The quasi-steady state approximation in photo-mediated process: From foundational concepts to practical applications

Light-mediated processes, prevalent in fields like catalysis and medicine, utilize photons to induce changes in matter. To comprehend these processes, one must have knowledge of light sources, light-absorbing molecules, environmental conditions, and reaction kinetics. Mathematical approximations, such as the quasi-steady-state approximation (QSSA), play a crucial role in unraveling the mechanisms of these intricate reactions. QSSA simplifies complex photochemical reactions by assuming short-lived intermediates and constant reactant concentrations. However, this reductionist approach can be inaccurate for complex reactions, particularly when dealing with long-lived species, photobleaching, or large-scale processes. While QSSA is valuable, its limitations necessitate a profound understanding of its mathematical and chemical foundations. In this tutorial review, we present the reader with foundational, historical, and practical examples on the use of QSSA that can be used in undergraduate and graduate training.