Room-temperature ionic liquids (RTIL) are important alternatives to aqueous electrolytes in electrocatalytic reactions, batteries, and fuel cells. They are known to reduce existing high overpotentials and increase CO2 solubility as well as product selectivity in CO2 reduction reactions (CO2RR). In our work, we have studied the activity for CO2RR of Au(111), Cu(111), and Cu-modified Au(111) electrodes with 1/3, 2/3, and 3/3 Cu monolayers, as well as of AuCu and AuCu3 intermetallics in contact with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][NTf2] electrolytes with 1.5 M H2O. Using offline gas chromatography (GC), we demonstrate the formation of H-2 and mainly CO as the only reduction products at Au(111), while exclusively H-2 is formed at Cu(111). Synergistic electronic and geometric effects lead to higher levels of CO formation at Cu-modified Au(111) electrodes in comparison to neat Au(111). Operando IR absorption spectroscopy (IRAS) of the bulk electrolyte shows the formation of a 2-imidazolium carboxylic acid intermediate that can lower the overpotential for CO2 reduction and does not require stabilization of a CO2- radical anion as an alternative intermediate at the interface. Systematic variation of the copper content at the catalysts' surfaces enables us to adjust the H-2/CO syngas ratio to a maximum of 1.8 for Cu-modified Au(111) electrodes and similar to 3.2 for AuCu3 catalysts at electrolysis times of 20 min, demonstrating a large tunability of the syngas ratio with electrode potential. The observed range of H-2/CO ratios includes the ideal ratio of 2 for the Fischer-Tropsch process to produce hydrocarbons and the ratio of 3 needed for methanation.

The functionality of organic electronic devices is governed by the dynamics of charge carriers and excited states in organic semiconductors. In particular, the relaxation of excitons and the transfer of charge carriers at metal electrodes crucially determine the performance of organic optoelectronic devices. In a combined experimental study we apply time-resolved photoluminescence and two-photon photoemission to reveal the ultrafast exciton dynamics and charge transfer at prototype organic/metal contacts comprising thin molecular films on single-crystalline noble-metal surfaces. On the basis of experiments with systematically varied film thicknesses, we relate the strong quenching of Frenkel excitons and charge transfer excitons to the wave function overlap with the metal, indicating charge transfer as the dominant relaxation pathway. Moreover, the presence of an electronic interface state is found to facilitate the transfer of excited carriers across the organic/metal interface.

Frontier orbitals determine fundamental molecular properties such as chemical reactivities. Although electron distributions of occupied orbitals can be imaged in momentum space by photoemission tomography, it has so far been impossible to follow the momentum-space dynamics of a molecular orbital in time, for example, through an excitation or a chemical reaction. Here, we combined time-resolved photoemission using high laser harmonics and a momentum microscope to establish a tomographic, femtosecond pump-probe experiment of unoccupied molecular orbitals. We measured the full momentum-space distribution of transiently excited electrons, connecting their excited-state dynamics to real-space excitation pathways. Because in molecules this distribution is closely linked to orbital shapes, our experiment may, in the future, offer the possibility of observing ultrafast electron motion in time and space.

Using time-resolved two-photon photoemission, we systematically study temperature effects on the Shockley-derived unoccupied interface states of perylene- and naphthalene-tetracarboxylic dianhydride (PTCDA and NTCDA) adsorbed on Ag(111). With increasing sample temperatures from 100 to 300 K, we observe a continuous decrease of the interface-state energy by 0.06 and 0.1 eV, respectively, for the two different molecule/metal systems. We relate this energy shift to a molecular lifting off the metal surface. The lifting is accompanied with an increase of the interface-state lifetime from 28 to 53 fs for PTCDA and from 60 to 110 fs for NTCDA. This is in qualitative agreement with the reduction of phase space for electron scattering associated with the thermal energy shift. The magnitude of the experimentally observed lifetime increase, however, is much stronger than would be expected from the energy shift alone.

