Ultrafast materials science and 4D imaging with atomic resolution both in space and time
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troduction The basic principle of many present-day time-resolved techniques can be described as follows: A reaction is triggered in the sample, and the time-delayed probing pulse of x-rays, electrons, or neutrons creates a distinct signal (diffraction pattern, image, absorption spectrum) that can be collected at specific time delays relative to the beginning of the triggered reaction. The idea is simple and plausible, however, the technical implementation, the experimental possibilities, instrumental performance, and limitations are multiple and often challenging. In the current scientific literature, the word “ultrashort” refers to the time domain ranging from femtoseconds (fs, 10–15 s) to picoseconds (ps, 10–12 s). Accordingly, the word “ultrafast” refers to physical, chemical, and biological events that occur on that time scale, and hence the terms femtophysics, femtochemistry, and femtobiology can be found in the recent scientific literature. Femto materials science seems to be less known, possibly because it overlaps with femtophysics and, in some aspects, with femtochemistry, because ultimately we are all trying to understand how atoms and electrons behave. Even faster processes than “femto” occur on the attosecond time scale. This article will concentrate on the materials science aspects, taking as a given that short laser pulses (in the visible and x-ray spectral domain) can be generated. The first part of this article will present the possibilities for ultrafast materials science
research. It needs to be stressed that the chief driving force for ultrafast science in physics, chemistry, biology, and materials science is the development of better, faster, brighter, and more stable lasers. Without pico-, femto-, and now attosecond laser pulses, ultrafast science would hardly be possible. The second part of this article will present two complementary and competing probing techniques (x-ray- and electron-based) that are trying to keep pace with the developments on the “laser front.”
From 3D to 4D Although time-resolved visualization of events is not particularly new—at the beginning of the 20th century the time scale was on the order of seconds—the fascination with the new possibilities of understanding previously untraceable ultrafast physical, chemical, and biological processes, expressed in many of the associated publications, is invigorating scientists to delve into the characterization of the dynamic processes of nature.1–84 In essence, one can now start to understand how the already attained three-dimensional (3D) structural information at the atomic scale changes with time—hence the terminology: structural dynamics or 4D characterization (x, y, z and time). Electron microscopy and diffraction, x-ray and neutron diffraction, x-ray absorption, nuclear magnetic resonance, and several other spectroscopy techniques have enabled research on the structure of matter at the atomic scale. They have had an enormous impact on modern scientific knowledge by making
Alexander Ziegler, Max-Planck Institute for Biochemistry;
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