X-ray Free-Electron lasers (XFELs) are new light sources, providing ultra short, high-intensity pulses in the x-ray domain. Compared to third generation synchrotron sources - the hitherto most powerful x-ray sources - XFELs are characterized by an increase of almost 9 orders of magnitude in intensity and extremely short pulse durations of only a few femtoseconds. Thus, XFELs are the first x-ray sources with characteristics comparable to those of an optical laser. Our group is interested in the fundamental interaction of this high-intensity x-ray radiation with atoms and molecules.
Quantum electronics with x-rays
The most prominent example of quantum optics is probably the laser itself - the amplification of light on electronic transitions in atoms and molecules. XFELs open the possibility to create such “real” lasers for the first time. We develop the theory and design experiments of such atomic and molecular x-ray lasers.
High-intensity x-ray pulse propagation
In addition to the microscopic quantum dynamics in strong x-ray fields, we simulate the macroscopic propagation and amplification of x-ray radiation in dense atomic or molecular gases and plasmas. This is crucial to determine the temporal and spectral characteristics of the created x-ray radiation.
Furthermore, resonant interaction of x-rays can result, just as it is the case with optical light, in strong dispersion. This can lead to interesting effects, like “slow light” in the x-ray domain. This effect can be used to control the delay of one x-ray pulse to another, which would open the pathway for well controlled pump-probe experiments in the x-ray regime.
Wave-packet dynamics in molecules
XFELs have pulse durations in the femtosecond range – the characteristic timescale of breaking and forming chemical bonds. In addition, x-rays can probe matter with element specificity. It is therefore possible to address a specific atom within a molecule and launch electronic wave packets at well defined positions in the molecule. Adding a second x-ray pulse, it is possible to induce and control vibrational dynamics in the system. We develop theoretical schemes to launch and observe these wave packets, which might ultimately lead to new experimental techniques to observe charge transport or light-induced structural changes in molecules.