Electronic Structure and Low-Lying Electronic States of Al₃O and Al₃O⁻:  Photoelectron Spectrum of Al₃O⁻

We have calculated the equilibrium geometries of the ground electronic state of Al3O and Al3O- using Hartree−Fock, density functional, and coupled-cluster doubles methods. These molecules are nominally equilateral triangles of Al atoms with an oxygen in the center which distort due to the Jahn−Teller effect. The calculated global minima had C2v symmetry with (b2)1 2B2 and (b2)2 1A1 configurations, respectively. The global minimum of the lowest triplet state of Al3O- was found to have D3h symmetry with an (e‘)2 3A2‘ electron configuration. While this was the lowest energy state of the molecule among D3h symmetry points, this triplet minimum had slightly higher energy than the 1A1 state at its C2v global minimum. The 1A1 (a1)2 and the 1B2 (b2)1(a1)1 configurations of Al3O- and the 2A1 (a1)1 configuration of Al3O lead to transition states of C2v symmetry on the respective potential energy surfaces for pseudorotation. Using the CCD geometries of (b2)2 1A1 and (e‘)2 3A2‘ Al3O-, configuration interaction calculations have been performed to determine the low-lying vertical excited states of Al3O, and those results have been utilized to interpret the recently reported experimental photoelectron spectrum of Al3O-. On the basis of the present CI results, new assignments have been made for some of the peaks.