The Fundamental Absorption Edge of Crystalline and Amorphous SiO 2

Kramers-Kronig-transformed reflectivity spectra of α-crystalline SiO 2 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 - E g ) 3/2 -dependence for the absorption coefficient with E g = (9.3 ± 0.1) eV in α-SiO 2 . Our results are compared to excitation spectra of the self-trapped exciton in α-SiO 2 and to the parity forbidden transition in Cu 2 O. Although qualitatively similar, the results in amorphous SiO 2 show a number of quantitative differences.