Toward Reliable Spectroscopic Analysis of Reaction Kinetics in Polaritonic Chemistry.

Recent reports suggest that chemical reaction rates can change inside an optical cavity. These effects have been attributed to the hybridization of molecular vibrational modes with cavity modes into polaritons, but the underlying mechanism remains debated. Recently, attempts to reproduce the key experiments have sometimes failed, which also poses ambiguity and impedes the determination of the possible mechanism. Without a reliable theoretical framework, polaritonic chemistry-seeking to use optical resonators as catalysts for reactions-has reached a pivotal stage. Standardized protocols for reproducible cavity experiments are therefore urgently needed. Here, we identify pitfalls in approaches that monitor reaction progress with UV/Vis spectroscopy. Using the transfer-matrix method, we analyze a model pseudo-first-order reaction and assess how transient cavity thickness variations, cavity inhomogeneity, and fitting protocols influence the extracted rate constant. We find that changes in cavity thickness upon reactant introduction can strongly distort apparent kinetics when monitoring at a single wavelength, an artifact that is mitigatable by spectral smoothing. Additionally, we demonstrate that, unlike in many studies, the asymptotic extinction should be treated as a fitting parameter rather than fixed to the final experimental value. By identifying these pitfalls, our work lays the foundation for more robust analyses and reliable measurements in polaritonic chemistry.