Direct observation of H\u3csub\u3e2\u3c/sub\u3e binding to a metal oxide surface

Inelastic neutron scattering is used to probe the dynamical response of H2 films adsorbed on MgO(100) as a function of film thickness. Concomitant diffraction measurements and a reduced-dimensionality quantum dynamical model provide insight into the molecule-surface interaction potential. At monolayer thickness, the rotational motion is strongly influenced by the surface, so that the molecules behave like quasiplanar rotors. These findings have a direct impact on understanding how molecular hydrogen binds to the surface of materials used in catalytic and storage applications

Layer-by-layer Growth of Solid Argon Films on Graphite as Studied by Neutron Diffraction

The layer-by-layer growth of solid argon films on graphite at T=10 K is studied using elastic neutron diffraction. The growth is characterized by individual layers with commensurate in-plane lattice constants. As the coverage is increased beyond two layers, evidence of the coexistence of ABC and ABA stacking is apparent, with the ABC sequence dominating as the film thickens. A continuous decrease in the Debye-Waller factor also occurs as the film thickness grows, indicating a crossover from two-dimensional to three-dimensional behavior. As the coverage is increased beyond about four nominal layers, there is evidence of bulk crystallite formation. The diffraction results are compared with equivalent measurements for methane films and with the recent computer simulations of Hruska and Phillips

Dust exposure and chronic respiratory symptoms among coffee curing workers in Kilimanjaro: a cross sectional study

Coffee processing causes organic dust exposure which may lead to development of respiratory symptoms. Previous studies have mainly focused on workers involved in roasting coffee in importing countries. This study was carried out to determine total dust exposure and respiratory health of workers in Tanzanian primary coffee-processing factories. A cross sectional study was conducted among 79 workers in two coffee factories, and among 73 control workers in a beverage factory. Personal samples of total dust (n = 45 from the coffee factories and n = 19 from the control factory) were collected throughout the working shift from the breathing zone of the workers. A questionnaire with modified questions from the American Thoracic Society questionnaire was used to assess chronic respiratory symptoms. Differences between groups were tested by using independent t-tests and Chi square tests. Poisson Regression Model was used to estimate prevalence ratio, adjusting for age, smoking, presence of previous lung diseases and years worked in dusty factories. All participants were male. The coffee workers had a mean age of 40 years and were older than the controls (31 years). Personal total dust exposure in the coffee factories were significantly higher than in the control factory (geometric mean (GM) 1.23 mg/m3, geometric standard deviation (GSD) (0.8) vs. 0.21(2.4) mg/m3). Coffee workers had significantly higher prevalence than controls for cough with sputum (23% vs. 10%; Prevalence ratio (PR); 2.5, 95% CI 1.0-5.9) and chest tightness (27% vs. 13%; PR; 2.4, 95% CI 1.1-5.2). The prevalence of morning cough, cough with and without sputum for 4 days or more in a week was also higher among coffee workers than among controls. However, these differences were not statistically significant. Workers exposed to coffee dust reported more respiratory symptoms than did the controls. This might relate to their exposure to coffee dust. Interventions for reduction of dust levels and provision of respiratory protective equipment are recommended

Melting mechanism in monolayers of flexible rod-shaped molecules

URL:http://link.aps.org/doi/10.1103/PhysRevLett.69.652 DOI:10.1103/PhysRevLett.69.652The melting of butane and hexane monolayers adsorbed on a graphite basal-plane surface has been studied by molecular-dynamics simulations and experimentally by neutron diffraction. The simulation results are qualitatively consistent with the observed diffraction patterns and suggest a general mechanism for melting in monolayers of flexible rod-shaped molecules. Melting requires the formation of vacancies in the monolayer by molecular motion perpendicular to the surface. This ''footprint reduction'' mechanism implies that strictly two-dimensional theories of melting are inapplicable to these systems.This work was supported by Danish Natural Science Research Council Grant No. M 11-7015, U.S. NSF Grants No. DMR-8704938 and No. DMR-9011069, and Pittsburgh Supercomputing Center Grant No. DMR-880008P

Neutron scattering investigations of the dynamics of thin films adsorbed on solid surfaces

While not commonly thought of as surface sensitive probes, high-resolution inelastic and quasielastic neutron scattering (INS) techniques can be used to investigate the dynamical properties of films adsorbed on solid surfaces. These studies are possible because the adsorption takes place on samples which exhibit a large surface-to-volume ratio. By combining dynamical measurements with data from structural, thermodynamic and computer simulation studies it is then possible to develop a comprehensive picture of the microscopic properties of an adsorbed film system. In order to illustrate the power of employing INS techniques we will discuss the continuous melting of monolayer ethylene and the layer-by-layer melting of multilayer argon films on the basal plane of graphite and the rotational tunneling of methane films on the MgO(100) surface

The construction of a high resolution crystal backscattering spectrometer HERMES I

There is a need in the United States for a state-of-the-art, cold-neutron, crystal backscattering spectrometer (CBS) designed to investigate the structure and dynamics of condensed matter systems by the simultaneous utilization of long wavelength elastic diffraction and high-energy-resolution inelastic scattering. Cold neutron spectroscopy with CBS-type instruments has already made many important contributions to the study of atomic and molecular diffusion in biomaterials, polymers, semiconductors, liquid crystals, superionic conductors and the like. Such instruments have also been invaluable for ultra high resolution investigations of the low-lying quantum tunneling processes that provide direct insight into the dynamical response of solids at the lowest energies. Until relatively recently, however, all such instruments were located at steady-state reactors. This proposal describes HERMES I (High Energy Resolution Machines I) a CBS intended for installation at the LANSCE pulsed neutron facility of Los Alamos National Laboratory. As explained in detail in the main text, the authors propose to construct an updated, high-performance CBS which incorporates neutron techniques developed during the decade since IRIS was built, i.e., improved supermirror technology, a larger area crystal analyzer and high efficiency wire gas detectors. The instrument is designed in such a way as to be readily adaptable to future upgrades. HERMES I, they believe, will substantially expand the range and flexibility of neutron investigations in the United States and open new and potentially fruitful directions for condensed matter exploration. This document describes a implementation plan with a direct cost range between $4.5 to 5.6 M and scheduled duration of 39--45 months for identified alternatives

[Visualization of convection in {sup 3}He-superfluid-{sup 4}He]

Low energy neutrons provide a unique opportunity for the visualization of interesting dynamical systems. Particularly for liquid helium mixtures, there are a number of very basic issues of current interest which can only be addressed by means of very novel techniques. The technique developed under this proposal is one of these and is designed to address specific issues for {sup 3}He-superfluid-{sup 4}He mixtures. These are: convection in these mixtures, and heat flow when the {sup 3}He concentration is very low. The neutron imaging device which the authors developed is potentially very useful for investigating the convection patterns when these mixtures are subjected to an adverse density gradient. There are a number of intriguing results obtained without the benefit of imaging. A full understanding of these results will depend crucially on obtaining images of the convection pattern. The second issue of interest, heat flow in very dilute superfluid mixtures, poses a particularly puzzling question. The expected divergence of the effective conductivity with vanishing {sup 3}He concentration, X, is not seen experimentally. This effect is intimately tied to the distribution of {sup 3}He in the direction of heat flow. However, no information on the distribution is available. The device which the authors constructed is potentially capable of addressing this need. The technique can also be used with any stem in which there is a spatially varying distribution of neutron absorbers. Hence, it has broad applicability