Optical Absorption and Nonradiative Decay Mechanism ofE′Center in Silica

We report ab initio configuration interaction calculations on the optical transitions of the $E^{\prime }$ center, a hole trapped at an oxygen vacancy, $(—\mathrm{O}{)}_3{\mathrm{Si}}^{•}$ ${}^{+}\mathrm{Si}(\mathrm{O}—{)}_3$, in silica. We found two competing excitation mechanisms: (1) promotion of one electron from an $\mathrm{O}\left(2p\right)$ valence band orbital to the singly occupied Si dangling bond; (2) charge transfer (CT) transition from $(—\mathrm{O}{)}_3{\mathrm{Si}}^{•}$ to ${}^{+}\mathrm{Si}(\mathrm{O}—{)}_3$. The two excitations occur at similar energies, $\approx 5.8–6\mathrm{eV}$ (5.85 eV in the experiment), but only the CT has a strong intensity. The excitation is followed by a complex nonradiative decay process which may explain the absence of luminescence for this center.