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Selective XANES spectroscopy from RIXS contour maps
Journal of Physics and Chemistry of Solids 66 (2005) 2168–2172 www.elsevier.com/locate/jpcs
Selective XANES spectroscopy from RIXS contour maps Hisashi Hayashi a,b,*, Masaki Kawata a, Rumi Takeda a, Atsushi Sato a, Yasuo Udagawa a, Naomi Kawamura c, Susumu Nanao d a
IMRAM, Tohoku University, Katahira, Sendai 980-8577, Japan b PRESTO, JST, 4-1-8 Honcho Kawaguchi, Saitama, Japan c JASRI, Mikazuki, Hyogo 679-5198, Japan d IIS, University of Tokyo, Komaba 4-6-1, Meguro, Tokyo 153-8505, Japan
Abstract Kb resonant inelastic X-ray scattering (RIXS) spectra of Mn and Ga compounds are studied to examine different types of selective XANES. RIXS spectra of MnO in Mn Kb1,3 region and of GaCl2 in Ga Kb2 region are plotted as contour maps over wide energy ranges in both excitation and emission. Through analyses of RIXS contour maps by the use of the Kramers–Heisenberg equation, spin-selective XANES spectra are deduced for MnO. In the case of a mixed valence compound GaCl2 selective XANES of Ga3C ion can be obtained by making use of a large difference in transition probabilities. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction X-ray absorption near-edge structure (XANES) spectroscopy is now utilized over wide research-fields as an element-specific characterization method. It is well known that XANES technique measures spectra that average over all the chemical states of an element in the sample. In many instances, e.g. heterogeneous catalyst studies, it is of vital importance to selectively probe different forms of the element. In order to tackle this issue, ‘selective’ XANES spectroscopy has been proposed [1,2], which is based upon chemical effects on X-ray emissions such as chemical shifts and/or appearance of satellites. Selective absorption spectra may be deduced from the selected components of resonant inelastic X-ray scattering (RIXS) [2–6], which we loosely define here as X-ray emissions excited with X-ray energies near absorption edge. Since chemical effects are generally large in outer-shell electrons, RIXS measurements of outershells/empty-core emissions are more effective than those of emissions between inner shells in order to observe selective XANES spectra. However, unfortunately, outer-shells/ empty-core emission intensities are intrinsically low. Thus selective XANES spectroscopy is not well developed yet,
* Corresponding author. Tel.: C81 22 217 5385; fax: C81 22 217 5337. E-mail address: [email protected] (H. Hayashi).
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.09.052
although its importance has been well recognized and several attempts have been carried out so far [1,2,7–13]. Recently, we have developed a high-resolution X-ray spectrometer with large acceptance [14], and applied it to measure RIXS of inner-shells/empty-core emissions for deducing lifetime-broadening-suppressed XANES [15–17]. The present work is an extension of the RIXS studies on inner-shells/empty-core emissions to outer-shells (3p or valence)/empty-core emissions. As examples to examine spin-selective and state-selective XANES, MnO (an antiferromagnet) and GaCl2 (a mixed valence compound, where GaC and Ga3C ions are embedded in different chemical environments [18]) are selected. Through RIXS measurements over wide energy ranges in both excitation and emission to make RIXS contour maps, possibilities of selective XANES spectroscopy are explored. 2. Experimental The experiments were carried out at the BL47XU at SPring-8. In this beamline is a two-crystal Si(111) monochromator. The flux at the sample position was about 1013 photons/s, and typical spot size was 0.5 mm (height)!0.5 mm (width). The multi-crystal spectrometer employed was described in detail elsewhere [14]. In measurements of RIXS in Mn Kb1,3/Ga Kb2 region, the scattered radiation was horizontally dispersed and vertically focused with five cylindrically-bent Ge (440)/Si (555) crystals having a 550/950 mm radius of curvature. Here the solid angle to collect emissions is about 0.06/0.02 sr, where the solid angle per eV is about 9!10K4/2!10K4 sr/eV.
