Estimation of the Electric Field in Atom Probe Tomography Experiments Using Charge State Ratios

Kingham [(1982). The post-ionization of field evaporated ions: A theoretical explanation of multiple charge states. Surf Sci116(2), 273-301] provided equations for the probability of observing higher charge states in atom probe tomography (APT) experiments. These "Kingham curves" have wide application in APT, but cannot be analytically transformed to provide the electric field in terms of the easily measured charge state ratio (CSR). Here we provide a numerical scheme for the calculation of Kingham curves and the variation in electric field with CSR. We find the variation in electric field with CSR is well described by a simple two- or three-parameter equation, and the model is accurate to most elements and charge states. The model is applied to experimental APT data of pure aluminium and a microalloyed steel, demonstrating that the methods described in this work can be easily applied to a variety of APT problems to understand electric field variations.