Short and long-range structural time evolution in optically excited solids probed by x-rays

This thesis is dedicated to the study of transient structural dynamics in solid state materials triggered by femtosecond laser pulses. The general goal of this work is to deepen the understanding of the electron-lattice coupling which in some specific cases leeds to emergence of various cooperative phenomena. Two types of experiments are described which are meant to probe directly longand short-range order of solids with a femtosecond time-resolution, namely x-ray diffraction and x-ray absorption spectroscopy. Respectively, two types of materials were studied. With femtosecond x-ray diffraction we probe the laser-induced dynamics of a structural order parameter in TiSe2 in the charge density wave phase. The main goal of these experiments is to clarify the nature of the CDW phase transition in this material, which has long been a subject of extensive debate. We investigate both the time and fluence dependence of the periodic lattice distortion associated with the CDW phase. The experimental data show that the laserinduced melting of the CDW phase occurs on a time scale faster than 250 fs. From the fluence dependence we determine the threshold energy required to melt the CDW non-thermally, which is four times lower than for thermal suppression of the superlattice. This observation stands in contrast to the previous studies of the laser induced superlattice suppression in strongly correlated systems, where the thermal and laser-driven thresholds are similar. We analyse the laser-induced lattice dynamics with a model based on Landau theory for second order phase transitions combined with a modified two-temperature model, accounting for the formation of excitons below the phase transition temperature. From this analysis we estimate exciton binding energies, which are close to values inferred from IR spectroscopy. Complementary all-optical pump-probe reflectivity measurements with a time resolution of ∼40 fs provide detailed information on the temperature and fluence dependence of the amplitude mode of the CDW phase and help to make the analysis of the lattice dynamics more quantitative. The second part of this thesis is devoted to the transient x-ray absorption of the excited Ti impurity ions in Al2O3, probed at the Ti K-edge. Here we address two questions. First, we investigate to which extent time-resolved XAS is applicable for study of diluted crystals with a low concentration of dopant ions. Second, we try to resolve a long-standing question concerning the type of JahnTeller distortion of the TiO6 octahedron, when the Ti 3+ ion is in the excited E state. We investigate the Ti K-edge fine structure with a time-resolution of 100 ns and analyse the data with the MXAN code, based on full multiple scattering theory. The analysis evidences two types of distortion a fully symmetric A1g mode and a tetragonal Jahn-Teller active Eg mode. The observation and determination of the magnitude of the A1g mode is unique as the other spectroscopies (e.g. optical, infrared and EPR) are insensitive to this type of distortion. We also determine the magnitude of the Eg mode, althought we can not distinguish between the compression and alongation of the oxygen octahedron. Additional excited state EXAFS measurements help to resolve this question. The EXAFS