Penning ionization electron and ultraviolet photoelectron spectroscopy of ultrathin bis(l,2-benzoquinonedioximato)platinum(II) films

Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 351–356

Penning ionization electron and ultraviolet photoelectron spectroscopy of ultrathin bis(l,2-benzoquinonedioximato)platinum(II) films Hiroyuki Ozaki a,∗ , Masanori Suhara a , Mai Taki a , Kouki Akaike a , Osamu Endo a , Keiki Takeda b , Ichimin Shirotani b a

Department of Organic and Polymer Materials Chemistry, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan b Division of Electrical and Electronic Engineering, Muroran Institute of Technology, Muroran, Hokkaido 050-8585, Japan Available online 30 November 2006

Abstract ˚ bis(1,2Penning ionization electron spectra (PIES) and ultraviolet photoelectron spectra (UPS) have been measured for ultrathin (3–10 A) benzoquinonedioximato)platinum(II) (Pt(bqd)2 ) films to investigate the molecular aggregation and electronic structure. Upon vapor deposition onto a cooled graphite (0 0 0 1) surface, Pt(bqd)2 forms either of two films A and B with completely different spectra. The PIES of film A exhibits enhanced bands due to ␲ and platinum 5d⊥ (dxz , dyz or dz2 )-derived MOs while that of film B is characterized by overwhelming bands due to oxygen nonbonding MOs, which indicates that the molecule lies flat in film A but the molecular short axis stands in film B. Moreover, the UPS features of film A do not correspond to those of film B and are essentially the same as those of an amorphous film prepared on a cooled metal surface. Nevertheless, films A and B afford almost the same features in both PIES and UPS when annealed to room temperature: the PIES become rather smooth to provide somewhat enhanced ␲ and d⊥ -derived bands whereas the UPS obtain an increased number of distinct bands. From these observations, it is considered that the electronic structure of a Pt(bqd)2 molecule is modified through the intermolecular interactions of the d⊥ -derived MOs, which operate differently depending on the molecular aggregation. © 2006 Elsevier B.V. All rights reserved. Keywords: Bis(l,2-benzoquinonedioximato)platinum(II); Ultrathin film; Electronic structure; Molecular aggregation; Penning ionization electron spectroscopy; Ultraviolet photoelectron spectroscopy

1. Introduction Plane or chain molecules vapor-deposited onto a graphite (0 0 0 1) surface are not pinned to the substrate but provided with mobility dependent on the temperature, experiencing moderate interactions with the substrate [1–3]. The environment enables ˚ aggregate comprising them to form an ultimately thin (3–4 A) lying molecules [1–7]. Sometimes they exhibit a definite orientation and a specific arrangement in a region as large as a submicrometer scale [3,7] or become thick layer by layer [2,6,8]. Since organic molecules are anisotropic unlike metal or semiconductor atoms forming superlattices, there exist various patterns of molecular aggregation and film growth [1–8], which encourages us to create new subnanomaterials by arranging tailored molecules on graphite and organize them. For example, if we choose a planar compound having polar groups as the

∗

Corresponding author. Fax: +81 42 385 7649. E-mail address: [email protected] (H. Ozaki).

0368-2048/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2006.11.045

monolayer constituent, a hydrogen-bonded network (organic monatomic layer) can be obtained by arranging the lying molecules [9,10]; we detected new states produced by the intermolecular orbital interactions taking part in the hydrogen bonding [10]. On the other hand, chain hydrocarbons once selfassembled to form a columnar structure can undergo surface reactions under appropriate conditions: we created a single sheet of a sashlike or a clothlike polymer (atomic sash [1,4,7,8,11] or atomic cloth [1,5,12]) by the intramonolayer photopolymerization of alkadiyne or alkatetrayne on graphite. Furthermore, we demonstrated that the atomic sash exhibits at least two typical structures with different conformations and they can be transformed to each other by changing the substrate temperature [7] or applying an adequate bias voltage from an STM tip [11]. These results tell us that the molecule-substrate interactions govern the geometrical structure and stability of the system to a considerable extent. Our studies in this line have been inclined to unite molecules laid flat in a monolayer [4,5,7–12]. We try here, however, to introduce new chemical bonds or specific interactions to a direction perpendicular to the molecular plane

