Effect of Xe2* light (7.2 eV) on the infrared and vacuum ultraviolet absorption properties of hydroxyl groups in silica glass

Changes in the optical-absorption properties of hydroxyl group (=SiOH) in silica glasses were studied to determine the specific form of =SiOH whose absorption takes place at around the intrinsic absorption edge of silica glass (similar to 8 eV). In the experiments, four kinds of fused silica were irradiated with 7.2 eV light from a xenon dielectric-barrier discharge lamp or they were heated at 1273 K in vacuum. In addition to the near-edge absorption, changes in the intensity, peak position and the shape of the infrared absorption bands due to a bond-stretching mode of =SiOH s at 3673 cm(-1) (2.72 mu m) and the first overtone of these stretching vibrations at 7230 cm(-1) (1.38 mu m) were monitored. The results of absorption spectral observation after the irradiation are summarized as two points: (i) a blue shift of the effective absorption edge took place (increasing in the transmissivity); (ii) the 3673 cm(-1) band broadened and shifted to the red for all the samples except the OH-free silicas, and similarly, the absorption intensity of the 7230 cm(-1) band decreased and the band shifted to the red, too. It was found through analysis of these changes that the blue shift of the absorption edge corresponded with the decrement of a particular form of =SiOH and the red shift of the 3673 cm(-1) band was caused by the decrement of the isolated =SiOH. The =SiOH absorption located at more than 7.4 eV region is tentatively ascribed to =SiOH existing as the isolated form in silica glass. It is argued that the isolated =SiOH was converted into the hydrogen bonded =SiOH by irradiation of the Xe-2* band emission.