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

Multi-reference configuration interaction (MRD-CI) computations are reported on nineteen potential curves of Al 2 3+ , a trication having three valence electrons. The ground state X 2 Sigma u + (4 sigma g 2 4 sigma u ), dissociating into ground Al + +Al 2+ , is repulsive. In spite of its triple positive charge, three excited states arising from configurations with maximal occupation of bonding orbitals are metastable: 1 2 Pi u (4 sigma g 2 2 pi u ), 1 4 Sigma g - and 1 2 Delta g (both 4 sigma g 2 pi u 2 ). In the same order, the potential barriers D eff , equilibrium geometries R e and vibrational frequencies omega e are: 0.28, 0.57, and 0.04 eV; 4.88, 4.79 and 5.20 a 0 ; and 298, 330 and 180 cm -1 . Experimental data for X 3 Pi u (4 sigma g 2 4 sigma u 2 5 sigma g 2 pi u ) of Al 2 correspond to 1.55 eV (D e ), 5.10 a 0 and 286 cm -1 , respectively, showing that the local bonding in Al 2 3+ is comparable to that in Al 2 . For low upsilon ', the main depopulation mechanism of 1 2 Pi u and 1 4 Sigma g - involves radiative emission of type bound-continuum. For upsilon '=0, each maximum of the bands 1 2 Pi u to 1 2 Sigma g + and 1 4 Sigma g - to 1 4 Pi u appears in the same energy region ( approximately= 5800 cm -1 ).