Giovanni Romanelli

Direct Measurement of Competing Quantum Effects on the Kinetic Energy of Heavy Water upon Melting

Even at room temperature, quantum mechanics plays a major role in determining the quantitative behavior of light nuclei, changing significantly the values of physical properties such as the heat capacity. However, other observables appear to be only weakly affected by nuclear quantum effects (NQEs); for instance, the melting temperatures of light and heavy water differ by less than 4 K. Recent theoretical work has attributed this to a competition between intra- and intermolecular NQEs, which can be separated by computing the anisotropy of the quantum kinetic energy tensor. The principal values of this tensor change in opposite directions when ice melts, leading to a very small net quantum mechanical effect on the melting point. This Letter presents thefirst direct experimental observation of this phenomenon, achieved by measuring the deuterium momentum distributionsn(p) in heavy water and ice using deep inelastic neutron scattering (DINS) and resolving their anisotropy. Results from the experiments, supplemented by a theoretical analysis, show that the anisotropy of the quantum kinetic energy tensor can also be captured for heavier atoms such as oxygen. (The iceberg image in the Table of Contents and Abstract graphics was used with permission of the NOAAs National Ocean Service, 2012 (http://commons.wikimedia.org/wiki/ File:Iceberg_-_NOAA.jpg).)

Evolution of Hydrogen Dynamics in Amorphous Ice with Density.

The single-particle dynamics of hydrogen atoms in several of the amorphous ices are reported using a combination of deep inelastic neutron scattering (DINS) and inelastic neutron scattering (INS). The mean kinetic energies of the hydrogen nuclei are found to increase with increasing density, indicating the weakening of hydrogen bonds as well as a trend toward steeper and more harmonic hydrogen vibrational potential energy surfaces. DINS shows much more pronounced changes in the O-H stretching component of the mean kinetic energy going from low- to high-density amorphous ices than indicated by INS and Raman spectroscopy. This highlights the power of the DINS technique to retrieve accurate ground-state kinetic energies beyond the harmonic approximation. In a novel approach, we use information from DINS and INS to determine the anharmonicity constants of the O-H stretching modes. Furthermore, our experimental kinetic energies will serve as important benchmark values for path-integral Monte Carlo simulations.

Electron-volt neutron spectroscopy: beyond fundamental systems

This work provides an up-to-date account of the use of electron-volt neutron spectroscopy in materials research. This is a growing area of neutron science, capitalising upon the unique insights provided by epithermal neutrons on the behaviour and properties of an increasing number of complex materials. As such, the present work builds upon the aims and scope of a previous contribution to this journal back in 2005, whose primary focus was on a detailed description of the theoretical foundations of the technique and their application to fundamental systems [see Andreani et al., Adv. Phys. 54 (2005) p.377] A lot has happened since then, and this review intends to capture such progress in the field. With both expert and novice in mind, we start by presenting the general principles underpinning the technique and discuss recent conceptual and methodological developments. We emphasise the increasing use of the technique as a non-invasive spectroscopic probe with intrinsic mass selectivity, as well as the concurrent use of neutron diffraction and first-principles computational materials modelling to guide and interpret experiments. To illustrate the state of the art, we discuss in detail a number of recent exemplars, chosen to highlight the use of electron-volt neutron spectroscopy across physics, chemistry, biology, and materials science. These include: hydrides and proton conductors for energy applications; protons, deuterons, and oxygen atoms in bulk water; aqueous protons confined in nanoporous silicas, carbon nanotubes, and graphene-related materials; hydrated water in proteins and DNA; and the uptake of molecular hydrogen by soft nanostructured media, promising materials for energy-storage applications. For the primary benefit of the novice, this last case study is presented in a pedagogical and question-driven fashion, in the hope that it will stimulate further work into uncharted territory by newcomers to the field. All along, we emphasise the increasing (and much-needed) synergy between experiments using electron-volt neutrons and contemporary condensed matter theory and materials modelling to compute and ultimately understand neutron-scattering observables, as well as their relation to materials properties not amenable to scrutiny using other experimental probes.

