Evidence of delocalized excitons in amorphous solids

We studied the temperature dependence of the absorption coefficient of amorphous SiO2 in the range from 8 to 17.5 eVobtained by Kramers-Kronig dispersion analysis of reflectivity spectra. We demonstrate the main excitonic resonance at 10.4 eV to feature a close Lorentzian shape redshifting with increasing temperature. This provides a strong evidence of excitons being delocalized notwithstanding the structural disorder intrinsic to amorphous SiO2. Excitons turn out to be coupled to an average phonon mode of 83 meV energy