Bound-free collective electron excitations in negatively charged silver clusters

We report measurements of the photodestruction cross sections of small sputtered negatively charged silver clusters (Ag-N(-), 5 less than or equal to N less than or equal to 19). Resonance-like peaks are observed in the optical spectra which can be attributed to collective electron excitations. In contrast to neutral and positively charged clusters, the resonance energies are far above the electron emission threshold. With decreasing cluster size the resonance positions distinctly shift towards lower energies. This red-shift is explained by an extension of a semi-empirical model by Liebsch which we previously used to interpret the blue-shift in Ag-N(+) [J. Tiggesbaumker, L. Koller, K.H. Meiwes-Broer and A. Liebsch, Phys. Rev. A 48 (1993) R1749], now the polarizabilities of small metal clusters.