Time-resolved photoemission from mass-selected metal clusters and their complexes with biomolecules at insulating surfaces

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Adding or removing one atom changes the physical and chemical properties of small metal clusters drastically. This is particularly true for small silver clusters with up to about 20 atoms. Especially the optical properties of supported and free atomic clusters of silver are intriguing and a strong current interest in this particular cluster system is due to two recent observations: (i) Irradiation of silver oxide particles leads to the photo-induced formation of stable distributions of surface-supported atomic silver clusters; (ii) a strong visible fluorescence was recently found, originating from unprecedented long-lived excited molecular states in atomic silver clusters. These properties uniquely qualify the small atomic silver clusters as a basis for advanced high-density optical storage (via (i)) and read-out (via (ii)) devices. However, neither the detailed electronic origin and cluster size dependence of the observed fluorescence is known up to now, nor any experimental data about the mechanism of the photoreductive formation of silver clusters. In order to get an insight into these fascinating properties of atomic silver clusters, we intend to apply the technique of time-resolved photoemission spectroscopy to a well-defined cluster-support system consisting of mass-selected silver clusters soft-landed onto an atomically flat insulating thin film of magnesia. The goal is to determine the electronic level structure of the supported silver clusters as a function of cluster size as well as the corresponding excited state lifetimes, in order to identify possible radiative and non-radiative transitions.