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The origin of the well-defined collective excitations found in liquid para-H 2 by recent experiments is investigated. The persistence of their relatively long lifetimes down to microscopic scales is well accounted for by calculations carried out by means of path-integral-centroid molecular dynamics. In contrast only overdamped excitations are found in calculations carried within the classical limit. The results provide fully quantitative evidence of quantum effects on the dynamics of a simple liquid. PACS numbers: 67.20. + k, 62.10. + s, 62.60. + v In spite of being composed of the simplest stable molecules, the transport and kinetic properties of liquid hydrogen still resist a quantitative understanding [1]. Such peculiar behaviors are usually thought to arise because of the light masses of the particles forming the liquid (M ഠ 2 amu) and the relatively low temperatures where it exists under its saturated vapor pressure. Quantum effects are thus expected to be noticeable although their relevance for understanding quantitatively most of the transport properties awaits to be established in full. These effects are first manifested by the appearance of a discrete spectrum of transitions between molecular rotational levels. The quantum nature of such motions imposes some symmetry constraints to the total molecular wave function. In particular, the rotational and nuclear spin states of the two protons forming the H 2 molecule are coupled, leading to two distinguishable species, para͑p͒-H 2 and ortho͑o͒-H 2 corresponding to molecules having antiparallel (I 0), and parallel (I 1) spin states, respectively. This has an immediate consequence on the symmetry of the interaction potential between H 2 molecules which is isotropic for p-H 2 molecules but angular dependent for o-H 2 . It is so because the orientational distribution of the internuclear bond in the laboratory frame is spherically symmetric (s-like) for the former and axially symmetric (p-like) for the latter. A noticeable manifestation of quantum behavior is expected concerning the spatial smearing of the molecular wave function. This comes because of the low molecular mass and the low temperatures in question (about 10-20 K) which makes the thermal (de Broglie ) wavelength l ͑2ph 2 ͞Mk B T ͒ to reach values larger than the molecular dimensions. In fact, close to triple point, l ഠ 3.3 Å which becomes comparable to the equilibrium separation between two p-H 2 molecules, r 0 ഠ 3.5 Å. In addition to the spatial spread of the wave function a delocalization time t d can be defined in terms of the fluid number density r and the particle mass through [2] t d M͞2ph͑r͞2.612͒ 2͞3 , which for the triple point density yields an estimate for t d ഠ 1.2 ps. Finally, exchange effects arising because of the indistinguishability of the molecules are deemed to play a very minor role since the temperatures in question are well above the quantum degeneracy temperature T 0 h͞k B t d 6.5 K and therefore most particles will be accommodated in excited levels [3].Two sets of recent in...

We determined the self part of the intermediate scattering function in liquid polyethyleneoxide ͑PEO͒ and PEO-alkali iodide complexes by means of neutron spin-echo spectroscopy and molecular dynamics ͑MD͒ computer simulations. We present the first accurate quantitative results on the segmental dynamics in the time range up to 1 ns and the wave-vector range from a few nm Ϫ1to approximately 20 nm Ϫ1 . We investigate the influence of polymer chain length, salt concentration, and cation type. We find that the neutron data and MD data for pure PEO agree very well. A relatively small concentration of dissolved salt ͑1 metal ion per 15 monomers͒ leads to a slowing down of the segmental motions by an order of magnitude. Here, the MD simulations agree qualitatively. Increasing the chain length from 23 to 182 monomers has no significant effect except at the highest salt concentration. Similarly, changing the cation from Li to Na hardly makes any difference. The Rouse model does not adequately describe our data. © 2000 American Institute of Physics. ͓S0021-9606͑00͒51525-6͔ Amorphous polymer electrolytes provide an environment-friendly alternative for liquid electrolytes used in batteries, fuel cells, electrochemical displays, and chemical sensors.1 A polymer electrolyte is a complex of a polar polymer with a metal salt. In order to optimize performance of applications, it is of fundamental importance to understand the mechanism of ion transport, which is closely coupled to the segmental motions of the polymer chain. The systems most studied are poly͑ethyleneoxide͒ ͑PEO͒ and poly͑propyleneoxide͒ ͑PPO͒ salt complexes.From Brillouin light scattering of PPO-salt systems 2 and MD simulations of PEO-NaI systems 3 it appears that the Na ϩ ions form crosslinks between different oxygen atoms within a polymer chain, which causes slowing down of movement of polymer segments. Quasielastic neutron scattering measurements on the PPO-LiClO 4 complex have confirmed this effect, but because of the limited energy resolution it was impossible to obtain quantitative results for the effect of solvated salt on the structural relaxation.4 Londono et al. 5 have performed neutron diffraction with isotopic Li substitution in combination with MD simulations in order to determine the partial pair distribution function g Li,O (r). They obtained a Li-O coordination number of about 3.5 for PEOLiI ͑O:Mϭ5, which is the number of ether oxygens of the polymer chain per metal ion͒, confirming crosslinking between cations and ether oxygens. It has been shown that the conductivity characteristics for PPO-Li salt and PPO-Na salt are very similar.6 Therefore, we expect that the influence of Li and Na on the polymer dynamics in PEO is similar.Until today, no quantitative results were available on the local dynamics of the backbone segments of the polymer nor on the influence of various parameters such as salt concentration, polymer chain length, and different ions. Neutron spin echo ͑NSE͒ is the technique of choice regarding energy resolution and wave-vector rang...

