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No Heading By means of inelastic neutron scattering measurements, we have extracted the incoherent scattering function of liquid para-hydrogen, pure and mixed with deuterium or neon, in various thermodynamic conditions. The low-energy parts of the neutron spectra recorded in forward scattering on TOSCA-II were analyzed in the framework of the Gaussian approximation and filled through a Levesque-Vertet model for the velocity auto-correlation function. These correlation functions derived from the forward-scattering data appear totally unable to properly describe the backward-scattering ones. These findings prove a breakdown of the Gaussian approximation for semi-quantum liquids in the kinematic range of our experiment. The present results appear of extreme interest and suggest further investigation on the limits of the widely-used Gaussian approximation. [ABSTRACT FROM AUTHOR]

Hydrogen clathrate hydrates are ice-like crystalline substances in which hydrogen molecules are trapped inside polyhedral cages formed by the water molecules. Small cages can host only a single H2 molecule, while each large cage can be occupied by up to four H2 molecules. Here, we present a neutron scattering study on the structure of the sII hydrogen clathrate hydrate and on the low-temperature dynamics of the hydrogen molecules trapped in its large cages, as a function of the gas content in the samples. We observe spectral features at low energy transfer (between 1 and 3 meV), and we show that they can be successfully assigned to the rattling motion of a single hydrogen molecule occupying a large water cage. These inelastic bands remarkably lose their intensity with increasing the hydrogen filling, consistently with the fact that the probability of single occupation (as opposed to multiple occupation) increases as the hydrogen content in the sample gets lower. The spectral intensity of the H2 rattling bands is studied as a function of the momentum transfer for partially emptied samples and compared with three distinct quantum models for a single H2 molecule in a large cage: (i) the exact solution of the Schrödinger equation for a well-assessed semiempirical force field, (ii) a particle trapped in a rigid sphere, and (iii) an isotropic three-dimensional harmonic oscillator. The first model provides good agreement between calculations and experimental data, while the last two only reproduce their qualitative trend. Finally, the radial wavefunctions of the three aforementioned models, as well as their potential surfaces, are presented and discussed., (© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).)

In the rich ice polymorphism landscape, ice XVII, metastable at ambient pressure and at temperatures below 130 K, is surely one of the most interesting from both fundamental and technological perspectives due to its porosity, i.e., its capability to repeatedly absorb and desorb molecular hydrogen by dosing the gas at pressures even below the ambient one. Here, owing to this exceptional key feature, we investigate the roto-vibrational dynamics of the H2 molecules trapped in the fully deuterated ice XVII structure. Making use of the high-resolution and brilliance of the TOSCA neutron vibrational spectrometer, combined with high-resolution Raman data, we are able to efficiently distinguish the center-of-mass translational bands from the rotational ones and to study them as a function of the guest filling of the ice structure, unraveling a peculiar behavior for the confined particle in a low-dimensional system. Moreover, we also report the study of the microscopic dynamics of confined nitrogen and oxygen, which are the most abundant molecular species in the atmosphere and are of paramount interest for technological applications. Finally, we show that the ice XVII porosity is a unique feature, especially in the low pressure regime, within the emptied-hydrate phases discovered to date., (© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).)

In this paper, we report the results of a centroid molecular dynamics (CMD) study of the canonical velocity autocorrelation functions (VACFs) in liquid Ne-D_{2} mixtures at a temperature of T=30K and in the full D_{2}-concentration range (0%≤x_{D_{2}}≤100%). This binary system was selected because of its moderate, although sizable, quantum effects which, as far as its equilibrium properties are concerned, are fully described by the path integral Monte Carlo (PIMC) simulations that have been also implemented. A comprehensive test of the VACF spectral moments carried out using three physical quantities (namely, mean kinetic energy, Einstein frequency, and mean-squared force) obtained from PIMC was performed revealing the potentialities, as well as the limitations, of the CMD approach to the single-particle dynamics in these low-T liquid mixtures. Additional physical information was extracted from the canonical VACFs by fitting their spectra via two distinct methods: the Levesque-Verlet model (LV, very flexible but highly heuristic) and the itinerant oscillator model (IO, based on the physical ground of a single particle rattling inside a short-lived diffusing pseudocage). Both provided good fits of the CMD outputs, with LV being always more adequate than IO in the case of the Ne VACFs, while, as for the D_{2} VACFs, the LV superiority is evident only at high x_{D_{2}} values. However, a peculiar and systematic effect was found after analyzing the IO-fitted parameters: the estimated pseudocage masses turned out to be at least one order of magnitude lower than the corresponding values inferred from the PIMC simulations. This outcome concerns both the Ne and the D_{2} rattling molecule and, as we also discovered, had already been observed (but promptly forgotten) in purely classical simulations of liquid Ar. The possible physical origins of this finding have been finally discussed in some detail, also in connection with the result of the more recent exponential expansion theory (EET), which manages to shed more light on the concept of single particles rattling inside short-lived pseudocages, ultimately demonstrating its untenability.

