Exciton-induced lattice relaxation and the electronic and vibrational spectra of silicon clusters.

The geometrical structures and electronic and vibrational spectra of silicon clusters with 20, 60, and 70 atoms are investigated. Using a first-principles quantum-molecular-dynamics method we obtain the equilibrium structures, and study the effects of an exciton-induced lattice relaxation in these systems. The relaxation induced by the exciton may result in a strong distortion of metastable structures, accompanied by changes in the electronic and vibrational spectra. This photoinduced deformation is associated with the presence of nearly degenerate electronic states in the metastable structure, similar to spontaneous Jahn-Teller distortions. The distortion yields lower symmetry and energy, suggesting the trapping of excitons in some of these structures.