Photoemission orbital tomography: There’s more to see

Photoemission orbital tomography (POT) makes electrons in molecules visible – but it can do more than that, as shown by Anja Meier in her award-winning doctoral thesis.

Eine Person in weißer Schutzkleidung steht vor einem Gerät mit Schläuchen und Glasbehältern.
For Anja Meier, it was already a fantastic achievement to experimentally demonstrate orbitals – the theoretically predicted locations of electrons within a molecule. What’s even better, however, is that she has extended the method used to achieve this.
Copyright: Forschungszentrum Jülich / Sascha Kreklau

Anja Meier is not easily discouraged – fortunately for science, as it now has a far more versatile tool at its disposal than before. Together with colleagues from the Peter Grünberg Institute (PGI-3), the University of Graz, and the National Metrology Institute of Germany in Berlin, Meier has helped to advance the development of photoemission orbital tomography (POT), a method that visualizes the physical properties of molecules and pinpoints where the electrons within them are located.

It has long been known that electrons in a molecule are not distributed evenly. Instead, they tend to occupy specific regions known as orbitals. Researchers can calculate the shape of these orbitals using mathematical equations. For several years now, POT has made it possible to experimentally verify whether these calculations are realistic. “As part of this process, we illuminate samples with photons in a targeted way,” explains Meier. This ejects electrons from the sample.

The method has so much potential. I’m certain there is still much more to come.

Meier and her team then measure their energy and emission angles. From the measured data, the researchers create “momentum maps”, essentially snapshots of the electron distribution revealing the shape of the orbitals. “Being able to actually visualize orbitals is fantastic for me as a chemist,” says the researcher.

Assumptions disproved

For many years, POT was considered to have only limited applications. In her doctoral thesis, Meier challenged these limitations and demonstrated that the method is capable of far more than originally assumed. “The common thread running through all of this was a paper by colleagues from Graz in 2009,” Meier recalls. “In the paper, Prof. Peter Puschnig and Prof. Michael Ramsey described for the first time which systems could be investigated using POT.” At the time, the two researchers – who now work closely with Meier – believed that the method could only be used in specific cases, for example to measure flat molecules. In her doctoral thesis, Meier experimentally tested these assumptions and was able to demonstrate that POT can be applied much more broadly.

“My most important finding was that we can measure different types of orbitals using POT,” says the researcher. According to computational models, molecular orbitals come in a variety of shapes. Meier has now succeeded for the first time in measuring s-type molecular orbitals using POT, even though these are particularly difficult to access experimentally and return highly complex data. Using POT, Meier was also able to answer a long-disputed question regarding the electron distribution in a ring-shaped molecule, the “superbenzene” kekulene.

The red and blue clouds schematically depict the spatial distribution of the electrons in one of the molecule’s orbitals. PTCDA is a dark red solid that can be used as a pigment.
The red and blue clouds schematically depict the spatial distribution of the electrons in one of the molecule’s orbitals. PTCDA is a dark red solid that can be used as a pigment.
Copyright: Image courtesy of Markus A. Huber, Regensburg Center for Ultrafast Nanoscopy

With her findings, the researcher even convinced the most sceptical leading figures in her field, such as the late British physicist Prof. Phil Woodruff. “He had doubts about whether POT could really deliver accurate results in all these new cases,” recalls Meier. She therefore carried out a series of experiments he had proposed and proved that the method works. The pioneer of solid-state physics congratulated her personally on this success. “I found this especially rewarding,” says Meier.

The scientific community recognized the many highly complex series of measurements and months of analysis with two separate awards: the ICSOS Young Scientist Prize 2023 and Forschungszentrum Jülich’s Excellence Prize 2025.

Thanks to Meier’s research, a whole range of possibilities for using the method has emerged. And from the researcher’s perspective, the possibilities are far from exhausted: “The method has so much potential. I’m certain there is still much more to come.” Meier also wants to continue contributing to this progress, albeit on a more limited basis. “I’ve realized that full-time research and starting a family are difficult for me to balance,” says the scientist. She now divides her time between working as a scientific coordinator and as a postdoc at PGI-3.

She is currently further developing POT together with colleagues as part of the “Orbital Cinema” project. In this project, the researchers illuminate samples with short laser pulses, which can be used to selectively break chemical bonds. “Using POT, we take snapshots at very short intervals of how the molecules change during the experiment. This allows us to create short film sequences of chemical reactions,” explains Meier. The results promise to be exciting: they may once again push the boundaries of what can be seen. Meier certainly plans to keep at it.

Anja Meier presents her work in this video: go.fzj.de/effzett-anjameier

This text is taken from the 1/26 issue of effzett. Text: Nora Lessing

Last Modified: 13.08.2026
effzett issue 1-26