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Crucial Breakthrough in Chemistry

Tracking Electrons in Time and Space

19. Februar 2021, 09:31 Uhr   |  Heinz Arnold

Tracking Electrons in Time and Space
© Philipps-Universität Marburg / Till Schürmann

The scientists tracked the orbital tomograms through time with ultrahigh resolution. To do this, the electrons in the molecules were excited into a different orbital with femtosecond laser pulses.

For the first time, scientists have been able to track electrons in a chemical reaction in both time and space. Countless applications are opening up.

For example, interfaces and nanostructures and the processors, sensors, displays, organic solar cells, catalysts based on them could be optimized. "There may even be opportunities for applications and technologies that we haven't even thought of yet," says Ulrich Höfer, professor of experimental physics at Philipps University in Marburg and spokesman for the German Research Foundation's Collaborative Research Center 1083.

"It has long been a dream of physical chemistry to accurately track the electrons in a chemical reaction in time and space," says Professor Stefan Tautz, head of the Institute for Quantum Nanoscience at Forschungszentrum Jülich

Until now, electrons could be tracked either only in space or only in time. The scientists from Jülich, Marburg and Graz have now combined the two: For the first time, they have observed electrons transferring through an interface between a molecular layer and a metal in space and time.

Such interfaces are being researched in Collaborative Research Center 1083 of the German Research Foundation at the University of Marburg, within the framework of which the experiments were carried out. "Interfaces initially appear to be nothing more than the juxtaposition of two layers - but they are the place where the functions of materials manifest themselves in the first place. They are therefore of paramount technological importance," explains Ulrich Höfer, professor of experimental physics at Philipps-Universität Marburg and spokesman for Collaborative Research Center 1083 of the German Research Foundation. In organic solar cells, for example, the combination of different materials at an interface allows the states excited by the irradiated light to be split more effectively, so that current can flow. Interfaces also play an important role in OLEDs.

Track orbital tomograms through time with ultra-high resolution

The scientists' experimental approach is based on a breakthrough in the spectroscopy of molecules that occurred only a few years ago: photoemission orbital tomography, which is based on the photoelectric effect. "In this process, a layer of molecules on a metal surface is bombarded with photons, i.e. light particles, whereupon the energetically excited electrons break free," explains Professor Peter Puschnig of Karl Franzens University in Graz. "However, they do not fly out into space at random afterwards, but allow conclusions to be drawn about the spatial distribution of electrons in the molecular orbitals based on their angular and energy distribution."

"The key success of our work is that we track the orbital tomograms through time with ultrahigh resolution," says Dr. Robert Wallauer, group leader and junior scientist at the University of Marburg.

Ultra-short laser pulses plus new pulse microscope

To do this, the scientists not only used a special laser with ultrashort pulses in the femtosecond range to excite the electrons in the molecules, but also a novel pulsed microscope that allowed them to simultaneously measure the direction and energy of the extracted electrons with high sensitivity. A femtosecond is a millionth of a billionth of a second. Such short pulses, like a kind of strobe light, are suitable for breaking down fast processes into individual images and allowed the researchers to follow electron transfer as if in slow motion. "This allowed us to spatially track electron excitation paths virtually in real time," explains Professor Stefan Tautz. "In our experiment, an electron is first excited from its initial state into an unoccupied molecular orbital by a first laser pulse before finally reaching the detector by a second laser pulse. Not only were we able to closely observe the time course of this process, but the tomograms also allowed us to clearly track where the electrons came from.”

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