Water and its relation to broken bond defects in fused silica

We have studied the effects of water and defects on the Raman spectrum of fused silica. Intensities of defect lines at 604 and 490 cm−1 were found to increase with fictive temperature and to decrease with increasing OH content. The results suggest that water in the melt is preferentially trapped at the defect sites. The increased sensitivity obtained by using an optical fiber as the sample enabled us to see for the first time the Si– (OH) stretching vibration at 970 cm−1. The Si–(OH) intensity agrees with that of the 1100 cm−1 dangling oxygen line in a silica fiber weakly doped with K2O. A comparison with other spectral features shows that the Raman cross section for the Si–O stretch is extremely weak. This weak cross section contributes to the weak Raman intensity of the 1100 cm−1 lines in both fused and crystalline SiO2 and also explains why the dangling oxygen stretching vibration is not observed by Raman scattering in neutron damaged silica. Measurements of the much stronger 3690 cm−1 OH stretching vibration in a series of bulk samples of known OH content suggest this line as a useful measure of OH in fibers. In addition we find a water induced low frequency (30 cm−1) band which is weakly Raman and strongly infrared active.