L. E. Berman

On the structure and evolution of the buried S/Au interface in self-assembled monolayers : x-ray standing wave results.

We describe a structural study of the S/Au interface for decanethiol monolayers (C10) on a Au(111) surface using the technique of X-ray standing waves ( XSWs). The XSW results for full-coverage monolayers are inconsistent with any model incorporating a single sulfur adsorption site, such as the widely assumed threefold hollow site on the Au(111) surface. Instead, the XSW results reveal two distinct sulfur head group sites, each with a distinct lateral and vertical location with respect to the underlying gold lattice. We discuss structural models that are consistent with these results. We have also studied the evolution of the structure versus coverage with XSW and X-ray photoelectron spectroscopy ( XPS) and have determined that the local S/Au interface structure of the ‘‘lying down’’ striped phase at low coverages (0.27 ML, 1 ML=4.62◊1014 molecules cm’2) is indistinguishable from that of the ‘‘standing up’’ c(4◊2) phase at saturation (1 ML). Some important implications concerning the structure and growth of these monolayers are discussed.

Liquid gallium cooling of silicon crystals in high intensity photon beams (invited)

The high‐brilliance, insertion‐device‐based photon beams of the next generation of synchrotron sources (Argonne’s APS and Grenoble’s ESRF) will deliver large thermal loads (1–10 kW) to the first optical elements. Considering the problems that present synchrotron users are experiencing with beams from recently installed insertion devices, new and improved methods of cooling these first optical elements, particularly when they are diffraction crystals, are clearly needed. A series of finite element calculations were performed to test the efficiency of new cooling geometries and various cooling fluids. The best results were obtained with liquid Ga metal flowing in channels just below the surface of the crystal. Ga was selected because of its good thermal conductivity and thermal capacity, low melting point, high boiling point, low kinetic viscosity, and very low vapor pressure. Its very low vapor pressure, even at elevated temperatures, makes it especially attractive in UHV conditions. A series of experiment...

Cryogenic cooling of monochromators

In this paper, we report on the design and x‐ray diffraction properties of cryocooled silicon single crystals exposed to the following wiggler beams: power density: 150 W/mm2, total power: 75 W and power density: 0.5 W/mm2, total power: 100 W. First, thermomechanical and engineering aspects of low‐temperature crystal cooling are discussed, leading to two basic cooling geometries: internal cooling and side cooling. Experimental tests of both these cooling schemes at NSLS on beam lines X25 and X17 are then described and discussed. Finally, engineering problems related to the integration of cryogenic cooling on ESRF beam lines are presented.

A High Resolution X-ray fluorescence spectrometer for near edge absorption studies

A high‐resolution fluorescence spectrometer using a Johann geometry in a backscattering arrangement was developed. The spectrometer, with a resolution of 0.3 eV at 6.5 keV, combined with an incident beam, with a resolution of 0.7 eV, form the basis of a high‐resolution instrument for measuring x‐ray absorption spectra. The advantages of the instrument are illustrated with the near‐edge absorption spectrum of dysprosium nitrate.

Diamond crystal X-ray optics for high-power-density synchrotron radiation beams

Man-made perfect single crystal isotopically-enriched diamond is demonstrated to be an excellent X-ray monochromator even when subjected to the highest incident power density expected at third-generation synchrotron source undulator beam lines. Double-crystal rocking curve tests of a diamond (400) wafer exposed to an X-ray power density of 207 W/mm2 (75 W total power) revealed just 1 arc sec of induced thermal distortion integrated across the beam footprint.