Charge and energy transport across organic/metal interfaces play a decisive role for the functionality of organic semiconductor devices. As well-defined model systems for such heterointerfaces, we consider thin films of the organic semiconductor perylene-tetracarboxylicacid- dianhydride (PTCDA) deposited on the single-crystalline (111)-surfaces of the noble metals silver and gold. By means of time-resolved photoluminescence we investigate the exciton dynamics of these systems in the energy and time domain. Systematic variation of the PTCDA film thickness enables us to follow the exciton relaxation rates as a function of the molecule-metal separation from several nanometers down to a few angstroms. Spatially localized excitations, such as excimers, are found to relax by nonradiative energy transfer to the metal. In contrast, the relaxation of charge-transfer (CT) excitons can be explained by exciton diffusion and subsequent annihilation at the organic/metal interface. Both mechanisms are found to be much more efficient on Ag(111) than on Au(111). For excimers, the faster relaxation on the silver substrate presumably involves the excitation of intraband transitions inside the metal. The higher relaxation rate of CT excitons is explained in terms of enhanced charge transfer across the PTCDA/Ag(111) interface, which is mediated by the electronic interface state inherent to this organic/metal interface.

The presence of a surrounding medium strongly affects the spectral properties of localized surface plasmons at metallic nanoparticles. Vice versa, plasmonic resonances have large impact on the electric polarization in a surrounding or supporting material. For applications, e.g., in light-converting devices, the coupling of localized surface plasmons with polarizations in semiconducting substrates is of particular importance. Using photoemission electron microscopy with tunable laser excitation, we perform single-particle spectroscopy of silver nanoclusters directly grown on Si(100). Two distinct localized surface plasmon modes are observed as resonances in the two-photon photoemission signals from individual silver clusters. The strengths of these resonances strongly depend on the polarization of the exciting electric field, which allows us to assign them to plasmon modes with polarizations parallel and perpendicular, respectively, to the supporting silicon substrate. Our mode assignment is supported by simulations which provide insight into the mutual interaction of charge oscillations at the particle surface with electric polarizations at the silver/silicon interface.

The unoccupied electronic structure of stacked layers of copper(II)phthalocyanine (CuPc) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on Ag(1 1 1) has been investigated by means of two-photon photoemission (2PPE). We find a rich electronic structure comprising at least five unoccupied electronic states which we identify based on their energetic position and their dispersion in momentum space. More specifically, we observe the first and the second image-potential states of the modified Ag(1 1 1) surface, as well as the metal-organic interface state (IS) inherent to the PTCDA/Ag(1 1 1) interface. Moreover, two additional molecular features are observed for the CuPc/PTCDA/Ag(1 1 1) system which we attribute to an unoccupied molecular orbital (LUMO + 2) of CuPc. The 2PPE intensity of the IS exhibits a pronounced dependence on the pump photon energy, which closely follows the optical absorption of the outer molecular layer. This strongly points to charge transfer from the optically excited molecules to the interface state.

The feasibility to couple surface plastrons with molecular excitations is an intriguing feature of plasmonic-organic hybrid systems. To date, investigations of plasmonic-excitonic coupling have largely focused on ensembles of nanoparticles and on purely optical methods. Here we present a single-particle approach based on laser-spectroscopic photoemission electron microscopy. Localized surface plasmons give rise to photoemission resonances in laser excitation spectra of individual silver nanoclusters. As a most striking manifestation of plasmon excitor coupling, upon deposition of a thin zinc tetraphenylporphyrin (ZnTPP) film, a second resonance near the ZnTPP Soret band appears in our spectra. In accordance with simulations, spectral repulsion of both resonances as well as intensity reditribution between both modes indicates their plasmonic-excitonic hybrid character.