Microscopic motions in molten potassium spanning three frequency decades are studied by neutronscattering techniques. These comprise well-defined density oscillations and stochastic particle rearrangements and both are modeled on microscopic grounds. While vibratory motions are shown to share characteristics with those of their parent crystals, dynamic correlations between a diffusing particle and its neighbors can be accounted for only semiquantitatively.

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A recent experiment ͓Phys. Rev. Lett. 80, 2141 ͑1998͔͒ showed heavily damped excitations in molten Li 4 Pb, within kinematic scales well beyond those of hydrodynamic sound. These findings pointed to the presence of short-lived out-of-phase atomic motions as the underlying microscopic phenomenon. A series of computer molecular dynamics studies are performed to investigate the details of the atomic motions. From an analysis of the simulated structure factors for molten Li 4 Pb, as well as by a comparison with those of liquid Li under different thermodynamic conditions, it is found that the high-frequency excitation found in the alloy shows characteristics remarkably different from those of pure Li. The relative phases of the atoms partaking in such motions, as well as the remarkably short excitation lifetimes, portray it as a fairly localized mode, with a frequency dependent polarization. ͓S1063-651X͑98͒07210-9͔PACS number͑s͒: 61.20. Lc, 61.25.Mv, 61.12.Ϫq

Heavily damped excitations are found in molten Li 4 Pb by inelastic neutron scattering. The experiment covered a kinematic range which enabled an unambiguous characterization of such excitations by means of the study of the wave vector dependence of their frequencies, lifetimes, and signal amplitudes. It is shown that the excitations being sampled exhibit features which substantially deviate from those expected for the propagation of an acoustic mode (which should involve in-phase atomic displacements). [S0031-9007(98) [7,8], and even some semimetallic liquids [9]. The most salient feature of such dynamic phenomena concerns their frequencies, which are well above those expected for a continuation to large wave vectors of hydrodynamic sound. In fact, kinetic-theory predictions [6] portray such excitations as being supported by the light component only so that they apparently travel with phase velocities close to those characteristic of the pure component, which are well above those given by the elastic constants of the mixture.Results from experiment and computer simulation on a variety of systems [1][2][3][4][5][6][7][8] can show well defined, heavily damped or overdamped features in S͑Q, v͒. If the motions being sampled are heavily damped or overdamped, only broad shoulders are visible, so that characteristic frequencies are usually obtained from peaks in the generalized susceptibility x͑Q, v͒~vS͑Q, v͒, or in the longitudinal current correlation function J 1 ͑Q, v͒ v͞Q 2 x͑Q, v͒ (for a comment on the meaning of such frequencies, see Ref.[10]). Such frequencies v Q are oftentimes converted to phase velocitieshv Q ͞Q. From the exploration of the phase velocity trend within the low-Q region, the presence of a mode of acoustic nature (where atoms involved execute in-phase displacements) propagating with a velocity well above that corresponding to hydrodynamic sound, has been inferred.The points which still are subject to controversy regard (a) the adequacy of discussing the nature on excitations appearing at relatively large-Q in terms of constructs which only retain full sense within the realm of hydrodynamics (i.e., a sound mode), and (b) the assignment of the observed frequencies, to a definite underlying microscopic mechanism, in the absence of further information such as the Q dependence of the excitation amplitude (which provides a direct insight into the phase relationships of the motions being sampled at a given frequency). Observation of clear nonacoustic modes at high frequencies was reported years ago for molten salts [4]. It was rationalized in terms of opticlike vibrations, but it has hardly ever been discussed within the present context. Also, excitations with frequencies well above hydrodynamic sound not involving propagation of longitudinal sound have been found in experiments in some molecular [7] or even metallic [9] liquids.The main thrust behind the substantial experimental effort witnessed in recent times for the scrutiny of this class of systems [1,2,5] was motivated by predictions made from ...

Coherent dynamic structure factors S(k,co) obtained by means of thermal neutron inelastic scattering are presented. The experiments were performed on liquid Ar at four densities along the 120-K isotherm covering a range of wave numbers k from 4.2 to 39.0 nm '. The neutron time-offlight spectra are corrected for all known experimental effects with an improved data-reduction system. Special attention is paid to corrections for multiple scattering, duty-cycle overlap, and instrumental resolution. The importance of various correction steps is shown. The reliability of the corrected data is assessed by means of two independent consistency checks, viz. , the detailed-balance condition and the first frequency moment of S(k,co). The S(k,co) data are presented both as a function of k at fixed co and as a function of co at fixed k. The peak height and full width at half maximum of S(k, co) at fixed k are shown for all densities together with the small-k (hydrodynamic) and large-k (free-gas) asymptotes. The frequency moments of S(k,co), evaluated up to the fourth moment, are consistent with results from computer simulations and from theoretical calculations. The longitudinal current correlation function CI(k, co), derived from the experimental S(k,co), is examined and both its peak position {yielding the dispersion curve for longitudinal current fluctuations) and its peak height (a measure of the lifetime of the fluctuations) are discussed.