A. Scafati

X-ray absorption near edge structures (XANES) in simple and complex Mn compounds☆

Abstract The X-ray Absorption Near Edge Structures (XANES) of Mn compounds at the Mn K-edge have been measured with high resolution at the Frascati Synchrotron Radiation Facility “PULS”. Molecular effects, such as “shape resonances”, have been identified in molecular complexes. Experimental evidence of solid state (or long-range order) effects have been found in spectra of simple oxides. The application of XANES for determining the local structure and chemical bonding of Mn ion in unknown compounds is discussed.

High-resolution x-ray spectrometer based on spherically bent crystals for investigations of femtosecond laser plasmas

Ultrashort-pulse, laser-produced plasmas have become very interesting laboratory sources to study spectroscopically due to their very high densities and temperatures, and the high laser-induced electromagnetic fields present. Typically, these plasmas are of very small volume and very low emissivity. Thus, studying these near point source plasmas requires advanced experimental techniques. We present a new spectrometer design called the focusing spectrometer with spatial resolution (FSSR-2D) based on a spherically bent crystal which provides simultaneous high spectral (λ/Δλ≈104) and spatial resolution (≈10 μm) as well as high luminosity (high collection efficiency). We described in detail the FSSR-2D case in which a small, near point source plasma is investigated. An estimate for the spectral and spatial resolution for the spectrometer is outlined based on geometric considerations. Using the FSSR-2D instrument, experimental data measured from both a 100 fs and a nanosecond pulse laser-produced plasma are pr...

X‐ray absorption near edge structure (XANES) of Cu(II)–ATP and related compounds in solution: Quantitative determination of the distortion of the Cu site

We have studied the Cu binding site in Cu(II)–ATP, Cu(II)–tripolyphosphate, and Cu(II)–D‐ribose complexes in aqueous solution at several pH values ranging from 3 to 12 by means of high‐resolution x‐ray spectroscopy. We have measured the spectra in the x‐ray absorption near edge structure (XANES) region to probe the tetragonal distortion of the Cu site in these compounds, as indicated by optical d–d spectroscopy. The spectrophotometric studies proposed that Cu(II) bind to the oxygen atoms of the phosphate groups of ATP with a distorted octahedral symmetry at low pH, while at high pH, the metal atom would be bound to the hydroxyl groups of the ribose moiety with an almost planar symmetry. We have compared the measured XANES spectra with multiple scattering calculations, by using as a structural model a simple tetragonal CuO6 cluster with fixed equatorial distance (1.95 A) and variable axial distance (1.95–2.7 A). The multiple scattering calculations fit well the experimental data of Cu–ATP in solution, and ...

X-ray radiation from ions with K-shell vacancies

Abstract New types of space resolved X-ray spectra produced in light matter experiments with high intensity lasers have been investigated experimentally and theoretically. This type of spectra is characterised by the disappearance of distinct resonance line emission and the appearance of very broad emission structures due to the dielectronic satellite transitions associated to the resonance lines. Atomic data calculations have shown, that rather exotic states with K-shell vacancies are involved. For quantitative spectra interpretation we developed a model for dielectronic satellite accumulation (DSA-model) in cold dense optically thick plasmas which are tested by rigorous comparison with space resolved spectra from ns-lasers. In experiments with laser intensities up to 10 19 W/cm 2 focused into nitrogen gas targets, hollow ion configurations are observed by means of soft X-ray spectroscopy. It is shown that transitions in hollow ions can be used for plasma diagnostic. The determination of the electron temperature in the long lasting recombining regime is demonstrated. In Light-matter interaction experiments with extremely high contrast (up to 10 10 ) short pulse (400 fs) lasers electron densities of n e ≈3×10 23 cm −3 at temperatures between kT e =200–300 eV have been determined by means of spectral simulations developed previously for ns-laser produced plasmas. Expansion velocities are determined analysing asymmetric optically thick line emission. Further, the results are checked by observing the spectral windows involving the region about the He α -line and the region from the He β -line to the He-like continuum. Finally, plasmas of solid density are characteristic in experiments with heavy ion beams heating massive targets. We report the first spectroscopic investigations in plasmas of this type with results on solid neon heated by Ar-ions. A spectroscopic method for the determination of the electron temperature in extreme optically thick plasmas is developed.

