Structural relaxation of hydrogen bonded systems investigated by time-resolved X-ray diffraction

• Applicant: Professor Dr. Alfred Laubereau
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2002 to 2006
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 5373921

Using time-resolved X-ray diffraction on an ultrashort time scale, the short-range order in molecular liquids can be monitored, providing information on structural relaxation. Of special interest for the planned investigations are the microscopic dynamics of H-bonded systems like water, aqueous electrolytic solutions, while simple alcohols may serve as model systems for comparison. It is clear that the ability of water to associate to a 3-dimensional H-bonded network results in the special features of this most important solvent. Recent experiments utilizing time-resolved infrared spectroscopy on a sub-picosecond time scale have opened the door to a more detailed picture of the hydrogen bond dynamics of water. An important disadvantage in these investigations however is that the measured spectral features have to be related to structural changes using up to now empirical rules which are debatable. A direct determination of the hydrogen bridge bond dynamics of water and related systems on the sub-picosecond time scale is lacking. A powerful method for this purpose is time-resolved small angle X-ray diffraction. The time-integrated version of the technique has demonstrated its potential in numerous cases during the past decades, providing time- and spatial averages of the short-range order in the liquid. To get access to the strutural dynamics of water a novel time-resolved version with sub-picosecond resolution has to be developed. A pump-probe approach will be utilized and built up in close collaboration with the group of W. Zinth at the Munich university. For the excitation process a femtosecond laser pulse in the UV or mid-infrared spectral range will be used initiating the breaking of hydrogen bonds in the sample. The subsequent structural changes are monitored by a sub-picosecond X-ray pulse emitted from a laser plasma generated by the help of the same high power femtosecond laser system that also delivers the pump pulse. The new experiment will provide key information on the ultrafast structural dynamics of H-bonded systems like bulk water and aqueous electrolytes relevant for biological systems. In a second stage of the project, water will be also studied under special boundary conditions, i.e. in microporous systems like zeolithes and in model systems of biological membranes.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1134: Transient structures in condensed matter investigated by ultrafast x-ray techniques
• Participating Person: Professor Dr. Robert Laenen