Multiple ionization of atomic argon irradiated by EUV free-electron laser pulses at 62 nm: evidence of sequential electron strip

We have investigated multiple ionization of atomic argon by extreme-ultraviolet light pulses (62 nm, 100 fs in width, <2 × 10 14 W cm −2 ) at the free-electron laser facility in Japan, and observed highly charged ions with the charge state up to +6. The measured laser power dependence of the highly charged ions indicates that the multiple ionization proceeds via the sequential stripping of electrons.