Al K-edge extended fine structures in X-ray emission spectra of aluminum metal and aluminum oxide measured by an electron probe microanalyzer (EPMA)

SPECTROCHIMICA ACTA PART B

ELSEVIER

Spectrochimica Acta Part B 53 (1998) 505-507

Letter

A1 K-edge extended fine structures in X-ray emission spectra of aluminum metal and aluminum oxide measured by an electron probe microanalyzer (EPMA) S. Tanuma*, M. Nishio Japan Energy Analytical Research Center Co. Ltd, 3-17-35 Niizo-Minami, Toda, Saitama 335, Japan

Received 7 October 1997; revised 11 November 1997; accepted 19 November 1997

Abstract

The radiative Auger satellite peaks of AI Kc~ for aluminum metal and aluminum oxide were measured over a small area using an electron probe microanalyzer (EPMA). The oscillation was found to be similar to the extended X-ray absorption fine structure (EXAFS) in the EPMA spectra, oscillation which was recently discovered by Hayashi et al. (1997) in the X-ray fluorescence (XRF) spectra. The measured EXAFS spectra with EPMA are in good agreement with those by Hayashi et al., but here the oscillation structure could be obtained in a few minutes over a small area by using EPMA. © 1998 Elsevier Science B.V. Keywords: X-ray emission spectra; Aluminum metal; Aluminum oxide; extended fine structures; electron probe microanalyzer

The extended X-ray absorption fine structure (EXAFS) method is widely used for the local structural analyses of amorphous, non-crystal materials, catalysts and so on. It is, however, very difficult to obtain the E X A F S spectra of light elements such as aluminum and silicon without a synchrotron radiation (SR) source. Furthermore, to measure the E X A F S of light elements in a small area, such as over a 1/xm region, is very difficult even if a SR source is used. Recently, Kawai et al. [1] and Hayashi et al. [2] pointed out that the profiles of the radiative Auger satellite peak profile in X-ray fluorescence (XRF) spectra resembled that of the X-ray absorption edge, and they predicted that the extended X-ray absorption * Corresponding author. Fax: +81 48 4332150; e-mail: [email protected]

fine structure spectroscopy would be able to be performed by line shape analysis of the radiative Auger effect in the XRF spectra. However, the time required for a spectrum strong enough for data analysis was longer than a few hours. Here, the same experiment was carried out using an electron excitation method. The measurement of the radiative Auger satellite peaks in characteristic X-ray spectra of A1 metal and A1203 are reported here over a small area with an electron probe microanalyzer (EPMA). The measurement conditions for E P M A were as follows: the accelerating voltage was between 15 and 20 kV; the probe current was 2 . 0 - 2 . 6 × 10 -6 A; the beam diameters were 1 and 50/~m; and the used analyzing crystal was TAP. These measurements were made using a JEOL 8900 EPMA. The measured low energy side of the X-ray spectra of Kc~ for aluminum metal and aluminum oxide are

0584-8547/981519.00 © 1998 Elsevier Science B.V. All rights reserved PII S0584-8547(97)00125-0

S. Tanuma, M. Nishio/Spectrochimica Acta Part B 53 (1998) 505-507

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Photon Energy (eV) Fig. 1. Measured satellite peaks in the low energy side in AI K X-ray emission spectra of AI metal and AI20~ with an EPMA. Measurement conditions for EPMA were as follows: the accerelating voltage = 20keV; beam current = 2#A; beam diameter = 1 ,am; measurement time per channel = 3 s; total measurement time = 20 min.

shown in Fig. 1. Small satellite peaks were found which were due to the radiative Auger effect at about 1380 eV on each spectrum, together with fine structures, as shown in Fig. 1. The aluminum metal can be completely distinguished from the oxide using the satellite peak shape. The spectrum of the radiative Auger satellite for aluminum metal taken by EPMA was compared with the XRF fine structure [2] and the genuine EXAFS spectrum [3], as shown in Figs 2 and 3. In Fig. 2, three peaks can be seen in the range 12801400 eV in the observed A1 spectrum by EPMA. The resulting spectrum is in excellent agreement with the XRF of Hayashi et al. [2]. Fig. 3 shows the comparison between a satellite peak, which was background subtracted, measured by EPMA and the genuine EXAFS spectra of aluminum metal measured with a SR source [3]. Four peaks are clearly found in the spectra taken by EPMA in Fig. 3. The two spectra also show great similarities in peak positions and shapes• Fig. 4 also shows the comparison between the radiative Auger satellite peak shape of A 1 2 0 3 taken by EPMA and that from XRF by Kawai et al. [4]. These spectra also closely resemble each other. Therefore, this method can be used for the EXAFS measurement of light elements over a small area.

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Photon Energy (eV) Fig. 2. Comparison of the AI-K edge extended X-ray absorption fine structures in the spectra taken by various techniques. The solid line shows the present result measured by EPMA. The measurement conditions were: acceretating voltage = 15keV; beam diameter = 50,am; total measurement time = 50 rain. Dashed line shows the XRF spectra measured by XRF [2]; dashed-and-dotted line shows the genuine X-ray absorption fine structure using a SR source [3].

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Fig. 3. Comparison of EXAFS oscillations between an X-ray emissionspectrum measured by EPMA and that of a genuine EXAFS spectrum with a SR source. The resulting oscillations by EPMA were obtained from the data shown in Fig. 2 using 7-point Savitzky-Golay smoothing after background subtraction using a

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S. Tanuma, M. Nishio/Spectrochimica Acta Part B 53 (1998) 505-507

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Although EPMA is an analytical tool for the elemental analysis in small regions of solid specimens, it is very difficult to carry out the chemical state analysis. However, this method gives such a capability, of small area analysis of the local structure, to the conventional EPMA. Furthermore, the measurement time is short (usually less than 10 min) compared to the XRF method (a few hours). This method can be practically applied to the EXAFS analyses of light elements over a small area.

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Photon Energy (eV) Fig. 4, Comparison of EXAFS oscillations between an X-ray emission spectrum measured by EPMA and that from XRF by Kawai et al. [4].

Since the efficiency of X-ray excitation of light elements by an electron probe is higher than that by X-ray fluorescence, the use of EPMA is more suitable for the measurement of EXAFS of light elements, such as aluminum, when compared with the XRF. Additionally, EPMA c a n m e a s u r e the small a r e a EXAFS easily,

The authors would like to thank Dr J. Kawai (Kyoto University) for sending preprints before publication.

References [I] J. Kawai, T. Nakajima, T. Inoue, H. Adachi, M. Yamaguchi, K. Maeda, S. Yabuki,, Analyst 119 (1994)601. [21 K. Hayashi, J. Kawai, Y. Awakura, Spectrochim, Acta B, in press. [3] A. Fontain, P, Lagarde, D. Raoux, J.M. Estva., J. Phys. F: Metal Phys. 9 (1978) 2143. [4] J. Kawai. K. Hayashi, Y. Awakura, J. Phys. Soc. Jpn,, in press.