352

H. Ozaki et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 351–356

on the cleavage planes of alkali halides [16]. Because of the slipped stacking of molecules with a longer Pt–Pt distance, the ␤-form exhibits different properties but transforms into the ␣form by heating [16]. In this study, temperature-induced changes in Penning ionization electron spectra (PIES) [6] and ultraviolet photoelectron spectra (UPS) have been measured for ultrathin ˚ Pt(bqd)2 films on graphite to survey the valence elec(3–10 A) tronic structures dependent on the molecular aggregations. 2. Experimental Pt(bqd)2 was synthesized and purified as reported previously [14]. The preparation of ultrathin films and the measurements of the PIES and UPS were carried out with an ultrahigh vacuum (UHV) electron spectrometer [10]. Two kinds of graphite substrates (12 mm × 10 mm) were used: Grafoil consisting of graphite crystallites with their (0 0 0 1) planes (face ˚ oriented parallel to the foil plane and highly size 100–2000 A) oriented pyrolytic graphite (HOPG) comprising much larger crystallites [17]. The latter was cleaved in the air and both substrates were cleaned by heating to 670 K in an UHV for 48 h. Then each substrate was cooled to 90 K and the sample was deposited. The amount of sample deposition ␦ was controlled in the unit of monolayer equivalence (MLE), which contains molecules necessary to form a monolayer with flat orientation and the arrangement in the ab plane of the ␣-form crystal. He* (2 3 S, 19.82 eV) metastable atoms and the He I (21.22 eV) resonance line were used as the excitation sources. 3. Results and discussion

Fig. 1. (a) Molecular structure of bis(l,2-benzoquinonedioximato)platinum(II) (Pt(bqd)2 ). (b) Crystal structure of ␣-form Pt(bqd)2 [13]. (c) Crystal structure of ␤-form Pt(bqd)2 [16].

of bis(l,2-benzoquinonedioximato)platinum(II) (Pt(bqd)2 ) (see Fig. 1(a)). In a crystal structure called ␣-form (Fig. 1(b)) [13], Pt(bqd)2 molecules are stacked parallel and rotated by 90◦ to the neighbors, forming a linear array of platinum with an exceptionally ˚ and exhibit pressure-induced short Pt–Pt distance 3.173 A, insulator to metal transition [14]. It is expected that interactions among Pt atoms contacting directly are responsible for the anomalous property of the single-component coordination compound. Though the electronic absorption spectra and the threshold ionization potential were measured for polycrystal˚ thick) prepared by vapor deposition onto line films (ca. 1000 A a quartz or a glass substrate [15], the electronic structure of an isolated Pt(bqd)2 molecule viewed with the MO picture, which must be a clue to explain the phenomenon, has been studied neither experimentally nor theoretically yet. Another type of crystal structure called ␤-form (Fig. 1(c)) is observed in Pt(bqd)2 films

The graphite substrates exhibited almost the same PIES and UPS after cleaning. On a Grafoil substrate, either of two Pt(bqd)2 films A and B with completely different PIES and UPS was obtained even when the same amount of sample was deposited onto the substrate maintained at the same temperature, probably due to some undetermined conditions. In contrast, film B was always obtained on an HOPG substrate and it exhibits the same PIES and UPS as film B prepared on a Grafoil substrate. Keeping these facts in mind, we will not distinguish the graphite substrates in what follows unless necessary. Fig. 2(a and b) compare the PIES and UPS of films A and B (δ = 3 MLE) formed on graphite at 90 K. Fig. 2(c) shows density of states (DOS) diagrams for a molecule and the aggregates of two and three molecules in a ␤-form column, obtained by RHF calculations with the Lanl2DZ basis set and Koopmans’ approximation. Orbital energies and characters for a single molecule are listed in Table 1. Though MOs 80 and 79 are too separated from deeper lying MOs in calculated energy, the following points should be noted: (i) MOs with the contribution of the Pt 5d AOs are scattered in a wide energy region; (ii) the HOMO has only a little 5dyz character; (iii) MOs 78, 71, 69 and 58 have a large contribution of the Pt 5dxz or 5dyz AOs, which is comparable to or predominant over the contribution of ligand ␲ orbitals, while MO 73 is mostly composed of the Pt 5dz2 AO (these MOs will be referred to as d⊥ -derived MOs together); (iv) nO MOs having oxygen lone pair character (74, 72, 70 and 68) are mainly con-

