The origin of the intrinsic 1.9 eV luminescence band in glassy SiO 2

Abstract

The current controversy over the nature of the centers giving rise to the 1.9 eV photoluminescence (PL) band (the R-band), the suggested defect models and the relevant experimental data are briefly reviewed. The luminescence emission, excitation and polarization spectra of neutron-irradiated synthetic silica were studied between 6 and 300 K using site-selective dye-laser and Ar ion laser excitation. Resonant zero-phonon lines (ZPL) were observed below 80 K both in luminescence emission and excitation spectra in the 1.9–2.1 eV region. A vibration line in emission spectra 890 cm −1 below the ZPL energy is attributed to the symmetric stretching vibration of the silicon-non-bridging oxygen bond in the ground electronic state of the non-bridging oxygen hole center. A similar line 860 cm −1 above the ZPL in the excitation spectra corresponds to the same vibration in the excited state. The intensities of the resonant ZPLs are dependent on the excitation energy and show a nearly Gaussian distribution with the peak at 1.935 ± 0.01 eV and halfwidth 82 ± 7 meV. In the zero approximation, this distribution describes the concentration distribution of the PL centers with the respective energies of the excited electronic state. The 4.8 eV excitation band of the 1.9 eV PL is complex, due to different electronic transitions. No ZPLs or vibrational structures are observed under excitation with KrF excimer laser or Xe lamp in this band. The optical absorption in this region is due to overlapping bands of several different defect centers. The low-temperature luminescence bands at 2.05–2.1 eV and 2.35–2.4 eV, excited by the green (2.41 eV) and blue (2.71 eV) Ar ion laser lines, have a nature different from the 1.9 eV band. Several different defects contribute to the 2.0 eV optical absorption band in irradiated glassy SiO 2.