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Hard X-ray photoelectron spectroscopy (HAXPES) provides numerous exciting opportunities to deepen our understanding of materials chemistry and electronic structure, as is evident from recent review...

We present an overview of the National Institute of Standards and Technology beamline X24A at the National Synchrotron Light Source at Brookhaven National Lab and recent work performed at the facility. The beamline is equipped for HAXPES measurements, with an energy range from 2.1 to 6 keV with Si(1 1 1) crystals. Recent measurements performed at the beamline include non-destructive depth dependent variable kinetic energy measurements of dielectric and semiconductor films and interfaces for microelectronics applications, band alignment at buried interfaces, and the electronic structure of bulk-like materials. The design and operation of the current beamline will be discussed, as well as the future NIST beamline at NSLS II.

We report hard x-ray photoemission spectroscopy measurements of the electronic structure of the prototypical correlated oxide ${\mathrm{Sr}}_{x}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{VO}}_{3}$. By comparing spectra recorded at different excitation energies, we show that 2.2 keV photoelectrons contain a substantial surface component, whereas 4.2 keV photoelectrons originate essentially from the bulk of the sample. Bulk-sensitive measurements of the O $2p$ valence band are found to be in good agreement with ab initio calculations of the electronic structure, with some modest adjustments to the orbital-dependent photoionization cross sections. The evolution of the O $2p$ electronic structure as a function of the Sr content is dominated by $A$-site hybridization. Near the Fermi level, the correlated V $3d$ Hubbard bands are found to evolve in both binding energy and spectral weight as a function of distance from the vacuum interface, revealing higher correlation at the surface than in the bulk.

We have measured the response of chromium-doped alumina screens to soft x-ray radiation and derived quantum efficiency curves for the energy range from 2.5 to 4.5 keV. Persistent luminescence (or afterglow) from this material is observed for several minutes following the removal of the x-ray source. It is also observed that the luminescence output rises gradually for several minutes before maximizing during x-ray irradiation. In the article we discuss possible sources of this delayed luminescence and its consequences for application of these screens as soft x-ray detectors. Such screens have found application as narrow-band radiation detectors for a hot fusion plasma diagnostic.

We present a detailed depth-sensitive study of the evolution in correlated electron behavior from the surface of the prototypical correlated oxide, SrxCa1-xVO3, to its bulk. Photoemission measurements of varying surface sensitivity are employed to directly compare both the spectral weight and energetics of the correlated electron features, and resonant soft x-ray emission spectroscopy is used as a bulk-sensitive reference. The surface component, which still contributes significantly to photoemission at 2.2 keV, is characterized by a transfer of spectral weight into the incoherent lower Hubbard band and the corresponding shift of these states towards lower binding energy.

We have measured the potassium {ital K}{sub {beta}} and chlorine {ital K}{sub {beta}} resonant Raman x-ray scattering from KCl. Strong {ital K}-shell absorption peaks are directly reflected in the resonant Raman spectra. The emission spectra are qualitatively evaluated in terms of the associated absorption spectra, based on a simple single-electron model of the scattering process. This model affords direct interpretation of the energy, line shape, and intensity variations of the emission spectra. In the resonant Raman regime, energy offsets between the data and the model predictions are attributed to binding energy differences between intermediate and final state excitons; thus these offsets indicate that the associated peaks in the absorption spectra are excitonic in nature. {copyright} {ital 1996 The American Physical Society.}

Near an absorption edge, x-ray emission cannot be treated separately from the absorption process itself; a scattering formalism must be used. Experimental data have been recorded showing x-ray emission from xenon following excitation by tunable synchrotron radiation below and above the {ital L}{sub 3} absorption edge. Complete data sets are presented for Xe {ital L}{alpha}{sub 1,2} and {ital L}{beta}{sub 2,15} emission from 10 eV below to 40 eV above the {ital L}{sub 3} edge. In accord with the resonant-inelastic-scattering model, the observed x-ray emission evolves from resonant Raman scattering into characteristic fluorescence as the excitation energy is scanned from below to above the absorption edge.

We report on the performance of an InSb/KDP monochromator mismatched crystal pair in use on beamline X24A at the National Synchrotron Light Source. This crystal pair provides extremely high spectral resolving power and throughput in the photon energy range 1750 to 2100 eV, which is below the physical limit of Si(111). By measuring the back-reflection from a Si(111) single crystal, we determine the resolution of the mismatched pair to be 0.40±0.05 eV at 1977 eV. The first Si 1s absorption spectrum (1840 eV) recorded with resolution better than the core-hole lifetime is also reported.

