Annealed, thin films of gold were studied by electron microscopy and optical transmission. The particles ranged in size from less than 5 to 100 nm (longest dimensions). The observation of two surface-plasmon peaks was analyzed by treating the particles as oblate spheroids in which the average shape, as determined by optical measurements, is governed by the heat treatment and evaporation thickness. The depolarizing effect of the substrate was clearly demonstrated by shifts in the frequencies of the resonance peaks. Two resonances associated with surface-plasmon modes of oscillation were observed for each of the two axes of the model oblate spheroids.

• Received 27 February 1986

DOI:https://doi.org/10.1103/PhysRevB.34.2246