Applicability of a single-particle picture for resonant photoelectron spectroscopy on molecule-metal interfaces

In this paper we discuss how far a single-particle picture can be applied for the description of photoelectron spectroscopy of large $\ensuremath{\pi}$-conjugated molecules on metal substrates measured near a $K$ edge. In this context we focus on particular spectroscopic features, which only exist at the immediate interface and show a constant kinetic energy when the photon energy is varied immediately above a resonance. We demonstrate that the kinetic energy of these features can be calculated on the basis of a single-particle picture for molecule-metal interface systems without charge transfer in the ground state. Similar features are also observed in the case of molecule-metal interfaces which show an occupation of the formerly lowest unoccupied molecular orbital in direct photoelectron spectroscopy as a result of charge transfer in the ground state. However, an analogous calculation of the kinetic energy of these signals fails here, indicating that a single-particle picture is not sufficient for these systems, for which many-body effects have to be adequately taken into account.