P. E. Sokol

A Quasi-Elastic and Inelastic Neutron Scattering Study of H2 in Zeolite

The diffusion of molecular hydrogen adsorbed in zeolite 13X at high coverages has been studied by quasi-elastic neutron scattering for temperatures ranging from 0 to 60 K. No diffusive motion was observed, within instrumental resolution, at temperatures below 20 K, well above the bulk melting point of H2, in contrast to recent NMR studies. The diffusive motion of the adsorbed H2could be described by a liquid like jump diffusion model above 35 K. The diffusion coefficient demonstrates an Arrhenius behavior: D = D0exp(E/kT), with D0 = 1.2 × 10−8m2/s and an activation energy of E = 62 K. The inelastic scattering spectrum has also been studied. The scattering consisted of several peaks superimposed on a broad background extending from 1 meV to above 100 meV and is consistent with rotational transitions for a hindered rotor in a strong orientational potential with a broad distribution of barrier heights.

Slow Diffusion of Molecular Hydrogen in Zeolite 13X

High-resolution quasi-elastic neutron scattering measurements have been performed on molecular hydrogen in zeolite 13X. Previous NMR measurements suggested that the freezing temperature is suppressed from 14 K down to 8 K. In contrast, previous intermediate resolution quasi-elastic neutron scattering studies suggested freezing occurred between 25 and 35 K. Unfortunately, the limited instrumental resolution available in the previous quasi-elastic neutron scattering study was not sufficient to show this point definitively. We report new quasi-elastic neutron scattering measurements with very high resolution that show no evidence of mobile hydrogen below 25 K, which is well above the bulk liquid-solid transition temperature for hydrogen. A quasi-elastic component appears between 25 and 30 K indicating the presence of mobile H2. However, the width and momentum dependence of the quasi-elastic scattering are much different than would be expected for the diffusive motion of liquid hydrogen in this temperature range. Instead, we find that a slow diffusive component representing jumps between well-defined sites appears first at low temperatures. As the temperature is raised, a faster liquid like diffusive component appears.

The structure of deuterium in Vycor

Neutron diffraction studies of normal deuterium in porous vycor glass have been carried out at temperatures in the range 4 to 20 K as a function of pore filling. For pore fillings, f, above f = 0.6 we find that the D2 in the pores forms a crystalline solid with a large amorphous component. The appearance of the crystalline phase is characterized by supercooling and hysteresis of the liquid-solid transition in agreement with previous thermodynamic studies. The crystalline phase has a structure which is different from the bulk and exhibits a large correlation length. Pore fillings below f = 0.6 show no sign of crystalline ordering, although enhanced short range ordering is observed.

A Neutron Scattering Study of Hydrogen in Vycor Glass

The elastic, quasi-elastic, and deep inelastic scattering of para-hydrogen in porous vycor glass have been measured in both the solid and liquid phase. At temperatures above 12 K, the H2 in the pores can be described as a bulk-like liquid with a single tightly bound layer at the pore surface. At temperatures below 8 K, the hydrogen in the pores is solidified into a crystalline structure different from that of the bulk. No indication of migration of the hydrogen out of the pores as recently proposed to explain torsional oscillator measurements, is observed. The momentum distribution of the hydrogen in the pores at both low and high temperatures is Gaussian and shows no indication of a Base condensate peak.

Two-dimensional excitations of superfluid confined in 72% porosity xerogel

Abstract We present neutron inelastic-scattering measurements of the low-energy excitation spectrum of superfluid 4 He confined in porous xerogel glass. In addition to the bulk-like excitation, we also observe a broad peak centred at energies of less than that of the bulk-like peak. The dependence on filling fraction of both peaks has been investigated and we find that the second broad peak exhibits behaviour that is consistent with that of an excitation confined to the high-density liquid layers near the pore walls, i.e. with that of a two-dimensional excitation.

Neutron inelastic scattering from 4He in restricted geometries

Abstract When liquid 4 He is condensed in porous aerogel glass (typical pore size ≈ 5000 A), many of the superfluid properties are significantly altered. Measurements have been made of the dynamic structure factor S ( Q, ω ) of liquid 4 He in restricted geometries. The collective phonon-roton excitations are shown to have an intrinsic broadening associated with the restricted geometry. The temperature dependence of the excitation spectrum is discussed.

High resolution measurements of the temperature dependence of the roton energy of superfluid 4He

New measurements of the roton excitations in superfluid 4He have been made using the IRIS spectrometer (energy resolution ∼20μeV at the roton energy) at the ISIS spallation neutron source. The roton energy was determined over a temperature range of 0.56 to 1.61 K. High resolution (energy resolution <1μeV) neutron inelastic scattering measurements by Andersen et al.2 of the dependence of the 4He roton energy on temperature have shown significant deviations from the expected behaviour; the temperature variation was found to be discontinuous and slower than predicted by the established roton-roton interaction theory due to Bedell, Pines and Zawadowski (BPZ).3 We find that our results can be described well by the BPZ theory over this temperature range and we do not observe the anomalous behaviour reported by Andersen et al. We offer some explanations as to the origin of this apparent disagreement.

An Inelastic Neutron Scattering Study of H2 in Vycor Glass

The orientational properties of n-H2confined in Vycor have been studied via inelastic neutron spectroscopy. We observe two distinct rotational transitions which we ascribe to H2in the center of the pore and H2strongly bound to the surface. The molecules on the surface can be modeled as rotationally hindered rotors, and a distribution of orientational potentials is extracted.

The momentum distribution of 3He

Abstract DINS measurements have been performed on liquid 3 He at different densities at 500 mK. The single-particle kinetic energy is extracted from the data and is found to be in disagreement with theoretical predictions.

High-resolution measurements of the temperature dependence of the roton energy of superfluid

New measurements of the roton excitations in superfluid 4He have been made using the IRIS spectrometer (energy resolution ∼20μeV at the roton energy) at the ISIS spallation neutron source. The roton energy was determined over a temperature range of 0.56 to 1.61 K. High resolution (energy resolution <1μeV) neutron inelastic scattering measurements by Andersen et al.2 of the dependence of the 4He roton energy on temperature have shown significant deviations from the expected behaviour; the temperature variation was found to be discontinuous and slower than predicted by the established roton-roton interaction theory due to Bedell, Pines and Zawadowski (BPZ).3 We find that our results can be described well by the BPZ theory over this temperature range and we do not observe the anomalous behaviour reported by Andersen et al. We offer some explanations as to the origin of this apparent disagreement.