Optical absorption of ultrafine metal spheres with dielectric cores

We compute the optical absorption of a glass containing a small fraction of silver particles with dielectric cores. The results are based on the Maxwell Garnett formalism. The dielectric permeability of the metallic shells is obtained by modifying experimental data for bulk silver to account for size dependent scattering of the conduction electrons. We find an absorption maximum in the visible range at a wavelength which, for sufficiently minute particles, depends rather strongly on the size of the cores and on their dielectric constant. The relevance of these results to recent experimental data by Smithard and by Genzel et al. is pointed out. We argue that their observed absorption maxima can be explained without invoking any quantum effects.