Electronic structure and lifetimes of GaX2+ (X = N, O, F) in the gas phase. Unraveling stability trends

Abstract

High-level single-reference CCSD(T) and multireference MS-CASPT2/CASSCFab initio calculations have been carried out to determine the electronic structure and the lifetimes of GaX²⁺ (X = N, F) doubly charged diatomic systems. Lifetimes were evaluated using the Exterior Complex Scaling (ECS) method implemented with basis sets of B-spline functions. The results obtained led to the conclusion that both species GaN²⁺ and GaF²⁺ can be considered as bound systems in the gas phase. GaN²⁺ is a kinetically stable dication, because although it is thermodynamically unstable with respect to its dissociation into Ga⁺ (¹S) + N⁺ (³P) the barrier to be surmounted is rather high and wide, so that the lowest 14 vibrational states of this system are bound. GaF²⁺ is found to be a thermodynamically stable species with respect to its dissociation into Ga⁺ (¹S) + F⁺ (¹D). With respect to its dissociation into Ga⁺ (¹S) + F⁺ (³P), strictly speaking, it is metastable. However, the crossing between the attractive PEC and the repulsive one occurs at rather large internuclear distances, and therefore it can be only explored by highly vibrationally excited molecules. Thus, in practical terms GaF²⁺ can be considered as a bound species. The behavior of GaN²⁺ and GaF²⁺ is in clear contrast with that predicted for GaO²⁺, which was found to be metastable with respect to its dissociation into Ga⁺ (¹S) + O⁺ (⁴S), with lifetimes in the range of ns to seconds. This seems to indicate that the primary factor governing the stability and lifetimes of these doubly charged diatomic species is the ionization energy of the atom X bound to Ga.