Physicists have experimentally reconstructed the quantum probability distribution of an exciton for the first time, using a technique that captures both the spatial and momentum spreads of these short-lived quasiparticles. The achievement, reported in Physical Review X, opens a new window into how excited states evolve in complex materials and could inform the design of more efficient organic solar cells.
Excitons form when light strikes a semiconductor or insulator, promoting an electron to a higher energy band and leaving behind a positively charged hole. The electron and hole bind together into a neutral quasiparticle that plays a central role in light harvesting in optoelectronic devices. But their quantum mechanical structure has been notoriously difficult to study because excitons decay within picoseconds, far too fast for standard spectroscopic techniques to simultaneously probe the spatial dimensions of their orbits and their momentum profiles.
To overcome this, a team led by Peter Puschnig of the University of Graz in Austria, working with colleagues at Marburg University and Forschungszentrum Jülich in Germany, turned to time-resolved photoemission orbital tomography (trPOT). This method combines pump-probe photoemission spectroscopy with momentum microscopy, offering femtosecond resolution that is well suited to capturing the exciton wavefunction.
The researchers began by applying an ultrashort laser pulse with an energy of 2.35 eV to alpha-sexithiophene, an organic semiconductor used in solar cells and other optoelectronic applications. This pulse generated the excitons. A second, linearly polarized laser pulse with a much higher energy of 21.7 eV then ejected electrons from the excitons through photoemission. By measuring the energy and direction of the ejected electrons, theoretical models allowed the team to infer their quantum-mechanical state. Varying the time between the two pulses produced snapshots of the exciton at different moments after its creation.
The team found that the exciton in the sexithiophene films initially extends over approximately 1.5 nanometers, roughly three molecules. Within 400 femtoseconds of its creation, however, its size contracts by about 25%. «What makes this particularly exciting is that we are not just measuring an energy or lifetime but are also gaining access to the quantum-mechanical wave function of the exciton itself, including its spatial structure and phase,» Puschnig said.
Puschnig noted that the experiment grew out of earlier work on photoemission orbital tomography, including a 2021 proof-of-principle study showing the method could be extended into the ultrafast regime. The main challenge, he said, was coordinating demanding experiments and theory: producing well-defined molecular films, transporting them under ultra-high vacuum from Jülich to Marburg in a vacuum suitcase, performing ultrafast photoemission measurements, and carrying out computationally intensive ab-initio calculations in Graz to interpret the data.
The work provides a new way to study excitons directly in real space and to follow their quantum-mechanical evolution in time. According to the researchers, it could help clarify how excited states evolve into charge-separated states in the donor/acceptor systems found in organic photovoltaics, where the electron and hole can begin to separate after photoexcitation.
The team's next goal is to move from the relatively simple exciton they studied to more complex systems. «We want to observe how charge-separation processes are controlled by the molecular and electronic structure of a material,» Puschnig said. «Doing this will be particularly interesting because charge separation is the crucial step between absorbing a photon and generating a usable electrical current in an organic solar cell.»
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