Daniele Colognesi

This study presents an analysis of the diffusional dynamics of H 2 impurities in liquid Ne using quasielastic neutron scattering (QENS) and quantum dynamical simulations. By applying a Lorentzian fitting procedure, we identify two key spectral features: peak width ( Γ ) and intensity ( A ), both of which are highly dependent on the momentum transfer Q . At low Q values, the experimental results for Γ align well with simulated self-diffusion coefficients. However, at higher Q , QENS data reveals a sublinear variation with respect to Q 2 that characterizes a jump diffusion process. Using the Hall and Ross model, we determine the mean residence time ( τ CM ) and the standard deviation of jump lengths ( l CM ). These values provide concrete evidence for the existence of Ne “pseudocages,” i.e., short-lived structures formed by neon atoms that trap H 2 molecules, confirming a hypothesis suggested in a previous research. The study highlights a strong coupling between the H 2 diffusion and its vibration within these pseudocages, driven by the high molecular density of the system. This finding underscores the necessity of moving beyond the well-known Gaussian approximation to accurately describe the microscopic dynamics of semiquantum fluids. In conclusion, this work improves the quality of available neutron spectra for H 2 in liquid Ne, correcting previous misinterpretations that were actually due to instrumental limitations rather than physical anomalies.

The fundamental properties of ice have always attracted a lot of interest due to omnipresence of ice in many different natural contexts. Since cubic ice recently become experimentally accessible from a low-density gas hydrate precursor [1, 2], it has been possible to measure its density as a function of temperature in the whole thermodynamic range of metastability. We found strong analogies with respect to the other ice I polytype, i.e., hexagonal ice Ih [3], including the presence of a negative thermal expansion behavior at low temperature. Based on these results, a new enthalpy calculation quantifies the metastable nature of the cubic form and, consequently its inaccessibility from a "normal" ice Ih precursor.

This work provides evidence for the crucial role played by the self-part of functions as important as the dynamic structure factor S(Q, ω) and the spectrum of the transverse current autocorrelation CT(Q, ω) in studies of liquid dynamics. By using ab initio molecular dynamics simulations of liquid Ag, we show that a multimode representation also accounts very well for the simulated self-dynamic structure factor Sself(Q, ω), which provides insight on the single-particle dynamics. In particular, we provide a convincing proof that Sself(Q, ω) is responsible for the observation of transverse-like contributions in a longitudinal quantity such as S(Q, ω). It is particularly significant that an analogous situation was found when considering the transverse current. In fact, its self-part, CT,self(Q, ω), was shown to be accountable for the presence of longitudinal-like modes in a transverse quantity, such as CT(Q, ω). Due to the single-particle origin of the transverse signal in the dynamic structure factor, we also show that the apparent Q-dependence of transverse-like frequencies, as inferred from S(Q, ω), differs from the true dispersion of transverse excitations appropriately derived from CT(Q, ω).

This work provides a comprehensive picture of the advances that the exponential expansion theory (EET) of autocorrelation functions relevant to liquids dynamics made possible in the last decade up to very recent times. The role of both longitudinal and transverse collective excitations in liquids is investigated by studying the main autocorrelation functions typically obtained either experimentally (when possible) or through molecular dynamics simulations. Examples for some classes of liquids are provided, especially intended for the understanding of dispersion curves, i.e., the collective mode frequencies as a function of the wavevector Q, which is inversely proportional to the length scale at which microscopic processes are probed. The main result of this work is the ubiquitous observation that the EET method works extremely well for all considered autocorrelation functions or spectra, either experimental or simulated. This paper provides also, in its final part, important hints for future research, based on an integration of the EET lineshape description within Bayesian inference analysis.

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.

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.

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.