Multiphoton and above-threshold ionization in the XUV energy range

Recent experiments at the free-electron laser in Hamburg FLASH have succeeded in measuring direct multiphoton ionization and above-threshold ionization (ATI) in the XUV. We present a theoretical description of the phenomena seen in argon and xenon in an ab initio approach using the time-dependent configuration interaction singles method. Upon solving the time-dependent Schr\"odinger equation for the N-electron problem the photoelectron spectrum is calculated. Multiphoton absorption cross sections are derived and employed to quantify the ATI process for different photon energies.