The ZnO2+ dication in the gas phase

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.

Dissociation of GaN2+ and AlN2+ in APT: Analysis of experimental measurements

The use of a tip-shaped sample for the atom probe tomography technique offers the unique opportunity to analyze the dynamics of molecular ions in strong DC fields. We investigate here the stability of AlN²⁺ and GaN²⁺ dications emitted from an Al_0.25Ga_0.75N sample in a joint theoretical and experimental study. Despite the strong chemical resemblance of these two molecules, we observe only stable AlN²⁺, while GaN²⁺ can only be observed as a transient species. We simulate the emission dynamics of these ions on field-perturbed potential energy surfaces obtained from quantum chemical calculations. We show that the dissociation is governed by two independent processes. For all bound states, a mechanical dissociation is induced by the distortion of the potential energy surface in the close vicinity of the emitting tip. In the specific case of GaN²⁺, the relatively small electric dipole of the dication in its ground 1³Σ^- and excited 1¹Δ states induces a weak coupling with the electric field so that the mechanical dissociation into Ga⁺ + N⁺ lasts for sufficient time to be observed. By contrast, the AlN²⁺ mechanical dissociation leads to Al²⁺ + N which cannot be observed as a correlated event. For some deeply bound singlet excited states, the spin-orbit coupling with lower energy triplet states gives another chance of dissociation by system inter-system crossing with specific patterns observed experimentally in a correlated time of flight map.