- Email: [email protected]
Ultraviolet photoelectron spectroscopy of multiple atoms encapsulated fullerenes
Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 239–242
Ultraviolet photoelectron spectroscopy of multiple atoms encapsulated fullerenes S. Hino a, b, ∗ , N. Wanita b , M. Kato b , K. Iwasaki a , D. Yoshimura c, d , T. Inoue e , T. Okazaki e , H. Shinohara e b
a Faculty of Engineering, Chiba University, Chiba 263-8522, Japan Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan c Institute for Molecular Science, Okazaki 444-8585, Japan d Research Center for Material Science, Nagoya University, Nagoya 464-8602, Japan e Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan
Available online 12 February 2005
Abstract Ultraviolet photoelectron spectra (UPS) of metallofullerenes with multiple atoms inside the cage, Ti2 @C80 , Ti2 @C84 and Y2 C2 @C82 , have been measured with a synchrotron orbital radiation light source. The spectra were compared with those of other metallofullerenes. Spectral onsets of these metallofullerenes are 0.7–0.8 eV, which is close to those of group II metal atom encapsulated metallofullerenes but much larger than those of other group III mono-metal atom encapsulated ones. The spectra are compared with molecular orbital calculation to estimate the amounts of transferred electrons and the cage structure. © 2005 Elsevier B.V. All rights reserved. Keywords: Photoelectron emission; Electronic structure; Fullerenes; Metallofullerenes
1. Introduction Since an existence of fullerenes containing metal atoms inside the cage is proposed [1], a series of fullerenes encapsulated metal atom(s) has been reported [2]. Except for Ti, Ta and Hf, encapsulated metal atoms are groups II and III including lanthanides and actinides. Upon encapsulation of the metal atoms, the electronic structure of fullerene cages suffers large change and electrons are transferred from the metal atoms to the cage. Determination of cage structure as well as the amounts of transferred electrons is an important issue in science of metallofullerene.The amounts of transferred electrons depend on the metal species. Group II metal atom in Ca@C82 or Ba@C82 gives two electrons whereas group III elements in M@C2n (M represents metal species) fullerenes give various amounts of electrons; two electrons in Sc@C82 but three in La@C82 or Gd@C82 . As for lanthanides and actinides, the situation is more complicated; from two to four ∗
Corresponding author. E-mail address: [email protected] (S. Hino).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.069
electrons are transferred. When multiple atoms are entrapped in the fullerene cage, estimation of the amounts of transferred electrons could be more complicated. Rietveld/maximum entropy method (MEM) analysis on X-ray diffraction pattern of metallofullerenes [3,4] could give information on the cage structure and the amounts of transferred electrons, but this method is not decisive because it depends on the starting carbon cages. Ultraviolet photoelectron spectroscopy combined with molecular orbital calculation has been helpful to estimate the amounts of transferred electrons [5,6] and the cage structure [7]. In this article, we will present UPS of multiple metal atom encapsulated fullerenes and compare them with those of other metallofullerenes and theoretically calculated simulation spectra.
2. Experimental Multiple atoms encapsulated metallofullerenes were synthesized by arc heating of graphite and metal composite
240
S. Hino et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 239–242
rods and isolated with multiple-stage high performance liquid chromatography (HPLC) separation method. Details are described elsewhere [8]. UPS were measured at Ultraviolet Synchrotron Orbital Radiation facility (UVSOR) beam line BL8B2, at Institute for Molecular Science, Japan. Metallofullerenes were vacuum deposited onto gold-coated Mo disks and their sublimation temperature was ranging between 800 and 920 K. Resolution of the spectra was 100 meV and the spectra were referred to the Fermi level.
photoelectron spectra, and this region could be used as a fingerprint of fullerenes. Principally, the UPS of these M@C82 s resemble; the difference among them is a small structure just below the Fermi level resulted from electron transfer from the entrapped metal atom to the cage. The UPS of M@C82 s also resembles to those of hollow cage C82 [9]. On the other hand, the UPS of Ti2 @C80 and Ti2 @C84 are quite different from those of M@C82 . This suggests that the electronic structures of these metallofullerenes differ so much.
