Optical properties of metal aggregates in Li+ and K+ irradiated MgO crystals

Optical absorption spectra of MgO crystals irradiated with high energy (250 to 450 keV) alkali ions (Li+, K+) present two bands of colloidal nature after thermal annealing of the crystal. Mie theory cannot represent all the features of this behaviour which are related to the very high dose (5 x 1016 to 1017 ions/cm2) possible to get with such a technique (up to 1022 cm-3 implanted ions). The results are interpreted using Maxwell-Garnett theory applied to a granular metal film embedded in a dielectric host and it is possible to attribute the two observed absorption bands to plasma resonance and optical conduction absorption of the metal precipitate. (author)