Temperature effects on the IR absorption bands of hydroxyl and deuteroxyl groups in silica glass

In this report we will show the changes of the band shape of the hydroxyl group (SiOH) infrared (IR) absorption band (～3670 cm^?1) in silica glass induced by lowering temperature in the range from 290 to 20 K. This band is considered as the overlap of several spectral components associated to the vibrational activity of hydroxyl groups in different bond configurations. By a suitable analysis of the experimental band profile in terms of different sub-bands, we studied the thermal evolution of each component and we reconsidered their assignations. For comparison we examined the SiOD absorption band (～2710 cm^?1) as a function of temperature as well. Our data can be interpreted as an evidence of a conversion process of free into H(D)-bonded SiOH(D) groups. Moreover, we estimated the ratio between the molar extinction coefficients of the two main species involved in this process finding that this ratio is appreciably influenced by the isotopic exchange.