Photon emission spectra of individual alumina-supported silver clusters have been measured for the first time. The light emission stimulated by electron injection from the tip of a scanning tunneling microscope can be assigned to the $(1,0)$ mode of the Mie-plasmon resonance in small silver particles. As cluster sizes decrease, the resonance position shifts to higher energies and the linewidth increases. In the size range examined (1.5–12 nm), intrinsic size effects are discussed as possible origins for the observed size dependence of the Mie resonance.

• Received 13 December 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.3994