SSRL Beam Line Wunder will be the first soft X-ray energy range synchrotron radiation beam line specifically designed to exploit the unique aspects of periodic insertion devices in the wiggler-undulator (wunder) regime. Aspects of the development of this beam line are described in this paper. We emphasize the joint Optical, Thermal, and Mechanical optimization studies that were required.
The undulator is a machine that induces amplitude fluctuations in normally-rectilinear electron trajectories, causing coherently-reinforced radiation to appear along the direction of electron motion. Presently, such machines operate on electrons travelling in straight sections of synchrotron rings. The inherently large number of variables characterizing the undulator-electron interaction makes it difficult to identify the principal parameters for c-)t-Lmization when designing synchrotron rings dedicated to undulator operation. In this paper we present the results of a variational analysis of the equation describing undulator radiation. In particular, our results identify the optimum electron trajec-tories in undulators, in the sense of maximizing the intensity of the emitted radiation in the forward direction, and establish the optimum amplitudes of the electron motion. These important results are presented in terms of the total electron velocity, 13c, and the average on-axis velocity 3*c. The results, besides providing a valuable insight into undulator operation, rigorously establish the principal optimization parameters for associated syn-chrontron ring design.
We have measured the reflectivity of several multilayer dispersion elements between 80 and 500 eV. Two samples of ReW-C and one of Ni-C with 2d spacing of approximately 70 a and 150 A . were tested at angles of incidence between 10 to 80°. Measurements were made by fixing the incident and reflected angles (Bragg) and scanning the photon energy. Theoretical analyses of these multilayers have also been made and the results are compared with the experimental measurements.
The effect of uncorrelated roughness on the reflectivity of layered synthetic microstructures (LSM's) is studied. A computer code which includes uncorrelated roughness is developed allowing the subtraction of roughness from experimental measurements. This allows the use of LSM's to measure the optical constants of materials in the x-ray region of the spectrum. The method is applied to an V/C LSM to measure the resonance correction to the atomic factor, f', across the vanadium K-absorption edge.
Core electron excitations of rare earth metals have been studied by electron energy loss spectroscopy (EELS) in reflection geometry. Electron energy loss spectra of 3d and 4d excitations are investigated along the rare earth series, and are compared to X ray absorption and synchrotron partial yield spectra. In the study of inner-shell transitions the electron energy loss technique has an important advantage over photon absorption studies, in that electric dipole forbidden transitions may be induced. In 4d→4f excitations new transitions are clearly identified for the light rare earths, while the heavy rare earths show strong enhancement of already dipole allowed levels of high J as the primary energy is lowered. Even stronger non-dipole effects are observed in 3d→4f excitation spectra, although the range of multiplet energies is much less than for 4d→4f. Finally, it is shown that core level excitations in EELS can be successfully used in the surface chemistry of rare earths as a valence monitor during surface reactions.
We have studied the electronic properties of the amorphous, graphitic, and carbidic phases of carbon layers on niobium. While features of the valence hand photoemission and absorption cross sec-tion spectra have been discussed in separate publications, this presentation focusses on the observation of resonant photoemisston and the photon-induced Auger spectra.
Photoemission with characteristic x-ray and synchrotron radiation has shown that the first atomic layers of the noble metals have electronic properties which differ from those of the bulk. The d bands are narrowed at the surface, resulting in reduced s-d hybridiza-tion and a transfer of charge from s- into d-states. These changes result in a decrease in the core electron binding energies at the surface which have been determined for all three metals.
Photoemission studies with synchrotron radiation of surface core-level shifts in metals are reviewed. Models of surface shifts in d transition metals are discussed together with applications to surface crystallography : reconstruction, chemisorption and photoelectron diffraction experiments.
Angle resolved photoemission spectra have been measured for the polar (III) and (100) surfaces of GaAs. Strong differences exist in both cases between Ga-rich and As-rich surfaces in the energy region near the valence band maximum. Comparison of the spectra for the As-rich GaAs(111) (2x2) surface with a calculation of the location expected for bulk-derived features shows that the bulk bands appear to be folded back into the (2x2) surface Brillouin zone. No such effect is seen for the GaAs(100) c(4x4) surface.
