Imaging ultrafast molecular processes with high-repetition-rate X-ray free electron lasers

The availability of high-repetition-rate XFEL facilities like the European XFEL and the upcoming LCLS-II opens up new exciting possibilities for time-resolved molecular imaging. In particular, techniques relying on multi-coincident charged-particle detection like Coulomb explosion imaging or molecular-frame photoelectron measurements now become feasible at XFELs. Recently, XFEL experiments of this type succeeded in obtaining complete momentum-space images of ring molecules, capturing the positions of all constituents including hydrogen atoms, enabled reconstruction of full three-dimensional geometry of smaller polyatomic molecules like halomethanes, and used photoelectron diffraction and spectroscopy to characterize X-ray-induced molecular dissociation. This talk will review recent progress in ion-ion and ion-electron coincident measurements at XFELs, discuss application of these techniques to time-resolved experiments and the relation to more conventional X-ray photoelectron spectroscopy. Specific examples will include XFEL-driven Coulomb explosion imaging of different C₇H₈ isomers and of strong-field driven dynamics in halomethanes, as well as potential applications to light-induced charge transfer reactions.