4D Imaging by Ultrafast Electron Diffraction

Direct observation of atomic-scale and sub-atomic-scale motion in matter is a key challenge in physics and chemistry, because rearrangements of atoms and electrons are fundamental to the function of materials, molecules, nanosystems, and devices using them. Visualization of atoms and electrons in motion in all four dimensions of space and time is the purpose of this project.
Our approach is ultrafast electron diffraction with single electrons at high repetition rate: Femtosecond laser pulses are used to initiate the changes of interest, and at a chosen delay ultrashort electron pulses of keV energy are diffracted into Bragg spots, Debye-Scherer rings, or inelastic contributions, in order to obtain a snapshot of the atomic-scale structure at that time. A sequence of such measurements is then combined into a movie of the atomic motion in all four relevant dimensions of space and time. The use of single electrons avoids space charge and the associated Coulomb explosion, and makes it possible to reach much better time resolutions than with conventional techniques.
The single electrons are generated at photon energies close to the work function; in addition, a microwave cavity compresses the pulses further. The de Broglie wavelength of keV electrons, for example 0.07 Angstrom at 30 keV, is suitable for imaging atomic distances, as well as electron densities. This allows for recording many of the primary and elementary steps of chemical reactions and condensed matter transformations. The key essentials of this project are

(1) realization of a novel regime of time resolution in electron diffraction, i.e. ~10 femtoseconds as compared to hundreds of femtoseconds before, and
(2) application of these capabilities to proof-of-principle experiments and novel types of complex systems, such as self-assembled surfaces, chemically activated surfaces, biomolecules or viruses, molecular crystals, and other specimen.
(3) In the future, we aim for extending 4D diffraction into the regime of electron densities. Processes like the motion of charge in nanostructures/metamaterials, the change of bonding to antibonding orbitals in molecules, or the displacement of electrons within dielectric materials in laser fields (nonlinear optics) are expected to be recorded in 4D.