  Inelastic x-ray scattering and Brillouin light scattering measurements of the
dynamic structure factor of liquid hydrogen fluoride have been performed in the
temperature range$ T=214\div 283 K$. The data, analysed using a viscoelastic
model with a two timescale memory function, show a positive dispersion of the
sound velocity $c(Q)$ between the low frequency value $c_0(Q)$ and the high
frequency value $c_{\infty \alpha}(Q)$. This finding confirms the existence of
a structural ($\alpha$) relaxation directly related to the dynamical
organization of the hydrogen bonds network of the system. The activation energy
$E_a$ of the process has been extracted by the analysis of the temperature
behavior of the relaxation time $\tau_\alpha(T)$ that follows an Arrhenius law.
The obtained value for $E_a$, when compared with that observed in another
hydrogen bond liquid as water, suggests that the main parameter governing the
$\alpha$-relaxation process is the number of the hydrogen bonds per molecule.
