Electronic structure and stability of the SiO²⁺ dications produced in tomographic atom probe experiments

The molecular electronic states of the SiO²⁺ dication have been investigated in a joint theoretical and experimental analysis. The use of a tip-shaped sample for tomographic atom probe analysis offers the unique opportunity to produce and to analyze the lifetime of some excited states of this dication. The perturbation brought by the large electric field of the polarized tip along the ion trajectory is analyzed by means of molecular dynamics simulation. For the typical electric fields used in the experiment, the lowest energy triplet states spontaneously dissociate, while the lowest energy singlet states do not. We show that the emission process leads to the formation of some excited singlet state, which dissociates by means of spin-orbit coupling with lower-energy triplet states to produce specific patterns associated with Si⁺ + O⁺ and Si²⁺ + O dissociation channels. These patterns are recorded and observed experimentally in a correlated time-of-flight map.