Design Of An Ultrafast Electron Diffractometer With Multiple Operation Modes*

Directly resolving structural changes in material on the atomic scales of time and space is desired in studies of many disciplines. Ultrafast electron diffraction (UED), which combines the temporal resolution of femtosecond-pulse laser and the spatial sensitivity of electron diffraction, is an advancing methodology serving such a goal. Here we present the design of a UED apparatus with multiple operation modes for observation of collective atomic motions in solid material of various morphologies. This multi-mode UED employs a pulsed electron beam with propagation trajectory of parallel and convergent incidences, and diffraction configurations of transmission and reflection, as well utilities of preparation and characterization of cleaned surface and adsorbates. We recorded the process of electron-phonon coupling in single crystal molybdenum ditelluride following excitation of femtosecond laser pulses, and diffraction patterns of polycrystalline graphite thin film under different settings of electron optics, to demonstrate the temporal characteristics and tunable probe spot of the built UED apparatus, respectively.