Temperature and size dependence of the optical response of small Ag-n clusters

The absorption spectra of small Ag-n(+) clusters are calculated at finite v ibrational temperature by using a microscopic tight-binding RPA method. We consider free clusters with sizes between n = 3 and n = 13 and take into ac count explicitly the degrees of freedom corresponding to the 4d-electrons. We analyze the optical absorption as a function of the cluster size. We sho w that the contribution of the cl-electrons has an important influence on t he size dependence of the energy of the Mie plasmon. We also perform ensemb le averages to obtain the absorption spectra for different vibrational temp eratures. We obtain relatively good agreement with experiment for a tempera ture T similar to 500 K. The dynamics of the 4d-electrons, which shows in s mall clusters an incipient delocalized character for n > 7, yields an impor tant contribution to the absorption spectrum already for n = 13. We find th at the strength of this contribution can be controlled by varying the vibra tional temperature.