14)X-ray Spectrometry– HVC Capacitor, HV Ceramic Capacitor to build All kinds of X-ray machine.
X-RAY ABSORPTION SPECTROMETRY
XAS is a widely used tool to provide local structural informa-
tion, including chemical bonding, charge distribution, and oxida-
tion -reduction of atoms and molecules, because the information
obtained by X-ray absorption is limited to the local environment
of a particular atom. Based on the development of laser-driven
X-ray sources, time-resolved XAS probing the transient chemical
and structure changes in solids and liquids can be carried out
using tabletop X-ray spectrometers. One possibility to observe
photochemical transients is an ultrafast optical pumpsX-ray probe
experimental setup. Tomov and Rentzepis ( I1 ) developed and
tested a femtosecond laser-plasma X-ray source employing poly-
capillary optics between the target and the sample. An amplified
laser pulse with duration of 130 fs at 10-Hz repetition rate was
focused onto a Cu wire target by an off-axis parabolic mirror,
resulting in an X-ray source of 50ím. The 5 ° divergence of the
beam after the focal point of the polycapillary was well suited to
the energy-dispersive detection system consisting of a Si dispers-
ing crystal and a CCD detector. Although the recording an EXAFS
spectrum requires a few hours exposure at 10-Hz repetition time,
acquiring signals from several thousands of pump -probe shots
using the same time delay after the laser pulse, irreversible
transient can also be studied. The authors claimed that time-
resolved EXAFS studies on photodissociation of CBr
4
are possible
at a 10-Hz repetition rate by flowing the sample solution allowing
the sample replacement after each shot. The characteristics of
broadband thermal X-ray spectra produced by femtosecond laser-
produced plasmas were thoroughly discussed (I2 ). In addition to
the target material, the production of hot electrons during the
interaction between the laser beam and a solid target has to be
taken into account for achieving a spectral range suitable for a
particular measurement. Hot electrons produce characteristic and
Bremsstrahlung X-rays and determine the characteristics of the
plasma. The spectral characteristics and the photon yield of laser-
plasma sources were found to be suitable for subpicosecond
EXAFS experiments. Time-resolved XAS is also a promising tool
in structural biology, since the oxidation-state and structure of
reaction intermediates are of high importance in understanding
enzymatic processes. Haumann et al. ( I3 ) presented a novel
approach termed “sampling-XAS”. The method utilizes time scans
at each energy point, using laser flashes for a stepwise advance
of the catalytic reaction, while the characteristic fluorescence
intensity was detected. The time resolution was limited by the
fluorescence detection system; a value of 200 ís was reached for
oxidation-state monitoring of the manganese complex of oxygenic
photosynthesis. However, the necessity of a new position of the
sample for each energy point may be a limitation of the technique.
Improvements of XAS measurement techniques for heteroge-
neous systems were reported in the review period. Ishii and
Uchihashi (I4 ) presented scanning capacitance microscopy as a
surface-sensitive technique for measuring XAS using a tunable
monochromatic X-ray beam at a synchrotron radiation source.
Since X-ray absorption is evaluated from the change in capacitance
caused by X-ray-induced photoemission of a localized electron, a
site-selective XAS analysis of trapping-center atoms with localized
electrons could be realized. Measurements of a GaAs surface
showed that resonant intratransition and resonant scattering of
emitted photoelectrons enhance the signal at particular X-ray
photon energy and reveal the local density of states of the gallium
oxide and the complex structure of the trapping centers. A depth
sensitivity of 1 nm is characteristic to the technique; the authors
expect a lateral resolution of the same order of magnitude. Another
possibility for surface-sensitive structural analysis is grazing
incidence XAS (GIXAS). Keil et al. ( I5 ) presented a new method
of GIXAS using an off-specular reflection geometry. In addition
to the specular reflected beam, the angular distribution of the
intensity reflected from a rough surface or interface shows a
second well-defined maximum called a Yoneda peak. Using such
an asymmetric setup, with the detector centered at the Yoneda
peak, this technique was found to be sensitive to lateral hetero-
geneities, which typically occur at surfaces or interfaces. Results
obtained for a Cu/CuO system indicated that the proposed
technique was capable of distinguishing between the contributions
of the upper copper oxide and the underlying metallic copper layer
that was not possible by using conventional GIXAS. The recent
rapid progress in materials research demands more information
on the atomic structure in inhomogeneous systems. Micro-XRF
combined with micro-XAS are useful techniques, when points of
interest for micro-XAS are selected from an image collected by
means of scanning micro-XRF at one or two energies. To decrease
the measurement time needed for XAS imaging, Sakurai and
Mizusawa (I6 ) presented a technique based on XRF projection-
type microscopy, which is a powerful new tool recently developed
for rapid imaging. A monochromatic X-ray beam irradiates a 8
8mm
2
area of the sample at a shallow angle of 2 °; a lateral
resolution of about 15-18 ím is determined by a collimator placed
just in front of the CCD detector. Chemical-state imaging for
copper corrosion using energy scan in the XANES region was
possible within a few minutes. Because of the limited lateral
resolution, combination of scanning-type and projection-type
imaging methods would be promising for studying realistic
inhomogeneous system in many sciences.