Ab-initio simulation of photoinduced transformation of small rings in   amorphous silica
  We have studied the photoinduced transformation of small rings (3-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' and a non-bridging-oxygen hole center (NBOHC), has been proposed experimentally to occur in silica exposed to F2 laser irradiation (at 7.9 eV). By using a new scheme for the simulation of rare events in ab-initio molecular dynamics (Iannuzzi, Laio and Parrinello, Phys. Rev. Lett. 90, 238303 (2003)), we have identified the transformation path for the opening of a 3-membered ring induced by a self-trapped triplet exciton, the migration of NBOHC and formation of a couple of stable E' and NBOHC paramagnetic defects. 
D. Donadio M. Bernasconi