3. Comparison of the UPS 4. Ti2 @C80 and Ti2 @C84 The UPS of metallofullerenes, M@C82 , Ti2 @C80 and Ti2 @C84 , are summarized in Fig. 1. Spectra onsets of the metallofullerenes vary from 0.2 eV (La@C82 ) to 0.8 eV (Ti2 @C84 ). The onset energy of trivalent metal atom encapsulated M@C82 is small whereas that of divalent metal is large. This could be understood as follows: three additional electrons on the C82 cage form an open shell molecule and unpaired electron is not stabilized so much, but two electrons are to occupy the LUMO of C82 and induces small change on the electronic structure of the cage. The onset energy of two metal atoms encapsulated metallofullerenes is almost equal to that of divalent metal encapsulated ones, which indicate that two or even number electrons are transferred to the cage. The upper valence region between 0 and 4.5 eV in the UPS are due to pseudo -electrons of the fullerene cage. This region is characteristic energy range for fullerenes
An NMR analysis of Ti2 @C80 [10] suggests that this metallofullerene could be a mixture of two isomers having symmetry of Ih and D3h with 1:3 ratio. Molecular orbital calculation using Fijutsu Win MOPAC program module (PM3 parameterisation) has been carried out on these geometry with varying the amounts of electrons on the fullerene cage. A simulated spectrum obtained by adding two spectra derived from Ih and D5h geometry with 1:3 ratio reproduces the UPS fairly well, which is in favour of the NMR analysis described above. However, the NMR spectrum could be also interpreted as a C2v –C78 fullerene with carbide type Ti2 C2 inside. A simulated spectrum obtained from the firstprinciple calculation using C2v geometry with additional four electrons on the cage also reproduces the observed one fairly well [11]. Therefore, the cage structure of Ti2 @C80 is not determined yet. Only the amounts of transferred electrons, namely four electrons, could be determined for the moment. As no NMR spectrum of two Ti2 @C84 isomers has been measured, there is no clue on the cage symmetry. Molecular orbital calculation on all 24 isolated pentagon rule satisfying C84 isomers with excess four, six or eight electrons on the cage has been carried out. The simulated spectra calculated from C1 (#12) and D2 (#21) structures with additional four electrons show no bad correspondence with the observed ones of isomers I and II, respectively, although it is not satisfactory.
5. Y2 C2 @C82
Fig. 1. Comparison of the UPS of metallofullerenes. The incident photon energy is 40 eV.
Fig. 2 shows incident photon energy dependent photoelectron spectra of Y2 C2 @C82 . Spectral onset is 0.75 eV below the Fermi level, which seems to indicate that even electrons are transferred to the cage as other multiple atoms encapsulated metallofullerenes. There are three structures between 0 and 4.5 eV and this is quite different from the UPS of other M@C82 fullerenes. Structures below 4.5 eV are mainly due to -electrons that constitute fullerene skeletal structure. The spectra of this region are more or less the same as those of other metallofullerenes; a distinct peak at about 5.5 eV and another one at 7.5 eV are easily
S. Hino et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 239–242
Fig. 2. The incident photon energy dependence of the UPS of Y2 C2 @C82 .
recognizable. Spectral intensity of each structure changes when the incident photon energy is changed. This is typical spectral response of fullerene cage structures. Absorption spectra of Y2 C2 @C82 and Sc2 @C84 (III) are virtually the same, although an NMR analysis on Y2 C2 @C82 indicates C3v –C82 symmetry and that on Sc@C84 (III) indicates D2d –C84 symmetry [8]. Fig. 3 shows the UPS of Sc2 @C84 (III) [12], Y2 C2 @C82 and hollow cage C84 [13].
241
The spectrum of C84 differs from those of two metallofullerenes, but the spectra of Sc2 @C84 (III) and Y2 C2 @C82 are almost identical. Resemblance is due to their analogous electronic structure derived from the same cage structure and the same amounts of transferred electrons. Probably, the NMR measurement or analysis of Sc2 @C84 (III) was not sufficient to deduce the cage symmetry; the cage symmetry of both metallofullerenes could be C3v as was proposed for Y2 C2 @C82 , and an actual formula of Sc2 @C84 might be Sc2 C2 @C82 . Molecular orbital calculation of two C3v –C82 cages with four and six excess electrons on them was performed. A simulated spectrum obtained from C3v –C82 4− (No. 8) cage structure gives reasonably good correspondence with the observed one. That is, each Y atom seems to give two electrons to the cage. The oxidation state of Sc in Sc2 @C84 (III) (Sc2 C2 @C82 ) was estimated to be +2.6 from an EELS measurement [12]. Y in Y2 C2 @C82 could be much easily ionized than Sc so that the oxidation state of Y could be around +3. This is contradicting to the deduction on the amounts of electron on the fullerene cage described above. Present results may suggest that either Y atoms give electrons to encapsulated carbon atoms or Y atoms form tight bonds with the entrapped carbon atoms.
