Microscopic self dynamics in liquid hydrogen and in its mixtures with deuterium and neon: a neutron scattering study

By means of inelastic neutron scattering measurements, we have extracted the incoherent scattering function of liquid para-hydrogen, pure and mixed with deuterium or neon, in various thermodynamic conditions. The low-energy parts of the neutron spectra recorded in forward scattering on TOSCA-II were analyzed in the framework of the Gaussian approximation and filled through a Levesque-Vertet model for the velocity auto-correlation function. These correlation functions derived from the forward-scattering data appear totally unable to properly describe the backward-scattering ones. These findings prove a breakdown of the Gaussian approximation for semi-quantum liquids in the kinematic range of our experiment. The present results appear of extreme interest and suggest further investigation on the limits of the widely-used Gaussian approximation.