No abstract available

[en] Spectroscopy with cold neutrons is one of the most important areas of current superiority of reactor based instruments over those at spallation sources. This is particularly due to the capability of continuous source time-of-flight spectrometers to use instrumental parameters optimally adapted for best data collection rate in each experiment. These parameters include the pulse repetition rate and the length of the pulses to achieve optimal balance between resolution and intensity. In addition, the disc chopper systems used provide perfect symmetrical line shapes with no tails and low background. We present a set of novel techniques making up the IN500 project at Los Alamos, which is based on the combined use of extended pulse length, coupled moderator, disc chopper system and advanced neutron optical beam delivery. This development will enable Lujan center to surpass the best reactor sources in cold neutron spectroscopy by realizing for the first time all of the above key capabilities of steady state instruments on a pulsed spallation source. (author)

[en] Inelastic-incoherent-neutron scattering can be a valuable nanostructural probe of H₂-doped porous materials, provided the spectral peaks can be interpreted in terms of crystal-field-split hydrogen-molecule energy levels, which represent a signature of the local symmetry. Inelastic-neutron-scattering measurements as well as extensive theoretical analyses have been performed on stage-2 Rb-intercalated graphite (Rb-GIC), with physisorbed H₂, HD, and D₂, a layered porous system with abundant spectral peaks, to assess whether the crystal-field-state picture enables a quantitative understanding of the observed structure. Potential-energy surfaces for molecular rotational and translational motion, as well as the intermolecular interactions of hydrogen molecules in Rb-GIC, were calculated within local-density-functional theory (LDFT). Model potentials, parameterized using results of the LDFT calculations, were employed in schematic calculations of rotational and translational excited state spectra of a single physisorbed H₂ molecule in Rb-GIC. Results of the analysis are basically consistent with the assignment by Stead et al. of the lowest-lying peak at 1.4 meV to a rotational-tunneling transition of an isotropic hindered-rotor oriented normal to the planes, but indicate a small azimuthal anisotropy and a lower barrier than for the isotropic case. Based on the experimental isotope shifts and the theoretically predicted states, they conclude that spectral peaks at 11 and 22 meV are most likely related to center of mass excitations

[en] The neutron diffraction pattern of 'C₂₄Cs'(CD₄)sub(x) has been studied as a function of filling, x, and temperature from 4 K up to the gas desorption point at 170 K. At low temperatures, peaks due to coexisting (√ 7 x √ 7) commensurate and a 3.2% expanded incommensurate phase have been detected in the ratio of ca. 3:1, respectively. At higher temperatures, as methane is desorbed, the lattice-vector changes are small but the expanded lattice peak loses intensity first, between 130 and 154 K. Above 154 K a single broad peak at the commensurate lattice position gradually weakens as gas is lost, indicating the persistence of this more stable phase. These data are compared with the results of sorption potential and molecular dynamics simulations. Evidence for two different c-axis stacking sequences in our C₂₈Cs sample before and after sorption is presented. (author)

[en] The intermediate valence compounds Yb₂M₃Ga₉ (M = Rh, Ir) exhibit an anisotropic magnetic susceptibility. We report measurements of the temperature dependence of the 4f occupation number, n_f(T), for Yb₂M₃Ga₉ as well as the magnetic inelastic neutron scattering spectrum S_mag(ΔE) at 12 and 300 K for Yb₂Rh₃Ga₉. Both n_f(T) and S_mag(ΔE) were calculated for the Anderson impurity model with crystal field terms within an approach based on the non-crossing approximation. These results corroborate the importance of crystal field effects in these materials; they also suggest that Anderson lattice effects are important to the physics of Yb₂M₃Ga₉

[en] Complete text of publication follows. The dynamics of the freezing transition of the rotator phase crystal of ethanol into its orientationally disordered counterpart (orientational glass) is monitored by measurements of molecular rotational component in the quasielastic neutron scattering spectrum. It is demonstrated that the observed phenomena can be mapped onto those shown by a model of infinitely thin hard needles rotating around body-centered-cubic lattice positions. The basic signatures across the orientational glass transition are similar in the needle model [1] and in the neutron scattering data for ethanol [2]. As the model glass transition is of purely dynamical origin, our findings support the idea that the glass transition is purely dynamical and not associated with any thermodynamic phase transition. (author)

[en] The reorientational dynamics within the rotationally disordered cubic plastic phase of solid ethanol is investigated by means of the concurrent use of computer molecular dynamics and quasielastic neutron scattering. Motions involving widely different time scales are shown to take place above the calorimetric 'glass transition' which is centered at T_g≅97 K. These correspond to well-defined reorientations belonging to the cubic point group. The dynamics of this solid exhibits features remarkably close to those of the supercooled liquid that can exist at the same temperature. Such similitude of dynamic behavior serves to provide some clues for the understanding of the nature of molecular motions at temperatures close to the canonical liquid→glass transition

[en] Vibrational motions of solid N-methylformamide (NMF) and its N-deuterated analogue are investigated using the inelastic neutron scattering (INS) technique at 15 K. The force field for obtaining the normal vibrational modes of the crystal is based on a quantum chemical calculation and a subsequent transfer of a harmonic force field of a smaller pentameric segment to a fragment of 11 NMF molecules. Two types of hydrogen bonds present in crystalline NMF are also modeled with dimers. The distinct bonding leads to a splitting of the N-hydrogen wagging mode in the spectrum. Although the hydrogen bonding has a profound effect on vibrational frequencies, the results indicate that an occurrence of a double-well potential for bonded hydrogen proposed previously is unlikely. Instead, a limited electronic conjugation along the hydrogen bonds in crystalline NMF is observed. Unlike in previous models, we simulate the relative INS intensity of each vibrational transition separately, which leads to a substantial improvement of the overall profile of the intensity pattern. The modeling allows one to assign most of observed INS bands to vibrational modes and the overall spectral profile that reproduced by the simulation compares well with the experiment. copyright 1998 American Institute of Physics