Two-Photon Excitation of Autoionizing States through Zeros in the Continuum

Abstract
A method is described for two-photon excitation of autoionizing states by use of two different-frequency photons in which the first photon reaches an energy which is near a maximum in the one-photon cross section but where the one-photon cross section is very nearly zero. This provides a large two-photon amplitude while suppressing the single-photon excitation. The application of the technique to study the Ba 6snd7snd transitions by detecting the total ionization produced is described. No discrimination between ionization from one- and two-photon processes was required.