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Thermal and UV photolytic behavior of Fe(CO)5 on evaporated Fe films: an infrared reflection absorption study
Vibrational Spectroscopy 18 Ž1998. 141–147
Thermal and UV photolytic behavior of Fe žCO /5 on evaporated Fe films: an infrared reflection absorption study T. Tanabe, T. Morisato, Y. Suzuki, Y. Matsumoto, T. Wadayama, A. Hatta
)
Department of Materials Science, Graduate School of Engineering, Tohoku UniÕersity, Aoba-yama 02, Sendai 980-8579, Japan Received 29 June 1998; revised 2 October 1998; accepted 5 October 1998
Abstract Infrared reflection absorption spectroscopy ŽIRRAS. has been used to observe the thermal and UV Ž254 nm. photolytic behavior of FeŽCO.5 initially deposited at 90 K on evaporated Fe films. A molecular distortion of FeŽCO.5 upon adsorption on the surface is suggested by the appearance of the equatorial C–O stretch mode ŽAX1 . that is infrared inactive in the original trigonal bipyramidal geometry. The approximate saturated coverage Ž1.0 L exposure. of adsorbed FeŽCO.5 at 90 K yields a dominant IRRAS band at 2065 cmy1 due to the axial C–O stretch mode ŽAY2 .. Increasing exposure to 1.2 L yields a shoulder at 2060 cmy1 which increases in intensity and shifts to 2065 cmy1 after 3.2 L exposure due to multilayer formation. Reconstruction of the multilayer is proposed to occur before molecular desorption at 155 K. Heating to 160 K results in small band features which we ascribe to a small amount of FeŽCO.5. UV photolysis of an FeŽCO.5 multilayer at 90 K yields a dominant band at 2084 cmy1 due to formation of FeŽCO.4 . Upon heating to 260 K this band decreases in intensity and shifts to 2064 cmy1 followed by disappearing at 270 K. Instead a major band is observed at 2050 cmy1, which disappears at 280 K. Formation of an Fe 3ŽCO.12 species at 260 K is proposed. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Surface photochemistry; Infrared absorption spectroscopy; Chemisorption; Physical adsorption; Surface chemical reaction; Iron; Iron pentacarbonyl
1. Introduction A number of investigations have been made to reveal thermal and photolytic products from metal carbonyls in low-temperature matrices w1x. However, similar information on solid surfaces is much less known. There is little doubt that the thermal and
) Corresponding author. Fax: q81-22-217-7318; E-mail: [email protected]
photolytic reactions of metal carbonyls depend on the nature of the substrate. Actually, while such reaction processes have been investigated for FeŽCO.5 on insulators w2–5x, semiconductors w6–8x, and metal surfaces w9–15x, effects of the substrate materials have not fully been understood. Most of these investigations have been made using infrared absorption spectroscopy to identify intermediates or products generating in the surface reactions. By means of infrared reflection absorption spectroscopy ŽIRRAS., Zaera w10x has shown that FeŽCO.5 on PtŽ111. decomposes into FeŽCO.4 at
0924-2031r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 8 . 0 0 0 5 7 - 5
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T. Tanabe et al.r Vibrational Spectroscopy 18 (1998) 141–147
185 K and then FeŽCO. 3 is produced at 240 K, both of which desorb as FeŽCO.5 after recombination with adsorbed CO at 195 and 265 K, respectively. However, such a recombination to form FeŽCO.5 does not occur in the thermal decomposition of FeŽCO.5 on NiŽ100. w14x. Although to date little attention has been given to thermal and photolytic reactions of FeŽCO.5 on iron metal surfaces, they appear of interest because the parent metal surface could interact much more directly to the species produced from FeŽCO.5 or precursor in such reaction processes. In the work described below IRRAS was used to study the adsorption of FeŽCO.5 as well as its thermal and UV photolytic reactions on evaporated Fe films. Possible surface species resulting from these reactions will be discussed in this paper.
2. Experimental The Iron films were deposited in a mass thickness of 30 nm onto room temperature SiŽ111. wafers by electron-beam evaporation in an ultrahigh vacuum ŽUHV. chamber, previously evacuated by a turbomolecular pump to a base pressure of approximately 1 = 10y1 0 Torr. The raw Fe material was of 99.99% purity and was evaporated at a rate of 0.1 nmrs. The vacuum chamber was equipped with a reflection high energy electron diffraction ŽRHEED. apparatus. RHEED observations of the Fe films revealed a lack of crystallographic order. The Fe-deposited Si wafer was mounted on a long-travel translational stage and then magnetically transferred to another chamber pumped to 2.5 = 10y1 0 Torr where Fourier-transform IRRAS measurements were performed. The Fe film could be cooled or heated by a combination of liquid nitrogen cooling and electrical resistive heating in the range 90–570 K as monitored with a chromel–alumel thermocouple. FeŽCO.5 was admitted into the vacuum chamber through a doser attached to an adjustable leak valve. The exposures were determined using an ion gauge to monitor the pressure in the chamber. The FeŽCO.5 sample was obtained from Kanto Chemical Ž95% purity., and was purified by vacuum distillation before use. A p-polarized infrared beam enters and exits the vacuum chamber through BaF2 windows mounted on conflat flanges. The infrared beam was incident at an
angle of 808 with respect to the surface normal. IRRAS spectra were obtained as the average of 500 scans with 4 cmy1 resolution using a Mattson RS-2 FT-IR spectrometer equipped with a liquidnitrogen-cooled HgCdTe detector. Each of the spectra was ratioed against the spectrum of the clean surface. UV photolysis was performed with 254-nm radiation from a low-pressure mercury lamp Ž12 W.
3. Results and discussion 3.1. Adsorption of Fe(CO)5 on Fe Fig. 1 shows IRRAS spectra in the C–O stretching region for FeŽCO.5 adsorbed on an evaporated Fe film at 90 K as a function of exposure. The spectrum after 0.1 L exposure Ž1 L s 10y6 Torr s. exhibits a very weak band at 2045 cmy1 which increases in intensity and shifts to 2065 cmy1 at 1.0 L exposure due to increased dipole–dipole coupling between the adsorbed molecules. In the spectra
Fig. 1. IRRAS spectra in the C–O stretching region as a function of exposure for FeŽCO.5 on an Fe film surface at 90 K.
