Ab initio photoabsorption spectra and structures of small semiconductor and metal clusters

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