title: Sampling measurement of soft-x-ray-pulse shapes by femtosecond sequential ionization of Kr+ in an intense laser field.
abstract: We propose a sampling technique for measuring the shape of ultrashort soft-x-ray pulses. The technique uses the transient state of Kr+ ions that is produced by the femtosecond sequential evolution of Kr ions during optical-field-induced ionization as an ultrafast x-ray-absorption sampling gate. We demonstrate the technique by measuring the pulse shape of the 51st harmonic (15.6 nm) generated by a 100-fs titanium:sapphire laser pulse. The measured pulse duration is 220 fs. Our experimental result confirms that the sequential evolution of Kr+ ions from neutral Kr to Kr2+ is the dominant contribution to the ionization process from the aspect of time-domain measurement.
authors.affiliation: NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosoto Wakamiya, Atsugi, Kanagawa 243-0198, Japan. oguri@nttbrl.jp
authors.forename: Katsuya
authors.initials: K
authors.lastname: Oguri
authors.forename: Tadashi
authors.initials: T
authors.lastname: Nishikawa
authors.forename: Tsuneyuki
authors.initials: T
authors.lastname: Ozaki
authors.forename: Hidetoshi
authors.initials: H
authors.lastname: Nakano