C.R. Natoli

Use of one‐electron theory for the interpretation of near edge structure in K‐shell x‐ray absorption spectra of transition metal complexes

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...

AB-INITIO MODELLING OF X-RAY ABSORPTION SPECTRA

Abstract A new method for analyzing X-ray absorption spectra using multiple-scattering theory is presented. The basic principles and some applications are briefly discussed focusing the presentation on the general relevance of this approach. The GNXAS set of programs, especially designed to perform structural analysis on absorption data of any system, is described showing the practical applicability of the theory. The applications presented show that reliable and unique structural information on pair and higher-order distribution functions can be obtained by using this method which takes proper account of the multiple-scattering contributions.

The Three-Body Correlation Function in Amorphous Silicon Probed by X-Ray Absorption Spectroscopy

In this paper we present structural results on the three-body correlation function in amorphous silicon as determined by the X-ray absorption spectroscopy (XAS). It is shown, for the first time, that the XAS can provide quantitative information beyond the pair correlation. The theoretical analysis is performed using an original formulation of the multiple-scattering theory of the XAS in terms of the n-body distribution functions. These results are compared with recent computer simulations of continuous random network models.

XANES (x ray absorption near edge structure): A new probe of higher order correlation function in amorphous semiconductors

Abstract The higher order correlation function of local atomic distribution in amorphous semiconductors have been probed directly by XANES (x ray absorption near edge structure) experiments using synchrotron radiation. We show that uniquestructural informations on the geometry of local atomic arrangeaments in amorphousvanadium oxide semiconductors and in silica glasses can be obtained. The multiple scattering theory has been applied to interpret the data.

X-ray absorption spectroscopy on amorphous silicon: A probe for the three-body correlation-function

Abstract We report for the first time the experimental determination of physical parameters involving three-body correlations in amorphous silicon using the x-ray absorption spectroscopy. The results are compared with recent computer simulations of continuous random network models.

Orientational Disorder In Amorphous-silicon Probed By Xanes (x-ray Absorption Near Edge Structure)

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.