Wayne R. McKinney

High-power terahertz radiation from relativistic electrons

Terahertz (THz) radiation, which lies in the far-infrared region, is at the interface of electronics and photonics. Narrow-band THz radiation can be produced by free-electron lasers and fast diodes. Broadband THz radiation can be produced by thermal sources and, more recently, by table-top laser-driven sources and by short electron bunches in accelerators, but so far only with low power. Here we report calculations and measurements that confirm the production of high-power broadband THz radiation from subpicosecond electron bunches in an accelerator. The average power is nearly 20 watts, several orders of magnitude higher than any existing source, which could enable various new applications. In particular, many materials have distinct absorptive and dispersive properties in this spectral range, so that THz imaging could reveal interesting features. For example, it would be possible to image the distribution of specific proteins or water in tissue, or buried metal layers in semiconductors; the present source would allow full-field, real-time capture of such images. High peak and average power THz sources are also critical in driving new nonlinear phenomena and for pump–probe studies of dynamical properties of materials.

IR spectroscopic characteristics of cell cycle and cell death probed by synchrotron radiation based Fourier transform IR spectromicroscopy

Synchrotron radiation based Fourier transform IR (SR-FTIR) spectromicroscopy allows the study of individual living cells with a high signal to noise ratio. Here we report the use of the SR-FTIR technique to investigate changes in IR spectral features from individual human lung fibroblast (IMR-90) cells in vitro at different points in their cell cycle. Clear changes are observed in the spectral regions corresponding to proteins, DNA, and RNA as a cell changes from the G(1)-phase to the S-phase and finally into mitosis. These spectral changes include markers for the changing secondary structure of proteins in the cell, as well as variations in DNA/RNA content and packing as the cell cycle progresses. We also observe spectral features that indicate that occasional cells are undergoing various steps in the process of cell death. The dying or dead cell has a shift in the protein amide I and II bands corresponding to changing protein morphologies, and a significant increase in the intensity of an ester carbonyl C===O peak at 1743 cm(-1) is observed. Biopolymers (Biospectroscopy) 57: 329-335, 2000

Performance of a high resolution, high flux density SGM undulator beamline at the ALS (invited)

The performance of ALS beamline 7.0 is described. This is an integrated system for delivering radiation from a 5 cm period undulator to spectroscopy and microscopy experiments across the range of photon energies from 60 to 1200 eV. The beamline is engineered to deliver the highest possible flux, with negligible deformation of the optic surfaces due to heating. Two experiment stations are served with rapid interchangeability. The measured operational parameters, the resolution and flux delivered, and the refocus of the light into a small spot at the experiment are all discussed.

Dependence of the fundamental band gap of AlxGa1−xN on alloy composition and pressure

Optical absorption studies were performed to investigate the dependence of the fundamental band gap of AlxGa1−xN epitaxial films on Al content and applied hydrostatic pressure. The results of absorption measurements performed at atmospheric pressure yielded the variation of the band-gap energy E(x)=3.43+1.44x+1.33x2 eV for the AlxGa1−xN system. Optical absorption edge associated with the direct Γ band gap shifts linearly towards higher energy under applied pressure. By examining the pressure dependence of the absorption edge in samples with different AlN mole fractions and taking into account the difference of compressibility between the epitaxial films and sapphire substrate, the pressure coefficients for the direct Γ band gaps of AlxGa1−xN were determined.

Suite of three protein crystallography beamlines with single superconducting bend magnet as the source.

At the Advanced Light Source, three protein crystallography beamlines have been built that use as a source one of the three 6 T single-pole superconducting bending magnets (superbends) that were recently installed in the ring. The use of such single-pole superconducting bend magnets enables the development of a hard X-ray program on a relatively low-energy 1.9 GeV ring without taking up insertion-device straight sections. The source is of relatively low power but, owing to the small electron beam emittance, it has high brightness. X-ray optics are required to preserve the brightness and to match the illumination requirements for protein crystallography. This was achieved by means of a collimating premirror bent to a plane parabola, a double-crystal monochromator followed by a toroidal mirror that focuses in the horizontal direction with a 2:1 demagnification. This optical arrangement partially balances aberrations from the collimating and toroidal mirrors such that a tight focused spot size is achieved. The optical properties of the beamline are an excellent match to those required by the small protein crystals that are typically measured. The design and performance of these new beamlines are described.