T. Tanabe et al.r Vibrational Spectroscopy 18 (1998) 141–147
recorded after small exposures very weak bands are seen near 2125 and 2085 cmy1 ; the latter band probably arises from a small amount of photolysis before starting measurements. As the exposure is increased to 1.2 L, a shoulder appears at 2060 cmy1 which grows and shifts to 2066 cmy1 after 3.2 L exposure, the maximum exposure investigated. In addition weak bands appear at 2013 and 1964 cmy1 for 1.2 L exposure. These bands hardly shift with further exposures. The integrated area of the dominant band observed in Fig. 1 is plotted as a function of exposure in Fig. 2. We can see that the band area Ži.e., integrated intensity. increases steeply first and then slowly with increasing exposure up to approximately 1.0 L, an exposure that likely corresponds to full coverage of the surface with an adsorbed monolayer of FeŽCO.5 . Above 1.0 L the band intensity increases linearly with exposure up to 3.2 L. This feature can be taken as evidence for multilayer condensation. Thus, the weak bands at 2013 and 1964 cmy1 observed for exposures above 1.2 L must be attributed to the condensed species on the monolayer. The 2125 cmy1 band observed at small exposures shifts to 2120 cmy1 at 3.2 L with little increase in intensity. This band is thought to arise from the monolayer. It is known that FeŽCO.5 has a trigonal bipyramidal structure with D 3h symmetry w16,17x for which one may expect two infrared active modes, AY2 Žaxial.
Fig. 2. IRRAS band areas Žintegrated intensities. as a function of exposure for iron pentacarbonyl on the Fe film at 90 K.
143
and EX Žequatorial., and three Raman active modes, 2AX 1 Žaxial, equatorial. and EX Žequatorial., for the C–O stretch vibrations. Cataliotti et al. w18x investigated the infrared absorption of crystalline FeŽCO.5 on KBr at 190–200 K and assigned bands at 2115, 2033, 2003, and a doublet at 1982 and 1977 cmy1 to the modes AX 1 Žequatorial., AX 1 Žaxial., AY2 Žaxial., and EX Žequatorial., respectively. The appearance of the AX 1 vibrations, which should be infrared inactive for the free molecule, has been explained w18x by the C 2 site symmetry of the single molecule in the crystalline state w19x. On the other hand, vapor phase FeŽCO.5 gives two infrared bands at 2034 and 2012 cmy1 due to the AY2 Žaxial. and EX Žequatorial. modes, respectively w18x. The corresponding bands are observed at 2002 and 1979 cmy1 in the liquid phase w17x. Thus, it is clear that the agreement between the infrared absorption data described above and our IRRAS data shown in Fig. 1 is far from complete. Accordingly, it seems very likely that some perturbations are imposed on the surface FeŽCO.5 molecules. FeŽCO.5 adsorbed on porous Vycor glass w4x and zeolites w20x exhibits a very weak infrared band near 2115 cmy1 due to the equatorial AX 1 mode which is infrared inactive on the basis of D 3h symmetry. Appearance of this mode has been explained by assuming the noncoplanarity of the equatorial CO groups ŽC 3v symmetry. due to adsorption w20x. We also adopt this assumption to explain the monolayer band at 2125 cmy1 observed in Fig. 1. In this case the noncoplanarity of the equatorial CO groups is justified when the molecule is aligned with its major axis perpendicular to the Fe surface. This is because the probing electric field is almost entirely polarized perpendicular to the metal surface and only those modes that produce a dynamic dipole normal to the surface should be active in IRRAS Žnormal-dipole selection rule.. The perpendicular alignment of the molecule may be responsible for the failure to observe the equatorial EX mode in Fig. 1. The equatorial AX 1 mode has also been observed in IRRAS for adsorbed FeŽCO.5 on AuŽ111. w15x and polycrystalline Ag w12x at 90–95 K. On these surfaces and on PtŽ111. at 110 K w10x as well, a C–O stretch band appears around 2060 cmy1 . This band is relatively close in position to the axial AY2 band in the gas phase Ž2033 cmy1 . w18x, but yet it is about
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T. Tanabe et al.r Vibrational Spectroscopy 18 (1998) 141–147
30 cmy1 higher than the gas phase band. This blue shift has been explained by a change in geometry from a trigonal bipyramid to a C 4v square pyramid w10,12,15x. This geometrical change seems unlikely on the Fe surface in light of the observation of the equatorial AX 1 mode. In our case the axial AY2 band is located at 2045 cmy1 in the low coverage limit Ži.e., 0.1 L., where the effect of dipole–dipole coupling is negligible. It then follows that the blue shift is less pronounced in our case. Because of no evidence for the presence of any other species on the Fe surface we propose that the blue shift of the AY2 mode could be due to adsorption via the oxygen end on the surface. A blue shift is expected if the associated dipole moment derivative could be negative w21x. The position of the AY2 band we observed at multilayer exposures is nearly the same as that observed at full coverage of the monolayer, indicative of the participation of dipolar coupling that is collective. In our Raman spectra for multilayer FeŽCO.5 on Ag w22x a doublet feature was observed at 1995 and 1985 cmy1 due to an equatorial EX mode in addition to the bands at 2115 Žequatorial AX 1 . and 2030 cmy1 Žaxial AX 1 .. Assignments for the 2013 and 1964 cmy1 bands observed in Fig. 1 are not straightforward, but the latter band may correspond to the EX mode because of its steady growth with prolonged exposures. 3.2. Thermal behaÕior of Fe(CO)5 without photolysis The FeŽCO.5 sample prepared by 3.2 L exposure at 90 K was then heated to successively higher temperatures. Fig. 3 shows IRRAS spectra measured in the C–O stretching region as a function of the indicated temperatures. It is clear that no spectral change occurs up to 130 K. Heating to 141 K results in a distinct spectrum that displays peaks at 2059, 1982, and 1970 cmy1 . At the same time two shoulders appear around 2050 and 2030 cmy1 . Additionally, the 2120, 2013, and 1964 cmy1 bands observed at 130 K cannot be seen in the spectrum. The 2059 cmy1 band arises from the 2066 cmy1 band of the multilayer at 130 K, and its red shift is due to a decrease in the overall effect of dipole–dipole coupling caused by diminution of the multilayer molecules as suggested by the concomitant decrease of the band intensity. Upon further heating to 150 K
Fig. 3. IRRAS spectra for 3.2 L iron pentacarbonyl on the Fe film at successively elevated temperatures. The spectrum of 3.2-L iron pentacarbonyl at 90 K is shown in Fig. 1.
