Direct and Sequential Multi-photon Ionization of Rare-gas Atoms in the XUV
The interaction of individual atoms with intense short-wavelength radiation is a fundamental issue in particular for atomic physics, but also in general for many other areas of FEL-based science. To understand the ionization dynamics and the role of sequential and direct multi-photon absorption processes, relevant information can be obtained experimentally from both ion and electron spectroscopy, supported by theoretical approaches.
In this report the results of a broad investigation are presented, performed at different XUV photon energies delivered by the Free electron LASer in Hamburg (FLASH), on systems with electronic structures of different complexities (Ar and Xe atoms). At first, unexpectedly high charge states are reported for Argon at different photon energies. The comparison of the intensity dependent ion yield spectra with the results of rate equation calculations reveals the importance of excited states during the different steps of the ionization ladder.
In addition, electron spectroscopy is used in combination to ab-initio calculations to provide a quantitative insight into the relevance of direct multi-photon absorption in the ionization process. The more complex and intriguing case of Xe shows, in contrast to the case of Ar, a more pronounced and photon energy dependent production of highly charged ions. The role of collective electronic response in the multiphoton ionization will be discussed considering the one photon and two-photon direct 4d electron emissions with respect to their dependence on the photon energy tuning in the region of the 4d->f giant resonance.