During 1998/99 three beamlines of the UE56-PGM type were built at BESSY-II to meet the increased need for synchrotron radiation of tuneable, i.e. linear to fully circular, polarization. We report on the results of characterization measurements, which were focused on spectral resolution, photon flux and polarization properties.The spectral resolution for all three beamlines. exceeds E/DeltaE =100 000 at 64.1 eV as obtained from absorption spectra of doubly excited He, The flux measurements resulted in values that closely follow the theoretical prediction of Up to 1 x 10(14) photons/(s 100 mA 0.1%) at 160 eV. The degree of polarization turned out to be close to 100% with circular and linear content, each of up to 100% depending on the undulator shift value, as predicted by theory. We also give first results on the double beam mode in which both undulator modules produce horizontally separated beams. This mode was used for fast switching of the polarization, utilizing a motorized chopper in the beamline. (C) 2001 Published by Elsevier Science B.V.

We present the first experimental data obtained from the new microfocus beamline U41-PGM recently put into operation. The optical layout of the beamline has been designed to conserve the high spectral brillance of the undulator source. The beamline consists of a collimated plane grating monochromator (PGM) and a subsequent refocusing stage. A toroidal mirror is used for refocusing onto the sample surface. With the 600 1/mm grating an absolute photon flux between 1012 and 1013 photons per second has been recorded over the whole energy range. Although the monochromator is not designed to deliver highest energy resolution, a resolving power of up to 6000 at the nitrogen K-edge has been obtained recently. The fixfocus constant cff is variable in a wide range. According to the experimental requirements it can be set to select either high photon flux or energy resolution.

This report describes a new method for monochromator energy calibration, which is based on the freedom to change the fix focus constant cff in the plane grating monochromator at BESSY. If the monochromator is not perfectly aligned, spectra taken at different cff-values show energy shifts (of 1–2%). These can be attributed to offsets in the grating angles. Measuring spectra at different cff-values, one can determine these offsets by employing a fitting procedure. If these offsets are inserted, cff variation measurements lead to relative energy scale changes of less than a few 10−4.

Spin resolved resonant photoemission from paramagnetic Gd(0001)/W(110) was performed in the 4d→4f excitation region using circularly polarized radiation from the new helical undulator UE 56/1 at BESSY II. The general spectral behaviour of the electron spin polarization, including changes of its sign, revealed in the Gd 7F photoemission peak coincides with the MCD (magnetic circular dichroism) asymmetry from ferromagnetic Gd. This similarity is substantially due to the spin–orbit interaction, which identically determines the (primary) excitation in both processes.

A beamline for circularly polarised radiation from a Sasaki type elliptical double undulator has been recently commissioned at BESSY-II. The PGM (Plane Grating Monochromator) based beamline works with collimated light and employs only one set of optical elements for steering and monochromatising the two (circularly) polarised, angularly separated, beams. The first commissioning results are presented here to illustrate the capabilities of this undulator-beamline concerning photon flux, photon energy resolution and polarisation characteristics.

We present a comparative study for a microfocus beamline at the U41 undulator, the most brilliant source at BESSY-II. High flux and an illuminated sample spot size between 1 and 10 μm are the primary design goals to bridge the spatial resolution gap attainable with imaging photoelectron emission microscopy and microspectroscopy applications. The basic layout of the beamline consists of a monochromatisation and a subsequent refocusing stage. For the former, a focused spherical grating monochromator (FSGM) design and a collimated plane grating monochromator (PGM) were optimised. Both monochromator make use of the same refocusing stage: For limited spatial resolution but large working distance between the last mirror chamber and focal spot, a horizontal deflecting toroid will give a spot of 10×20 μm2. The second system, a Kirkpatrick–Baez arrangement of bent plane-elliptical mirrors is expected to reach a 1×1 μm2 spot. The plane grating monochromator was finally chosen for its high flux and versatility. Ray trace calculations indicate a photon flux of ca. 3×1013 photons/s at a spectral resolution E/ΔE above 2300.

Beam line optics for synchrotron facilities are generally designed from geometric calculations wherein the bending magnet, undulator, or wiggler photon source is approximated by a high divergence point source. In this article we will, however, show that in the case of a synchrotron radiation source with very small divergence (such a source can be encountered at an undulator based third generation facility) the imaging properties resemble those of a laser source. This implies certain criteria for the focusing properties of the optical elements in the beam line. For instance, a beam waist is found close to the back focal plane of the optical element, with its position and size dependent on the divergence. If the divergence is determined predominantly by the diffraction part, this also means that the size and position of the beam waist is photon energy dependent. The study has been carried out by analytic treatment, numerical simulations as well as beam profile measurements carried out at beam line I511 at the MAX II synchrotron in Lund, Sweden.