the multilayer band at 2059 cmy1 completely disappears which places the most intense band at 2053 cmy1 . At 155 K the band shifts to 2050 cmy1 with a decrease in intensity, and after heating to 160 K small features are observed around 2050, 2030, and 2010 cmy1 . The two high frequency bands probably arise from a small amount of FeŽCO.5 . These bands disappear almost completely in the 170 K spectrum where the low frequency band at 2010 cmy1 is still observed. In view of its stability and position the 2010 cmy1 band is proposed to be due to CO adsorption w23x. It has been reported w24x that UV photolysis of silica-adsorbed FeŽCO.5 results in the appearance of a band at 2052 cmy1 , close in position to the 2053 cmy1 band at 150 K ŽFig. 3.. This band was assigned to Fe 3 ŽCO.12 w24x, but this species seems improbable in our case because it is rather stable even at room temperature w24x. Similarly, the formation of Fe 2 ŽCO. 9 is also unlikely w25x. It is clear from Fig. 3 that the 2053 cmy1 band at 150 K
T. Tanabe et al.r Vibrational Spectroscopy 18 (1998) 141–147
developed at the expense of the multilayer band near 2060 cmy1 observed at lower temperatures. The 2053 cmy1 band Žprobably the 2030, 1982, and 1970 cmy1 bands also. is proposed to correspond to a reconstructed multilayer since its intensity is too intense to be associated with a surface monolayer; its significant broadness is suggestive of relaxation of the molecules caused by annealing. Sato et al. w12x proposed that a dominant band at 2052 cmy1 observed on polycrystalline Ag at 190–230 K could be due to the conversion of FeŽCO.5 to FeŽCO.4 . Zaera w10x also suggested the formation of FeŽCO.4 on PtŽ111. at 185 K on the basis of the appearance of a band at 2035 cmy1 . However, it seems quite unlikely that such a thermal conversion could occur in the multilayer on the Fe surface at temperatures as low as 140 K. The monolayer and multilayer FeŽCO.5 molecules on AgŽ111. desorb at 182 and 170 K, respectively w11x. The corresponding temperatures are 190 and 160 K on RuŽ001. w26x. Moreover, the multilayer on PtŽ111. desorbs at 185 K w10x. Thus, it follows that the desorption temperatures are lower in our case where the multilayer begins to desorb around 155 K while only a small amount of adsorbed FeŽCO.5 remains at 165 K. 3.3. UV photolysis of Fe(CO)5 To examine the UV photolysis of FeŽCO.5 at 90 K, IRRAS spectra were recorded under 254-nm irradiation from a low-pressure Hg lamp. Fig. 4 shows the results as a function of UV irradiation time. As in the previous case, the sample was prepared by exposing FeŽCO.5 at 3.2 L to a 30-nm thick Fe film at 90 K. UV irradiation for 0.5 m results in the appearance of a new band at 2080 cmy1 , in addition to a shoulder at 2058 cmy1 due to the AY2 mode of unreacted FeŽCO.5 molecule. It is clear that a new species was generated by the irradiation at the expense of the multilayer FeŽCO.5 molecules. After 1 m of irradiation the AY2 band decreases in intensity and shifts to 2050 cmy1 . On the other hand, the new band is not significantly changed by prolonged irradiation except for a slight blue shift to 2084 cmy1 after 7 m of irradiation. Thus, it is clear that the UV photolysis of the FeŽCO.5 multilayer at 90 K leads to a single dominant band in the C–O stretching region.
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Fig. 4. IRRAS spectra for 3.2-L iron pentacarbonyl on an Fe film at 90 K as a function of UV Ž254 nm. irradiation time.
It is also noted that the resulting photoproduct is highly resistant to the UV photon. We now discuss the UV photoproduct from FeŽCO.5 based on Fig. 4. The complete UV-decarbonylation of FeŽCO.5 Ži.e., CO evolution and Fe deposition. occurs on SiŽ111. at 120 K w27x as well as on TiO 2 Žanatase. at room temperature w8x. If such a complete decomposition occurs in our case, the resulting CO would be adsorbed on the Fe surface. However this possibility can be ruled out. Firstly, our previous experiments w28x showed that the saturation coverage of adsorbed CO at 90 K on evaporated Fe films gives rise to a C–O stretch band at 2055 cmy1 that is sufficiently low compared to 2084 cmy1 . Secondly, a different spectrum is observed upon heating the Fe surface to 260 K after UV photolysis at 90 K, as shown later. It is therefore possible that the 2084 cmy1 band is associated with some polynuclear iron carbonyl such as Fe 2 ŽCO. 8 , Fe 2 ŽCO. 9 , and Fe 3 ŽCO.12 . Actually, these species have been suggested to be the main products of UV photolysis on silica w2x, porous Vycor glass w4x, and
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T. Tanabe et al.r Vibrational Spectroscopy 18 (1998) 141–147
alumina w29x. Furthermore, Fe 2 ŽCO. 9 is known to be the major product in the sunlight photolysis of gasphase FeŽCO.5 w30,31x. It is already established w32x that Fe 2 ŽCO. 9 has a structure of D 3h symmetry in which two FeŽCO. 3 trigonal pyramids are bridged by three carbonyls. This molecule in the solid state exhibits two terminal Ž2080 and 2034 cmy1 . and one bridging Ž1828 cmy1 . C–O stretch bands w31x in accordance with the geometry. However, the formation of Fe 2 ŽCO. 9 is unlikely in the present case because Fig. 4 shows no any band corresponding to the terminal 2034 cmy1 band that is far more intense than the 2080 cmy1 band w31x. Sato and Suzuki w15x have shown that the 320-nm photolysis of FeŽCO.5 adsorbed on AuŽ111. at 90 K produces a single intense band at 2081 cmy1 . They ascribed this band to an FeŽCO.4 species with its Fe atom bonded directly to the surface. According to the infrared spectrum of Fe 3 ŽCO.12 reported by Sheline w33x, the position of the highest Žalso intense. C–O stretch band is as low as 2043 cmy1 . Poliakoff and Turner w34x have shown that UV irradiation of Fe 2 ŽCO. 9 in an argon matrix results in Fe 2 ŽCO. 8 in both the bridged and unbridged forms; in either event the highest C–O stretching frequency is lower than 2066 cmy1 for Fe 2 ŽCO. 9 in the same matrix. These facts make it unlikely that Fe 2 ŽCO. 8 or Fe 3 ŽCO.12 was formed in our UV photolysis. We therefore propose that the band at 2084 cmy1 in Fig. 4 is associated with an intermediate FeŽCO.4 species as on AuŽ111. w15x. After completion of photolysis at 90 K for 7 m IRRAS spectra were taken at elevated temperatures, the results of which are shown in Fig. 5. The 2084 cmy1 band observed at 90 K increases a little in intensity and shifts to 2078 cmy1 after heating to 180 K, suggesting a realignment of the FeŽCO.4 photoproduct. Upon heating to 220 K the main band is reduced in intensity and shifts to 2074 cmy1 . After heating to 260 K this band is located at 2064 cmy1 accompanied by a shoulder at 2053 cmy1 . Heating to 270 K results in the appearance of a band at 2050 cmy1 and a broad feature centered at 2110 cmy1 ; the former band arises from the shoulder Ž2053 cmy1 . in the 260-K spectrum. The development of the 2050 cmy1 band at 270 K must be attributed to a new thermal product. A possible reaction that might begin to take place at 260 K is polymerization of the
Fig. 5. IRRAS spectra after UV Ž254 nm. photolysis of iron pentacarbonyl on the Fe film at 90 K followed by heating the substrate to the indicated temperatures. The spectrum just before heating Žafter 7-min UV photolysis. is shown in Fig. 4, but note the different scale.
