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

Modern X-ray and neutron diffraction techniques can give precise parameters that describe dynamic or static displacements of atoms in crystals. However, confusing and inconsistent terms and symbols for these quantities occur in the crystallographic literature. This report discusses various forms of these quantities, derived from probability density functions and based on Bragg diffraction data, both when the Gaussian approximation is appropriate and when it is not. The focus is especially on individual atomic anisotropic displacement parameters (ADPs), which may represent atomic motion and possible static displacive disorder. The first of the four sections gives background information, including definitions. The second concerns the kinds of parameter describing atomic displacements that have most often been used in crystal structure analysis and hence are most commonly found in the literature on the subject. It includes a discussion of graphical representations of the Gaussian mean-square displacement matrix. The third section considers the expressions used when the Gaussian approximation is not adequate. The final section gives recommendations for symbols and nomenclature.

/pdf/atomic-dispacement-parameter-nomenclature-report-of-a-97ndja1hr8.pdf

Atomic Dispacement Parameter Nomenclature. Report of a Subcommittee on Atomic Displacement Parameter Nomenclature

A new method for collecting complete three-dimensional electron diffraction data is described. Diffraction data is collected by combining electron beam tilt at many very small steps, with rotation of the crystal in a few but large steps. A number of practical considerations are discussed, as well as advantages and disadvantages compared to other methods of collecting electron diffraction data.

/pdf/collecting-3d-electron-diffraction-data-by-the-rotation-20uq6mt40l.pdf

Collecting 3D electron diffraction data by the rotation method

Towards automated diffraction tomography. Part II—Cell parameter determination

Flash desorption mass spectrometry is used to study the structures and kinetics of hydrogen adsorption on the (100) plane of tungsten. It is shown that there are two binding states, β1 and β2, in a saturation ratio of 2:1 which obey first‐ and second‐order kinetics, respectively. The activation energies of desorption are 26.3 and 32.3 kcal mole−1 and both vary with coverage by less than 0.5 kcal monolayer−1. Deuterium isotope effects, exchange, and replacement are also examined. These results combined with recent LEED measurements for this system are used to formulate a structure for the adsorbate which appears to quantitatively explain the presently available data. This structure is shown to be reasonable on the basis of the electronic structure of the substrate which predicts multiple states, a c(2 × 2) structure, and island formation at low coverage as observed.

Interaction of H2 with (100) W. I. Binding States

Structure Analysis by Electron Diffraction

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Structure Analysis by Electron Diffraction