Femtosecond photoemission catches an exciton shrinking by a quarter in an organic semiconductor

Excitons form when light lifts an electron into a higher energy band and the electron binds to the positively charged hole it leaves behind ^[18]. They decay within picoseconds. Standard spectroscopy is too slow to record their spatial extent and their momentum profile together inside that window ^[3]. Physics World describes the new study as the first experimental reconstruction of an exciton's quantum probability distribution ^[1].

The experiment uses two pulses. Puschnig, who led the work at the University of Graz with colleagues at Marburg University and Forschungszentrum Jülich ^[2], described the step after the second pulse ejects the electron. "If we then measure the energy and direction of the ejected electrons, theoretical models allow us to infer their quantum-mechanical state," he said ^[7]. Changing the gap between pulses gives a time series. "And by varying the time between the first and second laser pulses, we are able to obtain snapshots of the exciton at different times after its creation," he said ^[8].

Those snapshots show the exciton contracting by about 25% within 400 fs of its creation ^[10], to roughly 1.1 nm ^[1]. Four hundred femtoseconds is 0.4 ps ^[2], so the contraction is complete before the exciton reaches the end of a lifetime counted in picoseconds ^[3]. The Physics World report does not give error bars on either figure or say how many films were measured.

Calling this a direct measurement needs a qualification, and Puschnig's own account supplies it. The detector records the energies and directions of electrons. The wavefunction is what the models infer from them ^[7]. Interpreting the data required heavy ab-initio computation in Graz ^[13]. In my view the advance is that those calculations now have more to agree with. "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 ^[11].

For organic photovoltaics the case is still a forecast. Sexithiophene is used in solar cells ^[6], but these films hosted what the team calls a relatively simple exciton ^[16]. The step that matters for devices happens in donor/acceptor systems, where electron and hole begin to separate after photoexcitation. The researchers say the method could help explain how excited states evolve into those charge-separated states ^[15]. Those are the next experiments. "We want to observe how charge-separation processes are controlled by the molecular and electronic structure of a material," Puschnig told Physics World ^[17].

The procedure is demanding to run. "The main challenge 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," Puschnig said ^[13].