  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.