intermediate FeŽCO.4 species to yield Fe 2 ŽCO. 8 or Fe 3 ŽCO.12 . It is known that Fe 3 ŽCO.12 exhibits a major band at 2053 cmy1 and a minor band at 2109 cmy1 w2,4x, close in position to the bands observed in the 270-K spectrum. We therefore adopt this species as the most likely product. The broad features observed at 280 and 295 K suggest the presence of other carbonyl species on the surface.
4. Summary In this study IRRAS has been used to observe the thermal and UV Ž254 nm. photolytic behavior of FeŽCO.5 on evaporated Fe films. It is proposed that subcarbonylation into FeŽCO.4 occurs upon UV photolysis at 90 K followed by generation of Fe 3 ŽCO.12 rather than Fe 2 ŽCO. 8 at 260 K. IRRAS spectra for the condensed FeŽCO.5 multilayer before UV photolysis reveal that it is thermally stable up to 130 K
T. Tanabe et al.r Vibrational Spectroscopy 18 (1998) 141–147
but is reconstructed at around 140 K before molecular desorption at 155 K. Only a small amount of adsorbed FeŽCO.5 is observed at 160 K together with adsorbed CO which remains sufficiently stable at 170 K.
Acknowledgements This work has been supported in part by a Grantin-Aid for Scientific Research ŽA. from the Ministry of Education, Science, Sports and Culture.
References w1x M. Poliakoff, J.J. Turner, J. Chem. Soc. Dalton Ž1974. 2276, and references therein. w2x M.R. Trusheim, R.L. Jackson, J. Phys. Chem. 87 Ž1983. 1910. w3x F.G. Celii, P.M. Whitmore, K.C. Janda, J. Phys. Chem. 92 Ž1988. 1604. w4x M.S. Darsillo, H.D. Gafney, M.S. Paquette, J. Am. Chem. Soc. 109 Ž1987. 3275. w5x S. Sato, T. Ohmori, J. Phys. Chem. 95 Ž1991. 7778. w6x N. Bottka, P.J. Walsh, R.Z. Dalbey, J. Appl. Phys. 54 Ž1983. 1104. w7x N.S. Gluck, Z. Ying, C.E. Bartosch, W. Ho, J. Chem. Phys. 86 Ž1987. 4957. w8x Y. Ukisu, S. Sato, T. Ohmori, Appl. Organometal. Chem. 5 Ž1991. 243. w9x J.R. Swanson, C.M. Friend, J. Vac. Sci. Technol. A 6 Ž1988. 770. w10x F. Zaera, Surf. Sci. 255 Ž1991. 280.
147
w11x M.A. Henderson, R.D. Ramsier, J.T. Yates Jr., Surf. Sci. 275 Ž1992. 297. w12x S. Sato, Y. Ukisu, H. Ogawa, Y. Takasu, J. Chem. Soc. Faraday 89 Ž1993. 4387. w13x S. Sato, Y. Ukisu, Surf. Sci. 283 Ž1993. 137. w14x M. Xu, F. Zaera, Surf. Sci. 315 Ž1994. 40. w15x S. Sato, T. Suzuki, J. Phys. Chem. 100 Ž1996. 14769. w16x H. Stammreich, Osw. Sala, Y. Tavares, J. Chem. Phys. 30 Ž1959. 856. w17x M. Bigorgne, J. Organometal. Chem. 24 Ž1970. 211. w18x R. Cataliotti, A. Foffani, L. Marchetti, Inorg. Chem. 10 Ž1971. 1594. w19x J. Donohue, A. Caron, Acta Cryst. 17 Ž1964. 663. w20x T. Bein, P.A. Jacobs, J. Chem. Soc. Faraday 79 Ž1983. 1819. w21x T.L. Brown, D.J. Darensbourg, Inorg. Chem. 6 Ž1967. 971. w22x K. Yamatake, T. Tanabe, Y. Suzuki, Y. Matsumoto, T. Wadayama, A. Hatta, J. Raman Spectrosc. 29 Ž1998. 781. w23x T. Tanabe, Y. Suzuki, T. Wadayama, A. Hatta, unpublished data. w24x R.L. Jackson, M.R. Trusheim, J. Am. Chem. Soc. 104 Ž1982. 6590. w25x S.C. Fletcher, M. Poliakoff, J.J. Turner, Inorg. Chem. 25 Ž1986. 3597. w26x H.H. Huang, C.S. Sreekanth, C.S. Seet, G.Q. Xu, L. Chan, J. Phys. Chem. 100 Ž1996. 18138. w27x J.R. Swanson, C.M. Friend, Y.J. Chabal, J. Chem. Phys. 87 Ž1987. 5028. w28x T. Tanabe, Y. Suzuki, T. Wadayama, A. Hatta, to be published. w29x S. Sato, T. Ohmori, J. Chem. Soc., Chem. Commun. Ž1990. 1032. w30x M.H. Chisholm, A.G. Massey, N.R. Thompson, Nature 211 Ž1966. 67. w31x R.K. Sheline, K.S. Pitzer, J. Am. Chem. Soc. 72 Ž1950. 1107. w32x H.M. Powell, R.V.G. Ewens, J. Chem. Soc. Ž1939. 286. w33x R.K. Sheline, J. Am. Chem. Soc. 73 Ž1951. 1615. w34x M. Poliakoff, J.J. Turner, J. Chem. Soc. ŽA. Ž1971. 2403.
