Neutron-scattering study of the impulse approximation in ZrH2

A Python Algorithm to Analyze Inelastic Neutron Scattering Spectra Based on the y-Scale Formalism

This paper presents a Python-based algorithm, named INSCorNorm, to correct the inelastic neutron scattering (INS) spectra for both sample and container self-shielding and to normalize the experimental spectral intensity to an absolute physical scale (barn/energy unit) facilitating the comparison with computer simulations and interpretation. The algorithm is benchmarked against INS measurements of ZrH₂ performed on the TOSCA spectrometer at the ISIS Facility. We also apply the algorithm to the INS spectra from l-lysine, a system of broad interest in biology and medicine, and we discuss how corrected INS data provide an experimental benchmark for theoretical calculations of nuclear anisotropic displacement parameters in molecular systems. The total neutron sample cross section to use for the self-shielding corrections is discussed, as well as the best approach to derive experimentally the cross section at the VESUVIO spectrometer, together with the experimental value of the hydrogen nuclear mean kinetic energy, ⟨E_k⟩. The algorithm is made available to the neutron user community within the MANTID software.

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.

Proton momentum distribution and anomalous scattering intensities in a pseudo-spherical ammonium ion: a neutron Compton scattering study of (NH(4))(2)PdCl(6) and (NH(4))(2)TeCl(6)

Neutron Compton scattering (NCS) measurements on ammonium hexachloropalladate and hexachlorotellurate were performed at room temperature. Proton scattering intensities and momentum distributions, as measured in the NCS experiment, have been compared with results expected from the impulse approximation (IA) for both systems. The measurement shows that scattering intensity from protons is anomalous even though their momentum distribution has a second moment that agrees very well with the ab initio calculation for an isolated pseudo-spherical NH(4)(+) ion in the ground vibrational state. Detailed data analysis shows that there is no extra (beyond the IA expected value) broadening or peak shift of proton momentum distribution due to ultra-fast kinetics of the Compton scattering process leading to anomalous scattering intensities. This is most probably due to highly symmetric local potential in the NH(4)(+). Presented results have interesting implications for further theoretical work in the field.

Proton momentum distribution of liquid water from room temperature to the supercritical phase

Measurements of the proton momentum distribution n(p) in water from ambient conditions to above the supercritical point are compared with theoretical calculations based on a recently developed polarizable water model. The n(p) along the H-bond direction is narrower in the dense phases, and approaches that of the isolated molecule in the more dilute phases. The theoretical model, which includes only electrostatic interactions, is unable to explain the softening of the local potential experienced by the proton in the dense phases, but it accurately predicts the n(p) for the dilute phases.

Excess of proton mean kinetic energy in supercooled water

We find, by means of a deep inelastic neutron scattering experiment, a significant excess of proton mean kinetic energy E_(k) in supercooled water, compared with that measured in stable liquid and solid phases. The measured values of E_(k) at moderate degrees of supercooling do not fit the predicted linear increase with temperature observed for the water stable phases. This anomalous behavior is confirmed by the shape of the measured momentum distribution, thus supporting a likely occurrence of ground-state quantum delocalization of a proton between the O atoms of two neighboring molecules. These results strongly suggest a transition from a single-well to a double-well potential felt by the delocalized proton, with a reduced first neighbor O-O distance, in the supercooled state, as compared to ambient condition.

Anomalous neutron Compton scattering cross sections in ammonium hexachlorometallates

The authors have performed neutron Compton scattering measurements on ammonium hexachloropalladate (NH(4))(2)PdCl(6) and ammonium hexachlorotellurate (NH(4))(2)TeCl(6). Both substances belong to the family of ammonium metallates. The aim of the experiment was to investigate the possible role of electronic environment of a proton on the anomaly of the neutron scattering intensity. The quantity of interest that was subject to experimental test was the reduction factor of the neutron scattering intensities. In both samples, the reduction factor was found to be smaller than unity, thus indicating the anomalous neutron Compton scattering from protons. Interestingly, the anomaly decreases with decreasing scattering angle and disappears at the lowest scattering angle (longest scattering time). The dependence of the amount of the anomaly on the scattering angle (scattering time) is the same in both substances (within experimental error). Also, the measured widths of proton momentum distributions are equal in both metallates. This is consistent with the fact that the attosecond proton dynamics of ammonium cations is fairly well decoupled from the dynamics of the sublattice of the octahedral anions PdCl(6) (2-) and TeCl(6) (2-), respectively. The hypothesis is put forward that proton-electron decoherence processes are responsible for the considered effect. Decoherence processes may have to do rather with the direct electronic environment of ammonium protons and not with the electronic structure of the metal-chlorine bond.

Final-state effects in neutron Compton scattering measurements on zirconium deuteride and beryllium

We report inelastic neutron scattering measurements of the neutron Compton profile, J(y), for Be and for D in polycrystalline [Formula: see text] over a range of momentum transfers, q between 27 and [Formula: see text]. The measurements were performed using the inverse geometry spectrometer eVS which is situated at the UK pulsed spallation neutron source ISIS. We have investigated deviations from impulse approximation (IA) scattering which are generically referred to as final-state effects (FSEs) using a method described by Sears. This method allows both the magnitude and the q dependence of the FSE to be studied. Analysis of the measured data was compared with analysis of numerical simulations based on the harmonic approximation and good agreement was found for both [Formula: see text] and Be. Finally we have shown how [Formula: see text], where V is the interatomic potential, can be extracted from the antisymmetric component of J(y).