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

Abstract

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.

Unraveling the Metastability of Cn2+ (n = 2–4) Clusters

“…An efficient fragmentation channel for the 5 Σ u – state is the intersystem crossing due to SOC with the 3 Π u states. We have computed the lifetimes by means of perturbation theory using the method described in ref (53). In brief, the SOC lifetime for a given vibrational level is calculated from the vibrational wave function obtained from the MRCI potential energy curves and using a separate evaluation of the SOC matrix element, as described in the SI.…”

“…We have computed the lifetimes by means of perturbation theory using the method described in ref. 53 Briefly, the SOC lifetime for a given vibrational level is calculated from the vibrational wave function obtained from the MRCI potential energy curves and using a separate evaluation of the SOC matrix element as described in the supporting information. Subsequently, we simulated the dissociation process of 12 C2 2+ along the 3 Pu PEC using an idealized atom probe, 53,54 whose geometry is sketched in Figure 1 (a).…”

“…Both the tip of the field emitter and the LE are mimicked using a confocal paraboloid following the method given in. 53 The electric field at the tip apex was set at 46 V/nm, which is comparable to the electric fields applied in experiments. 55 The original 12 C2 2+ ions were oriented randomly with respect to the electric field.…”

Potential sources of compositional inaccuracy in the atom probe tomography of InxGa1-xAs

“…The detection of neutrals can indeed occur if these are generated far enough from the surface such that the molecule (and thus also the neutral fragment) has already acquired a substantial energy (velocity) such that the neutral fragment impinges on the detector with sufficient energy to generate a detectable event [26]. For instance, dissociative events producing neutral fragments have been detected in SiO2 [27], GaN [29] and Fe3O4 [26]. No such tracks are observed on the correlation histogram in this study, which is similar to the case of carbon clusters [40].…”

“…Dissociation may even result in neutral fragments [24], [26], [27], which are not always detected. Theoretically predicting whether the production of neutrals rather than charged fragments is more favorable during dissociation, in the presence of a high DC field, entails a dedicated study for each material/molecular-ion, which is a non-trivial process [27]- [29].…”