Photochemical Kinetics: Reaction Orders and Analogies with Molecular Beam Scattering and Cavity Ring-Down Experiments

Abstract

Despite its fundamental importance, photochemical kinetics is not often treated in much detail in physical chemistry courses and concepts often remain unclear. One topic of recent debate concerns reaction orders of photochemical reactions. In this article, we emphasize that a photochemical reaction system is composed of several elementary steps, each of which has a defined molecularity and reaction order. The elementary, primary absorption step can be considered a bimolecular reaction. Depending on the experimental conditions, the apparent total reaction order of the mechanism may have different values, but will still be defined in most cases. Possible conceptual difficulties may be avoided by realizing two analogies between experiments involving light and kinetic experiments not involving light: a standard absorption measurement has an analogy with a molecular beam scattering experiment, and cavity ring-down spectroscopy has an analogy with a conventional static reactor experiment.