A recent simulation study of the transverse current autocorrelation of the Lennard-Jones fluid [Guarini et al., Phys. Rev. E 107, 014139 (2023)] revealed that this function can be perfectly described within the exponential expansion theory [Barocchi et al., Phys. Rev. E 85, 022102 (2012)]. However, above a certain wavevector Q, not only transverse collective excitations were found to propagate in the fluid, but a second oscillatory component of unclear origin (therefore called X) must be considered to fully account for the time dependence of the correlation function. Here, we present an extended investigation of the transverse current autocorrelation of liquid Au as obtained by ab initio molecular dynamics in the very wide range of wavevectors 5.7 ≤ Q ≤ 32.8 nm-1 in order to also follow the behavior of the X component, if present, at large Q values. A joint analysis of the transverse current spectrum and its self-portion indicates that the second oscillatory component arises from the longitudinal dynamics, as suggested by its close resemblance with the previously determined component accounting for the longitudinal part of the density of states. We conclude that such a mode, albeit featuring a merely transverse property, fingerprints the effect of longitudinal collective excitations on single-particle dynamics, rather than arising from a possible coupling between transverse and longitudinal acoustic waves., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

A thorough analysis of the transverse current autocorrelation function obtained by molecular dynamics simulations of a dense Lennard-Jones fluid reveals that even such a simple system is characterized by a varied dynamical behavior with changing length scale. By using the exponential expansion theory, we provide a full account of the time correlation at wavevectors Q between the upper boundary of the hydrodynamic region and Q_{p}/2, with Q_{p} being the position of the main peak of the static structure factor. In the Q range studied, we identify and accurately locate the wavevector at which shear wave propagation starts to take place, and show clearly how this phenomenon may be represented by a damped harmonic oscillator changing, in a continuous way, from an overdamped to an underdamped condition. The decomposition into exponential modes allows one to convincingly establish not only the crossover related to the onset of transverse waves but, surprisingly, also the existence of a second pair of modes equivalent to another oscillator that undergoes, at higher Q values, a similarly smooth over to underdamped transition.

In this paper a combined neutron scattering and quantum simulation study of the collective dynamics in liquid Ne-D_{2} mixtures, at a temperature of T=30K and in the wave-vector transfer range 4 nm^{-1}

Proton dynamics in Pd 77 Ag 23 membranes is investigated by means of various neutron spectroscopic techniques, namely Quasi Elastic Neutron Scattering, Incoherent Inelastic Neutron Scattering, Neutron Transmission, and Deep Inelastic Neutron Scattering. Measurements carried out at the ISIS spallation neutron source using OSIRIS, MARI and VESUVIO spectrometers were performed at pressures of 1, 2, and 4 bar, and temperatures in the 330-673 K range. The energy interval spanned by the different instruments provides information on the proton dynamics in a time scale ranging from about 10 2 to 10 -4 ps. The main finding is that the macroscopic diffusion process is determined by microscopic jump diffusion. In addition, the vibrational density of states of the H atoms in the metal lattice has been determined for a number of H concentrations and temperatures. These measurements follow a series of neutron diffraction experiments performed on the same sample and thus provide a complementary information for a thorough description of structural and dynamical properties of H-loaded Pd-Ag membranes.

In the past few years, the exponential expansion analysis of time autocorrelation functions has provided profound insight into the leading microscopic processes driving the atomic-scale dynamics and has made it possible to highlight the presence and the role of various relaxation channels through which the fundamental correlation functions decay with time. Here we apply this method to the determination of the full time dependence of a correlation function c(t) in a quantum system at nonzero temperature, by making explicit its relationship with its Kubo transform c_{K}(t), which in some cases can be approximately computed with the presently available quantum simulation techniques. We obtain an exact expression for c(t) in terms of the exponential modes that describe the time behavior of c_{K}(t). The relative importance of the various modes in determining the overall shape of c(t) can then be studied in detail. This work extends to the full time domain the results of a previous paper [Guarini et al., Phys. Rev. Lett. 123, 135301 (2019)PRLTAO0031-900710.1103/PhysRevLett.123.135301], in which we employed the same method to calculate the zero time value of the velocity autocorrelation function, to obtain a microscopic description of the quantum mean kinetic energy in a fluid. In particular, we show that the decay constants and the frequencies of the dominant microscopic modes of c(t) are the same as those of c_{K}(t), but the dynamics of the quantum system also contains an additional term decaying on a time scale determined solely by temperature of the system.

