Quantum diffusion of molecular hydrogen in liquid Ne + H₂ mixtures: A quasielastic neutron scattering study

This study presents an analysis of the diffusional dynamics of H2 impurities in liquid Ne using quasielastic neutron scattering (QENS) and quantum dynamical simulations. By applying a Lorentzian fitting procedure, we identify two key spectral features: peak width (P) and intensity (A), both of which are highly dependent on the momentum transfer Q. At low Q values, the experimental results for P align well with simulated selfdiffusion coefficients. However, at higher Q, QENS data reveals a sublinear variation with respect to Q2 that characterizes a jump diffusion process. Using the Hall and Ross model, we determine the mean residence time (tau CM) and the standard deviation of jump lengths (lCM). These values provide concrete evidence for the existence of Ne "pseudocages," i.e., short-lived structures formed by neon atoms that trap H2 molecules, confirming a hypothesis suggested in a previous research. The study highlights a strong coupling between the H2 diffusion and its vibration within these pseudocages, driven by the high molecular density of the system. This finding underscores the necessity of moving beyond the well-known Gaussian approximation to accurately describe the microscopic dynamics of semiquantum fluids. In conclusion, this work improves the quality of available neutron spectra for H2 in liquid Ne, correcting previous misinterpretations that were actually due to instrumental limitations rather than physical anomalies.