Raman spectra of sulfur, selenium or tellurium clusters confined in nano-cavities of zeolite A

Solid State Communications, Vol. 107. No. 9, pp. 513-518, 1998 0 1998 Published by Elsevier Science Ltd Printed in Great Britain 0038-1098/98 $19.00+.00

PII: !iOO38-1098(98)00205-I

RAMAN SPECTRA

OF SULFUR, SEL.ENIUM OR TELLURIUM CLUSTERS NANO-C 4VITIES OF ZEOLITE A

CONFINED

IN

Vladimir V. Poborchii* Ioffe Physico-Technical (Received

23 January

1998;

in revised form

Institute,

St. Petersburg

194021, Russia

13 March 1998; accepted 20 April 1998 by H. Kamimura)

Raman spectra of sulfur, selenium and tellurium clusters confined in the large cavities of zeolite A with diameters of - 1.4 nm have been studied. It is shown that sulfur is stablilized in the form of Ss rings. Selenium is stabilized in the form of Se12 and Se8 rings. Se12 rings show dominant bands in the Raman spectra but they are less stable than Se8 rings under laser illumination with a wavelength of 514.5 nm. Tellurium is stabilized iy the form of Te8 rings. Low-frequency strong and broad bands at -40 cm and -29 cm observed in the spectra of zeolite A with sulfur and with selenium, respectively, are attributed to librations of the ring molecules in the zeolite cavities. 0 1998 Published by Elsevier Science Ltd. Keywords: A. nanostructures., inelastic light scattering.

B. nanofabrications,

1. INTRODUCTION Zeolite A (IUPAC Code LTA) possessing regular set of cavities with diameter of 1.14 nm is very attractive for preparation of 3-dimensional lattice of nano-clusters using free space of the cavities (Fig. 1). Chalcogens (S, Se and Te) are good materials for preparation of this kind of clusters because they can be easily adsorbed into zeolite A and their optical spectra can be studied in visible or near UV spectral regions, where zeolites are transparent. Samples of zeolite A containing sulfur (A-S), selenium (A-Se) or tellurium (A-Te) have been studied in a number of works [l-13]. However, molecular composition of the cavity-confined clusters has not been completely determined. Only for sulfur clusters, situation is quite clear. It has been shown by X-ray diffraction [l], Raman scattering [5] and optical absorption spectra [5] that the main molecular unit of sulfur in A-S is a most stable sulfur molecule, namely, Sg crown-like ring [Fig. l(a)]. (In the reference [5], it was shown that not only Sg rings but also S; anions can be

* Present address: JRCAT, c/o National Institute for Advanced Interdisciplinary Research, l-l-4 Higashi, Tsukuba, Ibaraki 305, Japan.

D. optical properties, E.

stabilized in the A-S crystals if sulfur is adsorbed at a temperature of -5OO”C, only Sg being stabilized at a temperature of -300°C.) However, a low-frequency part of the Raman spectrum of A-S has not been studied yet. A-Se has been examined by Raman scattering [3, 4, 6,7, 1 l-131, optical absorption spectra [2,4,8,9, 11, 131 and EXAFS [8, lo]. All data showed that some kind of Se ring molecules rather than chain molecules are stabilized. In a number of works [3, S-10, 131, authors declared (without forcible arguments) that the ring molecules in A-Se are Ses rings. However, Raman spectra of A-Se 16, 11, 121 displayed dominant features which cannot be attributed to the Se8 ring. In the works [ 11, 121, it was proposed that Selz ring is the main molecule in A-Se but detailed assignment of the Raman bands has not been made and a possibility of stabilization of other molecules has not been considered. A-Te has been studied by Raman scattering [3, 111, EXAFS [lo] and optical absorption spectra [ 111. Similar to Se clusters, experimental data showed that, probably, ring Te molecules are stabilized. In the work [3], it was proposed that Tes is stabilized but that idea was based on only one Raman band and as it was shown later, the frequency of the band has been determined incorrectly. In the work [ll], two reliable Raman bands has been 513

