Electronic and optical properties of polyanionic metal clusters

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The physical and chemical properties of finite systems often scale in a simple way with the particle size and show characteristic trends. In contrast, very small clusters show distinct quantum-size effects when changing the number of atoms. Recently developed experimental methods for generating mass-selected, polyanionic metal clusters in digital radio frequency traps have opened up opportunities to study (metallic) nanosystems on not yet accessible potential surfaces. An exciting aspect is the formation of a repulsive Coulomb potential as a characteristic feature, whereby a barrier is formed by the interplay between charge repulsion and electron-electron correlations. Binding energies above the vacuum level allows for a relaxation of the complex by electron tunneling through the barrier. In contrast to molecules and fullerenes, the photo-induced dynamics of polyanionic systems with delocalized electrons has not been studied in detail by experiment and theory so far. In addition to the electronic structure and possible anisotropies of the Coulomb barrier, the project focuses on the optical activity of polyanionic metal clusters with emphasis on thecollective excitations. Currently, the decay of the plasmon mode and the coupling to single-electron levels in nanoparticles is still under discussion. We will conduct experiments using tunable laser pulses, as well as the pump-probe technique in order to study the physical properties of polyanionic metal clusters, using angular-resolved spectroscopy as an extension of the already existing electrontime-of-flight diagnostics. We aim to characterize the electronic level structure, the optical response and the ultrafast dynamics. Tunneling and internal relaxation are relevant decay pathways, which have to be considered in the photoemission process. In the experiments we concentrate on copper, silver and gold clusters as they represent model systems due to their isoelectronic structure. Due to their plasmonic properties, the noble metal clusters are also interesting for technical applications.