Microscopic self-dynamics in liquid hydrogen and in its mixtures with deuterium

We have measured the dynamic structure factor of liquid parahydrogen, pure and mixed with deuterium, invarious thermodynamic conditions using incoherent inelastic neutron scattering. The experiments were carriedout on TOSCA-II, a new time-of-flight, inverse-geometry, crystal-analyzer spectrometer. After an accurate datareduction, the high-energy parts of the neutron spectra recorded in backward scattering were studied throughthe modified Young and Koppel model, from which the mean kinetic energy values for a hydrogen moleculewere estimated. In addition the low-energy parts of the neutron spectra recorded in forward scattering wereanalyzed in the framework of the Gaussian approximation and fitted through a Levesque-Verlet model for thevelocity autocorrelation function. Thus various physical quantities are determined and compared with accuratepath integral Monte Carlo simulations. Despite the excellent quality of these fits, the velocity autocorrelationfunctions derived from the forward-scattering data appear totally unable to properly describe the backwardscatteringones. These findings prove an unquestionable breakdown of the Gaussian approximation in semiquantumliquids. The present results appear of great interest and suggest further investigation on the limits ofthe widely used Gaussian approximation.