New generation of synchrotron radiation sources and ultra-high resolution spectrometers, as well as the X-ray free electron lasers (XFELs) generating powerful ultra-short pulses, have dramatically expanded the field of application of X-ray spectroscopy and made it possible to study the ultrafast physical and chemical processes. This significantly expanded class of problems and the growing complexity of the processes and objects under study require development of new theoretical concepts necessary for accurate modeling of the investigated phenomena, as well as for predicting new effects and design of new experiments.
It has become increasingly obvious that the knowledge of only static picture of the matter structures is insufficient in science and that dynamics effects plays more and more important role in different applications like structural dynamics of materials, engineering materials science, studies of matter under extreme conditions etc. That relates to the dynamics of electronic and nuclear motion, including rotations, vibrations as well as the fragmentation of molecules. During the last few decades, many new fundamental results have been obtained using high brilliance sources of soft X-ray radiation – synchrotron radiation (SR) and X-ray Free Electron Laser (XFEL) facilities. Resonant inelastic and elastic X-ray scattering, resonant photoemission and coincidence techniques, as well as time resolved X-ray pump-probe measurements have Table_R06 Page 3 of 20 become the hot techniques in current and forthcoming investigations. The advantage of X rays is that the wavelengths match typical bond lengths in molecular or condensed matter systems together with the high element selectivity of X-ray spectroscopies. The reach information extracted from RIXS and RAS spectra refers not only to a target element and its position but also to the functional group thanks to so called chemical shifts. This great advantage of X-ray spectroscopies is used to locally probe the local structure, as it is reported by research groups of A. Föhlisch, Ph. Wernet, and L. Weinhardt. Intrinsic features of resonant X-ray scattering-spectroscopy free from the lifetime broadening of core-excited state allow to resolve vibrational structure (e.g. see works from groups of A. Föhlisch and Y. Harada). In contrast to other experimental techniques, the dominant contribution to the vibrational progression observed in Resonant Inelastic X-ray scattering (RIXS) and resonant Auger scattering (RAS) of free molecules and liquids comes from the vibrational modes localized near the core excited atom. Contrary to another techniques (for example IR spectroscopy) the dynamics in intermediate core-excited state results quite often in long vibrational progression even in RIXS of liquids. The excitation of high vibrational states gives unique opportunity to probe the long-range part of the potential energy surfaces (see Fig. 1) which brings information about local surrounding of liquids.
Our group has long term experience in studying the X-ray—matter interaction, and is clearly aware of the significance of these effects, in particular associated with additional ionization and excitation channels, including those leading to the formation of double-hole states, as well as ultrafast fragmentation of molecules, for both linear and non-linear X-ray spectroscopic techniques. Our previous results obtained for free molecules led us to a deeper understanding of the underlying physics and naturally triggered to carry out further investigations of more complex quantum systems, like polycrystalline CuO, layered clay system kaolinite, and liquid water and ice, where we already have contributed with several outstanding results. Moreover, one of the most common experimental techniques for studying molecular systems and biomolecules on modern X-ray radiation facilities are carried out in solutions, which are delivered to the interaction region by means of microjets or droplets. Due to this fact, studies of the effects of X-ray—liquid interactions accompanied by redistribution of vibrational energy over local and collective modes and nonlinear optical effects become very important. One of the objectives of our projects is to study the dynamics of X-ray matter interaction, including the recoil effect, focusing on modeling quantum electron-nuclear effects in complex quantum systems (polyatomic molecules, molecular clusters and liquids) with the help of the modern advanced XFEL sources in the framework of pump-probe spectroscopy.
