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: 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 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. In contrast to spectroscopic approaches, the information is structural and the coordinates of motion are directly deduced.
Real-time observation of atomic and electronic structure calls for femtosecond/attosecond timing to be combined with picometer spatial resolution. 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. The time resolution, however, of state-of-the-art experiments is ~200 femtoseconds, which is not sufficient for recording many of the primary and elementary steps of chemical reactions and condensed matter transformations. In addition, 4D-imaging studies of rather complex systems have been scarce and many experiments so far concentrate on simple model systems, although ultrafast electron diffraction has proved particularly valuable for discovering fundamental reaction mechanisms.
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.

In combination, this will grant visualization of complex transition processes on the atomic scale and promises novel fundamental insight into many so far elusive processes within matter during changes.