Spectroscopy of extremely cold silver clusters in helium droplets

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The first highly resolved electronic spectra of small non-alkali metal clusters embedded in nanoscopic helium droplets (N≈20000) are presented. The helium droplets serve as an extremely cold liquid matrix with a temperature of 0.37 K. Resonant two-photon-ionization is used for size-selective spectroscopy on a silver cluster distribution (N≤10). Results are reported for Ag2, Ag3 and for Ag8. For the latter, a very narrow absorption spectrum is resolved corresponding to the Ag8 plasmon resonance. The linewidth of the observed resonance is a factor of two smaller than theoretical zero-temperature predictions for the plasmon line width of closed-shell metal clusters. In contrast, the lifetime of the Ag8 resonance is estimated to be of the order of nanoseconds, which is inconsistent with the plasmon picture and typical for molecular transitions.