Thermal and UV photolytic behavior of Fe žCO /5 on evaporated Fe films: an infrared reflection absorption study T. Tanabe, T. Morisato, Y. Suzuki, Y. Matsumoto, T. Wadayama, A. Hatta
)
Department of Materials Science, Graduate School of Engineering, Tohoku UniÕersity, Aoba-yama 02, Sendai 980-8579, Japan Received 29 June 1998; revised 2 October 1998; accepted 5 October 1998
Abstract Infrared reflection absorption spectroscopy ŽIRRAS. has been used to observe the thermal and UV Ž254 nm. photolytic behavior of FeŽCO.5 initially deposited at 90 K on evaporated Fe films. A molecular distortion of FeŽCO.5 upon adsorption on the surface is suggested by the appearance of the equatorial C–O stretch mode ŽAX1 . that is infrared inactive in the original trigonal bipyramidal geometry. The approximate saturated coverage Ž1.0 L exposure. of adsorbed FeŽCO.5 at 90 K yields a dominant IRRAS band at 2065 cmy1 due to the axial C–O stretch mode ŽAY2 .. Increasing exposure to 1.2 L yields a shoulder at 2060 cmy1 which increases in intensity and shifts to 2065 cmy1 after 3.2 L exposure due to multilayer formation. Reconstruction of the multilayer is proposed to occur before molecular desorption at 155 K. Heating to 160 K results in small band features which we ascribe to a small amount of FeŽCO.5. UV photolysis of an FeŽCO.5 multilayer at 90 K yields a dominant band at 2084 cmy1 due to formation of FeŽCO.4 . Upon heating to 260 K this band decreases in intensity and shifts to 2064 cmy1 followed by disappearing at 270 K. Instead a major band is observed at 2050 cmy1, which disappears at 280 K. Formation of an Fe 3ŽCO.12 species at 260 K is proposed. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Surface photochemistry; Infrared absorption spectroscopy; Chemisorption; Physical adsorption; Surface chemical reaction; Iron; Iron pentacarbonyl
1. Introduction A number of investigations have been made to reveal thermal and photolytic products from metal carbonyls in low-temperature matrices w1x. However, similar information on solid surfaces is much less known. There is little doubt that the thermal and
) Corresponding author. Fax: q81-22-217-7318; E-mail: [email protected]
photolytic reactions of metal carbonyls depend on the nature of the substrate. Actually, while such reaction processes have been investigated for FeŽCO.5 on insulators w2–5x, semiconductors w6–8x, and metal surfaces w9–15x, effects of the substrate materials have not fully been understood. Most of these investigations have been made using infrared absorption spectroscopy to identify intermediates or products generating in the surface reactions. By means of infrared reflection absorption spectroscopy ŽIRRAS., Zaera w10x has shown that FeŽCO.5 on PtŽ111. decomposes into FeŽCO.4 at
0924-2031r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 8 . 0 0 0 5 7 - 5
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T. Tanabe et al.r Vibrational Spectroscopy 18 (1998) 141–147
185 K and then FeŽCO. 3 is produced at 240 K, both of which desorb as FeŽCO.5 after recombination with adsorbed CO at 195 and 265 K, respectively. However, such a recombination to form FeŽCO.5 does not occur in the thermal decomposition of FeŽCO.5 on NiŽ100. w14x. Although to date little attention has been given to thermal and photolytic reactions of FeŽCO.5 on iron metal surfaces, they appear of interest because the parent metal surface could interact much more directly to the species produced from FeŽCO.5 or precursor in such reaction processes. In the work described below IRRAS was used to study the adsorption of FeŽCO.5 as well as its thermal and UV photolytic reactions on evaporated Fe films. Possible surface species resulting from these reactions will be discussed in this paper.
2. Experimental The Iron films were deposited in a mass thickness of 30 nm onto room temperature SiŽ111. wafers by electron-beam evaporation in an ultrahigh vacuum ŽUHV. chamber, previously evacuated by a turbomolecular pump to a base pressure of approximately 1 = 10y1 0 Torr. The raw Fe material was of 99.99% purity and was evaporated at a rate of 0.1 nmrs. The vacuum chamber was equipped with a reflection high energy electron diffraction ŽRHEED. apparatus. RHEED observations of the Fe films revealed a lack of crystallographic order. The Fe-deposited Si wafer was mounted on a long-travel translational stage and then magnetically transferred to another chamber pumped to 2.5 = 10y1 0 Torr where Fourier-transform IRRAS measurements were performed. The Fe film could be cooled or heated by a combination of liquid nitrogen cooling and electrical resistive heating in the range 90–570 K as monitored with a chromel–alumel thermocouple. FeŽCO.5 was admitted into the vacuum chamber through a doser attached to an adjustable leak valve. The exposures were determined using an ion gauge to monitor the pressure in the chamber. The FeŽCO.5 sample was obtained from Kanto Chemical Ž95% purity., and was purified by vacuum distillation before use. A p-polarized infrared beam enters and exits the vacuum chamber through BaF2 windows mounted on conflat flanges. The infrared beam was incident at an
angle of 808 with respect to the surface normal. IRRAS spectra were obtained as the average of 500 scans with 4 cmy1 resolution using a Mattson RS-2 FT-IR spectrometer equipped with a liquidnitrogen-cooled HgCdTe detector. Each of the spectra was ratioed against the spectrum of the clean surface. UV photolysis was performed with 254-nm radiation from a low-pressure mercury lamp Ž12 W.
3. Results and discussion 3.1. Adsorption of Fe(CO)5 on Fe Fig. 1 shows IRRAS spectra in the C–O stretching region for FeŽCO.5 adsorbed on an evaporated Fe film at 90 K as a function of exposure. The spectrum after 0.1 L exposure Ž1 L s 10y6 Torr s. exhibits a very weak band at 2045 cmy1 which increases in intensity and shifts to 2065 cmy1 at 1.0 L exposure due to increased dipole–dipole coupling between the adsorbed molecules. In the spectra
Fig. 1. IRRAS spectra in the C–O stretching region as a function of exposure for FeŽCO.5 on an Fe film surface at 90 K.
