Multiphoton Ionization of Atoms and Molecules with Soft and Hard X-rays

We have recently extended our previous investigations of the multiphoton ionization of heavy atoms, such as Kr and Xe [1, 2], and of high-Z atom containing molecules [3] from the soft into the hard X-ray range as well as into the XUV regime. Using the 100-nm focus environment at LCLS, we were able to reach peak intensities up to 10$^{19}$W/cm$^{2}$ at photon energies between 5 to 9 keV. This allows studying atomic and molecular ionization processes under unprecedented X-ray intensities and, in particular, under the identical conditions where typical coherent diffractive imaging experiments are performed. Our results are thus important benchmarks for calculating radiation damage effects in FEL-based X-ray imaging experiments. Using new micro-focusing capabilities at FLASH, we also extended our studies into the XUV range between 70 and 200 eV photon energy and observed significantly higher charge states than previously reported. I will present the results from our recent measurements at LCLS and FLASH and discuss the different multiphoton ionization mechanisms that play a role in the XUV, soft, and hard X-ray range. \\[4pt] [1] B. Rudek \textit{et al.}, Nature Photon. \textbf{6}, 858 (2012)\\[0pt] [2] B. Rudek \textit{et al.} Phys. Rev. A \textbf{87}, 023413 (2013)\\[0pt] [3] B. Erk \textit{et al.}, Phys. Rev. Lett. \textbf{110}, 053003 (2013)