The Fundamental Absorption Edge of Crystalline and Amorphous SiO 2

Kramers-Kronig-transformed reflectivity spectra of α-crystalline SiO2 are compared to absorption data (E ≤ 8.8 eV) and further analysed in the energy range 8 eV ≤ E ≤ 10.5 eV. We find a good agreement with the transmission measurements, and for 9.3 ≤ E ≤ 9.7 eV a (E - Eg)3/2-dependence for the absorption coefficient with Eg = (9.3 ± 0.1) eV in α-SiO2. Our results are compared to excitation spectra of the self-trapped exciton in α-SiO2 and to the parity forbidden transition in Cu2O. Although qualitatively similar, the results in amorphous SiO2 show a number of quantitative differences.