514

RAMAN SPECTRA

OF SULFUR, SELENIUM

OR TELLURIUM

Vol. 107, No. 9 222 I

03) 248

Fig. 1. A large cavity of the zeolite A with &membered chalcogen (S, Se or Te) ring (a) and 12-membered selenium ring (b) inside. For larger 8-membered rings in the row S, Se and Te, position of the ring moves down to the center of the cavity. observed and it was proposed that Tel2 ring can be stabilized. However, Raman data are still incomplete and a final conclusion about structure of Te clusters in A-Te cannot be made. To summarize the data of the works on A-Se and A-Te, it should be concluded that the molecular composition of Se and Te clusters confined in zeolite A is still unclear and further studies are needed. The aim of this work is to study Raman spectra of S, Se and Te clusters stabilized in zeolite A in more details than it was done before, to assign the observed Raman bands and to determine molecular composition of Se and Te clusters. In this work, Raman spectra of A-S, A-Se and A-Te are presented. New Raman features and band intensity dependencies vs illumination are found. Two kind of ring molecules (Se,;, and Ses) are determined for A-Se and one kind (Tes) is determined for A-Te. Low-frequency bands observed in the Raman spectra of A-S and A-Se are attributed to librations of the ring molecules in zeolite cavities. 2. EXPERIMENTAL Zeolite A crystals (unit cell framework formula is Na ,zAl &i 1204s) with cubic shape and size of 30-50 pm have been used. Zeolites have been dehydrated and then exposed to the chalcogen vapor at -300°C for sulfur, at -450°C for selenium and at -550°C for tellurium. Raman spectra excited with Ar laser 514.5 nm and Kr laser 647.1 nm lines have been studied using Dilor Z triple monochromator equipped with a photomultiplyer and Jasco double monochromator equipped with a CCD camera. Size of a laser spot on a sample was -50 pm and illumination power was varied in the interval -5-50 mW. For Raman measurements, we used zeolite samples as tablets or powders in a quartz capillary. 2.1. A-S Raman spectrum of A-S in full scale is shown in Fig. 2(a). There are five distinct bands in the spectrum at the frequencies of 40, 78, 153, 222 and 480 cm-‘. The

zcco~ LurJ 50

I

100

150

xc

250

RAMAN

300

3M

4cnl

450

500

550

SHIFT, I/cm

Fig. 2. Raman spectra of A-S excited with a 5 14.5 nm of Ar laser. Spectrum of the original sodium form is shown in a full scale (a). Low-frequency part of the spectra is shown for sodium and ion-exchanged lithium forms (b). bands at the frequencies 78, 153, 222 and 480cm-’ correspond well to the internal modes of the crown-like Ss ring and they should be attributed to the E2 torsion mode, E2 bond-bending mode, A, bond-bending mode and A, bond-stretching mode of the S s ring, respectively [14, 151. It should be noted that the 480 cm-’ band displays two shoulders. A shoulder on the low-frequency side of the band is due to the E2 bond-stretching mode of Ss. A shoulder on the high-frequency side of the band is due to the zeolite A strongest Raman band at -495 cm-’ [ 161. It should be noted that one more S s Raman band at 248 cm-’ (E3 bond-bending mode) is seen in Fig. 2(b) in the spectrum of A-S (NaA-S) with weak noise. A question is what is the origin of the low-frequency band at 40cm-‘. It cannot be attributed to an intramolecular vibration of the Ss ring because of too low frequency. Analysis of the Ss ring vibrations [14, 151 showed that the lowest intramolecular frequency is due to the torsion E2 mode. For orthorhombic sulfur, the band of this mode is observed at 85 cm-‘. We observe the band of this mode at the frequency 78 cm-‘. Low frequency of the 40 cm-’ band and a large width of the band compared to the widths of the bands of intramolecular vibrations suggest assignment of this band to the librations of the Ss rings inside zeolite cavities. It is possible to distinguish between Raman bands of the librations and that of the internal molecular vibrations if we change interaction between a molecule and zeolite matrix. Following this idea, we compared the spectrum of sulfur adsorbed into the sodium form of the zeolite A (NaA) with the spectrum of that adsorbed into the lithium form of the zeolite A (NaA) with the spectrum of that adsorbed into the lithium form of the zeolite A (LiA). Interaction between Ss ring and a matrix should be slightly different for these two zeolite forms and so the

frequency of librations of Ss rings in zeolite cavities should also be slightly different. The spectra of NaA-S and LiA-S are shown in Fig. 2(b). The frequencies of the intramolecular vibrations have been found to be the same for NaA-S and for LiA-S but the low-frequency band have been found to be shifted to higher frequencies for LiA-S (40cm-’ for NaA-S and 43 cm-’ for LiA-S). This observation supports assignment of the low-frequency band to the librations of Ss in zeolite cavities. 2.2. A-Se

