Ultrafast X-ray spectroscopy and scattering
The time scales on which systems can evolve are very diverse. With ultrafast X-ray methods, we concentrate on the range from quasi-instantaneous electronic transitions between different energy levels via the absorption of a photon, to electron dynamics like charge transfer and electronic scattering processes on the sub-100 femtosecond level and finally to time scales that involve rearrangements of the atomic nuclei on the picosecond level (vibrations, lattice ordering phenomena etc.). Dynamics on these time scales are very rich and can induce interesting changes in the material's properties, its chemistry and the functionality.
A fundamental limit to the spectroscopic access to ultrafast dynamics is given by the Fourier limit (dE*dt is on the order of the Planck constant h=4eV*fs). Low energetic excitations evolve on slower time scales. Specific X-ray sources or measurement techniques need to be employed to resolve ultrafast dynamics with X-ray tools. Typically the time scales slower than 100 fs are resolved with pump-probe methods, whereas the ultrashort life time of core holes allow to study even faster processes.