The VUV Free Electron Laser FLASH operates in soft X-ray range and produces high-intensive pulse trains with few tens femtoseconds duration. The transversely fully coherent beam will open new experiments in solid state physics which can not be studied with present radiation sources.
The study of the time dependent response of the multilayer to the X-ray pulse can provide insights into the process of interaction of highly intense FEL radiation with matter. To test the influence of electron excitation on the optical properties of boron carbide, the refractive index of B4C was measured near B K-edge by energy-resolved photon-in-photon-out method probing a Bragg reflection from periodical multilayers. The measured data clearly show that the variation of the fine structure of the K-absorption edges due to the chemical nature of the absorber element.
The knowledge obtained from experiments with continuous radiation was used to design the respective experiments with pulse from the FEL. In my thesis, it is proposed that the geometrical setup, where the incident pulse arrives from the FEL under the angle close to the 1st order ML Bragg peak, provides the most valuable information. Preliminary simulation considering form factors of neutral and ionized boron showed that due to ionization, pronounced changes in the reflectivity curve are expected. The proposed scheme can be the powerful tool to study the various processes within the electronic sub-system of the FEL pulse interaction with matter. This type of investigations gives a deep understanding of the nature of the electronic excitation and the recombination at the femtosecond scale.