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We present ab initio calculations of X-ray-absorption fine structure for the Re L 3 and L 1 edges in crystalline ReO 3 based on an exact curved-wave multiple-scattering approach. We show that (1) the contribution of multiple-scattering signals from linear chains, like Re – O – Re and O – Re – O, is very large both in the L 3 and L 1-edge spectra due to the strong focussing effect caused by the middle atom; (2) the geometrical dependence of multiple-scattering terms on the final l state leads to the additional difference between signals from L 3 and L 1 edges, especially for paths forming a right angle at the absorbing atom; (3) the fine structure above L 3 and L 1-edge is a superposition of two signals from L 1 and L 2 edges with a very significant contribution of this latter.

A mathematically transparent derivation of the multiple-scattering equations valid for a general non-muffin-tin potential, as applied to clusters of atoms with and without a surrounding outer sphere, is presented. These equations are shown to be a natural generalization of the analogous equations valid for muffin-tin potentials. An expression for the photoabsorption and electron scattering cross section in the framework of the multiple-scattering theory valid for a general potential is derived for what may be the first time, providing the necessary generalization for the similar expression valid in the muffin-tin case. A connection with the Green-function approach to the problem is also established via a generalized optical theorem.

We report the experimental extraction of the multiple-scattering contribution to the K-edge x-ray-absorption near-edge structure spectrum (XANES) of crystalline silicon. The multiple-scattering signal, is obtained by taking the difference ${\mathrm{\ensuremath{\chi}}}_{\mathrm{expt}}$(k)-${\mathrm{\ensuremath{\chi}}}_{2}$(k)=${\mathrm{\ensuremath{\chi}}}_{\mathrm{MS}}$(k)= where ${\ensuremath{\chi}}_{\mathrm{expt}(\mathrm{k})}$ is defined as [\ensuremath{\alpha}(k)-${\ensuremath{\alpha}}_{0}$(k)]/${\ensuremath{\alpha}}_{0}$(k), ${\ensuremath{\chi}}_{2}$ is the spherical-wave-calculated extended x-ray-absorption fine-structure (EXAFS) signal, and the ${\ensuremath{\chi}}_{n}$'s are the multiple-scattering contributions to the total absorption coefficient \ensuremath{\alpha}(k) assumed to have the form \ensuremath{\alpha}(k)=${\mathrm{\ensuremath{\alpha}}}_{0}$(k)[1+ over the full wave-vector range, ${\ensuremath{\alpha}}_{0}$(k) being the atomic absorption coefficient. The ${\ensuremath{\chi}}_{2}$ term has been calculated over the full energy range by using a spherical-wave formula and including mean-free-path and Debye-Waller effects. By using the subtraction procedure we find a ${\ensuremath{\chi}}_{\mathrm{MS}}$ oscillation of large amplitude in the first 70 eV of the spectrum. Comparison with the theoretical double scattering ${\ensuremath{\chi}}_{3}$(k) gives account of the main features of the ${\ensuremath{\chi}}_{\mathrm{MS}(\mathrm{k})}$ spectrum. We show that the main ${\ensuremath{\chi}}_{3}$ contribution comes from the double scattering paths involving atoms of the first as well as of the second shell and that higher terms of the series (ng3) are damped by the finite core hole lifetime and the inelastic losses of the photoelectron in the final state. Finally we analyze the K-edge absorption spectra of amorphous silicon. We find that the ${\ensuremath{\chi}}_{\mathrm{MS}}$ signal, contrary to the crystalline case, is suppressed. Its suppression is assigned to the orientational disorder in bond angles in the amorphous phase.

We describe progress towards the performance of SCF-SW-Xalpha calculations of photo-absorption cross-sections based on the theory of Natoli et al. for non-muffin-tin potentials. A crucial requirement is the accurate modelling of the electron-molecule potential using spherical harmonic expansions. We describe how this has been achieved and what difficulties are encountered. In the particular case of our model compound, chromium hexacarbonyl, we show what muffin-tin calculations produce and show that we may expect significant improvements from a non-muffin-tin calculation. Finally, we comment on the programming problems involved in these computations.

