The low-frequency dynamics of different solid phases of ethanol ͑fully ordered crystal, orientational glass, and true structural glass͒ is investigated by measurements of low-temperature specific heat as well as determination of the vibrational densities of states by inelastic neutron scattering. Such polymorphism provides a unique benchmark to study the relative importance of different kinds of disorder on glass-dynamics properties. The measurements are carried out both for hydrogenous and fully-deuterated samples. Large isotopic effects are found for the excess contributions ͑i.e., non-Debye͒ to the specific heat of the disordered solids, which have clear correlates in the low-frequency spectra. ͓S0163-1829͑98͒05626-4͔

Deep inelastic neutron scattering experiments using indirect time-of-flight spectrometers have reported a smaller cross section for the hydrogen atom than expected from conventional scattering theory. Typically, at large momentum transfers, a deficit of 20-40% in the neutron scattering intensity has been measured and several theories have been developed to explain these results. We present a different approach to this problem by investigating the hydrogen cross section in polyethylene using the direct geometry time-of-flight spectrometer MARI with the incident energy fixed at a series of values ranging from Ei=0.5 eV to 100 eV. These measurements span a much broader range in momentum than previous studies and with varying energy resolutions. We observe no momentum dependence to the cross section with an error of 4% and through a comparison with the scattering from metal foil standards measure the absolute bound cross section of the hydrogen atom to be σ(H)= 80 ± 4 barns. These results are in agreement with conventional scattering theory but contrast with theories invoking quantum entanglement and neutron experiments supporting them. Our results also illustrate a unique use of direct geometry chopper instruments at high incident energies and demonstrate their capability for conducting high-energy spectroscopy.

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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...

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