AB-INITIO PHOTOABSORPTION SPECTRA AND STRUCTURES OF SMALL SEMICONDUCTOR AND METAL-CLUSTERS

Ab initio photoabsorption cross sections of small silicon and alkali c lusters (Li,Na) have been calculated by a new method. Different struct ural isomers of a given silicon cluster give rise to clearly distingui shable spectra. Inclusion of screening effects is shown to be essentia l to describe the spectra. We propose that comparison of theoretical s pectra with measured ones would serve as a useful tool to discern the structure of small semiconductor clusters. Low temperature measurement s and/or line-shape analysis are needed to extract detailed structural information from the spectra of alkali clusters because of the collec tive character of the main resonance.