Femtosecond X-ray and electron diffraction for the study of the ultrafast structural dynamics in molecular systems

Direct information on structural changes characterising chemical reactions is expected from diffraction experiments with wavelengths of the radiation/particles in the 1-Å range, providing atomc scale resolution. Present experiments on fastest molecular dynamics are limited by the availability of short enough X-ray or electron pulses with high brilliance. The new MAP sources will open the way to a new class of experiments. Diffraction experiments with fs X-ray and electron pulses can resolve the molecular reaction dynamics induced by visible, UV and IR light. We will perform experiments on molecular crystals as well as small angle X-ray and electron diffraction experiments on surfaces, solutions or on evaporated molecules and biopolymers.

• For molecules in an ordered (crystalline) surrounding, novel information will be obtained on the structural changes during the course of a chemical reaction.
• The excitation-induced modifications of the crystalline composition, the intra- and intermolecular rearrangements and the melting processes will be studied.
• Fundamental dynamics of larger molecules, where the incorporation into a crystalline surrounding would modify or even prevent the reaction, will be investigated in solution and after evaporation.

Prototype scattering experiments on molecular crystals have already demonstrated the feasibility of the transient scattering experiments. Systematic investigations of the reaction dynamics, especially in large molecules or biological systems, require a considerable increase in the radiation flux and an adaption of the repetition rate. Here hard X-ray pulses of the MAP programme with low divergence and short pulse duration will open up the way to new, currently not accessible, experimental information on ultrafast chemical reaction and structural dynamics.