On the quantum contributions to phase transitions in Water probed by inelastic neutron scattering

This Thesis on the Physics of condensed matter collects some of my research activities performed during my Doctorate devoted to the measurement of nuclear quantum effects on the dynamics of hydrogen and oxygen in the water molecule. Indeed, the single-particle motion and in particular the kinetic energy are effective thermometers representing the probe to measure the deviations from classical or first-order quantum mechanics. In the introduction, I present the scientific case and highlight the complexity of the mechan ical and thermodynamic properties of water and their physical modelling. I also explain why neutron scattering has been chosen as the experimental technique to study this subject and how inelastic scattering at high energy and momentum transfers can access the single-particle motion in the case of the nuclear constituents of the molecule i.e., oxygen and hydrogen, the latter as a proton or deuteron. Then, a discussion on momentum distributions in statistical classical and quantum systems is presented, introducing models enabling a physical interpretation in the case of water or similar molecules. In particular, the use of multivariate distributions allows the measurement of the anisotropy of the effective potential which determines the nuclear motion. In particular, i) I optimized the data reduction and analysis routines reducing their running times and ii) I increased the reliability of the fitting parameters describing the shapes of the momentum distributions, reducing the number of free parameters through physical constraints. This leads to the determination of the components of the nuclear kinetic energy matrix. A harmonic model is then proposed to relate these observables to the translational, ro tational and vibrational frequencies of the molecule. This calculation is based on the model proposed by Moreh and co-workers for the interpretation of gamma nuclear resonance scatter ing from excited nuclear levels. I derived the analytical generalization to a molecule in any phase through knowledge of vibrational frequencies and molecular geometrical parameters. Moreover, I implemented the model in the data analysis routines in order to predict, guide or compare with experimental results. The results of inelastic neutron scattering experiments on the melting of light and heavy wa ter are then presented. Changes in the components of the kinetic-energy matrix are discussed in the case of phase transitions and the relevance of nuclear quantum effects is highlighted. The experimental results are compared to the harmonic model and to computer simulations based on path-integral molecular dynamics. Another set of experiments is presented on the measurement of the dynamic structure factor of supercritical water in thermodynamic states across the pseudo-critical line, i.e., the locus of points with a pressure greater than the critical pressure corresponding to a maximum of the specific heat for a temperature greater than the critical temperature. Moreover, the changes in the dynamic structure factor are studied in the amorphous-solid water phase across the transition from very-high-density to high-density amor phous ice, and then from the high-density to the low-density amorphous ice. I have been directly involved in the experiment preparation and set-up, measurements, analysis and interpretation of the experiments on heavy water, supercritical water and amorphous ice. I contributed to the data analysis and interpretation of inelastic neutron scattering experiments in super-cooled, stable liquid, supercritical water and poly-crystalline ice. This work has been carried out at the ISIS Neutron and Muon Pulsed Source at the Ruther ford Appleton Laboratory in the UK for about four months for the experiments and data analysis, at the SNS Spallation Neutron Source at Oak Ridge National Laboratory in the US for data reduction and analysis, and in Tor Vergata for the data analysis, interpretation and modelling. During the three years of Doctorate I attended to the School of Neutron Scattering Francesco Paolo Ricci (XI Edition) on the neutron investigation of bio-systems in Taormina as a student; I participated in the International Neutron Scattering Instrumentation School at the Laboratori Nazionali di Frascati as a student and local organizer; I attended the International Conference on Neutron Scattering 2013 in Edinburgh contributing to the poster session with the results on the oxygen dynamics that was for the first time measured through neutron Compton scattering; I contributed to the VI Workshop in Electron-Volt Neutron Spectroscopy meeting in Abingdon (UK) with a talk on competing quantum effects on the melting of heavy water. Since September 1st, I steadily work at ISIS in Rutherford Appleton Laboratory to the computational and experimental development of eV neutron scattering techniques.