Ultra-fast electrons in imaging of microscopic structural changes

• Zahra Rezaei (School of particles and accelerators, IPM, Tehran)

Alongside ultrafast X-ray techniques, time-resolved ultra-fast electron microscopy, and diffraction will yield unprecedented insights into the rapid physical, chemical, and biological processes. We follow various imaging approaches with ultrashort, highly coherent electron pulses to investigate the dynamics of ultrafast structural phenomena. The Ultrafast electron diffraction (UED) technique, is a pump-probe experimental method based on the combination of optical pump-probe spectroscopy and electron diffraction. It is very similar to time-resolved crystallography, but instead of using X-rays as the probe, it uses electrons. In the UED technique, a femtosecond (fs) laser optical pulse excites (pumps) 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 fs electron pulse incident upon the sample. The electron pulse undergoes diffraction as a result of interacting with the sample. The scattered electrons will form a diffraction pattern (image) on a CCD camera. This pattern contains 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 to produce a motion picture of the changes in the data.