Site-selective luminescence study of defects in gamma-irradiated glassy germanium dioxide

Luminescence of γ-irradiated samples of GeO₂ glass was studied using dye laser excitation in the wavelength region between 590 and 656 nm. An inhomogeneously broadened luminescence band with a peak around 1.86 eV, halfwidth of 0.16 eV and decay constant τ ≈ 5.1 μs was found. At temperatures below 20 K a resonance ZPL and a vibronic peak 710 cm^-1 below the laser photon energy emerge when laser is tuned to energies between 1.95-1.89 eV. The site-selective luminescence spectra are qualitatively similar to those of the non-bridging oxygen hole center (NBOHC) in SiO₂ glass and are attributed to NBOHC in glassy GeO₂. The measured ratio between the energies of the observed local vibrations in GeO₂ (710 cm^-1) and SiO₂ (890 cm^-1) is close to the ratio, calculated on the basis of the known Si-O and Ge-O central force constants and taking the reduced mass of Si-O and Ge-O molecules for the mode mass. This confirms the assignment of the 710 and 890 cm^-1 vibronic lines to the non-bridging oxygen stretching modes of the NBOHC in glassy GeO₂ and SiO₂ respectively.