M. García-Hernández

Collective excitations in liquid methanol: A comparison of molecular, lattice‐dynamics, and neutron‐scattering results

The collective dynamics of liquid methanol‐d4 is studied by means of molecular‐dynamics simulation. The model potential is validated by means of lattice energy calculations and shows a very good agreement with the experimentally obtained crystal structure. Center‐of‐mass density and momentum fluctuations are investigated in the (Q,ω) region which is also accessible to inelastic neutron‐scattering (INS) techniques. A simple viscoelastic model previously used for the analysis of INS data is tested against the dynamic structure factor computed from the simulation. A direct comparison with the INS results themselves is also made and qualitative agreement is found. Also, a tentative assignment of the peaks appearing in the current–current correlations is made on the basis of lattice‐dynamics calculations for the polycrystalline low‐temperature α phase.

H-Bond in methanol: a molecular dynamics study

Abstract A set of molecular dynamics (MD) simulations of methanol- d 4 at three temperatures in the liquid range (200, 250 and 300K) have been carried out. The equations of motion of 256 molecules interacting through a potential model due to Haughney et al. [J. Phys. Chem., 91 (1987) 4934] were solved using the velocity version of the Verlet algorithm. This rather large number of molecules was required for studying the behaviour of the system at momentum transfers as low as 0.25 A −1 . It was found that the system experiences long period fluctuations, and therefore very long MD runs (of the order of 100 ps) are necessary in order to obtain accurate statistical averages. Computed static properties are in good agreement with those reported by Haughney et al. and the neutron weighted g ( r ) and the static structure factor compare favourably with available neutron diffraction data. The study of time-dependent properties through centre-of-mass autocorrelation functions (VACF, F s ( Q,t ) and F ( Q,t )) and their memory functions reveals features unknown in simple liquids and very similar to those found in liquid water. A close agreement between centre-of-mass single-particle autocorrelation functions and the translational part of QENS data is also observed. The dynamic structure factor for the centres of mass show distinctive side peaks in the same region of the ( Q,ω ) plane where recently collective excitations have been studied using coherent neutron scattering thus establishing the presence of propagating short wavelength modes. fa]Presented at the International Symposium on Hydrogen Bond Physics held at Il Ciocco, Barga, Italy, 11–14 September 1990.

Coherent Inelastic Neutron Scattering Response from Liquid Methanol

Inelastic spectra of deuteromethanol at 200 K measured by triple-axis neutron spectroscopy are reported. A reconstruction of the dynamical structure factor from the observed spectral intensity has been achieved by means of a deconvolution procedure. A complementary analysis of the spectra in terms of simple phenomenological models (hydrodynamic and viscoelastic) is also presented. The experimental data indicates the existence of a propagating mode which becomes overdamped for wave vectors ≥ 0.6 A−1.

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.

Collective low-frequency excitations in a molecular glass

The excitations below 40 meV of a glass-former mixture (CD3OD 6% D2O) have been studied by neutron spectroscopy. A well-resolved inelastic peak with a dispersion behaviour reminiscent of those found in metallic glasses has been observed for temperatures above 10 K. The presence of such peak is correlated with an abrupt enhancement of the total vibrational density of states (TDOS). The nature of the excitations is analysed by means of their wave vector dependence. Finally, the TDOS are tentatively interpreted in terms of a coupling between sound waves and localized excitations, which becomes effective for temperatures above 10 K.

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.

Observation of high frequency excitations in a molecular glass

Abstract Neutron inelastic triple-axis spectra of CD 3 OD/6% D 2 O in the glassy and liquid phases are reported. Both the dispersion and damping of the sound modes show characteristic features as the temperature is lowered down into the glassy regime. Also the analysis of the observed scattering intensities evidences the presence in the glass of a second component centered at zero frequency which is not observed in the liquid.

Observation of propagating collective excitations in liquid SO2

The spectrum of collective excitations in liquid sulphur dioxide at two thermodynamic states has been measured by neutron triple‐axis spectroscopy for momentum transfer ranges of 0.35≤Q≤2.0 A−1. Well‐resolved inelastic peaks indicating the presence of propagating modes are found for Q values below 0.5 A−1 at T=266 K and below 0.7 A−1 at T=210 K. A precise value for the hydrodynamic limit has been found in both cases and the measured ‘‘dispersion relations’’ are discussed in relation to predictions made by recent theoretical approaches.

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.