Neutron diffraction and inelastic scattering from adsorbed molecules

Neutron scattering measurements on the structure and dynamics of adsorbed phases at the gas solid interface have proliferated in the last five years and this paper reports some recently obtained results for the systems methane-graphite and ammonia-graphite, which show BET type I and type III isotherms respectively. These systems clearly illustrate the differences in both structure and dynamics to be expected in other examples where either wetting of the surface or non-wetting behaviour occurs upon adsorption. Evidence for phase transitions in both types of system coming from diffraction is reported, and this is substantiated by measurements using inelastic scattering to follow the molecular dynamics. The neutron inelastic scattering from molecules adsorbed on platinum black and in zeolites reveals some of the virtues of neutron inelastic scattering for the study of chemisorbed species. Finally, some preliminary experiments on neutron small angle scattering at the solid liquid interface in polystyrene latex-water sols are reported.     

Dynamics of water molecules adsorbed by silica and resin SGK-7

This paper has presented neutron spectroscopy data on the dynamics of light water molecules adsorbed in the cation exchanger (ion-exchange resin) SGK-7 and on the surface of aerosils (highly dispersed pyrogenic silica) with different levels of hydration. The measurements have been performed on a DIN-2PI spectrometer (Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research, Dubna, Russia). The characteristics of the diffusive and vibrational motions of adsorbed water molecules have been determined from the experimental neutron scattering spectra. The data obtained in the quasi-elastic neutron scattering region have been analyzed using a model accounting for the effects of restricted translational and rotational diffusion. The results have demonstrated a significant decrease in the diffusion mobility of adsorbed water molecules as compared to conventional (bulk) water. In particular, the self-diffusion coefficient decreases several times, and the diffusion rate is the lower, the smaller is the thickness of the hydration layer. The dependences of the intensity and half-width of the quasi-elastic scattering peak on the magnitude of the neutron momentum transfer q in the scattering process exhibit a nonmonotonic character. This indicates manifestation of the effects of restricted translational diffusion, rotational diffusion, and jump diffusion. The partial distributions of vibrational frequencies of hydrogen atoms of water molecules adsorbed by the cation exchanger and aerosils have been obtained from the inelastic neutron scattering data.     

Signature of a type-a glass transition and intrinsic confinement effects in a binary glass-forming system

We study dynamically highly asymmetric binary mixtures comprised of small methyl tetrahydrofuran (MTHF) molecules and polystyrene. Combined use of dielectric spectroscopy, ^{2}H nuclear magnetic resonance, incoherent quasielastic neutron scattering, and depolarized dynamic light scattering allows us to selectively probe the dynamics of the components in a broad dynamic range. It turns out that the mixtures exhibit two glass transitions in a wide concentration range although being fully miscible on a macroscopic scale. In between both glass transition temperatures, the dynamics of the small molecules show strong confinement effects, e.g., a crossover from Vogel-Fulcher to Arrhenius behavior of the time constants. Moreover, the dynamical behavior of small molecules close to the slow matrix is consistent with mode coupling theory predictions for a type-A glass transition, which was expected from recent theoretical and simulation studies in comparable systems.     

Status of experiments probing the dynamics of water in confinement

In many relevant situations, water is not in its bulk form but is instead attached to some substrates or filling some cavities. We shall call water in the latter environment confined or interfacial water as opposed to bulk water. This confined water is essential for the stability and function of biological macromolecules. In this review paper, we present the more recent up to date account of the dynamics of confined water as compared with that of bulk water. Various techniques are used to study the dynamics of confined water. Among them, quasi-elastic and inelastic neutron scattering is a powerful tool to study translational and rotational diffusion as well as vibrational density of states of confined water. Various examples involving water confined in porous media, adsorbed on surface of ionic crystals, in the presence of organic solutes and at the surface of biological molecules are presented. The combined effects of the hydration level and the temperature on the retardation of the water molecules motions are discussed on the basis of phenomenological models as well as of power law fits based on the Mode Coupling Theory.Received: 1 January 2003, Published online: 8 October 2003PACS: 61.12.Ex Neutron scattering (including small-angle scattering) - 66.30.Pa Transport properties of condensed matter (nonelectronic): Diffusion in nanoscale solids     

Levitation methods for structural and dynamical studies of liquids at high temperatures

