Nonlinear light-matter interaction with intense XUV free-electron lasers

The optical response of matter to external light stimuli is encoded in their absorption spectra and gives access to the quantum dynamics of atoms and molecules. In the extreme ultraviolet (XUV) and x-ray domain this includes site-specific resonant transitions, making a clear identification possible and attributing them to individual excitations of bound electrons. With the availability of intense XUV/x-ray light from free-electron lasers these transitions can be further modified, thus opening up new possibilities for deliberately controlling the underlying quantum dynamics in a site-specific manner. In this regard we have performed a series of experiments employing all-XUV-optical pump-probe transient absorption spectroscopy with the FLASH free-electron laser in Hamburg, which is based on self-amplified spontaneous emission (SASE), in order to probe and control XUV nonlinear light-matter interaction in atoms and small molecules [1]. For instance, we have successfully demonstrated the direct dressing of a two-electron transition in helium, resolving Fano lineshape asymmetry changes that can be attributed to the strong coupling of the ground state to an autoionizing doubly excited state [2]. In neon, FEL-induced Stark shifts can be identified in the measured absorption spectra as well as resolving coherence signatures on the few-meV spectral and few-fs temporal scale [3]. These measurements thus also allowed us to quantify the average spectro-temporal chirp of SASE-FEL pulses [4]. With proven sensitivity to spectral interference structures and combining both high temporal and spectral resolution, nonlinear all-XUV-optical transient absorption spectroscopy can be regarded as a key step towards the implementation of multi-color multi-dimensional spectroscopy methodologies in the XUV and x-ray domain.