Ultrafast electronic charge redistribution processes in ionic crystals by femtosecond x-ray diffraction

点击翻译按钮获取中文摘要

This project aims at understanding fundamental microscopic mechanisms which underlie ultrafast atomic motions and electronic charge redistribution processes in crystalline matter. The main scientific goal is a detailed spatial characterization (with atomic resolution) of the so called soft mode phonons in displacive ferroelectrics. The soft-mode is a complex interplay of ionic motions and inter-ionic electronic charge transfer processes. It is a sort of precursor of the phase transition with symmetry breaking changes of the electron density map which are inherent to coherent soft mode oscillations of ferroelectrics. The planned experiments aim at studying such changes using femtosecond x-ray diffraction by means of the rotation method. To this end, we will set up a new detector end station for the femtosecond x-ray rotation method which will allow for measuring up to 12 reflections simultaneously. Our planned experiments are based on a laser plasma source for hard x-ray pulses in combination with an intense terahertz source for applying strong electric field transients to the crystal. The proposed method will be sensitive to both field-driven anisotropic changes of the atomic positions within the unit cell, i.e. structural changes, and to electron transfer processes between various ions within the unit cell. A spatially resolved characterization of the soft-mode will clarify the interplay of electronic and ionic motions of this hybrid mode which is expected to lead in turn a better understanding of the ferro- to para-electric phase transition.A second series of experiments aims at electric field-induced quasi-instantaneous deformations of the electron density map in ionic crystals made of light elements (e.g. LiBH4, LiH, LiOH, NaBH4 etc.). The femtosecond x-ray rotation method on a multitude of equivalent reflections will give new experimental insights into ground state electron correlations of the materials. Furthermore, applying strong THz fields instead of optical fields to the crystals will give new insights in the relative contributions of spatial ionic shifts vs. electronic charge transfer to the macroscopic polarization.