We present an exponential mode analysis of the dynamical processes determining the time behavior of the Kubo velocity autocorrelation function (KVAF) of fluid para-H_{2}, as obtained by ring polymer molecular dynamics simulations at various fluid densities. The mechanisms contributing to the decay of the KVAF are thoroughly characterized at a slightly supercritical temperature, in a density interval ranging from the critical point to the fluid-solid transition. We show that the quantum nature of the system does not influence the specific phenomena and decay channels through which a loss in velocity correlation takes place, since these are the same as found in classical fluids. Similarly, a dynamical crossover is observed with increasing density, signaling the onset of a transverse-like dynamics like in classical systems. We also investigate the effect of density on the processes contributing to the most relevant property of a quantum fluid, namely, the large values of the total and zero-point kinetic energy arising through the Heisenberg uncertainty principle.

By applying an exponential mode analysis to ring polymer molecular dynamics simulations of dense fluid parahydrogen, we find that the dynamical processes establishing the time behavior of the Kubo velocity autocorrelation function display the same nature as those already observed in high-density classical fluids. This result permits us to demonstrate that the exponential mode decomposition is a unique tool to identify which dynamical processes lead to one of the most notable properties of quantum fluids: the large value of the mean kinetic energy per particle and the importance of the zero-temperature quantum effects in determining it.

We report the results of a ring polymer molecular dynamics study of the Kubo velocity autocorrelation function of a quantum fluid as para-hydrogen aimed at the comparison with its classical counterpart. Quite different density conditions were considered for both the classical and quantum cases, in order to compare the two systems before and after the dynamical crossover typically undergone by the velocity autocorrelation function (VAF) of fluids at densities around the triple point, where the shape of the function changes from a monotonic to an oscillatory behavior with a negative minimum. A detailed study of the phase diagram of classical para-hydrogen was necessary for a reasonable choice of the classical states to be taken into consideration, in the spirit of the classical principle of corresponding states. The shape of the quantum and classical VAF was thoroughly analyzed, exhibiting at all studied densities clear differences that might be taken as evidence of quantum effects. We show that these differences are substantially reduced by applying a state-dependent time scaling with respect to a reference time identified with the inverse of the collision rate. An even better coincidence in shape is found by comparing the two systems at slightly non-corresponding reduced densities, suggesting that the quantum system behaves almost like the classical one, but at systematically less dense reduced states of the latter. We also find an unexpected and quite interesting density trend of the collision rate of both classical and quantum para-hydrogen, which accounts for the effectiveness of the scaling throughout the explored density range. The mean kinetic energy and the diffusion coefficients are also discussed in some detail.

We have measured the dynamic structure factor of liquid neon-hydrogen mixtures (T=30.1 K) at two different H_{2} concentration levels (namely, 3.4% and 10%) making use of inelastic neutron scattering. This system has been selected since the presence of heavy Ne atoms strongly influences the self-dynamics of the H_{2} centers of mass via the formation of short-lived cages, which act both on the vibrational and the diffusive parts of the single-particle motion. After operating a standard data reduction and the subtraction of the Ne signal, experimental neutron spectra were analyzed through a generalization of the Young and Koppel model, and the H_{2} center-of-mass self-dynamic structure factor was finally extracted for the two liquid samples. Important physical quantities (namely, single-particle mean kinetic energy and self-diffusion coefficient) were estimated from the experimental data and then compared with quantum dynamical calculations, which also provided simulations of the velocity autocorrelation functions for Ne atoms and H_{2} centers of mass. The latter estimates, in the framework of the well-known Gaussian approximation, were used to simulate the H_{2} center-of-mass self-dynamic structure factor in the same kinematic range and thermodynamic conditions of the neutron scattering one. The comparison between measured and calculated spectra turned out to be qualitatively good, but some discrepancies, especially in the low-frequency part, seem to reinforce the idea of the existence of relevant non-Gaussian effects as in the case of pure hydrogen and H_{2}-D_{2} mixtures.

VESPA, Vibrational Excitation Spectrometer with Pyrolytic-graphite Analysers, aims to probe molecular excitations via inelastic neutron scattering. It is a thermal high resolution inverted geometry time-of-flight instrument designed to maximise the use of the long pulse of the European Spallation Source. The wavelength frame multiplication technique was applied to provide simultaneously a broad dynamic range (about 0-500 meV) while a system of optical blind choppers allows to trade flux for energy resolution. Thanks to its high flux, VESPA will allow the investigation of dynamical and in situ experiments in physical chemistry. Here we describe the design parameters and the corresponding McStas simulations.

