Ab initiocalculations for the interconversion of optically active defects in amorphous silica

Using ab initio calculations on clusters, we have identified a new reaction path between the dicoordinated silicon atom defect and the paramagnetic E_γ^' center in amorphous silica. Under ionizing irradiation, the dicoordinated silicon atom interacts with a nearby oxygen atom relaxing into a structure in which the unpaired electron is distributed between two threefold-coordinated silicon atoms. The transformation into the E_γ^' center follows by a subsequent formation of a puckered configuration.