Ultrafast soft X-ray absorption spectroscopy

This paper demonstrates time resolved X-ray absorption spectroscopy with a resolution in the sub-20 fs range around the L-edge (100 eV) of amorphous silicon (XANES) and gathered information beyond the L-edge about the atomic structure with EXAFS. This setup can be easily adapted for other materials such as carbon, and open the way to gain insight into fast dynamical processes of large organic molecules.

The development of reliable femtosecond solid-state lasers brought new possibilities into time-resolved spectroscopy. For the first time it became possible, in principle to monitor the nuclear motion of molecules, crystal lattices and other out-of-equilibrium structures. However, usually it is very difficult to map the experimental observations to the structural dynamics. Therefore, experimental approaches are needed that can overcome the limitation of optical studies for structural determination, while the high temporal resolution of femtosecond lasers is maintained. Structural techniques such as X-ray diffraction (XRD) or X-ray absorption spectroscopy (XAS) deliver much more direct information about the structure[1]. An impressive effort in the last years has culminated in the demonstration of time-resolved XRD and XAS with a resolution in the order of a few 100 femtoseconds. The key to the successful realization was the development of laser driven x-ray sources. Various schemes have been demonstrated including laser driven x-ray diodes, high harmonic generation, and laser generated plasma sources[1]. In this contribution we report on the generation of soft x-ray pulses via high harmonic generation (HHG) and their first use for time resolved XAS to investigate the structural dynamic of amorphous silicon with a temporal resolution of about 20fs. To our knowledge this is highest temporal resolution ever demonstrated in XAS.