Towards novel soft X-ray spectroscopic tools
atom specific electronic structure and dynamics
To investigate dynamic processes in complex systems it is of equal importance to address the temporal evolution of a wave packet and the question between which atomic centers the dynamic evolution takes place. In the spectral regime of optical transitions this atom specific information can not be obtained. Thus, the future development lies in the combination of high temporal resolution in the femtosecond range with the elemental and chemical state selectivity of X-rays.
Due to the rapid evolution of pulsed X-ray sources, the field of X-ray physics is currently undergoing a revolutionary development towards the investigation of ultra fast dynamics in an atom specific way, which will give insights of fundamental importance to physics, chemistry and biology.
The concept of Free-Electron-Laser (FEL) sources yields high-brilliance, ultra short radiation pulses (~20-200 fs) tunable over a wide photon energy range from the vacuum ultraviolet (VUV: 10-200 eV), the soft-X-rays (100-1500 eV) to the X-rays (above 1500 eV). The FELs at Hamburg (VUV, started 2005) and the Linac Coherent Light Source (X-ray: LCLS) at Stanford (start ~2008) will provide excellent conditions for this research.
Independently, time resolved pump/probe spectroscopy using high harmonic VUV pulses has been revolutionized by unprecedented pulse definition on the attosecond timescale, through the development of high precision phase stabilized Lasers and innovative optics. At the same time ps-pulsed X-ray Laser plasma sources (i.e. K lines of Cu, Al) become available. These Laser based X-ray sources have an excellent temporal synchronization between the pump and probe pulses, however at rather low intensity and peak brilliance.
A fusion of the advantages of the FEL (brilliance) and the optical Lasers (pulse definition and synchronization) is aimed at in the concept of a high gain high harmonic seeded FEL (planned at BESSY), where high harmonics of an optical fs-Laser system are amplified in a cascaded FEL scheme. Thus, the powerful tools of fs-Laser physics (pulse shaping, phase stabilization etc.) will be transposed and imprinted onto ultra short pulses in the soft-X-ray spectral range. In particular experiments using the high peak brilliance of the Free-Electron-Lasers and time resolved pump-probe experiments are key areas of future research.