A study of the structure and bonding of small aluminum oxide clusters by photoelectron spectroscopy: AlxOy− (x=1–2, y=1–5)

The structure and bonding of aluminum oxide clusters, AlxOy (x=1–2, y=1–5), are studied with anion photoelectron spectroscopy (PES) and are compared with preliminary ab initio calculations. The spectra were obtained at four detachment photon energies: 2.33, 3.49, 4.66, and 6.42 eV. The 6.42 eV spectrum for AlO− reveals the X 2Σ+ ground state and two excited states of AlO. The 6.42 eV spectrum for AlO2− also shows three states for AlO2: X 2Πg ground state and the A 2Πu and BΣg+2 excited states. The spectra for Al2Oy− clusters show vibrationally resolved ground states which come from Al sp-type orbitals and also high binding energy excited states, which are mainly of oxygen 2p character. Al2O2, which has a D2h rhombus structure, has an electron affinity (EA) of 1.88 eV and its singlet–triplet excitation energy is measured to be 0.49 eV. Much higher EAs are measured for the larger Al2Oy clusters. The PES spectra of Al2O3−, Al2O4−, and Al2O5− show very similar electronic and vibrational structure. Furthermore, the ground state vibrational frequencies of these three molecules are also similar. These observations lead us to suggest that these molecules all have a rhombuslike structure, similar to Al2O2, with the oxygen atoms sequentially attaching to the terminal aluminum atoms. The spectra are consistent with an ionic bonding view of these clusters and the vibrational frequencies are in good agreement with the theoretical results. Significant information about the structure and bonding of these small aluminum oxide clusters is obtained and discussed.