  The article explores optical properties of nanostructures containing
spherical gold nanoparticles of various radii. We explore the particle radius
as a criterion to select a permittivity model aimed at describing optical
absorption spectra of gold granules. The experiments showed splitting of the
absorption band of granular gold films to form a second absorption peak. The
first peak is associated with the phenomenon of plasmon resonance, while the
second one reflects quantum hybridization of energy levels in gold. Quantum
effects were shown to prevail over size effects at a granule diameter of about
5-6 nm. The Mie theory gives a rigorous solution for the scattered
electromagnetic field on a sphere taking into account optical properties of the
latter, however, it does not specify criteria of selecting a model to calculate
dielectric permittivity. Both calculations and experiments confirmed the
limiting diameter of gold nanoparticles where the Hampe-Shklyarevsky model is
applied. Meanwhile, this model was still unable to predict splitting of the
plasma absorption band. The data presented in the article can be used for a
predetermined local field enhancement in composite media consisting of a
biolayer and metal nanoparticles.
