Tip influence on plasmon excitations in single gold particles in an STM

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.