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

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

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

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