<title>Ultrafast x-ray absorption spectroscopy: observing the equilibrium structure and structural dynamics of solvated molecules</title>

Abstract

Ultrafast high-intensity laser pulses incident upon condensed matter targets can generate solid-density plasmas that emit x-ray pulses with sub-picosecond temporal structure and significant spatial coherence. Such ultrafast laser-driven plasma x-ray sources based on solid and liquid targets are currently under construction in our laboratory. Performance details at several kilohertz laser pulse repetition rates are discussed. As an application of the temporal structure of laser-generated x-ray pulses, ultrafast x-ray absorption fine structure (UXAFS), currently under development, is discussed. It allows, in principle, to measure the structural dynamics of atoms during a chemical process in solution. An overview over UXAFS is presented and properties of our ultrafast x-ray absorption spectrometer are discussed. First calculations of time dependent UXAFS-spectra for ironpentacarbonyl are presented. Ultrafast molecular dynamics depend on the structure of the solvated molecule at the moment of photo-excitation. This structure depends on the solute's interaction with the solvent. Furthermore, the solute's vibrational modes and structure are correlated, solvent dependent, and can be measured by mid-infrared and x-ray absorption spectroscopy. Such measured spectra are presented and correlated with semi-empirical quantum calculations in order to elucidate the solvation environment of transition metal coordination complexes in various solvents.