19W 10 17M leo0 @) 1500 14X

(c)

1200 12w

@)

IIW

(a)

lcal

Raman spectra of A-Se have been excited with a red light of 647.1 nm line of Kr-laser (weakly absorbed by the sample) or with a green light of 514.5 nm line of Ar-laser (strongly absorbed by the sample). Raman spectrum of A-Se, which was excited with a 647.1 nm line of a Kr-laser with a power of - 10 mW, is shown in Fig. 3. There are distinct bands at 29,56. 76,88, 111,121,142 and 258 cm-’ with a shoulder at 267 cm-‘. We have found no significant dependence of the relative intensities of the bands in the spectrum on the duration of the illumination or increase in the laser power in the range of lo-50 mW. A situation is different at the excitation with a 514.5 nm line of Ar-laser, namely, relative intensities of the bands change depending on the illumination conditions. As it is shown below, a variety of bands in the spectrum can be assigned to two groups depending on their behavior under illumination. Raman spectra of A-Se obtained at d.ifferent intensities of excitation with a wavelength 5.14.5 nm are shown in Fig. 4. Relative intensities of the bands in A-Se spectrum depends on the intensity and duration of illumination. Obviously, intensities of the bands at 76, 111, 12 1 and 267 cm-’ (frequencies determined from the spectra excited with 647.1 nm line (Fig. 3) which coincide with those of Fig. 4(a) are given) become stronger compared to other bands with increase in the excitation power or duration of illumination. These

15001.‘.‘.“‘.‘.‘.‘.‘.‘.“I.‘.‘.’ 20

515

RAMAN SPECTRA OF SULFUR, SELENIUM OR TELLURIUM

Vol. 107, No. 9

40

60

80

loo

120 140

loo

180 200 220 240 260 280 300

RAMAN SHIFT, l/cm

Fig. 3. Raman spectrum of A-Se excited with a 647.1 nm line of Kr laser with a power -10 mW.

20

40

80

Bo

loo

120

140

feu

180

200

220

240

260

280

300

RAMAN SHIFT, l/cm

Fig. 4. Raman spectra of A-Se excited with a 514.5 nm line of Ar laser with a power -5 mW (a), -20 mW (b), -20 mW and preliminary exposed during 20 min (c), -50 mW (d). Exposure time during Raman signal collection by CCD camera is -1 min. bands are indicated with a label “i” (increasing) in Fig. 4. Intensities and frequencies of these bands agree well with intensities and frequencies of a crown-like Ses ring molecule in another zeolite AlPOd- [ 171. Most intensive characteristic bands of Ses in AlPOd- have been observed at the frequencies 76 (Ez bond-bending mode), 115 (A, bond-bending mode) and 268 cm-’ (Al bond-stretching mode) which correspond well to the frequencies of the bands in the spectrum of A-Se at 76,111 and 267 cm-‘, respectively. A weaker band at 121 cm-’ in A-Se spectrum should be attributed to the E3 bond-bending mode of Ses similar to similar mode of Ss at the frequency 248 cm-‘. The bands labeled with a letter “d” (decreasing) at 56, 88 and 258 cm-‘, which display decrease in the relative intensity compared to the Ses bands with increase of the laser power, cannot be attributed to any known Se molecule in gaseous, liquid or solid state. The frequencies and intensities of the bands correspond well to the calculated frequencies and intensities of the Se ,2 ring molecule [ 1l] of DJd point group symmetry which is similar to the dodecasulfur S ‘2 [ 181. According to this interpretation, we should assign the 56 cm-’ band to the A Igbond-bending mode (for S l2 a “strong” band of this mode is at 128 cm-’ (18]), the 88 cm-’ to the E, bond-bending mode (for S ‘2 a “middle” band of this mode is at 177 cm-’ [18]) and the 258 cm-’ band to the A,, bond-stretching mode (for S ‘2 a strong band of this mode is at 459 cm-’ [ 181). It is also reasonable to assign a weak 142 cm-’ band (Fig. 3) to the second A Ig symmetric bond-bending mode of Se ‘2 ring which corresponds well to the calculated frequency of this mode 137 cm-’ [ll] (for S12 a “weak” band of this mode is at 288 cm-‘). One more band of Selz with the same level of intensity as that of 142 cm-’ band is expected in the range of 115- 125 cm-’ but it is probably

