SKM 2023 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 34: Ultrafast Electron Dynamics at Surface and Interfaces III

O 34.6: Topical Talk

Dienstag, 28. März 2023, 11:45–12:15, TRE Phy

Photoemission orbital tomography for excitons — •Peter Puschnig, Andreas Windischbacher, Melvin Hodžić, and Christian S. Kern — Institute of Physics, NAWI Graz, University of Graz, Austria

In photoemission orbital tomography (POT), the photoemission angular distribution (PAD) of oriented molecular layers is interpreted in terms of the Fourier transform of the initial molecular orbital from which the electron is emitted. Recently, it has been demonstrated that POT can in principle also be applied to optically excited states using a femtosecond pump-probe setup, which images the excited electron-hole pairs of a system, the excitons, on ultrafast time scales [1]. However, a rigorous and generally applicable connection between the measured PAD and the spatial structure of the excitons is lacking. By considering the expansion of the exciton wave function in the product basis of valence and conduction state orbitals, as typically done when solving the electron-hole Bethe-Salpeter equation or Casida's equations in the framework of TDDFT, we demonstrate in this contribution that the PAD is given by the Fourier transform of a coherent sum of the electronic part of the exciton wave function. This relation, which is based on a plane wave final state, as well as the unexpected consequences of the hole for the measured kinetic energy spectrum an exciton is illustrated for a series of organic molecules in the gas phase for which the PAD is also simulated explicitly, and without resorting to a plane-wave final state, by means of a real-time, real-space TDDFT approach.

[1] Wallauer et al., Science 371, 1056-1059 (2021).