서강대학교

초록/요약

In the 90's, the advent of insertion devices in X-ray sources resulted in an improvement of the coherence of X-rays and offered a large range of new experimental possibilities. In this thesis we use coherent X-rays in two different experimental techniques and present corresponding results. Coherent X-rays are available both at 3rd generation synchrotrons and X-ray free electrons lasers, despite major differences in concepts and beam properties which are discussed. The first study presented in this thesis is the demonstration of the feasibility of the X-ray photon correlation spectroscopy (XPCS) technique with XFEL source. XPCS is based on the correlation of speckle patterns collected from a glassy sample by coherent X-rays at different delay times. The fact that a technique well established at 3rd generation synchrotrons could be transferred to XFEL is not trivial and this study paves the way for faster dynamics studies. Furthermore, the characteristics of the XFEL beam itself are worth to study and need to be understood for an experiment to be successful. The first generation of XFEL being based on the self-amplified spontaneous emission (SASE) process, random by nature, fluctuations in energy, intensity and beam position form the essence of the X-ray beam. We use here the XFEL beam to retrieve the dynamics of gold nanoparticles in a polystyrene matrix above its melting temperature, and deduce in the meantime several beam characteristics such as the number of transverse modes and the average beam size, and estimate the sample heating. The second part of this thesis concerns the study of the internal deformation change in a crystalline catalyst responding to various input gases during a catalytic reaction using the coherent X-ray diffraction technique in Bragg geometry. The coherent X-ray beam is used here to measure the evolution of the diffraction pattern of a platinum single crystal during the catalytic oxidation of methane, a reaction that takes place in car exhaust systems. The diffraction pattern centered on a Bragg peak shows radically different behaviors depending on the gas ambiance and the temperature. Using a phase retrieval algorithm, we are able to deduce both the displacement and the strain distribution in the nanocrystal. Whereas in this case catalysis takes place essentially on the surface, our results show for the first time that the strain generated propagates through the entire crystal lattice.