Whether in solar cells or during photosynthesis – when light strikes a surface, its particles, the photons, are absorbed by the material and thereby energise electrons. In the process, their spatial distribution – the so-called wave function – changes, and a bond is formed between an electron and the resulting electron hole. These ‘excited pairs’ are known as excitons. They play a key role in modern optoelectronic materials. However, although they have been known for decades, their internal quantum-mechanical structure has remained largely hidden until now. “We have now succeeded for the first time in experimentally reconstructing the spatial distribution and temporal evolution of an exciton’s wave function in the very first moments of its existence,” reports Peter Puschnig, Professor of Electronic Structure of Nanomaterials at the Department of Physics, University of Graz. “The measurements show that, after its formation, the electron-hole pair extends across approximately three molecules and then shrinks by around 25 per cent within the first 400 femtoseconds – a quadrillionth of a second,” explains the researcher.
To film this process, the scientists first excited the exciton with an ultrashort light pulse and then ejected the electrons from the pair bond using a second high-energy laser pulse. The latter process is as known photoemission. “If we then measure the energy and direction of the electrons, theoretical models allow us to infer their quantum-mechanical state. By varying the time delay between the excitation and the subsequent laser pulse, one obtains different snapshots of the exciton, which can be pieced together to form a video of the quantum world,” explains Puschnig.
Leading international research
The work was carried out in close collaboration between three leading international research groups. Stefan Tautz and his team at Forschungszentrum Jülich produced the organic semiconductor samples, characterised them and transported them to Marburg under ultra-high vacuum conditions, ensuring that their exceptional quality was maintained right up to the measurement. “The samples consist of wafer-thin, ordered films of the rod-shaped 6T molecule that have been deposited onto a specially prepared copper surface. The precise alignment of the molecules and their targeted decoupling from the substrate are important for maintaining the exciton long enough to make its formation visible,” explains Monja Stettner, who prepared the samples as part of her dissertation and assisted with the experiments in Marburg. The group led by Ulrich Höfer at Marburg University carried out the high-precision photoemission experiments and analysed the measurement data.
At the University of Graz, Peter Puschnig’s team developed the theoretical concepts for describing photoemission from excitons, carried out the quantum mechanical simulations and developed an analytical model that enabled the interpretation of the experimental data. “Using our model, it is possible to deduce the spatial shape and the internal quantum-mechanical phase of the exciton wave function directly from measured photoelectron images,” explains Siegfried Kaidisch, who made a key contribution to this work as part of his PhD thesis. The experimental method underlying the research – Photoemission Orbital Tomography (POT) – was also developed in Peter Puschnig’s group and has, in recent years, been expanded in collaboration with Stefan Tautz’s team at Forschungszentrum Jülich into an internationally established tool for investigating electronic states in organic materials.
EU research project
The current publication marks one of the most significant milestones to date in the EU project ‘Orbital Cinema’. Funded by an ERC Synergy Grant, the research project aims to visualise the dynamics of electrons in materials with unprecedented spatial and temporal resolution. “In the next step, we want to observe the separation of electrons and holes in so-called donor-acceptor systems. This process determines how efficiently light can be converted into electrical current and is therefore central to future developments in organic photovoltaics,” says Puschnig.
Publication
Observing the spatial and temporal evolution of exciton wave functions in organic semiconductors
Marcel Theilen, Siegfried Kaidisch, Monja Stettner, Sarah Zajusch, Eric Fackelman, Alexa Adamkiewicz, Robert Wallauer, Andreas Windischbacher, Christian S. Kern, Michael G. Ramsey, François C. Bocquet, Serguei Soubatch, F. Stefan Tautz, Ulrich Höfer, and Peter Puschnig
Physical Review X, 28 August 2026
https://doi.org/10.1103/3zmg-276c
Video: The evolution of an exciton
The animation is based on data calculated using the model developed by Peter Puschnig’s research group at the University of Graz. It shows an amplified version of what was measured in the experiment, in order to make the processes more clearly visible. In principle, however, the wave function (right) can be inferred from experimental photoemission data (left). © Siegfried Kaidisch, University of Graz
If you are interested in the quantum world, you can study physics at the University of Graz.
