Photoionization of excited atoms by low intensity light: experimental test of the effective cross section.

The effective cross-section concept, analogous mathematically to Panarella's effective photon concept, is confirmed experimentally. Uniform spatial profiles of excited atom cross sections yield linear response to incident light, unlike previous results with Gaussian spatial profiles. Implications include potentially improved wavelength tuning sensitivity with the optogalvanic effect, obtaining response linearity in gas discharge detectors of uv and visible light and, because of the analog, modeling the geometrical aspects of Panarella's effective photon concept.