Optical properties of aggregated metal systems: Interband transitions

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Abstract
In previous publications we reported on the optical properties of aggregated metal systems, where the metal particles were assumed to be free-electron type with damping, and where they were assumed to be dispersed in a dielectric medium with a varying index of refraction. The main results were the shift of bulk and surface plasma frequencies with aggregation density and the appearance of an extra resonance, called the optical conduction resonance (OCR), on the low-energy side of the plasma frequency. The OCR was shown to be a transverse collective mode of the free electrons in the aggregates which could be excited by incident light. In the present work the effects of electronic interband transitions on the free-electron OCR and the free-electron plasma frequencies are examined. Important new results are obtained including frequency shifts and splitting of the OCR absorption peak and the loss function by interband transitions. These results are used in the analysis of the dielectric functions of some real metals in aggregated form.

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