The structures of Ag,Fe,Mn, and Ge microclusters were determined using EXAFS. The measurements were performed over a wide range of clusters sizes.The clusters were prepared using the gas aggregation technique and isolated in solid argon at 4.2 K. A strong contraction of the interatomic distances was observed for Ag dimers and multimers. Silver clusters larger than 17 A mean diameter show a small contraction of the nn distance and a structure consistent with an fcc lattice. By contrast smaller clusters show the presence of a small expansion and a strong reduction or absence of nnn in the EXAFS signal. This points towards a different crystallographic structure for Ag microclusters. In iron clusters we observed a gradual reduction of the nn distance as the cluster size decreases. The interatomic distance for iron dimers was determined to be 1.94 A, in good agreement with earlier measurements. The iron microclusters show a bcc structure down to a mean diameter of 9 A. Iron clusters with 9 A mean diameter show a structure consistent with an fcc or hcp lattice.Mn microclusters show a simpler crystallographic structure than α-Mn. The measurements on Ge clusters show the presence of only nearest neighbors. There was clear evidence of temporal annealing as determined by variations in the near edge structure of the K-absorption edge. Absorption edge measurements were also performed on free Ge clusters traveling perpendicular to the direction of the synchrotron radiation beam.

The carbon K-edge Reflection-Extended X-Ray Absorption Fine Structure (refl-EXAFS) spectra from graphite, diamond and glassy carbon have been investigated. There is good phase shift transferability between the two well-known bonding types in diamond and graphite, provided that appropriate inner potential corrections to the K-edge energy E. are made. The model spectra from diamond and graphite were used to investigate the nature of glassy carbon. It was found that, for the particular form of glassy carbon used in this study, the bonding more closely resembled sp3 than sp2. This result is preliminary pending our evaluation of the influence of surface oxygen.

Small perturbations of the structure and composition of the 90K superconductor Y1Ba2Cu3O7−δ (YBCO) have profound effects on its superconducting properties [1]. In an effort to understand the interaction between the structure of YBCO and its superconducting properties, we have made an extensive investigation of YBCO doped with Fe and Co, using x-ray absorption spectroscopy.

An instrument to undertake grazing incident EXAFS has been developed. This is discussed along with some applications of its use on industrially important systems.

Some elements in some chemical states exhibit strong dichroism and birefringence near x-ray absorption edges. The atomic scattering factor is a complex tensor. This polarization anisotropy has profound effects on the transmission and scattering of x-rays even when the incident radiation is unpolarized. The linear polarization of synchrotron radiation makes it easier to study the effects and to use them for new methods of structure determination. Several of these anomalous scattering tensors have been measured by absorption spectroscopy and in diffraction experiments. New polarization terms enter the calculation of diffraction intensities, with interesting consequences. Reflections forbidden by a screw-axis rule are observed in sodium bromate near the Br K edge and permit direct observation of the structure factor phases of their second order reflections. This technique is a method of selective diffraction in which atoms of single element in a single chemical state contribute to the signal, and it can reveal their positions with precision. These effects can be a handicap for some applications of near-edge anomalous scattering in the study structures of crystals and amorphous materials.

Synchrotron radiation sources allow a new generation of parallel beam instruments to be developed for polycrystalline xray diffraction. Spectroscopy can be combined with diffraction in either angle or energy scanning modes. The high intensity narrow instrument profile and freedom from aberrations make possible a wide range of measurements; of anomalous dispersion, of peak-shape analysis to determine particle size and strain, of preferred orientation and structure as a function of depth and of lattice parameters with a precision approaching 1 part per million. The parallel beam instrument which we have developed is now to be installed on a regular basis at SSRL and at Daresbury.

The dispersion of x-rays (XR), soft x-rays (SXR) and extreme ultraviolet (EUV) light by multilayer structures is dependent on the scattering and absorption cross-sections of the elements used to synthesize the multilayer. In this paper it will be shown that this dependence provides a means for the accurate experimental determination of the optical constants of the multilayer constituents. Two specific approaches will be presented and discussed. First, it will be shown that detailed analysis of the energy dependence of the reflectivity of a simple depth periodic multilayer allows the unfolding of the optical constants. Secondly a new optic structure, the multilayer diffraction grating, will be described and it will be demonstrated that such combined microstructure optics allow the scattering cross-sections of the multilayer constituents to be accurately determined over broad spectral ranges.

