Argonne National Laboratory is preparing to build a new synchrotron radiation source, the 7-GeV Advanced Photon Source (APS), that will provide the world's most brilliant x-ray beams for research. The APS will produce x-rays for materials research, condensed-matter physics, chemistry, and biological and medical studies by researchers from industry, universities, and national laboratories.

In this paper we concentrate on fluctuation phenomena encountered in interacting multilayered fluid membranes using synchrotron x-ray scattering as the primary tool. These systems consisting of surfactant or lipidic surfaces, are prototype models for understanding the statistical behavior of fluctuating surfaces embedded in three-dimensions. In elucidating the nature of fluid membranes we stress a unique intrinsic property: in contrast to usual surfaces with finite shear moduli, where surface tension plays a central role, the free energy of these essentially tensionless surfaces is governed by their geometrical shape and its fluctuations. We present data in a new regime of stability for very dilute and flexible membranes with interlayer separations of order hundreds of Angstroms. This is in contrast to most rigid membranes where the interlayer interactions are dominated by detailed microscopic interactions such as hydration and van der Waals. We show that the stability of these dilute lamellar phases is associated with violent out-of-plane fluctuations of the membranes giving rise to an effectively large long-range repulsive interaction theoretically elucidated by Helfrich. Because of its entropic origin, this interaction is universal and we present data for two different surfactant systems. Finally, we show that this new regime is distinct from the classical regime in which largely separated membrane sheets are stabilized because of their mutual electrostatic repulsion.

X-rays incident on a surface under grazing angle may undergo total external reflection and excite an interior wave field damped exponentially into the bulk. These evanescent waves are a sensitive probe for the study the real structure in the near surface region. We report results on the influence of implantation defects on Bragg diffracted and on diffuse intensities. By detailed comparison of Bragg intensities with predictions of dynamical scattering theory we detect the presence of amorphous layers and determine their thickness. For the first time defect induced diffuse scattering underconditions of grazing incidence andexit is observed and compared to recent theoretical results. Strength and symmetry of implantation induced defects can be determined as well as their depth distribution which is compared to results of a TRIM simulation: the defect distribution is found to agree with that of the deposited collisional energy.

Total electron yield (TEY) of an InAs/GaAs heterojunction due to soft x-ray excitation has been studied. This heterojunction was prepared by an overgrowth of a 600 Å InAs layer on a GaAs substrate using molecular beam epitaxy. Experimental data are compared with theoretical analysis based on a modified Fresnel formulation to calculate the wave field distribution in stratified media with interfacial roughness. The TEY angular profiles obtained at a given x-ray energy reveal information on the interfacial roughness, secondary electron escape length, attenuation length of elastically scattered photo-electrons, and optical constants of the epilayer in the xray regime.

The study of ultrathin films spread on liquid substrates has been limited to macroscopic observations of general film behavior. Synchrotron radiation has provided the means of observing the two-dimensional crystalline properties of monolayers. Our recent experiments performed on NSLS X-23B beam line was the trial run of a new liquid surface diffractometer. Major emphasis has been placed on vibrational and environmental isolation of the film, in addition to 0.1 micron resolution in the control of beam positioning. The film is examined in a closed, ultraclean environment consisting of a fused silica trough, purified N2 atmosphere, 0.1°C temperature control and chromatography grade or better solvents and surfactants. Surface properties (surface tension, and visco-elastic moduli) are monitored by surface laser light scattering spectroscopy. The x-ray scattering pattern from the liquid surface contains the expected specular reflection as well as scattered peaks at small wave numbers. We believe these to be caused by high frequency surface ripple fields.

Surface x-ray scattering has been used to study in-situ the structure of Pb monolayers electrochemically adsorbed on Ag (111) electrodes. Pb forms an incommensurate, hexagonal two-dimensional (2D) solid, which is rotated approximately 4.5° from the substrate symmetry directions and compressed relative to bulk Pb. Between monolayer formation and bulk deposition, the Pb-Pb near neighbor distance decreases linearly with applied potential. Due to the chemical equilibrium between the Pb monolayer and the Pb in solution, the isothermal compressibility of the monolayer can be measured and is in good agreement with that calculated for a 2D non-interacting free electron gas model of the monolayer. It is observed that the intensity of surface diffraction from the Ag substrate (the Ag crystal truncation rod) decreases when the Pb monolayer is adsorbed, although the cause of this is not known.