We have measured the dynamic structure factor of liquid para-hydrogen mixed with normal deuterium (T=20 K) at two different concentration levels using incoherent inelastic neutron scattering. This choice has been made since the presence of D(2} modifies the self-dynamics of H(2) in a highly nontrivial way, acting both on its pseudophononic and its diffusive parts in a tunable way. After an accurate data reduction, recorded neutron spectra were studied through the modified Young and Koppel model and the H(2) center-of-mass self-dynamics structure factor was finally extracted for the two mixtures. Some physical quantities (i.e., self-diffusion coefficient and mean kinetic energy) were determined and compared with accurate quantum calculations, which, in addition, also provided estimates of the velocity autocorrelation function for the H(2) centers of mass. These estimates, in conjunction with the Gaussian approximation, were used to simulate the H(2) center-of-mass self-dynamics structure factor in the same range as the experimental one. The agreement between measured and calculated spectra was globally good, but some discrepancies proved the unquestionable breakdown of the Gaussian approximation in these semiquantum systems at a level comparable to that already observed in pure liquid para-hydrogen.

We report inelastic neutron scattering (INS) measurements on molecular hydrogen deuteride (HD) trapped in binary cubic (sII) and hexagonal (sH) clathrate hydrates, performed at low temperature using two different neutron spectrometers in order to probe both energy and momentum transfer. The INS spectra of binary clathrate samples exhibit a rich structure containing sharp bands arising from both the rotational transitions and the rattling modes of the guest molecule. For the clathrates with sII structure, there is a very good agreement with the rigorous fully quantum simulations which account for the subtle effects of the anisotropy, angular and radial, of the host cage on the HD microscopic dynamics. The sH clathrate sample presents a much greater challenge, due to the uncertainties regarding the crystal structure, which is known only for similar crystals with different promoter, but nor for HD (or H2) plus methyl tert-butyl ether (MTBE-d12).

The P2X7 receptor (P2X7R) is attracting increasing attention for its involvement in cancer. Several recent studies have shown a crucial role of P2X7R in tumour cell growth, angiogenesis and invasiveness. In this study, we investigated the role of the two known human P2X7R functional splice variants, the full length P2X7RA and the truncated P2X7RB, in osteosarcoma cell growth. Immunohistochemical analysis of a tissue array of human osteosarcomas showed that forty-four, of a total fifty-four tumours (81.4%), stained positive for both P2X7RA and B, thirty-one (57.4%) were positive using an anti-P2X7RA antibody, whereas fifteen of the total number (27.7%) expressed only P2X7RB. P2X7RB positive tumours showed increased cell density, at the expense of extracellular matrix. The human osteosarcoma cell line Te85, which lacks endogenous P2X7R expression, was stably transfected with either P2X7RA, P2X7RB, or both. Receptor expression was a powerful stimulus for cell growth, the most efficient growth-promoting isoform being P2X7RB alone. Growth stimulation was matched by increased Ca(2+) mobilization and enhanced NFATc1 activity. Te85 P2X7RA+B cells presented pore formation as well as spontaneous extracellular ATP release. The ATP release was sustained in all clones by P2X7R agonist (BzATP) and reduced following P2X7R antagonist (A740003) application. BzATP also increased cell growth and activated NFATc1 levels. On the other hand cyclosporin A (CSA) affected both NFATc1 activation and cell growth, definitively linking P2X7R stimulation to NFATc1 and cell proliferation. All transfected clones also showed reduced RANK-L expression, and an overall decreased RANK-L/OPG ratio. Mineralization was increased in Te85 P2X7RA+B cells while it was significantly diminished in Te85 P2X7RB clones, in agreement with immunohistochemical results. In summary, our data show that the majority of human osteosarcomas express P2X7RA and B and suggest that expression of either isoform is differently coupled to cell growth or activity.

We have performed high-resolution inelastic neutron scattering (INS) measurements on binary hydrogen clathrate hydrates exhibiting the hexagonal structure (sH). Two samples, differing only in the ortho/para fraction of hydrogen, were prepared using heavy water and methyl tert-butyl ether as the promoter in its perdeuterated form. The INS spectrum of the translation-rotation (TR) excitations of the guest H2 molecule was obtained by subtracting the very weak signal due to the D2O lattice modes. By means of a subtraction procedure, it has been possible to obtain separately the spectra of caged p-H2 and o-H2. sH clathrates are comprised of three distinct types of cages, two of which, differing in shape and size, are each occupied by one H2 molecule only. Both contribute to the measured INS spectrum which is, therefore, rather complex and challenging to assign unambiguously. To assist with the interpretation, the INS spectra are calculated accurately utilizing the quantum methodology which incorporates the coupled five-dimensional TR energy levels and wave functions of the H2 molecule confined in each type of nanocage. The computed INS spectra are highly realistic and reflect the complexity of the coupled TR dynamics of the guest H2 in the anisotropic confining environment. The simulated INS spectra of p-H2 and o-H2 in the small and medium cages are compared with the experimental data, and are indispensable for their interpretation.