The x‐ray standing wave technique has been used to determine the adatom positions on the Si(111) 7×7 and Si(111) √3×√3–In surfaces. By monitoring the intensity of the surface sensitive Si 1s photopeak as a function of photon energy around the Si(111) Bragg backreflection condition, we determine the Si adatoms of the clean 7×7 surface to reside 1.69±0.1 A above the extrapolated Si(111) bulk plane. In contrast, the In adatoms on the 1/3 monolayer √3×√3–In surface are found to remove the Si displacements and lie at a position of 2.11±0.05 A. High‐resolution Si 2p core‐level measurements support the T4 geometry for the In adatom location.

Triangulation from x‐ray standing wave applied to three Bragg planes, together with symmetry considerations, have established the geometrical structure of epitaxial Bi overlayers on GaAs (110). Our results are consistent with the epitaxial continued layer structure model. Our analysis is based on prior experimental results that indicate two equally populated Bi sites.

Through a unique combination of surface sensitive extended x‐ray absorption fine structure (EXAFS) and x‐ray standing waves (XSW), we have determined the pertinent structural parameters of the clean InP(110) surface reconstruction. We find a rotation angle of 27° between the P–In chains, no change of the first neighbor P–In bond length, and a small but measurable, ∼0.1 A, expansion of the average P–P second neighbor distance. The general application of the EXAFS and XSW techniques to the study of clean surfaces will be discussed.

An x‐ray beam position monitor has been developed and is currently operational at the NIST beam line X‐24A of the National Synchrotron Light Source. It was developed to accurately measure the vertical position of the synchrotron beam and to allow precise positioning of beam line optics. It is an area‐type monitor positioned 5.6 m from the tangent point and precedes all beam line instrumentation. Vertical beam position is given by the differential photoemission current from two grids mounted on a Macor frame. The monitor also senses the total photon beam flux.

The strain and relaxation of InAs and InGaAs films grown on GaAs(001) have been examined by the x‐ray standing wave and extended x‐ray absorption fine‐structure techniques. While 1 monolayer (ML) films of both InAs and InGaAs are found to be tetragonally distorted in accordance with the prediction of macroscopic‐elastic theory, thicker InAs films are found to collapse to their natural‐lattice constant past a critical thickness Tc, of ∼2 ML’s. By 8 ML’s, bond‐length strain is no longer evident, and a large degree of structural disorder is observed.

A new ‘‘Cowan type’’ double‐crystal monochromator, based on the boomerang design used at National Synchrotron Light Source (NSLS) beamline X‐24A, has been developed for beamline 9.3.1 at the Advanced Light Source (ALS), a windowless ultrahigh vacuum beamline covering the 1–6 keV photon‐energy range. Beamline 9.3.1 is designed to simultaneously achieve the goals of high energy resolution, high flux, and high brightness at the sample. The mechanical design of the monochromator has been simplified, and recent developments in technology have been included. Measured mechanical precision of the monochromator shows significant improvement over existing designs. In tests with x‐rays at NSLS beamline X‐23A2, maximum deviations in the intensity of monochromatic light were just 7% during scans of several hundred eV in the vicinity of the Cr K edge (6 keV) with the monochromator operating without intensity feedback. Such precision is essential because of the high brightness of the ALS radiation and the overall length of beamline 9.3.1 (26 m).

The x‐ray standing wave technique has been used to determine the geometric structure of the monolayer Sb/GaAs(110) interface. Using the backreflection diffraction geometry, we find the average perpendicular distance of Sb atoms from the (220), (400), and (111) diffracting planes to be 2.27±0.05, 1.60±0.05, and 3.00±0.1 A, respectively. On the basis of these data, the atomic coordinates of the overlayer have been quantitatively established. The results are in agreement with theoretical calculations for the epitaxial continued layer structure and inconsistent with other models of zigzag chains.

