Ab initio calculations are employed to understand the photoisomerization process in small Al3O3- clusters. This process is the first example of a photoinduced isomerization observed in an anion cluster gas-phase system. Potential energy surfaces for the ground state and the excited state (S-1 and T-1) are explored by means of B3LYP, MP2, CI-singles, and CASSCF methods. We demonstrate that the isomerization process occurs between the global minimum singlet state Book structure (C-2v,(1)A(1)) and the triplet state Ring structure (C-2v,B-3(2)). The calculated vertical excitation energy is 3.62 eV at the CASSCF level of approximation, in good agreement with the experimental value (3.49 eV). A nonplanar conical intersection, which hosts the intersystem crossing between the S-1 and T-1 surfaces is identified at the region of around R(1,6)=2.4 Angstrom. Beyond the experimental results, we predict, that this isomerization is reversible upon absorption of a phonon with energy of 1.92 eV. Our results describe a unique system, whose structure depends on its spin multiplicity; it exists as the Book structure on singlet states and as the Ring structure on triplet states. (C) 2002 American Institute of Physics. [References: 35]
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