We propose a sampling technique for measuring the shape of ultrashort soft-x-ray pulses. The technique uses the transient state of Kr<sup>+</sup> 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<sup>+</sup> ions from neutral Kr to Kr<sup>2+</sup> is the dominant contribution to the ionization process from the aspect of time-domain measurement.
© 2004 Optical Society of America
Katsuya Oguri, Tadashi Nishikawa, Tsuneyuki Ozaki, and Hidetoshi Nakano, "Sampling measurement of soft-x-ray-pulse shapes by femtosecond sequential ionization of Kr<sup>+</sup> in an intense laser field," Opt. Lett. 29, 1279-1281 (2004)