H. Hayashi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2168–2172
Analyzed X-rays from different crystals were focused on different vertical positions of a two-dimensional positionsensitive proportional counter. All data were taken at room temperature at a constant scattering angle of w908 in the horizontal plane to reduce extraneous non-resonant scatterings. The sampling times were less than 1 h/spectrum. The overall energy resolution in the Mn Kb1,3/Ga Kb2 measurements was 1.4/2.9 eV as determined by the FWHM of elastic lines. MnO powder was purchased from Kanto Kagaku, and was employed after compressing into a pellet. GaCl2 powder was obtained from Sigma-Aldrich, and was packed between two 50 mm-thick Kapton windows under a dry Ar atmosphere. For checking the sample status, fluorescence XANES spectra were measured by using a PIN-photo diode. As shown later, the white-line feature is clearly split into two components separated by about 3 eV. The low- and the high-energy component have been assigned to transitions associated with GaC and Ga3C, respectively [19], but the intensity ratio of the split white-lines was different from that of pure GaCl2, suggesting that the present sample is a mixture of GaCl2 and GaCl3 (w1:1). The intensity ratio of the two changed with irradiation time, and after 10 h, the XANES profile became almost the same as that of GaCl3, that is, oxidation proceeds quickly with irradiation. Hence the beam position on the Ga sample was changed every 1 h, and always XANES was observed before and after a RIXS measurement in order to ensure that the part newly moved had not been damaged by irradiation and that the sample degradation was not so large as to prevent the analyses described later. 3. Results and discussion 3.1. Spin-polarized XANES Spin-polarized XANES (SPXANES) gives a direct approach to resolve the spin dependence of excited electronic states of non-ferromagnetic materials. It is based on an assumption that one can separate the Kb1,3 (3p/1s) emission spectrum into an internally referenced spin-up part and a spindown part [3,20,21], and hence one can achieve a local-spin selectivity in the K-edge XANES by measuring the emission from either the main (spin-down, Kb1,3) or satellite (spin-up, Kb 0 ) lines as a function of incident X-ray energy. Fig. 1(a) shows a Kb1,3 (spin-down)- and Kb 0 (spin-up)RIXS contour map of MnO [14,22], where the abscissa (u1) and the ordinate (u2) are excitation and emission energies, respectively. Here the 1s/3d resonant excitation appears as an island, and an existence of two prominent ridges extending parallel to the abscissa, indicated by broken lines, is evident. The energy u2 of each ridge is the same as that of regular Kb1,3 and Kb 0 emission. In addition, three ridges stretching diagonally, indicated by chains, are also conspicuous. By traversing the contour map along a constant u2 axis one obtains an excitation spectrum at the u2, which is equivalent to measuring excitation spectra while monitoring the part of fluorescence spectra, the conventional method to observe SPXANES [7–10]. Shown in Fig. 1(b) are two such excitation
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spectra obtained along the lines u2Z6491 eV (Kb1,3 peak) and u2Z6476 eV (Kb 0 peak), which are essentially the same as the SPXANES of MnO reported previously [7]. Here, non-vanishing components are noted in the energy region of 1s/3d quadrupolar transition in the spin-up polarized spectrum. Since all the five 3d-electrons of Mn2C in MnO are known to be spin-up, the transition is forbidden from Hund’s rule. This casts some doubts on the validity of the underlying assumption of the conventional SPXANES method. In addition, theoretical calculation of SPXANES on Mn K-edge of MnO by Soldatov et al. has shown a significant discrepancy from the observed spectra for both spin components [23]. In order to make full use of the information incorporated in RIXS spectra, instead of one-dimensional scans along some horizontal ridges, whole Kb-RIXS contour map should be analyzed by the following equation based on the Kramers– Heisenberg formula [22]; ð up down dsðu1 Þ ðU1s C uÞfðdgup C ðdgdown g 1s =duÞf 1s =duÞf f du 2 du2 U1s C uKu1 C G21s =4Z2 (1) Here Zu refers to the energy of the excited electron in the intermediate state, and G1s is the widths of the 1s levels, the down energy of which is represented by ZUls. dgup 1s =du and dg1s =du are spin-dependent oscillator strength distributions for spin-up and spin-down 1s electrons, respectively. The shape of the Kb1,3-RIXS spectra is determined also by final state effects, such as lifetimes and multiplet structure. In Eq. (1), they are taken into consideration by functions fup and fdown. The instrumental resolution function has been convoluted in the fup down and fdown. Starting from assumed dgup 1s =du and dg1s =du, which are often observed absorption spectra, calculations of RIXS spectra are iterated by the use of Eq. (1) by modifying down dgup 1s =du and dg1s =du until satisfactory agreement is reached between observed and calculated RIXS spectra at every excitation energy. Analyses of the contour map in terms of Eq. (1) have already been described in detail [22], and hence only the results are down shown in Fig. 1(c) as the ‘best-fit’ dgup 1s =du and dg1s =du. Both curves are considerably different from the excitation spectra obtained by monitoring Kb 0 or Kb1,3 bands in Fig. 1(b). Major features are labeled; 1s/3d (pre-edge peak), A and B (shoulders), C and E (peaks), and D (valley). It is evident that 1s/3d quadrupole transition is completely missing in the dgup 1s =du, as is expected from Hund’s rule. It is immediately understood from the RIXS map of Fig. 1(a) that the nonvanishing intensity apparent in the Kb 0 excitation spectrum is due to the diagonal component which originates from the main peak of the Kb1,3 ridge. Although not shown here, every feature down of the best fit dgup 1s =du and dg1s =du has its counterpart in the SPXANES spectra calculated by Soldatov et al. [23]. These results indicate that analyses of RIXS maps in terms of Kramers–Heisenberg equation are crucial for correct determination of SPXANES.
