Optical Properties of Small Gold Particles

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Abstract
The optical absorption of dispersions of gold particles is studied as a function of particle size, temperature, and method of preparation. The absorption spectra for gold sols in glass and water with small spread in particle size are compared to spectra calculated from Mie theory and the most reliable of recently measured optical constants of gold. The comparisons indicate that the real part of the dielectric constant ε1 for particles from 85 to 200 Å in diameter is the same as for bulk gold, and that the imaginary part ε2 is similar for particles and bulk, although some discrepancies exist. Gold sols in glass have optical properties closer to bulk gold than sols in water, probably because of defects and adsorbed material on the particles in water. Between wavelengths of about 0.40 and 0.45 μ the optical properties of the particles in glass are unchanged down to particles containing very few atoms. The optical absorption peak at 0.525 μ is unaffected by particle size down to a particle diameter of about 85 Å, showing that this absorption cannot be described by the simple free‐electron model. This inadequacy of the model in this wavelength region was also deduced from the bulk optical constants. Below a diameter of about 85 Å the absorption peak is proportional to the particle diameter, as required by the free electron theory for particles much smaller than the mean free path of the electrons, but quantitative agreement with this theory is lacking. The absorption peak is shifted slightly with temperature because of the small change in electron concentration, and the peak is broadened as the temperature is increased for particles both larger and smaller than 85 Å in diameter.