516

RAMAN SPECTRA OF SULFUR,

overlap with the 121 cm-’ band of Ses and in the A-Se spectrum of Fig. 3 contributions of Ses ring E3 mode and Se’* ring E, mode to the 121 cm-’ are roughly equal, but after green light illumination [Fig. 4(b), (c) and (d)], contribution of Ses ring E3 mode becomes dominant. The size and symmetry of both Selz and Ses rings correspond well to the size and symmetry of the large cavities of the zeolite A. Possible locations of the Se12 and Ses rings in the cavities are shown in Fig. 1. It is reasonable to suppose that the Se I1 molecule is oriented by its 3-fold axis along the 3-fold axis of the zeolite and the Ses ring is oriented by its 4-fold axis along the 4-fold axis of the zeolite. (X-ray diffraction of A-S shows location of Ss similar to that of Ses in Fig. l(a)). It is still unclear about possibility of double Ses ring structure inside one zeolite cavity. Theoretically it is possible but experimental data do not give clear evidence for this. Data of different works about Se-loading limit of zeolite A give different results [6, 11, 131. Probably, most careful experiment which gives a value of the Se-loading limit - 10.5 Se atoms per cavity was performed in [ 111. However, this value can be obtained in the case of arrangement of single Ses and Se’2 molecules in the cavities without double Ses rings. Our preliminary study of Raman spectra of A-Se with different Se-loading densities showed that the Se,#es concentration ratio only slightly varies and this ratio display minimums at the lowest and highest Se-loading densities. Increase of the relative concentration of Se* rings at high Se-loading densities confirms possibility of stabilization of double Ses ring structure but additional experiments are needed to solve this problem completely. It is interesting to note that some shift of the Raman bands with increase of the laser power is taking place. The band at 76 cm-’ is probably a doublet and one component of the doublet is stronger at the low-power excitation [Fig. 4(a)] but another one becomes stronger at the high-power excitation [Fig. 4(d)]. The band of the AI bond-bending mode of Ses shifts from 111 cm-’ to 107 cm-’ and becomes sharper with increase in the laser power. These effects are probably associated with significant heating of the sample and change in the state of the molecules (probably, delocalization) in zeolite cavities at the high-power excitation. The effect of the relative decrease in the intensities of Se,* bands is associated with a splitting of this molecule to smaller Se molecules and their evaporation from zeolite. This is confirmed by observation of bleaching of A-Se tablet at the place exposed to the laser illumination. Probably, not only heating but also optical excitation contributes to the effect of destruction of Se12. The origin of the low-frequency strong band at 29 cm-’ in the spectrum of A-Se is probably the same

SELENIUM

OR TELLURIUM

Vol. 107, No. 9

as the origin of the 40 cm-’ band in the spectrum of A-S. According to both experimental data and calculations for Ses and according to calculations for Se,*, there is no strong Raman bands of intramolecular vibrations at the frequencies -30 cm-‘. Moreover, the band at 29 cm-’ is broader than the bands of intramolecular vibrations of Se I2 and Ses. Thus, it is reasonable to attribute the band at 29 cm-’ to the librations of the molecules in the cavities. We think that the librations of both Sell and Ses contribute to the 29 cm-’ band. This is a reason why the intensity of this band is higher than the intensities of the bands of intramolecular vibrations of Se,* and Ses rings. (In the A-S spectrum, only Ss librations contribute to the low-frequency band and the intensity of the band is roughly equal to the intensities of the strongest bands of the intramolecular vibrations.) 2.3. A-Te The Raman spectrum of A-Te is shown in Fig. 5. Three distinct bands are displayed at 45, 62 and 182 cm-‘. It is reasonable to attribute these bands to the E2 bond-bending, A, bond-bending and A, bond-stretching modes of the crown-like Tes ring, respectively, because of similarity of these bands to the 153,222 and 480 cm-’ bands of Ss and to the 76,111 and 267 cm-’ bands of Ses. To check this assignment, let us consider a dependence of the frequencies of the most Raman active modes (E2 bond-bending, A, bondbending and Al bond-stretching) of Ss, Ses and Tes determined from the spectra of A-S, A-Se and A-Te correspondingly vs the inverse square root of atomic mass (Fig. 6). Clearly, the bond stretching mode frequencies as well as bond-bending mode frequencies display roughly linear dependences vs the inverse square root of atomic mass. This is a strong argument for the assignment of the considered Raman bands to species with similar structures and bondings, namely, to the 8-membered rings (Ss, Ses, Tes).

