Reflection spectra of dense amorphousSiO2in the vacuum-uv region

Vacuum ultraviolet (vuv) reflection spectra have been measured from amorphous ${\mathrm{SiO}}_2$(a-${\mathrm{SiO}}_2$) plates of varying densities. Two bands were observed at 10.3 and 11.4 eV in the vuv reflection spectra for as-received a-${\mathrm{SiO}}_2$. Upon densifying the a-${\mathrm{SiO}}_2$ with a multianvil-type high-pressure apparatus, both the strong band at 10.3 eV and the weaker shoulder at 11.4 eV were found to shift toward lower energies and to broaden. The shift of these bands during densification can be explained either by a unification of the extended point-dipole theory and harmonic oscillator model, or by the relationship between the band-gap energy and Si-O bond length. The broadening of the bands during densification, and thus the Si-O-Si bond-angle decrease, may be due to either the distortion of ${\mathrm{SiO}}_4$ tetrahedra or a widening of the distribution of rotational arrangements between connected ${\mathrm{SiO}}_4$ tetrahedra causing a distortion of the electronic structure. A shift of the band at 10.3 eV toward higher energy is observed upon cooling in undensified a-${\mathrm{SiO}}_2$, but this shift gradually disappears with increasing densification. Although the reason for the temperature dependence of band position for the undensified a-${\mathrm{SiO}}_2$ is not yet clear, it appears that the disappearance of this dependence with densification is related to an increase of the structural randomness of the glass.