Ab InitioSimulations 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 $E^{\prime }$ centers) induced by UV irradiation in $a-{\mathrm{SiO}}_2$. By dynamical simulations in the excited state of a periodic model of $a-{\mathrm{SiO}}_2$ we have identified the reaction path and activation barrier for the $\mathrm{Si}—\mathrm{Si}\to =\mathrm{S}\mathrm{i}:$ interconversion. A new competitive transformation of the excited, neutral Si—Si bond into two $E^{\prime }$ 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.