Spectroscopy of extremely cold silver clusters in helium droplets

Abstract.

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.