Low energy vibrational dynamics of hydrogenated silica gels

A comparative study of low temperature specific heat (1.5-25 K) and low frequency Raman scattering (<150 cm-1) in sol-gel hydrogenated silica samples has been carried out to analyse the variations of the dynamical properties of a disordered system as a function of the changes of the network connectivity. Variations of the connectivity in the range from 3 to 3.75, obtained by decreasing the hydrogen content or by increasing the density of the as-prepared samples (achieved by thermal treatments at various temperatures) alter the magnitude of the quasi-elastic scattering, as well as the density of low energy vibrations determining the low temperature specific heat. Using as a reference observations in quenched vitreous silica, which has the largest connectivity, it is shown that decreasing connectivity leads to (i) a softening of the network and (ii) a smaller excess of additional lower energy vibrations over the predictions of the Debye theory.