Time-resolved diffraction of single molecules
with spatial localisation, cooling and orientation:
structure of short-lived intermediates

Investigation of the dynamics within a molecule during an ultrafast structural change is coming into reach with the new generation of MAP light sources (more than 10¹¹ X-ray photons per pulse). This should enable us to record a diffraction pattern from a single molecule without need of crystalline periodicity. The time-resolved 3D structure can be retrieved from the diffraction patterns, if the destroyed molecule can be replaced by identically prepared copies supplied one after the other. To achieve this goal, we will develop reproducible and precise methods at the laser repetition rate to prepare and handle the molecular target.

We propose to study photo-triggered isomerisation on single molecular ions to elucidate the structural changes during fundamental chemical reactions. We will handle undisturbed single molecular ions (m < 10⁴ μ) by sympathetically laser cooling the external degrees of freedom in a Coulomb crystal confined in a Paul trap, cooling the internal rovibrational degrees of freedom by black-body radiation close to their ground states, separate single molecular ions into a diffraction zone by controlling electrical fields on electrodes and light pressure and 3D-orient them in space by additional light pulses. Since we will investigate charged molecules, we will take advantage of elaborate schemes to append functional groups that can be ionised without disturbing the dynamics of the molecule. The alternative protonation will simulate biological surroundings. This approach will provide cold and oriented molecular ions with a repetition rate of 10 kHz, positioned to better than 1 μm, and 3D-oriented in space. It should enable the almost background-free investigation of dynamical effects on the fs scale by X-ray diffraction fully synchronised with the optical excitation. Already available laser-produced and accelerated electrons and the related diffraction patterns should allow us to elucidate processes in molecules on a time scale well below 100 fs.