In recent years, levitation methods have been increasingly used to study the atomic structure and dynamics of high-temperature liquids, in particular metallic melts. These methods provide a containerless and, consequently, high-purity sample environment. No corrections for signals due to a crucible need to be made, and deep undercoolings of the liquid become possible, reducing the effect of thermal fluctuations. On the other hand, the sample position and, hence, the scattering geometry is not fixed and the free sample surface exhibits capillary waves. Nevertheless, the combination of levitation techniques with x-ray or neutron sources has proven to be possible and successful. This paper reviews the progress made in this field during the last 10 years or so. It discusses the different levitation techniques: aerodynamic, electromagnetic, electrostatic, as well as the applied spectroscopic techniques: x-ray and neutron diffraction, x-ray absorption and quasi-elastic neutron diffraction. Some recent results are also highlighted.     

Scattering of Polarized and Natural Light by a Monolayer of Spherical Homogeneous Spatially Ordered Particles under Normal Illumination

We consider distortions of the shapes of absorption bands of strongly scattering samples in the spectra of molecules adsorbed on the surface of dispersed solids. We show that the influence of the scattering on IR spectra of dispersed samples is not reduced merely to a weakening of the intensity of the transmitted light, but, rather, the scattering affects the contour of a spectral band due to changes in the refractive index of the substance in the range of the absorption band. Using carbon dioxide adsorbed on a NaX zeolite as an example, we demonstrate two methods for taking into account the contribution of the scattering to the spectra of surface compounds: the registration of the “diffuse-transmission” spectrum and placing a sample into an immersion liquid, for which liquid oxygen is used. The obtained spectra indicate that, if the zeolite is saturated with the adsorbate, the band of the antisymmetric stretching vibration of CO₂ molecules broadens and reveals a complex structure as a result of the resonant dipole–dipole interaction between adsorbed molecules.     

Phase transitions of interfacial water at 165 and 240 K. Connections to bulk water physics and protein dynamics

We are considering water adsorbed as a monolayer on Vycor, a porous silica glass. The interfacial water molecules interact with the substrate through hydrogen bonding with the numerous silanol (Si-OH) groups present all over the surface. This special form of water exhibits peculiar dynamical properties. A combined calorimetric, diffraction, high resolution quasi-elastic and inelastic neutron scattering study shows that interfacial water experiences a glass transition at 165 K and a liquid-liquid transition at 240 K from a low-density to a high density-liquid. We show that this unusual behaviour, compared to the bulk, is due to a strong weakening of the hydrogen-bond strength, possibly due to the reduced number of hydrogen-bonds engaged by water molecules when they are in an interfacial two dimensional situation. The connections of these findings to the physics of bulk water and protein dynamics are discussed.     

生物分子结合水的结构与动力学研究进展

生物结合水在维护生物大分子的结构、稳定性以及调控动力学性质和生理功能等方面起着决定性的作用.从分子水平上理解生物结合水分子的结构与性质及其影响生物结构和功能的本质与规律,是揭示生物大分子生理功能机理的关键.目前生物结合水的结构与动力学相关研究尚处于初步阶段.本文从三个方面介绍当前生物结合水的相关研究及其进展:首先介绍结合水对蛋白质折叠、质子给予与迁移、配体结合与药物设计以及变构效应等生物结构和功能的影响;然后介绍生物分子周围的水分子结构研究情况;最后从时间尺度、动力学属性、生物分子与水分子之间的动力学耦合作用、蛋白质表面结合水次扩散运动等角度介绍生物分子水合动力学的研究进展,并归纳出一些目前尚待进一步解决的科学问题.     

Melting of thin films of alkanes on magnesium oxide

Recent incoherent neutron scattering investigations of the dynamics of thin alkane films adsorbed on the Magnesium Oxide (100) surface are reported. There are marked differences in the behaviour of these films, as a function of temperature and coverage, compared to similar measurements on graphite. In particular, it has previously been shown that adsorbed multilayer films on graphite exhibit an interfacial solid monolayer that coexists with bulk-like liquid, well above the bulk melting point. In contrast, these studies show that the alkane films on MgO exhibit no such stabilization of the solid layer closest to the substrate as a function of the film thickness, even though the monolayer crystal structures are remarkably similar. These studies are supported by extensive thermodynamic data, a growing body of structural data from neutron diffraction and state of the art computer modelling     

