F. Barocchi

Line shape and moment analysis in depolarized induced light scattering

We compare the two methods of deriving interaction-induced pair polarizabilities from collision-induced, depolarized spectra, namely classical line shape and moment analysis. We discuss their limitations and show that the joint use of line shape and moment analysis leads to the determination of an empirical pair polarizability anisotropy model which has maximal consistency with the available experimental results. We have applied the combined method to the pair spectrum of argon at room temperature (298 K) as an example.

Three-body potential effects in the structure of fluid krypton

The low-density expansion and a new integral equation have been used to calculate the pair correlation function and several related quantities for krypton, using realistic pair potentials and several models for the three-body potentials: the Axilrod-Teller-Muto model, the multipolar model up to the triple quadrupole term and the Loubeyre model. The results are compared with experimental data in some detail. The models describe the three-body effects in correlation functions in a qualitative sense, but quantitative differences are observed, suggesting that shorter-ranged effects must also be included.

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.

Induced pair polarizability anisotropy in mercury from depolarized interaction-induced light scattering spectra

Abstract Depolarized interaction-induced light scattering spectra of mercury between T = 873 and 1173 K have been used for deriving a pair polarizability anisotropy. Absolute zeroth and second moments have been measured and compared with theoretical calculations using various models for the pair polarizability anisotropy. The results show that gaseous mercury behaves quite differently from noble gases, since during collision the mercury pair develops a large intermediate-range positive anisotropy, revealing the breakdown of the classical dipole-induced-dipole approximation and the onset of specific electronic interactions.

Comment on the detailed balance symmetrization procedure of classical spectra

By means of the Wigner theory we derive an expression for the first approximation with respect to ħ of the spectrum related to a general correlation function and its zero-th moment. We then show that the symmetrization of spectra with simply the application of the detailed balance term to the classical spectra is valid in general only for ω ≪ 2kT/ħ .

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.

The microscopic structure of liquid mercury from neutron and X-ray diffraction

Abstract We report good agreement between neutron and X-ray measurements of the static structure factor S(Q) of liquid mercury, as well as the first determination of its isothermal density derivative from neutron measurements.

Intermolecular structure of liquid deuterium : a new neutron diffraction investigation

The static centre of mass structure factor of liquid deuterium, already determined by means of time of flight neutron diffraction (as reported in Physical Review E in 1993), has been re-investigated with a two-axis diffractometer at a reactor source. A detailed analysis of the data is presented, with special attention devoted to the correction for inelastic scattering effects. The comparison of the two sets of data reveals the presence of discrepancies, which cannot be attributed to statistical inaccuracies. The density and the temperature derivatives of the structure factor have also been measured with high accuracy. For these quantities, the two experiments are in good agreement with each other. This suggests that the discrepancy in the structure factor is due to systematic effects which vanish in a differential measurement.

Self-dynamics and collective dynamics of liquid mercury

Abstract The low-momentum dynamics of liquid mercury has been investigated by means of inelastic neutron scattering and molecular dynamics (MD) simulations. Because of the rather high incoherent cross-section and the high mass of mercury, the measured dynamic structure factor is dominated by the self-correlation function, although the low-momentum collective dynamics are made accessible by the very good energy resolution of the experiment. Collective modes are clearly visible against the incoherent scattering up to a momentum transfer of 0.4Å−1 and the associated velocity is found to be 2200 ± 200 m s−1, well in excess of the sound velocity which is equal to 1470 m s−1. The self-dynamics, as resulting from either the experiment or the MD simulations, turn out to be characterized by two time scales.