Attosecond Transient Absorption Spectroscopy

Motion in the microcosm unfolds via the formation of wavepackets, resulting from a coherent superposition of quantum states. Pioneered by A. H. Zewail, nuclear motion in (bio-)molecules as well as the formation and rupture of chemical bonds have been accessed extensively by femtosecond (1 fs = 10 seconds) pump-probe spectroscopy. For these outstanding experiments, A. H. Zewail was awarded the Nobel prize in 1999, establishing the research area of femtochemistry. However, the resolution offered by femtosecond spectroscopy is insufficient to track the dynamics of electronic motion in atoms or molecules since they evolve on an attosecond (1 as = 10 s) to few-fs time scale and thus remain elusive so far. This thesis establishes attosecond transient absorption spectroscopy as a new approach for the exploration of electronic motion. By combining quasimonocycle (1.5 optical cycles) near infrared (NIR) laser pulses as an initiation event and non-invasive isolated extreme ultraviolet attosecond pulses in a unique exertion as probe pulses, several proof-of-concept experiments are presented. Strong-field ionization of noble gas atoms via the ultrashort NIR laser pulse produces several charged states whose formation during the ionization process has been tracked by the attosecond probe pulse and yielded the first real-time, state-resolved observation of atomic ionization, indicating a delayed formation of higher charged states with respect to lower ones. Strong-field ionization of krypton atoms—performed with quasimonocycle laser pulses, limiting the ionization window to less than 3.2 fs—creates singly charged ions in a coherent superposition of quantum states. The subsequent evolution of the valence electron motion has been traced for the first time. Attosecond transient absorption spectroscopy, in combination with appropriate modeling, has enabled the complete reconstruction of valence electron motion, including its degree of coherence, which is not accessible by conventional time-integrated spectroscopy. Besides the investigation of prototypical open systems, this new tool has also been successfully applied to study more complex systems. For instance various quantum beats in xenon ions have been measured in different charge states, indicating complex multi-electron dynamics. It is shown that attosecond transient absorption spectroscopy further expands the horizon of attosecond science and holds promises to precisely access state-resolved sub-cycle ionization dynamics during strong-field ionization of matter and to explore sophisticated multi-electron dynamics including hole-hole and multi-hole correlations.