A comparative study of low temperature specific heat (1.5-25 K) and low fre
quency Raman scattering (<150 cm(-1)) in sol-gel hydrogenated silica sample
s has been carried out to analyse the variations of the dynamical propertie
s of a disordered system as a function of the changes of the network connec
tivity. Variations of the connectivity in the range from 3 to 3.75, obtaine
d 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. Us
ing 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 e
nergy vibrations over the predictions of the Debye theory. (C) 1999 Elsevie
r Science B.V. All rights reserved.