New opportunities for research on magnetic materials are emerging as a result of quiet revolutions in several areas including: materials synthesis techniques, surface characterization capabilities, new magnetic sensitive detectors and spectroscopic techniques, improved synchrotron radiation instrumentation, and predictive modeling based on first principals calculations. This paper describes some of the more recent advances and assesses some of the new opportunities that are emerging in the field of magnetic materials research.

High-resolution core-level photoemission spectroscopy allows the distinction of atoms in different layers and in inequivalent sites by their binding energy shifts. By comparison with model structures and reference samples, the number of atoms in each distinct chemical configuration can be determined. The chemical shifts induced by adsorption can be correlated with the electronegativity difference between the substrate and the adsorbate atoms. These observations provide a quantitative description of the interaction and reaction between adsorbates and surfaces, and important information about the atomic structure and the electronic properties can be deduced. Results from several representative systems including the adsorption of In, Ag, and Sn on Si(100) will be discussed.

Epitaxy of CoSi2 layers on Si crystal surfaces can be strongly influenced by growing appropriate template layers. The electronic structure of thin (∼7Å) epitaxial CoSi2 films on Si(100) has been studied with angle-resolved photoemission to investigate atomic bonding in the layers and at their boundaries. Most Co atoms in the layers are in a CoSi2-like environment, including those Co atoms near the free surface. Cobalt atoms at the Si-CoSi 2 interface seem to have fewer Si neighbors.

Synchrotron radiation has been used to perform resonant photoemission measurements across the 3p→3d photoabsorption threshold from cleaved FexO (x ≃0.945), Fe3O4, and α-Fe2O3 single crystal surfaces. The resonant enhancement of the Fe 3d photoelectrons allows the Fe 3d-derived final states in the valence band to be distinguished from the overlapping O 2p states. Using well-characterized single crystals, the distributions of Fe 3d-derived states associated with the ferrous (Fe2+) and ferric (Fe3+) cations have been identified. The Fe 3d-derived states are found to extend about 18 eV below the Fermi level in each oxide, which can be attributed to a significant amount of hybridization between the Fe 3d and O 2p orbitals.

Photoemission EXAFS (PEXAFS) studies of InP(110), InP(110) + 1 Å Al and InP(110) + 3 Å Al are presented. In each case, photoemission from the P 2p core-level was monitored using a cylindrical mirror analyzer by a two-point CIS (constant-initial-state spectroscopy) method in 150–280 eV photon energy range. The data were analyzed by conventional Fourier analysis procedures using the theoretical phase function of McKale et al. plus absorber phase function of Teo and Lee. No diffraction effect other than EXAFS is observed. For the clean InP(110) surface, the PEXAFS results show relaxation (contraction) of the surface in excellent agreement with the LEED results. But a small contraction in the P-P bond length (surface unit mesh parameter, a0) has also been measured, which was not reported in the LEED studies. For the InP(110) surface covered with 1 Å Al, metal (cluster)-induced surface structural changes in the substrate are determined which include removal of relaxation of the surface unit mesh in combination with change in the P-In bond length within the surface unit mesh. At the 3 Å Al-coverage on the InP(110) surface, the interface resembles AlP.

The stress in the surface of a thin (0–10 monolayer) film of mercury can be relieved by either changing the substrate and consequently changing adlayer lattice constants or by growing thicker films. An empirical relationship between strain and the valence electronic structure of mercury overlayers can be demonstrated, though we present evidence that this relationship can be altered and destroyed by crystallography changes.

Photon stimulated desorption of fluorine ions from silicon surfaces was studied via excitation of the Si 2p core level with synchrotron radiation. These results showed that the process is chemically selective in that the removal of a fluorine ion from a silicon species in a given oxidation state can be enhanced by tuning the photon energy to the excitation wavelength corresponding to a transition from the 2p core level of the bonding atom to the conduction band minimum. This process was studied as a possible means for the production of surfaces with selected compositions of species. The results of selective exposures of fluorinated surfaces to monochromatized radiation indicated that secondary desorption processes and the inherent chemistry of the surface reactions can override the effects of selective desorption. Other possibilities for selective surface reactions via core-level excitations are discussed.

