Spectroscopic parameters related to non-bridging oxygen hole centers in amorphous-SiO 2 M. Cannas a, * , L. Vaccaro a , B. Boizot b a Dipartimento di Scienze Fisiche ed Astronomiche dell’ Universita ` di Palermo, Via Archirafi 36, I-90123 Palermo, Italy b Laboratoire de Solides Irradie ´s, UMR 7642 CEA CNRS Ecole Polytechnique, Palaiseau, France Received 14 June 2005; received in revised form 21 November 2005 Available online 6 January 2006 Abstract The relationship between the luminescence at 1.9 eV and the absorption bands at 2.0 eV and at 4.8 eV were investigated in a wide variety of synthetic silica samples exposed to different c- and b-ray irradiation doses. We found that the intensities of these optical bands are linearly correlated in agreement with a model in which they are assigned to a single defect. This finding allows the determination of spectroscopic parameters related to the optical transitions efficiencies. In this case the absorption oscillator strength at 4.8 eV is 200 times higher than that at 2.0 eV; while the 1.9 eV luminescence quantum yield under 4.8 eV excitation is lower (by a factor 3) than that under 2.0 eV excitation. These results are consistent with the energetic level scheme proposed in the literature for the non-bridging oxy- gen hole center. Moreover, they account for the excitation ! luminescence pathways occurring after UV and visible absorption. Ó 2005 Elsevier B.V. All rights reserved. PACS: 71.55.Jv; 78.40.Pg; 78.55.Qr; 61.43.Fs Keywords: Optical spectroscopy; Defects; Absorption; Luminescence; Silica 1. Introduction The influence of ionizing radiation on the optical prop- erties of amorphous-SiO 2 (silica) is a timely research field strongly motivated by the large use of silica materials in many technologies requiring stable optical transparency (e.g. fibers, laser optics and radioactive environments) [1,2]. A large number of works over the last decades have confirmed that irradiation induces intrinsic defects (dan- gling bonds, oxygen-deficiency or oxygen-excess) which, in turn cause optical transitions producing absorption (OA) and photoluminescence (PL) (see review papers by Griscom [3] and Skuja et al. [4], and references therein). Owing to the complexity of OA and PL spectra over a wide range extending from visible to vacuum-ultraviolet (V-UV), the identification of defects and the assignment of the opti- cal bands to their electronic structure remains an incom- pletely solved problem. One of the most controversial aspects concerning radia- tion induced defects in silica regards the optical transitions associated with the non-bridging oxygen hole center (NBOHC) whose structure is identified by its paramagnetic properties and is denoted by „Si–O Å [5];(„) stands for bonds with three oxygen and ( Å ) indicates un unpaired elec- tron. It is well accepted that this defect gives rise to a weak OA band at 2.0 eV (oscillator strength f 2.0eV  1.5 · 10 4 ), that is not easily measured. The inverse transition is asso- ciated with a PL band at 1.9 eV with a lifetime of 14 ls [6–8]. One of the main experimental proofs that supports the assignment of these visible transitions to the NBOHC is the observation of a side band of the zero-phonon-line emission at 1.9 eV [6,9], whose frequency (890 cm 1 ) is due to coupling to the stretching vibration of the dangling oxygen. The PL at 1.9 eV can also be excited in the UV 0022-3093/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.12.001 * Corresponding author. Tel.: +39 0916234298; fax: +39 0916162461. E-mail address: cannas@fisica.unipa.it (M. Cannas). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 352 (2006) 203–208