SER Scattering by Metal Colloids

Abstract

With respect to their optical properties, metal colloids and other particles of dimensions near the wavelength of light represent an interesting state of division of metals. They lie in size between molecules containing clusters of twenty or more metal atoms [1], which exhibit optical absorption due to one-electron excitation between quantized levels, and bulk metal samples which show collective conduction-electron phenomena. The optical response of colloidal metal particles, containing perhaps 10⁵ atoms, also involves collective electronic motions, but because of the spatial confinement of the electron gas the electronic motions show normal mode behaviour, and the modes (plasma oscillations) may be resonantly excited by light of the appropriate frequency. Because of resistive damping of the induced currents, the particles exhibit absorption of light at the resonance frequencies, and the study of the attractive colours and the mechanism of the absorption shown by finely divided metals, particularly the yellow and wine-red colloidal silver and gold dispersions, has a long history [2]. Interest in the optical properties of small metal particles has recently attracted renewed interest however because of their role in helping establish the significance of metal plasma resonances in the surface-enhanced Raman (SER) effect.