M.J. Zwanenburg

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.

X-ray diffraction studies of potassium dihydrogen phosphate (KDP) crystal surfaces

We have studied the surface atomic structure of KDP crystals using X-ray scattering. These crystals were grown from an aqueous solution and we have done measurements both ex situ and in situ. The ex situ measurements were performed in vacuum or in air. In order to be able to do in situ measurements, we designed and built a crystal growth chamber which is compatible with X-ray diffraction experiments. The atomic arrangement of the two naturally existing faces of KDP has been determined. Preliminary results are presented of measurements performed during growth. Furthermore, the influence of metal impurities on the atomic structure of the growing interface is examined.

Surface atomic structure of the (3×3)R30°-Sb reconstructions of Ag(111) and Cu(111)

Abstract We present an X-ray diffraction structure analysis of the ( 3 × 3 )R30° reconstructions of Ag(111)-Sb and Cu(111)-Sb surfaces. We find these structures to be very similar. Contrary to previous reports, we find that all top layer atoms reside at stacking fault positions. Each ( 3 × 3 )R30° surface unit cell contains one substitutional Sb atom. We determined the out-of-plane relaxations of the top layer atoms and the in-plane distortions in the second layer. For coverages below 1/3 monolayer, the Sb atoms are embedded randomly at fcc positions in the top surface layer.

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.

Propagation of a partially coherent focused X-ray beam within a planar X-ray waveguide

A linear-transmission Fresnel-zone-plate lens is used for coupling a monochromatic X-ray beam of 13.2 keV into a planar X-ray waveguide. The zone plate focuses the beam parallel to the entrance of the waveguide, by which means a flux enhancement of a factor of 54 inside the waveguide is obtained. This substantially enlarges the range of X-ray diffraction experiments that can be performed on samples confined within the waveguide. The coherent properties of the beam, as obtained at a third-generation synchrotron facility, are investigated by comparing the experimental data with numerical calculations in which partial coherence is taken into account by propagation of the mutual intensity function. The conditions for which the X-rays travelling through the waveguide are coherent are discussed.

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.

Propagation of coherent x rays in a multistep-index x-ray waveguide

We have developed and tested a multistep-index x-ray waveguide. The waveguide geometry allows for a controlled positioning of the waveguide gap down to the nanometer range. The waveguide consists of two closely spaced flat fused-silica substrates, coated with an aluminum layer. On top of the aluminum layer we deposited an optically transparent spacer layer. The aluminum layers are the mirrors of an optical interferometer, which is used to measure the separation and parallelism of the waveguide surfaces. The x rays are guided within the gap between the spacer layers. We have observed so-called cladding modes and demonstrate waveguiding in a multistep index structure having a gap width of 59 nm over a length of 5 mm.