物质动态结构的非弹性散射研究

在静态结构的基础上，考虑原子的振动，分子键的振动、转动和振－振运动的结构称为动态结构，它是用非弹性散射方法来测定的。本文评述研究动态结构的实验方法：中子非弹性散射，非弹性X射线散射、核非弹性散射和Raman散射以及红外吸收谱。简单介绍了声子散射理论基础，继后，分三节描述了用这些方法来研究动态结构的若干结果。1）结晶物质，包括晶内，表面和界面，高纯近完整晶体中杂质引起的，多晶中、薄膜和纳米晶体中的点阵动力学研究；2）非晶物质，包括非晶固体，高聚合物，生物大分子，准晶和液体的动态结构研究；3）高温超导体的点阵动力学研究，文末给出了小结和最后评论。     

Melting mechanism of monolayers adsorbed in cylindrical pores: An influence of the pore wall roughness

We have analyzed the mechanism of melting of layers adsorbed in cylindrical pores of porous materials. The goal was to understand the melting mechanism of simple fluids adsorbed in pores with heterogeneous wall surface. The studied system was a monolayer of methane molecules adsorbed in MCM-41 pore of diameter d = 4 nm. Both experimental (neutron scattering) and simulation (Monte Carlo) results proved extremely strong influence of the wall roughness on the melting mechanism. The most striking difference between melting on smooth and rough surfaces was in the temperatures of the transition. The transformation between solid-like and liquid-like monolayer phases adsorbed on a rough surface was observed in a very large temperature range and the solid like properties were observed even above the bulk methane melting temperature.     

Multiscale structures and phase transitions in metallic glasses:A scattering perspective

Amorphous materials are ubiquitous and widely used in human society, yet their structures are far from being fully understood. Metallic glasses, a new class of amorphous materials, have attracted a great deal of interests due to their exceptional properties. In recent years, our understanding of metallic glasses increases dramatically, thanks to the development of advanced instrumentation, such as in situ x-ray and neutron scattering. In this article, we provide a brief review of recent progress in study of the structure of metallic glasses. In particular, we will emphasize, from the scattering perspective, the multiscale structures of metallic glasses, i.e., short-to-medium range atomic packing, and phase transitions in the supercooled liquid region, e.g., crystallization and liquid-to-liquid phase transition. We will also discuss, based on the understanding of their structures and phase stability, the mechanical and magnetic properties of metallic glasses.     

The backscattering of low energy ions and surface structure

Low energy ion scattering (< 2 keV) in combination with LEED allows surface structure analysis in the case of gases adsorbed on single crystal surfaces. Strong shadowing and anisotropic effects observed are used to estimate the position of the adsorbed species. These techniques may also be extended to study adsorption on polycrystals. For the study of the faces of polar crystals multiple scattering effects in conjunction with shadowing offer a method of surface structure analysis. Surface relaxation effects may possibly be successfully measured by high energy (100 keV – 2 MeV) ion scattering making use of channelling and blocking effects.     

Some recent results using spin echo resolved grazing incidence scattering (SERGIS)

The advantages of neutrons for probing bulk structures are well known: they provide statistically averaged correlation functions over a large range of length scales and they are sensitive to light atoms such as hydrogen. These same qualities are, in principle, useful in the study of surfaces and buried morphologies in thin films, especially when the films are polymeric or biological. However, because of the limited sample volume for such systems, the scattering is weak, especially if the neutron beam has to be severely collimated in order to resolve distances of interest (typically 10 to several 100 nm parallel to the surface of the sample). SERGIS is a technique that can potentially overcome these limitations by allowing high resolution measurements of lateral surface structure without requiring tight beam collimation. In this paper we discuss recent progress towards implementing SERGIS both at the Low Energy Neutron Source at Indiana University and on the Asterix reflectometer at the Los Alamos Neutron Science Center (LANSCE). The architecture we use exploits a robust symmetry-related cancellation of Larmor phase aberrations. The spatial resolution it achieves closely mimics that of the ideal magnetic Wollaston prisms. To make progress in understanding SERGIS, we have applied it to the measurement of simple diffraction gratings and developed a dynamical theory that accounts quantitatively and without adjustable parameters for all of the data sets we have measured to date. We argue here that, if SERGIS is to be applied to the study of weakly scattering thin films, it will be necessary to develop the technique of dark-field spin echo scattering angle measurement.     

