Optical Absorption and Nonradiative Decay Mechanism ofE′Center in Silica

We report ab initio configuration interaction calculations on the optical transitions of the E′ center, a hole trapped at an oxygen vacancy, ( - O)3Si• +(O - )3, in silica. We found two competing excitation mechanisms: (1) promotion of one electron from an O(2p) valence band orbital to the singly occupied Si dangling bond; (2) charge transfer (CT) transition from ( - O)3Si• to +Si(O - )3. The two excitations occur at similar energies, ≈5.8-6 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