Abstract
Shot-by-shot photoelectron spectroscopy has been performed to study the three-photon double ionization of Ar in intense extreme-ultraviolet free-electron laser fields over a photon energy range of 19.6--22.0 eV. It is found that the double ionization to Ar${}^{2+}$ 3${p}^{\ensuremath{-}2}$ proceeds sequentially via the formation of singly charged Ar${}^{+}$ 3${p}^{\ensuremath{-}1}$ states and is enhanced around photon energies of 20.5 and 21.5 eV. Two types of resonance are identified in the two-photon ionization of the Ar${}^{+}$ 3${p}^{\ensuremath{-}1}$ states: (i) resonances to intermediate states at the one-photon energy level and (ii) those to autoionizing states above the Ar${}^{+}$ ionization threshold.
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