science is to directly observe atomic motions as they occur. Femtosecond electron diffraction provided the first 'light' of sufficient intensity to achieve this goal by attaining atomic resolution to structural changes on the relevant timescales. This review covers the technical progress that made this new level of acuity possible and gives a survey of the new insights gained from an atomic level perspective of structural dynamics. Atomic level views of the simplest possible structural transition, melting, are discussed for a number of systems in which both thermal and purely electronically driven atomic displacements can be correlated with the degree of directional bonding. Optical manipulation of charge distributions and effects on interatomic forces/bonding can be directly observed through the ensuing atomic motions. New phenomena involving strongly correlated electron?lattice systems are also discussed in which optically induced changes in the potential energy landscape lead to ballistic structural changes. Concepts such as the structural order parameters are now directly observable at the atomic level of inspection to give a remarkable view of the extraordinary degree of cooperativity involved in strongly correlated electron?lattice systems. These recent examples, in combination with time-resolved real space imaging now possible with electron probes, are truly defining an emerging field that holds great promise to make a significant impact in how we understand structural dynamics.This article is dedicated to the memory of Professor David John Hugh Cockayne, a world leader in electron microscopy, who sadly passed away in December.

https://iopscience.iop.org/article/10.1088/0034-4885/74/9/096101/pdf

Femtosecond electron diffraction: heralding the era of atomically resolved dynamics

A review. Ultrafast x-ray techniques using diffraction and absorption are discussed with an emphasis on the absorption techniques. Ultrafast x-ray sources and detectors for use in x-ray absorption spectroscopy are also reviewed. [on SciFinder (R)]

Ultrafast X-ray absorption spectroscopy

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.

Ultrafast electron microscopy in materials science, biology, and chemistry

This paper, the fifth in a series, is concerned with the experimental description of ultrafast electron diffraction
and its application to several isolated chemical systems. We present a detailed description of the Caltech apparatus, which consists of a femtosecond laser system, a picosecond electron gun, and a two-dimensional charge-coupled device ( CCD) detection system. We also discuss the analysis of the scattering patterns. Ultrafast diffraction images from several molecules (CCl_4, I_2, CF_3l, C_2F_4I_2) are reported. For our first study of a chemical reaction in a molecular beam, we show the change in the radial distribution function following the formation of CF_3 radical after dissociation of CF_3l. The total experimental temporal resolution is discussed in terms of the electron pulse width and velocity mismatch. The electron pulse was characterized temporally with a streaking technique that yielded the width as a function of the number of electrons per pulse. Experimental results show that the electron source produces picosecond (or less) pulses at densities of 100 electrons per pulse and 10-ps pulses at 1000 electrons per pulse. We also report our observation of a novel photoionization-induced lensing effect on the undiffracted electron beam, which we have used to establish time zero for UED when reactions are initiated by a laser pulse.

Scott B. Kim

Papers

Ultrafast Electron Diffraction. 5. Experimental Time Resolution and Applications