A. Chahid

Collective excitations in liquid deuterium studied by inelastic neutron scattering

Abstract Well defined collective excitations up to a momentum transfer value of 3.8 A -1 have been found in liquid deuterium 0.5 K above the melting point. The derived dispersion curve shows a remarkable similarity with the one observed for liquid 4 He in the normal liquid phase, with a well defined minimum at a momentum transfer of 2.08 A -1 corresponding to a gap of 30 K. A qualitative comparison is made with recent calculations on liquid para-hydrogen at the triple point.

Single-particle dynamics of liquid CCl4: a comparison of molecular dynamics and neutron quasi-elastic scattering results

The molecular translation and rotational motions of liquid CCl4 are studied by means of the concurrent use of computer molecular dynamics simulations as well as neutron quasi-elastic scattering. The relevant correlation functions have been analysed in terms of simple analytical models and the total spectra are compared with the measured ones. The data confirms the substantial departure from simple rotational or diffusion behaviour encountered in a previous work.

Collective dynamics of liquid carbon tetrachloride studied by inelastic neutron scattering and computer simulation

An inelastic neutron scattering (INS) study of carbon tetrachloride (CCl4) at two different temperatures within its liquid range is reported. The analysis of the spectra reveals the presence of a collective inelastic response throughout the explored Q‐range (0.3 A−1≤Q≤2.2 A−1). The wave vector dependence of the excitation energies (‘‘dispersion relations’’) is investigated by means of the analysis of the measured spectral intensities in terms of a phenomenological (viscoelastic) model. In order to help with the analysis of the experimental intensities a molecular dynamics study has been carried out. The analysis of the calculated dynamical factors is also performed within the same approximation and the results are tested by comparison with those stemming from the experimental observations.

Magnetic Dynamics in Liquid Oxygen

The magnetic and structural dynamics in liquid oxygen are studied by means of high-resolution inelastic neutron scattering and computer molecular-dynamics simulations. The former technique enables an approximate separation between nuclear and magnetic responses from the total, unpolarized neutron double differential cross-sections. The magnetic scattering spectra are then shown to reproduce the wave vector dependence of the inelastic intensities associated with the magnetic excitations measured in a previous study. Finally, the magnetic spectral functions are shown to be well reproduced by a model of exchange-coupled paramagnet and estimates of the exchange parameter and correlation length are obtained from the analysis of the frequency moments.

Spin dynamics in beta -oxygen

The magnetic and structure-related excitations in solid beta -oxygen are investigated by means of high-resolution inelastic neutron scattering. An approximate separation of the magnetic response is achieved by means of modelling the structural excitations from a lattice dynamics calculation, and the reliability of such a separation assessed by comparison of the static magnetic susceptibility with that derived from polarized neutron diffraction. In addition, the analysis of the experimental strictly elastic intensity reveals the presence of static short-range magnetic order, evidencing the presence of two different characteristic correlation lengths.

Magnetic dynamics in the disordered phases of condensed oxygen

A high-resolution inelastic neutron scattering study of the liquid and plastic crystal phases of oxygen is presented. The separation of the magnetic response from the total cross sections is achieved by means of a computer molecular dynamics simulation which enables an approximate estimation of the magnetic spectral weight functions. The magnetic dynamics of the orientationally disordered plastic crystal is shown to exhibit a characteristic behaviour corresponding to an exchange-coupled paramagnet. The magnetic response of the liquid phase retains the most salient characteristics of the rotationally disordered solid phase.

Inelastic neutron and light scattering studies on liquid sulphur dioxide

The constant-volume specific heat and adiabatic sound velocity in liquid sulphur dioxide are studied by means of high resolution inelastic neutron and light scattering experiments. The generalized densities of states Z(E) are obtained from the inelastic neutron scattering spectra and show a marked temperature dependence, evidenced as quasicrystalline behaviour at temperatures just above melting, which develops a substantial intensity at zero and low frequencies as the temperature is raised. The hypersonic velocity derived from the light scattering study is shown to be consistent with previous ultrasound and thermal neutron data.

Magnetic dynamics in the disordered phases of oxygen

Abstract The results of a neutron scattering investigation on the spin and structural dynamics of oxygen in the liquid and orientationally disordered phases are presented. The presence of magnetic excitations is inferred from the analysis of the widths of the unpolarized spectra as well as from the wave vector dependence of the inelastic intensities.

Coherent excitations in liquid deuterium

The coherent dynamical response of liquid deuterium 0.5 K above the melting temperature has been studied by means of triple-axis inelastic scattering and time-of-flight techniques. A model scattering law to represent the single-particle and low energy collective dynamics has been tested by analysing the time-of-flight spectra. On the other hand, well defined collective excitations up to a momentum transfer of 3.8A-1 have been found in the triple-axis experiments. The derived dispersion curve shows a well defined minimum at 2.08A-1 with a gap of 30 K.

Neutron quasielastic scattering from molecular liquids and glasses

An overview of the present status of the theory of neutron scattering from disordered molecular systems (liquids and glasses), with the main emphasis on the theoretical modelling of spectral lineshapes, is presented. Applications to several specific cases are described in order to illustrate the capabilities of Quasielastic Neutron Scattering (QENS) for obtaining reliable information about the stochastic dynamics of these systems.