Size effects in the optical properties of Au_nAg_n embedded clusters

The optical properties of bimetallic silver-gold clusters are investigated in the size range 1.9–2.8 nm. Clusters, produced by laser vaporization of an alloy target Au-Ag (50%-50% atomic composition) are embedded with a low concentration in an alumina matrix. The mixed particles have the same stoichiometry as the rod. Absorption spectra exhibit a plasmon resonance band between those of pure Au and Ag embedded clusters, shifting towards higher energies with decreasing cluster size. Theoretical calculations in the frame of the time-dependent-local-density-approximation, including an inner skin of ineffective screening and the porosity of the matrix, are in good agreement with experimental results.