Theoretical study of the ground and excited states of silicon clusters: Si₈H_x

The electronic structures of small silicon clusters in the ground and excited states have been studied based on ab initio configuration interaction by the single-substitutions approximation method. We consider Si₈H_x clusters of chain, branch, ladder, and cubic types as model compounds, due to their variety of molecular structures. Calculated Stokes shifts and oscillator strengths successfully reproduce the experimental electronic spectrum. These phenomena are well explained by the detailed analysis of the orbital patterns and energy level changes related to the excitation. The optical properties of the cubic Si cluster are considerably different from those of the other Si₈ clusters. It shows zero oscillator strength, i.e., "forbidden transition," from the first to 20th excited states because of its high symmetry.