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Fig. 1. (a) A Kb-RIXS contour map of MnO; emission intensities as a function of excitation (u1) and emission (u2) energies are plotted. Two horizontal and three diagonal ridges are indicated by broken lines and chains, respectively. (b) Cross sections of the map at u2Z6476 eV (solid squares) and u2Z6491 eV (open circles). (c) The best-fit dg1s/du’s (SPXANES) obtained by the analysis using Eq. (1) [22].
3.2. State-selective XANES In order to obtain state-selective XANES (SSXANES), desirable are accurate measurements of RIXS of valence/ empty-core emissions [1,2,24,25], which exhibit large chemical effects. However, intrinsic low intensities of these transitions have made valence-RIXS experiments extremely difficult. Thus inner shell/1s emissions, which show only small chemical shift (e.g. Mn Kb1,3 [11], Cu Ka1 [12], and Fe Kb1,3 [13]), should have been employed in previous SSXANES studies at a sacrifice of state-selectivity. A use of efficient multi-crystal spectrometer equipped with a twodimensional detector can alleviate problems associated with
low intensities and makes it possible to challenge the SSXANES by the use of valence/empty-core emissions. Fig. 2(a) shows a Kb2 RIXS contour map of the present Ga sample, a mixture of GaCl2 (GaCCGa3C compound) and GaCl3 (Ga3C compound), where the abscissa and ordinate are the same as those in Fig. 1(a). Similar to Fig. 1(a), an existence of two ridges extending parallel to the abscissa is evident. One is a prominent ridge at u2Z10365 eV that corresponds to the energy of Ga Kb2 (valence/1s) [26,27], and another, less prominent one is at u2Z10350 eV, which corresponds to 3d/1s transition energy [26]. They are indicated by broken lines in the figure. In addition, two ridges stretching diagonally, indicated by chains, can also be observed. The existence of these diagonal ridges
H. Hayashi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2168–2172
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Fig. 3. Comparisons of integrated intensities of the Ga Kb2 (valence/1s) and the low-energy satellite (3d/1s) with fluorescence XANES spectra of GaCl2CGaCl3 (GaCCGa3C) and GaCl3 (Ga3C). Here the satellite intensity is multiplied by 22.
Fig. 2. (a) A Kb-RIXS contour map and (b) RIXS spectra of the Ga chloride studied here.
clearly demonstrates that, as in the case of MnO, fluorescence excitation spectra obtained by monitoring a part of fluorescence spectrum are not adequate to extract SSXANES and that analyses in terms of Kramers–Heisenberg equation are required. Here, however, a more versatile method is examined as described below. The Kb-RIXS spectra constituting the RIXS map are plotted in Fig. 3(b), which tell us that relative intensities of Kb2 versus 3d/1s satellite band vary with excitation energy. In Fig. 3, integrated intensities of the Kb2 and the satellite bands are plotted against u1. Normalization was made by the use of the integrated excitation X-ray intensities. Intensities of the satellite bands are about 1/20 of those of Kb2 and hence experimental errors are accordingly larger. It is evident that the former has a threshold at around 10373 eV with a single peak at 10375 eV, while the latter has threshold at around 10370 eV with a shoulder at 10372 eV in addition to a peak at around 10375 eV; SSXANES spectra of some kind have been obtained. In Fig. 3, conventional (total fluorescence monitored) fluorescence XANES spectra of pure GaCl3 (Ga3C) and
the present Ga sample (GaCl2CGaCl3: GaCCGa3C) are also shown. The former is the same as the XANES spectrum of GaCl3 in the literature [19] with a single peak at 10375 eV. The latter has two peaks near the threshold; 10372.5 and 10375 eV. Since it has been reported that GaC shows a sharp peak at 10372.5 eV [19], the low energy peak can be attributed to the single-valence Ga ion. The fact that the intensity of this peak in the present sample decreases with irradiation supports the assignment. It is evident from Fig. 3 that the excitation energy dependence of the intensity of the Kb2 band almost overlaps with the fluorescence XANES spectrum of pure GaCl3; XANES of Ga3C has been selectively picked out by monitoring Kb2 emission in the present sample. On the other hand, the excitation energy dependence of the 3d/1s transition intensity closely follows that of total fluorescence XANES, having a shoulder at 10372.5 eV and a distinct peak at 10375 eV. Evidently this satellite transition, unlike Kb2 emission, shows no state selectivity. Since chemical effects of inner shells such as 1s and 3d are small, no marked state selectivity of this satellite is not very surprising. On the other hand, why the Kb 2 shows Ga 3C selectivity is worth consideration, because it is evident that the state sensitivity observed here can not be attributed to chemical shifts. In a simple ionic-crystal picture, the valence electrons of the Ga chlorides can be considered to be ClK 3p. Thus the Ga Kb2 is regarded to arise from the crossover transition [2,24,25] Cl3p/Ga 1s as is shown in Fig. 4. Intensity of such a transition should strongly depend on the interaction between Ga and Cl ions and hence on the bond-lengths between the metal and the ˚ ) is just ligand. In the case of GaCl2, the Ga3C–Cl- length (2.19 A ˚ ) [28], and, therefore, it w2/3 of GaC–Cl- lengths (3.18–3.27 A can be expected that the intensity of the crossover transition of Cl-/Ga3C is much stronger than that of Cl-/GaC. This is a possible explanation on the Ga3C sensitivity of the Kb2 band. Preliminary calculations using DVXa method on Ga3CClK 4 and GaCClK clusters depicted in Fig. 4 indeed indicate that 8 chemical shift of Cl 3p orbital energies is small but that overlaps between Cl 3p and Ga 4p orbitals differ by an order of magnitude
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H. Hayashi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2168–2172
Fig. 4. Schematic energy diagrams and the cluster models for GaC and Ga3C components in GaCl2.
