Elementary steps of photo-chemical processes
in the condensed phase revealed by ultrafast
structure-sensitive vibrational spectroscopy

Although structure-resolving ultrafast diffraction experiments are currently under way for prototype reactions, they can only be applied for a limited class of molecular reactions, i. e. in the crystalline state (x-ray diffraction) or in vacuum (electron diffraction). However, the surrounding solvent environment plays a key role for many chemical processes in condensed matter. Such processes will be addressed in this project. To this end, novel approaches to ultrafast vibrational spectroscopy will be developed with superior time resolution and measuring accuracy. Femtosecond infrared and femtosecond stimulated Raman spectroscopy (FSRS) can identify transient reaction intermediates and reveal the motion of nuclei and the energy flow in the molecule. Vibrational spectroscopy in the MIR and THz range is highly sensitive to the rearrangement of hydrogen atoms and hydrogen bonds that are practically invisible in x-ray and electron diffraction experiments. Switching of hydrogen bonds in chemical reactive systems by intense IR pulses and following the changes of the reaction path is an important issue of the project.

In one sub-project we will address structural changes connected with ultrafast reactions (isomerisation, ring opening, proton transfer) in optically triggered molecular switches. We will use UV/visible and IR pump and infrared of Raman probe for the identification of reaction intermediates and molecular interactions in the various intermediate states. Multidimensional infrared experiments will improve significance and sensitivity of the experiments. Shaped infrared pump pulses are used to selectively trigger molecular changes. In a second sub-project we study the role of hydrogen bonds in a wide range of molecular systems extending from hydrogen-bonded organic liquids and water to biomolecular model systems. The breaking of hydrogen bonds after deposition of infrared quanta and energy migration via hydrogen bonds will be studied. The experiments will be complemented by the diffraction experiments (C.2.1) and benefit from the intense IR pulses generated in A.1.3. The interpretation will be carried out in close collaboration with C.2.6.