Special problems affecting the design of spectrometers used to acquire synchrotron light excited soft x-ray emission spectra are discussed, and a high efficiency, high resolution spectrometer especially designed for this use is described. The special advantages of soft x-ray emission spectroscopy and of synchrotron light for exciting emission are outlined. Several types of experimental investigations that would benefit from the use of photon excitation and a high efficiency spectrometer are discussed.
The continuum of the conventional x-ray tube despite of its comparatively low intensity provides the possibility to record high resolution fluorescence and absorption spectra in the ultrasoft x-ray region. A system is described which uses a concave grating grazing incidence spectrometer with automatic photoelectric recording in conjunction with a high power rotating anode allowing high currents (up to 4 A) at relatively low voltages (4 to 10 kV). The device permits recording emission and absorption spectra of oxygen, nitrogen, carbon, and other elements at resolutions comparable with the inherent widths of the inner levels involved in the transitions.
The technique of photoelectron diffraction is reviewed with special emphasis on comparisons with LEED and SEXAFS in surface structural determinations. The review is intended to serve as a guide in choosing photoelectron diffraction over these other techniques. The application of the various data acquisition modes to well-defined systems as well as to more complex overlayer structures is demonstrated. In particular, the potential of photo-electron diffraction in structural determinations of molecular and multi-site overlayer systems is emphasized.
Core level angle-resolved photoemission intensity oscillates sinusoidally with increasing photoelectron momentum. Interference between direct and scattered photo-emission causes this angle-resolved photoemission extended fine structure (ARPEFS). We will discuss an analytic single-scattering theory which quantitatively describes the oscillations. The procedures for extracting surface geometry information from photo-emission measurements will be illustrated with S(1s) ARPEFS from S on Ni(100) and Cu(100) obtained with the soft X-ray double crystal monochromator at the Stanford Synchrotron Radiation Laboratory. Building on the surface sensitivity and chemical selectivity of photoemission, ARPEFS analysis provides direct geometrical information from the oscillation frequencies (derived with auto-regressive Fourier analysis), from intensity changes with polarization and analyzer position, and from analysis of scattering phase-shift zero-crossings.
The wide energy range and tunability of synchrotron radiation provide soft x-ray photoemission spectroscopy (SXPS) with several effective methods for characterizing metal-semiconductor interfaces on an atomic scale. These SXPS techniques reveal that metal-semiconductor interfaces are in general not abrupt and that the detailed atomic structure is a controlling factor in determining interface electronic structure.
We discuss the x-ray absorption near edge structure (XANES) above the K-edges of light elements in atomic and molecular adsorption on single-crystal surfaces. We concentrate on the systems 0 on Ni(100) and CO on Ni(100), and compare experimental data with the results of multiple-scattering calculations. The r6le of multiple-scattering and the use of lesser approximations is briefly discussed. The atomic adsorption of oxygen provides a reasonably sensitive test of adsorption site, but the near edge structure is influenced by as many as 30 neighbour atoms. By contrast, for the molecular adsorption of CO, the XANES is dominated by intra-molecular scattering with negligible contributions from the substrate atoms. Monitoring the XANES for molecular adsorbates is shown to provide an extremely accurate measure of changes in intra-molecular distances upon chemisorption: an accuracy of 0.01 A is readily achieved, far better than the results of any other surface structural method, and sufficient for the deduction of chemical trends in bonding.
The direct structure determination of the silicide formed from < 1 monolayer of Ni deposited on room temperature Si(111) leads to a model for silicide growth and interface formation. The model forms a basis for understanding many of the photoemission, ion scattering, and microscopy results in this coverage regime.
Three soft x-ray monochromators at the Berlin electron storage ring BESSY are described: a plane grating monochromator (SX-700) for the photon energy range 9 eV to about 2000 eV at high resolution, a high energy toroidal grating monochromator which delivers high photon flux up to - 1000 eV at medium resolution and a double crystal monochromator for the range 800 eV to about 4000 eV. The first experimental results obtained with these instruments are presented: resonant photoemission from europium intermetallic compounds, SEXAFS spectra of oxygen on copper and Cls photoelectron spectra from gaseous CO including satellite structure up to 20 eV excitation energy.
Within the last decade several spectroscopic techniques have been developed for probing the electronic structure of organic molecules, solids and surfaces over large binding energy ranges using synchrotron radiation. We discuss results from several of these techniques, including absorption spectroscopy, soft X-ray fluorescence and photoemission with tunable excitation. Among the problems presently studied are the determination of valence band structure of bulk and adsorbate systems, resonant intershell interaction, Carbon is core level spectroscopy and final state effects including X-ray absorption near edge structure (XANES).
