Multi-photon ionization and excitation of the rare gases by free electron laser radiation

Full text: Multi-photon processes in the vacuum ultraviolet and soft x-ray regions became experimentally accessible only very recently when the free electron laser FLASH at DESY in Hamburg was put into operation. The high brilliance of its light pulses makes it possible to study non-linear processes in the photoionization of atoms, molecules and clusters. First priority in the study of these processes has the spectroscopy of atoms, in particular rare gas atoms. The study of non-linear processes in theses atoms was due to their high ionization thresholds with normal laser radiation not feasible; only the SASE-FEL with its ultra-bright VUV radiation made this now possible. Studying the intensity dependence proved the well-known relationship between the number of involved photons and the exponential behavior of photon intensity versus ionization probability. More interesting was the relationship between simultaneous and sequential double photo-ionization. Our preliminary data analysis points to a predominance of the latter by two photons, if the necessary threshold energy for this process is exceeded. The last in a way most interesting aspect in this regard is the angular distribution of the corresponding photoelectrons. Multi-photon processes are characterized by photoelectron angular distributions, which reflect the number of photons involved in the ionization process. This general rule is clearly exhibited in our angular distribution results. However, the different photolines show very distinct differences in this respect, which needs further theoretical explanation. (author)