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Technical applications using electrostatic field ionization (ESFI) need aformula for the rate-constant K_e for the free-space ESFI of a hydrogenic atomin its ground electronic state. This formula must indicate the dependence onionization energy I. Existing formulae were derived using atomic units.However, many scientists work with ESFI as an important technical process, butare not familiar with the Gaussian or atomic units systems. In the 1970s, thepresent International System of Quantities (ISQ), which includes the equationsbehind SI units, was designated as the main system for university teaching andthe communication of scientific equations. 40 years on, ISQ derivations of ESFIrate-constant formulae are still not easily found, but are now needed. Thistutorial paper gives a detailed ISQ derivation of a formula for K_e. Thederivation is closely modelled on the Landau and Lifshitz (LL) approach used intheir 1958 work (in atomic units) on hydrogen atom ESFI. The ISQ derivationconfirms that, for hydrogenic atoms, ionization energy appears in thepre-exponential as I^(5/2), and defines a universal "field ionizationconstant". It also shows how the ISQ formula relates to the Gurney and Condon"attempt frequency" form for tunnelling rate-constants, and an ISQ formula isgiven for the motive energy in the related JWKB integral. The ISQ derivationuses a motive-energy transformation analogous to a transformation used by LL.The need for this transformation in ESFI theory raises questions as to thecorrectness of theoretical treatments of field electron emission fromnon-planar emitters, which do not make this transformation.

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