Three-photon double ionization of Ar studied by photoelectron spectroscopy using an extreme ultraviolet free-electron laser: manifestation of resonance states of an intermediate Ar+ ion

Three-photon double ionization of an Ar atom has been investigated by photoelectron spectroscopy using an extreme ultraviolet free-electron laser. When the photon energy of the laser has been tuned to the resonances of the intermediate Ar+ ion, photoelectron peaks due to the resonant two-photon single ionization of the Ar+ ion have been observed. And it has been revealed that the appearance of the peaks is controlled by the mixing ratios of 3p(4) core- configurations of the multi-configuration states. Furthermore, it has been shown that the laser intensity dependence of the Ar2+ ion production rate due to the resonant two-photon single ionization strongly deviates from that expected by perturbation theory.