Hiromi Ikeura-Sekiguchi

In-line phase-contrast imaging of a biological specimen using a compact laser-Compton scattering-based x-ray source

Laser-Compton scattering (LCS) x-ray sources have recently attracted much attention for their potential use at local medical facilities because they can produce ultrashort pulsed, high-brilliance, and quasimonochromatic hard x rays with a small source size. The feasibility of in-line phase-contrast imaging for a “thick” biological specimens of rat lumbar vertebrae using the developed compact LCS-X in AIST was investigated for the promotion of clinical imaging. In the higher-quality images, anatomical details of the spinous processes of the vertebrae are more clearly observable than with conventional absorption radiography. The results demonstrate that phase-contrast radiography can be performed using LCS-X.

SOFT X-RAY MICROSCOPY OF SOFT MATTER HARD INFORMATION FROM TWO SOFTS

Scanning transmission X-ray microscopy (STXM) and X-ray photoelectron emission microscopy (X-PEEM) provide quantitative chemical analysis at a spatial resolution well below 100 nm. Soft X-ray absorption or near edge X-ray absorption (NEXAFS) contrast provides sensitive differentiation of species which have similar elemental composition but are chemically distinct. Due to the ability of soft X-rays at wavelengths below the O K-edge to penetrate water, and on account of lower radiation damage, soft X-ray microscopy is an ideal tool for providing quantitative information about soft matter in the context of biological, polymer and environmental studies. Examples are given from recent studies of: phase segregation in polyurethanes and polymer blends, protein adsorption on polymers relating to biomaterial optimization, and metal mapping in biofilms. These examples show that it is indeed possible to get quantitative (hard) information by combining soft X-rays and soft materials.

Fragmentation and charge-neutralization pathways depending on molecular orientation at surfaces

In order to elucidate how the fragmentation dynamics differs depending on chemical-bond orientation on the surface, we have measured the polarization-angle dependence of the photon-stimulated ion-desorption (PSID) yield spectra from condensed formic acid around the C Is region. The measurements were done using linearly polarized synchrotron radiation combined with a newly constructed time-of-flight mass spectrometer. Photoabsorption (or X-ray absorption near edge structures) and PSID yield spectra showed considerably different polarization dependencies. Fragment-ion yields for C 1s → σ + CH resonance were greatly increased when the direction of the electric field vector (E) of the light became close to perpendicular to the surface. The results imply that the fragments formed from the perpendicularly protruding molecular plane or bond axis preferentially survive the charge-transfer de-excitation by surrounding molecules or underlayers and lead to desorption.

Molecular ordering effect of regioregular poly(3-hexylthiophene) using sulfur K-edge X-ray absorption spectroscopy

The electronic structure of the unoccupied conduction band of regioregular poly(3-hexylthiophene) (RR-P3HT) was investigated by X-ray absorption spectroscopy (XAS) near the sulfur K-edge. Angle-dependent XAS studies revealed that polymer chains in films are well aligned and oriented edge-on with respect to the Si substrate. It was clearly observed that the bottom of the conduction band near the Fermi level in a π–π stacking film is lower (0.3 eV) than that in a powder. It was demonstrated that π–π stacking interactions improve the power conversion efficiency through the reduction of the lowest unoccupied molecular orbital (LUMO) energy level.

Orientation effect on photo-fragmentation and ion-desorption from the top most layers of molecular solids

Using a newly developed rotatable-time-of-flight mass spectrometer (TOF-MS) and polarized synchrotron radiation, we have investigated orientation effect on fragmentation and desorption pathways occurring at the top-most layers of molecular solids. Polarization-angle dependencies electron- and ion-NEXAFS of a condensed layer of partially deuterated formamide (HCOND 2 ) are reported. A marked orientation effect was observed for enhanced H + -yields following C1s → σ * CH core-resonance. This effect was attributed to the existence of direct photodissociation as well as to the change of charge-neutralization probability. The product yields of some fragments, however, showed no polarization dependence especially when dissociation sites were far from core-excited atoms. This is the case for N-D breaking and D + -desorption following C1s → σ * ND resonance. It suggests that higher Rydberg states with plural symmetries are mixed.

