One of the key milestones in advancing the performance of molecular electronic and photonic devices is to gain a comprehensive understanding of the electronic properties and rich excited state dynamics of this class of materials. In this context, momentum-resolved photoemission in combination with photoemission orbital tomography (POT) has been established as a powerful tool to study the band structure of molecular films and to reveal the degree of localization of molecular valence orbitals by their characteristic emission pattern in momentum space.

In this contribution, we exploit these capabilities of POT to study the valence and excited states of fullerenes grown on noble metal surfaces. For the most prototypical fullerene, the buckyball C$_{60}$, we will show that the valence states show signatures of an atomic crystal-like band structure with delocalized $\pi$- and localized $\sigma$-orbitals [1]. This observation differ significantly from our results for thin films of the endohedral fullerene Sc$_3$N@C$_{80}$, where the valence states are strongly localized on the carbon cage of the molecules.

Finally, we provide a first insight into the momentum space signatures of the excited state dynamics of C$_{60}$ thin films obtained by time-resolved two-photon momentum microscopy. For an optical excitation with $3.1\,$eV photons, we are able to identify three characteristic emission patterns even in the small momentum space range accessible by our experiment. These signatures are discussed in the context of the recently proposed charge transfer and Frenkel exciton character of these states [2].

[1] N. Haag, D. Lüftner, F. Haag, J. Seidel, L. Kelly, G. Zamborlini, M. Jugovac, V. Feyer, M. Aeschlimann, P. Puschnig, M. Cinchetti, B. Stadtmüller. Signatures of an atomic crystal in the band structure of a C60 thin film, Phys. Rev. B 101, 2020.

[2] B. Stadtmüller, S. Emmerich, D. Jungkenn, N. Haag, M. Rollinger, S. Eich, M. Maniraj, M. Aeschlimann, M. Cinchetti, S. Mathias. Strong modification of the transport level alignment in organic materials after optical excitation, Nat. Commun. 10, 2019.

The photoemission orbital tomography (POT) technique, a variant of angle-resolved photoemission spectroscopy, has been very useful in the characterization of the electronic properties of molecular films. It is a combined experimental and theoretical approach that is based on the interpretation of the photoelectron angular distribution in terms of a one-electron initial state. This includes the unambiguous assignment of emissions to specific molecular orbitals, their reconstruction to real space orbitals in two and three dimensions, the deconvolution of complex spectra into individual orbital contributions beyond the limits of energy resolution, the extraction of detailed geometric information such as molecular orientations, twists and bends, the precise description of the charge balance and transfer at interface, and the detection of momentum-selective hybridization with the substrate, to name only a few examples. In its simplest form, POT relies on the plane-wave approximation for the final state. While this works surprisingly well in many cases, this approximation does have its limitations, most notably for small molecules and with respect to the photon-energy dependence of the photoemission intensity. Regarding the latter, a straightforward extension of the plane wave final state leads to a much-improved description while preserving the simple and intuitive connection between the photoelectron distribution and the initial state.

Charge transfer across molecular interfaces is reflected in the population of electronic orbitals. For ordered organic layers, time-resolved photoemission orbital tomography (tr-POT) is capable of spectroscopically identifying the involved orbitals and deducing their population from the measured angle-resolved photoemission intensity with high temporal resolution [1]. As examples, I will present recent results obtained for PTCDA and CuPc adsorbed on Cu(100)-2O. We observe two distinct excitation pathways with visible light. While the parallel component of the electric field makes a direct HOMO-LUMO transition, the perpendicular component can transfer a substrate electron into the molecular LUMO. The experimental data are modelled by a density matrix description of the excitation and photoemission process. We find similar LUMO lifetimes for both excitation pathways, whereas the true dephasing times differ by two orders of magnitude.

Future tr-POT experiments will employ a two-pulse coherent control excitation scheme to steer the charge transfer. In some cases, this scheme will allow us to deduce the relative phase of the involved orbitals directly from the experiment. Furthermore, the combination with strong THz excitation and subcycle time resolution will make it possible to monitor charge transfer processes and hybridization during surface bond formation with POT.

[1] R. Wallauer, M. Raths, K. Stallberg, L. Münster, D. Brandstetter, X. Yang, J. Güdde, P. Puschnig, S. Soubatch, C. Kumpf, F. C. Bocquet, F. S. Tautz, U. Höfer. Tracing orbital images on ultrafast time scales, Science 371: 1056-1059, 2021. https://doi.org/10.1126/science.abf3286

Time-resolved photoemission orbital tomography is a promising technique for the characterization of light-matter interaction in organic semiconductors. However, its state-of-the-art analysis approach based on density functional theory and the plane-wave model of photoemission cannot account for the correlated many-body nature of excitonic wavefunctions, which nevertheless represent the dominant optoelectronic response of organic semiconductors. Building on the many-body interaction formalisms of the $GW$ approach and the Bethe-Salpeter equation, we present a complete description of the angle-resolved exciton photoemission spectrum, and apply this model to the exemplary exciton relaxation cascade in multilayer C$_{60}$ crystals to investigate an intriguing property of the excitonic wavefunction: In C$_{60}$, and more generally in organic semiconductors, excitons can be of multiorbital nature, with both the electron and hole spread over multiple orbitals. We elucidate how photoemission orbital tomography is uniquely sensitive to this multiorbital nature and exploit it to directly access the hole part of the excitonic wavefunction in addition to its electron counterpart. With this capability, exciton photoemission orbital tomography provides a versatile probe of key exciton properties such as localization, charge transfer, and relaxation dynamics.