We studied the reflectivity spectra of amorphous silicon dioxide detectedunder vacuum UV synchrotron radiation as a function of temperature between 10and 300 K. Kramers-Kronig dispersion analysis of reflectivity spectra allowedus to determine the absorption coefficient in the range from 8 to 17.5 eV.Spectra show four main peaks, the spectral positions of which are consistentwith literature data. An appreciable dependence of the line-shape ontemperature is observed for the first two peaks only. We demonstrate theexciton peak at 10.4 eV to have a very good Lorentzian band-shape at all theexamined temperatures. Based on existing theoretical models, this allows toargue excitons in SiO2 to be weakly scattered by phonons, thus retaining theirmobility properties notwithstanding the effects of exciton-phonon coupling andof intrinsic structural disorder of amorphous SiO2. Moreover, the observedtemperature dependence of the peak position together with the features of theUrbach absorption tail and of self-trapped exciton emission allow us toestimate the main parameters ruling exciton dynamics in SiO2. The features ofthe intrinsic Urbach absorption tail can be satisfactorily explained as aconsequence of those of the first excitonic peak, supporting the interpretationof the Urbach tail in SiO2 as a consequence of the momentary self-trapping ofthe 10.4 eV exciton. Finally, the characteristics of the other energy peaks arediscussed and an excitonic origin also for the 11.6 eV peak is put forward. Onthe whole, our results show that exciton dynamics accounts for all opticalproperties of pure silicon dioxide from 8 up to 11 eV.
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