1 arXiv: 1412.1821v4 PRESENTED AS A TUTORIAL PAPER ISQ derivation ("derivation in SI units") of a formula for the electrostatic field ionization rate-constant for a hydrogenic atom in its ground electronic state Richard G. Forbes Advanced Technology Institute & Department of Electronic Engineering, University of Surrey, Guildford, Surrey GU2 7XH, UK Permanent e-mail alias: r.forbes@trinity.cantab.net As part of their theory, technological applications involving electrostatic field ionization (ESFI), such as gas field ion sources and atom probe tomography, need a formula for the rate-constant K e for free- space ESFI of a hydrogenic atom in its ground electronic state. This formula needs to explicitly show the dependence on ionization energy I (or, equivalently, the charge-number Z). Most existing formulae for hydrogenic atom ESFI were derived in some variant of the atomic units system. However, large numbers of applied scientists and engineers work with ESFI as a process of technological importance, but cannot nowadays be expected to have familiarity with the Gaussian or atomic units equation systems. In the 1970s, what is now called the International System of Quantities (ISQ), which includes the equation system behind SI units, was internationally adopted as the primary system for university teaching and for communication of scientific equations between theoreticians and applied scientists and engineers. 40 years on, derivations of ISQ versions of basic ESFI rate- constant formulae are still not easily found in the literature. Transparent ISQ derivations are now needed. This paper presents a detailed ISQ derivation of a formula for K e , using a method that is modelled closely on the conceptual approach used by Landau and Lifschitz (LL) in their well-known and widely accepted 1958 work (in the atomic units system) on ESFI of the hydrogen atom. This ISQ derivation confirms that, for a hydrogenic atom, the ionization energy appears in the pre-exponential as I 5/2 , and defines a universal "field ionization constant" that also appears. It is also shown how the