Optical absorption and nonradiative decay mechanism of E′ 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)(3)Si-. Si+(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)(3)Si-. to Si+(O-)(3). The two excitations occur at similar energies, approximate to 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.