Refinements in the theory of XAFS (x-ray absorption fine structure) are discussed. These include spherical wave corrections and multiple scattering contributions. The conventional theory, with these refinements included, can be recovered using effective scattering amplitudes, appropriate Debye-Waller factors, and many-body amplitude reduction factors.

Extended x-ray absorption fine structure (EXAFS) spectra have been acquired experimen-tally for a number of fcc metals with atomic number Z between 28 (Ni) and 90 (Th). The backscattering amplitude and phase shift functions have been extracted from the data and compared with calculations based on a single scattering theory, using for the ejected electron, the exact curved wave function. The calculated functions obtained with the curved wave formalism provide increased accuracy, particularly at low k, i.e. k < 4 Å-1, compared to those obtained with the plane wave approximation. Agreement between the calculated and measured backscattering and phase functions is excellent for Ni, Cu, and Yb, but less satisfactory for the heavier elements, Pt, Au, Pb, and Th. Experiment confirms the prediction that very rapid changes of the the phase occur as a function of k for high Z elements.

We describe recent measurements which have provided, in unprecedented detail, insights into the electronic mechanisms through which energy carried into a material by photon irradiation is absorbed, localized and rechanneled to produce desorption, surface modification, erosion and damage. The specific object of these studies has been desorption induced by electronic transitions in alkali halide crystals, with particular emphasis on the dynamics of changes in the surface and near-surface regions. In our experiments, the irradiating ultraviolet photons are provided by a synchrotron storage ring, and the dynamical information about desorption products is obtained from optical measurements of the quantum states, yields and velocity distributions of neutral ground-state and excited-state atoms ejected from the surface of the irradiating material. These studies have shown that the dominant exit channels in photon-induced particle emission are those producing ground-state and excited-state neutral atoms. Using dynamical information about these desorbing neutral species, obtained, for example, by laser-induced fluorescence and laser Doppler spectroscopy, we are generating an increasingly comprehensive picture of the dynamics of electronic energy flow into and out of pure crystalline surfaces in these prototypical dielectrics. We are also beginning to be able to relate desorption dynamics to specific materials properties, and to discriminate between pure surface and near-surface effects in these materials. Applications of these techniques to the problem of photon-induced surface damage and to analysis of surface dynamics in dielectric materials are discussed, and the relationships between these nearly ideal model materials and the non-crystalline, covalently bonded materials more typical of real optical elements are pointed out.

Preliminary experimental and theoretical studies have been undertaken of the time dependent x-ray reflectivity of a germanium crystal under pulsed laser illumination. The integrated x-ray reflectivity is used as a temperature probe of the crystal. The Chroma laser system at KMS Fusion provided a 1.054 μm (IR) heating pulse with a 25 ps (FWHM) width. Fluences on the germanium (333) crystal varied from 0 to 2 Vcm2. The germanium crystal was simultaneously irradiated with high intensity x-rays from a laser plasma. The emitting plasma was formed by directing a high energy, 0.527 μm laser at a manganese target. The time history of the x-rays reflected from the crystal was detected by an x-ray streak camera. A computer code was written which models temperature dependent dynamical x-ray diffraction. The code takes into account the heating beam absorption profile, depth-dependent Debye-Waller effect and thermal strain profile. The present setup is sensitive to reflectivity changes of 20%. With some improvements, the technique appears to have some promise for real-time studies of crystal behavior.

Measurements of soft x-ray production by transition radiation have been performed in a series of experiments at the Lawrence Livermore National Laboratory. The results have shown that transition radiation is an intense and predictable source of photons in the soft x-ray energy range. This paper will give a brief review of the general properties of the x-ray distributions generated by these sources.

X-rays at glancing angles have a short controllable penetration depth which can be used to advantage in studying surfaces and interfaces. Data are presented for Cu-Al and Ag-Au bilayers which demonstrate the utility of x-ray reflectivity and extended x-ray absorption fine structure (EXAFS) measurements. X-ray reflectivity measurements are sensitive to layer thickness and interface roughness, while the EXAFS probes the atomic scale envir'onment of the interface atoms. As the samples are annealed clear changes are observed in both techniques indicative of the growth of interfacial CuAl2 for the Cu-Al samples and interdiffusion for Ag-Au. Interface sensitivity is verified for the Cu-Al samples by the observations that the EXAFS signal changes from Cu-like to CuAl2-like with the growth of ~ 100 A of CuAl2. Quantitative fits to the x-ray reflectivity data are used to obtain interface roughness, and indicate that the growing CuAl2 layer has a roughness comparable to its thickness. This is consistent with compound growth proceeding predominantly along grain boundaries.

Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy is a recently developed tool which is used to determine angular orientation and bond lengths of small molecules which adsorbed onto surfaces. Polarized soft x-rays from a synchrotron radiation source cause electronic transitions from core levels into unfilled molecular orbitals near the vacuum level. Many small molecules have unfilled it and a symmetry orbitals, and dipole selection rules cause these transitions to be highly polarization dependent. By rotating the sample with respect to the photon beam, one observes this polarization dependence, and from this the angular orientations of the chemical bonds are inferred. The technique is less model dependent than conventional techniques such as LEED and HREELS, and has been used to determine a number of orientations with confidence. The spectra also yield information about the lengths of the bonds from which structural models may be constructed for the surface + adsorbate system.

EXAFS measurements of ion-damaged amorphous Ge (a-Ge) show that low temperature annealing causes a structural relaxation in the as-implanted a-Ge. It is found that there is a sharpening of the first shell in the radial distribution but no change occurs in the first-shell distance or coordination number. No higher shells in the radial distribution are observed, either before or after annealing, indicating that these shells remain highly disordered. The observed structural relaxation is an amorphous-amorphous transition; no nucleation of microcrystals takes place. EXAFS measurements are made using conversion electron detection (CEEXAFS), which is essentially total electron yield detection in ambient conditions, allowing the EXAFS measurements to be near-surface sensitive with a sampling depth of 600 to 800 Å.

Results of Extended X-Ray Absorption Fine Structure (EXAFS) studies of aperiodic alloys of AlMn and related alloys are summarized. By comparing the EXAFS of icosahedral AlMnSi and AlMn with that of α(AlMnSi), it was found that Mn icosahedra of edge length ti 5 Å are the structure units in the icosahedral phase. The connection among these icosahedral units is different from the connection in the a phase. Comparing the Fe and Mn sites in icosahedral AlMnFeSi and decagonal AlMnFe, indicates that Fe and Mn occupy the same sites in the icosahedral phase but different sites in the decagonal phase. We discuss these results in terms of the randomly connected icosahedra model (RCI) proposed for the icosahedral phase.

Extended X-ray absorption fine-structure (EXAFS) of the iron atoms in Cu98Fe2, Cu67Au30Fe3, and Cu69Au3oFel are used to provide direct structural information about the Fe enrivonments in these alloys. Fe in Cu98Fe2 is face-centered cubic γ-Fe, whereas Fe in the Cu-Au-Fe alloys is body-centered cubic α-Fe. These results are in excellent agreement with the results deduced from the Mossbauer studies of the hyperfine field distributions in these systems.

The available synchrotron radiation sources and those proposed for construction in the near future in the US and abroad can produce fluxes of x-radiation high enough that the fluorescent and scattered flux even from biological samples approach and will exceed acceptable levels of counting losses even in fast photon counting detector system. Ionization chambers in current integration mode can afford very high flux and large apertures. But they suffer time limitations in the fraction of the millisecond region, microphonics, and the necessity of a gas supply of very constant pressure. We have developed an alternative detector system consisting of a photomultiplier tube equipped with a highly efficient ZnS (Ag) scintillator in current integration mode. It can have apertures up to 5 inches in diameter and a time resolution adequate for rapid reaction studies using synchrontron radiation (70 ns decay time to 10%). In initial tests, we did not detect any saturation effects with the fluxes available. The advantages of these detectors seem to be simplicity and reliability in addition to freedom from environmental effects and the relatively low cost compared to other devices. These detectors have been used successfully at the Photon Factory, Japan and at CHESS.

The use of x-ray photoelectron diffraction (XPD) for studies of surface and epitaxial overlayer structures is reviewed. XPD is found to provide several direct and rather unique types of structural information, including the sites and positions of adsorbed atoms, the orientations of small molecules or fragments bonded to surfaces, the layer thicknesses and morphologies of epitaxial or partially epitaxial overlayers, and short-range spin order in magnetic materials. A rather straightforward single scattering model also proves capable of quantitatively describing such data for near-surface species. New directions in such studies also include measurements with high angular resolution and expanded use of synchrotron radiation.

This paper reviews the principles of X-ray photoelectron spectroscopy (XPS) critical to the interpretation of angle resolved measurements. A simple model appropriate for a heterogeneous overlayer on a flat surface is presented. An outline of the quantitative determination of overlayer characteristics is also included. The technique described is applied to the naturally occurring layers on an air exposed surface of a silicon wafer.

Angular Dependedent X-Ray Photoelectron Spectroscopy (ADXPS) provides a method for inves-tigating thin (less than 10A) surface layers without the need for destructive depth profiling. In addition, surface chemical environments can be identified and their chemical shifts monitored as a function of angle. The ability to examine samples at very low angles (less than 10°) is important if one is to distinguish the type and extent of surface coverage. A study of passivation methods on beryllium surfaces by ADXPS shows that conventional treatments passivate the surface with a thinner oxide layer than air exposure but leave additional chemical residues. A study of thin native oxides on silicon by ADXPS was compared with different mathematical models for the surface. The experimental data for two different sample treatments could be reasonably well fit by simple models.

