Angle-resolved photoemission spectroscopy has been developed to a very high accuracy. However, effects that depend sensitively on the state of the emitted photoelectron were so far hard to compute ...for real molecules. We here show that the real-time propagation approach to time-dependent density functional theory allows us to obtain final-state effects consistently from first principles and with an accuracy that allows for the interpretation of experimental data. In a combined theoretical and experimental study we demonstrate that the approach captures three hallmark effects that are beyond the final-state plane-wave approximation: emission perpendicular to the light polarization, circular dichroism in the photoelectron angular distribution, and a pronounced energy dependence of the photoemission intensity.
Charge carrier mobilities in molecular condensates are usually small, as the coherent transport, which is highly effective in conventional semiconductors, is impeded by disorder and the small ...intermolecular coupling. A significant band dispersion can usually only be observed in exceptional cases such as for π-stacking of aromatic molecules in organic single crystals. Here based on angular resolved photoemission, we demonstrate on the example of planar π-conjugated molecules that the hybridization with a metal substrate can substantially increase the delocalization of the molecular states in selective directions along the surface. Supported by ab initio calculations we show how this mechanism couples the individual molecules within the organic layer resulting in an enhancement of the in-plane charge carrier mobility.
Several experimental methods allow measuring the spatial probability density of electrons in atoms, molecules and solids, that is, the absolute square of the respective single-particle wave function. ...But it is an intrinsic problem of the measurement process that the information about the phase is generally lost during the experiment. The symmetry of this phase, however, is a crucial parameter for the knowledge of the full orbital information in real space. Here, we report on a key experiment that demonstrates that the phase symmetry can be derived from a strictly experimental approach from the circular dichroism in the angular distribution of photoelectrons. In combination with the electron density derived from the same experiment, the full quantum mechanical wave function can thus be determined experimentally.
We present a comprehensive study of the adsorption behavior of iron phthalocyanine on the low-index crystal faces of silver. By combining measurements of the reciprocal space by means of ...photoelectron momentum mapping and low energy electron diffraction, the real space adsorption geometries are reconstructed. At monolayer coverage ordered superstructures exist on all studied surfaces containing one molecule in the unit cell in case of Ag(100) and Ag(111), and two molecules per unit cell for Ag(110). The azimuthal tilt angle of the molecules against the high symmetry directions of the substrate is derived from the photoelectron momentum maps. A comparative analysis of the momentum patterns on the substrates with different symmetry indicates that both constituents of the twofold degenerate FePc lowest unoccupied molecular orbital are occupied by charge transfer from the substrate at the interface.
•We observe ordered structures of FePc on Ag(100), Ag(110) and Ag(111).•The superstructure unit cells and the arrangement of FePc are derived.•We find a charge transfer to both of the degenerate LUMOs.
Fascinating pictures that can be interpreted as showing molecular orbitals have been obtained with various imaging techniques. Among these, angle resolved photoemission spectroscopy (ARPES) has ...emerged as a particularly powerful method. Orbital images have been used to underline the physical credibility of the molecular orbital concept. However, from the theory of the photoemission process it is evident that imaging experiments do not show molecular orbitals, but Dyson orbitals. The latter are not eigenstates of a single-particle Hamiltonian and thus do not fit into the usual simple interpretation of electronic structure in terms of molecular orbitals. In a combined theoretical and experimental study we thus check whether a Dyson-orbital and a molecular-orbital based interpretation of ARPES lead to differences that are relevant on the experimentally observable scale. We discuss a scheme that allows for approximately calculating Dyson orbitals with moderate computational effort. Electronic relaxation is taken into account explicitly. The comparison reveals that while molecular orbitals are frequently good approximations to Dyson orbitals, a detailed understanding of photoemission intensities may require one to go beyond the molecular orbital picture. In particular we clearly observe signatures of the Dyson-orbital character for an adsorbed semiconductor molecule in ARPES spectra when these are recorded over a larger momentum range than in earlier experiments.
The adsorption of aromatic molecules on metal surfaces leads to a complex reorganization of the molecular and metal wave functions. Various processes such as charge transfer, hybridization between ...molecular and metallic states, and the formation of dispersing bands within the interface have been demonstrated fororganometallic interface systems. For the model molecule 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), we compare highly ordered monolayers on Ag(110) and Cu(100), which allows us to identify changes of the interfacial electronic structure when altering the coupling strength with the substrate by means of angle-resolved photoelectron spectroscopy. The stronger coupling to the Ag(110) substrate goes along with a shorter photohole lifetime and a stronger hybridization of the NTCDA lowest unoccupied molecular orbital with metal states. Supported by ab initio calculations, we show that the observed band dispersion is greatly enhanced due to the interaction with Ag(110) while the laterally denser adsorption geometry of NTCDA on Cu(100) entails a larger intermolecular wave-function overlap, and the presence of the substrate results in no further bandwidth enhancement.
We report the discovery of a charge transfer (CT) related low binding energy feature at a molecule-metal interface by the application of resonant photoelectron spectroscopy (RPES). This interface ...feature is neither present for molecular bulk samples nor for the clean substrate. A detailed analysis of the spectroscopic signature of the low binding energy feature shows characteristics of electronic interaction not found in other electron spectroscopic techniques. Within a cluster model description this feature is assigned to a particular eigenstate of the photoionized system that is invisible in direct photoelectron spectroscopy but revealed in RPES through a relative resonant enhancement. Interpretations based on considering only the predominant character of the eigenstates explain the low binding energy feature by an occupied lowest unoccupied molecular orbital, which is either realized through CT in the ground or in the intermediate state. This reveals that molecule-metal CT is responsible for this feature. Consequently, our study demonstrates the sensitivity of RPES to electronic interactions and constitutes a new way to investigate CT at molecule-metal interfaces.
Graphene's peculiar electronic band structure makes it of interest for new electronic and spintronic approaches. However, potential applications suffer from quantization effects when the spatial ...extension reaches the nanoscale. We show by photoelectron spectroscopy on nanoscaled model systems (disc-shaped, planar polyacenes) that the two-dimensional band structure is transformed into discrete states which follow the momentum dependence of the graphene Bloch states. Based on a simple model of quantum wells, we show how the band structure of graphene emerges from localized states, and we compare this result with ab initio calculations which describe the orbital structure.
The experimental imaging of electronic orbitals has allowed one to gain a fascinating picture of quantum effects. We here show that the energetically high-lying orbitals that are accessible to ...experimental visualization in general differ, depending on which approach is used to calculate the orbitals. Therefore, orbital imaging faces the fundamental question of which orbitals are the ones that are visualized. Combining angular-resolved photoemission experiments with first-principles calculations, we show that the orbitals from self-interaction-free Kohn-Sham density functional theory are the ones best suited for the orbital-based interpretation of photoemission.
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