Spherical-wave corrections in x-ray-absorption fine structure (XAFS) are incorporated using a novel high-energy approximation which replaces the plane-wave approximation and is found to be in excellent agreement all the way to the absorption edge. The single-scattering theory of XAFS is recovered in terms of distance-dependent backscattering amplitudes. Application to the $K$-shell XAFS of Cu yields results in close agreement with the corresponding exact treatment. Multiple-scattering contributions in XAFS and spherical-wave corrections in angle-resolved photoemission are treated similarly.

The various schemes for calculating inner shells X-ray absorption spectra are reviewed and shown to be mutually equivalent. The unifying framework is provided by the multiple scattering (MS) theory. In p articular t he formulation based on the Green' s function approach allows one under certain conditions to write the absorption coefficient as a sum of an infinite number of terms which have a direct physical meaning. The conditions under which this expansion is possible is discussed and as a consequence a unifying scheme of interpretation of X-ray absorption spectra is proposed. Finally an approximate formula for t he r apid evaluation of the n-th order term of the expansion is given.

We report the results of multiple scattered wave SCF X‐alpha calculations of the one‐electron cross section for K‐shell photoabsorption in the molecular complexes MoO4−−, CrO4−−, and MoS4−−. We show that the method can successfully account for energy separations and relative cross sections of spectral features both below and above the K‐shell ionization threshold. Furthermore, we show: (a) that the first fairly intense peak on the low energy side of the rising edge for molybdate and chromate is due to a dipole allowed transition to a bound antibonding state of mainly nd character on the metal ion; this transition is possible because of the mixing with the ligand p orbitals having the proper T2 symmetry induced by the tetrahedral molecular potential; (b) the shoulder on the rising absorption edge can be explained by the beginning of the steplike continuum absorption when convolved with a Lorentzian function of frequency to imitate lifetime and monochromator broadening: (c) the main absorption peak is due t...

The problem of the additional structure exhibited by samarian sulfide is attacked theoretically and experimentally. Both approaches lead to the conclusion that it is due to multiple-scattering effects and not to mixed-valence or many-body process.

Near edge structures of X-ray absorption spectra from deep core levels, both in molecules and in extended media, have attracted, because of their prominence, the attention of the researchers in the field ever since they were experimentally observed.

By making a comparison between the Mn K-edge absorption of (${\mathrm{MnO}}_{4}$${)}^{\mathrm{\ensuremath{-}}}$ and [Mn(${\mathrm{H}}_{2}$O${)}_{6}$${]}^{2+}$ complexes in aqueous solution we obtain an experimental determination of the energy extent of the type-II multiple-scattering (MS) regime that is substantially wider than expected. Theoretical calculations based on the MS formalism support this conclusion. We also recognize three energy regions in the absorption spectra of these complexes: a full MS region, where numerous or an infinite number of MS paths of high order contribute (depending on whether the MS series converges or not), an intermediate MS region, where only a few MS paths of low order are relevant, and a single-scattering region where the photoelectron is backscattered only once by the ligands [extended x-ray-absorption fine-structure (EXAFS) regime]. Theoretical considerations show that this must be a general situation in x-ray-absorption spectra and opens the way to a unified scheme for their interpretation. The energy extent of the three regions is obviously system dependent. We also show how to generalize to MS contributions the usual EXAFS analysis using curved-wave propagators and indicate how to extract geometrical information from the spectra of the two clusters investigated. In particular the method is used to derive the Mn---O---O---Mn path length in the (${\mathrm{MnO}}_{4}$${)}^{\mathrm{\ensuremath{-}}}$ complex.