1250

=j

12cQ.

182 1

1050 I 40

60

eo

loo

120

140

169

180

.,

200

220

RAMAN SHIFT. l/cm

Fig. 5. Raman spectrum 647.1 nm line of Kr laser.

of A-Te

excited

with

a

RAMAN SPECTRA

Vol. 107, No. 9 5W-

OF YJLFUR, .

450400'8

5. 350:. a-- 3cQy

250-

% u

200-

;

150* loo,

. Tea

-1

50

1' 0.08

:

I 0.10

'

I 0.12

'

I 0.14

.

I. 0.16

I 0.18

.1

0.20

l/(atomic massj”

Fig. 6. Dependence of frequencies of the bands in the spectra of A-S, A-Se and A-Te assigned to the E2 bond-bending (triangles), A I bond-bending (circles) and A, bond-stretching (squares) of Ss, Ses and Tes. correspondingly, vs the inverse square root of the atomic mass. Another argument supporting this interpretation is shown in the Table 1. In this table, the frequencies of the bands of A-S(Se, Te) attributed to the E2 bond-bending, A, bond-bending and A i bond-stretching modes of Ss, Ses and Tes are compared with the corresponding calculated frequencies. (The method of the calculation is the same as that of [ll]. Details will be given elsewhere [ 191.) Agreement between the experimental frequencies and calculated ones is good. The conclusion about Tes stabilization in A-Te is consistent with Te-loading density data (we never succeeded in incorporation of more than eight Te atoms per cavity into zeolite A) and geometrical consideration (the size of Tes is of -0.9 nm less than the size of the cavity of - 1.14 nm). It should be noted that the bands at 45 and 182 cm-’ have been reported in [ 111. It has been proposed to assign the bands at 45 and 182 cm-’ to the A Ig bond-bending and A Ig bond-stretching modes of Tel2 although the authors have noted that the size of Tel2 is a little bit large (- 1.18 nm) for the zeolite A cavity and they have not strongly insisted on this interpretation because of shortage of experimental data. It has been suppose,d that the strongest Raman band of Tes in the bond-bending Table 1. Experimental Mode

Al stretch. A, bend. E2 bend.

and calculated

frequencies

SELENIUM

mode region can be associated only with A I bondbending mode. However, the frequency of -45 cm-’ has been found to be too low for the A, bond-bending mode of Tes and the 45 cm-’ band of A-Te has been assigned to the A Ig bond-bending mode of Te Q. Thus, the assignment of the 45 and 182 cm-’ bands to Tel2 has been made because the authors have found a contradiction for assignment of the 45 cm-’ band to Tes. Data of the present work provide two important arguments for change of interpretation of [ll]. The first argument is that the Raman spectra of A-Se (Figs 3 and 4) display the band of the E2 bond-bending mode at 76 cm-’ which is stronger than the band of the A, bond-bending mode at 111 cm-’ (Figs 3 and 4). It means that (similar to Ses in A-Se) the strongest band in the bond-bending mode region of A-Te at 45 cm-’ can be attributed not to the A 1 mode but to the E2 mode of Tes. This point removes a contradiction discussed in [ 111. The second argument is an observation in this work of a new band at -62 cm-’ in the A-Te spectrum. This band can be easily attributed to the Ai bond-bending mode of Tes. Moreover, in the bond-stretching mode region of the A-Te spectrum, a shoulder at the lowfrequency side of the 182 cm-’ band at 160-170 cm-’ can be assigned to the E2 bond-stretching mode of Tes. Similar shoulder at the low-frequency side of the Ai bond-stretching mode band is observed for Ss in A-S and Ses in AFI-Se [17]. This shoulder is an important characteristic feature of the eight-membered rings. To summarize this section, we should note that in spite of new results obtained for A-Te, some shortage of the experimental data still takes place. Most of the data are consistent with stabilization of Tes ring in the zeolite A. However, a possibility of alternative interpretations cannot be completely excluded. Further studies are still necessary. 3. CONCLUSIONS Raman spectra show evidence for stabilization of Ss, Ses, Selz and Tes ring molecules in zeolite A. Ss and Ses molecules are well known in condensed state but there is no information about stabilization of Selz and Tes rings in conditions different from the zeolite A cavities. A possible reason for this stability is a good compatibility

of S(Se, Te)s Te8

Se8

s8

517

OR TELLURIUM

Exp.

