Showing papers by "J. Mayers published in 2011"

Probing the binding and spatial arrangement of molecular hydrogen in porous hosts via neutron Compton scattering.

Abstract: The adsorption of molecular hydrogen (H2) in the alkali-graphite intercalate KC24 has been studied using simultaneous neutron diffraction and Compton scattering. Neutron Compton scattering data for the (H2)xKC24 system (x = 0–2.5) were measured at T = 1.5 K as a function of the relative orientation between the neutron beam and the intercalate c-axis. Synchronous with the above proton-recoil measurements, high-resolution diffraction patterns were measured in back-scattering geometry. From these diffraction measurements, the intrinsic mosaicity of the Papyex-based intercalate was determined to be ∼15° half-width-at-half-maximum, in good agreement with previous studies [Finkelstein et al., Physica B, 2000, 291, 213]. Hydrogen uptake by the intercalate leads to a distinct and readily detectable broadening of the isotropic Compton profile compared to bulk H2, indicative of an enhanced interaction of the H2 molecule with the surrounding solid-state environment. Total proton-recoil intensities also scale linearly with the amount of adsorbed hydrogen. Taking as our starting point previous experimental and theoretical results, the isotropic widths of the proton momentum distributions can be explained on the basis of three energy scales, namely, intramolecular H–H vibrations, followed by H–H librations and H2 centre-of-mass translations. From the coverage dependence of these neutron data, we also establish an upper bound of ∼10 meV for intermolecular hydrogen–hydrogen interactions. Finally, we observe a weak anisotropy of the width of the proton momentum distributions. Comparison of these experimental data with first-principles predictions indicates that subtle quantum mechanical effects associated with particle delocalisation and exchange lie at the heart of the observed behaviour. Overall, these results demonstrate the suitability and largely untapped potential of neutron Compton scattering to explore H2 uptake by solid-state hosts.

Calibration of an electron volt neutron spectrometer

Abstract: The procedure for calibrating the VESUVIO eV neutron spectrometer at the ISIS neutron source is described. VESUVIO is used primarily to measure the momentum distribution n(p) of atoms, by inelastic scattering of very high energy (5–150 eV) neutrons. The results of the calibrations show that measurements of n(p) in atoms with masses lower than 16 amu can be measured with a resolution width ∼25% of the intrinsic peak widths in the current instrument configuration. Some suggestions as to how the instrument resolution could be significantly improved are made.

Comment on "High-energy neutron scattering from hydrogen using a direct geometry spectrometer"

Abstract: The paper by Stock et al. [Phys. Rev. B 81, 024303 (2010)] reports measurements of neutron scattering from hydrogen in the 1--100 eV range of energy transfers, using the direct geometry MARI spectrometer at ISIS. Stock et al. claim that their measurements have comparable energy resolution to those on the inverse geometry VESUVIO spectrometer at ISIS. Most importantly, the main conclusions of Stock et al. with regard to VESUVIO are not valid unless this claim is true. Here we present overwhelming evidence that the energy resolution of the measurements in Phys. Rev. B 81, 024303 (2010) is one to two orders of magnitude worse than on VESUVIO. It follows that the conclusion of Stock et al. that anomalous neutron cross sections measured on VESUVIO [Chatzidimitriou-Dreismann et al., Phys. Rev. Lett. 79, 2839 (1997)] are ``the result of experimental issues using indirect geometry spectrometers'' is unfounded.