_applied sciences_  
O2 Loaded Germanosilicate Optical Fibers: Experimental In Situ Investigation and Ab Initio Simulation Study of GLPC Evolution under Irradiation  
**[PERSON] 1**, **[PERSON] 2,3,* 2**, 3**, 4**, 5**, 6**, 7**, 8**, 9**, 10**, 11**, 12**, 13**, 14**, 15**, 16**, 17**, 18**, 19**, 20**, 21**, 22**, 23**, 24**, 25  
the generation mechanisms and the conversion of defects were the main topics of a great number of investigations [3, 8, 9, 10, 11, 12, 13, 14, 15, 16]. Moreover, a few experimental techniques, such as H\({}_{2}\) or O\({}_{2}\) loading of optical fibers, have been designed with the aim to enhance the radiation resistance by passivating radiation-induced point defects, and to enhance the duration of the optical fiber in a harsh environment [17, 18]. One of the most important defects related to applicative and basic research relevance is the Germanium Lone Pair Center (GLPC [19]) in Ge-doped silica, which is employed to produce one of the most common optical fiber types in telecommunications and sensing applications [20, 21]. It is accepted that the GLPC is an electron donor and that its presence affects the sensitivity of the silica to radiation or laser exposure [11, 16, 22]. The microscopic structure of such defect must be mentioned as well. The GLPC is a twofold coordinated Ge atom with two electrons forming a lone pair [19, 23]. This defect is responsible for absorption bands at about 5.12 eV (related to a singlet-singlet transition) and at about 3.8 eV (related to a singlet-triplet transition) [19, 23]. Furthermore, it is responsible for two emission bands at 4.3 eV (inverse singlet-singlet transition) and at \(\sim\)3.1 eV (triplet-singlet transition, the triplet state can be populated by intersystem crossing processes) [19, 23]. In particular, the latter emission at \(\sim\)3.1 eV (i.e., \(\sim\)400 nm) could be exploited to study the kinetics of creation or bleaching of GLPC centers in silica by means of cathodoluminescence (CL) spectroscopy [24, 25]. In the present investigation, other aspects related to the GLPC must be remembered. The GLPC defect is usually present in high concentrations in nonirradiated optical fibers because of the manufacturing processes, and its concentration decreases when the fibers are exposed to radiation [26]. Furthermore, a recent investigation has proven that O\({}_{2}\) loading of the optical fiber is able to destroy these GLPC, then modify their response to consecutive irradiation [10], suggesting a possible method to tune the radiation response of germanosilicate optical fibers in the ultraviolet and visible domains. However, in this previous investigation, the basic mechanisms responsible for the GLPC bleaching in O\({}_{2}\) loaded fibers were not deeply investigated. In this work, to better understand how O\({}_{2}\) loading bleaches the GLPC centers (before irradiation) and how they can be generated during irradiation in treated optical fibers, we completed our experimental work with ab initio calculations. These simulations enabled us to identify the relevant structures resulting from the reaction of the GLPC with molecular oxygen, and to evaluate the stability of such bulk defects and their possible back conversion to GLPC, or alternatively their permanent bleaching. In particular, the present work provides strong support in favor of the existence and stability of dioxagermirane (DIOG) as a bulk point defect in O\({}_{2}\)-loaded Ge-doped fibers. The dioxagermirane defect is a germanium-based analogue of the dioxasilyrane [27], a point defect which was first detected as a surface defect on reactive silica [28] and has recently been found in luminescent siliceous materials and silica nanoparticles [29, 30]. Moreover, in a previous theoretical study, the DIOG (i.e., the (O\({}_{2}\))Ge= defect) was investigated as a surface defect in germanium oxides [31].