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Abstract

This thesis investigates the interaction of ultrashort, extreme-ultraviolet (XUV) and soft x-ray laser pulses with atoms and molecules in the gas phase. In total, the subject is explored from four different perspectives, which are all based on the short- lived–coherent electronic responses to the laser pulses, and measured with transient absorption spectroscopy. First, a theoretical study reveals how transient energy shifts of electronic dressed states in atoms driven by an intense XUV Free-Electron Laser (FEL) lead to temporal dipole phase shifts and absorption-line changes. Second, a follow-up study investigates the electronic-population Rabi-cycles corresponding to the absorption-line changes of the first study. A convolutional neural network is employed to reconstruct the temporal population dynamics from the simulated spectral absorption modifications. The inversion from an absorption to an emission line is described and a potential experimental demonstration in helium is discussed. Third, dense gas targets enable amplification of the otherwise improbable, non-linear process of stimulated resonant inelastic x-ray scattering (RIXS), as well as x-ray FEL propagation-based spatial-spectral reshaping. To this end, a new experimental setup is built and utilized in an x-ray FEL driven RIXS experiment in dense neon gas. Fourth, a novel experiment combining XUV pulses from high-order harmonic generation (HHG) and XUV-FEL pulses is demonstrated by time-resolving a photochemical reaction in molecular oxygen. An FEL pulse initiates coupled nuclear-electronic dissociation pathways from molecular oxygen ions, which are time-resolved on femto- and picosecond time scales by identifying the reaction products in the time-delayed HHG absorption spectra. A FEL-photon-energy–resolved study of the fragments is performed to compare findings from absorption spectroscopy with kinetic energy release spectra recorded in parallel with a reaction microscope.