Saturation Effects in Resonant Three Photon Ionization of Potassium

The intensity and polarization dependence of the ion production due to three quantum ionization was investigated in potassium. The energy of two of the photons was tuned into resonance with n2D levels for 4 < n < 22. The ratio of the ion signals produced with circular polarization to that found with linear polarization was significantly less than that predicted by a single rate approximation. The discrepancy is explained by the increased linewidth of the resonant 2D level due to the ionization process itself. Single quantum photoionization cross sections for the excited levels calculated by means of quantum defect theory are shown to be consistent with the experimental results.