We report inelastic neutron scattering (INS) measurements on molecular hydrogen trapped in simple (D2O) and binary (D2O plus perdeuterated tetrahydrofuran) clathrate hydrates, performed at a low temperature using two different neutron spectrometers to probe both energy and momentum transfer. The INS spectra of binary clathrate samples exhibit a rich structure containing sharp bands arising from both the rotational transitions and the rattling modes of the guest H2 molecule. They agree well with the rigorous fully quantum simulations, which account for the subtle effects of the anisotropy, angular and radial, of the host cage on the H2 microscopic dynamics and the resulting spectra. The simple clathrate samples present a much greater challenge, due to the multiple H2 occupancy of the large cages, which makes the quantum calculations an extremely difficult task. In addition, we discuss in detail various physical aspects of the experimental and simulated INS spectra, such as their temperature dependence, the effects of the cage geometry, and the different features associated with the ortho-hydrogen and para-hydrogen species.

In the present study we report neutron spectroscopic measurements on polycrystalline lithium imide, namely, incoherent inelastic neutron scattering at 20 K, and neutron Compton scattering from 10 K up to room temperature. From the former technique the H-projected density of phonon states up to 100 meV is derived, while the latter works out the spherically averaged single-particle (i.e., H, Li, and N) momentum distributions and, from this, the mean kinetic energies. Only for H at the lowest investigated temperature, non-gaussian components of its momentum distribution are detected. However, these components do not seem directly connected to the system anharmonicity, being fully compatible with the simple N-H bond anisotropy. Neutron data are also complemented by ab initio lattice dynamics simulations, both harmonic and, at room temperature, carried out in the framework of the so-called "quantum colored noise thermostat" method. The single-particle mean kinetic energies in lithium imide as a function of temperature show a quite peculiar behavior at the moment not reproduced by ab initio lattice dynamics methods, at least as far as H and Li are concerned. As matter of fact, neither their low temperature values nor their temperature trends can be precisely explained in terms of standard phonon calculations.

We have measured the Raman spectrum of ammonia borane at low temperature (T = 15 K) and across the orthorhombic-to-tetragonal phase transition at T = 225 K. A comprehensive study of the low frequency lattice modes using Raman spectroscopy has been carried out. Data analysis has been complemented by a density functional theory calculation of which the results have been used for a detailed assignment of the Raman active modes. The analysis of the spectroscopic measurements taken across the phase transition seems to be consistent with the increasing orientational disorder of the molecular components and seems to be compatible with the equalization of the a and b lattice constants characteristic of the tetragonal phase.

Inelastic neutron scattering and Raman scattering spectra of a melt-infiltrated composite of NaAlH(4) and active carbon fibers have been measured at low temperature for two sample conditions: as prepared and subjected to hydrogen desorption-absorption cycling. After a careful data analysis, the present experimental results have been compared to the corresponding spectroscopic data taken from bulk NaAlH(4) and Na(3)AlH(6). Evident signatures induced by infiltration process onto the NaAlH(4) phonon bands have been detected, showing up as a strong peak broadening and smoothing together with, in some cases, an energy shift. Traces of Na(3)AlH(6), appearing as an extra intensity between 130 and 200 meV, seem also confirmed. A substantial agreement between neutron and Raman measurements has been found for the as-prepared melt-infiltrated sample, while for the cycled sample the two techniques produced rather dissimilar results. However, this apparent discrepancy can be explained by considering the different penetration depths of the two spectroscopic probes. Further work, both experimental and based on ab initio simulations, is surely needed in order to rationalize the finding of the present measurements.

A Raman investigation of the AlH(3) polymorph has been carried out at a low temperature (20 K) under helium atmosphere (2 bar). The pristine material was composed of three polymorphs, namely, the α, β, and γ phases. The β phase has been removed by warming the sample to 70 °C, while further heating at 100 °C was used to remove the γ phase. This allowed us to evidence, on a purely experimental basis, the characteristic Raman spectrum for each phase. Raman spectra, for the three phases, have been also calculated using density functional theory, and the results have been compared to the present experimental data, allowing for a univocal assignment, to each phase, of its characteristic spectral features.