Abstract
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 ${E}^{\ensuremath{'}}$ and a nonbridging-oxygen hole center (NBOHC), has been proposed experimentally to occur in silica exposed to ${\mathrm{F}}_{2}$ laser irradiation (at $7.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$). 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 ${E}^{\ensuremath{'}}$ and NBOHC paramagnetic defects.
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