Speaker
Description
Photoemission orbital tomography (POT) is a powerful technique, by which the electron distribution of orbitals of well-ordered molecules at solid surfaces can be imaged in momentum space [1]. Recently, we combined the method with laser pump-probe techniques to investigate the dynamics of charge transfer processes at molecular interfaces [2,3]. In this talk I will discuss how time-resolved POT (tr-POT) can image the momentum-space distribution and temporal evolution of molecular excitons [4]. These bound states of electrons and holes govern light-matter interactions in organic semiconductors, yet their full quantum mechanical wavefunctions have remained experimentally elusive.
For the model system α-sexithiophene (6T) on a Cu(110)-p(2x1)O surface, we determine a spatial extent of 9 Å for the exciton. It is seen to span about three neighboring molecules with a distinct phase modulation. From the temporal evolution of the recorded photoemission momentum maps, we derive a reduction of size of the exciton by about 25% during its 420-fs lifetime, which we attribute to self-trapping [4]. Our results resolve a long-standing debate on the exciton character in organic semiconductors and establish tr-POT as a general method to experimentally access exciton wavefunctions with spatial, phase and time resolution.
References
[1] P. Puschnig et al., Science, 326, 702 (2009)
[2] R. Wallauer et al., Science, 371, 1056 (2021)
[3] A. Adamkiewicz et al., J. Phys. Chem. C., 127, 20411 (2023)
[4] M. Theilen et al., arXiv:2511.23001 (2025)