Direct Measurements of Quantum Kinetic Energy Tensor in Stable and Metastable Water near the Triple Point: An Experimental Benchmark

This study presents the first direct and quantitative measurement of the nuclear momentum distribution anisotropy and the quantum kinetic energy tensor in stable and metastable (supercooled) water near its triple point, using deep inelastic neutron scattering (DINS). From the experimental spectra, accurate line shapes of the hydrogen momentum distributions are derived using an anisotropic Gaussian and a model-independent framework. The experimental results, benchmarked with those obtained for the solid phase, provide the state of the art directional values of the hydrogen mean kinetic energy in metastable water. The determinations of the direction kinetic energies in the supercooled phase, provide accurate and quantitative measurements of these dynamical observables in metastable and stable phases, that is, key insight in the physical mechanisms of the hydrogen quantum state in both disordered and polycrystalline systems. The remarkable findings of this study establish novel insight into further expand the capacity and accuracy of DINS investigations of the nuclear quantum effects in water and represent reference experimental values for theoretical investigations.

Robust measurement of para-ortho H2 ratios to characterise the ISIS hydrogen moderators

We discuss an experimental approach to determine the concentration of para-H2 in the hydrogen moderators serving the Target Stations 1 and 2 at ISIS. The outcome of the measurements based on this approach is crucial for the Target Station 1 project, where a detailed characterisation of the present performance is needed to test neutronics simulations. The approach described here is based on the measurement of neutron transmission spectra over a wide energy range of samples extracted from the moderators. Pilot experiments were performed on the VESUVIO spectrometer at ISIS and were combined with measurements of thermal conductivity of the same samples using the ISIS para-H2 rig.

A tale of two foils: ISIS TS-1 water moderators

Currently there are two, decoupled and poisoned, water moderators at the ISIS Target Station 1 (TS-1), located above the target, operating at room temperature and providing neutrons for the experiments in the fields of spectroscopy, diffraction, etc. In this paper, the details about neutronics model of ISIS Target Station 1, simulation results related to TS-1 water moderators and comparison with corresponding measurement results are presented. The importance of the quality neutronics calculations for planning the upgrade/changes in the target station (as in removal of the poisoning foil in upstream water moderator) and locating the operational issues (disappearance of poisoning foil in downstream water moderator) is discussed.

Hydrogen mean force and anharmonicity in polycrystalline and amorphous ice

The hydrogen mean force from experimental neutron Compton profiles is derived using deep inelastic neutron scattering on amorphous and polycrystalline ice. The formalism of mean force is extended to probe its sensitivity to anharmonicity in the hydrogen-nucleus effective potential. The shape of the mean force for amorphous and polycrystalline ice is primarily determined by the anisotropy of the underlying quasi-harmonic effective potential. The data from amorphous ice show an additional curvature reflecting the more pronounced anharmonicity of the effective potential with respect to that of ice Ih.

Neutrons Matter – VII International Workshop on Electron-Volt Neutron Spectroscopy

Science is a superb way of transcending national boundaries and political circumstances. The long-standing agreement between the Italian Consiglio Nazionale delle Ricerche (CNR) and the British Sci...

Neutrons matter: VII international workshop on electron-Volt neutron spectroscopy – A preface to the workshop proceedings

We present here a collection of works reporting on the recent experimental and theoretical activities taking advantage of epithermal neutron spectroscopy, and in particular focusing on recent results presented during the VII International Workshop on Electron-Volt Neutron Spectroscopy held in Rome on 7-8 November 2018.

VESUVIO+: The Current Testbed for a Next-generation Epithermal Neutron Spectrometer

We present an overview of ongoing developments in epithermal neutron spectroscopy using the VESUVIO+ beam line at the ISIS Facility. In its current incarnation, VESUVIO+ provides a suitable platform for further and much-needed progress in the judicious exploitation of epithermal neutrons at a pulsed spallation source, as well as constitutes a necessary milestone towards a next-generation station for Epithermal and Thermal Neutron Analysis, hereafter ETNA. In particular, we discuss recent improvements in capability relative to its predecessor VESUVIO. These include the concurrent use of mass-resolved neutron spectroscopy, transmission, and diffraction to explore the properties of complex functional materials, as well as the implementation of techniques unique to pulsed neutron sources such as γ-ray dopplerimetry and energy-resolved prompt-γ activation analysis.