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=214-283 K. The data, analyzed using a viscoelastic model with a two time-scale 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(infinityalpha)(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 hydrogen bonds per molecule.