between the two clusters. Thus, the present results suggest a new possibility of SSXANES; even though chemical shift in energy is not sufficient, difference in transition probability can be employed to extract SSXANES. More detailed analysis for the Ga Kb-RIXS in terms of Kramers-Heisenberg together with DVXa calculations is in progress. In summary, it has been demonstrated that outer-shells/1s emissions can now be studied in detail by the use of highsensitive X-ray spectrometer. RIXS analysis in terms of Kramers–Heisenberg equation provide genuine site or state selective XANES. In addition, it was demonstrated that chemical shifts are not necessarily required to obtain stateselective XANES. Acknowledgements The experiments were carried out at SPring-8 under the proposals, No.2002B0475-NX-np, No.R03A47XU-0017N, and No.2003B0143-NXa-np. Part of the present work is supported by MEXT Grant-in Aid for Young Scientists, No.14703012. References [1] F. de Groot, Chem. Rev. 101 (2001) 1779. [2] P. Glatzel, U. Bergmann, Coord. Chem. Rev. 249 (2005) 65. [3] A. Kotani, S. Shin, Rev. Mod. Phys. 73 (2001) 203.
[4] P. Glatzel, U. Bergmann, J. Yano, H. Visser, J.H. Robblee, W. Gu, F.M.F. de Groot, G. Christou, V.L. Pecorano, S.P. Cramer, V.K. Yachandra, J. Am. Chem. Soc. 126 (2004) 9946. [5] W.M. Heijboer, P. Glatzel, K.R. Sawant, R.F. Lobo, U. Bergmann, R.A. Barrea, D.C. Koningsberger, B.M. Weckhuysen, F.M.F. de Groot, J. Phys. Chem. B 108 (2004) 10002. [6] P. Glatzel, U. Bergmann, W. Gu, H. Wang, S. Stepanov, B.S. Mandimutsira, C.G. Riordan, C.P. Horwitz, T. Collins, S.P. Cramer, J. Am. Chem. Soc. 124 (2002) 9668. [7] K. Ha¨ma¨la¨inen, C.-C. Kao, J.B. Hastings, D.P. Siddons, L.E. Berman, V. Stojanoff, S.P. Cramer, Phys. Rev. B 46 (1992) 14274. [8] F.M.F. de Groot, S. Pizzini, A. Fontaine, K. Ha¨ma¨la¨inen, C.-C. Kao, J.B. Hastings, Phys. Rev. B 51 (1995) 1045. [9] Q. Qian, T.A. Tyson, C.-C. Kao, M. Croft, A.Yu. Ignatov, Appl. Phys. Lett. 80 (2002) 3141. [10] Q. Qian, T.A. Tyson, S. Savrassov, C.-C. Kao, M. Croft, Phys. Rev. B 68 (2003) 14429. [11] M.M. Grush, G. Christou, K. Ha¨ma¨la¨inen, S.P. Cramer, J. Am. Chem. Soc. 117 (1995) 5895. [12] Y. Izumi, F. Kiyotaki, H. Nagamori, T. Minato, J. Electron Spectrosc. Relat. Phenom. 119 (2001) 193. [13] P. Glatzel, L. Jacquamet, U. Bergmann, F.M.F. de Groot, S.P. Cramer, Inorg. Chem. 41 (2002) 3121. [14] H. Hayashi, M. Kawata, R. Takeda, Y. Udagawa, Y. Watanabe, T. Takano, S. Nanao, N. Kawamura, J. Electron Spectrosc. Relat. Phenom. 136 (2004) 191. [15] H. Hayashi, R. Takeda, M. Kawata, Y. Udagawa, N. Kawamura, Y. Watanabe, S. Nanao, Phys. Rev. B 70 (2004) 155113. [16] H. Hayashi, R. Takeda, Y. Udagawa, T. Nakamura, H. Miyagawa, H. Shoji, S. Nanao, N. Kawamura, Phys. Rev. B 68 (2003) 45122. [17] H. Hayashi, R. Takeda, M. Kawata, Y. Udagawa, Y. Watanabe, T. Takano, S. Nanao, N. Kawamura, T. Uefuji, K. Yamada, J. Electron Spectrosc. Relat. Phenom. 136 (2004) 199. [18] P.A. Cox, The electronic structure and Chemistry of solids, Oxford University Press, Tokyo, 1987. [19] A.P. Wilkinson, A.K. Cheetham, J. Appl. Cryst. 