SLAC

A research group at SLAC headed by Wang has been conducting research and development on a new instrument, known as Ultrafast Electron Diffraction (UED) to be used along with X-ray to study material structures. Electrons have many advantages compared to x-ray such as (Wang, 2015), 

  1.  Electrons scatter off both the atomic nuclei and electrons of the sample, while x-ray interacts only with the electrons of the sample, so both electrons and x-ray can be used as a complementary to study the ultrafast dynamics of the materials.
  2. The scattering cross-section of the electrons is larger than that for x-ray scattering, paves the way for observing the fine details at nanometer-scale.
  3. Electrons have several hundred less damaging than x-ray to samples per elastic scattering events (R. Hendreson, 1995)
  4. Electrons can be accelerated and focused.

 

A mega-electron-volt ultrafast electron diffraction (MeV UED) system has been established at SLAC in which the electron pulses are accelerated in radio frequency (rf) field, allows extraction of much brighter electron beams from the cathode and reduces the space-charge effects comparing to the electron pulses generated from the photocathodes in the ultrafast transmission electron can be accelerated  electrostatic potential up to 200 KeV

Applications

The ultrafast structural dynamics over localized crystalline domain has been resolved using a time-resolved mega-electron-volt electron microdiffraction which achieves a temporal resolution of 109 fs with pulses of 10k electrons at 4.2 MeV energy (X. Shen, 2017)

Visualizing the temporal evolution of phonons in 20 nm ultrathin gold films excited by 30 fs Laser pulse at 800 nm central wavelength and probed by UED with 3.3 MeV  electron pulse of 200 fs duration (Chase2016) showed a significant background reduction of the multiple scattering, owing to the high  ultrafast electron pulse energy (MeV) compared to Kev electron energies allows detecting the temporal evolution of phonons in more complex material throughout Brillion zone.

The outlook

Researchers at SLAC are working on the improvement of electron pulse time resolution to less than one femtosecond time scale to realize the motion of electrons in chemical bonds 

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