H. Ozaki et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 351–356

353

Table 1 Orbital energies and characters of a single Pt(bqd)2 molecule obtained by RHF/Lanl2DZ calculations No. of MO 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50

−ε/eV

Character

Remark

7.5743 8.0203 10.5082 11.0633 11.1207 11.4486 11.5319 11.7681 12.4247 13.0266 13.0696 13.2353 13.6541 14.2928 14.3842 14.4822 14.9429 15.4855 15.7138 15.9617 16.0487 16.2806 17.4488 17.5024 17.6705 17.7111 17.7361 17.7840 17.9062 18.1685 18.6586

␲(N) > ␲(O,B)a ,

HOMO

b2g au b1g au b2g b3u b2u ag b3g b1g b1u b2g ag b1g b3u b2u ag b3g b1u ag b2u au b2g b3g b1u b3u b3g ag b2u b1g ag

dyz ␲(N,O) > ␲(B)a dxz , ␲(B) > ␲(O)a ␲(B,O)a ␲(B) > ␲(O)a > dyz ␲(B,O)a nO > ␴CN dz2 nO > dxy dxz , ␲(O)a nO , hbb dyz > ␲(O)a dx2 −y2 > nO , ␴NO > dz2 ␲(B) > ␲(O)a , dxz ␲(B,O)a ␴CH (s)c , ␴CC > ␴CH (1)c , ␴NO ␴CH (s)c , ␴CC > ␴CH (1)c , dx2 −y2 ␴CH (1)c , ␴CC ␴CH (1)c , ␴CC nO , hbb , ␴CN , ␴NPt [dx2 −y2 , dz2 ] ␴CC , ␴NO , ␴CH (s)c ␲(N,O)a dyz , ␲(N,O)a nO , ␴NO ␴CH (s)c > ␴CC ␲(N,O,C1,2 )a ␴CH (s)c > ␴CC ␴CH (l)c , ␴CC , hbb ␴CH (s)c , ␴CC > ␴CN , ␴NO , nO ␲(N)a > dxz > ␲(C1,2 ,O)a ␴CH (s)c , ␴CC > dx2 −y2

d⊥ -derived

d⊥ -derived d⊥ -derived d⊥ -derived

d⊥ -derived

The long and the short axis of the molecule are placed parallel to the x and the y-axis, respectively (see Fig. 1(a)). a Symbols N, O, B and C1,2 in the parentheses indicate that the ␲ MO is mainly distributed on the nitrogens, oxygens, benzene rings and carbons at the 1- and 2-positions (adjacent to the nitrogens), respectively. b Abbreviation ‘hb’ denotes that the MO has electron distribution around the hydrogen bonding O· · ·H· · ·O. c Symbols ‘s’ and ‘l’ in the parentheses indicate that the ␴ CH MO is distributed mainly along the short and the long axis of the molecule, respectively.

centrated for calculated ionization potential (IP) 11.5–13.7 eV but there exist deeper lying nO MOs (61, 57 and 52) as well. Referring to the DOS diagrams and Table 1, and focusing our attention on the relative intensities of the PIES bands (see below), we have made assignments as indicated in Fig. 2(a and b). In Fig. 2(a), both PIES are free from the graphite band g at 3.3 eV, meaning that the substrate surface is completely covered with molecules because He* metastable atoms do not penetrate into the solid and interact with the outermost layer selectively. The two PIES differ entirely from each other, indicating that the outermost molecules are oriented differently because metastables probe the local electron distribution of individual MOs at the externally exposed portion of the molecules [6]. In the PIES of film A, the high kinetic energy (Ek ) region with high DOS for the ␲ and d⊥ -derived MOs are markedly enhanced compared to the low Ek region with that for ␴ MOs. Therefore, the molecules in film A are laid flat on average so that the ␲ and d⊥ derived MOs spreading perpendicularly to the molecular plane are predominantly probed by metastables. The PIES of film B exhibits enhanced bands at Ek < 7 eV, where the PIES of film A has only weak features. These bands

