Abstract
The effect of the size and shape of small particles on plasmon excitations has been investigated, as possible indicators for the physical properties of supported metal catalyst particles. The plasmon frequencies of a small metal sphere, which is covered with an oxide shell and is half embedded in some kind of support, are calculated using hydrodynamic theory as functions of the radius of the particles. The presence of the support introduces new plasmon frequencies. In addition to frequencies close to those for isolated particles, there are three more frequencies if the support is of the same metal as the particle, and one frequency is added if the support is an insulator. All the surface plasmon frequencies show the same type of increase for particle sizes smaller than about 4 nm, but the magnitude of the increase varies considerably. Classical dielectric theory has been used to calculate the surface plasmon excitation frequencies for the supported hemispheres. A general formula has been derived and applied to some cases of special geometry. It has been shown theoretically that only the two neighboring even and odd modes are coupled in the supported hemisphere system. A shift of 3 eV for the surface plasmon for the supported hemisphere metal particle (11 eV) with respect to that in the supported metal sphere particle (8 eV) has been predicted and observed in the specimen Al AlF 3 .
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