Surface plasma resonances in small spherical silver and gold particles

With 51 keV electrons surface plasma losses have been investigated on small spherical Ag and Au particles embedded in a medium with dielectric constant? _a=2.37. The surface loss of particles with radii of about 50 Å is found at 2.99 ±0.03 eV for silver and 2.34±0.03 eV for gold being in good agreement with calculated values. For larger radii the loss shifts to higher energy values which agrees qualitatively with the theory of Fujimoto and Komaki for the free electron gas. The optical extinction bands of the particles have been measured, too. Their maxima are shifted to lower energies, in accordance with Mie's theory, if the particle size increases sufficiently. Comparison of the energy values of the “electric” extinction bands with those of the surface plasmons shows, that they correspond in theory and experiment, if the particles are small. This confirms, that the optical colloid extinction of Ag and Au may be interpreted as plasma resonance absorption and emission of the particles.