27.03.2013

How to freeze-frame electrons with X-rays

Electrons are the glue that keeps atoms together in matter, and their motion plays a key role in the function and transformation of materials. During complex chemical and biological reactions, electrons undergo ultrafast rearrangements, determining the later properties of the generated substance or material. One of the key characteristics of the new X-ray lasers like FLASH or European XFEL is to trace these ultrafast electronic rearrangements in detail, in order to understand and eventually change the behaviour of the reacting partners, to develop new functional materials. For ordinary synchrotron light sources, this motion of electrons has always been too fast for taking an image. But with the intense and ultra short light pulses of X-ray lasers, these facilities should be able to freeze-frame the electronic motion during a chemical reaction, even if the sample consists only of a handful of molecules.

The scientists simulated the measurement of rearranging electrons in their orbitals. (Single still image)

But such high-speed films do not only call for new detectors and equipment, it also needs new theoretical approaches: How can one describe and interpret the interaction of ultra short and highly intense X-ray with a non-stationary electronic system? And how can one be sure that the intense laser pulse does only probe the current arrangement and does not disturb the sample too much to destroy the measurement? Robin Santra and co-workers from the Center for Free-Electron Laser Science at DESY have now investigated this very basic problem. In their publication now being published at Physical Review Letters, the work lead by Gopal Dixit proposes a new strategy to visualize these rearrangements in time.

For their work, the team simulated a disordered electronic system rearranging in a few femtoseconds time, the usual timescale for electronic movements. In their simulations, the scientists described the incoming light pulse as a quantum packet interacting with the electrons and demonstrated that the laser pulses are really able to shoot different states of the ultrafast electronic motions. On the other hand, the team showed that the problem of light-induced changes during the snapshot of the motion can be overcome by using the phase variation of X-rays. Phase contrast imaging is a very common method for imaging samples with synchrotron light. It makes use of the fact that interaction of light with matter does not only change the intensity of the light wave but also the phase. The phase difference of the transmitted light and the scattered light can be detected and it contains the information of the sample´s structure.

“Although this is a very basic theoretical investigation, our approach opens the possibility, for the first time, to take snapshots of electronic motion at different instants and make a movie of electrons,” claims Jan Malte Slowik, one of the authors. “Once the method can be applied in experiments, it will shed light on, and thus improve our understanding of ultrafast processes in nature,” as Gopal Dixit points out.

Original Paper

The scientists simulated the measurement of rearranging electrons in their orbitals. (Complete sequence)