Sequential two-photon double ionization of the 4d shell in xenon

The sequential two-photon double ionization of 4d electrons of xenon in intense free-electron laser (FEL) radiation is theoretically investigated for photon energies of 70-200 eV. At these energies, which are available at the free-electron laser FLASH, the 4d photoionization dominates over the ionization of the valence shells, showing a giant resonance. The 4d vacancy produced by the first photon strongly couples to the (Auger) autoionization continuum with a lifetime of only ∼6 fs and thus the Auger decay competes with the second photoionization in a typical FEL pulse. The photoelectron angular distributions and angular correlation between emitted electrons are investigated as associated with the peculiar 4d character of the photoionization and Auger decay.