Calc.

Exp.

Calc.

Exp.

Calc.

480 222 153

480 220 162

267 111 76

269 109 82

182 62 45

182 65 48

518

RAMAN SPECTRA

OF SULFUR.

of size and symmetry of Se,* and Tes with size and symmetry of large cavities of zeolite A. Se,* rings coexist with Ses rings in zeolite A but they cannot occupy the same cavity. Se12 rings show dominant bands in the Raman spectrum of A-Se but they are less stable than Ses rings under illumination by the green line 514.5 nm of Ar laser. Strong and broad low-frequency bands in the Raman spectra of the zeolite A with sulfur and selenium are attributed to the librations of ring molecules in the cavities. Acknowledgements-The author is grateful to Prof. Tanaka and Dr. Kolobov for possibility to finish this work during staying at Joint Research Center for Atom Technology, Tsukuba, Japan, to Dr. Petranovskii for supplying zeolite A crystals and to Prof. Onari for kind providing of Jasco spectrometer for Raman measurements.

REFERENCES 1. Seff, K., J. Phys. Chem., 76, 1972, 2601. 2. Bogomolov, V.N., Lutsenko, E.L., Petranovskii, V.P. and Kholodkevich, S.V., JETP Lett., 23, 1976,482. 3. Bogomolov, V.N., Zadorozhnii, AI., Petranovskii, V.P., Fokin, A.V. and Kholodkevich, S.V., JETP Lett., 29, 1979, 373.

4. 5.

Bogomolov, V.N., Poborchii, V.V., Kholodkevich, S.V. and Shagin, S.I., JETP Letters, 38, 1983, 532. Bogomolov, V.N., Petranovskii, V.P., Poborchii, V.V. and Kholodkevich, S.V., Sov. Phys. Solid State, 25, 1983, 1415.

SELENIUM 6.

OR TELLURIUM

Vol. 107, No. 9

Bogomolov, V.N., Poborchii, V.V. and Kholodkevich, S.V., JETP Lett., 42, 1985, 517. Bogomolov, V.N., Poborchii, V.V., Romanov, S.G. and Shagin, S.I., J. Phys., C18, 1985, L313. Parise, J.B., MacDougall, J., Herron, N., Fat-lee, R., W. Sleight, A., Wang, Y., Bein, T., Moller, K. and Moroney, L.M., Inorg. Chem., 27, 1988,221. Nozue, Y., Kodaira, T., Terasaki, O., Yamazaki, K., Goto, T., Watanabe, D. and Thomas, J.M., J. Phys. Condensed

Matter, 2, 1990, 5209.

1O. Endo, H., Maruyama,

K., Tsuzuki, T. and Yao, M.,

Jap. J. Appl. Phys., 32, 1993 (Suppl. 32-2), 773.

Poborchii, V.V., Ivanova, M.S., Petranovskii, V.P., Barnakov, Yu.A., Kasuya, A. and Nishina, Y.,

’”

Materials Science 1996, 129.

12 ’

13.

14. 15. 16. 17.

&

Engineering,

A2171218,

Poborchii, V.V., in Progress in Zeolite and Microporous Materials, Studies in Surface Science and Catalysis, Vol. 105 (Edited by H. Chon, S.-K. Ihm and Y.S. Uh), 631. Elsevier Science, 1997. Lin, Z., Wang, Z., Chen, W., Lin, L., Li, G., Liu, Z., Han, H. and Wang, Z., Solid State Commun., 100, 1996, 841. Scott, D.W., McCullough, J.P. and Kruse, F.H., J. Mol. Spectrosc., 32, 1969, 13. Steudel, R., Spectrochimica Acta, 31A, 1975, 1065. Smirnov, KS., Le Maire, M., Bremard, C. and Bougeard, D., Chem. Phys., 179, 1994,445. Poborchii, V.V., Kolobov, A.V., Caro, J., Zhuravlev, V.V. and Tanaka, K., Chem. Phys. Lett., 280, 1997, 17.

18. 19.

Steudel, R. and Rebsch, M., J. Mol. Spectrosc., 1974, 189. Poborchii, V.V., in preparation.

51,