Behavior of molecules and molecular ions near a field emitter

Abstract

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