Ultrafast Spectroscopy with an EdgeBasic Concepts and Tabletop Applications of Femtosecond X‑ray Transient...

Electronic absorption spectroscopy tools have long been part of the arsenal of molecular spectroscopy techniques to investigate the properties of valence electrons and chemical bonds. However, in recent years, this toolkit has rapidly expanded into the shallow and inner core orbitals, permitting a nuanced understanding of reaction dynamics in excited electronic states from the vantage points of distinct atomic sites. As these tools continue to be innovated, particularly in tabletop implementations, we provide an overview of how such versatile light pulses are (i) generated and (ii) applied in basic research laboratories, not only to answering long-standing questions in molecular spectroscopy but also to tackling emerging problems in energy conversion and quantum information science. In this tutorial review, we dive into a conceptual understanding of how femtosecond extreme-ultraviolet and soft X-ray pulses provide mechanistic understanding of ultrafast chemical dynamics through tracking wide regions of the molecular phase space with atomic-site fidelity. We further explain the nuts and bolts of a typical tabletop apparatus that control the parameters that determine the spectral and temporal resolution from the experimentalist's standpoint. Finally, we circle back to spectroscopic selection rules from first-principles, connecting to several benchmark studies where femtosecond time-resolved X-ray absorption spectroscopy provides a cutting "edge" in the real-time mapping of nonadiabatic processes that operate far away from chemical equilibrium. Potential pitfalls and complementary methodologies that map other details of the evolution of electronic and nuclear structures on ultrafast time scales are discussed.