Time-Resolved Structural Imaging of Chemical Transition State Dynamics

For bonds to be broken and new bonds to be formed, chemical reactants have to come close to each other and evolve through transient intermediate configurations known as the transition state. Transition state dynamics is closely related to reaction mechanisms and of fundamental importance in chemistry.

Much work has been done to unravel these dynamics, which often involve major structural rearrangement of atoms.

However, there are a lot of open questions since to date none of the applied spectroscopic techniques has directly delivered the time-dependent transition state structure.

I propose to develop a novel probe that images, one molecule at a time, the full three-dimensional atomic configuration of individual transition states as they evolve: Reaction precursors are prepared using molecular ions and small (ionic) clusters with defined initial structure and tunable internal temperature.

Starting from these well-defined initial configurations, chemical dynamics will be initiated by a femtosecond laser pulse.

Timed Coulomb Explosion Imaging, induced by extremely short intense laser or X-ray pulses together with full coincidence momentum imaging of all fragments is then applied as a probe.

The latter yields the evolving transition state structure by mapping the position of all atoms as a function of time-delay between the two pulses.

I argue that the progress in laser technology in recent years was imperative to make such a scheme feasible now, since it allows measurements at high repetition rate, which is absolutely crucial.

I propose work packages of increasing complexity to study ground state chemical reactions in the presence of solvent molecules and under the influence of a control pulse.

The structural information obtained from Coulomb Explosion Imaging gives direct insight into how reaction mechanisms change under such conditions. I anticipate that light will be shed on some of the long-standing open questions surrounding transition state dynamics.