Microscopic dynamics and relaxation processes in liquid Hydrogen Fluoride

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.