Neutron scattering study of thermally excited density fluctuations in a dense classical fluid

Postol, T. A.; Pelizzari, Charles A.

doi:10.1103/physreva.18.2321

Cited by 28 publications

(10 citation statements)

References 33 publications

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“…Moreover, investigating the nature of the collective excitations at high-Q values, we find that the three mode description can still be used, and it gives a minimum in the dispersion curve around Q m , as reported previously in the liquid [6]. Up to now, similar experiments on the S͑Q, E͒ of dense gases have been performed with neutrons at Q substantially lower than Q m , and at densities lower than those of the liquid [8,9].…”

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confidence: 53%

“…Up to now, only neutron Brillouin scattering techniques have provided the opportunity to study the dynamics in a Q region below and around Q m , where the S͑Q, E͒ is deeply influenced by the microscopic structure. This has been done in liquid neon [5,6] and in liquid and gaseous argon [7][8][9].The development of the inelastic x-ray scattering (IXS) technique with meV energy resolution opens up new opportunities to explore microscopic dynamic properties of dense fluids in Q-E regions of difficult access to neutron spectroscopy. Furthermore, the small x-ray beam sizes give one the opportunity to explore thermodynamic states which can be produced only in small volumes, as noble gases at temperatures well above the critical point and at densities either comparable or higher than the triple point.…”

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confidence: 99%

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confidence: 99%

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Dynamics of Dense Supercritical Neon at the Transition from Hydrodynamical to Single-Particle Regimes

et al. 1998

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The dynamic structure factor, S͑Q, E͒, of neon has been studied by inelastic x-ray scattering and by molecular dynamic simulation in the momentum transfer region Q 1 25 nm 21 at T 295 K and P 3 kbar. At this density, comparable to that of the liquid at ambient pressure, the shape of S͑Q, E͒ evolves from a Brillouin triplet towards a complex single line shape, which precurs the shape expected for single-particle behavior. The data, analyzed using the three mode model of the molecular hydrodynamics, show the absence of positive dispersion in the sound velocity and a minimum in the dispersion curve. [S0031-9007(98)05931-6] PACS numbers: 51.10. + y, 61.10.Eq, 61.20.Ja, 78.70.Ck During the past decades, much effort has been devoted to studying the dynamic structure factor, S͑Q, E͒, of dense fluids in both liquid and gaseous phases. One of the aims is the understanding of the transition from the hydrodynamics regime towards the so-called kinetic one [1], as a function of the density r and of the exchanged momentum Q. These two different regimes can be distinguished by a length scale characteristic of the system. This is often chosen as the Enskog mean free path l E defined as l E l B ͞g͑r 0 ͒, where l B is the Boltzmann mean free path, l 21 B prr 2 0 ͞2, and g͑r 0 ͒ is the pair distribution function evaluated at the particles' radius r 0 . For excitations with wavelength 2p͞Q much larger than l E , the fluid appears as a continuum, and the Navier-Stokes equation can be used to derive the S͑Q, E͒. At intermediate Q values Ql E ഠ 1, the breakdown of the hydrodynamics theory is expected to occur. For Ql E ¿ 1, the dynamics becomes that of a single free particle between two collisions with its neighbors. In the two limit cases the dynamic structure factor has a well-known shape and a simple physical interpretation: (i) In the small Ql E limit there are three modes. These are, respectively, the two Stokes and anti-Stokes compression (sound) modes, which disperse linearly with a slope corresponding to the adiabatic velocity of sound, and the heat diffusion mode, which is centered at zero energy transfer and has a width proportional to D T Q 2 , D T being the thermal diffusion coefficient. (ii) In the high Ql E limit, within the impulse approximation, the line shape reflects the initial state momentum distribution, i.e., the Boltzmann distribution. Here, the S͑Q, E͒ reduces to a Gaussian centered at the recoil energyh 2 Q 2 ͞2M, where M is the particle mass.The extension of a three mode description of S͑Q, E͒ beyond the hydrodynamic limit, where Ql E approaches unity, has been suggested by kinetic theory [2] and several molecular dynamics studies performed with both hard spheres [3] and Lennard-Jones potentials [4]. Firm experimental data covering in detail the whole transition region up to Q m , the Q value of the first maximum in the static structure factor, are not yet available in high density fluids above the critical point, in spite of the speculative interest to have a complete overview of all dynamic processes at b...

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confidence: 53%

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confidence: 99%

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Dynamics of Dense Supercritical Neon at the Transition from Hydrodynamical to Single-Particle Regimes

et al. 1998

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“…Unfortunately, the predictions performed using the kinetic theory usually give poor results at high density due to the increasing weight of correlated collisions. The INS measurements of Postol and Pelizzari on supercritical Ar [60] proposed an experimental test of the kinetic theory predictions based on the generalized Enskog approach. As a result, no quantitative agreement was demonstrated between theoretical and experimental results, although at the low density (10.5 atoms/nm 3 ) and high temperature (295 K) probed in the experiment it was reasonable to assume the kinetic theory description to hold validity.…”

Section: -1975: First Signatures Of a Viscoelastic Behavior In Rementioning

confidence: 99%

The Spectrum of Density Fluctuations of Noble Gases Probed by THz Neutron and X-ray Spectroscopy

Cunsolo

2016

Applied Sciences

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Approximately 50 years of inelastic scattering studies of noble gases are reviewed to illustrate the main advances achieved in the understanding of the THz dynamics of simple systems. The gradual departure of the spectral shape from the hydrodynamic regime is discussed with an emphasis on the phenomenology of fast (sub-ps) relaxation processes. This review shows that relaxation phenomena in noble gases have an essentially collisional origin, which is also revealed by the parallelism between their characteristic timescale and the interatomic collision time. Additionally, recent THz spectroscopy results on noble gases at extreme thermodynamic conditions are discussed to illustrate the need for a revision of our current understanding of the supercritical phase.

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“…Moreover, it is an exceptionally convenient sample for neutron studies due to the very large, totally coherent, scattering cross-section of the 36 Ar isotope. Well-known pioneering neutron works on the collective dynamics of argon are, among others, those of references [29,30] in the liquid and [44] in the compressed gas phase, while [45] reports on a light scattering experiment. Though sound modes were in some cases detected, depending on the investigated Q range, and obviously with particular evidence in the light Brillouin case where Q = 0.017 nm −1 (see figure 3 of [45]), the first quantitative analysis of acoustic excitations was carried out in the neutron study of reference [46].…”

Section: Rare Gasesmentioning

confidence: 99%

Collective dynamics in noble-gas and other very simple classical fluids

Bafile¹,

Barocchi²,

Guarini³

2008

Condens. Matter Phys.

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Rare gases and their liquids are the simplest systems to study for accurate investigations of the collective dynamics of fluid matter. Much work has been done using different spectroscopic techniques, moleculardynamics simulations, and theoretical developments, in order to gain insight into the microscopic processes involved, in particular, in the propagation of acoustic excitations in gases and liquids. Here we briefly review the interpretation schemes currently applied to the characterization of such excitations, and recall a few results obtained from the analysis of rare-gas fluids and other very simple systems.

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Neutron scattering study of thermally excited density fluctuations in a dense classical fluid

Cited by 28 publications

References 33 publications

Dynamics of Dense Supercritical Neon at the Transition from Hydrodynamical to Single-Particle Regimes

Dynamics of Dense Supercritical Neon at the Transition from Hydrodynamical to Single-Particle Regimes

The Spectrum of Density Fluctuations of Noble Gases Probed by THz Neutron and X-ray Spectroscopy

Collective dynamics in noble-gas and other very simple classical fluids

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