J.F. van der Veen

Layering of a liquid metal in contact with a hard wall

When a liquid makes contact with a solid wall, theoretical studies indicate that the atoms or molecules will become layered adjacent to the wall, giving rise to an oscillatory density profile. This expectation has not, however, been directly verified, although an oscillatory force curve is seen for liquids compressed between solid surfaces. Here we present the results of an X-ray scattering study of liquid gallium metal in contact with a (111) diamond surface. We see pronounced layering in the liquid density profile which decays exponentially with increasing distance from the wall. The layer spacing is about 3.8 å, which is equal to the repeat distance of (001) planes of upright gallium dimers in solid α-gallium. Thus it appears that the liquid near thewall assumes a solid-like structure similar to the α-phase, which is nucleated on freezing at lower temperatures. This kind of ordering should significantly influence flow, capillary osmosis, lubrication and wetting properties,, and is likely to trigger heterogeneous nucleation of the solid.

Are glycoproteins and glycosaminoglycans components of the eukaryotic genome

THE eukaryotic genome is composed primarily of DNA and a complex and heterogeneous array of proteins. Evidence is rapidly accumulating to suggest that these chromosomal proteins are important in determining structural as well as functional properties of the genome1–6. In addition to being implicated in packaging of the cells genetic information7–12, chromosomal proteins render limited and defined genetic sequences available for transcription13–23. But the specific properties and mode of interaction of genome components remain to be resolved. We present here evidence that several molecular weight classes of chromosomal proteins are glycoproteins. Furthermore, we report the association of glycosaminoglycans, another class of carbohydrate-containing macromolecules, with the eukaryotic genome.

Melting and freezing at surfaces

Although melting and freezing are common phenomena in nature, the microscopic mechanisms underlying these phase transformations are poorly understood. In the past decade, the possible role of surface and interfaces in inducing melting of the solid or freezing of the liquid has been the subject of intense research. Here, we present a brief status report.

Stable Nitroxides as Lipid Antioxidants

STABLE nitroxides have been shown by Neiman et al.1,2 and Brownlie and Ingold3 to be inhibitors of polymerization. Hoffman4 mentioned that di-t-butyl nitroxide was an effective antioxidant, polymerization inhibitor and radical trap, but no details were given. According to Neiman et al.1 and Brownlie and Ingold3 the nitroxide radical reacts with an alkyl radical, stopping the chain reaction as follows The nitroxides compete favourably with oxygen for the R radical1, and propagation is said to be prevented with little or no peroxide formation. Work in this laboratory5 suggested that nitroxide radicals were involved in the antioxidant activity of secondary aliphatic hydroxyl-amines toward unsaturated lipids. Some of the known stable nitroxide radicals have now been examined.

Focusing of coherent X-rays in a tapered planar waveguide

Abstract Focusing of X-rays in one dimension is achieved in a planar waveguide with linear taper. The waveguide consists of two plates with a variable tilt angle and an air gap between them. Compression of the beam and coherent mode coupling inside the waveguide result in a line focus of 26 nm height at the exit.

A new x-ray diffraction method for structural investigations of solid-liquid interfaces

A synchrotron x-ray diffraction method is presented for structural investigations of interfaces between low-Z substrates and heavier liquids. The method, similar to methods used in neutron scattering, is based on illuminating the interface through the solid substrate. The backgrounds arising from bulk scattering and the signal-to-background ratio are estimated and compared with experimental results. An ultrahigh vacuum (UHV) setup is described in which the atomic arrangement and roughness of clean interfaces can be studied in situ. Our first results illustrate the possibilities for both out-of-plane and in-plane diffraction studies. The specular reflectivity of the Ga/diamond(111)-2×1 interface was measured for perpendicular momentum transfers up to 2.2 A−1. In an in-plane study of Ga/Si(111)-7×7 the in-plane structure factor of Ga liquid within a depth of ∼50 A was compared to the structure factor of the bulk liquid.

Structure and morphology of the as-polished diamond(111)-1 × 1 surface

Abstract We present an X-ray diffraction study of the polished, unreconstructed diamond(111) surface. By measuring the distribution of diffracted intensities along the ( hk )=(10) crystal truncation rod, we have determined the structure of the unreconstructed surface. We find that after heating to 650°C, the polished diamond (111)-(1 × 1) surface exhibits a single-bond cleavage plane terminated by hydrogen atoms and, for 15% of the surface area, by adsorbed oxygen, most likely in the form of hydroxyl groups. We observe relaxations in the first layers, most notably a 3% contraction between the first and second bilayer. AFM results are presented that confirm the morphology found in X-ray analysis. Medium-energy ion scattering was used to determine the oxygen coverage.

A planar x-ray waveguide with a tunable air gap for the structural investigation of confined fluids

A tunable x-ray waveguide with an air gap as the guiding medium is presented. Discrete transverse-electric modes excited in the air gap propagate almost undisturbed. Filling the air gap with a fluid allows for studies of ordering phenomena in a confined geometry. Since the guided modes are mainly confined to the guiding layer, background scattering from the plates is very low. Starting from the propagation characteristics of the modes in the empty waveguide, requirements on the x-ray source and on the positioning accuracy of the plates are derived. The construction of the waveguide is described and measurements of the far-field angular distributions of intensity exiting the waveguide are presented which illustrate the waveguide’s properties.

Evidence for tilted chains on the diamond (111)-(2 × 1) surface

We present the first X-ray diffraction structure analysis of the reconstructed diamond (111) surface. The atomic geometry is obtained from the distribution of diffracted intensities along integer-order rods of Bragg scattering perpendicular to the surface. Of all the geometries considered, an atomic arrangement featuring tilted chains of π-bonded surface atoms with distortions in subsurface layers is found to provide the best agreement between the calculated and measured structure factors. The tilted-chain topology explains the origin of the observed surface band gap, but it is at variance with recent theoretical predictions based on the local density approximation.

Structure of the diamond (111)-(2 × 1) surface revisited with ion scattering

Abstract We present a medium-energy ion scattering study of the reconstructed diamond (111)-(2 × 1) surface. Measured blocking patterns were compared with patterns calculated for a variety of structure models. We find that a π-bonded chain model featuring tilted chains best describes our data. The tilt explains the observed electronic band gap at the surface, but is at variance with the results of recent energy minimization calculations based on the local density approximation. Our results are in disagreement with a previous ion scattering study of the reconstructed diamond (111) surface.