Multiphoton ionization of atoms with soft x-ray pulses

Abstract new soft x-ray Free-Electron Laser in Hamburg (FLASH) has opened the doorway to totally new experiments of materials research on nanometer and femtosecond scales. However, the mechanisms of photon-matter interaction are not well understood under the conditions of ultra-high photon intensities in conjunction with short wavelengths. In this context, we have quantitatively investigated nonlinear photoionisation of rare gas atoms at FLASH by ion mass-to-charge spectroscopy and, thus, also the limits for the application of gas-ionisation detectors for the characterisation of x-ray lasers. By strong beam focusing, we have achieved irradiance levels beyond 10 13 W cm -2 at about 40 eV photon energy and up to 10 16 W cm -2 in the Extreme Ultra-Violet (EUV) at about 90 eV. Here, surprisingly high degrees of photoionisation were observed on Xe atoms. By comparison with other rare gas targets, it emerged that the excitation of inner-shell resonances might play a significant role on the degree of atomic perturbation by the radiation field.