Diffusive and rotational dynamics of condensed n-H2 confined in MCM-41.

In this paper, we report an inelastic neutron scattering study of liquid and solid n-H2 confined within MCM-41. This is a high surface area, mesoporous silica glass with a narrow pore size distribution centered at 3.5 nm. The scattering data provides information about the diffusive and rotational dynamics of the adsorbed n-H2 at low temperatures. In the liquid state, the neutron scattering data demonstrates that only a fraction of the adsorbed o-H2 is mobile on the picosecond time scale. This mobile fraction undergoes liquid-like jump diffusion, and values for the residence time τ and effective mean-squared displacement 〈u(2)〉 are reported as a function of pore filling. In the solid state, the rotational energy levels of adsorbed H2 are strongly perturbed from their free quantum rotor behavior in the bulk solid. The underlying orientational potential of the hindered rotors is due to the surface roughness and heterogeneity of the MCM-41 pore walls. This potential is compared to the hindering potential of other porous silicas, such as Vycor. Strong selective adsorption makes the interfacial layer rich in o-H2, leaving the inner core volume consisting of a depleted mixture of o-H2 and p-H2.

Confinement-driven phase separation of quantum liquid mixtures.

We report small-angle neutron scattering studies of liquid helium mixtures confined in Mobil Crystalline Material-41 (MCM-41), a porous silica glass with narrow cylindrical nanopores (d=3.4 nm). MCM-41 is an ideal model adsorbent for fundamental studies of gas sorption in porous media because its monodisperse pores are arranged in a 2D triangular lattice. The small-angle scattering consists of a series of diffraction peaks whose intensities are determined by how the imbibed liquid fills the pores. Pure (4)He adsorbed in the pores show classic, layer-by-layer film growth as a function of pore filling, leaving the long range symmetry of the system intact. In contrast, the adsorption of (3)He-(4)He mixtures produces a structure incommensurate with the pore lattice. Neither capillary condensation nor preferential adsorption of one helium isotope to the pore walls can provide the symmetry-breaking mechanism. The scattering is consistent with the formation of randomly distributed liquid-liquid microdomains ∼2.3 nm in size, providing evidence that confinement in a nanometer scale capillary can drive local phase separation in quantum liquid mixtures.

The new cold neutron chopper spectrometer at the Spallation Neutron Source: design and performance.

The design and performance of the new cold neutron chopper spectrometer (CNCS) at the Spallation Neutron Source in Oak Ridge are described. CNCS is a direct-geometry inelastic time-of-flight spectrometer, designed essentially to cover the same energy and momentum transfer ranges as IN5 at ILL, LET at ISIS, DCS at NIST, TOFTOF at FRM-II, AMATERAS at J-PARC, PHAROS at LANSCE, and NEAT at HZB, at similar energy resolution. Measured values of key figures such as neutron flux at sample position and energy resolution are compared between measurements and ray tracing Monte Carlo simulations, and good agreement (better than 20% of absolute numbers) has been achieved. The instrument performs very well in the cold and thermal neutron energy ranges, and promises to become a workhorse for the neutron scattering community for quasielastic and inelastic scattering experiments.

Dynamics of small-molecule glass formers confined in nanopores.

We report a comparative neutron scattering study of the molecular mobility and nonexponential relaxation of three structurally similar glass-forming liquids, isopropanol, propylene glycol, and glycerol, both in bulk and confined in porous Vycor glass. Confinement reduces molecular mobility in all three liquids, and suppresses crystallization in isopropanol. High-resolution quasielastic neutron scattering spectra were fit to Fourier transformed Kohlrausch functions exp[-(t∕τ)(ß)], describing the α-relaxation processes in these liquids. The stretching parameter ß is roughly constant with wavevector Q and over the temperature range explored in bulk glycerol and propylene glycol, but varies both with Q and temperature in confinement. Average relaxation times <τ(Q)> are longer at lower temperatures and in confinement. They obey a power law <τ(Q)> ∝ Q(-γ), where the exponent γ is modified by confinement. Comparison of the bulk and confined liquids lends support to the idea that structural and∕or dynamical heterogeneity underlies the nonexponential relaxation of glass formers, as widely hypothesized in the literature.

Mobility transition of solid rare gases in confined environments.

We report the results of x-ray diffraction and small angle scattering studies of Ar and Kr confined in sol-gel and Vycor glasses. The confined liquid crystallizes in a disordered hcp structure on freezing. Upon further cooling a sharp transition occurs at a reduced temperature of T/T(m) approximately 0.65, where the crystalline structure disappears and the total scattering decreases. This behavior marks the onset of a well-defined mobility transition, where the confined sample migrates out of the pore space.

The structure of confined oxygen in silica xerogels.

The microscopic structure of oxygen confined in silica xerogels has been studied as a function of temperature. In large pores, a crystalline solid forms with a structure consistent with that of the bulk. The size of the crystallites is much larger than the pore size, indicating that cooperative effects among pores are important in freezing. As the pore size is decreased, a crossover occurs where solidification results in an amorphous phase in the pores. The resulting amorphous phase is solid but is less ordered than the liquid phase.