25 (1992) 654. [20] K. Tsutsumi, H. Nakamori, K. Ichikawa, Phys. Rev. B 13 (1976) 929. [21] C. Suzuki, J. Kawai, H. Adachi, T. Mukoyama, Chem. Phys. 247 (1999) 453. [22] H. Hayashi, M. Kawata, Y. Udagawa, N. Kawamura, S. Nanao, Phys. Rev. B. 70 (2004) 134427. [23] A.V. Soldatov, T.S. Ivanchenko, A.P. Kovtun, S. Della Longa, A. Bianconi, Phys. Rev. B 52 (1995) 11757. [24] U. Bergmann, C.R. Horne, T.J. Collins, J.M. Workman, S.P. Cramer, Chem. Phys. Lett. 302 (1999) 119. [25] U. Bergmann, J. Bendix, P. Glatzel, H.B. Gray, S.P. Cramer, J. Chem. Phys. 116 (2002) 2011. [26] I. Topol, J. Tilgner, G. Leonhardt, A. Meisel, J. Phys. Chem. Solids 35 (1974) 1657. [27] G. Dra¨ger, W. Czolbe, E. Schulz, A. Simunek, J. Drahokoupil, Yu.N. Kucherenko, V.V. Nemoshkalenko, Phys. Stat. Sol. 131 (1985) 193. [28] R.W.G. Wyckoff, R.W.G. Wyckoff, Crystal Structures, 1, Interscience publisher, New York, 1965.
Selective XANES spectroscopy from RIXS contour maps Hisashi Hayashi a,b,*, Masaki Kawata a, Rumi Takeda a, Atsushi Sato a, Yasuo Udagawa a, Naomi Kawamura c, Susumu Nanao d a
IMRAM, Tohoku University, Katahira, Sendai 980-8577, Japan b PRESTO, JST, 4-1-8 Honcho Kawaguchi, Saitama, Japan c JASRI, Mikazuki, Hyogo 679-5198, Japan d IIS, University of Tokyo, Komaba 4-6-1, Meguro, Tokyo 153-8505, Japan
Abstract Kb resonant inelastic X-ray scattering (RIXS) spectra of Mn and Ga compounds are studied to examine different types of selective XANES. RIXS spectra of MnO in Mn Kb1,3 region and of GaCl2 in Ga Kb2 region are plotted as contour maps over wide energy ranges in both excitation and emission. Through analyses of RIXS contour maps by the use of the Kramers–Heisenberg equation, spin-selective XANES spectra are deduced for MnO. In the case of a mixed valence compound GaCl2 selective XANES of Ga3C ion can be obtained by making use of a large difference in transition probabilities. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction X-ray absorption near-edge structure (XANES) spectroscopy is now utilized over wide research-fields as an element-specific characterization method. It is well known that XANES technique measures spectra that average over all the chemical states of an element in the sample. In many instances, e.g. heterogeneous catalyst studies, it is of vital importance to selectively probe different forms of the element. In order to tackle this issue, ‘selective’ XANES spectroscopy has been proposed [1,2], which is based upon chemical effects on X-ray emissions such as chemical shifts and/or appearance of satellites. Selective absorption spectra may be deduced from the selected components of resonant inelastic X-ray scattering (RIXS) [2–6], which we loosely define here as X-ray emissions excited with X-ray energies near absorption edge. Since chemical effects are generally large in outer-shell electrons, RIXS measurements of outershells/empty-core emissions are more effective than those of emissions between inner shells in order to observe selective XANES spectra. However, unfortunately, outer-shells/ empty-core emission intensities are intrinsically low. Thus selective XANES spectroscopy is not well developed yet,
* Corresponding author. Tel.: C81 22 217 5385; fax: C81 22 217 5337. E-mail address: [email protected] (H. Hayashi).