Acknowledgements This study was a joint research program of UVSOR and Institute for Molecular Science. This work was financially supported from a Grant-in-Aid for Scientific Research on Priority Areas of Molecular Conductors (No. 15073203) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
References
Fig. 3. Comparison of the UPS of hollow cage C84 [13], C82 [14], Y2 C2 @C82 and Sc2 @C84 [8].
[1] Y. Chai, T. Guo, C. Gin, R.E. Haufler, L.P.F. Chibante, J. Fure, L. Wang, J.M. Alford, R.E. Smalley, J. Phys. Chem. 95 (1991) 7564. [2] For example, a review by H. Shinohara, Rep. Prog. Phys. 63 (2000) 843. [3] M. Takata, E. Nishibori, B. Umeda, M. Sakata, E. Yamamoto, H. Shinohara, Phys. Rev. Lett. 78 (1997) 3330. [4] C.-R. Wang, T. Kai, T. Tomi-yama, T. Yoshida, Y. Kobayashi, E. Nishibori, M. Takata, M. Sakata, H. Shinohara, Angew. Chem. Int. Ed. 40 (2001) 397. [5] S. Hino, H. Takahashi, K. Iwasaki, K. Matsumoto, T. Miyazaki, S. Hasegawa, K. Kikuchi, Y. Achiba, Phys. Rev. Lett. 71 (1993) 4261. [6] S. Hino, K. Umishita, K. Iwasaki, T. Miyamae, M. Inakuma, H. Shinohara, Chem. Phys. Lett. 281 (1997) 115. [7] S. Hino, K. Umishita, K. Iwasaki, M. Aoki, K. Kobayashi, S. Nagase, T. John, S. Dennis, T. Nakane, H. Shinohara, Chem. Phys. Lett. 227 (2001) 65. [8] T. Inoue, T. Tomiyama, T. Sugai, H. Shinohara, Chem. Phys. Lett. 382 (2003) 226, and references therein. [9] S. Hino, K. Matsumoto, S. Hasegawa, K. Iwasaki, K. Yakushi, T. Morikawa, T. Takahashi, K. Seki, K. Kikuchi, S. Suzuki, I. Ikemoto, Y. Achiba, Phys. Rev. B 48 (1993) 8418.
242
S. Hino et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 239–242
[10] B. Cao, M. Hasegawa, K. Okada, T. Tomiyama, T. Okazaki, K. Suenaga, H. Shinohara, J. Am. Chem. Soc. 123 (2001) 9679. [11] M. Otani, S. Okada, A. Oshiyama, 28th Fullerrene-Nanotubes General Symposium, January 7–9, 2005, Nagoya, Aichi, Japan. [12] T. Pichler, Z. Hu, C. Grazioli, S. Legner, M. Knupfer, M.S. Golden, J. Fink, F.M.F. de Groot, M.R.C. Hunt, P. Rudolf, R. Follath, Ch.
Jung, L. Kjeldgaard, P. Br¨uhwiler, M. Inakuma, H. Shinohara, Phys. Rev. B 62 (2000) 13196. [13] S. Hino, K. Matsumoto, S. Hasegawa, K. Kamiya, H. Inokuchi, T. Morikawa, T. Takahashi, K. Seki, K. Kikuchi, S. Suzuki, I. Ikemoto, Y. Achiba, Chem. Phys. Lett. 190 (1992) 169. [14] S. Hino, K. Matsumoto, S. Hasegawa, K. Iwasaki, K. Yakushi, T. Morikawa, T. Takahashi, K. Seki, K. Kikuchi, S. Suzuki, I. Ikemoto, Y. Achiba, Phys. Rev. B 48 (1993) 8418.
