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.

The alloying of Ag and Au has been investigated on the level of individual clusters by analyzing the light emission excited by electron injection from an STM tip. Different Ag-Au alloy and shell-core clusters have been prepared at room temperature on a thin Al 2 O 3 film on NiAl͑110͒ by simultaneous and successive deposition of both noble metals. For simultaneous deposition, one Mie-plasmon resonance has been detected with a wavelength position shifting from the pure Au to the Ag value with increasing Ag content. The results are in agreement with calculations based on Mie theory indicating a complete mixing of both materials. For successive deposition, two Mie resonances have been observed, attributed to plasmon excitations in the shell and core of the clusters. Comparing these results to model calculations, a considerable intermixing of the core and shell materials is concluded, which is especially strong in Au shell-Ag core clusters.

Photon-emission spectra have been measured for individual titania-supported gold particles excited by the electron current from an STM tip. Depending on the interaction strength between tip and cluster, the emission behavior changes from excitations with Mie-plasmon character at 2.3 eV to modes of tip-induced plasmons at 1.8 eV. The strong tip influence on plasmon excitations in Au particles results from the almost constant dielectric function of gold between 1.8 and 2.3 eV, making the plasmon resonance sensitive to small changes in the electromagnetic environment. The lifetime of collective modes, derived from homogeneous linewidths, is governed by dielectric losses in gold, in particular, by gold interband transitions.

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