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

Abstract

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.A large number of experimental and theoretical dynamical studies of liquid water near the triple point are available in literature; 1−9 nevertheless, a full and accurate characterization of hydrogen dynamics is still lacking. The latter is of vital importance for clarifying thermodynamic properties and the key to expand our understanding of some of the mysterious characteristics of water, supercooled water (SW), and glassy water, the latter being its viscous counterparts, known as amorphous ice. 10 Nuclear quantum effects (NQEs) play an important role in water, ice, and hydrogen-bonded systems and directly influence their microscopic structure and dynamical properties. In most of these cases, the hydrogen atoms are localized in potential wells with pronounced zero point motion. The equilibrium hydrogen dynamics is reflected in the quantum momentum distribution, n(p), a quantity which provides complementary information to what is garnered from diffraction techniques. Because of NQEs, n(p) markedly differs from the classical Maxwell−Boltzmann distribution, being determined almost entirely by the quantum mechanics of the vibrational ground state properties. 11−17 This makes n(p) a highly sensitive probe of the local environment, fingerprinting any changes occurring both in the structure of the hydrogen-bonded network as well as in the local symmetry. Thus, n(p) together with the mean kinetic energy, ⟨E K ⟩, provide key insights into the hydrogen local environment to rationalize the puzzling feature of liquid water near the triple point. DINS is the unique experimental technique that directly access the n(p). 18 The basic principles of data interpretation of the DINS technique are based on the validity of the impulse approximation (IA), 19 which is exact in the limit of infinite momentum transfer, ℏq. 20,21 Within the IA, the inelastic neutron sc...

The onset of the tetrabonded structure in liquid water

“…NCS measurements were performed at four temperatures, 300, 315, 320, 325 K and at atmospheric pressure. Temperature was controlled with the precision of 0.1 K. Details of the experimental apparatus and measurement technique are reported in previous papers [22,53,54]. NCS is described within the framework of the impulse approximation (IA) [20,55], valid at high energy and momentum transfers [21,56,57], with the scattering intensities centred along the atomic recoil lines [21].…”

The road to a station for epithermal and thermal neutron analysis

“…State-of-the-art computer simulations including NQEs, based on the path integral formalism [20], have found in DINS experiments a much-needed benchmark, such as in [21,9]. However, the time-consuming nature of first-principles simulations has pushed the community to look for new strategies [22].…”

“…Looking at the proceedings of the VI (2014) [1] and VII (2017) international workshops on eV neutron spectroscopy [2], one can notice how the use of Deep Inelastic Neutron Scattering (DINS) to characterise Nuclear Quantum Effects (NQEs) mostly on hydrogen dynamics [3] has lead to MAss-selective Neutron SpEctroscopy (MANSE), whereby the Nuclear Momentum Distributions (NMDs) of several elements in a given sample are accessed at the same time [4,5,6]. Furthermore, a number of recent investigations carried out on the VESUVIO spectrometer [7], the flagship instrument for eV neutron spectroscopy at the ISIS Facility [8], successfully combined DINS with concurrent Neutron Diffraction (ND) measurements [9,10], or employed broad-range Neutron Transmission (NT) [11,12]. Moreover, a novel trend of concurrent characterisation of condensed-matter systems by means of DINS, ND, and Neutron Resonance Capture Analysis (NRCA) is proving to be a potent strategy and thus is gaining momentum [13].…”

The quantum nature of hydrogen

“…In terms of more direct experimental probes, deep inelastic neutron scattering (DINS) has emerged as an approach that can directly measure the momentum distribution of protons [9]. It can provide insights into the local environment of H atoms, which can help elucidate NQEs in hydrogen bond (HB) networks, such as in structures of crystalline and amorphous ice [9][10][11][12].…”