Microscopic dynamics and relaxation processes in liquid hydrogen fluoride

Inelastic x-ray scattering and Brillouin light scattering measurements of the dynamic structure factor ofliquid hydrogen fluoride have been performed in the temperature range T=214-283 K. The data, analyzedusing a viscoelastic model with a two time-scale memory function, show a positive dispersion of the soundvelocity c(Q) between the low frequency value c0(Q) and the high frequency value cinfinitoalpha(Q). This finding confirms the existence of a structural sad relaxation directly related to the dynamical organization of the hydrogen bonds network of the system. The activation energy Ea of the process has been extracted by theanalysis of the temperature behavior of the relaxation time taualpha(T) that follows an Arrhenius law. The obtainedvalue for Ea, when compared with that observed in another hydrogen bond liquid as water, suggests that themain parameter governing the a-relaxation process is the number of hydrogen bonds per molecule.