Dissociation of GaN²⁺ and AlN²⁺ in APT: Electronic structure and stability in strong DC field

We investigate from a theoretical point of view the stability of AlN²⁺ and GaN²⁺ dications produced under high static electric fields like those reached in Atom Probe Tomography (APT) experiments. By means of quantum chemical calculations of the electronic structure of these molecules, we show that their stability is governed by two independent processes. On the one hand, the spin-orbit coupling allows some molecular excited states to dissociate by inter-system crossing. On the other hand, the action of the electric field lowers the potential energy barrier, which ensures the dication stability in standard conditions. We present a detailed example of field emission dynamics in the specific case of the 1¹Δ states for a parabolic tip, which captures the essentials of the process by means of a simplified model. We show that the dissociation dynamics of AlN²⁺ and GaN²⁺ is completely different despite the strong resemblance of their electronic structure.