Transient molecular dynamics and structure; ultrafast Optical Kerr Effect and computer simulations

The ultrafast dynamics including intra- and intermolecular modes in liquids and crystal lattice vibrations takes place on femtosecond to picosecond time scales. Direct observation of such motions in time domain became available by the invention of femtosecond pulsed laser sources. In this thesis a femtosecond optical Kerr effect (OKE) setup, built from scratch at the Department of Chemistry, University of Warsaw, is presented. The unique construction of the setup allows to perform very fast measurements of the optically induced birefringence with a high signal to noise ratio. The technical details of the detection and synchronisation scheme, the details of the construction of the dedicated balanced detector and the methods of the signal distortions suppression are given. The optically induced birefringence is modulated by intra- and intermolecular modes of the sample, coherently excited by a pump laser pulse and probed by another pulse entering the sample at a precisely controlled time delay with respect to the first pulse. The described setup was used in order to measure the OKE response for a set of the following samples: methanol/acetone and ethanol/water liquid mixtures at 300 K, alexandrite and YAG (pure, Pr3+ and Nd3+ doped) crystals at different temperatures. In the case of the liquid samples the OKE response was also obtained by means of the molecular dynamics simulations and decomposed into the molecular, interaction induced and cross components. The obtained results were compared with experiment, where a very good agreement was found for the methanol/acetone system. In the case of ethanol/water system a special version of the software, dedicated for highly parallel calculation on graphical processors, was written in C using CUDA. The computation speed was improved by a factor of 10, compared to the CPU version. The understanding of the simulation results was deepened by the analysis of velocity, angular velocity and orientational correlation functions. The analysis of the first two allowed to distinguish different intermolecular modes involving stretching and bending of hydrogen bonds, stretching of acetone dimer or rotation of acetone molecule in the cage of its neighbours. In the case of water a non-monotonic behavior of the strength of the neighbour cage was found on its dilution in ethanol. The analysis of the orientational correlation allowed to find the reorientation times of molecules in the investigated systems. The simulation data were also analysed in order to obtain information on the hydrogen bond network structure, like the probabilities of different hydrogen bonding configurations or the lifetimes of hydrogen bonds. In the case of methanol/ethanol system additional experimental investigation was made using IR, Raman and time resolved 2DIR spectroscopies. The OKE signals for crystalline samples were obtained at different temperatures in the range of 263-373 K. For both alexandrite and YAG crystals the OKE signals of the coherent phonon oscillations were fitted in order to obtain the temperature dependence of the frequency and inverse lifetime. Those appeared to be linear in the considered temperature range, indicating a 3-phonon relaxation process. The obtained results were used to obtain the anharmonic coupling constants. In the case of the YAG crystals the dependence of phonon requency and lifetime on Pr3+ and Nd3+ dopant concentration was investigated and interpreted in terms of small, dopant induced alterations in the crystal structure, where the most nfluenced modes involve motion of the substituted ion. Since such changes are small and hus hard to measure, high precision of the OKE experiment shows to be of great value, particularly when only low doping is possible, which is the case for Pr:YAG and Nd:YAG crystals.