Estimation of Ionization Potentials by Comparison with Neighboring Elements

If a spectrum contains an unperturbed series of at least three members, the ionization potential can be found with fair accuracy from its limit. If but two members are available, serious errors occur if the Ritz correction is ignored. This correction can often be closely estimated by comparison with elements of neighboring atomic numbers. This method is familiar, but published estimates have not been made on a uniform system. Such estimates for spark spectra from Ca ii to Zn ii give ionization potentials differing from a smooth formula by ±0.02 volt, while the differences in the three cases where the Ritz correction was not applied, average ±0.39 volt.