Three major developments in the theory of electronic excitations in organic solids have occurred during the past decade. First, the role of electron correlation in localizing photoinduced molecular ion states within molecules has been analyzed in detail. Second, it has been recognized that under most circumstances structural disorder localizes photogener-ated charges as individual molecular ions in organic solids. Third, the collective behavior associated with quasi-one-dimensional organic solids (e.g., polyacetylene) has been studied intensively. This paper is a brief review of the central concepts underlying these three developments with emphasis on their implications for soft-X-ray photoemission.
Soft x-ray excitation involving C ls electrons in CO, as well as aliphatic, and aromatic hydrocarbons is found to result in ionic fragmentation of the original molecule. The frag-mentation pattern changes whether the C ls electron gets ionized or excited into a Rydberg-like orbital, or into an unfilled molecular orbital. In this study we correlate the near edge C ls absorption spectra with the observation of specific ionic fragments of the origi-nal molecules.
A summary is presented of typical gas-phase photoemission studies based on synchrotron radiation in the 50-5000 eV range, using beam lines at the Stanford Synchrotron Radiation Laboratory. Three topics are addressed: atomic inner-shell photoelectron cross sections and asymmetries, correlation peaks in rare gases, and core-level shape resonances in molecules. Photoelectron cross-section a(nZ) and asymmetry-parameter a(n0 studies in mercury vapor at photon energies up to 270 eV (up to 600 eV for a4f) extend coverage of these parameters to n<5 and 5<3. Comparison with Dirac-Slater and relativistic random-phase approximation calculations reveals systematic discrepancies. For example, distinct Cooper minima in a(n iZ,) are observed but not predicted, while predicted a(n9) values are typically too high. Correlation satellites have been studied for the K shells of helium (hv = 68-90 eV), neon (hv = 870-960 eV) and argon (hv = 3200-3320 eV). In helium the n=2 satellite peak was shown to have mainly 2p character at threshold, and its asymmetry was measured through the autoionizing resonance region. Tentative evidence was obtained that the neon satellites are less intense near threshold than in the high-energy limit, and that their intensities stay constant or decrease with increasing energy near threshold. A new satellite was observed in argon at 24.6 eV which appears to increase in intensity with energy. Molecular core-level shape resonances were observed for the first time by photoemission, yielding a(hv) and a(hv) for core levels from 180 eV binding energy (S 2p in SF6 and OCS) through C is in CO, CO2 and CF4, N ls in N2 and NO, and 0 is in CO and CO2 to 2490 eV (S ls in SF6). Several conclusions can be drawn about the photoelectron and Auger cross sections and asymmetry parameters.
We have built a scanning transmission soft X-ray microscope located at the National Synchrotron Light Source (NSLS). Images of biological specimens have been formed with submicron resolution. A Fresnel zoneplate serves as the focusing element.
Two imaging techniques, soft x-ray contact microscopy using a stationary source and transmission electronmicroscopy (TEM) were used in parallel for the study of human blood platelets. Thin sections of fixed cell preparations on EM grids were examined by the two methods and images of the same individual cells could be compared. Not all electrondense structures could be seen in contact x-ray images. However, other structures in the same cells, not identified by TEM, could be seen in soft x-ray contact pictures. In addition, whole cell mounts of intact platelet microtubules were examined by the two techniques. These electrondense structures were not photondense and disappeared in the x-ray pictures. Whole cell mounts of partially activated human platelets were also examined in parallel, by the use of the scanning electron microscope (SEM). In the x-ray images, a new platelet cytoskeleton could be seen in the partially activated cells. This was made of radial, filamentous structures departing from a centralized and retracted bunch of platelet organelles and, at times, extending into pseudopods outside the cell perimeter. The gas puff Z pinch method was also used in the study of live and partially activated human platelets. An image of multiple craters and undulating cell cords, in addition to a polarized and contracted bunch of platelet organelles, could be seen, while few pseudopods were seen extending beyond the cell perimeter. Some individual platelet granules could be identified in the cell craters, others were seen in the medium, outside the cell. A tear in the cell surface structure could also be identified. The latter was interpreted as a possible portal for the partial extrusion of platelet granules during platelet activation.
