Observing the Spatial and Temporal Evolution of Exciton Wave Functions in Organic Semiconductors

Authors: M. Theilen, S. Kaidisch, M. Stettner, S. Zajusch, E. Fackelman, A. Adamkiewicz, R. Wallauer, A. Windischbacher, C. S. Kern, M. G. Ramsey, F. C. Bocquet, S. Soubatch, F. S. Tautz, U. Höfer, P. Puschnig

Phys. Rev. X Accepted – Published 28. August 2026

Abstract: Excitons, the correlated electron-hole pairs governing optical and transport properties in organic semiconductors, have long resisted direct experimental access to their full quantum-mechanical wave functions. Here, we use femtosecond time-resolved photoemission orbital tomography (trPOT) combining high-harmonic probe pulses with time- and momentum-resolved photoelectron spectroscopy to directly image the momentum-space distribution and ultrafast dynamics of excitons in α-sexithiophene thin films. We introduce a model that enables reconstruction of the exciton wave function in real space, including both its spatial extent and its internal phase structure. The reconstructed wave function reveals coherent delocalization across approximately three molecular units and exhibits a characteristic phase modulation, consistent with ab initio calculations within the framework of many-body perturbation theory. Time-resolved measurements further indicate an approximately 25% contraction of the exciton radius within 400 fs, suggesting self-trapping driven by exciton-phonon coupling. These results establish trPOT as a general and experimentally accessible approach for resolving exciton wave functions—with spatial, phase, and temporal sensitivity—in a broad class of molecular and low-dimensional materials.

(a) Structural model of four monolayers of 6T (yellow) on the Cu(110)-(2 × 1)O substrate in which molecular planes are tilted by 32° with respect to the sample surface (gray, copper atoms; red, oxygen atoms). The normal-incidence pump (blue) and p-polarized probe at 55° incidence (red) pulses as well as the angular distribution of the emitted electrons are illustrated. (b) Chemical structure of 6T. (c) Illustration of the lowest optically allowed exciton state S₁ with the primary HOMO to LUMO and the secondary HOMO−1 to LUMO+1 contributions.

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Last Modified: 04.09.2026