The scientists tracked the orbital tomograms with ultra-high resolution over a period of time. For this purpose, the electrons in the molecules were excited into a different orbital using femtosecond laser pulses.
They are fast as lightning, tiny and almost impossible to capture: electrons. Tracking their movements closely could reveal a lot about chemical reactions. Previously seeming impossible, a mix of laser and electron spectroscopy could now change that.
Electrons play a central role in chemistry: they bind atoms. In chemical reactions, electrons move, breaking bonds and forming new ones in the process. Reaction equations can be used to describe exactly which substance donates or accepts how many electrons. However, researchers would like to know more details. “For many years, it has been a goal to precisely track electrons in time and space over the course of a reaction,” says Prof. Stefan Tautz from the Peter Grünberg Institute (PGI-3). Beneath this lies a big dream: to explain chemical reactions based solely on the spatial distribution of electrons in molecules.
Picture above: The scientists tracked the orbital tomograms with ultra-high resolution over a period of time. For this purpose, the electrons in the molecules were excited into a different orbital using femtosecond laser pulses.
Researchers from Jülich, Graz and Marburg have taken an important step towards this dream: they observed electrons transferring through an interface between a molecular layer and a metal. Using a combination of innovative methods, they captured the excitation paths of the blazingly fast particles in a series of individual images and were thus able to follow the process in slow motion.
This is a complex challenge, as electrons in a molecule do not have a fixed location at any point in time. It is only possible to specify areas where they are very likely to be: the orbitals. With the help of photoemission orbital tomography, which was developed jointly in Graz and Jülich a few years ago, researchers can record such orbitals experimentally. To do this, they bombard a molecular layer with light particles. This releases energetically excited electrons from the layer – not randomly, but according to a specific pattern that directly reflects the spatial distribution of electrons in the orbitals of the molecular layer.
However, this only provides a snapshot. In order to obtain a time sequence, the team used a special laser that generates ultra-short pulses with sufficiently high energy in the femtosecond range. Lasting only a quadrillionth of a second, each pulse produces a new image. The researchers used the organic molecule PTCDA on a copper substrate as a sample, with an extremely thin oxide layer between the two substances. A highly sensitive momentum microscope detected the direction and energy of the released electrons.
“The experiments have shown that, with our combination, it is possible in principle to follow the excitation paths of the electrons in space and time. We now want to prove this with other samples as well,” says Stefan Tautz. For example, the findings could help to optimize interfaces and nanostructures for processors, organic solar cells and catalysts, among others.
Photos: Philipps-Universität Marburg/Till Schürmann, Forschungszentrum Jülich/Sascha Kreklau, Video: Philipps-Universität Marburg / Till Schürmann