Reflectance spectrum of crystalline and vitreous SiO2at low temperature

The vacuum ultraviolet reflectance spectrum of crystalline and vitreous Si${\mathrm{O}}_2$ has been measured at low temperature. We find only minor differences between the room-temperature and 5-K spectra of both crystalline and vitreous Si${\mathrm{O}}_2$. These observations impose restrictions on the models to explain the electronic structure of the ordered as well as the disordered phase of Si${\mathrm{O}}_2$. The electronic structure of both forms of Si${\mathrm{O}}_2$ is mainly determined by the short-range structure of the Si${\mathrm{O}}_4$ tetrahedra. The strong reflectance band at 10.3 eV is described as an excitonic resonance in interaction with a background of interband transitions and with optically forbidden exciton states.