Ultraviolet photoelectron spectroscopy of multiple atoms encapsulated fullerenes S. Hino a, b, ∗ , N. Wanita b , M. Kato b , K. Iwasaki a , D. Yoshimura c, d , T. Inoue e , T. Okazaki e , H. Shinohara e b
a Faculty of Engineering, Chiba University, Chiba 263-8522, Japan Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan c Institute for Molecular Science, Okazaki 444-8585, Japan d Research Center for Material Science, Nagoya University, Nagoya 464-8602, Japan e Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan
Available online 12 February 2005
Abstract Ultraviolet photoelectron spectra (UPS) of metallofullerenes with multiple atoms inside the cage, Ti2 @C80 , Ti2 @C84 and Y2 C2 @C82 , have been measured with a synchrotron orbital radiation light source. The spectra were compared with those of other metallofullerenes. Spectral onsets of these metallofullerenes are 0.7–0.8 eV, which is close to those of group II metal atom encapsulated metallofullerenes but much larger than those of other group III mono-metal atom encapsulated ones. The spectra are compared with molecular orbital calculation to estimate the amounts of transferred electrons and the cage structure. © 2005 Elsevier B.V. All rights reserved. Keywords: Photoelectron emission; Electronic structure; Fullerenes; Metallofullerenes
1. Introduction Since an existence of fullerenes containing metal atoms inside the cage is proposed [1], a series of fullerenes encapsulated metal atom(s) has been reported [2]. Except for Ti, Ta and Hf, encapsulated metal atoms are groups II and III including lanthanides and actinides. Upon encapsulation of the metal atoms, the electronic structure of fullerene cages suffers large change and electrons are transferred from the metal atoms to the cage. Determination of cage structure as well as the amounts of transferred electrons is an important issue in science of metallofullerene.The amounts of transferred electrons depend on the metal species. Group II metal atom in Ca@C82 or Ba@C82 gives two electrons whereas group III elements in M@C2n (M represents metal species) fullerenes give various amounts of electrons; two electrons in Sc@C82 but three in La@C82 or Gd@C82 . As for lanthanides and actinides, the situation is more complicated; from two to four ∗
Corresponding author. E-mail address: [email protected] (S. Hino).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.069
electrons are transferred. When multiple atoms are entrapped in the fullerene cage, estimation of the amounts of transferred electrons could be more complicated. Rietveld/maximum entropy method (MEM) analysis on X-ray diffraction pattern of metallofullerenes [3,4] could give information on the cage structure and the amounts of transferred electrons, but this method is not decisive because it depends on the starting carbon cages. Ultraviolet photoelectron spectroscopy combined with molecular orbital calculation has been helpful to estimate the amounts of transferred electrons [5,6] and the cage structure [7]. In this article, we will present UPS of multiple metal atom encapsulated fullerenes and compare them with those of other metallofullerenes and theoretically calculated simulation spectra.
2. Experimental Multiple atoms encapsulated metallofullerenes were synthesized by arc heating of graphite and metal composite
240
S. Hino et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 239–242
rods and isolated with multiple-stage high performance liquid chromatography (HPLC) separation method. Details are described elsewhere [8]. UPS were measured at Ultraviolet Synchrotron Orbital Radiation facility (UVSOR) beam line BL8B2, at Institute for Molecular Science, Japan. Metallofullerenes were vacuum deposited onto gold-coated Mo disks and their sublimation temperature was ranging between 800 and 920 K. Resolution of the spectra was 100 meV and the spectra were referred to the Fermi level.
photoelectron spectra, and this region could be used as a fingerprint of fullerenes. Principally, the UPS of these M@C82 s resemble; the difference among them is a small structure just below the Fermi level resulted from electron transfer from the entrapped metal atom to the cage. The UPS of M@C82 s also resembles to those of hollow cage C82 [9]. On the other hand, the UPS of Ti2 @C80 and Ti2 @C84 are quite different from those of M@C82 . This suggests that the electronic structures of these metallofullerenes differ so much.
