Using model ionization efficiency curves to evaluate the methods for determining appearance potentials

Six methods of estimating appearance potentials from ionization efficiency curves of ions arising from single, competitive, consecutive and isolated ionic reactions are evaluated, using computed electron-impact ionization efficiency curves as ‘experimental’ data. Of the linear extrapolation, semi-log, second derivative, energy compensation, Warren and critical slope methods, a modified critical slope method gives the most accurate appearance potentials for fragment ions. Experimental data are presented to support this conclusion.