Mathias Richter

In the past decade, photoemission orbital tomography (POT) has evolved into a powerful tool to investigate the electronic structure of organic molecules adsorbed on surfaces. Here we show that POT allows for the comprehensive experimental identification of all molecular orbitals in a substantial binding energy range of more than 10 eV. Making use of the angular distribution of photoelectrons as a function of binding-energy, we exemplify this by extracting an orbital-resolved projected density of states for 15 π and 23 σ orbitals from the experimental data of the prototypical organic molecule bisanthene ( C 28 H 14 ) on a Cu(110) surface. These experimental results for an essentially complete set of orbitals within the given binding-energy range serve as stringent benchmarks for electronic structure methods, which we illustrate by performing density functional calculations employing four frequently used exchange-correlation functionals. By computing the respective molecular-orbital-projected densities of states, a one-to-one comparison with experimental data for an unprecedented number of 38 orbital energies became possible. The quantitative analysis of our data reveals that the range-separated hybrid functional HSE performs best for the investigated organic/metal interface. At a more fundamental level, the remarkable agreement between the experimental and the Kohn-Sham orbital energies over a binding-energy range larger than 10 eV suggests that—perhaps unexpectedly—Kohn-Sham orbitals approximate Dyson orbitals, which would rigorously account for the electron extraction process in photoemission spectroscopy but are notoriously difficult to compute, in a much better way than previously thought.

The Physikalisch-Technische Bundesanstalt (PTB) is the national metrology institute of the Federal Republic of Germany and has been using synchrotron radiation (SR) for more than 40 years. Starting at the former facility BESSY I, the storage ring BESSY II (see corresponding article in this volume) has been used for this purpose since 1998, especially in the X-ray range, as well as PTB's own metrology light source (MLS) for lower photon energies since 2008. Both facilities are located in Berlin-Adlershof and are operated by the Helmholtz-Zentrum Berlin within the framework of contractual agreements with PTB. The basic motivation of the work is radiometry. The two storage rings MLS and BESSY II are used as primary source standards and cryogenic radiometers as primary detector standards. The article gives a brief overview of the present status, scientific highlights, the technological development and upgrade plans for the decade 2023–2033 as well as a perspective of next scientific challenges.

The dependency on the excitation energy of ultrafast multi-photon ionization of xenon by intense, short extreme ultraviolet pulses (XUV) was investigated in the vicinity of the 4$d$ 'giant' resonance using ion time-of-flight spectroscopy. The yields of the high charge states of xenon show strong variations with the excitation energy. With reference to simulated absorption spectra, we can link the photon energy dependency to resonance structures of single-electron excitations mainly in the xenon N-shell and purely sequential multi-photon absorption.

Tracing the modifications of molecules in surface chemical reactions benefits from the possibility to image their orbitals. While delocalized frontier orbitals with π character are imaged routinely with photoemission orbital tomography, they are not always sensitive to local chemical modifications, particularly the making and breaking of bonds at the molecular periphery. For such bonds, σ orbitals would be far more revealing. Here, we show that these orbitals can indeed be imaged in a remarkably broad energy range and that the plane wave approximation, an important ingredient of photoemission orbital tomography, is also well fulfilled for these orbitals. This makes photoemission orbital tomography a unique tool for the detailed analysis of surface chemical reactions. We demonstrate this by identifying the reaction product of a dehalogenation and cyclodehydrogenation reaction.

Metalation and self‐metalation reactions of porphyrins on oxide surfaces have recently gained interest. The mechanism of porphyrin self‐metalation on oxides is, however, far from being understood. Herein, we show by a combination of results obtained with scanning tunneling microscopy, photoemission spectroscopy, and DFT computations, that the self‐metalation of 2H‐tetraphenylporphyrin on the surface of ultrathin MgO(001) films is promoted by charge transfer. By tuning the work function of the MgO(001)/Ag(001) substrate, we are able to control the charge and the metalation state of the porphyrin molecules on the surface.

Das Interesse an Metallierungs‐ und Selbstmetallierungsreaktionen auf Oxidoberflächen ist in jüngster Zeit ständig gewachsen. Der Mechanismus der Selbstmetallierungsreaktion ist jedoch nicht vollständig geklärt. Hier zeigen wir mithilfe von Rastertunnelmikroskopie, Photoemissions‐Spektroskopie und Dichtefunktionaltheorie‐Rechnungen, dass die Selbstmetallierung von 2H‐Tetraphenylporphyrin auf der Oberfläche von ultradünnen MgO(001)‐Filmen durch Ladungstransfer ermöglicht wird. Es wird gezeigt, dass der Ladungszustand und dadurch der Metallierungszustand der Porphyrin‐Moleküle durch die Austrittsarbeit des MgO(001)/Ag(001)‐Substrats gezielt eingestellt werden können.