It is now clear that the initial geometrical distribution of primary radiation products in irradiated biological matter is fundamental to the observed end-point (cell killing, mutation induction, chromosome aberrations, etc.) In recent years much evidence has accumulated, indicating that, for all radiations, physical quantities averaged over cellular dimensions (micrometers) are not good predictors of biological effect, and that energy deposition processes at the nanometer level are critical. Thus irradiation of cells with soft x rays whose secondary electrons have ranges of the order of nanometers is a unique tool for investigating different models for predicting the biological effects of radiation. We demonstrate techniques whereby the biological response of the cell, and the physical details of the energy deposition processes may be separated or factorized, so that given the response of a cellular system to, say soft x rays, the response of the cell to any other radiation may be predicted. The special advantages of soft x rays for eliciting this information and also information concerning the geometry of the radiation sensitive structures within the cell are discussed.
The theoretical aspects of using line radiation sources for soft x-ray microscopy are discussed. High resolution, absolutely calibrated spectra of a gas-puff z-pinch source are presented. A qualitative discussion of radiation damage mechanisms and the time scales involved indicate that any picture taken in less than a millisecond may be less damaged by the radiation. Results of a first attempt to use a 100J, nanosecond laser to produce a carbon plasma as a source for very short time-scale microscopy are presented.
Soft x-ray microscopy was proposed as an exciting new technique almost two decades ago. However, it is only recently with the advent of high resolution recording substrates such as polymethyl methacrylate and the development of synchrotron and pulsed x-ray sources (that can deliver a tuneable x-ray dose in nanoseconds or minutes) that soft x-ray contact microscopy has developed into a significant biological research tool. X-ray photos between the carbon and nitrogen absorption edges (3.1 - 4.4nm) penetrate short airpaths, water and thick specimens without significant diffraction or scattering effects and at dosages and exposure times that produce significantly less damage than charged particle imaging methods. Because the x-ray contact replica is formed by the differential absorption of soft x-rays by the biological specimen itself, small intracellular structures (such as membranes and intracellular organelles) can be clearly imaged at spatial resolutions approaching those of conventional scanning and transmission electron microscopy. This paper presents results obtained from the imaging of hydrated biological subcellular structures (isolated macromolecules found in the ground substance of cartilage and isolated muscle filaments) using soft x-rays from a stationary target and pulsed plasma x-ray source.
As two-dimensional x-ray microscopy develops, it is of interest to consider also possible three-dimensional imaging methods, in view of the thick-specimen capability of soft x-rays. The highest-quality 3-dimensional images of atomic assemblies today are produced by Fourier inversion of lA x-ray or neutron diffraction patterns of crystalline specimens. We consider whether a similar form of imaging may be obtained with very small non-crystalline specimens using soft x-rays. Success would allow microscopic objects, such as biological cells, to be imaged in three dimensions and at resolutions of the order of 15A. As x-ray wavelengths increase from lA, elastic scattering persists, Compton scattering decreases, and photoelectric absorption increases. The first and third of these give rise to coherent scattering and are effective in diffraction, while the second contributes an incoherent background and does not contribute to diffraction. Thus we should expect that diffraction will exist in the soft x-ray region and with lower intrinsic background than in the shorter wavelength region. Small-angle diffraction patterns from multiple small objects (latex spheres) have indeed been observed in the soft x-ray region1 . Here, however, we are concerned with the possibility of measuring large-angle patterns, and from single small objects.
Soft x-ray holographic microscopy is discussed from an experimental point of view. Three series of measurements have been carried out using the Brookhven 750 MeV storage ring as an x-ray source. Young slits fringes, Gabor (in line) holograms and various data pertaining to the soft x-ray performance of photographic plates are reported. The measurements are discussed in terms of the technique for recording them and the experimental limitations in effect. Some discussion is also given of the issues involved in reconstruction using visible light.
A high resolution Vector Scan electron beam lithography system for fabrication of structures with minimum dimensions below 100 nm is described. A selection was made from a variety of processes suitable for high resolution fabrication, in order to provide the desired properties of apodized Fresnel zone plates used in the Stony Brook X-ray scanning microscope. Experimental characterization of the zone plates with regard to resolution and efficiency in the microscope is described.