Ultrafast Diffraction Techniques

Abstract

AbstractThe need for femtosecond time resolution in the analysis of processes governed by atomic motion is well established. In biology, chemistry, and physics different times are of interest, but the shortest relate to the vibration of matter, being typically about 10−13 s (100 femtoseconds). Time‐resolved experiments are used to monitor time‐dependent processes. Depending on the spectral range of observation, different properties can be studied. Usually, visible or near‐visible radiation provides information on the electronic properties, but atomic motion can be directly monitored only through very short wavelength radiation, such as X‐rays. The electronic changes associated with vibrations in single molecules, liquids or crystal lattices, isomerization and the breaking or formation of chemical bonds, can be monitored by ultrafast optical spectroscopy techniques, but the accompanying ultrafast structural rearrangements cannot be directly observed. In the detailed characterization of structures at the atomic level, X‐ray and electron diffraction is an unparalleled tool, allowing reaction intermediates to be followed.Synchrotrons have become essential to the study of structures. Nevertheless, the shortest durations attainable with the latest generation of synchrotrons, hold the time resolution of experiments around 50 ps. This article shows how subpicosecond time resolution can be achieved in the analysis of very short‐lived transient structures. The two emerging techniques in subpicosecond time‐resolved X‐ray (electron) diffraction are (1) ultrashort X‐ray (electron) probe pulses, produced through a femtosecond laser system, and (2) fast X‐ray detection during quasicontinuous synchrotron‐produced X‐ray probe illumination.