Multiphoton ionization of He by using intense high-order harmonics in the soft-x-ray region

We report on the multiphoton ionization processes in the soft-x-ray region (lambdaless than or equal to30 nm). On the basis of the measured time-of-flight spectra for both ions and electrons obtained using intense soft-x-ray pulses produced by high-order harmonics, the cross sections of the two-photon double ionization and above-threshold ionization of He are estimated. The high-intensity soft-x-ray radiation achieved by phase-matched high-order harmonics enables the investigation of these nonlinear optical processes, which were beyond the reach of conventional light sources.