Shadow monochromatic backlighting: Large-field high resolution X-ray shadowgraphy with improved spectral tunability

The shadow monochromatic backlighting (SMB) scheme, a modification of the well-known soft X-ray monochromatic backlighting scheme, is proposed. It is based on a spherical crystal as the dispersive element and extends the traditional scheme by allowing one to work with a wide range of Bragg angles and thus in a wide spectral range. The advantages of the new scheme are demonstrated experimentally and supported numerically by ray-tracing simulations. In the experiments, the X-ray backlighter source is a laser-produced plasma, created by the interaction of an ultrashort pulse, Ti:Sapphire laser (120 fs, 3–5 mJ, 10 16 W/cm 2 on target) or a short wavelength XeCl laser (10 ns, 1–2 J, 10 13 W/cm 2 on target) with various solid targets (Dy, Ni + Cr, BaF 2 ). In both experiments, the X-ray sources are well localized spatially (∼20 μm) and are spectrally tunable in a relatively wide wavelength range (λ = 8–15 A). High quality monochromatic (δλ/λ ∼ 10 −5 –10 −3 ) images with high spatial resolution (up to ∼4 μm) over a large field of view (a few square millimeters) were obtained. Utilization of spherically bent crystals to obtain high-resolution, large field, monochromatic images in a wide range of Bragg angles (35° < Θ < 90°) is demonstrated for the first time.

Development and characterisation of an XeCl excimer laser-generated soft-X-ray plasma source and its applications

SummaryA large-aperture, long-pulse XeCl excimer laser has been used to generate a soft-X-ray plasma source. Several laser optical configurations have been employed to optimise X-ray emission, including positive-branch unstable resonators and injection by seeding the gain region of the laser with a small commercial excimer laser, resulting in power densities in the range 1012–1014 W cm−2. The characteristics of the plasma source for each different laser configuration and for different target materials are investigated. The most suitable source conditions (spectral energy distribution, time duration, etc.) for specific applications are discussed.

Na-like autoionization states of copper ions in plasma, heated by excimer laser

A 100 ns excimer laser-produced copper plasma was used to investigate the Na-like Cu XIX n = 4-2 satellite emission in a higher density and lower temperature regime than previously studied. Theoretical model calculations were used to identify a number of radiative transitions from autoionizing Na-like copper levels. These transitions were measured with an accuracy of 0.001 A. A comparison of these spectral measurements to spectra obtained from lower density Nd-glass laser-produced copper plsmas demonstrated the changing role of different excitation mechanisms for different plasma conditions.

Dominant role of dielectronic satellites in the radiation spectra of a laser plasma near the target surface

It is shown that in a dense, not very hot, multiply charged plasma the satellite structures of resonance lines can become more intense than the resonance lines themselves. Experimental and theoretical investigations show that the conditions under which the satellite structures dominate in the emission spectrum of the plasma are quite easily realized experimentally and, furthermore, apparently they will be the most typical case in investigations of compressed plasma regions in inertial-confinement fusion experiments and in the study of plasma produced by high-contrast pico-and femtosecond laser pulses.

Line formation of high-intensity -Rydberg dielectronic satellites in dense laser-produced plasmas

On the basis of experimental and theoretical investigations it is demonstrated for the first time that in cold dense optically thick laser-produced plasmas, created near the target surface, the capture into the He-like ground state is negligible for line formation and that observed high-intensity -Rydberg satellite intensities are correlated with highly populated He-like excited states . X-ray emission spectra with simultaneous high spectral and spatial resolution provide a direct verification of the proposed excitation mechanism. Atomic data calculations for all configurations with n = 3 - 6 have been carried out and were employed in spectral modelling. Taking into account the proposed excitation channels, excellent overall agreement is found. Successful cross-checks with the spectral interval near the -line are demonstrated. The spectral modelling is proposed for sensitive density (electron density, excited state population) and temperature diagnostics near the target surface.

Features of plasma produced by excimer laser at low intensities

A plasma, created at interaction of short-wavelength excimer laser radiation with flat targets was investigated (tlas = 12 ns, λlas = 0.308 μm, qlas = 4 – 8 × 1012 W/cm2) with the help of various x-ray spectroscopic methods. The comparison of shapes and intensities of some observable spectral lines of H-, He and Li-like ions of Na, Mg and Al with results of model calculations has allowed to determine space distributions of laser plasma parameters up to distances of 0.4 mm from the target surface. Comparison of obtained results with theoretical models of absorption of short-wavelength radiation in a plasma shows, that the absorption of short-wavelength laser radiation in a plasma (at considered values of laser flux density) is executed due to inverse bremsstrahlung process in the areas with Ne < Ne, crit..