Optical Properties of Si-n(+) (n=6-100) Clusters: The Approach to Bulk Behavior

Photoabsorption spectra of isolated Si-6-100(+) clusters are presented in the energy range of h omega = 1.9-5.4 eV. By measuring the absolute photodissociation cross sections, this work attempts to bridge the gap between the optical properties of few-atom-sized silicon clusters to the well-studied regime of nanoparticles and the condensed matter. This is demonstrated by probing the effective optical band gaps in dependence of the size of the silicon clusters. From comparison with the corresponding bulk properties predicted by Mie theory, structural transitions between spherical and prolate clusters as well as quantum confinement effects for larger clusters are stressed. The impact of the dissociation threshold on the measured absorption spectra is considered carefully.