are ascribed to the ␴ MOs hardly probed in the flat molecular orientation. Furthermore, there exist an extremely weak first band at 12.9 eV and overwhelmingly enhanced bands between the first and the ␴ bands. The overwhelming bands, which do not correspond to the ␲ and d⊥ -derived bands of film A, are attributed to the nO MOs. Since the ␲ MOs 80 and 79 responsible for the first band, mainly composed of N 2pz and O 2pz AOs and distributed normally to the molecular plane, are scarcely observed while the nO MOs projecting from the O atoms along the plane are exclusively detected in the high Ek region, the molecules must be oriented with the short axes almost perpendicular to the surface and the O atoms exposed outside. It should be emphasized that such an orientation of plane molecules has been found for the first time on graphite. From this orientation, the ␴ bands enhanced at lower Ek must be due to ␴CH MOs of a C H bonding character. We have demonstrated that ultrathin films comprising the same kind of molecules but having different molecular aggregations provide completely different PIES whereas the UPS are essentially the same under our conditions if the contribution of the substrate bands is neglected [2,6]; such observations repre-

354

H. Ozaki et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 351–356

Fig. 2. He* (2 3 S, 19.82 eV) PIES (a) and He I (21.22 eV) UPS (b) of ultrathin films A and B prepared by depositing 3 MLE of Pt(bqd)2 on graphite substrates held at 90 K. See text for the film types. The abscissa for the PIES is shifted by 1.40 eV (the difference between the excitation energies) relative to that for the UPS. (c) Density of states (DOS) diagrams for an isolated molecule and the aggregates of two and three molecules in a ␤-form column, obtained by RHF calculations with the Lanl2DZ basis set and Koopmans’ approximation.

sent the conservation of molecular individuality regardless of the aggregation. But this is not the case for films A and B, which tells us that the electronic structures as well as the aggregations are not alike. In Fig. 2(b), the graphite bands g, G1 and G2 contribute to the UPS of both films to a similar extent. As for features due to

Pt(bqd)2 , one becomes aware that (i) the first and the second band of film B are shifted to higher Ek than those of film A, (ii) their relative intensities are reversed and (iii) the remaining bands of both films do not correspond. Since there is little regularity in the aggregation of molecules in film A as will be elucidated below, these findings indicate that molecules in film B are arranged in a specific manner to bring about directional intermolecular orbital interactions and modify the electronic structure: the UPS can reflect the DOS of the split MOs. From the discussion on the relative intensities of the PIES bands for film B, we tentatively suppose that ␤-form crystallites are grown with the ac plane parallel to graphite. As can be seen from Fig. 1(c), the short axes of the surface molecules stand almost vertically and the O atoms are exposed outside in this case, In Fig. 2(c), (i) the first, the fourth and the fifth band shift to lower IP, (ii) the second and the fourth band are intensified and (iii) the third and the fifth band are weakened, with the increasing number of molecules arranged in the ␤-form column. The tendencies seem to be related to the differences between the UPS of films A and B for Ek > 11 eV. Fig. 3(a and b) show temperature-induced changes in the spectra of films A (δ = 3 MLE) and B (δ = 1.5 MLE). Upon raising the substrate temperature, there is no apparent change in the PIES of film A at 170 K, but the PIES loses the enhanced bands and obtains new broad bands and a trace of band g at 220 K. The high Ek shoulder of the band around 11 eV is a little upheaved at room temperature (RT). The intensified band g means that about 20% of the substrate surface is exposed outside. At any rate, since the newly enhanced bands exhibit Ek larger than 8 eV and the lower Ek region in which some nO MOs and many ␴ MOs exist is less enhanced, it is considered that the molecules lie flat so that the ␲ and d⊥ -derived MOs are observed selectively. The UPS features for film A observed at 90 K are almost maintained at 170 K, but drastically altered at 220 K into new distinct ones. They become slightly weakened at RT in accordance with the intensified band g, which indicates that the number of molecules existing on the substrate is reduced by desorption. The apparent enhancement of the band at 13.8 eV is ascribable to the increased intensity of band G2 . In Fig. 3(b), all the PIES exhibit band g, indicating the presence of some uncovered parts of the substrate and, correspondingly, all the UPS bear intense band g. During annealing, the nO and ␴ bands markedly enhanced in the PIES of film B at 90 K are gradually weakened whereas the first PIES band faintly observed at 90 K grows in relative intensity. Above 220 K, the nO and ␴ bands disappear and new bands due to the ␲ and d⊥ derived MOs appear at Ek > 8 eV; the resultant PIES is similar to that of the annealed film A. These findings indicate that the short molecular axis standing at 90 K becomes tilted with the temperature to yield the flat orientation at RT. Film B gradually loses the low-temperature bands in the UPS as well and obtains new bands to exhibit almost the same UPS as the annealed film A at RT. It is noteworthy that 1.5 MLE molecules remain in film B during annealing because bands g are almost unchanged in both spectra. It makes a striking contrast with a fact that about half the molecules in film A desorb at RT; the number of molecules on the RT substrate seems a little smaller in film A than in film B, judging from the intensity of band g in the UPS. From these

