Electrons In Quantum Fluid Fill Up With Laser Energy

For the first time it has been possible to observe the absorption of energy from laser light by free electrons in a liquid. Up to now this was only possible in the gas phase. The findings under the direction of TU Graz open new doors for ultrafast electron microscopy.

When examining and developing materials, it is crucial to take a very quick look at the very small things. The necessary spatial resolution for investigations in the (sub) atomic range can be achieved with electron microscopic methods. But it is difficult with the speed: The fastest processes in solids take only a few femtoseconds (the quadrillionth part of a second). The time resolution of conventional electron microscopes is too low for this. In order to improve the duration of the electron pulses, electrons would have to be selected within a shorter time window - in analogy to the camera shutter, which controls the exposure time during photography.

This time selection is in principle possible with extremely short laser pulses. The process is called Laser-Assisted Electron Scattering (LAES, German laser-assisted electron scattering ). During a collision with atoms of the material sample, electrons can absorb energy from the incident light field. “Structural information is provided by all electrons - but those with a higher energy level can be assigned to the time window in which the light pulse was radiated. With this method, it is possible to take a short time window out of the long electron pulse and thus improve the time resolution, ”explains Markus Koch, professor at the Institute for Experimental Physics at Graz University of Technology. So far, LAESProcesses, although investigated for around 50 years, have only been observed in the gas phase.

Markus Koch and his team, in collaboration with researchers from the Institute for Photonics at TU Wien and the Institute for Chemistry at Tokyo Metropolitan University, have now demonstrated for the first time that laser-assisted electron scattering can also be observed in condensed matter, specifically in superfluid helium.

Superfluid helium as a success factor
The TU Graz researchers carried out the experiment in a superfluid helium droplet with a diameter of only a few nanometers (3-30 nm), into which they introduced individual atoms (indium or xenon) or molecules (acetone) that acted as an electron source - one am Institute already tried and tested technology . "The free electrons can move perfectly in the droplet and absorb more energy in the light field than they give off through collisions with the helium atoms," says Leonhardreiber, doctoral student at Markus Koch and responsible for the work. This acceleration enables much faster electrons to be observed.

The experiments were interpreted in cooperation with Markus Kitzler-Zeiler, an expert on strong field processes at the Vienna University of Technology . To validate the LAES effect, the interactions were simulated by the simulation expert from Tokyo Metropolitan University Reika Kanya under the same conditions as in the experiment. The researchers have published their results in Nature Communications.

In the future, the LAES process will be investigated on thin layers of different materials, which can also be produced in helium droplets, in order to determine important parameters for such experiments: for example, the optimal layer thickness or the intensity of the laser pulses for use in an electron microscope.

This research project was funded by an FWF project and is anchored in the Field of Expertise “ Advanced Materials Science ”, one of five strategic focus areas at TU Graz. Participating researchers are members of NAWI Graz Physics.