Quantum dynamics in weak and strong fields measured by XUV nonlinear spectroscopy

In this work fundamental nonlinear dynamics inside the neon atom are studied in two respects: At first, correlations between electronic inner-shell excitations in the extreme-ultraviolet (XUV) spectral range are probed on their natural sub-femtosecond time scale. To this end, the concept of attosecond transient absorption with a high-harmonic generated (HHG) attosecond and a single time-delayed moderately strong near-infrared (NIR) pulse was extended by a third, perturbative NIR pulse to perform time resolved four-wave-mixing spectroscopy. This allowed to retrieve coupling dynamics between states of odd and even parity in a two-dimensional spectral representation. While the first part of this work explores the sequential interaction of several weak and moderately strong, fully coherent laser pulses with the target neon, the second part addresses the impact of strong, partially-coherent fields delivered by the XUV free-electron laser in Hamburg (FLASH). For this purpose, a novel beamline setup was developed and assembled at FLASH which allowed to perform first XUV-pump-XUV-probe transient absorption measurements. The measurements revealed the time-delay and intensity-dependent control of sequential ionization processes, coherence-enhancement effects, and strong coupling signatures of bound—bound transitions in doubly-ionized neon. For the interpretation of the experimental results numerical simulations based on quantum mechanical few-level models were employed. Future applications of this method involve the two-dimensional spectroscopy both with HHG and FLASH pulses to probe site-specific information of electronic processes in molecules