REFLECTION SPECTRA OF DENSE AMORPHOUS SIO2 IN THE VACUUM-UV REGION

Vacuum ultraviolet (vuv) reflection spectra have been measured from am orphous SiO2(a-SiO2) plates of varying densities. Two bands were obser ved at 10.3 and 11.4 eV in the vuv reflection spectra for as-received a-SiO2. Upon densifying the a-SiO2 With a multianvil-type high-pressur e apparatus, both the strong band at 10.3 eV and the weaker shoulder a t 11.4 eV were found to shift toward lower energies and to broaden. Th e shift of these bands during densification can be explained either by a unification of the extended point-dipole theory and harmonic oscill ator model, or by the relationship between the bandgap energy and Si-O bond length. The broadening of the bands during densification, and th us the Si-O-Si bond-angle decrease, may be due to either the distortio n Of SiO4 tetrahedra or a widening of the distribution of rotational a rrangements between connected SiO4 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-SiO2, but this shift gradually disappears with increasing densification. Although the reas on for the temperature dependence of band position for the undensified a-SiO2 is not yet clear, it appears that the disappearance of this de pendence with densification is related to an increase of the structura l randomness of the glass.