Al 3 O y (y=0–5) clusters: Sequential oxidation, metal-to-oxide transformation, and photoisomerization

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Abstract
Photoelectron spectra of a series of Al 3 O y − clusters (y=0–5) are presented at several photon energies: 532, 355, 266, and 193 nm. The electron affinities and low-lying electronic states of the Al 3 O y clusters are reported. The photoelectron spectra clearly reveal a sequential oxidation behavior and how the electronic structure of the clusters evolves from that of a metal cluster at Al 3 to that of a complete oxide cluster at Al 3 O 5 : Two valence electrons of Al 3 are observed to be transferred to each additional O atom until Al 3 O 5 , where all the nine valence electrons of Al 3 are transferred to the five O atoms. The anion, Al 3 O 5 − , which can be viewed as (Al 3+ ) 3 (O 2− ) 5 , is found to be a closed shell cluster, yielding an extremely high electron affinity for Al 3 O 5 (4.92 eV). The electron affinities of the remaining clusters are: 1.90 ( Al 3 ), 1.57 eV ( Al 3 O ), 2.18 eV ( Al 3 O 2 ), 2.80 eV ( Al 3 O 3 ), and 3.58 eV ( Al 3 O 4 ). An electronic excited state of Al 3 − is also observed at 0.40 eV above the Al 3 − ground state. Isomers are observed for all the oxide clusters with lower electron affinities. Particularly, vibrational structures are observed for the two isomers of Al 3 O 3 − , as well as a photoisomerization process between the two isomers. The structure and bonding of the oxide clusters are discussed based on the experimental data and the known structures for Al 3 and Al 3 O .