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.09.052
although its importance has been well recognized and several attempts have been carried out so far [1,2,7–13]. Recently, we have developed a high-resolution X-ray spectrometer with large acceptance [14], and applied it to measure RIXS of inner-shells/empty-core emissions for deducing lifetime-broadening-suppressed XANES [15–17]. The present work is an extension of the RIXS studies on inner-shells/empty-core emissions to outer-shells (3p or valence)/empty-core emissions. As examples to examine spin-selective and state-selective XANES, MnO (an antiferromagnet) and GaCl2 (a mixed valence compound, where GaC and Ga3C ions are embedded in different chemical environments [18]) are selected. Through RIXS measurements over wide energy ranges in both excitation and emission to make RIXS contour maps, possibilities of selective XANES spectroscopy are explored. 2. Experimental The experiments were carried out at the BL47XU at SPring-8. In this beamline is a two-crystal Si(111) monochromator. The flux at the sample position was about 1013 photons/s, and typical spot size was 0.5 mm (height)!0.5 mm (width). The multi-crystal spectrometer employed was described in detail elsewhere [14]. In measurements of RIXS in Mn Kb1,3/Ga Kb2 region, the scattered radiation was horizontally dispersed and vertically focused with five cylindrically-bent Ge (440)/Si (555) crystals having a 550/950 mm radius of curvature. Here the solid angle to collect emissions is about 0.06/0.02 sr, where the solid angle per eV is about 9!10K4/2!10K4 sr/eV.
H. Hayashi et al. / Journal of Physics and Chemistry of Solids 66 (2005) 2168–2172
Analyzed X-rays from different crystals were focused on different vertical positions of a two-dimensional positionsensitive proportional counter. All data were taken at room temperature at a constant scattering angle of w908 in the horizontal plane to reduce extraneous non-resonant scatterings. The sampling times were less than 1 h/spectrum. The overall energy resolution in the Mn Kb1,3/Ga Kb2 measurements was 1.4/2.9 eV as determined by the FWHM of elastic lines. MnO powder was purchased from Kanto Kagaku, and was employed after compressing into a pellet. GaCl2 powder was obtained from Sigma-Aldrich, and was packed between two 50 mm-thick Kapton windows under a dry Ar atmosphere. For checking the sample status, fluorescence XANES spectra were measured by using a PIN-photo diode. As shown later, the white-line feature is clearly split into two components separated by about 3 eV. The low- and the high-energy component have been assigned to transitions associated with GaC and Ga3C, respectively [19], but the intensity ratio of the split white-lines was different from that of pure GaCl2, suggesting that the present sample is a mixture of GaCl2 and GaCl3 (w1:1). The intensity ratio of the two changed with irradiation time, and after 10 h, the XANES profile became almost the same as that of GaCl3, that is, oxidation proceeds quickly with irradiation. Hence the beam position on the Ga sample was changed every 1 h, and always XANES was observed before and after a RIXS measurement in order to ensure that the part newly moved had not been damaged by irradiation and that the sample degradation was not so large as to prevent the analyses described later. 3. Results and discussion 3.1. Spin-polarized XANES Spin-polarized XANES (SPXANES) gives a direct approach to resolve the spin dependence of excited electronic states of non-ferromagnetic materials. It is based on an assumption that one can separate the Kb1,3 (3p/1s) emission spectrum into an internally referenced spin-up part and a spindown part [3,20,21], and hence one can achieve a local-spin selectivity in the K-edge XANES by measuring the emission from either the main (spin-down, Kb1,3) or satellite (spin-up, Kb 0 ) lines as a function of incident X-ray energy. Fig. 1(a) shows a Kb1,3 (spin-down)- and Kb 0 (spin-up)RIXS contour map of MnO [14,22], where the abscissa (u1) and the ordinate (u2) are excitation and emission energies, respectively. Here the 1s/3d resonant excitation appears as an island, and an existence of two prominent ridges extending parallel to the abscissa, indicated by broken lines, is evident. The energy u2 of each ridge is the same as that of regular Kb1,3 and Kb 0 emission. In addition, three ridges stretching diagonally, indicated by chains, are also conspicuous. By traversing the contour map along a constant u2 axis one obtains an excitation spectrum at the u2, which is equivalent to measuring excitation spectra while monitoring the part of fluorescence spectra, the conventional method to observe SPXANES [7–10]. Shown in Fig. 1(b) are two such excitation
