Fig. 4-12 Outgoing photoelectron from an x-ray absorbing atom The outgoing photoelectron will be backscattered by neighboring atoms. An EXAFS oscillation originates from the interference between the outgoing and the incoming waves. The lifetime of an inner-shell vacancy must be longer than the transit time for the photoelectron. The smearing effect of the EXAFS signal due to the finite-lifetime of the core hole becomes more serious for heavy elements with many higher-shell electrons to drop into the K-hole.

Fig. 4-13 x-ray absorption spectrum near the Pt-K edge of Pt foil The x-ray absorption coefficient increases at 78.4 keV for excitation of the photoelectron from the K-shell. The EXAFS function is extracted from the oscillatory part of the spectrum, which is surrounded by dashed lines in the figure.

Fig. 4-14 Local structure around the Pt atom calculated by Fourier transformation of the EXAFS function The peak at r=2.77 angstrom is due to the nearest neighboring Pt atoms.

The radiation spectrum from a bending magnet at the SPring-8 storage ring shows sufficient photon density to observe qualitative x-ray absorption spectra, even at 100 keV. This advantage makes it possible to measure x-ray absorption spectra near the K-absorption edges for almost all heavy elements. The fine structure of the x-ray absorption coefficient just above the edges is known as EXAFS (Extended x-ray Absorption Fine Structure). We can learn about local structure around the x-ray absorbing atoms by analyzing an EXAFS spectrum. An EXAFS spectrum near the LIII-absorption edges should have been measured for heavy elements and are usually restricted by the following LII-absorption edge. EXAFS spectra near the K-absorption edges are anticipated in order to improve the accuracy of local structure parameters for heavy elements, e.g. lanthanoids. However, it was theoretically pointed out that the finite lifetime of a core hole smears out EXAFS oscillation for heavy elements. In this study, we have succeeded in observing EXAFS spectra in the high energy region. Figure 4-13 shows an EXAFS spectrum near the Pt-K edge (78.4 keV) of a Pt foil (0.1 mm thick). The edge energy is higher than those of lanthanoids and is the highest one in the world at present. Figure 4-14 shows the local structure around the Pt atoms obtained by Fourier transformation of the EXAFS signal. The precise analysis of local structure is expected for compounds including heavy elements such as lanthanoids.

Reference Y. Nishihata et al., EXAFS in the High-Energy Region, J. Synchrotron Rad., 5, 1007 (1998).

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