3. Comparison of the UPS 4. Ti2 @C80 and Ti2 @C84 The UPS of metallofullerenes, M@C82 , Ti2 @C80 and Ti2 @C84 , are summarized in Fig. 1. Spectra onsets of the metallofullerenes vary from 0.2 eV (La@C82 ) to 0.8 eV (Ti2 @C84 ). The onset energy of trivalent metal atom encapsulated M@C82 is small whereas that of divalent metal is large. This could be understood as follows: three additional electrons on the C82 cage form an open shell molecule and unpaired electron is not stabilized so much, but two electrons are to occupy the LUMO of C82 and induces small change on the electronic structure of the cage. The onset energy of two metal atoms encapsulated metallofullerenes is almost equal to that of divalent metal encapsulated ones, which indicate that two or even number electrons are transferred to the cage. The upper valence region between 0 and 4.5 eV in the UPS are due to pseudo -electrons of the fullerene cage. This region is characteristic energy range for fullerenes
An NMR analysis of Ti2 @C80 [10] suggests that this metallofullerene could be a mixture of two isomers having symmetry of Ih and D3h with 1:3 ratio. Molecular orbital calculation using Fijutsu Win MOPAC program module (PM3 parameterisation) has been carried out on these geometry with varying the amounts of electrons on the fullerene cage. A simulated spectrum obtained by adding two spectra derived from Ih and D5h geometry with 1:3 ratio reproduces the UPS fairly well, which is in favour of the NMR analysis described above. However, the NMR spectrum could be also interpreted as a C2v –C78 fullerene with carbide type Ti2 C2 inside. A simulated spectrum obtained from the firstprinciple calculation using C2v geometry with additional four electrons on the cage also reproduces the observed one fairly well [11]. Therefore, the cage structure of Ti2 @C80 is not determined yet. Only the amounts of transferred electrons, namely four electrons, could be determined for the moment. As no NMR spectrum of two Ti2 @C84 isomers has been measured, there is no clue on the cage symmetry. Molecular orbital calculation on all 24 isolated pentagon rule satisfying C84 isomers with excess four, six or eight electrons on the cage has been carried out. The simulated spectra calculated from C1 (#12) and D2 (#21) structures with additional four electrons show no bad correspondence with the observed ones of isomers I and II, respectively, although it is not satisfactory.
5. Y2 C2 @C82
Fig. 1. Comparison of the UPS of metallofullerenes. The incident photon energy is 40 eV.
Fig. 2 shows incident photon energy dependent photoelectron spectra of Y2 C2 @C82 . Spectral onset is 0.75 eV below the Fermi level, which seems to indicate that even electrons are transferred to the cage as other multiple atoms encapsulated metallofullerenes. There are three structures between 0 and 4.5 eV and this is quite different from the UPS of other M@C82 fullerenes. Structures below 4.5 eV are mainly due to -electrons that constitute fullerene skeletal structure. The spectra of this region are more or less the same as those of other metallofullerenes; a distinct peak at about 5.5 eV and another one at 7.5 eV are easily
S. Hino et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 239–242
Fig. 2. The incident photon energy dependence of the UPS of Y2 C2 @C82 .
recognizable. Spectral intensity of each structure changes when the incident photon energy is changed. This is typical spectral response of fullerene cage structures. Absorption spectra of Y2 C2 @C82 and Sc2 @C84 (III) are virtually the same, although an NMR analysis on Y2 C2 @C82 indicates C3v –C82 symmetry and that on Sc@C84 (III) indicates D2d –C84 symmetry [8]. Fig. 3 shows the UPS of Sc2 @C84 (III) [12], Y2 C2 @C82 and hollow cage C84 [13].
241
The spectrum of C84 differs from those of two metallofullerenes, but the spectra of Sc2 @C84 (III) and Y2 C2 @C82 are almost identical. Resemblance is due to their analogous electronic structure derived from the same cage structure and the same amounts of transferred electrons. Probably, the NMR measurement or analysis of Sc2 @C84 (III) was not sufficient to deduce the cage symmetry; the cage symmetry of both metallofullerenes could be C3v as was proposed for Y2 C2 @C82 , and an actual formula of Sc2 @C84 might be Sc2 C2 @C82 . Molecular orbital calculation of two C3v –C82 cages with four and six excess electrons on them was performed. A simulated spectrum obtained from C3v –C82 4− (No. 8) cage structure gives reasonably good correspondence with the observed one. That is, each Y atom seems to give two electrons to the cage. The oxidation state of Sc in Sc2 @C84 (III) (Sc2 C2 @C82 ) was estimated to be +2.6 from an EELS measurement [12]. Y in Y2 C2 @C82 could be much easily ionized than Sc so that the oxidation state of Y could be around +3. This is contradicting to the deduction on the amounts of electron on the fullerene cage described above. Present results may suggest that either Y atoms give electrons to encapsulated carbon atoms or Y atoms form tight bonds with the entrapped carbon atoms.
