Spectroscopy of extremely cold silver clusters in helium droplets

The first highly resolved electronic spectra of small non-alkali metal clus ters embedded in nanoscopic helium droplets (N approximate to 20000) are pr esented. 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-se lective spectroscopy on a silver cluster distribution (N less than or equal to 10). Results are reported for Ag-2, Ag-3 and for Ag-8. For the latter, a very narrow absorption spectrum is resolved corresponding to the Ag-8 pla smon 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 Ag-8 resonance is estimated to be of the order of nanoseconds, which is inconsi stent with the plasmon picture and typical for molecular transitions.