Zero-Point Motion of Liquid and Solid Hydrogen

TL;DR

This study uses inelastic neutron scattering to investigate zero-point motion in liquid and solid hydrogen, revealing temperature-dependent quantum effects and confirming theoretical models with experimental data.

Contribution

It provides empirical estimates of molecular displacement and kinetic energy in hydrogen, highlighting the importance of quantum effects near phase transitions.

Findings

Molecular mean-squared displacement increases with temperature.

Kinetic energy drops significantly upon melting.

Good agreement with quantum Monte Carlo simulations.

Abstract

We present an inelastic neutron scattering study of liquid and solid hydrogen carried out using the wide Angular Range Chopper Spectrometer at Oak Ridge National Laboratory. From the observed dynamic structure factor, we obtained empirical estimates of the molecular mean-squared displacement and average translational kinetic energy. We find that the former quantity increases with temperature, indicating that a combination of thermal and quantum effects is important near the liquid-solid phase transition, contrary to previous measurements. We also find that the kinetic energy drops dramatically upon melting of the crystals, a consequence of the large increase in molar volume together with the Heisenberg indeterminacy principle. Our results are compared with quantum Monte Carlo simulations based on different model potentials. In general, there is good agreement between our findings and theoretical predictions based on the Silvera-Goldman and Buck potentials.