Two- and three-photon double ionization of helium by soft x-ray free-electron laser pulses

The multiphoton double ionization of He was investigated using a soft x-ray free-electron laser (SX-FEL), in the perturbative intensity regime (<10(13) W cm(-2)). The photon energy was tuned to the transition between the 1s and 2p levels of He+ ions. At this resonance energy of 40.8 eV, both sequential three-photon double ionization (S3PDI) and two-photon double ionization (2PDI) were measured by electron spectroscopy and by ion time-of-flight mass spectroscopy. We distinguished S3PDI from 2PDI by electron spectroscopy. Based on these experimental data, SX-FEL pulse-energy dependences of electron and ion yields for both S3PDI and 2PDI were discussed. The angular distribution of 2p photoelectrons emitted from He+ (2p) ions in the S3PDI process was examined as an example of the simplest photoemission dynamics.