H. Ozaki et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 351–356

355

Fig. 3. Temperature-induced changes in the He* (2 3 S) PIES and He I UPS of Pt(bqd)2 films A (a) (3 MLE) and B (b) (1.5 MLE) prepared on graphite substrates held at 90 K. A DOS diagram for a molecular pair in an ␣-form column is inserted on the top of the UPS for film B.

observations, films A and B must be transformed into similar films at RT with the same aggregation and electronic structure, which are different from those of both films at low temperature. It is natural to expect that the molecular rearrangement brings about MO splitting due to the introduction of new or another type of orbital interactions among adjacent molecules, In fact, the UPS of the RT films resemble the DOS diagram for a molecular pair in an ␣-form column (see the middle of Fig. 3(b)). Fig. 4 compares temperature-induced changes in the UPS of film A with those of “film C”, prepared by depositing 20 MLE of Pt(bqd)2 onto a copper substrate (without a crystallographic surface) held at 170 K [18]. Film C exhibits distinct UPS features at 170 K, but they are smeared out at RT. Surprisingly, the features for film C at 170 K and RT closely correspond to those for film A at 90 K and RT, respectively. It is well known that planar molecules vapor-deposited onto a metal substrate held at low temperature form amorphous films while those at RT polycrystalline films, and the former are transformed into the latter upon annealing [6,19–21]. Moreover, an amorphous Pt(bqd)2 film was prepared on a rough substrate at liquid nitrogen temperature whereas a polycrystalline one at RT [15]. According to such wisdom, it is probable that film C is amorphous at 170 K and the electronic structure reflected in the UPS is close to that of an isolated molecule. Hence, we consider that the molecules in film A exhibiting the UPS features characteristic of the amorphous film

below 170 K (Fig. 3(a)) lack high regularity in the arrangement although they are oriented flat on average as described before; they cannot undergo modifications in the electronic structures by directional orbital interactions among neighbors. At RT, on the contrary, film C is very likely to be polycrystalline, not merely

Fig. 4. Temperature-induced changes in the UPS of film A (3 MLE) and film C prepared by depositing 20 MLE of Pt(bqd)2 onto a copper substrate held at 170 K [18].

356

H. Ozaki et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 351–356

because of the above wisdom but also because of the smooth and flat UPS features reminding us of the formation of band structures in an early stage: if the molecules are piled up in an ␣-form column, the d⊥ -derived MOs and some ␲ MOs are effectively split to afford new MOs, broadening the UPS features. Since a one-to-one correspondence is found at RT between the UPS features of films A and C, the aggregation and electronic structure of Pt(bqd)2 in the annealed film A (and the annealed film B providing the same spectra) are related to those in the crystalline phase. But the following three facts must be taken into account: (i) there are only 1.5 MLE or less molecules laid flat in the annealed films A and B; (ii) the UPS bands are not so broadened for the annealed films A and B as for the polycrystalline film C; (iii) the DOS calculated for a molecular pair in an ␣-form column represents the UPS features of the annealed films A and B to a certain degree. Therefore, we consider bilayer domains in the ␣-form arrangement as well as monolayer domains major aggregates in the annealed films A and B, which is supported by our recent STM studies [22]. There are two types of domains I and II exhibiting different patterns in the STM images of film B (on an HOPG substrate) at 180 K and the ratio of domain II to domain I area increases with the temperature. At RT, domain I is found to be of monolayer with periodicities formed by molecules laid flat, but it does not have the molecular arrangement in the ab plane of the ␣-form structure. In contrast, domain II is considered to be of bilayer since the STM image indicates the presence of upper layer molecules removed by the tip. The ab plane structure of the ␣-form does not contradict the contrast pattern. 4. Conclusion A rare example of aggregation-dependent valence electronic structures has been provided for ultrathin films comprising one kind of rigid molecules on graphite. When deposited onto a graphite (0 0 0 1) surface held at 90 K, Pt(bqd)2 forms an ultrathin film A or B. In film A, molecules are oriented flat on average, but lack regularity in the arrangement. Because of the amorphous aggregation, they essentially maintain the electronic structure of an isolated molecule. In film B, on the contrary, molecules are oriented with the standing short axes, which is accompanied by intermolecular interactions modifying the character of a single molecule. When annealed to RT, each film is transformed into a new common one mainly composed of flat molecules in

