Researchers have reconstructed an exciton's full quantum-mechanical wavefunction for the first time, tracking a spatial contraction of about 25% within 400 femtoseconds of creation in thin films of alpha-sexithiophene.

The result, published in Physical Review X, comes from time-resolved photoemission orbital tomography (trPOT), a pump-probe variant of an established surface-science method now extended into the femtosecond regime.

Excitons are bound, neutral electron-hole pairs formed when photons promote electrons across the band gap of organic semiconductors. They carry energy in light-harvesting devices but typically decay within picoseconds (10⁻¹² s), faster than standard spectroscopy can image spatially and in momentum at the same time.

How does the instrument capture both dimensions at once?

The team, led by Peter Puschnig at the University of Graz with collaborators at Philipps-Universität Marburg and Forschungszentrum Jülich, drove each film with a 2.35 eV normal-incidence pump pulse that created the excitons.

A second, linearly polarized pulse at 21.7 eV ejected the bound electron. Momentum microscopy then mapped its energy and emission direction, and ab-initio calculations converted those measurements into a wavefunction snapshot at each delay time.

"If we then measure the energy and direction of the ejected electrons, theoretical models allow us to infer their quantum-mechanical state," Puschnig explains. Scanning the pump-probe delay produced femtosecond-resolved images of the evolving quasiparticle.

What did the snapshots show?

The exciton's initial wavefunction spanned roughly three alpha-sexithiophene molecules, or about 1.5 nm. Within 400 fs of formation, its spatial extent contracted by roughly a quarter — a structural change no prior technique could resolve in real time.

"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 says.

What were the experimental bottlenecks?

Three sites had to act as one. Films were grown under ultra-high vacuum at Jülich, then shuttled to Marburg in a vacuum suitcase for femtosecond photoemission. Interpretation then required computationally intensive ab-initio calculations back in Graz.

The trPOT workflow extends a 2021 proof-of-principle study in Science, where Puschnig's group demonstrated that photoemission orbital tomography could be pushed into the ultrafast regime. The current work is the first to apply that capability to an exciton rather than a ground-state orbital.

Why should an organic-photovoltaic R&D team care?

Charge separation — the step that turns an absorbed photon into a usable current — occurs when a bound exciton splits into free carriers at a donor/acceptor interface. It remains a dominant loss channel in many organic solar cells, where efficiencies continue to plateau below those of silicon.

Until now, the dynamics of that step could only be inferred from indirect transport measurements. Imaging the exciton wavefunction directly could let materials groups screen molecular candidates by how their structure steers charge separation, potentially shortening the design loop for organic photovoltaic devices.

What comes next?

"We want to observe how charge-separation processes are controlled by the molecular and electronic structure of a material," Puschnig tells Physics World. "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."

The group plans to move from the relatively simple alpha-sexithiophene films — a single molecule studied in isolation — to prototypical donor/acceptor blends, where the same femtosecond-snapshot workflow could capture the precise moment an exciton dissociates into free charges.

Whether that benchmark translates into improved device yields will depend on extending the technique beyond ultra-high vacuum to interfaces that more closely resemble working solar cells.