Time-resolved low temperature luminescence of non-bridging oxygen hole centers in silica glass

The spectral, temporal and polarization characteristics of the non-bridging oxygen (NBO) hole center luminescence band at 1.9 eV in neutron irradiated silica glass has been studied by site selection techniques. A resonant zero-phonon line and vibrational structure 890 cm^-1 apart emerge at temperatures below 65 K under monochromatic 632.8 nm excitation. The vibrational structure is attributed to the symmetric stretching mode of silicon - NBO bond. The highly polarized zero-phonon line is due to the charge transfer transition between the 2p-like non-bonding orbital of the NBO and the filled 2p lone pair orbital of one of the 3 ligand oxygen atoms in the same SiO₂ tetrahedron. The well-known absorption band at 4.8 eV corresponds to the transition from σ NBO-Si bonding state to the non bonding 2p orbital of NBO.