  The purpose of this work is to identify the field evaporation mechanism
associated with charge density distribution under extreme fields, linking atom
probe tomography (APT) experiments with density functional theory (DFT)
modeling. DFT is used to model a materials surface bonding, which affects the
evaporation field of the surface atoms under high electric fields. We show how
the evaporation field of atoms is related to the charge density by comparing
the directionality and localization of the electrons with the evaporation of
single ions versus dimers. This evaporation mechanism is important for the
reconstruction of APT data, which is partially dependent on the input
evaporation fields of the atoms. In $L1_{2}-Al_{3}Sc$, $Al-Al$ surface atoms
are more likely to evaporate as dimers than $Al-Sc$ surface atoms. We find that
this is due to $Al-Al$ having a shared charge density, while $Al-Sc$ has an
increased density localized around the $Sc$ atom. Further, the role of
subsurface layers on the evaporation physics of the surface atoms as a function
of charge density is considered. Beyond the practical considerations of
improving reconstruction of APT data, this work provides an approach for design
of surface chemistry for extreme environments.
