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We report initial NMR studies of (i) xenon gas diffusion in model heterogeneous porous media and (ii) continuous flow laser-polarized xenon gas. Both areas utilize the pulsed gradient spin-echo (PGSE) techniques in the gas phase, with the aim of obtaining more sophisticated information than just translational self-diffusion coefficients--a brief overview of this area is provided in the Introduction. The heterogeneous or multiple-length scale model porous media consisted of random packs of mixed glass beads of two different sizes. We focus on observing the approach of the time-dependent gas diffusion coefficient, D(t) (an indicator of mean squared displacement), to the long-time asymptote, with the aim of understanding the long-length scale structural information that may be derived from a heterogeneous porous system. We find that D(t) of imbibed xenon gas at short diffusion times is similar for the mixed bead pack and a pack of the smaller sized beads alone, hence reflecting the pore surface area to volume ratio of the smaller bead sample. The approach of D(t) to the long-time limit follows that of a pack of the larger sized beads alone, although the limiting D(t) for the mixed bead pack is lower, reflecting the lower porosity of the sample compared to that of a pack of mono-sized glass beads. The Pade approximation is used to interpolate D(t) data between the short- and long-time limits. Initial studies of continuous flow laser-polarized xenon gas demonstrate velocity-sensitive imaging of much higher flows than can generally be obtained with liquids (20-200 mm s-1). Gas velocity imaging is, however, found to be limited to a resolution of about 1 mm s-1 owing to the high diffusivity of gases compared with liquids. We also present the first gas-phase NMR scattering, or diffusive-diffraction, data, namely flow-enhanced structural features in the echo attenuation data from laser-polarized xenon flowing through a 2 mm glass bead pack.

Elastic neutron diffraction has been used to study the structure of N2 films adsorbed on the (0001) surfaces of an exfoliated graphite substrate at coverages 0 between 1.0 and 1.67 layers and at temperatures below 11 K. For 0=1.0, the diffraction patterns can be fit by the in-plane rectangular V 3 X 3 herringbone structure denoted C (commensurate), previously inferred from low-energy electron diffraction (LEED) experiments. Analysis of the relative Bragg-peak intensities in the neutron diffraction pattern of the C phase extends the LEED results by yielding a value of /=45'+5 for the angle between the N -N bond and the short axis of the unit cell. Also, we find a substantially smaller Debye-Wailer factor for this phase than previously inferred from x-ray experiments. At coverages 8=1.13 and 1.27, the diffraction patterns are consistent with the uniaxial incommensurate (UI) phase seen by LEED. The patterns can be fit with the same molecular orientational parameters as for the C phase. We find the compression of the monolayer to be complete at 6=1.67 where the film density is -10% greater than for the C phase. Fits to the diffraction pattern at this coverage indicate a slight oblique distortion of the unit cell from hexagonal symmetry. For this nearly triangular incommensurate (TI) phase, we obtain orientational parameters in the ranges 30' & P &45' and 10 & P & 20 where l3 is the tilt angle of the N -N bond with respect to the surface.At coverages between 6=1.27 and 1.40, there is evidence of coexistence of the UI and TI phases.

We report simultaneous measurements of the permeability and effective porosity of oil-reservoir rock cores using one-dimensional NMR imaging of the penetrating flow of laser-polarized xenon gas. The permeability result agrees well with industry standard techniques, whereas effective porosity is not easily determined by other methods. This NMR technique may have applications to the characterization of fluid flow in a wide variety of porous and granular media.

We report initial NMR studies of continuous flow laser-polarized xenon gas, both in unrestricted tubing, and in a model porous media. The study uses Pulsed Gradient Spin Echo-based techniques in the gas-phase, with the aim of obtaining more sophisticated information than just translational selfdiffusion coefficients. Pulsed Gradient Echo studies of continuous flow laser-polarized xenon gas in unrestricted tubing indicate clear diffraction minima resulting from a wide distribution of velocities in the flow field. The maximum velocity experienced in the flow can be calculated from this minimum, and is seen to agree with the information from the complete velocity spectrum, or motion propagator, as well as previously published images. The susceptibility of gas flows to parameters such as gas mixture content, and hence viscosity, are observed in experiments aimed at identifying clear structural features from echo attenuation plots of gas flow in porous media. Gas-phase NMR scattering, or position correlation flow-diffraction, previously clearly seen in the echo attenuation data from laserpolarized xenon flowing through a 2 mm glass bead pack is not so clear in experiments using a different gas mixture. A propagator analysis shows most gas in the sample remains close to static, while a small portion moves through a presumably near-unimpeded path at high velocities.

Elastic neutron diffraction and Mossbauer spectroscopy have been used to study the structure, orientational-disordering (OD} transition, and melting of an Fe(CO)5 submonolayer adsorbed on Grafoii. The OD transition occurs between 150 and 167 K from a two-subiattice {J7 x 421) structure to a nearly (J7 x J7) phase in which the molecuies are believed to rotate about the surface normal. Mossbauer spectra exhibit a more abrupt variation with temperature near melting at 170 K than through the OD transition.Recently, there has been increased interest in physisorbed monolayers of nonspherical molecules on graphite. " In contrast to rare gases which are known to condense in hexagonal layers, little is known of the monolayer structure of these moleculesparticularly their orientational ordering.Moreover, such films exhibit phase transitions which do not occur in monolayers of spherical atoms. One of these is the orientational-disordering (OD) transition to a phase in which there is long-range translational order but in which each molecule rotates at its equilibrium site. Experimental evidence of OD transitions has been found for nitrogen'3 and ethane'4 monolayers on graphite, and they have begun to be investigated theoretically by computer simulation. 5 6 The computer models assume the molecular centers to be fixed at their lattice sites and for rotation to occur about an axis perpendicular to the surface.Depending on lattice symmetry and intermolecular interactions, both first order and continuous OD transitions have been predicted.%e have recently begun to study the structure and phase transitions of iron pentacarbonyl [Fe(CO)s] monolayers on graphite. This bipyramid-shaped molecule was selected because it is favorable for structural investigations by neutron diffraction while it can also be probed dynamically by Mossbauer spectroscopy. ' %e describe here a model for the Fe(CO) 5 monolayer structure at low temperatures inferred from neutron diffraction and show it to be consistent with calculations based on empirical atomatom potentials. %e then present evidence from both neutron and Mossbauer experiments for an OD transition between 150 and 167 K followed by melting at 170 K. %e find the complementarity of neutron diffraction as a probe of orientational order and Mossbauer spectroscopy with its sensitivity to translational motion of the adsorbed molecules to be very useful in developing a model of the OD transition.The neutron scattering experiments were performed at the University of Missouri Research Reactor Facilit on a two-axis spectrometer (wavelength X=4.07 ) equipped with a five-counter multidetector-data acquisition system. 9 The Mossbauer spectrometer located at the Technion was operated in a constant acceleration mode and used a 25-mci Rh:"Co y-ray source. '0 Both neutron and Mossbauer samples consisted of a Grafoil' substrate having a surface area of -20 m'/g. The neutron sample was oriented with the scattering vector Q parallel to the foil planes while in the Mossbauer experiments the gamma k vector (k") cou...

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