High Energy Electron diffraction

For ultrafast electron diffraction (UED) experiments DESY is developing and operating the accelerator-based high energy electron facility REGAE. It provides 2 – 5 MeV electron pulses with a charge of typically 100 fC and a pulse duration down to 20 fs and is ideally well suited for time-resolved diffraction experiments. REGAE is currently the only working MeV UED facility in Europe. While other institutes are planning similar facilities, with REGAE DESY is several years ahead of these and taking a lead position in Europe.
A major difference between diffraction experiments withs electrons and X-rays is their different interaction with matter. Electrons interact with the nucleus instead of inner shell orbitals which increases the sensitivity towards lighter atoms and here in particular hydrogen atoms, which provide relatively poor contrast in X-ray experiments. These makes electrons the ideal probe for thin samples of low-Z materials, such as biomolecules or many battery materials, which provide only weak scattering contrast in X-ray experiments.

With radiation damages effects being reduced by about a factor of 1000 compared to X-rays and the better visibility of hydrogen atoms, REGAE offers a great potential for diffraction experiments with biological macromolecules.

In close collaboration with DESYs machine division our group therefore explores the potential of high-energy electron diffraction experiments for structure determination from biological macromolecules at room temperature as an alternative to commonly used synchrotron-based X-ray crystallography. The biggest challenge for these electron diffraction experiments with biological macromolecules is sample preparation, as the maximum sample thickness is limited to one micrometer or ideally even thinner. Therefore, we are currently exploring different sample preparation and growth techniques to produce such extremely thin but laterally extended samples.

In collaboration with the DESY’s laser group FS-LA we are additionally implementing a 3 GHz bunch train mode at REGAE. Such a bunch train will contain more than 3000 micro bunches and has an overall duration of about 1 - 1.5 μs. Spreading the bunch charge over a longer time allows to generate a microfocus beam still providing enough coherence for diffraction experiments with protein crystals.
  
The overall goal of our efforts is to develop REGAE into a user facility for time resolved electron diffraction experiments with biological macromolecules and other samples. Different laser pumping schemes such as UV/VIS and THz will be available for the experiments.