Relativistic effective core potential calculations have been employed in the framework of a spin-orbit configuration interaction to compute the lowest-lying electronic states of the CaBr2+ and CaI2+ dications, and the results are compared with the data for the isovalent CaCl2+ system studied earlier. The ground X 2H state in all three dications arises from a strong polarization of X(2Po)(X = Cl, Br or I) by the Ca2+(1S) ion and is bound by 0·96-1·55 eV with respect to the corresponding diabatic dissociation limits. It is split by the spin-orbit interaction into the X1 2Π3/2 and X2 2Π1/2 components, with the energy splittings calculated to be 647 cm-1 (CaCl2+), 2115 cm-1 (CaBr2+) and 3544cm-1 (CaI2+). The X1 and X2 states are found to be thermodynamically stable in CaCl2+ and CaBr2+, while in CaI2+ the lowest dissociation limit, Ca+(2S) + I+(3P2), lies 1700 and 5200 cm-1 lower than the X1 and X2 minima respectively. The X1 and X2 states in CaI2+ are extremely long-lived, however, owing to the high and very broad potential barriers to dissociation. The first electronic excited state, A2Σ+, is also bound in all the above systems, although it is pre-dissociated in CaBr2+ and CaI2+ at large internuclear distances. All other low-lying electronic states of CaX2+ are repulsive. Electric-dipole moments are calculated for the A → X1,X2 transitions. The corresponding radiative lifetimes are found to be very long in CaCl2+: τ(A → X1) = 19·3 ms and τ(A → X2) = 9·9 ms (the values are given for v′ = 0), and become very significantly shorter for CaBr2+ and CaI2+ because of the stronger spin-orbit interaction in the heavier systems. This effect is especially noticeable for the A → X2 transitions, for which the τ values are computed to be 364 μs in CaBr2+ and 50·3 us in CaI2+. The theoretical data obtained should aid in the future spectroscopic detection of these species. To data no experiment of this type has been successfully carried out for any of the thermodynamically stable diatomic dications.

Molecular Physics
Department of Chemistry

Alekseyev, A.B. (Aleksey B.), Liebermann, H.-P. (Heinz-Peter), Lingott, R. (Rainer), Buenker, R.J. (Robert J.), & Wright, J.S. (1997). Long-lived diatomic dications: Potential curves and radiative lifetimes for CaBr2+ and CaI2+ including relativistic effects. Molecular Physics, 91(5), 777–787.