  A major challenge for Atom Probe Tomography (APT) quantification is the
inability to decouple ions which possess the same mass/charge-state ($m/n$)
ratio but a different mass. For example, $^{75}{\rm{As}}^{+}$ and
$^{75}{\rm{As}}{_2}^{2+}$ at ~75 Da or $^{14}{\rm{N}}^+$ and
$^{28}{\rm{Si}}^{2+}$ at ~14 Da, cannot be differentiated without the
additional knowledge of their kinetic energy or a significant improvement of
the mass resolving power. Such mass peak overlaps lead to ambiguities in peak
assignment, resulting in compositional uncertainty and an incorrect labelling
of the atoms in a reconstructed volume. In the absence of a practical
technology for measuring the kinetic energy of the field-evaporated ions, we
propose and then explore the applicability of a post-experimental analytical
approach to resolve this problem based on the fundamental process that governs
the production of multiply charged molecular ions/clusters in APT, i.e.,
Post-Field Ionization (PFI). The ability to predict the PFI behaviour of
molecular ions as a function of operating conditions could offer the first step
towards resolving peak overlap and minimizing compositional uncertainty. We
explore this possibility by comparing the field dependence of the
charge-state-ratio for Si clusters ($\rm{Si}_2$, $\rm{Si}_3$ and $\rm{Si}_4$)
with theoretical predictions using the widely accepted Kingham PFI theory. We
then discuss the model parameters that may affect the quality of the fit and
the possible ways in which the PFI of molecular ions in APT can be better
understood. Finally, we test the transferability of the proposed approach to
different material systems and outline ways forward for achieving more reliable
results.
