Ultrafast Electron Diffraction Technology for Exploring Dynamics of Molecules

With the recent successful development of X-ray free electron lasers (X-FELs), it became possible to explore sub-nano structure dynamics of materials with 100-fs temporal accuracy. Ultrafast electron diffraction (UED) can achieve similar performance at a much lower cost and on a smaller scale by using ultrashort and low-energy electron beams. The UEDs are suitable for studying thin films, surfaces, and gas samples that are difficult to study with the X-FELs. Starting from non-relativistic UEDs using low-energy electron beams of less than 100 keV, it led to the development of relativistic UEDs using a-few-MeV electron beams. These efforts have contributed to the identification of the unexplored mechanism of matter by observing the dynamics of atoms with higher temporal accuracy. Electron beam is easier to handle than X-rays, and various technologies are being developed to improve the performance of UED. We review UEDs historically based on the development of core technologies. And application researches with the UEDs will be outlined in this paper.