A focusing quartz crystal monochromator system. has been developed that provides a bright X-ray spot at 1486.6ev. The X-ray spot size may he varied from one-hundred fifty (150) to one-thousand (1000) microns in four (4) steps. The X-ray system is combined with a high. performance electron lens that collects greater than five (5) percent of the photoemitted electrons and passes them into a hemispherical capacitor energy analyzer. The electrons are detected at the exit plane with a two (2) dimensional position sensitivity detector. Using this system, XPS has been carried out on a wide variety of practical samples. We will provide a brief review of the system, design followed by examples of the solution to unique problems. Samples which have been CLosen for discussion include: Crash Zone on a Winchester Disk An interocular Lens . Single 5 micron Carbon. Fiber Plasma Modified Polymeric Say The unique benefits of a focused. source with high energy resolution will also be discussed.

We have used X-ray photoelectron spectroscopy (XPS) to study the variation of surface pyrite density with coal particle size (53 4m - 250 4μm). We also detect and monitor pyrite oxidation to sulfate, an important process influencing the surface-dependency of coal-cleansing methods such as flotation. It is very likely that as coal is crushed as part of the processes employed to rid it of prospective pollutants one eventually reaches a pyrite size which may be called "characteristic". It is this parameter that we examine here. Good correlations are established between (i) the liberation of pyrite and particle size, (ii) surface pyrite/sulfate ratio, and (iii) oxidized and non-oxidized sulfur in a typical Canadian coal. For "non-oxidized", or "fresh" coal, the dispersion of pyrite on the coal surface is inversely proportional to coal particle radius, and the tangents of this curve intersect at a particular particle size (106±5 4μm). Although, for the oxidized coal, the appearance of the curves depend on oxidation time intervals at low temperature with humid air, there is an "optimum" particle size which exhibits maximum surface pyrite. Notably, this "optimum" size corresponds to the tangent's intersection for the non-oxidized coal, and hence the "characteristic" size of constituent pyrite. This should allow prediction of pyrite occurrence, a parameter of paramount interest in coal processing and cleaning technology. Coal surface characterization obtained by XPS after various conditioning steps and during flotation, allow both a functional analysis via the study of chemical shifts and a semi-quantitative analysis based on relative intensity measurements.

The combination of surface science, and X-ray photoelectron spectroscopy in particular, with electrochemical studies provides a powerful complementary method for the analysis of materials, as well as a useful way to adjust and monitor the surface properties of the material. Examples of such an application are provided by studies of carbon fibers and other materials including zirconium boride, aluminum and its alloys, and tungsten. The electrochemical behavior of electrodes such as aluminum and tungsten under a variety of conditions are shown to be very surface dependent. The value of a special in situ electrochemical cell that can be incorporated into the vacuum system of a surface spectrometer is discussed.

Problems in lineshape analysis for quantitation of XPS data on metals are reviewed. Comparison is made of deconvolution versus curve fitting approaches. Deconvolution methods successfully applied to AES data, using electron-energy-loss spectra, are not similarly successful with XPS data. An optimum approach is data fitting using previously formulated lineshapes that account for the large intrinsic energy losses which cause significant distortion of metal core level spectra.

A description of the Grazing Incidence X-ray Scattering technique is presented including examples of the wide variety of work being performed in the areas of ordered overlayer structure determination, phase transition studies of ordered overlayers and structural studies of thin amorphous films.

A (0,0,1) epitaxial layer of CdTe on a (0,0,1) GaAs substrate has been studied as a function of temperature by x-ray diffraction. Lattice parameters and integrated intensities of Bragg reflections were measured between 10 K and 360 K using a wavelength of 0.7093 Å (Mo Kα). The lattice parameters were measured parallel and perpendicular to the interface. The changes of the integrated intensities with temperature give information about the thermal vibrations. Average Debye temperatures for the substrate and epilayer are 232±2 K and 142±2 K, respectively. These data are compared with data from CdTe and GaAs single crystals in order to understand how strain is propagated and/or relieved in the composite system.

Presented here is a technique to structurally depth profile multilayer films using a Seemann-Bohlin triple axis x-ray diffractometer and a 12 kW rotation anode x-ray source. This paper discusses the Seemann-Bohlin geometry and aberrations related to depth profiling and refraction. The technique was successfully applied to multilayer metallic-thin films a few hundred angstroms thick and demonstrated to have an excellent sensitivity to ultra thin-metallic films less than 25 angstroms thick.

We have developed a technique for measuring the energy-resolved angular distribution of longitudinally coherent transition radiation generated in multiple-foil targets. This paper will demonstrate how data generated by these measurements can be used to determine the dielectric constants of materials in the soft x-ray spectral region.

Time-resolved X-ray diffraction has been used to investigate phase transitions in lipid-water systems. A variety of lamellar and non-lamellar phases have been examined and the transit time of the transitions between phases determined. A comparison is made between the characteristic phase transition transit times for phospholipids, galactolipids and surfactants. Transitions have been measured which span times of less than one second to orders of minutes and longer.