An empirical formula for the electron-impact ionization cross-section
An empirical formula with three free parameters is given for the representation of cross-sections for single ionization of atoms and ions from the ground state by electron-impact. Almost all experimental results can be approximated by this formula within 10% over the whole energy range between threshold and 10 keV; all experimental results can be approximated within experimental error. The formula proposed is not suitable to reproduce the exact contour of fine structures in the ionization cross-section curve.