Real-Time Characterization of Biogeochemical Reduction of Cr(VI) on Basalt Surfaces by SR-FTIR Imaging

Synchrotron radiation-based (SR) Fourier-transform infrared (FTIR) spectromicroscopy in the mid-infrared region is a surface analytical technique that can provide direct insights into the localization and real-time mechanisms for the reduction of the (CrO4)2- chromate [Cr(VI)] species on surfaces of geologic materials. Time-resolved SR-FTIR spectra indicate that, in the presence of endoliths (mineral-inhabiting microorganisms), microbial reduction of Cr(VI) to Cr(III) compounds on basaltic mineral surfaces is the key mechanism of Cr(VI) transformation. It proceeds in at least a two-step reaction with Cr(V) compounds as possible intermediate products, with the reduction of Cr(VI) increasing during the concomitant biodegradation of a dilute organic vapor (toluene). Analyses of spatially resolved SR-FTIR spectra show that the maximum reduction of Cr(VI) to Cr(III) compounds occurs on surfaces densely populated by microorganisms. The oxidation state of Cr(III) compounds was confirmed by micro-x-ray absorption...

High-resolution, high-transmission soft x-ray spectrometer for the study of biological samples

We present a variable line-space grating spectrometer for soft x-rays that covers the photon energy range between 130 and 650 eV. The optical design is based on the Hettrick-Underwood principle and tailored to synchrotron-based studies of radiation-sensitive biological samples. The spectrometer is able to record the entire spectral range in one shot, i.e., without any mechanical motion, at a resolving power of 1200 or better. Despite its slitless design, such a resolving power can be achieved for a source spot as large as (30 x 3000) microm2, which is important for keeping beam damage effects in radiation-sensitive samples low. The high spectrometer efficiency allows recording of comprehensive two-dimensional resonant inelastic soft x-ray scattering (RIXS) maps with good statistics within several minutes. This is exemplarily demonstrated for a RIXS map of highly oriented pyrolytic graphite, which was taken within 10 min.

Synchrotron-Based FTIR Spectromicroscopy: Cytotoxicity and Heating Considerations.

Synchrotron radiation-based Fouriertransform infrared (SR-FTIR)spectromicroscopy is a newly emergingbioanalytical and imaging tool. This uniquetechnique provides mid-infrared (IR)spectra, hence chemical information, withhigh signal-to-noise at spatial resolutionsas fine as 3 to 10 microns. Thus it enablesresearchers to locate, identify, and trackspecific chemical events within anindividual living mammalian cell. Mid-IRphotons are too low in energy (0.05–0.5eV) to either break bonds or to causeionization. In this review, we show thatthe synchrotron IR beam has no detectableeffects on the short- and long-termviability, reproductive integrity,cell-cycle progression, and mitochondrialmetabolism in living human cells, andproduces only minimal sample heating (<0.5°C). These studies haveestablished an important foundation forSR-FTIR spectromicroscopy in biological andbiomedical research.

High resolution soft X-ray bending magnet beamline 9.3.2 with circularly polarized radiation capability at the Advanced Light Source

Abstract Bending magnet beamline 9.3.2 at the Advanced Light Source (ALS) was designed for high resolution spectroscopy in the soft x-ray energy region, covering a range from 30 eV to 1500 eV with three gratings. The monochromator itself is a standard fixed included angle 55 m spherical grating monochromator and was originally used at the Stanford Synchrotron Radiation Laboratory (SSRL) as a prototype for later insertion device based monochromators for the ALS. For operations at the ALS, the toroidal pre-mirror used at SSRL to vertically focus onto the entrance slit and horizontally focus onto the exit slit was replaced by two separate crossed mirrors (Kirkpatrick-Baez configuration). Circularly polarized radiation is obtained by inserting a water-cooled movable aperture in front of the vertically focusing mirror to allow selecting the beam either above or below the horizontal plane. To maintain a stable beam intensity through the entrance slit, the photocurrent signals from the upper and lower jaws of the entrance slit are utilized to set a feedback loop with the vertically deflecting mirror Piezoelectric drive. The beamline end station has a rotatable platform (through 60°) that accommodates two experimental chambers, enabling the synchrotron radiation to be directed to either one without breaking vacuum.

Tracking chemical changes in a live cell: Biomedical applications of SR‐FTIR spectromicroscopy

Synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectromicroscopy is a newly emerging bioanalytical and imaging tool. This unique technique provides mid-infrared (IR) spectra, hence chemical information, with high signal-to-noise at spatial resolutions as fine as 3 to 10 microns. Thus it enables researchers to locate, identify, and track specific chemical events within an individual living mammalian cell. Mid-IR photons are too low in energy (0.05 - 0.5 eV) to either break bonds or to cause ionization. In this review, we show that the synchrotron IR beam has no detectable effects on the short- and long-term viability, reproductive integrity, cell-cycle progression, and mitochondrial metabolism in living human cells, and produces only minimal sample heating (< 0.5 degrees C). We will then present several examples demonstrating the application potentials of SR-FTIR spectromicroscopy in biomedical research. These will include monitoring living cells progressing through the cell cycle, including death, and cells reacting to dilute concentrations of toxins.