4D ultrafast electron diffraction, crystallography, and microscopy

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

Femtochemistry: the role of alignment and orientation

Some aspects of alignment and orientation have been considered for femtochemistry experiments. Elementary theoretical descriptions of the time evolution of alignment and angular momenta have been discussed and related to the radial and angular parts of the potential-energy surface. Applications to ICN unimolecular dissociation, H + CO2oriented bimolecular reaction, and crossings between different potential-energy curves in alkali-metal halide reactions (M + X) are give. Emphasis is on the femtosecond dynamics of the transition-state region and the clocking of fragment separation in real time

Picosecond-jet spectroscopy and photochemistry. Energy redistribution and its impact on coherence, isomerization, dissociation and solvation

The development of the picosecond-jet technique is presented. The applications of the technique to the studies of coherence (quantum beats), photodissociation, isomerization and partial solvation of molecules in supersonic-jet beams are detailed with emphasis on the role of intramolecular energy redistribution. Experimental evidence for intramolecular threshold effect for rates as a function of excess molecular energy is given and explained using simple theory for the redistribution of energy among certain modes. Comparison with R.R.K.M. calculation is also made to assess the nature of the statistical behaviour of the energy redistribution

Micrographia of the twenty-first century: from camera obscura to 4D microscopy

In this paper, the evolutionary and revolutionary developments of microscopic imaging are overviewed with a perspective on origins. From Alhazen’s camera obscura, to Hooke and van Leeuwenhoek’s two-dimensional optical micrography, and on to three- and four-dimensional (4D) electron microscopy, these developments over a millennium have transformed humans’ scope of visualization. The changes in the length and time scales involved are unimaginable, beginning with the visible shadows of candles at the centimetre and second scales, and ending with invisible atoms with space and time dimensions of sub-nanometre and femtosecond. With these advances it has become possible to determine the structures of matter and to observe their elementary dynamics as they unfold in real time. Such observations provide the means for visualizing materials behaviour and biological function, with the aim of understanding emergent phenomena in complex systems

Picosecond Laser Chemistry in Supersonic Jet Beams

In this review we describe the development and the applications of the picosecond-jet technique, which utilizes a picosecond laser and a supersonically-cold jet beam of large molecules. The applications include studies of coherence (quantum beats), photodissociation, isomerization, and partial solvation. The results emphasize the role of intramolecular energy redistribution, and provide evidence for intramolecular threshold effect for rates vs. excess molecular energy. Simple theory for this redistribution of energy among certain modes in molecules is given, and comparison with RRKM calculation is also made to assess the nature of the statistical behavior for the redistribution

Femtosecond transition-state dynamics

This article presents the progress made in probing femtosecond transition–state dynamics of elementary reactions. Experiments demonstrating the dynamics in systems characterized by a transition region and by a saddle-point transition state are reported, and comparison with theory is made

4D Scanning Transmission Ultrafast Electron Microscopy: Single-Particle Imaging and Spectroscopy

We report the development of 4D scanning transmission ultrafast electron microscopy (ST-UEM). The method was demonstrated in the imaging of silver nanowires and gold nanoparticles. For the wire, the mechanical motion and shape morphological dynamics were imaged, and from the images we obtained the resonance frequency and the dephasing time of the motion. Moreover, we demonstrate here the simultaneous acquisition of dark-field images and electron energy loss spectra from a single gold nanoparticle, which is not possible with conventional methods. The local probing capabilities of ST-UEM open new avenues for probing dynamic processes, from single isolated to embedded nanostructures, without being affected by the heterogeneous processes of ensemble-averaged dynamics. Such methodology promises to have wide-ranging applications in materials science and in single-particle biological imaging

Attosecond electron pulses for 4D diffraction and microscopy

In this contribution, we consider the advancement of ultrafast electron diffraction and microscopy to cover the attosecond time domain. The concept is centered on the compression of femtosecond electron packets to trains of 15-attosecond pulses by the use of the ponderomotive force in synthesized gratings of optical fields. Such attosecond electron pulses are significantly shorter than those achievable with extreme UV light sources near 25 nm ({approx}50 eV) and have the potential for applications in the visualization of ultrafast electron dynamics, especially of atomic structures, clusters of atoms, and some materials

New Light on Molecular and Materials Complexity: 4D Electron Imaging

In this Perspective, 4D electron imaging is highlighted, after introducing some concepts, with an overview of selected applications that span chemical reactions, molecular interfaces, phase transitions, and nano(micro)mechanical systems. With the added dimension of time in microscopy, diffraction, and electron-energy-loss spectroscopy, the focus is on direct visualization of structural dynamics with atomic and nanoscale resolution in the four dimensions of space and time. This contribution provides an expose of emerging developments and an outlook on future applications in materials and biological sciences

Femtosecond dynamics of flavoproteins: Charge separation and recombination in riboflavine (vitamin B_2)-binding protein and in glucose oxidase enzyme

Flavoproteins can function as hydrophobic sites for vitamin B_2 (riboflavin) or, in other structures, with cofactors for catalytic reactions such as glucose oxidation. In this contribution, we report direct observation of charge separation and recombination in two flavoproteins: riboflavin-binding protein and glucose oxidase. With femtosecond resolution, we observed the ultrafast electron transfer from tryptophan(s) to riboflavin in the riboflavin-binding protein, with two reaction times: approx 100 fs (86% component) and 700 fs (14%). The charge recombination was observed to take place in 8 ps, as probed by the decay of the charge-separated state and the recovery of the ground state. The time scale for charge separation and recombination indicates the local structural tightness for the dynamics to occur that fast and with efficiency of more than 99%. In contrast, in glucose oxidase, electron transfer between flavin-adenine-dinucleotide and tryptophan(s)/tyrosine(s) takes much longer times, 1.8 ps (75%) and 10 ps (25%); the corresponding charge recombination occurs on two time scales, 30 ps and nanoseconds, and the efficiency is still more than 97%. The contrast in time scales for the two structurally different proteins (of the same family) correlates with the distinction in function: hydrophobic recognition of the vitamin in the former requires a tightly bound structure (ultrafast dynamics), and oxidation-reduction reactions in the latter prefer the formation of a charge-separated state that lives long enough for chemistry to occur efficiently. Finally, we also studied the influence on the dynamics of protein conformations at different ionic strengths and denaturant concentrations and observed the sharp collapse of the hydrophobic cleft and, in contrast, the gradual change of glucose oxidase