Band-to-Band Excitation and Decay in SiO2 – Can We Get Evidences of such a Path?

We have investigated in a wide variety of amorphous silica systems – distinct by synthesis, purity, or doping induced modification – the relations among the different photoluminescence emissions arising from excitation in the band-to-band region from 12 to 20 eV, or within the band-edge exciton absorption at about 10 eV, or below the band gap via localized states. Measurements have been performed at different temperatures from 10 to 300 K, studying the competitive radiative decay paths of the excitation. The analysis gets an improved picture of the excitation-decay mechanisms in silica, showing that band-to-band excitation gives rise to non-bridging oxygen sites - identified by the 1.9 eV emission - which constitute the first step of a decay path that includes thermally activated configurations emitting at about 2.5 eV, with a luminescence usually ascribed to self-trapped-excitons.