The theory of Aden and Kerker on the scattering of electromagnetic waves from two concentric spheres has been applied to the interaction of light with two-layer metallic colloids. Starting with gold nuclei 59 Å in ``diameter,'' silver layers 5 to 220 Å in thickness have been grown epitaxially. The optical absorption spectra of these hydrosols containing two-layer particles are compared with the spectra calculated from the theory of Aden and Kerker. The calculated and experimentally observed spectra are in relatively good agreement in the wavelength range 380–700 mμ. Large discrepancies occur between 300 and 380 mμ for hydrosols containing particles with silver layers less than 15 Å. This is attributed to a deviation of the silver refractive index from that of bulk silver for these layer thicknesses. The experimental absorption peaks are generally found to be broader than the calculated results. This is due in part to size and shape anisotropy. Other possible factors are lattice imperfections and trapped ions at the gold—silver interface.

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A common nonequilibrium form exhibited by gold microcrystals grown from solution is a thin (111) platelet of trigonal or hexagonal symmetry. From moiré patterns it has been established that these crystals are essentially free of dislocations if the growth temperature is kept constant. Using both bright- and dark-field electron microscopy and electron diffraction at various crystal orientations it has been shown that the major imperfection is coherent twinning in the large (111) planes of the crystals. The layered growth can be explained on the basis of a twin-accelerated growth mechanism involving multiple twinning on parallel (111) planes. Anomalous ED spots of the type ⅓{4̄22} can be explained on the basis of a nonzero structure facture resulting from a stacking disorder in the (111) planes. An electron microscopic and diffraction study of various stages of growth indicates that multiple twinning occurs very early in the growth process. An intermediate structure is exhibited by the gold platelets during the initial 40% of the growth process. It is speculated that the intermediate phase involves a gold compound of the type Au2X.