A new ``Cowan type`` double-crystal monochromator, based on the boomerang design used at NSLS beamline X-24A, has been developed for beamline 9.3.1 at the ALS, a windowless UHV beamline covering the 1-6 keV photon-energy range. Beamline 9.3.1 is designed to simultaneously achieve the goals of high energy resolution, high flux, and high brightness at the sample. The mechanical design has been simplified, and recent developments in technology have been included. Measured mechanical precision of the monochromator shows significant improvement over existing designs. In tests with x-rays at NSLS beamline X-A, maximum deviations in the intensity of monochromatic light were just 7% during scans of several hundred eV in the vicinity of the Cr K edge (6 keV) with the monochromator operating without intensity feedback. Such precision is essential because of the high brightness of the ALS radiation and the overall length of beamline 9.3.1 (26 m).

X-ray photoelectron spectroscopy is an established technique for quantitative chemical analysis requiring accurate peak intensity analysis. We present evidence of focus∕alignment dependence of relative peak intensities for peaks over a broad kinetic energy range with a hemispherical electron analyzer operated in a position imaging mode. A decrease of over 50% in the Ag 2p₃/₂ to Ag 3d ratio is observed in a Ag specimen. No focus∕alignment dependence is observed when using an angular imaging mode, necessitating the use of angular mode for quantitative chemical analysis.

X-ray fluorescence spectroscopy with monochromatic photon excitation is presented as a tool for studies of charge-transfer excitations in correlated systems, using CeO{sub 2} and UO{sub 3} as examples. Ce 4{ital f}{r_arrow}3{ital d} and U 5{ital f}{r_arrow}3{ital d} x-ray fluorescence, with excitation near the 3{ital d} thresholds, probes states as eigenvalues for the ground state Hamiltonian from the Anderson impurity model. Sweeping the excitation energy across 3{ital d} absorption edges enhances contributions of different electronic configurations to fluorescence so that observed resonances indicate the charge-transfer origin of the absorption satellites. {copyright} {ital 1996 The American Physical Society.}

X-ray standing-wave and extended x-ray-absorption fine-structure measurements have determined the strain and bond distortions in a buried InAs monolayer grown epitaxially on GaAs(001). The In atoms are found to reside 1.64{plus_minus}0.03 A above the last-As plane of the GaAs substrate with an In-As bond length of 2.57{plus_minus}0.02 A. Relative to bulk InAs, this corresponds to an 8% expansion in the In-As planar distance perpendicular to the interface and a 0.05-A compression in the In-As bond length. This experiment indicates that macroscopic-elastic theory describes the distortions in InAs/GaAs(001) films even in the monolayer limit.

The x-ray standing-wave technique has been used to determine the geometrical structure of Sb on GaAs(110). Using the back reflection diffraction geometry from (220) and (400) planes, we find the average vertical distance of Sb atoms from the GaAs surface to be 2.27±0.05 A in agreement with theoretical calculations for the epitaxial continued layer structure. Other models of buckled zigzag chains are found inconsistent with our results. Additionally, our data indicate less than 10% disorder in the Sb overlayer

The x-ray-standing-wave technique has been applied to the (1 monolayer Sb)/GaP (110) interface. Using the back-reflection geometry from (200) and (220) planes, the buckling of the Sb chains was determined to be 0.04\ifmmode\pm\else\textpm\fi{}0.10 \AA{}, and the perpendicular displacement of these chains from the extrapolated bulk (220) plane was determined to be 2.31\ifmmode\pm\else\textpm\fi{}0.10 \AA{}. These measurements favor a structure such that Sb adatoms occupy sites analogous to those of the next extrapolated substrate layer.

The x-ray standing-wave technique through detection of surface-sensitive Auger electrons has determined the surface reconstruction of clean InP(110). Using the back-reflection diffraction geometry from (220) planes, we find the perpendicular displacements of the surface P and In atoms to be +0.18\ifmmode\pm\else\textpm\fi{}0.1 and -0.48\ifmmode\pm\else\textpm\fi{}0.08 \AA{}, respectively, from their unrelaxed bulk positions. The use of low-energy x rays combined with a novel method of data analysis opens the general problem of surface reconstruction to study by this technique.

