METASTABILITY IN DIATOMIC TRICATIONS - AN AB-INITIO CASE-STUDY OF AL-2(3+)

Multi-reference configuration interaction (MRD-CI) computations are reported on nineteen potential curves of Al2(3+), a trication having three valence electrons. The ground state X 2SIGMA(u)+ (4sigma(g)(2)4sigma(u)), dissociating into 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: 1(2)PI(u) (4sigma(g)(2)2pi(u)), 1(4)SIGMA(g)- and 1(2)DELTA(g) (both 4sigma(g)2pi(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 a0; and 298, 330 and 180 cm-1. Experimental data for X 3PI(u) (4sigma(g)(2)4sigma(u)(2)5sigma(g)2pi(u)) of Al2 correspond to 1.55 eV (D(e)), 5.10 a0 and 286 cm-1, respectively, showing that the local bonding in Al(2)3+ is comparable to that in Al2. For low upsilon', the main depopulation mechanism of 1(PI)u and 1(4)SIGMA(g)- involves radiative emission of to type bound-continuum. For upsilon'= 0, each maximum of the bands 1(2)PI(u) --> 1(2)SIGMA(g)+ and 1(4)SIGMA(g)- --> 1(4)PI(u) appears in the same energy region (almost-equal-to 5800 cm-1). The fluorescence lifetimes tau0 of 1(2)PI(u) and 1(4)SIGMA(g)- the in the 20-30 mus range, which might be long enough to allow detection of Al(2)3+ by accelerator mass spectrometry as recently reported for B2(3+). The slightly bound 1(2)DELTA(g) dissociates quickly via tunnelling, a feature making experimental detection of this state difficult. The triple ionization potentials of Al2 (X 3PI(u)) appear at relatively low energies: 39.8 eV (X 2 SIGMA(u)+, 4sigma5sigma(g)2pi(u) --> infinity, vertical), 43.6 eV (1(2)PI(u), 4sigma(u)(2)5sigma(g) --> infinity, adiabatic) and 49.3 eV (1(4)SIGMA(g)-, 4sigma(g)4sigma(u)(2)5sigma(g) --> infinitypi(u), adiabatic).