Ultrafast electron diffraction

Ultrafast electron diffraction (UED), also known as femtosecond electron diffraction, is a pump-probe experimental method based on the combination of optical pump-probe spectroscopy and electron diffraction. UED provides information on the dynamical changes in the structure of materials such as those undergoing phase transitions or chemical reactions. It is conceptually similar to time-resolved crystallography, but instead of using X-rays as the probe, it uses electrons. UED can provide a wealth of dynamics on charge carriers, atoms, and molecules.

The technique uses a femtosecond (10^–15 second) laser optical pulse to promote (pump) a sample into an excited, usually non-equilibrium state. The pump pulse may induce chemical, electronic, or structural transitions. After a finite time interval, a short (with a duration in femtoseconds or picoseconds) electron pulse is incident upon the sample, which interacts with and diffracts off the underlying atomic structure. The diffraction signal is, subsequently, detected by an electron counting instrument such as a charge-coupled device camera or a direct electron detector similar to detection methods in electron microscopy. Specifically, the detectors measure diffraction patterns, which contain structural information about the sample. By adjusting the time difference between the arrival (at the sample) of the pump and probe beams, one can obtain a series of diffraction patterns as a function of the various time differences. The diffraction data series can be concatenated in order to produce a motion picture of the changes that occurred in the data.