Electronic Structure and Properties of the O2+2, SO2+ and S2+2 Diatomic Dications

Multiply charged ions show novel energetic features which can potentially have useful photophysical properties. In order to investigate these interesting type systems, energy interaction curves for the ground and excited electronic states of O²⁺₂, SO²⁺, and S²⁺₂ have been generated by the ab initio complete active space multiconfiguration self-consistent field (CAS-MCSCF) method. The calculations were carried out in a triple-zeta sp plus double-zeta d Gaussian function basis set using compact effective potentials to replace the core electrons. In order to gauge the accuracy of the results, analogous calculations were carried out on the valence isoelectronic N₂ and NO⁺ systems for which experimental information is available for comparison of geometric and spectroscopic properties. Diverse high energy spectroscopy experiments on O²⁺₂ are interpreted and reconciled using the calculated ground and excited state energy interaction curves. Special attention is paid to the ₃Σ₊^u state, whose location and character have been particularly puzzling. Almost all the dication molecular states studied show the characteristic avoided crossing of diabatic states which gives a trapped equilibrium structure and a barrier to dissociation. These features make the dication kinetically stable, but thermodynamically unstable with an exothermic dissociation energy. The spectroscopy experiments on O²⁺₂ show that these states are experimentally attainable.