Quantum diffusion of molecular hydrogen in liquid Ne+H_{2} mixtures: A quasielastic neutron scattering study

This study presents an analysis of the diffusional dynamics of H_{2} 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 (Γ) and intensity (A), both of which are highly dependent on the momentum transfer Q. At low Q values, the experimental results for Γ align well with simulated self-diffusion coefficients. However, at higher Q, QENS data reveals a sublinear variation with respect to Q^{2} that characterizes a jump diffusion process. Using the Hall and Ross model, we determine the mean residence time (τ_{CM}) and the standard deviation of jump lengths (l_{CM}). These values provide concrete evidence for the existence of Ne "pseudocages," i.e., short-lived structures formed by neon atoms that trap H_{2} molecules, confirming a hypothesis suggested in a previous research. The study highlights a strong coupling between the H_{2} 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 H_{2} in liquid Ne, correcting previous misinterpretations that were actually due to instrumental limitations rather than physical anomalies.