R. Magli

Two-body depolarized cils spectra of krypton and xenon at 295 K

Abstract We have experimentally determined the two-body depolarized CILS spectra of krypton and xenon at room temperature between 2 and 120 cm −1 . Comparison of the first three even experimental moments of the spectra with theoretical calculations shows, as in argon, the necessity of introducing a short-range negative contribution to the induced pair polarizability.

The pair polarizability anisotropies of Kr and Xe from depolarized interaction-induced light scattering spectra

Abstract The joint use of moment analysis and comparison between calculated and experimental DILS (depolarized interaction-induced light scattering) lineshapes allows the determination of the most probable empirical pair polarizability anisotropy. This method has been applied here to the case of Kr and Xe.

The pair correlation function of krypton in the critical region: theory and experiment

We present the results of high-precision measurements of the structure factor S(k) of krypton in the near-critical region of the liquid - vapour phase transition for values of k ranging from 1.5 up to . The experimental results are compared with a theoretical calculation based on the hierarchical reference theory (HRT) with an accurate potential which includes two- and three-body contributions. The theory is based on a new implementation of HRT in which we avoid the use of hard spheres as a reference system. With this soft-core formulation we find a generally good agreement with experiments both at large k, where S(k) probes the short-range correlations, as well as at small k, where critical fluctuations become dominant. Also, for the density derivative of the pair correlation function there is an overall good agreement between theory and experiment.

Static structure of dense krypton and interatomic interaction

The static structure factor S(k) of liquid krypton has been measured by neutron diffraction at temperatures T=130, 169 and 199 K and at several densities. The extended k range of the measurements, 3.6-160 nm-1, and the high accuracy of the data allow an unambiguous computation of the radial distribution function g(r) and of the direct correlation function c(r). All three functions, S(k), g(r) and c(r), are in remarkably good agreement with the theoretical result when the Aziz pair interaction plus the triple-dipole three-body interaction are used in a triplet MHNC equation. The authors obtain definite evidence that the Lennard-Jones pair potential is not a good representation of the interatomic forces. The small remaining deviation between theory and experiment indicates the presence of some additional many-body force which is repulsive at short distance and attractive at intermediate distance.

Density evolution of the atomic momentum distribution in liquid and solid hydrogen

Abstract The density evolution of the atomic momentum distribution has been measured in liquid and solid hydrogen at constant temperature T = 20.4K and in the pressure range between 0 and 1180 bar. The experiment was carried out on the eVS spectrometer, at the pulsed neutron source ISIS, where the energy range of the available neutrons allows one to look directly at the proton wave function. The resulting kinetic energy of the protons is checked against other experimental results where the kinetic energy of centers of mass was measured.

Three-Body Effects from the Density Dependence of the Neutron Structure Factor of Gaseous 36Ar at 140 K

We present an analysis of the density dependence of the static structure factor in low density 36Ar gas at T = 140 K, from which we derive the three-body contribution. Within the experimental accuracy, the three-body contribution in the linear density expansion for the Fourier transform of the direct correlation function C(k) agrees, for 3 < k < 100 nm-1, with theoretical calculations based on a pair potential and the Axilrod-Teller three-body potential, eventually modified for short-range effects; for k < 3 nm-1 the neutron diffraction data show a behaviour significantly different with respect to the theoretical predictions.

Neutron diffraction study of long-range interactions in gaseous krypton

The low-Q part of the static structure factor S(Q) of has been derived at five low densities and room temperature by a small-angle neutron diffraction measurement and a careful treatment of experimental corrections. From the data the small-Q dependence of the Fourier transform c(Q) of the direct correlation function c(r) has been obtained and has allowed the first accurate experimental determination of the long-range three-body Axilrod-Teller potential strength in a classical fluid.

Two- and three-body long-range forces in argon and xenon by means of SANS

Abstract The long-range interaction in fluids is dominated by the attractive pair contribution U2; nevertheless it is well known and experimentally demonstrated that an accurate investigation requires the inclusion of the three-body contribution U3 even in the low-density regime. the characterization of both U2 and U3 is usually obtained by means of semi-empirical methods, the only exception being presently represented by SANS experiments on rare gases fluids. In this case the knowledge of S(k) in the low k-range allows a direct experimental determination of the U2 and U3 intensities, namely C6 and v. We report here the results of SANS experiments on low-density Ar and Xe performed at the PAXE diffractometer (LLB, Saclay). The effects of three-body forces are clearly separated from the pair contribution: the first experimental determination of C6 and v in those systems has been obtained, showing good agreement with the corresponding values found in literature.

Structure factor of liquid deuterium: a neutron diffraction experiment

Abstract The structure factor of liquid deuterium has been measured, for the first time, by means of a nuetron diffraction experiment. The experiment has been carried out at the pulsed neutron source ISIS (UK) on the SANDALS diffractometer. Five thermodynamic points were selected, in the liquid phase, near the triple point, three of them on the isotherm at 20.7 K, and the other two on the same isochore of the lowest density (25.4 nm 3 ). Owing to the SANDALS design, the low scattering angle geometry greatly reduced the effects of inelastic scattering. The high Q region of the structure factor is dominated by the single molecule term which is well approximated with a rigid rotor model, corrected by a Debye—Waller factor. As expected, this region of the diffraction pattern is independent of both density and temperature. The lower Q region contains information on the intermolecular structure factor which is modulated by the molecular form factor.

Three-body effects in simple insulating liquids: a small-angle neutron scattering study in Kr

A neutron diffraction experiment has been performed to measure the low-q structure of Kr at liquid densities. Deviations from a pure two-body behaviour are unambiguously determined in the explored q-range (1<q/nm−1<4) and are examined through the comparison between experimental results and Modified Hyper-Netted Chain (MHNC) calculations.