Timing dissociative dynamics of small molecules and dimers with internal clocks applying coincident particle momentum imaging

One of the central goals of modern ultrafast science is to understand the coupled motion of electrons and nuclei in dissociation processes of molecular and cluster targets. The challenges for a comprehensive experimental investigation are to simultaneously enable select electronic transitions that initiate the dynamics, distinguish and isolate the various reaction channels that occur, trace the dissociation pathways on the potential energy surfaces, measure the emission patterns and energies of the reaction products, and time the fragmentation steps.

In this presentation I will show how this demanding task can be accomplished with reaction microscopy while applying different internal clock mechanisms instead of pump-probe interrogation schemes. In particular, the change in charge state during sequential dissociation processes of small molecules and dimers is monitored in real-time in three examples: Upon photo double ionization with synchrotron radiation (a) the electron transfer via spin-orbit coupling in water molecules and (b) the interatomic decay of transient states in NeKr dimers is followed on the potential energy landscape and timed. Moreover, (c) the sequential emission of electrons during structural changes of oxygen molecules during charge-up after two X-ray absorption at the EuXFEL are traced, and the femtosecond dynamics of the dissociating ion is revealed.