Microscopic dynamics of gas molecules confined in porous channel-like ice structure.

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.

Density and time scaling effects on the velocity autocorrelation function of quantum and classical dense fluid para-hydrogen.

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.

Proton vibrational dynamics in lithium imide investigated through incoherent inelastic and Compton neutron scattering.

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.

Raman spectra of ammonia borane: low frequency lattice modes.

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.

Raman and inelastic neutron scattering study on a melt-infiltrated composite of NaAlH4 and nanoporous carbon.

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.

Temperature behavior of the AlH3 polymorph by in situ investigation using high resolution Raman scattering.

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, ß, and γ phases. The ß 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.

High resolution Raman and neutron investigation of Mg(BH4)2 in an extensive temperature range.

Raman spectra of Mg(BH(4))(2) have been measured in an extensive temperature range, from 15 to 473 K. Taking into account the high temperature conversion from the alpha to the beta phase, we have observed evident signatures of this phase transition and determined the Raman vibrational spectrum of each phase. The neutron scattering spectra of the beta phase sample were also recorded. The present experimental results have been compared to the density functional theory calculations available in the literature, and a substantial agreement has been found.

First principle calculations of alkali hydride electronic structures.

Electronic structure, volume optimization, bulk moduli, elastic constants, and frequencies of the transversal optical vibrations in LiH, NaH, KH, RbH, and CsH are calculated using the full potential augmented plane wave method, extended with local orbitals, and the full potential linearized augmented plane wave method. The obtained results show some common features in the electronic structure of these compounds, but also clear differences, which cannot be explained using simple empirical trends. The differences are particularly prominent in the electronic distributions and interactions in various crystallographic planes. In the light of these findings we have elaborated some selected experimental results and discussed several theoretical approaches frequently used for the description of various alkali hydride properties.

Signatures of quantum behavior in the microscopic dynamics of liquid hydrogen and deuterium.

We discuss the microscopic dynamics and structure of liquid hydrogen and deuterium, as probed by inelastic x-ray scattering measurements. Samples are kept in corresponding thermodynamic conditions, at which classical systems are expected to exhibit the same dynamic and static responses. On the contrary, we observe clear differences revealing the onset of quantum deviations, both in the broadening of inelastic excitations and in the position of the first sharp diffraction peak. These features are discussed, compared to path-integral Monte Carlo simulations, and finally associated with the different de Broglie wavelengths of the two isotopes.

Density of phonon states in solid parahydrogen from inelastic neutron scattering.

We have measured the inelastic neutron scattering spectrum of solid parahydrogen (at low pressure and T=13.3 K) using the thermal original spectrometer with cylindrical analyzers spectrometer at the ISIS pulsed neutron source (UK). From the experimental spectrum we have obtained the parahydrogen density of phonon states which has been compared with the estimates available in the literature. The present determination improves substantially the previous experimental scenario from the point of view of both statistics and accuracy. The comparison with the most recent estimate obtained from a quantum mechanical simulation of the molecular dynamics calls for an improvement of the computational methods..

Microscopic self-dynamics in liquid hydrogen and in its mixtures with deuterium.

We have measured the dynamic structure factor of liquid parahydrogen, pure and mixed with deuterium, in various thermodynamic conditions using incoherent inelastic neutron scattering. The experiments were carried out on TOSCA-II, a new time-of-flight, inverse-geometry, crystal-analyzer spectrometer. After an accurate data reduction, the high-energy parts of the neutron spectra recorded in backward scattering were studied through the modified Young and Koppel model, from which the mean kinetic energy values for a hydrogen molecule were estimated. In addition the low-energy parts of the neutron spectra recorded in forward scattering were analyzed in the framework of the Gaussian approximation and fitted through a Levesque-Verlet model for the velocity autocorrelation function. Thus various physical quantities are determined and compared with accurate path integral Monte Carlo simulations. Despite the excellent quality of these fits, the velocity autocorrelation functions derived from the forward-scattering data appear totally unable to properly describe the backward-scattering ones. These findings prove an unquestionable breakdown of the Gaussian approximation in semiquantum liquids. The present results appear of great interest and suggest further investigation on the limits of the widely used Gaussian approximation.

Direct experimental access to microscopic dynamics in liquid hydrogen.

We have obtained the double-differential incoherent neutron scattering cross section of liquid and solid parahydrogen in various thermodynamic conditions using TOSCA, a time-of-flight, inverse geometry, crystal analyzer spectrometer, operating at the pulsed neutron source ISIS. The measured cross section provides direct experimental access to the self part of the center-of-mass inelastic structure factor of the parahydrogen molecules in the system. Data have been corrected for the experimental effects and then analyzed in the framework of the Young-Koppel model and the Gaussian approximation. The velocity autocorrelation functions and their energy spectra have been obtained from a fitting procedure, making use of the quantum generalized Langevin equation and of model memory functions, and finally compared to the most recent results of both molecular centroid dynamics and self-consistent quantum mode-coupling theory. Some dynamic quantities were also related to simple equilibrium properties and simulated through a standard path integral Monte Carlo code. Results are very interesting but still urge for further developments of theoretical and dynamic simulation approaches, as well as for more extensive experimental efforts.

Deep-inelastic neutron scattering determination of the single-particle kinetic energy in solid and liquid 3He.

An experimental determination of the single-particle mean kinetic energies for 3He along the T = 2.00 K isotherm in the dense liquid and in the solid hcp and bcc phases is reported. Deep-inelastic neutron scattering measurements at exchanged wave vectors ranging from 90.0 to 140.0 A(-1) have been performed in order to evaluate, within the framework of the impulse approximation, the molar volume dependence of the kinetic energy. The results are found in excellent agreement with recent diffusion Monte Carlo simulations.