The Harmonic Picture of Nuclear Mean Kinetic Energies in Heavy Water

The effective isotropy of the hydrogen local potential in biphenyl and other hydrocarbons

We present an experimental investigation of the hydrogen nuclear momentum distribution in biphenyl using deep inelastic neutron scattering. Our experimental results suggest that the local potential affecting hydrogen is both harmonic and isotropic within experimental uncertainties. This feature is interpreted as a consequence of the central limit theorem, whereby the three-dimensional momentum distribution is expected to become a purely Gaussian function as the number of independent vibrational modes in a system increases. We also performed ab initio phonon calculations on biphenyl and other saturated hydrocarbons, from methane to decane. From the results of the simulations, one can observe that the nuclear momentum distribution becomes more isotropic as the number of atoms and normal modes in the molecule increases. Moreover, the predicted theoretical anisotropy in biphenyl is clearly larger than in the experiment. The reason is that the total number of normal modes necessary to reproduce the experimental results is much larger than the number of normal modes encompassed by a single unit cell due to the presence of structural disorder and intermolecular interactions in the real crystal, as well as coupling of different normal modes. Finally, experimental data were collected, over a subset of detectors on the VESUVIO spectrometer at ISIS, with a novel setup to increase the count rate and signal-to-background ratio. We envision that such an optimized experimental setup can provide faster measurements and more stringent constraints for phonon calculations.

Hydrogen Dynamics in Supercritical Water Probed by Neutron Scattering and Computer Simulations

In this work, an investigation of supercritical water is presented combining inelastic and deep inelastic neutron scattering experiments and molecular dynamics simulations based on a machine-learned potential of ab initio quality. The local hydrogen dynamics is investigated at 250 bar and in the temperature range of 553-823 K, covering the evolution from subcritical liquid to supercritical gas-like water. The evolution of libration, bending, and stretching motions in the vibrational density of states is studied, analyzing the spectral features by a mode decomposition. Moreover, the hydrogen nuclear momentum distribution is measured, and its anisotropy is probed experimentally. It is shown that hydrogen bonds survive up to the higher temperatures investigated, and we discuss our results in the framework of the coupling between intramolecular modes and intermolecular librations. Results show that the local potential affecting hydrogen becomes less anisotropic within the molecular plane in the supercritical phase, and we attribute this result to the presence of more distorted hydrogen bonds.

Hydrogen nuclear mean kinetic energy in water down the Mariana Trench: Competition of pressure and salinity

The Mariana Trench is one of the most famous and extreme environments on our planet. We report experimental values of the hydrogen nuclear mean kinetic energy in water samples at the same physical and chemical conditions than in the Challenger Deep within the Mariana Trench: a pressure of 1092 bars, a temperature of 1 °C, and a salinity of 35 g of salt per kg of water. Results were obtained by deep inelastic neutron scattering at the VESUVIO spectrometer at ISIS. We find that the effect of pressure is to increase the hydrogen nuclear mean kinetic energy with respect to ambient conditions, while ions in the solution have the opposite effect. These results confirm the recent state-of-the-art simulations of the nuclear hydrogen dynamics in water. The changes in the nuclear mean kinetic energy likely correspond to different isotopic fractionation values in the Challenger Deep compared to standard sea water.

Optimization of detection strategies for epithermal neutron spectroscopy using photon-sensitive detectors

In this work, we discuss an improved detection procedure for the photon-sensitive yttrium-aluminum-perovskite detectors installed on the VESUVIO spectrometer at the ISIS pulsed neutron and muon source. By decreasing the low-level energy threshold of detected photons, we observe an increased count rate up to a factor ∼3, and a decrease of relative error bars and noise of ∼40% and 35%, respectively, for deep inelastic neutron scattering measurements. In addition, we demonstrate how the reported optimization may increase the accuracy in the line shape analysis of neutron Compton profiles, as well as in the application of the mean-force approach to detect the anisotropy and anharmonicity in the single-particle local potential. We envisage that such an upgrade of the detection procedure would have a substantial impact on the VESUVIO scientific programme based on deep inelastic neutron scattering investigations.

Nuclear Quantum Effects in Water at the Triple Point: Using Theory as a Link Between Experiments

One of the most prominent consequences of the quantum nature of light atomic nuclei is that their kinetic energy does not follow a Maxwell-Boltzmann distribution. Deep inelastic neutron scattering (DINS) experiments can measure this effect. Thus, the nuclear quantum kinetic energy can be probed directly in both ordered and disordered samples. However, the relation between the quantum kinetic energy and the atomic environment is a very indirect one, and cross-validation with theoretical modeling is therefore urgently needed. Here, we use state of the art path integral molecular dynamics techniques to compute the kinetic energy of hydrogen and oxygen nuclei in liquid, solid, and gas-phase water close to the triple point, comparing three different interatomic potentials and validating our results against equilibrium isotope fractionation measurements. We will then show how accurate simulations can draw a link between extremely precise fractionation experiments and DINS, therefore establishing a reliable benchmark for future measurements and providing key insights to increase further the accuracy of interatomic potentials for water.