Photoelectron spectroscopy of sequential three-photon double ionization of Ar irradiated by EUV free-electron laser pulses

We have investigated the ionization of the Ar atom by 51 nm extreme-ultraviolet light pulses at the free-electron laser facility, SPring-8 Compact SASE Source test accelerator, in Japan. The angle-resolved photoelectron spectra contain lines due to sequential three-photon double ionization with the second ionization step proceeding via the resonantly enhanced two-photon absorption. The relative intensities of the corresponding photoelectron peaks and their angular dependence are explained in the framework of a three-step model of the process.