Photoabsorption of AgN(N ∼ 6-6000) Nanoclusters Formed in Helium Droplets: Transition from Compact to Multicenter Aggregation

AgN clusters with up to thousands of atoms were grown in large He droplets and studied by optical spectroscopy. For N <= 10(3) the spectra are dominated by a surface plasmon resonance near 3.8 eV and a broad feature in the UV, consistent with absorption by individual metallic particles. Larger clusters reveal unexpectedly strong broad absorption at low frequencies, extending down to approximate to 0.5 eV. This suggests a transition from single-center to multicenter formation, in agreement with estimates of cluster growth kinetics in He droplets. Moreover, the spectra of large clusters develop a characteristic dispersion profile at 3.5-4.5 eV, indicative of the coexistence of localized and delocalized electronic excitations in composite clusters, as predicted theoretically.