Attosecond transient absorption spectroscopy: an ultrafast optical probe for revealing electron dynamics

Attosecond transient absorption spectroscopy (ATAS) is an all-optical pump-probe technique that employs attosecond pulses (from the extreme ultraviolet to soft X-ray) to excite or probe a system, enabling real-time tracking of electronic transitions, quantum state evolution, and energy transfer processes. This approach possesses some key advantages: 1) ultrafast temporal resolution (sub-femtosecond) combined with high spectral resolution (millielectronvolt level); 2) broadband excitation of multiple quantum states, allowing simultaneous detection of multiple energy levels; and 3) element- and site-specific insights provided by the measurements of inner-shell to valence transition reveal charge transfer dynamics, spin state changes, and local structural evolution. To date, significant breakthroughs have been achieved in atomic/molecular physics, electronic coherent dynamics, and strong-field physics by using ATAS. This paper systematically reviews the technical principles and theoretical models related to ATAS by using medium intensity near-infrared pulses, analyzes the recent progress of the applications in gas-phase systems and condensed-phase systems, and explores their future prospects in ultrafast physical chemistry and quantum materials. In gas-phase environments, the ATAS has demonstrated significant capabilities in probing energy level shifts and population transfers in atomic systems, as well as capturing nonadiabatic dynamics and charge migration in diatomic and polyatomic molecules. While in condensed-phase systems, this technique has been effectively used to study the ultrafast dynamics of carriers in semiconductors and to examine the interaction dynamics of localized electrons in insulators and transition metals. Given the rapid evolution of attosecond laser technologies and the unique advantages of the ATAS detection method, this paper also outlines potential future directions. These prospects are expected to further expand the frontiers of ultrafast spectroscopy and drive advancements in a range of disciplines in basic research and technological applications.