Analytical parametrization for the shape of atomic ionization cross sections

The behavior of the ionization cross section of atoms is known classically in the limits of threshold energy and at high energies. These two limits are used to construct a simple analytical formula for the ionization cross section that depends upon two parameters: the magnitude of the maximum of the cross section and its position in energy. The parametrization has been tested for electron- and positron-impact ionization as well as for proton- and antiproton-impact ionization. It reproduces in all cases the shape of the cross section and offers a unified treatment for ionization by bare projectiles irrespectively of their charge and mass.