表/界面水的扫描探针技术研究进展

表面和界面水在自然界、人们的日常生活以及现代科技中无处不在.它在物理、化学、环境学、材料学、生物学、地质学等诸多基础学科和应用领域起到至关重要的作用.因此,表面和界面水的功能与特性的研究,是水基础科学的一项核心任务.然而,由于水分子之间氢键相互作用的复杂性,及其与水-固界面相互作用的竞争,使得表(界)面水对于局域环境的影响非常敏感,往往需要深入到分子层次研究其微观结构和动力学过程.近年来,新型扫描探针技术的发展使得人们可以在单分子甚至亚分子尺度上对表(界)面水展开细致的实空间研究.本文着重介绍几种代表性的扫描探针技术及其在表(界)面水体系中的应用,包括:超高真空扫描隧道显微术、单分子振动谱技术、电化学扫描隧道显微术和非接触式原子力显微术.此外,本文还将对表(界)面水扫描探针技术研究面临的挑战和未来发展方向进行了展望.     

Critical dynamics of magnets

We review our current understanding of the critical dynamics of magnets above and below the transition temperature with focus on the effects due to the dipole-dipole interaction present in all real magnets. Significant progress in our understanding of real ferromagnets in the vicinity of the critical point has been made in the last decade through improved experimental techniques and theoretical advances in taking into account realistic spin-spin interactions. We start our review with a discussion of the theoretical results for the critical dynamics based on recent renormalization group, mode coupling and spin-wave theories. A detailed comparison is made of the theory with experimental results obtained by different measuring techniques, such as neutron scattering, hyperfine interaction, muon spin resonance, electron spin resonance, and magnetic relaxation, in various materials. Furthermore we discuss the effects of dipolar interaction on the critical dynamics of three-dimensional isotropic antiferromagnets and uniaxial ferromagnets. Special attention is also paid to a discussion of the consequences of dipolar anisotropies on the existence of magnetic order and the spin-wave spectrum in two-dimensional ferromagnets and antiferromagnets. We close our review with a formulation of critical dynamics in terms of nonlinear Langevin equations.     

Intramolecular diffusive motion in alkane monolayers studied by high-resolution quasielastic neutron scattering and molecular dynamics simulations

Molecular dynamics simulations of a tetracosane (n-C24H50) monolayer adsorbed on a graphite basal-plane surface show that there are diffusive motions associated with the creation and annihilation of gauche defects occurring on a time scale of approximately 0.1-4 ns. We present evidence that these relatively slow motions are observable by high-energy-resolution quasielastic neutron scattering (QNS) thus demonstrating QNS as a technique, complementary to nuclear magnetic resonance, for studying conformational dynamics on a nanosecond time scale in molecular monolayers.     

Water dynamics in a lithium chloride aqueous solution probed by Brillouin neutron and x-ray scattering

We studied the collective excitations in an aqueous solution of lithium chloride over the temperature range of 270-205 K using neutron and x-ray Brillouin scattering. Both neutron and x-ray experiments revealed the presence of low- and high-frequency excitations, similar to the low- and high-frequency excitations in pure water. These two excitations were detectable through the entire temperature range of the experiment, at all probed values of the scattering momentum transfer (0.2 ?(-1) < Q < 1.8 ?(-1)). A wider temperature range was investigated using elastic intensity neutron and x-ray scans. Clear evidence of the crossover in the dynamics of the water molecules in the solution was observed in the single-particle relaxational dynamics on the μeV (nanosecond) time scale, but not in the collective dynamics on the meV (picosecond) time scale.     

Adsorption of iodoalkanes on graphite

In this work, we present a study of linear iodoalkanes physisorbed from their bulk liquid and at submonolayer coverages onto a graphite substrate by combining calorimetry, incoherent neutron scattering and synchrotron diffraction techniques. We identify that there is solid monolayer formation for these molecules for alkyl chain lengths of 5–12 carbons at high surface coverages. The monolayer structures of the odd homologues (5, 7, 9 and 11) have been addressed using synchrotron X-ray and neutron scattering at sub-monolayer coverages and indicate that the molecules lie with their carbon backbone parallel to the graphite surface with a Pgb plane group. Evidence for a non-bonded I–I interaction in the monolayers is also discussed.     

Direct observation of H2 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.