Ultrafast Charge-Transfer Exciton Dynamics in C<sub>60</sub> Thin Films
Sebastian Emmerich, Sebastian Hedwig, Benito Arnoldi, Johannes Stöckl, Florian Haag, Ralf Hemm, Mirko Cinchetti, Stefan Mathias, Benjamin Stadtmüller, Martin Aeschlimann
Abstract
thin films depending on the molecular coverage and the light polarization of the optical excitation. Using time- and momentum-resolved photoemission with femtosecond extreme ultraviolet (fs-XUV) radiation, we follow the exciton dynamics in the excited states while simultaneously monitoring the signatures of the excitonic charge character in the renormalization of the molecular valence band structure. Optical excitation with visible light results in the instantaneous formation of charge-transfer (CT) excitons, which transform stepwise into Frenkel-like excitons at lower energies. The number and energetic position of the CT and Frenkel-like excitons within this cascade process are independent of the molecular coverage and the light polarization of the optical excitation. In contrast, the depopulation times of the CT and Frenkel-like excitons depend on the molecular coverage, while the excitation efficiency of CT excitons is determined by the light polarization. Our comprehensive study reveals the crucial role of CT excitons for the excited-state dynamics of homomolecular fullerene materials and thin films.