J . Phys. Chem. 1988, 92, 50-56
50
progress can be made. To begin with, there seems little doubt that the H F will bind somewhere along the C=C=C backbone, giving rise to an essentially T-shaped structure. This is consistent with the fact that the spectrum observed for the H F stretch is a parallel band. Figure 6 shows two possible structures of this type with the HF bonded to the center of a carbon-carbon bond, in analogy with C2H,-HF, and to the central carbon atom. Unfortunately, with the A constant being so poorly determined, we are unable to differentiate between these two structures or any that lie between these extremes. Work is in progress to obtain a spectrum for the C-H stretches of this complex. Since these bands will be perpendicular, a much more accurate A value should be obtained. In both of the calculated spectra a Lorentzian line width of 1400 MHz fwhm was used. The lifetime of the excited vibrational state is clearly shorter than that of the ethylene-HF complex discussed above. Once again, we are unable to associate the width of the observed transitions with the vibrational predissociation lifetime of the complex, since it is not clear whether IVR is important or not. However, it is interesting to note that in all of the systems studied with the optothermal detection method, there is not a single example of a system in which there is unambiguous homogeneous broadening of the lines and yet a positive bolometer signal is observed. In principle, of course, if IVR is fast and dissociation is slow on the time scale of the molecular flight time from the laser crossing point to the bolometer, this situation could occur. This indicates, therefore that when the lifetime of the excited
vibrational state is short enough to observe homogeneous broadening of the transitions, the vibrational predissociation lifetime of the complex is at least short enough to ensure that the signal on the bolometer is negative. Unfortunately, this upper limit on the predissociation lifetime is rather long, namely, 3 X s.
Summary We have reported here the infrared-molecular beam spectra for ethylene-HF and allene-HF corresponding to the excitation of the HF stretching vibration. In both cases the spectra are well represented by a conventional rigid rotor, asymmetric top Hamiltonian facilitating the accurate determination of rotational constants and vibrational origins. For the case of allene-HF the transitions are heavily blended owing to extensive homogeneous broadening. As a result, the constants obtained for this system are more poorly determined than those for ethylene-HF so that a completely reliable structure cannot be obtained. Future work on the C-H stretching vibrations of this molecule should be of considerable help in clarifying this issue. Acknowledgment. We are grateful to A. C. Legon for providing us with the microwave results prior to publication and to the following agencies for their support of this research: the National Science Foundation (CHE-86-03604), the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Research Corp. Registry No. HF, 7664-39-3; allene, 463-49-0; ethylene, 74-85-1.
Laser-Induced Optical Emission Studies of Eu3+ Sites in Polycrystalline Powders of Monoclinic and Body-Centered Cubic Eu,03 K. C. Sheng and G . M. Korenowski* Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 121 80-3590 (Received: April 8, 1987; In Final Form: July 1 , 1987)
Laser-induced emission from the visible wavelength f-f transitions of Eu3+is used to determine the number of distinct europium ion sites and their site symmetries in powdered monoclinic and body-centered cubic (bcc) Eu203. Identical studies are also carried out for a higher surface area catalytic bcc Eu203material. For the monoclinic sample, the f-f emission spectrum is found to result from three distinct europium sites, all of which possess C, symmetry. Individual emission lines are assigned to the separate emitting sites for the 'Do 'FJ series for J = 0, 1, and 2. Emission spectra from the regular and higher surface area bcc Euz03materials are found to originate from two distinct Eu3+sites in the oxide lattice. The strongest emitting Eu3+site is assigned to the known C2symmetry site of the bcc lattice. No emission was observed from the known S6 symmetry site. The second and weaker emitting site, which is observed in the spectra of both the regular and higher surface area materials, is determined to result from an Eu3+ site with either C, or C, symmetry.
-
Introduction The unique physical and chemical properties of the lanthanide oxides make these materials useful in a variety of diverse applications. Some well-known areas of application for the oxides include use their as laser materials' and phosphors.z A lesser known application and an area of growing general interest is the use of the oxides as heterogeneous chemical catalyst^.^.^ Besides the practical importance of developing new catalysts, systematic studies of catalytic reactions on these oxides are also possible, and such studies promise to yield fundamental insights into the basis of catalytic activity. The ability to perform these studies with the lanthanide oxides stems from the regular decrease in surface basicity of the oxides as the lanthanide atomic number increases (a result of the lanthanide contraction). This regular variation of basicity, coupled with the fact that relatively few structures are found for the oxides but that these structures are exhibited *Address correspondence to this author
0022-3654/88/2092-0050$01.50/0
throughout the series, enables one to use a series of oxides to determine the importance of surface basicity versus structural effects on the catalytic activity. It is toward providing structural information that will be of future use in such systematic studies of catalysis that the spectroscopic studies of this paper are directed. The site symmetries occupied by the lanthanide ions, interaction between neighboring lanthanide ions, and interactions with the oxide lattice are important factors with respect to the observed physical and chemical behavior of the oxides. For the sesquioxides ( 1 ) Weber, M. J. In Handbook on the Physics and Chemistry of Rare Earrhs; Gschneider, K . A,, Jr.; Eyring, L., Eds.; North-Holland: New York, 1979; Vol. 4, Chapter 35, pp 275-315. (2) Blesse, G. In Handbook on the Physics and Chemistry of Rare Earths; Gschneider, K. A,, Jr.; Eyring, L., Eds.; North-Holland: New York, 1979; Vol. 4, Chapter 34, pp 237-274. (3) Netzer, F. P.; Bertel, E. In Handbook on the Physics and Chemistry of Rare Earths, Gschneider, K. A,, Jr.; Eyring, L., Eds.: North-Holland: New York, 1983; Vol. 5, Chapter 43, pp 217-320. (4) Rosynek, M . P.Catal. Rev. Sci. Eng. 1977, 16, 1 1 1 .
