A roadmap towards nonlinear x-ray spectroscopy

X-ray free-electron lasers (XFELs) sources have the potential to revolutionize our ways to probe complex chemical reaction dynamics. XFELs provide fs x-ray pulses of unprecedented high intensities, opening the pathway of obtaining high temporal resolution in optical pump x-ray probe experiments. Addressing molecules in the gas or liquid phase, time-resolved photoelectron, Auger-electron and x-ray emission spectroscopy are indispensable tools for studying the dynamics of the electronic valence state, that is typically initiated by an optical laser pulse. Yet, the full potential of XFELs to study these processes is far from being explored. XFELs open an intriguing future perspective, to advance current x-ray spectroscopic techniques, beyond simple absorption and emission spectroscopy. The main building block of a new class of nonlinear optical spectroscopy is stimulated inelastic x-ray scattering. Recent experimental results of the first demonstration of stimulated resonant inelastic x-ray scattering in atoms, along with feasibility studies to transfer this technique to molecules in the gas phase will be presented. Moreover, I will discuss the future theoretical and experimental hurdles on the road to developing a novel, time-resolved nonlinear x-ray spectroscopic technique, that ultimately should be applicable to a broad range of scientific problems at present XFEL sources.