The ultrafast X-ray spectroscopic revolution in chemical dynamics

The past two decades have seen rapid developments in short-pulse X-ray sources, which have enabled the study of nuclear and electronic dynamics by ultrafast X-ray spectroscopies with unprecedented time resolution ranging from nanoseconds to attoseconds. In this Perspective, we discuss some of the major achievements in the study of nuclear and electronic dynamics with X-ray pulses produced by high-harmonic, free-electron-laser and synchrotron sources. The particular dynamic processes probed by X-ray radiation highlighted in this Perspective are electronic coherences on attosecond to femtosecond timescales, chemical reactions, such as dissociations, and pericyclic ring-openings, spin-crossover dynamics, ligand-exchange dynamics and structural deformations in excited states. X-ray spectroscopic probing of chemical dynamics holds great promise for the future owing to the ongoing developments of new spectroscopies, such as four-wave mixing, and the continuous improvements in emerging laboratory-based, high-harmonic sources and large-scale, facility-based, free-electron lasers. Ultrafast X-ray spectroscopies enable the investigation of fast chemical dynamics with time resolutions reaching the order of attoseconds. Processes such as spin crossover, structural deformations in excited states and dissociation reactions can now be studied through the use of short X-ray pulses produced by high-harmonic, free-electron-laser and synchrotron sources.