Based on the surface extended x-ray-absorption fine-structure (SEXAFS) absolute surface coordination numbers, Woicik et al. [Phys. Rev. B 43, 4331 (1991)] argued that Sb trimers adsorb on the Si(111) surface in a modified ``bridge'' configuration rotated by 60\ifmmode^\circ\else\textdegree\fi{} relative to the atop ``milk stool'' geometry proposed by Abukawa et al. [Surf. Sci. Lett. 201, 513 (1988)]. Using the accurate near-neighbor bond lengths as determined from SEXAFS, these models predict perpendicular distances of 1.76 and 2.60 \AA{} between the top Si atoms and the Sb layer, respectively. New back-reflection x-ray standing wave data find the Sb atoms to reside 2.53\ifmmode\pm\else\textpm\fi{}0.1 \AA{} above the Si surface, in clear support of the milk stool geometry. Polarization-dependent SEXAFS relative coordination numbers support the milk stool model as well.

We report on the use of synthetic multilayers as diffracting elements in the double-crystal monochromator of an X-ray beamline to obtain focused, monochromatized synchrotron radiation below 1000 eV. A resolving power > 100 ( E / ΔE ) was observed at the fluorine K edge (690 eV) and the copper L 2.3 edges (930–950 eV) as demonstrated by absorption spectra of thin films.

The soft x‐ray standing‐wave technique has been used to study ordered monolayers of Sb on GaAs(110) and InP(110). Using the back‐reflection diffraction geometry from (220) planes, we determine the perpendicular distances of Sb atoms to the substrate and compare these with theoretical calculations and elastic low‐energy electron diffraction determinations. The various models of Sb chemisorption are evaluated on the basis of our data.

The bifilar pendulum is often used for indirect measurements of mass moments of inertia of bodies that possess complex geometries. The exact equation of motion of the bifilar pendulum is highly nonlinear, and has not been solved in terms of elementary functions. Extensive use has been made, however, of the linearized approximation to the exact equation, and it has been assumed that the simple harmonic oscillator adequately describes the motion of the bifilar pendulum. It is shown here that such is generally not the case. Numerical solutions to the exact nonlinear differential equations of motion are obtained for a range of values of initial angular displacement, filament length, and radius of gyration. The filament length and the radius of gyration are normalized with respect to the half-spacing between the filaments. It is shown that the approximate solution gives good results only for small ranges of the system parameters.

An efficient, high‐resolution secondary x‐ray spectrometer is critical to studies of x‐ray‐excited fluorescent x‐ray emission spectra. Even with the highest available incident flux of x rays, the signal count rate can become unacceptably low when dispersed by an analyzing crystal. This problem is most serious in studies of gas targets, or at low energies where fluorescence yields are low. We have characterized the performance of a spectrometer based on the Rowland circle geometry with a variable‐radius curved (Johann) analyzing crystal. A position‐sensitive detector was used so that counts at a range of points on the Rowland circle corresponding to different wavelengths can be recorded in parallel. The efficiency of the spectrometer permits the observation of weak processes, such as subthreshold elastic and elastic x‐ray scattering from gases. Energy resolution at low energies is sufficient to allow observations of spectral peak widths which are narrower than lifetime broadening widths. Polarization dependence of the fluorescence can also be studied.

Beamline X‐24A at the National Synchrotron Light Source was designed for optimal performance in the x‐ray spectral region 500–5000 eV. This choice of energy range placed a number of constraints on the beamline design, requiring a crystal monochromator in a windowless UHV environment. Although this increased the complexity of the design, there were compelling scientific reasons for our desire to work in this range. In addition to tunability over the selected energy range, a primary goal was to obtain the highest possible energy resolution in the primary beam. We have achieved incident energy resolution significantly better than the typical core‐level lifetime broadening for this energy range. This has permitted studies of processes that are not broadened by lifetimes, such as resonant scattering and back‐reflection x‐ray standing‐wave effects. In addition to high resolution, it was designed to collect and focus as much flux as possible from the bending magnet source.

Multilayer-coated mirrors, rather than conventional total-reflection mirrors, have been proposed as a means to reduce power incident on the first optical element of high resolution monochromators. We have designed, fabricated, installed and characterized a multilayer pre-mirror specifically for the 800 to 4000 eV range for the X24-A bending magnet beamline at the National Synchrotron Light Source. Various aspects of this application are discussed, including power and thermal considerations, beamline layout considerations and constraints, choice of multilayer materials and substrate, and techniques to ensure lateral uniformity of the multilayer. Results of a preliminary characterization of a mirror coated with a SiC/V multilayer and installed in the beamline are also discussed. 9 refs., 5 figs.

