The XAFS analysis and the extraction of the multielectron excitation (MEE) spectrum from the XAFS spectrum using conventional XAFS analysis programs, XANADU have been presented. Two electron transitions have been found in the extracted MEE spectrum. The manner of the extraction has been discussed.
X-ray absorption fine structure, XAFS observed in the X-ray absorption spectrum brings the information for both the local structure around a specified atom and the electronic state of a specified atom. The former is mainly obtained by the Fourier transformation and the curve-fitting calculation of the extended X-ray absorption fine structure, EXAFS, which extends from about 50 to 1000 eV above the absorption edge. Another near edge part which is observed up to about 50 eV above the absorption edge is called X-ray absorption near-edge structure, XANES and reflects both the electronic and the coordination state for a specified atom.
The X-ray absorption spectrum contains not only the XAFS but so-called multielectron excitation (MEE) process, where the removal of a core electron by photoabsorption causes excitation of additional electrons in the same atoms. Both the understanding of the transition process involving MEE process and the removal of the contribution of MEE structure from the XAFS spectrum are very important to interpret the XAFS spectrum. Theoretical approach to the explanation of the MEE process has also progressed but has not provided sufficient information to remove a contribution of the MEE structure from the XAFS spectrum yet. Attempts to extract such MEE structure from the XAFS spectrum, therefore, have been made experimentally and the multielectron transitions in the extracted MEE spectrum have been identified1). This paper presents the procedure of extracting the MEE structure by both removing non-EXAFS feature and comparing observed and curve-fitted EXAFS function through the XANADU2) package which contains the conventional XAFS analysis programs.
The XAFS spectrum for KOH were measured using the beam line BL-7C of the Photon Factory in National Laboratory for High Energy Physics, KEK, Tsukuba with 2.5 GeV positrons. The sample was prepared by mixing KOH solution and polyvinyl alcohol by heating, and the foil was formed for the XAFS experiment.
XANADU used for analyzing XAFS spectrum has been constructed by Sakane3). Some additional programs have been provided by Takahashi1) so as to analyze the MEE process in the XAFS spectrum. Figure shows the flow of the analysis for K K-edge XAFS of KOH. The background absorption due to lower energy absorption edges, Compton scattering and other processes is subtracted from the observed spectrum and the resultant spectrum is normalized using the absorption coefficient at the absorption threshold energy, E0.
A temporary chi(k) is calculated by subtracting a temporary atomic absorption coefficient, µtmp0 from the normalized absorption coefficient, µnorm; µtmp0 is calculated by using the kn-weighted least-squares fitting of 3 - 6 order poly nominal of k applied to µnorm above E0. After second-order 11 - 15 points smoothing is applied to k3-weighted temporary chi(k), two cubic spline interpolation curves, one connecting the maxima in the smoothed spectrum and the other connecting the minima, and the average of the two splines are calculated. The calculated average curve is subtracted from µnorm if such curve seems to be non-atomic absorption coefficient, ie., low-frequency or locally bouncing components such as MEE structures (step 1). The same procedure except that the subtraction of the average curve from the spectrum is repeated as step 2 and the resultant average curve which is obtained on step 2 is used as the atomic absorption coefficient. After then, calculated EXAFS spectrum chicalc(k) is subjected to the Fourier transform calculation. Next, the Fourier transform of EXAFS is back-transformed and the Fourier-filtered experimental EXAFS spectrum is used for the EXAFS curve fitting calculation, which provides the information on the local structures of the absorbing atoms. Finally, the MEE spectrum is calculated by summing the non-atomic absorption coefficient, which is eliminated on the step 1 of the extraction of chi(k) procedure, and the residue obtained by subtracting the chicalc(k) from the observed EXAFS spectrum chi(k). The threshold onsets at 17.06, 45.63 and 333.7 eV in the evaluated MEE spectrum of KOH, which have been interpreted as two-electron transitions for K atoms, ie., [1s3p], [1s3s] and [1s2p], respectively.
Supplying more advanced X-ray from SPring-8 and improved Photon Factory should require the more precise analysis. In turn, a great deal of XAFS studies have been carried out and the several excellent analyses have also been conducted in Germany. These means the necessity of the exchanges between German and Japanese investigators in this field.
Poster image (680×944 pixels)
The 7th German-Japanese Workshop on X-Ray Software, Nara (1997. 4).
Questions or comments are welcome.