monolayer and bilayer domains; the molecules in the second (upper) layer are probably superposed parallel but rotated by 90◦ to those in the first layer, affording another electronic structure caused by the splitting of the d⊥ -derived MOs. A series of STM studies are now under investigation to elucidate the molecular arrangements in more detail, which helps us to further advance the assignments of the bands in the PIES and UPS. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

[10] [11] [12]

[13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

H. Ozaki, J. Electron Spectrosc. Relat. Phenom. 76 (1995) 377. H. Ozaki, J. Chem. Phys. 113 (2000) 6361. O. Endo, N. Toda, H. Ozaki, Y. Mazaki, Surf. Sci. 545 (2003) 41. H. Ozaki, T. Funaki, Y. Mazaki, S. Masuda, Y. Harada, J. Am. Chem. Soc. 117 (1995) 5596. H. Ozaki, M. Kasuga, T. Tsuchiya, T. Funaki, Y. Mazaki, M. Aoki, S. Masuda, Y. Harada, J. Chem. Phys. 103 (1995) 1226. Y. Harada, S. Masuda, H. Ozaki, Chem. Rev. 97 (1997) 1897, and references therein. O. Endo, H. Ootsubo, N. Toda, M. Suhara, H. Ozaki, Y. Mazaki, J. Am. Chem. Soc. 126 (2004) 9894. H. Ozaki, T. Magara, Y. Mazaki, J. Electron Spectrosc. Relat. Phenom. 88–91 (1998) 867. H. Ozaki, M. Kasuga, S. Kera, M. Aoki, H. Tukada, R. Suzuki, N. Ueno, Y. Harada, S. Masuda, J. Electron Spectrosc. Relat. Phenom. 88–91 (1998) 933. H. Ozaki, M. Suhara, T. Ohashi, N. Toda, O. Endo, H. Tukada, J. Electron Spectrosc. Relat. Phenom. 137–140 (2004) 151. O. Endo, M. Suhara, H. Ozaki, Y. Mazaki, e-J. Surf. Sci. Nanotech. 3 (2005) 470. T. Takami, H. Ozaki, M. Kasuga, T. Tsuchiya, Y. Mazaki, D. Fukushi, A. Ogawa, M. Uda, M. Aono, Angew. Chem. Int. Ed. Engl. 36 (1997) 2755. M.M.- B´elomb´e, J. Solid State Chem. 27 (1979) 389. I. Shirotani, A. Kawamura, K. Suzuki, W. Utsumi, T. Yagi, Bull. Chem. Soc. Jpn. 64 (1991) 1607. I. Shirotani, T. Kudo, N. Sato, H. Yamochi, G. Saito, J. Mater. Chem. 5 (1995) 1357. T. Yaji, K. Yoshida, S. Isoda, T. Kobayashi, N. Sato, I. Shirotani, Thin Solid Films 393 (2001) 319. Y. Niimi, T. Matsui, H. Kambara, K. Tagami, M. Tukada, H. Fukuyama, Phys. Rev. B 73 (2006) 085421. M. Taki, H. Ozaki, S. Suhara, O. Endo, K. Takeda, I. Shirotani, unpublished results. Y. Kamura, K. Seki, H. Inokuchi, Chem. Phys. Lett. 30 (1975) 35. K.O. Lee, T.T. Gan, Chem. Phys. Lett. 51 (1977) 120. Y. Maruyama, N. Iwasaki, Chem. Phys. Lett. 24 (1974) 26. K. Akaike, M. Suhara, H. Ozaki, O. Endo, K. Takeda, I. Shirotani, in preparation.