Ab initiosimulation of photoinduced transformation of small rings in amorphous silica

We have studied the photoinduced transformation of small rings (three membered) in amorphous silica by Car-Parrinello simulations. The process of ring opening leading to the formation of a couple of paramagnetic centers, namely an $E^{\prime }$ and a nonbridging-oxygen hole center (NBOHC), has been proposed experimentally to occur in silica exposed to ${\mathrm{F}}_2$ laser irradiation (at $7.9\mathrm{eV}$). By using a scheme for the simulation of rare events in ab initio molecular dynamics [M. Iannuzzi, A. Laio, and M. Parrinello, Phys. Rev. Lett. 90, 238303 (2003)], we have identified a transformation path for the opening of a three-membered ring induced by a self-trapped triplet exciton, the migration of NBOHC and formation of a couple of stable $E^{\prime }$ and NBOHC paramagnetic defects.