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spectra obtained along the lines u2Z6491 eV (Kb1,3 peak) and u2Z6476 eV (Kb 0 peak), which are essentially the same as the SPXANES of MnO reported previously [7]. Here, non-vanishing components are noted in the energy region of 1s/3d quadrupolar transition in the spin-up polarized spectrum. Since all the five 3d-electrons of Mn2C in MnO are known to be spin-up, the transition is forbidden from Hund’s rule. This casts some doubts on the validity of the underlying assumption of the conventional SPXANES method. In addition, theoretical calculation of SPXANES on Mn K-edge of MnO by Soldatov et al. has shown a significant discrepancy from the observed spectra for both spin components [23]. In order to make full use of the information incorporated in RIXS spectra, instead of one-dimensional scans along some horizontal ridges, whole Kb-RIXS contour map should be analyzed by the following equation based on the Kramers– Heisenberg formula [22]; ð up down dsðu1 Þ ðU1s C uÞfðdgup C ðdgdown g 1s =duÞf 1s =duÞf f du 2 du2 U1s C uKu1 C G21s =4Z2 (1) Here Zu refers to the energy of the excited electron in the intermediate state, and G1s is the widths of the 1s levels, the down energy of which is represented by ZUls. dgup 1s =du and dg1s =du are spin-dependent oscillator strength distributions for spin-up and spin-down 1s electrons, respectively. The shape of the Kb1,3-RIXS spectra is determined also by final state effects, such as lifetimes and multiplet structure. In Eq. (1), they are taken into consideration by functions fup and fdown. The instrumental resolution function has been convoluted in the fup down and fdown. Starting from assumed dgup 1s =du and dg1s =du, which are often observed absorption spectra, calculations of RIXS spectra are iterated by the use of Eq. (1) by modifying down dgup 1s =du and dg1s =du until satisfactory agreement is reached between observed and calculated RIXS spectra at every excitation energy. Analyses of the contour map in terms of Eq. (1) have already been described in detail [22], and hence only the results are down shown in Fig. 1(c) as the ‘best-fit’ dgup 1s =du and dg1s =du. Both curves are considerably different from the excitation spectra obtained by monitoring Kb 0 or Kb1,3 bands in Fig. 1(b). Major features are labeled; 1s/3d (pre-edge peak), A and B (shoulders), C and E (peaks), and D (valley). It is evident that 1s/3d quadrupole transition is completely missing in the dgup 1s =du, as is expected from Hund’s rule. It is immediately understood from the RIXS map of Fig. 1(a) that the nonvanishing intensity apparent in the Kb 0 excitation spectrum is due to the diagonal component which originates from the main peak of the Kb1,3 ridge. Although not shown here, every feature down of the best fit dgup 1s =du and dg1s =du has its counterpart in the SPXANES spectra calculated by Soldatov et al. [23]. These results indicate that analyses of RIXS maps in terms of Kramers–Heisenberg equation are crucial for correct determination of SPXANES.
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Fig. 1. (a) A Kb-RIXS contour map of MnO; emission intensities as a function of excitation (u1) and emission (u2) energies are plotted. Two horizontal and three diagonal ridges are indicated by broken lines and chains, respectively. (b) Cross sections of the map at u2Z6476 eV (solid squares) and u2Z6491 eV (open circles). (c) The best-fit dg1s/du’s (SPXANES) obtained by the analysis using Eq. (1) [22].
3.2. State-selective XANES In order to obtain state-selective XANES (SSXANES), desirable are accurate measurements of RIXS of valence/ empty-core emissions [1,2,24,25], which exhibit large chemical effects. However, intrinsic low intensities of these transitions have made valence-RIXS experiments extremely difficult. Thus inner shell/1s emissions, which show only small chemical shift (e.g. Mn Kb1,3 [11], Cu Ka1 [12], and Fe Kb1,3 [13]), should have been employed in previous SSXANES studies at a sacrifice of state-selectivity. A use of efficient multi-crystal spectrometer equipped with a twodimensional detector can alleviate problems associated with
low intensities and makes it possible to challenge the SSXANES by the use of valence/empty-core emissions. Fig. 2(a) shows a Kb2 RIXS contour map of the present Ga sample, a mixture of GaCl2 (GaCCGa3C compound) and GaCl3 (Ga3C compound), where the abscissa and ordinate are the same as those in Fig. 1(a). Similar to Fig. 1(a), an existence of two ridges extending parallel to the abscissa is evident. One is a prominent ridge at u2Z10365 eV that corresponds to the energy of Ga Kb2 (valence/1s) [26,27], and another, less prominent one is at u2Z10350 eV, which corresponds to 3d/1s transition energy [26]. They are indicated by broken lines in the figure. In addition, two ridges stretching diagonally, indicated by chains, can also be observed. The existence of these diagonal ridges
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Fig. 3. Comparisons of integrated intensities of the Ga Kb2 (valence/1s) and the low-energy satellite (3d/1s) with fluorescence XANES spectra of GaCl2CGaCl3 (GaCCGa3C) and GaCl3 (Ga3C). Here the satellite intensity is multiplied by 22.
