Abstract: The critical electronic rearrangement processes underlying photochemical reactions typically unfold on the femtosecond to picosecond timescale, rendering them inaccessible to conventional steady-state spectroscopic techniques and posing a conceptual challenge in pedagogical settings. Femtosecond transient absorption spectroscopy, an ultrafast technique based on the pump-probe principle, enables direct tracking of transient intermediates and their dynamical evolution over timescales ranging from femtoseconds to nanoseconds. Using excited-state intramolecular proton transfer as a representative photochemical reaction, this article systematically reviews the development, working principles, and three types of characteristic signals of this technique, along with their physical interpretations. Employing the Cbz-HPNI-COOH molecule as a model system, comparative kinetic analyses under neutral and acidic conditions vividly illustrate how the intramolecular hydrogen-bonding environment governs the proton transfer rate. This contribution aims to equip students and educators in chemistry and allied disciplines with an intuitive grasp of ultrafast photochemical dynamics and to underscore the role of transient absorption spectroscopy as a pivotal bridge between molecular structure and reaction kinetics.

Key words: Femtosecond transient absorption spectroscopy, Pump-probe, Excited-state intramolecular proton transfer, Ultrafast dynamics, Photochemistry