Laser-induced luminescence in glassy SiO₂ and neutron-irradiated alpha-quartz: Three types of non-bridging oxygen hole centers

Site-selective photoluminescence (PL) spectra of α-quartz samples irradiated by 10¹⁸ and 10¹⁹ fast neutrons/cm² were studied at temperatures between 6 and 300 K under dye-laser excitation in the spectral region between 1.9 and 2.1 eV. At the room temperature an irradiation-induced PL band with the peak at 1.9 eV and decay time 17 μs was found. It is similar to the PL band of the non-bridging oxygen hole centers (NBOHC's) characteristic of amorphous forms of SiO₂. The low temperature (T<80 K) site-selective PL spectra show a number of sharp lines. Two of them, at 1.933 and 1.883 eV are identified as zero-phonon lines (R₁ and R₂ lines) due to two distinct variants of NBOHC's, embedded in an ordered crystalline environment and undergoing relatively small inhomogeneous broadening. They are superimposed by a broad continuum of significantly broadened zero-phonon lines, which are similar to the ones observed for NBOHC's in irradiated glassy SiO₂. Vibronic sidebands at 897 cm^-1 in emission spectra and at 849 cm^-1 in emission spectra are due to the silicon-(nonbridging oxygen) stretching mode of NBOHC.