A high-flux, scanning microspectroscope using the very intense, direct radiation from an undulator is introduced and its performance is discussed with various examples. This instrument utilizes the quasi-monochromatic beam from the first undulator at BESSY without monochromatization. Thus, X-ray induced Auger spectroscopy using all available photons above the respective threshold or photoemission using the 2-eV-wide first harmonic of this undulator can be performed. Spatial resolution is achieved in different ways, either simply by an adjustable aperture in front of the sample, yielding a diffraction-limited resolution of, for example, 3–4 μm for the C-Auger spectra, or by two mirrors, a toroidal condenser mirror and a demagnifying ellipsoidal mirror, which focus the aperture onto the sample yielding a resolution in the submicrometer range. Photoemission spectra from a selected spot can be obtained in seconds, or structures can easily be written using photon-induced reactions, because of the extremely high photon density of 1013 photons μm−2. Finally, line scans or 2D images can be recorded by scanning the sample across the focused beam.

For the high brilliance synchrotron radiation source BESSY II a couple of systems performing microscopy and spectromicroscopy at both high spatial and high energy resolution are planned or already under construction. The desired spatial resolution is in the nm-range, and thus monochromators are required which conserve the brilliance of the source throughout the beamline onto the analysed area. To serve the ultrahigh-resolution spectromicroscope SMART, a Petersen monochromator will be installed at an insertion device of 49 mm period length. When operated in highest spatial resolution mode, the field of view of the microscope is about 5 jtm in diameter. To optimize the photon flux density in the microscopes field of view, we studied the performance of the Petersen monochromator for various horizontal demagnification factors. The resulting optical layout produces a monochromatic image of the undulator source on the sample surface, which has a horizontal size of about 9 pm full width at half maimum. The vertical size of 5itm is determined by the exit slit width.

A new UHV spectromicroscope called SMART (spectromicroscope for all relevant techniques) is currently under construction for a soft X-ray undulator beamline at BESSY II. The instrument consists of a plane-grating monochromator with an aspherical focusing mirror and an ultrahigh-resolution, low-energy electron microscope containing an energy filter. It can be used as a photoemission microscope for a variety of electron spectroscopies (XAS, XPS, UPS, XAES) and has a calculated spatial resolution of better than 1 nm. A maximum energy resolution of about 0.1 eV will be provided by a corrected omega filter. The high lateral resolution of the electron microscope will be achieved through the correction of the chromatic and spherical aberrations of the objective lens by means of an electrostatic mirror in combination with a corrected magnetic beam separator. An additional electron source placed on the other side of the beam separator opposite the electrostatic mirror will also allow LEEM, MEM and small-spot LEED investigations to be carried out. The basic ideas, the various modes of operation and the electron optical design of the instrument are outlined.

The orientation of molecular ${\mathrm{H}}_{2}$O in a bilayer on defect-free NaCl(100) and of chemisorbed OH adsorbed at F centers on the (100) surface of NaCl was investigated using angle-resolved O K edge near-edge x-ray-absorption fine-structure spectroscopy (NEXAFS). High-quality single crystalline thin films of NaCl(100) of about four monolayers thickness were grown on Ge(100). These films were bombarded with 250 eV electrons so that F and F\ensuremath{'} centers of a maximum density of about 10% of a monolayer were created. Whereas on the defect-free surface only molecular water is adsorbed, dissociation occurs at the F centers and ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ is bound. NEXAFS of molecular water shows almost no angular dependence even in the monolayer. This finding is consistent with formation of a hydrogen bonded bilayer of water suggested earlier. The OH species formed at the F centers is found to be inclined with respect to the surface normal by 39\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}4\ifmmode^\circ\else\textdegree\fi{}. An adsorption model is proposed. \textcopyright{} 1996 The American Physical Society.

A coherent optical processor is constructed using two-wave mixing in a BaTiO3 crystal. This system is capable of performing autocorrelation of an input image in two successive steps. Theoretical considerations of the system are successfully compared with the experimental results. Influences of the mixing angle and intensity ratios on the result are discussed.

A method for increasing the visibility of Young fringes in Laser Speckle Velocimetry of up to 20 is proposed utilizing optical autocorrelation with photorefractive crystal BaTiO3. 1.