The structural composition and its influence on the optical properties of gold blacks and gold smokes

A model is developed for the optical properties of gold blacks and gold smokes within the realm of effective medium theory. This model assimes that the strands of gold black can be approximated by ellipsoids for which the depolarization factor is well known from electrostatic theory. This model is applied to several gold blacks and smokes produced by inert gas evaporation- It is found that excellent agreement between experimental and theoretical transmittance can be achieved in the solar spectrum using a distribution of ellipsoids (gold black strands) that is closely approximated by a log-normal function of the ellipsoid semimajor to semiminor axis ratio. An interpretation of the physical significance of this distribution is proposed. Throughout this work, the utmost attention has been placed on the physical significance of the model and mere curve fitting has been avoided as much as possible. Experimental results for parameters such as packing factor and sample thickness were used without modification. Experimental data for the optical properties of bulk gold was carefully examined and the only significant modification that was found was the need to account for an increase in the electron collision frequency due toscattering from the surfaces of the small gold crystallites. This size effect was accounted for on the basis of particle size measurements made using transmission electron micrographs and X-ray particle size broadening. The model developed was compared to the capacitor theory of the optical properties of gold blacks and was found to be a generalization of the latter. It was also compared to a recently developed model for the optical properties of gold smokes and it was found to be more consistent with experimental findings than the gold smoke model.