Incorporation of H2 in vitreous silica, qualitative and quantitative determination from Raman and infrared spectroscopy

Incorporation mechanisms of H₂ in silica glass were studied with Raman and infrared (IR) microspectroscopy. Hydrogenated samples were prepared at temperatures between 800°C and 955°C at 2 kbar total pressure. Hydrogen fugacities (f_H2) were controlled using the double capsule technique with the iron–wüstite (IW) buffer assemblage generating f_H2 of 1290–1370 bars corresponding to H₂ partial pressures (P_H2) of 960–975 bars. We found that silica glass hydrogenated under such conditions contains molecular hydrogen (H₂) in addition to SiH and SiOH groups. H₂ molecules dissolved in the quenched glasses introduce a band at 4136 cm⁻¹ in the Raman spectra which in comparison to that of gaseous H₂ is wider and is shifted to lower frequency. IR spectra of hydrogenated samples contain a band at 4138 cm⁻¹ which we assign to the stretching vibration of H₂ molecules located in non-centrosymmetric sites. The Raman and IR spectra indicate that the dissolved H₂ molecules interact with the silicate network. We suggest that the H₂ band is the envelope of at least three components due to the occupation of at least three different interstitial sites by H₂ molecules. Both, Raman and IR spectra of hydrogenated glasses contain bands at 2255 cm⁻¹ which may be due to the vibration of SiH groups. Under the assumption that the reaction Si–O–Si + H₂ → Si–H + Si–O–H describes adequately the ‘chemical dissolution' of H₂ molecules, the SiH concentrations in our samples were determined and the molar extinction coefficient for the SiH absorption band in the infrared (₂₂₅₅(SiH)) could then be estimated to be 45 ± 3 l/mol cm. The solubility of molecular H₂ in our hydrogenated samples was determined using the IR absorption band at 4138 cm⁻¹ and the extinction coefficient given by Shelby [J. Non-Cryst. Solids 179 (1994) 138]. Samples quenched with different cooling rates gave nearly identical Raman and IR spectra, suggesting that the chemical dissolution of hydrogen (SiH and SiOH) can be quenched to room temperature without changing relative concentrations and that no exsolution of hydrogen occurred during the quench.