SIMULTANEOUS GENERATION OF THE 7.6-EV OPTICAL-ABSORPTION BAND AND F2 MOLECULE IN FLUORINE DOPED SILICA GLASS UNDER ANNEALING

We examined chemical state of fluorine doped silica glasses and its thermal behavior. Almost all of the fluorine atoms were found to have the = SiF structure with Raman spectroscopy. No optical absorption in the region of 3-9 eV was detected in the glass. When the glasses were annealed in a He atmosphere at 1000-degrees-C, absorption bands peaking at 7.6 and 4.3 eV appeared. These two bands are attributed to the = SiSi = structure and to F2 molecules, respectively. We proposed a thermal decomposition reaction expressed as = SiF + FSi = --> = SiSi = + F2. The concentrations of the reaction products, = SiSi = and F2, estimated from the absorption cross sections were equal to each other within the errors of measurements. We also examined the radiation damage with gamma-ray. The concentration of E' center was almost the same for the same dose in silica glasses having different concentrations of FSi = and = SiSi =. We suggest that FSi = and = SiSi = were found to be stable for gamma-irradiation at room temperature.