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

Abstract

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

Test of the consistency of various linearized semiclassical initial value time correlation functions in application to inelastic neutron scattering from liquid para-hydrogen

“…This is not necessary for Panel (a) since the background has already been removed from experimental forward scattering data 78,101 . Fig.…”

“…In the experiment 101 that we consider in Section IIID, the neutron scattering spectrometer-the TOSCA-II apparatus 136 …”

“…Section III presents the LSC-IVR simulation results for the incoherent dynamic structure factor of liquid para-hydrogen at 14K T = (under nearly zero external pressure) using different correlation functions, along with the spectral moment test and the comparison to other methods and the recent inelastic neutron scattering experiment data 101 .…”

“…The purpose of this paper is to present an additional challenging application and test of the LSC-IVR approximation to quantum mechanical time correlation functions, namely the incoherent dynamic structure factor for inelastic neutron scattering from liquid para-hydrogen 100,101 , with special emphasis on how consistent the results are when obtaining this quantity from various time correlation functions. I.e., in most cases the physical quantity of interest can be expressed in terms of different time correlation functions, which would all give the same result if the calculations could be carried out exactly: e.g., diffusion coefficients can be obtained from position-position or velocity-velocity correlation functions, rate constants can be obtained from flux-flux or side-side correlation functions, etc.…”

Neutron study of non-Gaussian self dynamics in liquid parahydrogen

“…A first important result came a few years ago from a neutron scattering experiment [24] carried out on the TOSCA spectrometer at the pulsed neutron source ISIS. The validity of GA was questioned without resorting to calculations based on a quantum-simulated VACF.…”