Disentangling Ultrafast Electronic and Structural Dynamics with X-Ray Lasers.

Directing the functionality of molecules, materials and biophysical systems is challenging both from fundamental and applied standpoints. For example, understanding the elementary processes responsible for light-induced transformations require watching electronic and structural reorganizations on their intrinsic timescales. The X-ray free electron lasers (X-FEL) represent a new generation of incredibly short and ultra-bright X-ray source, which open new possibilities for developing the multidisciplinary field of ultrafast science. Experiments around X-FEL provide probes, sensitive to electronic and structural reorganizations, able to monitor transformations on the femtosecond timescale (1 fs=10-15  s). Recent years have seen terrific successes in providing a detailed view on light-induced processes, compared to what was understood from conventional optical pump-probe spectroscopy. This Concept article aims at illustrating, through recent studies mainly focussing on light-induced excited spin state trapping, how these X-FEL based techniques can help understanding light-activated functions, by monitoring elementary electronic and structural processes that may occur beyond the Born-Oppenheimer approximation.