Using a modified mode-coupling approach, incorporating exchange and correla
tion contributions due to the electron-electron interaction, the optical ab
sorption of an ensemble of nanoparticles in the presence of microscopic dis
order is calculated. It is shown, that microscopic disorder generates a sig
nificant broadening and redshift of all resonances in the regime where fluc
tuations of the transitions induced by disorder exceed the electron-electro
n correlation energy. At lower impurity concentrations, the electron-electr
on interaction can effectively narrow the resonance lines close to the band
gap of the nanoparticles. In the latter regime the absorption spectrum loo
ks like the absorption spectrum of a high-quality bulk semiconductor with t
he difference that the level spacing between resonances is enlarged due to
quantum-size effects. This spectral feature is measured in a system consist
ing of GaAs nanoparticles in a crystalline AlAs matrix as grown by molecula
r beam epitaxy, while in the former regime also the lowest resonances are s
trongly broadened as usually measured by other methods. (C) 1999 Acta Metal
lurgica Inc.