T. Tanabe et al.r Vibrational Spectroscopy 18 (1998) 141–147
recorded after small exposures very weak bands are seen near 2125 and 2085 cmy1 ; the latter band probably arises from a small amount of photolysis before starting measurements. As the exposure is increased to 1.2 L, a shoulder appears at 2060 cmy1 which grows and shifts to 2066 cmy1 after 3.2 L exposure, the maximum exposure investigated. In addition weak bands appear at 2013 and 1964 cmy1 for 1.2 L exposure. These bands hardly shift with further exposures. The integrated area of the dominant band observed in Fig. 1 is plotted as a function of exposure in Fig. 2. We can see that the band area Ži.e., integrated intensity. increases steeply first and then slowly with increasing exposure up to approximately 1.0 L, an exposure that likely corresponds to full coverage of the surface with an adsorbed monolayer of FeŽCO.5 . Above 1.0 L the band intensity increases linearly with exposure up to 3.2 L. This feature can be taken as evidence for multilayer condensation. Thus, the weak bands at 2013 and 1964 cmy1 observed for exposures above 1.2 L must be attributed to the condensed species on the monolayer. The 2125 cmy1 band observed at small exposures shifts to 2120 cmy1 at 3.2 L with little increase in intensity. This band is thought to arise from the monolayer. It is known that FeŽCO.5 has a trigonal bipyramidal structure with D 3h symmetry w16,17x for which one may expect two infrared active modes, AY2 Žaxial.
Fig. 2. IRRAS band areas Žintegrated intensities. as a function of exposure for iron pentacarbonyl on the Fe film at 90 K.
143
and EX Žequatorial., and three Raman active modes, 2AX 1 Žaxial, equatorial. and EX Žequatorial., for the C–O stretch vibrations. Cataliotti et al. w18x investigated the infrared absorption of crystalline FeŽCO.5 on KBr at 190–200 K and assigned bands at 2115, 2033, 2003, and a doublet at 1982 and 1977 cmy1 to the modes AX 1 Žequatorial., AX 1 Žaxial., AY2 Žaxial., and EX Žequatorial., respectively. The appearance of the AX 1 vibrations, which should be infrared inactive for the free molecule, has been explained w18x by the C 2 site symmetry of the single molecule in the crystalline state w19x. On the other hand, vapor phase FeŽCO.5 gives two infrared bands at 2034 and 2012 cmy1 due to the AY2 Žaxial. and EX Žequatorial. modes, respectively w18x. The corresponding bands are observed at 2002 and 1979 cmy1 in the liquid phase w17x. Thus, it is clear that the agreement between the infrared absorption data described above and our IRRAS data shown in Fig. 1 is far from complete. Accordingly, it seems very likely that some perturbations are imposed on the surface FeŽCO.5 molecules. FeŽCO.5 adsorbed on porous Vycor glass w4x and zeolites w20x exhibits a very weak infrared band near 2115 cmy1 due to the equatorial AX 1 mode which is infrared inactive on the basis of D 3h symmetry. Appearance of this mode has been explained by assuming the noncoplanarity of the equatorial CO groups ŽC 3v symmetry. due to adsorption w20x. We also adopt this assumption to explain the monolayer band at 2125 cmy1 observed in Fig. 1. In this case the noncoplanarity of the equatorial CO groups is justified when the molecule is aligned with its major axis perpendicular to the Fe surface. This is because the probing electric field is almost entirely polarized perpendicular to the metal surface and only those modes that produce a dynamic dipole normal to the surface should be active in IRRAS Žnormal-dipole selection rule.. The perpendicular alignment of the molecule may be responsible for the failure to observe the equatorial EX mode in Fig. 1. The equatorial AX 1 mode has also been observed in IRRAS for adsorbed FeŽCO.5 on AuŽ111. w15x and polycrystalline Ag w12x at 90–95 K. On these surfaces and on PtŽ111. at 110 K w10x as well, a C–O stretch band appears around 2060 cmy1 . This band is relatively close in position to the axial AY2 band in the gas phase Ž2033 cmy1 . w18x, but yet it is about
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30 cmy1 higher than the gas phase band. This blue shift has been explained by a change in geometry from a trigonal bipyramid to a C 4v square pyramid w10,12,15x. This geometrical change seems unlikely on the Fe surface in light of the observation of the equatorial AX 1 mode. In our case the axial AY2 band is located at 2045 cmy1 in the low coverage limit Ži.e., 0.1 L., where the effect of dipole–dipole coupling is negligible. It then follows that the blue shift is less pronounced in our case. Because of no evidence for the presence of any other species on the Fe surface we propose that the blue shift of the AY2 mode could be due to adsorption via the oxygen end on the surface. A blue shift is expected if the associated dipole moment derivative could be negative w21x. The position of the AY2 band we observed at multilayer exposures is nearly the same as that observed at full coverage of the monolayer, indicative of the participation of dipolar coupling that is collective. In our Raman spectra for multilayer FeŽCO.5 on Ag w22x a doublet feature was observed at 1995 and 1985 cmy1 due to an equatorial EX mode in addition to the bands at 2115 Žequatorial AX 1 . and 2030 cmy1 Žaxial AX 1 .. Assignments for the 2013 and 1964 cmy1 bands observed in Fig. 1 are not straightforward, but the latter band may correspond to the EX mode because of its steady growth with prolonged exposures. 3.2. Thermal behaÕior of Fe(CO)5 without photolysis The FeŽCO.5 sample prepared by 3.2 L exposure at 90 K was then heated to successively higher temperatures. Fig. 3 shows IRRAS spectra measured in the C–O stretching region as a function of the indicated temperatures. It is clear that no spectral change occurs up to 130 K. Heating to 141 K results in a distinct spectrum that displays peaks at 2059, 1982, and 1970 cmy1 . At the same time two shoulders appear around 2050 and 2030 cmy1 . Additionally, the 2120, 2013, and 1964 cmy1 bands observed at 130 K cannot be seen in the spectrum. The 2059 cmy1 band arises from the 2066 cmy1 band of the multilayer at 130 K, and its red shift is due to a decrease in the overall effect of dipole–dipole coupling caused by diminution of the multilayer molecules as suggested by the concomitant decrease of the band intensity. Upon further heating to 150 K
Fig. 3. IRRAS spectra for 3.2 L iron pentacarbonyl on the Fe film at successively elevated temperatures. The spectrum of 3.2-L iron pentacarbonyl at 90 K is shown in Fig. 1.
