Modification of vacuum-ultraviolet absorption of SiOH groups in SiO₂ glass with temperature, F₂ laser irradiation, and H-D isotope exchange

Variations of vacuum-ultraviolet (VUV) absorption of silanol (SiOH) groups in synthetic wet SiO₂ glass with temperature, exposure to F₂ laser light, and with H-D isotope exchange were examined at photon energies below 8 eV. The intensity of the VUV absorption band decreases with cooling or with exposure to F₂ laser light. The spectral changes in both cases are qualitatively similar and are attributed to an alternation of the bonding configuration of SiOH groups from isolated into hydrogen-bonded states. However, the resultant states are distinctly different: the hydrogen-bonded state formed on cooling is restored reversibly to the isolated state on heating, while that induced by F₂ laser light is metastable and does not decay below 300 °C. The VUV absorption is also reduced by replacing SiOH groups with SiOD groups due to the decrease of the zero-point energy of the ground state associated with OH groups.