X-ray tomographic imaging of crystal structure at the atomic level.

A direct nondiffractive tomographic algorithm is proposed for the determination of the crystal structure from real-space projections obtained by illuminating the sample with white x rays. This approach was applied to the pattern of the directional fine structure in absorption of white x rays recorded for a GaP crystal and allowed for a determination of the electron density distribution within the unit cell.

Measurements of photon interference X-ray absorption fine structure (piXAFS).

Experimental data are presented which demonstrate the existence of a fine structure in extended X-ray absorption spectra due to interference effects in the initial photon state (piXAFS). Interference occurs between the incident electromagnetic wave and its coherently scattered waves from neighboring atoms. Using fine platinum and tungsten powders as well as polycrystalline platinum foil, piXAFS was measured in high-precision absorption experiments at beamline X1 at HASYLAB/DESY over a wide energy range. piXAFS is observed below and above absorption-edge positions in both transmission and total-electron-yield detection. Based on experimental data it is shown that piXAFS is sensitive to geometric atomic structure. Fourier-transformed piXAFS data carry information, comparable with that of EXAFS, about the short-range-order structure of the sample. Sharp structures occur in piXAFS when a Bragg backscattering condition of the incident X-rays is fulfilled. They allow precise measurement of long-range-order structural information. Measured data are compared with simulations based on piXAFS theory. Although piXAFS structures are similarly observed in two detection techniques, the importance of scattering off the sample for the measurements needs to be investigated further. Disentangling piXAFS, multielectron photoexcitations and atomic XAFS in high-precision measurements close to absorption edges poses a challenge for future studies.

Theory of photon interference X-ray absorption fine structure.

The theory of photon interference x-ray absorption fine structure (piXAFS) is described. Due to coherent x-ray scattering from atoms, a spatial variation of the x-ray intensity is produced inside the sample. The intensity at the x-ray absorbing atom changes according to the incident energy. Thus piXAFS in extended absorption spectra is produced. It extends in a wide energy range over absorption edges. For powders the piXAFS formula has equivalent form as the EXAFS formula, and the Fourier transform provides distances of neighboring atoms from the absorbing atom. Due to a long mean free path of the photon, piXAFS for powders contains sharp structures. They are explained as a crystal grain orientation averaging of the x-ray standing wave effect.

Backscattering X-ray standing waves in the XUV region.

It is demonstrated that Bragg reflection of XUV radiation can be used to study structural properties of crystalline materials with large unit cells. A standing-wave field is formed in a layered TiSe2 single crystal for a near-backscattering geometry (theta = 88.5 degrees). The partial electron yield is measured as a function of photon energy across the (001) Bragg reflection condition (hv approximately equal to 1033 eV) and its characteristic modulation is compared with the results derived from dynamical diffraction theory in the two-wave approximation. The data reveal a large amount of disorder along the c-axis.

Real-space imaging of atomic structure with white x rays.

The first real-space x-ray image of an atomic structure was obtained by illuminating a crystal with white synchrotron radiation. The internal photocurrent signal served as a probe of the x-ray interference field strength at the atomic sites and was accordingly measured as a function of illumination direction to record the two-dimensional image. This novel method of real-space imaging makes use of the fact that the interference field intensity is energy independent with respect to contributions from those scattering atoms which are brought via sample rotation into the forward scattering condition. In contrast, contributions from other atoms oscillate with energy and vanish for broadband illumination.

Complex gamma-ray hologram: solution to twin images problem in atomic resolution imaging.

A new technique for high fidelity three-dimensional imaging of atomic structure with gamma-ray holography is demonstrated. A complex hologram was constructed from holograms recorded for different values of the nuclear scattering amplitude on both sides of the (57)Fe Mössbauer resonance. The holographic reconstruction was applied to this complex hologram resulting in a twin-image-free image of the bcc Fe local structure. The proposed procedure allows the removal of the twin images for all real space, making gamma-ray holography an unambiguous tool for atomic and magnetic structure imaging.

Parametric down conversion of X-ray photons.

Parametric down conversion of X-ray photons in diamond crystals was detected in two experiments, both using the phase-matching scheme first employed in the X-ray regime by Eisenberger & McCall [Phys. Rev. Lett. (1971), 26, 684-688]. The conversion events were detected by a combination of time-correlation spectroscopy and energy discrimination, using Si drift-chamber detectors. The time-correlation spectra give a direct comparison of the conversion rate over the accidental coincidence rate. Mechanisms for possible detection of false events and ways to cross check against them are discussed in detail.

A novel experimental technique for atomic X-ray holography.

A new experimental technique for reciprocal X-ray holography has been developed. The experimental set-up makes it possible to measure a reciprocal hologram without unwanted mixture of the X-ray fluorescence holography signal. The data are recorded during continuous rotation of the sample, and are accumulated over many revolutions. Thus the sensitivity to fluctuations of the source characteristics is reduced. A very high resolution over a large angular range is also achieved, which allows recording of X-ray standing-wave shapes near Bragg angles. The reconstruction of the measured hologram of a Cu(3)Au crystal gives positions of the nearest and next-nearest neighbours of a fluorescing Cu atom.

X-ray Holography for Structural Imaging.

X-ray atomic resolution holography is a new method for direct evaluation of three-dimensional electron density distribution in solids. The practical implementation of the multiple-energy technique on a synchrotron radiation source as well as image reconstruction from the experimental data are described. Holograms at several different energies were processed together to suppress twin images and artifacts from long-range-order effects in the experimental data sets. Reconstructed images of copper atoms in Cu(2)O crystals are presented.

X-ray standing wave analysis of highly perfect cu crystals and electrodeposited submonolayers of cd and tl on cu surfaces.

Experimental requirements for measuring the structure and coverage of adsorbates in the monolayer regime on single crystals with x-ray standing wavefields are discussed in detail along with a thorough description of the theory. The near-surface region of Cu crystals was probed depth selectively by detecting K as well as L fluorescence as a function of fluorescence escape angle. The effects of crystalline imperfections and of dispersive crystal arrangements on the spatial resolution of standing wave measurements are described. Copper crystals with (111) and (100) surface orientation were used as substrates for electrodeposition of Cd and Tl from an aqueous electrolyte using cyclic voltammetry. Submonolayer amounts of Cd and Tl deposited in the underpotential range were investigated on the emerged electrodes with x-ray standing waves keeping the samples under atmospheric pressure, in air, or in inert atmosphere.