Exciton–phonon interaction in crystalline and vitreous SiO₂

An investigation of the Urbach rule in crystalline SiO₂ (α‐quartz), crystalline SiO₂Ge, and vitreous SiO₂ in the framework of the models of Dow and Redfield and of Toyozawa shows that the fundamental optical absorption edge of SiO₂ is determined by strong exciton–phonon interactions. In the crystalline and vitreous SiO₂ low‐energy excitons with E = 9.1 eV (α‐quartz) and E = 8.7 eV (glass) interact mainly with longitudinal optical phonons. In the crystalline SiO₂Ge alloy the interaction with phonons is realized by excitons localized on germanium impurities. The exciton–phonon interaction in SiO₂Ge and vitreous SiO₂ is stronger than in crystalline SiO₂ due to the higher degree of localization of low‐energy excitons in these disturbed SiO₂ forms. The strong exciton–phonon interaction leads to a momentary self‐trapping of the low‐energy excitons in crystalline and non‐crystalline SiO₂ and to a corresponding relaxation of the SiO₂Ge excitons localized on Ge impurities. Numerical values of the fundamental exciton parameters are deduced and discussed.