Collective dipole oscillations in small silver clusters embedded in rare-gas matrices

The optical-absorption spectra of small mass-selected ${\mathrm{Ag}}_{\mathit{N}}$ clusters (N=2–21) embedded in solid argon are measured in the energy range 2.5–6.2 eV. Investigation of a continuous range of cluster sizes reveals the size development of the photoabsorption behavior. The main features of the spectra are in agreement with predictions based on the Nilsson-Clemenger shell model. The measured photoabsorption cross sections are consistent with a sum-rule calculation involving only s electrons. In addition, the medium influence on the spectra is studied by changing the matrix gas from argon to krypton and xenon for the cluster sizes N=7, 11, 15, and 21.