In der vorliegenden Arbeit werden dunne Porphyrinschichten und plasmonische Silberstrukturen mitMehr-Photonen-Photoemissionselektronenmikroskopie (nP-PEEM) untersucht. Der erste Teil der Arbeit befast sich mit optischen und elektronischen Eigenschaften dunner Porphyrinschichten in Abwesenheit plasmonischer Anregungen. In elektronenspektroskopischen Untersuchungen der Mehr-Photonen-Photoemission (nPPE) dieser Schichten ergeben sich Energieverteilungen der Photoelektronen, die sich nicht in einem traditionellen Modell erklaren lassen, welches auf Untersuchungen mit Ultraviolett-Photoemissionsspektroskopie (UPS) basiert. Aufbauend auf den Ergebnissen aus energie-, zeit- und ortsaufgelosten nPPEUntersuchungen der Porphyrinschichten wird ein alternatives Modell entwickelt, das eine konsistente Beschreibung der komplexen elektronischen Anregungs- und Relaxationsprozesse innerhalb der Schicht ermoglicht. Der zweite Teil der Arbeit hat plasmonische Anregungen in Silberstrukturen zum Thema. Dabei werden sowohl rein plasmonische Anregungen als auch Plasmon-Exziton-Wechselwirkungen in Porphyrin/Silber-Hybridstrukturen untersucht. Propagierende Oberflachenplasmonen werden mit nP-PEEM auf einer ausgedehnten Silberinsel abgebildet. Eine Auswertung des im PEEM-Bild beobachteten Interferenzmusters, welches durch Uberlagerung des plasmoninduzierten elektrischen Nahfeldes mit dem elektrischen Feld der anregenden Laserstrahlung entsteht, ermoglicht es, die Dispersion des Oberflachenplasmons auf der Silberoberflache zu bestimmen. Mit der gleichen Methode wird der Einflus einer dunnen Porphyrinschicht auf die Plasmonendispersion untersucht. Der Verlauf der Dispersionskurve weicht fur Anregungswellenlangen in der Nahe der molekularen Soret-Bande vom Verlauf fur eine rein plasmonische Anregung ab, was einer Kopplung des Oberflachenplasmons mit der exzitonischen Anregung in der Porphyrinschicht zugeschrieben wird. Lokalisierte Oberflachenplasmonen (LSP) werden mit nP-PEEM auf Silbernanopartikeln untersucht. Dazu wird eine Methode entwickelt, welche quantitative Laseranregungsspektroskopie der nPPE von Einzelpartikeln ermoglicht. Die Natur der LSP-Resonanzen, welche in den Anregungsspektren der Partikel beobachtet werden, wird durch Vergleich der experimentellen Spektren mit Simulationen aufgeklart. Der Einflus dunner Porphyrinschichten auf die LSP-Resonanzen wird mit Laseranregungsspektroskopie untersucht, mit dem Ergebnis, das die Porphyrinschichten eine spektrale Verschiebung der Partikelresonanzen verursachen. Simulationen sagen fur die Porphyrin/Silber-Hybridstrukturen eine weitere, porphyrinabgeleitete LSP-Resonanz voraus, welche sich unter geeigneten Bedingungen auch experimentell beobachten last.%%%%Thin porphyrin films and plasmonic silver structures are investigated with multi-photon photoemission electronmicroscopy (nP-PEEM). The first part of this work deals with optical and electronic properties of thin porphyrin films in absence of plasmonic excitations. Multiphoton photoelectron spectroscopy…

Dye molecules like porphyrins alter their optical properties upon condensation to a solid film. Besides intermolecular interactions, specific substrates can influence their optical characteristics, especially if the molecular film is in contact to plasmonic nanostructures. We apply multiphoton photoemission electron microscopy (nP-PEEM) with tunable laser excitation to the laterally resolved spectroscopy of magnesium-tetraphenylpoiphyrin (MgTPP) films deposited on nanostructured silver substrates. The high molecular specificity of nP-PEEM is demonstrated by the observation of a strong resonance at about 425 nm caused by optical excitation of the S-0 -> S-2 transition (Soret band) of the MgTPP molecules. This molecular excitation gives rise to remarkably strong three-photon photoemission. The spectral position of the Soret resonance in our nP-PEEM laser spectra points to reduced excitonic coupling between the surface molecules of the MgTPP film. By comparison of the photoemission intensities from MgTPP on nanostructured and unstructured silver regions, we conclude noticeable plasmon-mediated enhancement of photoemission at resonant Soret excitation even under off-resonance conditions for the localized surface plasmons (LSPs). Combining molecular specificity with high sensitivity to field enhancements, we demonstrate that multiphoton PEEM is an excellent tool for the investigation of solid dye films and their interaction with plasmonic substrates.