Electric field dependence of the photoionization cross section of Rb

The relative photoionization cross section of ground-state Rb is measured near threshold as a function of applied electric field strength. The cross section is found to have a wavelength-dependent resonance structure that changes with the strength of the electric field. This resonance structure is observed not only for energies above the classical field ionization limit, but for the case of $\pi$ excitation, beyond the zero-field ionization limit as well. An analysis of the data is presented that connects the concepts of "strong-field mixing" and the perturbation calculations of the energies of hydrogenic Stark components.