Physicists led by Peter Puschnig at the University of Graz have experimentally reconstructed the quantum probability distribution of an exciton for the first time. An exciton is the short-lived, neutral quasiparticle formed when a photon promotes an electron in a semiconductor or insulator, leaving behind a positively charged hole that the electron then binds to.
These states are central to light harvesting in optoelectronics, but they are notoriously hard to measure: excited electrons decay back within picoseconds, and standard spectroscopy cannot simultaneously resolve the spatial extent of the particle's orbit and its momentum profile inside that window. The team got around this with time-resolved photoemission orbital tomography (trPOT), which combines pump-probe photoemission spectroscopy with momentum microscopy and reaches femtosecond resolution. An ultrashort laser pulse at 2.35 eV generates the excitons in an organic semiconductor called alpha-sexithiophene; a second, linearly polarised pulse at 21.7 eV knocks electrons out. Measuring the energy and direction of those ejected electrons lets the researchers infer their quantum-mechanical state, and varying the delay between the two pulses yields snapshots of the exciton at successive moments after its creation.
What they saw: the exciton in the sexithiophene films initially spreads over roughly 1.5 nanometres, about three molecules, and contracts by about 25 per cent within 400 femtoseconds of forming. "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.
The work, published in Physical Review X, was carried out with colleagues at Marburg University and Forschungszentrum Jülich. Puschnig describes the main challenge as coordinating demanding experiment and theory: producing well-defined molecular films, transporting them under ultra-high vacuum in a vacuum suitcase from Jülich to Marburg, performing the ultrafast photoemission measurements, and running computationally intensive ab-initio calculations in Graz to interpret the data. The group's next goal is to move from this relatively simple exciton to more complex systems — above all charge separation, the step between absorbing a photon and producing a usable current in organic solar cells.