Polarized X-ray absorption studies have been carried out at the Cu K-edge to study the effect of Sr doping in La2CuO4 and oxygen doping in YBa2Cu3O6+x. These measurements help to elucidate the transitions giving rise to the absorption edges. We offer an explanation of the polarization shifts of features in terms of the results of our embedded cluster calculations of electronic structure.

Photoemission study of the Ga/InP(110) interface, in particular at the In 4d cooper minimum (CM) reveals that the growth of the deposited Ga on InP(110) at room temperature (RT) has two modes: chemisorption at low coverage and metallic island formation at high coverage, whereas the Ga overlayer is much more uniform at 80K low temperature (LT). A replacement reaction between Ga and InP is found to take place only underneath the Ga islands. Metal screening from the Ga islands is suggested to weaken the substrate bonds and enhance the replacement reaction. Distinct behavior of Fermi level pinning has been observed at different temperatures. This is correlated with the temperature dependence of the overlayer morphology as well as the interfacial reaction.

X-ray absorption spectra (XAS) contain information in the LIII near-edge region on filling of the absorber d-band, and in the extended fine-structure region on the physical environment of the absorber. We report here an evaluation of the effect on platinum LIII edges of preparation in clusters with a high fraction of Pt atoms at the surface. We also report the effects on platinum and rhenium LIII edges from addition of copper. These effects are surprisingly small.

We have also re-evaluated extended x-ray absorption fine-structure spectra (EXAFS) of platinum and rhenium in alumina-supported platinum-rhenium bimetallic catalysts. A novel feature of this new analysis was the requirement that interatomic distances, coordination numbers, and Debye-Waller type factors maintain certain physically necessary relationships among themselves. This procedure decreased the number of free variables and increased the amount of information returned by the analysis.

X-ray Absorption Spectroscopy has proved to be a powerful tool to elucidate a huge number of questions in materials science. Great interest exists in time-resolved experiments achieved with extreme energy resolution and energy scale stability to take a full benefit of the strong correlation between the stereochemical environment of the absorbing atom and the exact shape and position of the absorption edge.

Fast energy dispersive X-ray spectroscopy allows in-situ observations with data collected in a short time. Nowadays the main limitation concerns very low-concentration samples since It is no longer possible to use the dispersive geometry because detection of the signal via the decay channels is no more possible.

Important for the understanding of multilayer materials is a determination of their interface structure. The extended x-ray absorption fine structure (EXAFS) technique can be useful, particularly for interfaces with a high degree of structural disorder. This paper reviews the application of EXAFS to multilayers, and describes the standing wave enhancement of the EXAFS from multilayer interfaces. Examples are given for W-C and Ni- Ti multilayers.

Fluorescence Yield Near Edge Spectroscopy (FYNES) of ultra low Z materials represents a synchrotron radiation milestone for in-situ determination of local structure for a range of materials problems from monolayers to bulk samples even in the presence of a reactive atmosphere. Two examples will be presented highlighting the broad range of materials problems addressed by the FYNES technique. First a study of the kinetics of CO displacement on Ni(100) by hydrogen at pressures up to.1 torr. These kinetic results highlight the unique capabilities of FYNES to directly characterize surface reaction rates in the presence of reactive gases. Second, a pioneering fluorescence EXAFS study characterizing low concentration fluorine materials will be discussed. Finally new opportunities in FYNES presented by increased photon flux from insertion-device- based sources will be explored.

The early stages of cobalt silicide formation in high dose (1.0 to 8.0× 1017Co/cm2) cobalt implanted Si(100) are studied by extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD) and Rutherford backscattering spectroscopy (RBS). Locally ordered silicide that is not detectable in XRD has been observed with EXAFS in the as-implanted samples. Long-range ordered phases are observed in the 3 × 1017Co/cm2 samples. After thermal annealing at 700–750°C, single phase CoSi2 with (400) orientation is formed in all implants.