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In the past few years, the development of light sources of the 4th generation, namely XUV/X-ray Free Electron Lasers provides to the scientific community outstanding tools to investigate matter under extreme conditions never obtained in laboratories so far. As theory is at its infancy, the analysis of matter via the self-emission of the target is of central importance. The characterization of such dense matter is possible if photons can escape the medium. As the absorption of K-shell X-ray transitions is minimal, it plays a key role in this study. We report here the first successful observation of K-shell emission of Nitrogen at 430 eV using an XUV-Free Electron Laser to irradiate solid Boron Nitride targets under exceptional conditions: photon energy of 92 eV, pulse duration of ∼20 fs, micro focusing leading to intensities larger than 1016 W/cm2. Using a Bragg crystal of THM coupled to a CCD, we resolved K-shell line emission from different charge states. We demonstrate that the spectroscopic data allow characterization of electron heating processes when X-ray radiation is interacting with solid matter. As energy transport is non-trivial because the light source is monochromatic, these results have an important impact on the theory. © 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

We investigated single shot damage of Mo/Si multilayer coatings exposed to the intense fs XUV radiation at the Free-electron LASer facility in Hamburg - FLASH. The interaction process was studied in situ by XUV reflectometry, time resolved optical microscopy, and "post-mortem" by interference-polarizing optical microscopy (with Nomarski contrast), atomic force microscopy, and scanning transmission electron microcopy. An ultrafast molybdenum silicide formation due to enhanced atomic diffusion in melted silicon has been determined to be the key process in the damage mechanism. The influence of the energy diffusion on the damage process was estimated. The results are of significance for the design of multilayer optics for a new generation of pulsed (from atto- to nanosecond) XUV sources. (C)2010 Optical Society of America

We exposed standard Mo/Si multilayer coatings, optimized for 13.5 nm radiation to the intense femtosecond XUV radiation at the FLASH free electron laser facility at intensities below and above the multilayer ablation threshold. The interaction process was studied in-situ with reflectometry and time resolved optical microscopy, and ex-situ with optical microscopy (Nomarski), atomic force microscopy and high resolution transmission electron microscopy. From analysis of the size of the observed craters as a function of the pulse energy the threshold for irreversible damage of the multilayer could be determined to be 45 mJ/cm2. The damage occurs on a longer time scale than the XUV pulse and even above the damage threshold XUV reflectance has been observed showing no measurable loss up to a power density of 1013 W/cm2. A first explanation of the physics mechanism leading to damage is given.