Structural and dynamical properties of an aqueous gelatin solution (5 wt%, 0.1M NaCi, pH=7) in a sol-gel transition were studied by time-resolved small angle x-ray scattering (SAXS) and dynamic light scattering (DLS) after quenching the gelatin sol at ∼45”C to 11°C. SAXS intensity measurements suggested the presence of gel fibrils which grew initially in cross-section. The average cross-section of the gel fibrils reached a constant value after an initial growth period of ∼800 sec. Further increase in SAXS intensity could be attributed to the increase in the length of the gel fibrils. Photon correlation, on the other hand, clearly showed two relaxation modes in both the sol and the gel (∼1 hr after the quenching process) states: a fast cooperative diffusion mode which remained constant from the sol to the gel state after correction for the temperature dependence of solvent viscosity; and a slow mode that could be attributed to the self-diffusion of the “free” gelatin chains and aggregates. The slow mode contribution to the time correlation function was reduced from ∼40% in sol to ∼20% in gel signaling a decrease but not the elimination of “free” particles in the gel network. The decrease in the intensity contribution by the slow mode is, however, accompanied by a large increase in the characteristic line-width distribution.

We demonstrate the simultaneous acquisition of high-resolution x-ray absorption spectra and scattering data, using a combination of energy-dispersive optics and a twodimensional CCD detector. Results are presented on the optical constants of Pt and on the reflectivity of a platinum-carbon multilayer at the LIII absorption edge of Pt.

In an x-ray diffraction experiment performed on a germanium (111) crystal, both the rod-like and the diffuse-like scattering from the surface have been observed on a nonspecular crystal truncation rod. These scattering contributions can be explained using existing theory on surface roughness. Two treatments to the Ge (111) surface have been used to provide examples with different roughness characteristics for this study. Quantitative analysis results in a surface roughness of 2.5+0.3Å for a clean surface passivated with iodine and 4.3+0.5Å for a Syton polished surface covered with a naturally grown oxide layer. A typical lateral scale of flat surface regions has also been obtained from the transverse width of the diffuse-like scattering peak, and found to be 200 A and 400 Å respectively.

The unique properties of X rays offer many advantages over those of electrons and other charged particles for the microcharacterization of materials. X rays are more efficient in exciting characteristic X-ray fluorescence and produce higher fluorescent signal-to-background ratios than obtained with electrons. Detectable limits for X rays are a few parts per billion which are 10−3 to 10−5 lower than for electrons. Energy deposition in the sample from X rays is 10–3 to 10–4 less than for electrons for the same detectable concentration. High-brightness storage rings, especially in the 7 GeV class with undulators, will have sources as brilliant as the most advanced electron probes. The highly collimated X-ray beams from undulators simplify the X-ray optics required to produce submicron X-ray probes with fluxes comparable to electron sources. Such X-ray microprobes will also produce unprecedentedly low levels of detection in diffraction, EXAFS, Auger, and photoelectron spectroscopies for structural and chemical characterization and elemental identification. These major improvements in microcharacterization capabilities will have wide-ranging ramifications not only in materials science but also in physics, chemistry, geochemistry, biology, and medicine.

A review of recent advances in soft X-ray imaging using synchrotron radiation is given.

Grain boundary migration has been investigated in prestrained monocrystalline specimens of aluminum in situ, continuously and at temperatures ranging from 415 to 610°C by synchrotron (polychromatic) x-ray topography (SXRT). In general, new (recrystallized) grains nucleate at prepositioned surface indentations and expand into the prestrained matrix, revealing complex evolution of crystallographic facets and occasional generation of (screw) dislocations in the wake of the moving boundaries. Analysis of corresponding migration rates for several faceted grain boundaries yields activation energies ranging from 56 to 125 kCal/mole, depending on grain boundary character. it is concluded that grain boundary mobility is a sensitive function of grain boundary inclination, resulting in ultimate survival of low-mobility (faceted) inclinations as a natural consequence of growth selection. Advantages and disadvantages associated with measurement of grain boundary migration by SXRT are enumerated and corresponding results are interpreted in terms of fundamental relationships between grain boundary structure and corresponding migration kinetics.

White Beam Synchrotron Topography has been used to determine the role of localized stress fields in the solid state polymerization of single crystals of the diacetylene PTS. Results indicate that the stress fields due to grown in dislocations can accelerate local reaction kinetics in thermally induced polymerization reactions, although no such effects were previously observable in photolytically or radiolytically induced reactions. Results are analyzed in an analogous fashion to the treatment of the nucleation of solid state phase transformations at dislocations. Good agreement was found between approximate theoretical treatments and experimental observation. The response of the monomer crystal to the inhomogeneous stresses generated as a result of inhomogeneous reaction and the implications regarding local reaction kinetics are discussed in detail.