the multilayer band at 2059 cmy1 completely disappears which places the most intense band at 2053 cmy1 . At 155 K the band shifts to 2050 cmy1 with a decrease in intensity, and after heating to 160 K small features are observed around 2050, 2030, and 2010 cmy1 . The two high frequency bands probably arise from a small amount of FeŽCO.5 . These bands disappear almost completely in the 170 K spectrum where the low frequency band at 2010 cmy1 is still observed. In view of its stability and position the 2010 cmy1 band is proposed to be due to CO adsorption w23x. It has been reported w24x that UV photolysis of silica-adsorbed FeŽCO.5 results in the appearance of a band at 2052 cmy1 , close in position to the 2053 cmy1 band at 150 K ŽFig. 3.. This band was assigned to Fe 3 ŽCO.12 w24x, but this species seems improbable in our case because it is rather stable even at room temperature w24x. Similarly, the formation of Fe 2 ŽCO. 9 is also unlikely w25x. It is clear from Fig. 3 that the 2053 cmy1 band at 150 K
T. Tanabe et al.r Vibrational Spectroscopy 18 (1998) 141–147
developed at the expense of the multilayer band near 2060 cmy1 observed at lower temperatures. The 2053 cmy1 band Žprobably the 2030, 1982, and 1970 cmy1 bands also. is proposed to correspond to a reconstructed multilayer since its intensity is too intense to be associated with a surface monolayer; its significant broadness is suggestive of relaxation of the molecules caused by annealing. Sato et al. w12x proposed that a dominant band at 2052 cmy1 observed on polycrystalline Ag at 190–230 K could be due to the conversion of FeŽCO.5 to FeŽCO.4 . Zaera w10x also suggested the formation of FeŽCO.4 on PtŽ111. at 185 K on the basis of the appearance of a band at 2035 cmy1 . However, it seems quite unlikely that such a thermal conversion could occur in the multilayer on the Fe surface at temperatures as low as 140 K. The monolayer and multilayer FeŽCO.5 molecules on AgŽ111. desorb at 182 and 170 K, respectively w11x. The corresponding temperatures are 190 and 160 K on RuŽ001. w26x. Moreover, the multilayer on PtŽ111. desorbs at 185 K w10x. Thus, it follows that the desorption temperatures are lower in our case where the multilayer begins to desorb around 155 K while only a small amount of adsorbed FeŽCO.5 remains at 165 K. 3.3. UV photolysis of Fe(CO)5 To examine the UV photolysis of FeŽCO.5 at 90 K, IRRAS spectra were recorded under 254-nm irradiation from a low-pressure Hg lamp. Fig. 4 shows the results as a function of UV irradiation time. As in the previous case, the sample was prepared by exposing FeŽCO.5 at 3.2 L to a 30-nm thick Fe film at 90 K. UV irradiation for 0.5 m results in the appearance of a new band at 2080 cmy1 , in addition to a shoulder at 2058 cmy1 due to the AY2 mode of unreacted FeŽCO.5 molecule. It is clear that a new species was generated by the irradiation at the expense of the multilayer FeŽCO.5 molecules. After 1 m of irradiation the AY2 band decreases in intensity and shifts to 2050 cmy1 . On the other hand, the new band is not significantly changed by prolonged irradiation except for a slight blue shift to 2084 cmy1 after 7 m of irradiation. Thus, it is clear that the UV photolysis of the FeŽCO.5 multilayer at 90 K leads to a single dominant band in the C–O stretching region.
145
Fig. 4. IRRAS spectra for 3.2-L iron pentacarbonyl on an Fe film at 90 K as a function of UV Ž254 nm. irradiation time.
It is also noted that the resulting photoproduct is highly resistant to the UV photon. We now discuss the UV photoproduct from FeŽCO.5 based on Fig. 4. The complete UV-decarbonylation of FeŽCO.5 Ži.e., CO evolution and Fe deposition. occurs on SiŽ111. at 120 K w27x as well as on TiO 2 Žanatase. at room temperature w8x. If such a complete decomposition occurs in our case, the resulting CO would be adsorbed on the Fe surface. However this possibility can be ruled out. Firstly, our previous experiments w28x showed that the saturation coverage of adsorbed CO at 90 K on evaporated Fe films gives rise to a C–O stretch band at 2055 cmy1 that is sufficiently low compared to 2084 cmy1 . Secondly, a different spectrum is observed upon heating the Fe surface to 260 K after UV photolysis at 90 K, as shown later. It is therefore possible that the 2084 cmy1 band is associated with some polynuclear iron carbonyl such as Fe 2 ŽCO. 8 , Fe 2 ŽCO. 9 , and Fe 3 ŽCO.12 . Actually, these species have been suggested to be the main products of UV photolysis on silica w2x, porous Vycor glass w4x, and
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alumina w29x. Furthermore, Fe 2 ŽCO. 9 is known to be the major product in the sunlight photolysis of gasphase FeŽCO.5 w30,31x. It is already established w32x that Fe 2 ŽCO. 9 has a structure of D 3h symmetry in which two FeŽCO. 3 trigonal pyramids are bridged by three carbonyls. This molecule in the solid state exhibits two terminal Ž2080 and 2034 cmy1 . and one bridging Ž1828 cmy1 . C–O stretch bands w31x in accordance with the geometry. However, the formation of Fe 2 ŽCO. 9 is unlikely in the present case because Fig. 4 shows no any band corresponding to the terminal 2034 cmy1 band that is far more intense than the 2080 cmy1 band w31x. Sato and Suzuki w15x have shown that the 320-nm photolysis of FeŽCO.5 adsorbed on AuŽ111. at 90 K produces a single intense band at 2081 cmy1 . They ascribed this band to an FeŽCO.4 species with its Fe atom bonded directly to the surface. According to the infrared spectrum of Fe 3 ŽCO.12 reported by Sheline w33x, the position of the highest Žalso intense. C–O stretch band is as low as 2043 cmy1 . Poliakoff and Turner w34x have shown that UV irradiation of Fe 2 ŽCO. 9 in an argon matrix results in Fe 2 ŽCO. 8 in both the bridged and unbridged forms; in either event the highest C–O stretching frequency is lower than 2066 cmy1 for Fe 2 ŽCO. 9 in the same matrix. These facts make it unlikely that Fe 2 ŽCO. 8 or Fe 3 ŽCO.12 was formed in our UV photolysis. We therefore propose that the band at 2084 cmy1 in Fig. 4 is associated with an intermediate FeŽCO.4 species as on AuŽ111. w15x. After completion of photolysis at 90 K for 7 m IRRAS spectra were taken at elevated temperatures, the results of which are shown in Fig. 5. The 2084 cmy1 band observed at 90 K increases a little in intensity and shifts to 2078 cmy1 after heating to 180 K, suggesting a realignment of the FeŽCO.4 photoproduct. Upon heating to 220 K the main band is reduced in intensity and shifts to 2074 cmy1 . After heating to 260 K this band is located at 2064 cmy1 accompanied by a shoulder at 2053 cmy1 . Heating to 270 K results in the appearance of a band at 2050 cmy1 and a broad feature centered at 2110 cmy1 ; the former band arises from the shoulder Ž2053 cmy1 . in the 260-K spectrum. The development of the 2050 cmy1 band at 270 K must be attributed to a new thermal product. A possible reaction that might begin to take place at 260 K is polymerization of the
Fig. 5. IRRAS spectra after UV Ž254 nm. photolysis of iron pentacarbonyl on the Fe film at 90 K followed by heating the substrate to the indicated temperatures. The spectrum just before heating Žafter 7-min UV photolysis. is shown in Fig. 4, but note the different scale.
