Direct experimental access to microscopic dynamics in liquid hydrogen

We have obtained the double-differential incoherent neutron scattering cross section of liquid and solidparahydrogen in various thermodynamic conditions using TOSCA, a time-of-flight, inverse geometry, crystalanalyzer spectrometer, operating at the pulsed neutron source ISIS. The measured cross section provides directexperimental access to the self part of the center-of-mass inelastic structure factor of the parahydrogen moleculesin the system. Data have been corrected for the experimental effects and then analyzed in the frameworkof the Young-Koppel model and the Gaussian approximation. The velocity autocorrelation functions and theirenergy spectra have been obtained from a fitting procedure, making use of the quantum generalized Langevinequation and of model memory functions, and finally compared to the most recent results of both molecularcentroid dynamics and self-consistent quantum mode-coupling theory. Some dynamic quantities were alsorelated to simple equilibrium properties and simulated through a standard path integral Monte Carlo code.Results are very interesting but still urge for further developments of theoretical and dynamic simulationapproaches, as well as for more extensive experimental efforts.