Fig. 2: Structural changes in amorphous silica.

Representative background-corrected Raman spectra from amorphous silica (blue spectrum) formed within the first hours of the experiment near the glass surface, and from the silicate glass (light grey spectrum), showing the typical glass bands with second-order bands between 2300 and 2900 cm⁻¹. Raman modes for silica are assigned after Aguiar et al.²⁸. The stars (*) mark bands from O₂ (1555 cm⁻¹) and N₂ (2331 cm⁻¹) of the air between glass and objective. Inset diagram shows a series of Raman spectra in the frequency range between 200 and 600 cm⁻¹ that were taken at different times at the same location (cf. Fig. 3a). The blue and yellow spectra represent the first and last measurement at that location, respectively. Note that with increasing reaction time, the intensity near 430 cm⁻¹ increases significantly, reflecting the formation of five- and sixfold silica rings with time. Moreover, the ν_s(SiO–H) band is not visible in all spectra (purple spectrum), indicating an inhomogeneous distribution of silica inner surface OH groups that are linked via hydrogen to a water molecule (cf. Fig. 3b).