Verification of NEXAFS assignments by a comparison of fragmentation between gaseous and condensed methyl formate.

According to the ion desorption from condensed methyl formate (DCOOCH3) by C 1s electron excitation, C+ yield was strongly enhanced at 289.1 eV, a higher energy side of C1s(DC) --> pi*(C=O) peak. This feature could not be clearly assigned. In this work, to clarify the feature, fragment-ion yield curves of gas-phase methyl formate were measured and compared with those of condensed one, since gas-phase and condensed systems are presumed to show more or less similar fragment-ion yield curves under similar excitations. The results demonstrate that fragment-ion yield curves are useful for the assignments of NEXAFS features in both gaseous and condensed polyatomic molecules.

Characterization of x-ray photocathode in transmission mode for imaging applicationa)

The performance of an Al x-ray transmission photocathode was experimentally characterized by measuring the total electron yield from backsurface photocathodes of varying thickness. It was demonstrated that the backsurface electron yield is proportional to the x-ray photocurrent transmitted through the photocathode for thicknesses thicker than the optimum thickness. The optimum photocathode thickness with the highest conversion efficiency was found to be approximately 70nm at 2.963keV. An escape depth of the secondary electrons was determined to be approximately 13nm from the yield-versus-thickness data fitted with a semiempirical equation. Using this parameter, the dependence of the optimum photocathode thickness on the x-ray energy was calculated for the 30–10000eV range. As one example of imaging applications, different photoemission images in the transmission and reflection modes are also presented.

A short-pulse hard x-ray source with compact electron linac via laser-Compton scattering for medical and industrial radiography

An intense, quasi-monochromatic hard X-ray beam has been generated via the laser-Compton scattering of an electron bunch with a laser pulse. An s-band linear accelerator of 40 MeV and Ti:sapphire femtosecond terawatt laser were used to generate X-rays. We plan to increase the X-ray intensity up to two-orders than the current one until FY2008. Specifications of the electron accelerator and the laser systems are presented, together with the developments and modifications being undergone.

Photo-oxidative doping in π-conjugated zig-zag chain of carbon atoms with sulfur-functional group

Photo-oxidative doping processes were studied for the trans-polyacetylene backbone with the –SCH3 side group as a chemically representative of the precisely controlled S-functionalized zig-zag graphene nanoribbon edge. Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy indicates that photochemical reaction of S–CH3 with atmospheric O2 forms selectively oxidized products such as –S(O)CH3 and –SO3– bound to the polyacetylene (PA) backbone. Using the correlation between the oxidation states of sulfur and the XANES peak positions, the partial charge distribution of CH3Sδ+–PAδ– has been estimated. Such positively charged sulfur atoms can attract higher electronegative oxygen atoms and expect to enhance the photooxidization capabilities. The formation of the –SO3– side group is evidently responsible for hole doping into the PA backbone. The results can provide some strategy for area-selective and controllable doping processes of atomic-scale molecular systems with the assistance of UV light.

Observation of Oriented Organic Semiconductor using Photo-Electron Emission Microscope (PEEM) with Polarized Synchrotron

We have developed photoelectron emission microscope (PEEM) system with excitation of linearly polarized synchrotron X-rays at 30° incidence angle arrangement. The morphology, electronic structure, orientation for P3HT:PCBM hybrid films have been investigated using the PEEM instrument. The sulphur 1s near-edge X-ray-absorption fine structure (NEXAFS) spectra at micro-domains were obtained, where sulphur atoms of thiophene ring were detected. The nature and symmetry in the S 1s excitations of P3HT model cluster were elucidated by DFT calculations that we performed to assign resonant peaks in absorption spectra. We demonstrate that the orientation at micro-domains can be deduced from photon-energy dependencies of PEEM images.