Ab Initio Simulations of Photoinduced Interconversions of Oxygen Deficient Centers in Amorphous Silica

We have studied by ab initio molecular dynamics the interconversion between oxygen deficient centers (Si—Si bond, dicoordinated silicon =Si:, and E^′ centers) induced by UV irradiation in a-SiO₂. By dynamical simulations in the excited state of a periodic model of a-SiO₂ we have identified the reaction path and activation barrier for the Si—Si→=Si: interconversion. A new competitive transformation of the excited, neutral Si—Si bond into two E^′ centers has been identified. Our results provide strong theoretical support to the viability of these processes, proposed experimentally on the basis of optical data only.