Time-resolved X-ray diffraction with accelerator- and laser-plasma-based X-ray sources

Femtosecond X-ray pulses are a powerful tool to investigate atomic motions triggered by femtosecond pump pulses. This thesis is dedicated to the production of such pulses and their use in optical pump – X-ray probe measurement. This thesis describes the laser-plasma-based sources available at the University of Duisburg-Essen. Part of it consists of the description of the design, built-up and characterization of a new “modular” X-ray source dedicated to optimize the X-ray flux onto the sample under investigation. The acoustic wave generation in femtosecond optically excited semiconductor (gallium arsenide) and metal (gold) was performed using the sources of the University of Duisburg-Essen. The physical answer of the material was modeled by a simple strain model for the semiconductor, pressure model for the metal, in order to gain information on the interplay of the electronic and thermal pressures rising after excitation. Whereas no reliable information could be obtain in gallium arsenide (principally due to the use of a bulk), the model for gold achieved very good agreement, providing useful information. The relaxation time of the electron to lattice energy was found to be (5.0±0.3) ps, and the ratio of the Grüneisen parameters was found to be e / i = (0.5±0.1). This thesis also describes the Sub-Picosecond Pulse Source (SPPS) which existed at the (formally) Stanford Linear Accelerator Center, an accelerator-based X-ray source, and two measurements performed with it. The first one is the detailed investigation of the phonon softening of the A1g mode launch in bismuth upon fluence excitation. Detailed information concerning the new equilibrium position and phonon frequency were obtained over extended laser pump fluences. The second measurement concerned the study of the liquid phase dynamics in a newly formed liquid phase following ultrafast melting in indium antimonide. The formation of the liquid phase and its development for excitations close to the ablation threshold were revealed. Such results were possible to obtain, due to the unprecedented combination of a short X-ray pulse duration and brightness at the SPPS

Unraveling the mechanism of NO ligand photoisomerism by time-resolved infrared spectroscopy

International audienceUV-Vis- and infrared femtosecond spectroscopy makes it possible to reveal all different steps of photochemical reactions after the electronic excitation. The electronic relaxations are observed in the UV-Vis spectral range whereas the nuclear motions are monitored in the infrared spectral range. We used femtosecond time-resolved infrared spectroscopy to demonstrate the photoisomerization of the NO ligand photoinduced by a visible femtosecond pulse in a Na2[Fe(CN)5NO]*2H2O single crystal occurs in about 350 fs. The analysis of data makes it possible to unravel the mechanism leading to the photoisomerization of the NO ligand

Time-resolved diffraction with an optimized short pulse laser plasma X-ray source

We present a set-up for time-resolved X-ray diffraction based on a short pulse, laser-driven plasma X-ray source. The employed modular design provides high flexibility to adapt the set-up to the specific requirements (e.g. X-ray optics, sample environment) of particular applications. The configuration discussed here has been optimized towards high angular/momentum resolution and uses K$_{\alpha}$-radiation (4.51 keV) from a Ti wire-target in combination with a toroidally bent crystal for collection, monochromatization and focusing of the emitted radiation. $2\times 10^5$ Ti-K$_{\alpha1}$ photons per pulse with $10^{-4}$ relative bandwidth are delivered to the sample at 10 Hz repetition rate. This allows for high dynamic range ($10^4$) measurements of transient changes of the rocking curves of materials as for example induced by laser-triggered strain waves.Comment: 29 pages, 8 figure

Femtosecond x-ray diffraction reveals a liquid–liquid phase transition in phase-change materials

In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag4In3Sb67Te26 and Ge15Sb85 at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics

Ultrafast Photovoltaic Response in Ferroelectric Nanolayers

We show that light drives large-amplitude structural changes in thin films of the prototypical ferroelectric PbTiO[subscript 3] via direct coupling to its intrinsic photovoltaic response. Using time-resolved x-ray scattering to visualize atomic displacements on femtosecond time scales, photoinduced changes in the unit-cell tetragonality are observed. These are driven by the motion of photogenerated free charges within the ferroelectric and can be simply explained by a model including both shift and screening currents, associated with the displacement of electrons first antiparallel to and then parallel to the ferroelectric polarization direction.United States. Dept. of Energy (contract DE-FG02-07ER46431)United States. Office of Naval Research (grant N00014-06-1- 0459