intermediate FeŽCO.4 species to yield Fe 2 ŽCO. 8 or Fe 3 ŽCO.12 . It is known that Fe 3 ŽCO.12 exhibits a major band at 2053 cmy1 and a minor band at 2109 cmy1 w2,4x, close in position to the bands observed in the 270-K spectrum. We therefore adopt this species as the most likely product. The broad features observed at 280 and 295 K suggest the presence of other carbonyl species on the surface.
4. Summary In this study IRRAS has been used to observe the thermal and UV Ž254 nm. photolytic behavior of FeŽCO.5 on evaporated Fe films. It is proposed that subcarbonylation into FeŽCO.4 occurs upon UV photolysis at 90 K followed by generation of Fe 3 ŽCO.12 rather than Fe 2 ŽCO. 8 at 260 K. IRRAS spectra for the condensed FeŽCO.5 multilayer before UV photolysis reveal that it is thermally stable up to 130 K
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but is reconstructed at around 140 K before molecular desorption at 155 K. Only a small amount of adsorbed FeŽCO.5 is observed at 160 K together with adsorbed CO which remains sufficiently stable at 170 K.
Acknowledgements This work has been supported in part by a Grantin-Aid for Scientific Research ŽA. from the Ministry of Education, Science, Sports and Culture.
References w1x M. Poliakoff, J.J. Turner, J. Chem. Soc. Dalton Ž1974. 2276, and references therein. w2x M.R. Trusheim, R.L. Jackson, J. Phys. Chem. 87 Ž1983. 1910. w3x F.G. Celii, P.M. Whitmore, K.C. Janda, J. Phys. Chem. 92 Ž1988. 1604. w4x M.S. Darsillo, H.D. Gafney, M.S. Paquette, J. Am. Chem. Soc. 109 Ž1987. 3275. w5x S. Sato, T. Ohmori, J. Phys. Chem. 95 Ž1991. 7778. w6x N. Bottka, P.J. Walsh, R.Z. Dalbey, J. Appl. Phys. 54 Ž1983. 1104. w7x N.S. Gluck, Z. Ying, C.E. Bartosch, W. Ho, J. Chem. Phys. 86 Ž1987. 4957. w8x Y. Ukisu, S. Sato, T. Ohmori, Appl. Organometal. Chem. 5 Ž1991. 243. w9x J.R. Swanson, C.M. Friend, J. Vac. Sci. Technol. A 6 Ž1988. 770. w10x F. Zaera, Surf. Sci. 255 Ž1991. 280.
147
w11x M.A. Henderson, R.D. Ramsier, J.T. Yates Jr., Surf. Sci. 275 Ž1992. 297. w12x S. Sato, Y. Ukisu, H. Ogawa, Y. Takasu, J. Chem. Soc. Faraday 89 Ž1993. 4387. w13x S. Sato, Y. Ukisu, Surf. Sci. 283 Ž1993. 137. w14x M. Xu, F. Zaera, Surf. Sci. 315 Ž1994. 40. w15x S. Sato, T. Suzuki, J. Phys. Chem. 100 Ž1996. 14769. w16x H. Stammreich, Osw. Sala, Y. Tavares, J. Chem. Phys. 30 Ž1959. 856. w17x M. Bigorgne, J. Organometal. Chem. 24 Ž1970. 211. w18x R. Cataliotti, A. Foffani, L. Marchetti, Inorg. Chem. 10 Ž1971. 1594. w19x J. Donohue, A. Caron, Acta Cryst. 17 Ž1964. 663. w20x T. Bein, P.A. Jacobs, J. Chem. Soc. Faraday 79 Ž1983. 1819. w21x T.L. Brown, D.J. Darensbourg, Inorg. Chem. 6 Ž1967. 971. w22x K. Yamatake, T. Tanabe, Y. Suzuki, Y. Matsumoto, T. Wadayama, A. Hatta, J. Raman Spectrosc. 29 Ž1998. 781. w23x T. Tanabe, Y. Suzuki, T. Wadayama, A. Hatta, unpublished data. w24x R.L. Jackson, M.R. Trusheim, J. Am. Chem. Soc. 104 Ž1982. 6590. w25x S.C. Fletcher, M. Poliakoff, J.J. Turner, Inorg. Chem. 25 Ž1986. 3597. w26x H.H. Huang, C.S. Sreekanth, C.S. Seet, G.Q. Xu, L. Chan, J. Phys. Chem. 100 Ž1996. 18138. w27x J.R. Swanson, C.M. Friend, Y.J. Chabal, J. Chem. Phys. 87 Ž1987. 5028. w28x T. Tanabe, Y. Suzuki, T. Wadayama, A. Hatta, to be published. w29x S. Sato, T. Ohmori, J. Chem. Soc., Chem. Commun. Ž1990. 1032. w30x M.H. Chisholm, A.G. Massey, N.R. Thompson, Nature 211 Ž1966. 67. w31x R.K. Sheline, K.S. Pitzer, J. Am. Chem. Soc. 72 Ž1950. 1107. w32x H.M. Powell, R.V.G. Ewens, J. Chem. Soc. Ž1939. 286. w33x R.K. Sheline, J. Am. Chem. Soc. 73 Ž1951. 1615. w34x M. Poliakoff, J.J. Turner, J. Chem. Soc. ŽA. Ž1971. 2403.