Excitons exist as quantum mechanical wave functions that resist direct observation. That's why understanding how these electron-hole pairs behave within organic semiconductors has proven challenging.

A recent study led by Marcel Theilen, Siegfried Kaidisch, Monja Stettner, and colleagues now changes that picture by demonstrating a way to visualize exciton wave functions in real time and space, offering new clarity on how these quasiparticles form, move, and evolve.

The team worked with thin films of the organic semiconductor α-sexithiophene and used femtosecond time-resolved photoemission orbital tomography, which combines ultrafast laser pulses with momentum-resolved photoelectron spectroscopy. Through this approach, they reconstructed the full momentum-space distribution of excitons and translated it into real-space wave functions. 

The study showed that excitons in α-sexithiophene are delocalized across roughly three molecular units. This finding contradicts earlier assumptions, claiming that excitons are confined to a single molecule.

Time-resolved measurements also showed that excitons evolve quickly after formation, with their radius contracting by about 20 percent within approximately 400 femtoseconds. This observation provides direct experimental evidence of self-trapping driven by exciton–phonon coupling, where interactions with molecular vibrations gradually localize the exciton. 

The reconstructed wave functions further display clear phase modulation, which closely matches predictions from many-body perturbation theory using the GW approximation, including a determined 5.3 eV molecular orbital gap.

These results establish time-resolved photoemission orbital tomography as a practical and applicable method for studying excitons. 

Read the full article here to learn more about exciton wave function evolution.

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