X-ray - Based Studies of Structural Dynamics in Solids and Liquids

Abstract: The wavelength of x-ray radiation is much shorter than that of visible light. In fact, it is comparable to the distances between atoms in solids, which is on the order of one tenth of a nanometer. Using light of such a short wavelength, it is possible to study the structure of materials on the atom level. Scattering of x-rays has been developed into an invaluable tool to analyze various characteristics of matter, from the lattice structure of crystals to the structure of large biological molecules. In this way, we found that many of the properties of materials depend on their internal structure. To learn about changes in the structure of materials on the atom-level, time resolved x-ray scattering has proven a powerful technique. A wide variety of processes can be studied: from phase transitions in materials to vibrations in crystal lattices and pathways of chemical reactions. The aim of time-resolved studies is to follow these processes in real time. The timescale for changes in structure varies considerably depending on the underlying mechanism. Processes involving neighboring atoms typically take about 100,fs. Structure changes involving large groups of atoms or molecules occure on a timescale of picoseconds to nanoseconds. Different mechanisms can be used to trigger changes in structure. Laser pulses with a duration of less than 100,fs can be produced routinely and are used to initiate ultrafast changes in the structure. Alternatively, short electrical pulses can be used to trigger structural changes in piezo-electric materials. In this work, the main focus has been on experimental studies in order to deepen the understanding of structural changes in matter. The picosecond dynamics involved in the melting and recrystallization of a semiconductor, acoustic and thermal response of laser-excited solids, and the dynamics in the structure of a piezo-electric material have been studied. Additionally, instrumentation required for time-resolved x-ray scattering experiments has been developed.

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