Behavior of molecules and molecular ions near a field emitter

TL;DR

This paper investigates the behavior of molecules and ions near a field emitter in atom probe tomography, combining literature review, data analysis, trajectory modeling, and DFT simulations to understand emission mechanisms.

Contribution

It introduces new methods for analyzing APT data, models ion trajectories, and uses DFT to assess molecular ion energetics, clarifying neutral molecule emission processes.

Findings

Neutral molecules are unlikely to be thermally emitted directly.

Neutral molecules can form through dissociation of metastable molecular ions.

The study clarifies the mechanisms behind molecular and neutral ion emissions in APT.

Abstract

The cold emission of particles from surfaces under intense electric fields is a process which underpins a variety of applications including atom probe tomography (APT), an analytical microscopy technique with near-atomic spatial resolution. Increasingly relying on fast laser pulsing to trigger the emission, APT experiments often incorporate the detection of molecular ions emitted from the specimen, in particular from covalently or ionically bonded materials. Notably, it has been proposed that neutral molecules can also be emitted during this process. However, this remains a contentious issue. To investigate the validity of this hypothesis, a careful review of the literature is combined with the development of new methods to treat experimental APT data, the modelling of ion trajectories, and the application of density-functional theory (DFT) simulations to derive molecular ion energetics. It is shown that the direct thermal emission of neutral molecules is extremely unlikely. However, neutrals can still be formed in the course of an APT experiment by dissociation of metastable molecular ions.