Metastability in diatomic trications. An ab initio case study of Al23+

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Abstract
Multi-reference configuration interaction (MRD-CI) computations are reported on nineteen potential curves of Al23+, a trication having three valence electrons. The ground state X 2 Sigma u+ (4 sigma g24 sigma u), dissociating into ground Al++Al2+, is repulsive. In spite of its triple positive charge, three excited states arising from configurations with maximal occupation of bonding orbitals are metastable: 12 Pi u (4 sigma g22 pi u), 14 Sigma g- and 12 Delta g (both 4 sigma g2 pi u2). In the same order, the potential barriers Deff, equilibrium geometries Re and vibrational frequencies omega e are: 0.28, 0.57, and 0.04 eV; 4.88, 4.79 and 5.20 a0; and 298, 330 and 180 cm-1. Experimental data for X 3 Pi u (4 sigma g24 sigma u25 sigma g2 pi u) of Al2 correspond to 1.55 eV (De), 5.10 a0 and 286 cm-1, respectively, showing that the local bonding in Al23+ is comparable to that in Al2. For low upsilon ', the main depopulation mechanism of 12 Pi u and 14 Sigma g- involves radiative emission of type bound-continuum. For upsilon '=0, each maximum of the bands 12 Pi u to 12 Sigma g+ and 14 Sigma g- to 14 Pi u appears in the same energy region ( approximately= 5800 cm-1).

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