1) The article introduced the concept of an optimum suction dredge operating condition defined as a condition ensuring, for a given heading, the maximum soil output at the minimum powerenergy expenditure per 1 m3 of soil. 2) The hydraulic mix specific gravity is the only defining programming parameter for the automated suction dredge. 3) The method offered here permits calculating the specific gravity values ensuring the optimum operating conditions for the given case. 4) The calculation (using the method offered here) made for an automated 300–40 suction dredge of the “Gidromekhanizatsiya” trust produced the results coinciding with the direct measurement data on the same suction dredge during its operation at the heading.

Polarization of Cl K\ensuremath{\beta} x-ray fluorescence following selective excitation of gaseous ${\mathrm{CH}}_{3}$CL with synchrotron radiation is reported. The degree of polarization of the fluorescence depends sensitively on the chosen incident excitation energy in the Cl K-edge region. Theoretical considerations indicate that the fluorescence-polarization measurements can provide directly absorption and emission anisotropies, molecular-orbital symmetries, and relative fluorescence transition strengths.

The coupling of high-energy-resolution x-ray absorption and emission spectroscopies with a high-intensity and high-resolution synchrotron-radiation beamline has opened up several new avenues of research in inner-shell molecular physics. Of particular importance is the ability to tune the incident photon energy throughout the near-threshold region for the atomic core levels of the molecules. Sub- and supra-threshold excitations have revealed a diversity of phenomena, including screening effects below threshold, satellite-free fluorescence above threshold, and fluorescence polarization in both energy regions. All of these phenomena can be used in a complementary way to uncover some of the underlying molecular dynamics which occur upon inner-shell excitation and ionization. Some of the latest results from the NBS X-Ray Spectroscopy Beamline at NSLS will be presented to highlight these effects.

Using a combination of x-ray standing waves and surface extended x-ray-absorption fine structure, we have determined the interlayer relaxation of a metal surface in the presence of an adsorbate. The Cu(001) surface was studied with Cl c(2\ifmmode\times\else\texttimes\fi{}2) and S p(2\ifmmode\times\else\texttimes\fi{}2) overlayers using a back-reflection diffraction geometry from (111) planes at 2.9 keV. For the Cl-covered surface we find a 0.07\ifmmode\pm\else\textpm\fi{}0.04 A\r{} outward relaxation while for the S-covered surface the relaxation is more difficult to determine because of possible substract reconstruction.

The Medical Illness Severity Grouping System--called MEDISGRPS--is an admission-oriented patient severity grouping system that uses objective, key clinical findings to place patients in one of five severity groupings. Data from five hospitals indicate that severity level at admission is an important predictor of resource use, is essential for analysis of patients who deteriorate and/or respond poorly to therapy, and is useful to further specify DRGs. Measures of effectiveness and efficiency using MEDISGRPS are suggested.

X-ray standing wave (XSW), x-ray diffraction, and photoluminescence (PL) measurements were used to assess the layer perfection and positions of 1 and 1/2 monolayer (ML) InAs quantum wells buried in GaAs(001). Local structure in the 1-ML films was evaluated using x-ray absorption fine structure (XAFS) measurements. Growth temperature effects were studied in a series of samples produced by metal organic vapor phase epitaxy (MOVPE) between 400 and $600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ The XSW coherent position of the In atoms decreases with increasing temperature in the 1-ML samples, and the optimal growth temperature is near $550\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ as evidenced by the coherent position of $1.15\ifmmode\pm\else\textpm\fi{}0.02$ and the relatively high coherent fraction of $0.72\ifmmode\pm\else\textpm\fi{}0.08.$ With decreasing growth temperature the XSW measurements may suggest segregation of In atoms, which results in an incorporation of the In into multiple layers. The segregation appears to be reduced at the higher temperatures due to the favorable kinetic conditions created in the MOVPE environment. Low-temperature PL measurements indicate that the sharpest and most intense In-excitonic emission is obtained from the 1-ML sample grown at $530\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ For the $\frac{1}{2}$-ML samples, growth temperatures of 400 and $600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ produce similar standing wave results, although the PL reveals the higher temperature sample to be of far superior quality, due to excessive carbon incorporation at $400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ In-As bond-length distortions found in the XAFS measurements agree with a macroscopic elastic description of the pseudomorphic epitaxy.