Synchrotron radiation can be utilized to obtain Laue back reflection patterns of microscopic areas with very short exposure time. A method is presented to fully automatically evaluate Laue patterns. The experimental setup and procedure are outlined to determine the orientations of a large number of contiguous grains, and consequently to enable orientation mapping. Preliminary experimental results are presented.

The specular reflectivty of soft x-rays has been used to investigate the surface behavior of mixtures of poly(vinylidene fluoride), PVF2, and poly(methyl methacrylate), PMMA. Mixtures where the concentration of PMMA is 0.8 where investigated as a function of the preparation and annealing conditions. It was found that mixtures prepared by rapidly casting from dimethylformamide, DMF, exhibited a gradient in the concentration of the two components. The gradient extended over large distances and was such that PMMA, the higher surface energy component was located preferentially at the surface. With annealing at 165°C, the width of the gradient diminished but the concentration of PMMA remained high at the surface. Films of the mixtures prepared under a slower solvent evaporation procedure produced a reflectivity profile characteristic of a phase separated mixture. The extent of phase separation was reduced with annealing. In all cases the roughness of the surface of the films was found to be 5.OÅ or less.

X-ray computed tomography (CT) uses absorption profiles from many different viewing directions to reconstruct the two-dimensional distribution of x-ray absorptivity within a slice of the sample. The tunability, high brightness and parallelism of synchrotron radiation are critical to high resolution (0.001mm), high contrast (1%) CT or microtomography. In situ study of samples multiple times during the course of an experiment is exciting to consider.

Continuous fiber SiC/Al composites were deformed under three-point bending, and the resulting damage and fiber arrangement were revealed with synchrotron microtomography. Several hundred slices of 0.012 mm thickness were recorded simultaneously using 25 key radiation and a phosphor screen/charge coupled device (CCD) detector. Reconstruction was with the filtered back projection method. Low density regions were observed in the matrix in regions of highest stress where cracking is expected.

As a spectroscopic technique, x-ray absorption near-edge structure (XANES) of core-levels using synchrotron radiation is in wide-spread use for the determination of the molecular composition of solid surfaces. A common detection method measures the yield of secondary electrons, which is proportional to the x-ray absorption coefficient for sufficiently high photon energy. In the experiments reported here, we show how the secondary electrons emitted as a result of photoabsorption can be used to generate a magnified image of the sample surface, with fundamental spatial limits determined by the deBroglie wavelength of the emitted electrons. Contrast in the secondary electron spatial distribution contains both topographical and chemical information about the sample surface. The use of tunable synchrotron radiation enables us to separate these contributions to the microscopic image, and to spatially resolve surface chemical composition as reflected in micro-XANES spectra.

Synchrotron x-ray fluorescence experiments were performed using a prototype undulator for the Advanced Photon Source installed on the CESR storage ring at Cornell University during a run in May, 1988. Fluorescence spectra were collected from a number of standard references and unknowns. Thick target minimum detectable limits (MDL) were about a factor of two higher than those obtained using white bending magnet radiation at the NSLS. The higher MDLs could be due to lower polarization and/or imperfect alignment of the Si(Li) detector. Thin target MDLs were about 10 times lower than the NSLS since the undulator produced a usable spot size which was also 10 times smaller. Several one dimensional multi-elemental scans and two dimensional images were made with 10 μm resolution and 30 ppm MDL. These experiments demonstrate that undulators on the proposed Advanced Photon Source will be ideal for a trace element x-ray fluorescence microprobe with excellent elemental sensitivity and spatial resolution.

Of fundamental importance in understanding melting is the question of premelting (i.e. do surfaces melt at a lower temperature than the bulk). We report the results of an x-ray scattering experiment that has directly measured liquid correlations in Pb(110) and Pb(111) surfaces at temperatures substantially below the bulk melting point of Pb. Surprisingly, significant liquid scattering is observed at ≈230°K or >1/2Tm on both the (110) and the (111) surfaces. On the (110) surface, analysis of the scattered intensity yields 2–3 monolayers of liquid from 575°K to 599°K while substantially less than 1 monolayer of liquid is present on the (111) surface at 599°K.