Physicists film exciton formation in organic
A research team has captured the first ultrafast video of an exciton forming and shrinking within 400 femtoseconds after light hits an organic
Physicists have filmed the quantum-mechanical birth of an exciton. A team from the University of Graz, with collaborators from Marburg University and Forschungszentrum Jülich, used ultrafast laser pulses to capture the first moments after light strikes an organic semiconductor material. Their work, published in Physical Review X, provides a fundamental view of processes critical for improving organic solar cells.
For the first time, researchers have experimentally reconstructed the spatial distribution and temporal evolution of an exciton's wave function immediately after its creation. An exciton is a bound pair of an energized electron and the electron hole it leaves behind. Peter Puschnig, a professor at the University of Graz, reported the findings. "The measurements show that, after its formation, the electron-hole pair extends across approximately three molecules and then shrinks by around 25% within the first 400 femtoseconds," he explained. A femtosecond is one quadrillionth of a second.
Filming the quantum world
The team used a technique called time-resolved Photoemission Orbital Tomography. They first excited the material with an ultrashort laser pulse to generate an exciton. A second, high-energy ultraviolet laser pulse then ejected an electron from this bound pair. By measuring the energy and direction of these ejected electrons and varying the time delay between the two laser pulses, the scientists assembled a series of snapshots. Puschnig said these snapshots can be "pieced together to form a video of the quantum world." Theoretical models developed by his group allowed them to infer the exciton's quantum-mechanical state from the data.
A collaborative international effort
The breakthrough resulted from close collaboration between three specialized groups. Stefan Tautz's team at Forschungszentrum Jülich produced and characterized the ultra-pure samples. These samples consisted of ordered, wafer-thin films of rod-shaped alpha-sexithiophene (6T) molecules on a copper surface. Monja Stettner, who prepared the samples, noted that precise molecular alignment and decoupling from the substrate were important for preserving the exciton long enough to measure it.
The high-precision photoemission experiments were conducted and analyzed by Ulrich Höfer's group at Marburg University. At the University of Graz, Puschnig's team developed the theoretical framework. Ph.D. Student Siegfried Kaidisch created an analytical model that directly deduces the spatial shape and internal quantum-mechanical phase of the exciton wave function from the photoelectron images.
Path to more efficient photovoltaics
This publication is a major milestone for the EU research project "Orbital Cinema," which aims to visualize electron dynamics with unprecedented resolution. The next target for the researchers is observing the separation of electrons and holes in donor-acceptor systems. Puschnig stated this process "determines how efficiently light can be converted into electrical current and is therefore central to future developments in organic photovoltaics." The foundational understanding gained from filming the exciton's first moments provides essential building blocks for optimizing sustainable energy production from light.