0 1988 American Chemical Society
Optical Emission Studies of Eu3+ Sites
The Journal of Physical Chemistry, Vol. 92, No. I , 1988 51
centered cubic (bcc) Eu203. Eu203is chosen for study because (Ln203),the structural polymorphism exhibited by these materials of its central position in the lanthanide series and because two provides several crystalline structures, each with a variety of lanthanide ion sites.s The number and symmetry of the metal characteristic sesquioxide structures are readily produced in pure or mixed-crystal form. Studies are also performed on a bcc Eu203 ion sites in any one structural form'can be deduced from X-ray powder after its conversion to a higher surface area and catacrystallographic studies or optical studies of single-crystal systems. lytically active form of the oxide. Catalytic applications, however, employ powders of the oxides. In this particular application, defect sites, mixed crystal effects, In the work reported on here, the elimination of trace hydroxide and surface effects may have important consequences with respect species and adsorbed contaminants from the samples permits an unambiguous identification of the Eu3+ sites responsible for the to the catalytic activity. It is important for such applications to observed f-f emission lines from the pure oxides. For the be able to characterize the polycrystalline powders with respect monoclinic samples, three distinct Eu3+sites each of which posto the lanthanide ion sites following various material preparation sesses C, symmetry are found to contribute to the f-f emission procedures. 7 F J series with J = 0, 1, and 2, the spectrum. For the 'Do Lanthanide ion site spectroscopy has long been an active area individual emission lines are assigned to the separate C, emitting of investigation and is a powerful method of identifying lanthanide sites. The higher surface area and regular bcc Eu203powders ion sites in solids. The field is too extensive to be reviewed here are found to have identical emission spectra for the 5Do 7FJ in detail, and the reader is directed to several published works series. Two Eu3+ emitting sites are found in the bcc materials. for additional information and references to the subject matter.6%7 The strongest emitting site is assigned to a site of C2 symmetry. Elegant experimental techniques such as fluorescence line narSince the bulk structure of bcc Eu203possesses only C2 and S6 rowing (FLN) spectroscopy have been developed for the purpose sites for Eu3+, one might expect that the second and weaker of studying lanthanide ion sites in solids.*-12 These techniques are primarily used to determine ion sites in solids such as glasses emitting site to be of S6 symmetry. However, it is found from or transparent host crystals into which the lanthanide ions are the character of the emission spectrum that the second emitting lightly doped (typically several percent or less in concentration). site must possess either C2 or Ci symmetry. The results of these Lightly doped solids exhibit the strongest lanthanide ion emission studies clarify previously existing confusion with respect to the spectra and afford the best potential laser materials and phosphors. optically emitting Eu3+sites in polycrystalline powders of Eu203. Solids with a high concentration of lanthanide ions, for example, Experimental Section the pure oxides that are used for catalytic applications, are relEuzO3 was obtained as a 99.9% pure oxide from Alpha and as atively unexplored systems. Strong ion-ion interactions and a 99.99% pure oxide from AESAR and Aldrich. Prior to the phonon-assisted ion-ion interactions between neighboring lanoptical studies, the samples were cleaned of adsorbed contaminants thanide ion sites results in extremely rapid energy transfer between ion sites and dramatic quenching of lanthanide e m i s s i ~ n . ~ ~ ' ~ - ' and ~ water by heating the oxides at 600 OC for 24 h. The cleaning procedure was performed with the materials under vacuum. While Consequently, pure oxides, like the catalyst powders, are very still under vacuum, the Eu203samples were sealed in optical-grade different systems from the lightly doped solids where ion-ion quartz tubes for the spectroscopic studies. interaction is much weaker. For the pure oxides, site-selective Higher surface area catalytic powders of bcc E u z 0 3 were emission techniques (FLN methods) are in general not applicable prepared by the thermal dehydration of E u ( O H ) ~at 600 "C because of the rapid energy migration between the ion sites and according to the procedures outlined in the l i t e r a t ~ r e . ~ ,The '~ the associated spectroscopic line b r ~ a d e n i n g . ~ JNevertheless, ~.'~ E u ( O H ) ~starting material was obtained commercially from the information from studies of lightly doped systems may be of AESAR as a 99.9% pure material and also prepared in-house. value in the interpretation of spectra from highly concentrated Although surface areas were not determined for the Eu203powlanthanide systems. At present, it has not been established whether ders, the effect of the hydration/dehydration procedure on the or not this wealth of existing information will be of use to those surface area of other lanthanide oxides has been studied in detail.I6 individuals studying the pure oxides and catalytic powders. The This previous work serves to give an idea of the expected surface work of this paper demonstrates that this existing information is areas before and after processing. La203as received from compotentially of great value, when properly used, in aiding in the mercial suppliers was reported to have a surface area of
