A beamline for attosecond time-resolved soft-X-ray spectroscopy up to the oxygen K-edge (~540 eV) has been developed. This beamline has been applied to realize the first attosecond transient-absorption experiments at the carbon K-edge. We have studied the dynamics following strong-field ionization of ethylene in the gas phase. This has led to the observation of the D1D0 electronic relaxation of the ethylene cation in less than 7 fs, which is the fastest electronic relaxation dynamics observed to date. A second beamline for X-ray absorption spectroscopy equipped with a flat microjet has also been developed. This setup has been used to demonstrate extreme-ultraviolet high-harmonic generation for the first time. The properties of high-harmonic generation in liquids have been characterized in detail. More recently, the polarization and the wavelength dependence of HHG in liquids have also been studied. Moving on from high-harmonic spectroscopy to X-ray absorption spectroscopy, we have obtained the first femtosecond time-resolved spectra in the soft-X-ray domain from a liquid target. Specifically, we have studied the electronic and nuclear dynamics following the ionization of liquid methanol and ethanol. Next, we have studied the ionization-induced proton transfer in ionized urea solutions and have found that the transfer occurs very selectively and rapidly from the ionized urea molecule to another hydrogen-bonded urea molecule, but not to water. This work moreover revealed that X-ray absorption spectroscopy is capable of distinguishing electronic from structural rearrangements while simultaneously providing site-resolved information. Most interestingly, we have directly compared the ultrafast electronic-relaxation dynamics on photoexcited pyrazine molecules in the gas and solution phases. The gas-phase measurements have shown that the electronic relaxation pathway involves coherent population oscillations. These oscillations are absent in the liquid phase, which could be traced back to the change in topology of the associated conical intersections, resulting from different solvation shifts of the intersecting electronic states. Turning from absorption to photoelectron spectroscopy, we have completed the first attosecond time-resolved measurements in liquids by measuring photoemission delays of 50-70 attoseconds between liquid and gaseous water. The interpretation of this measurements has shown that the delays are dominated by solvation effects, i.e. they probe the influence of the liquid environment on the photoionization delays of water molecules. In a complementary experiment based on electron-ion-coincidence spectroscopy, we have measured photoionization delays as a function of the size of water clusters. We have found that the delays correlate linearly with the spatial extension of the electron hole. This result confirmed the interpretation of the liquid-phase results and provided a molecular-level understanding of attosecond photoionization dynamics of liquid water. Using electron-electron coincidence spectroscopy, we have realized the first observation of intermolecular Coulombic decay in liquid water. Finally, returning to the gas phase, we have observed attosecond charge migration in a neutral molecule, its de- and recoherence caused by nuclear wave-packet motion and the transfer of electronic coherence through a conical intersection. Most of these results have been published (2 Nature, 2 Science, 1 Nature Physics, 1 Nature Chemistry, 1 PRL, 1 Chemical Science, 1 JPC Letters, 1 Scientific Reports, 1 Optics Express, etc.) and some are currently under review or in preparation. These results have been presented at more than 20 talks and numerous poster presentations. Most of this material will be reviewed in an invited manuscript that is in preparation for Nature Reviews Chemistry.
