A study of the structure and bonding of small aluminum oxide ...

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1316 Desai et al.: Aluminum oxide clusters to the aluminum atom that is attached only to the two bridging oxygen atoms. This would create the symmetric D 2h Al 2 O 4 molecule. A similar structure was proposed for the Al 2 O 4 cation, in an experiment to study the stabilities and reactivities of aluminum oxide clusters. 23 Other isomers that were considered in our calculations include the C 2v ‘‘twisted pair’’ minimum Fig. 4f, with scaled frequencies of 968 and 914 cm 1 at the HF and MP2 levels, respectively. Another C 2v minimum, but with longer Al–O bonds and at a higher energy, was found with scaled frequencies of 1019 and 964 cm 1 at the HF and MP2 levels, respectively. It is important to note that assignment of the D 2h form to the experimentally observed species is tentative, since the calculated energy ordering of the D 2h and ‘‘twisted pair’’ C 2v structures changes with the inclusion of electron correlation. Calculations at higher levels of theory, and also of electron affinities, will provide more definitive identification of the observed isomer. F. Al 2 O 5 There have been no previous theoretical or experimental studies of Al 2 O 5 . Figures 3a and 3b show the PES spectra of Al 2 O 5 at 266 and 193 nm, respectively. The spectra display a broad, vibrationally resolved band at lower BE and a broad, unresolved band at higher BE, at 193 nm. A well resolved hot band is observed at 266 nm yielding a vibrational frequency of 1170 60 cm 1 for the anion. The first band corresponds to the ground state of Al 2 O 5 with an EA of 3.75 eV and with a vibrational frequency of 900 60 cm 1 . The relative intensities of the vibrational progression appear to be different at the two detachment wavelengths: the vibrational features at 4 eV and above are enhanced in the 193 nm spectrum. This is likely due to imperfect background subtractions or autodetachment transitions at 193 nm. The high BE feature, at approximately 5.1 eV, corresponds to the first excited state of Al 2 O 5 . The ground state is a result of the removal of an electron from the LUMO, which is likely to be of Al sp character and is expected to be bonding in nature. The similarity among the ground and the excited states in Al 2 O 5 ,Al 2 O 4 , and Al 2 O 3 suggests that the three molecules have similar structures and the corresponding ground and excited states have similar origins. As is seen in our previous studies, 35,36 when the total valency of the oxygen atoms exceeds that of the metal atoms in the M x O y oxide clusters, the molecule becomes electron deficient and begins to form O–O bonds. In other words, an O 2 unit tends to replace an oxygen atom in these types of clusters. It is expected that Al 2 O 5 would be made by replacing a terminal O atom in a D 2h Al 2 O 4 with an O 2 unit as seen in Fig. 4g. The decrease in the EA between Al 2 O 4 and Al 2 O 5 is similar to our previous observations involving Cu 2 O y clusters. 36 In our calculations, the only minimum found thus far for Al 2 O 5 is a twisted C 2v structure with one aluminum tetrahedrally bonded and the other aluminum atom bound to three oxygens. This structure is exactly the same as that proposed above, and is based on the similarity among the spectra of Al 2 O 3 ,Al 2 O 4 , and Al 2 O 5 . The totally symmetric scaled vibrational frequencies calculated are 1023 and 964 cm 1 ,at the HF and MP2 levels, respectively. The correct prediction of the Al 2 O 5 structure and the similarity among the PES spectra of Al 2 O 3 ,Al 2 O 4 , and Al 2 O 5 lends strong credence to the rhombus-type structures proposed for these clusters. VI. CONCLUSIONS The photoelectron spectra of small aluminum oxide clusters involving one and two aluminum atoms are reported at several photon energies. The spectra for AlO and AlO 2 at 193 nm showed additional excited states which were not previously observed at lower photon energies. We obtained vibrationally resolved spectra for the ground states for all the Al 2 O y species y2–5. Additionally, high BE features are observed at 193 nm due to the removal of electrons from oxygen 2p-type orbitals. The electron affinities and vibrational frequencies of these clusters are obtained and are compared with existing theoretical calculations and those that are in progress for this work. Probable structures are proposed for all the clusters consistent with the experimental observations and the various theoretical studies, which have shown these Al x O y species to be predominantly ionic with significant configuration mixing and strong correlation effects. Structural and bonding information are obtained from the current experiments, which can be compared with more accurate calculations. The Al 2 O 2 rhombus is a major structural feature in bulk aluminum oxide materials, formed between two AlO 4 units sharing two O atoms. We show evidence that this bulk structural feature exists in the Al 2 O x series of clusters. ACKNOWLEDGMENTS This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division and is conducted at the Pacific Northwest National Laboratory, operated for the U.S. Department of Energy by Battelle under Contract No. DE-AC06-76RLO 1830. 1 V. E. Henrich and P. A. Cox, The Surface Science of Metal Oxides Cambridge U.P., New York, 1994. 2 M. J. Linevsky, D. White, and D. E. Mann, J. Chem. Phys. 41, 542 1966. 3 A. Snelson, J. Phys. Chem. 74, 2574 1970. 4 D. M. Makowiecki, J. D. A. Lynch, and K. D. Carlson, J. Phys. Chem. 75, 1963 1971. 5 L. B. Knight and W. Weltner, J. Chem. Phys. 55, 5066 1971. 6 C. P. Marino and D. White, J. Phys. Chem. 77, 2929 1973. 7 P. A. Finn, D. M. Gruen, and D. L. Page, Adv. Chem. 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