Fig. 2. (a) A Kb-RIXS contour map and (b) RIXS spectra of the Ga chloride studied here.
clearly demonstrates that, as in the case of MnO, fluorescence excitation spectra obtained by monitoring a part of fluorescence spectrum are not adequate to extract SSXANES and that analyses in terms of Kramers–Heisenberg equation are required. Here, however, a more versatile method is examined as described below. The Kb-RIXS spectra constituting the RIXS map are plotted in Fig. 3(b), which tell us that relative intensities of Kb2 versus 3d/1s satellite band vary with excitation energy. In Fig. 3, integrated intensities of the Kb2 and the satellite bands are plotted against u1. Normalization was made by the use of the integrated excitation X-ray intensities. Intensities of the satellite bands are about 1/20 of those of Kb2 and hence experimental errors are accordingly larger. It is evident that the former has a threshold at around 10373 eV with a single peak at 10375 eV, while the latter has threshold at around 10370 eV with a shoulder at 10372 eV in addition to a peak at around 10375 eV; SSXANES spectra of some kind have been obtained. In Fig. 3, conventional (total fluorescence monitored) fluorescence XANES spectra of pure GaCl3 (Ga3C) and
the present Ga sample (GaCl2CGaCl3: GaCCGa3C) are also shown. The former is the same as the XANES spectrum of GaCl3 in the literature [19] with a single peak at 10375 eV. The latter has two peaks near the threshold; 10372.5 and 10375 eV. Since it has been reported that GaC shows a sharp peak at 10372.5 eV [19], the low energy peak can be attributed to the single-valence Ga ion. The fact that the intensity of this peak in the present sample decreases with irradiation supports the assignment. It is evident from Fig. 3 that the excitation energy dependence of the intensity of the Kb2 band almost overlaps with the fluorescence XANES spectrum of pure GaCl3; XANES of Ga3C has been selectively picked out by monitoring Kb2 emission in the present sample. On the other hand, the excitation energy dependence of the 3d/1s transition intensity closely follows that of total fluorescence XANES, having a shoulder at 10372.5 eV and a distinct peak at 10375 eV. Evidently this satellite transition, unlike Kb2 emission, shows no state selectivity. Since chemical effects of inner shells such as 1s and 3d are small, no marked state selectivity of this satellite is not very surprising. On the other hand, why the Kb 2 shows Ga 3C selectivity is worth consideration, because it is evident that the state sensitivity observed here can not be attributed to chemical shifts. In a simple ionic-crystal picture, the valence electrons of the Ga chlorides can be considered to be ClK 3p. Thus the Ga Kb2 is regarded to arise from the crossover transition [2,24,25] Cl3p/Ga 1s as is shown in Fig. 4. Intensity of such a transition should strongly depend on the interaction between Ga and Cl ions and hence on the bond-lengths between the metal and the ˚ ) is just ligand. In the case of GaCl2, the Ga3C–Cl- length (2.19 A ˚ ) [28], and, therefore, it w2/3 of GaC–Cl- lengths (3.18–3.27 A can be expected that the intensity of the crossover transition of Cl-/Ga3C is much stronger than that of Cl-/GaC. This is a possible explanation on the Ga3C sensitivity of the Kb2 band. Preliminary calculations using DVXa method on Ga3CClK 4 and GaCClK clusters depicted in Fig. 4 indeed indicate that 8 chemical shift of Cl 3p orbital energies is small but that overlaps between Cl 3p and Ga 4p orbitals differ by an order of magnitude
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Fig. 4. Schematic energy diagrams and the cluster models for GaC and Ga3C components in GaCl2.
between the two clusters. Thus, the present results suggest a new possibility of SSXANES; even though chemical shift in energy is not sufficient, difference in transition probability can be employed to extract SSXANES. More detailed analysis for the Ga Kb-RIXS in terms of Kramers-Heisenberg together with DVXa calculations is in progress. In summary, it has been demonstrated that outer-shells/1s emissions can now be studied in detail by the use of highsensitive X-ray spectrometer. RIXS analysis in terms of Kramers–Heisenberg equation provide genuine site or state selective XANES. In addition, it was demonstrated that chemical shifts are not necessarily required to obtain stateselective XANES. Acknowledgements The experiments were carried out at SPring-8 under the proposals, No.2002B0475-NX-np, No.R03A47XU-0017N, and No.2003B0143-NXa-np. Part of the present work is supported by MEXT Grant-in Aid for Young Scientists, No.14703012. References [1] F. de Groot, Chem. Rev. 101 (2001) 1779. [2] P. Glatzel, U. Bergmann, Coord. Chem. Rev. 249 (2005) 65. [3] A. Kotani, S. Shin, Rev. Mod. Phys. 73 (2001) 203.
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