Test of annealed Czochralski grown silicon crystals as X-ray diffraction elements with 145 keV synchrotron radiation

Abstract The X-ray diffraction properties of annealed Czochralski grown silicon single crystals have been studied on a triple axis diffractometer using 145 keV synchrotron radiation from the 30 keV critical energy wiggler at CHESS. The silicon crystals, containing approximately 20 ppm oxygen atoms, were annealed at 1200°C for 8 h in order to create a homogeneous distribution of defects. A 10 mm thick crystal produced a Laue diffraction rocking curve with a 4 arcsec wide plateau at 50% reflectivity, and a full width at half maximum of approximately 8 arcsec. The shape of the rocking curve could be reproduced by calculations based on Darwins extinction theory, using a mosaic distribution determined from high resolution γ-ray rocking curves measured from the same crystal. The momentum transfer resolution function of the triple axis spectrometer using these deformed crystals as monochromator and analyzer was also measured. It was found to be enlarged by factors of 11 and 18 in the perpendicular and parallel directions, respectively, compared with the resolution using perfect crystals. This enlarged resolution element can be advantageous in diffuse scattering experiments.

Adaptive crystal optics for high power synchrotron sources

Abstract We report successful operation of the first X-ray monochromator with built-in correction for the thermal strain field created by a high power X-ray beam. The 27 pole wiggler magnet at the National Synchrotron Light Source provides a total power load of 330 W in a beam 60 mm wide. A (220) silicon monochromator system has been tested; the compensated deformation field contributes only 2 arc sec full width at half maximum intensity to the monochromator Darwin width of high order Bragg reflections.

Performance of water jet cooled silicon monochromators on a multipole wiggler beam line at NSLS

Abstract A 27 pole wiggler on the X25 beam line at the National Synchrotron Light Source (NSLS) provides a horizontal X-ray power density of 350 W/mrad with a critical energy of 4.6 keV. Jet cooled silicon crystal monochromators are shown to be capable of almost perfect performance at the experimental position where the available power density is about 45 W/cm of beam width and the total power is about 250 W. Both Si(111) and Si(220) monochromators have been tested and the Bragg angle variation across the beam thermal footprint is shown to be only 2.5 arc sec at the highest power levels.

Performance of a gallium-cooled 85° inclined silicon monochromator for a high power density X-ray beam

We have made double crstal rocking curve measurements on a gallium-cooled silicon monochromator in both the normal flat geometry and an 85° inclined geometry on the X-25 focused wiggler beamline at the National Synchrotron Light Source. At 192 mA ring current, the focused wiggler delivers about 37.7 W of power into a spot size of FWHM 0.4 × 0.8 mm2, resulting in an average power density of about 118 W/mm2. The inclined crystal geometry spreads the beam footprint on the surface of the crystal while maintaining a b = −1 symmetric Bragg reflection. At an 85° inclination angle, the beam footprint is 11.5 times larger than that for the flat geometry. In the case of the flat geometry at a ring current of 156 mA, we see, via an infrared camera, an increase in temperature of 56°C above the nominal silicon temperature. The rocking curve this case were significantly broadened (FWHM for 15 keV Si(333) = 35 arcsec) due to the thermally induced strain in the silicon. In the inclined crystal, the thermal peak on the crystal was only about 2.7°C above the nominal silicon temperature. In this case, the rocking curve width for the 15 keV Si(333) reflection was measured to be FWHM = 2.7 arc sec compared with the theoretical width of FWHM = 1.0 arcsec. The residual strain is totally due to the mounting of the crystals and not the heating from the X-ray beam.

Transmission-mode diamond white-beam position monitor at NSLS

Two transmission-mode diamond X-ray beam position monitors installed at National Synchrotron Light Source (NSLS) beamline X25 are described. Each diamond beam position monitor is constructed around two horizontally tiled electronic-grade (p.p.b. nitrogen impurity) single-crystal (001) CVD synthetic diamonds. The position, angle and flux of the white X-ray beam can be monitored in real time with a position resolution of 500 nm in the horizontal direction and 100 nm in the vertical direction for a 3 mm × 1 mm beam. The first diamond beam position monitor has been in operation in the white beam for more than one year without any observable degradation in performance. The installation of a second, more compact, diamond beam position monitor followed about six months later, adding the ability to measure the angular trajectory of the photon beam.