The difference between the K-edges XANES (X-ray Absorption Near Edge Structure) spectra of crystal and amorphous silicon is discussed. We show that the multiple scattering signal gives a large contribution to the total absorption in crystalline silicon in an energy range of about 70eV. The double scattering term probing the triplet distribution function gives the major contribution to the XANES. We show that the difference between the crystalline and amorphous absorption spectra is due mainly to the crystalline multiple scattering signal which is quenched in the amorphous spectrum. The suppression of the signal due to the triplet distribution function in the amorphous silicon is assigned to the large orientational disorder.

A theoretical calculation of the K-edge spectrum of silicon was performed. A good fit with the experimental EXAFS spectrum has been obtained by using the spherical wave formalism, the actual values of the mean free path and the Debye-Waller terms.The relevant multiple scattering contribution in the first 70 eV of the spectrum has been obtained by subtraction of a simulated EXAFS spectrum from the experimental one. INTRODUCTION. - In order to establish the role of the basic physical processes in the EXAFS and XANES pans of K-edge absorption spectra such as inelastic losses, thermal vibration and multiple scattering we have performed a careful quantitative analysis of the silicon K-edge absorption spectrum. A good fitting of the EXAFS oscillations from 50 to 450 eV above the absorption threshold (in the range of wave vector 4-11 A-l, with ET=1838 eV) has been obtained using spherical The spectrum of the mean free path h(E) of the photoelectron has been determined from photoemission experiments(3) and it has been included in the EXAFS calculations. The EXES Debye-Waller factors (4) have been also considered. They have been calculated for each shell using a dispersive model of lattice vibrations of Walker and eati in^(^) starting from X-ray diffraction measures on vibrational amplitudes in Silicon (6J). An investigation on multiple scattering effects has been carried out by subtracting the calculated spectrum from the experimental one; the difference spectrum shows an oscillation which can be explained considering the first path of multiple scattering. E

The binding of Mn2+ ions to ATP molecules has been studied by means of the fine-structure analysis of X-ray absorption spectra (EXAFS and XANES) at the K-edge of Mn using the synchrotron, radiation facility PULS in Frascati. The results obtained in both freeze-dried and liquid samples of aqueous solutions at room temperature of the Mn-ATP complex under diffrent values of pH and Mn:ATP ratio are reported and discussed. The Mn2+ ion appears to be octahedrally coordinated, the phosphate oxygen atoms being in the first co-ordination shell. Values of (2.15±0.05) A and (3.4±0.05) A, respectively, have been found for Mn-O and Mn-P distances. Theoretical considerations together with experimental results do not support the existence of a direct binding of the Mn to the N(7) atom of the adenine ring. The coordination numbers for the first and second shell are consistent with the formation of a Mn(ATP)2 complex in solution at pH=9 and Mn: ATP=1:10, while a Mn-ATP 1:1 complex is found at lower pH and higher Mn: ATP ratios.

We present an exact and efficient method to calculate both thermal and structural damping of the multiple scattering (MS) contributions to the x-ray absorbtion coefficient. Our method, which accouts for the correlations, gives some results which indicate that only for the EXAFS signal the dominat damping is given by a simple Debye-Waller type of correction in the amplitude. In the higher order contribution to total cross section phase effects become important and can alter the effective damping. In the case of structural disorder we show how the MS signal is capable of providing new structural information in such a way to allow us to distinguish between various amorphous structural model.

Vaccines are one of the main arms of preventive medicine. Recently a large series of experiments with cancer-prone genetically engineered mice have shown that preventive vaccines are also extremely efficacious inhibitors of the progression of carcinogenesis. Early vaccination affords significant and persistent protection, whereas its efficacy fades when neoplastic lesions become more advanced. Our current attempts to use combination strategies and technological advances to make vaccines effective in cancer prevention able to cure more advanced stages of cancer lesions are based on the temporary and systemic T(reg) removal, the preparation of new bimodular plasmids for DNA vaccination, and the search for fresh target oncoantigens.