Acknowledgements This study was a joint research program of UVSOR and Institute for Molecular Science. This work was financially supported from a Grant-in-Aid for Scientific Research on Priority Areas of Molecular Conductors (No. 15073203) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
References
Fig. 3. Comparison of the UPS of hollow cage C84 [13], C82 [14], Y2 C2 @C82 and Sc2 @C84 [8].
[1] Y. Chai, T. Guo, C. Gin, R.E. Haufler, L.P.F. Chibante, J. Fure, L. Wang, J.M. Alford, R.E. Smalley, J. Phys. Chem. 95 (1991) 7564. [2] For example, a review by H. Shinohara, Rep. Prog. Phys. 63 (2000) 843. [3] M. Takata, E. Nishibori, B. Umeda, M. Sakata, E. Yamamoto, H. Shinohara, Phys. Rev. Lett. 78 (1997) 3330. [4] C.-R. Wang, T. Kai, T. Tomi-yama, T. Yoshida, Y. Kobayashi, E. Nishibori, M. Takata, M. Sakata, H. Shinohara, Angew. Chem. Int. Ed. 40 (2001) 397. [5] S. Hino, H. Takahashi, K. Iwasaki, K. Matsumoto, T. Miyazaki, S. Hasegawa, K. Kikuchi, Y. Achiba, Phys. Rev. Lett. 71 (1993) 4261. [6] S. Hino, K. Umishita, K. Iwasaki, T. Miyamae, M. Inakuma, H. Shinohara, Chem. Phys. Lett. 281 (1997) 115. [7] S. Hino, K. Umishita, K. Iwasaki, M. Aoki, K. Kobayashi, S. Nagase, T. John, S. Dennis, T. Nakane, H. Shinohara, Chem. Phys. Lett. 227 (2001) 65. [8] T. Inoue, T. Tomiyama, T. Sugai, H. Shinohara, Chem. Phys. Lett. 382 (2003) 226, and references therein. [9] S. Hino, K. Matsumoto, S. Hasegawa, K. Iwasaki, K. Yakushi, T. Morikawa, T. Takahashi, K. Seki, K. Kikuchi, S. Suzuki, I. Ikemoto, Y. Achiba, Phys. Rev. B 48 (1993) 8418.
242
S. Hino et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 239–242
[10] B. Cao, M. Hasegawa, K. Okada, T. Tomiyama, T. Okazaki, K. Suenaga, H. Shinohara, J. Am. Chem. Soc. 123 (2001) 9679. [11] M. Otani, S. Okada, A. Oshiyama, 28th Fullerrene-Nanotubes General Symposium, January 7–9, 2005, Nagoya, Aichi, Japan. [12] T. Pichler, Z. Hu, C. Grazioli, S. Legner, M. Knupfer, M.S. Golden, J. Fink, F.M.F. de Groot, M.R.C. Hunt, P. Rudolf, R. Follath, Ch.
Jung, L. Kjeldgaard, P. Br¨uhwiler, M. Inakuma, H. Shinohara, Phys. Rev. B 62 (2000) 13196. [13] S. Hino, K. Matsumoto, S. Hasegawa, K. Kamiya, H. Inokuchi, T. Morikawa, T. Takahashi, K. Seki, K. Kikuchi, S. Suzuki, I. Ikemoto, Y. Achiba, Chem. Phys. Lett. 190 (1992) 169. [14] S. Hino, K. Matsumoto, S. Hasegawa, K. Iwasaki, K. Yakushi, T. Morikawa, T. Takahashi, K. Seki, K. Kikuchi, S. Suzuki, I. Ikemoto, Y. Achiba, Phys. Rev. B 48 (1993) 8418.