How does the photochemical equivalence law relate photons to reacting molecules?

The photochemical equivalence law, also known as the Stark–Einstein law, establishes a fundamental relationship between light and chemical reactions. It states that for every quantum (or photon) of radiation absorbed, one molecule of the substance undergoes a primary chemical change.12

In terms of moles, this means that for every mole of a substance that reacts, Avogadro’s number ($6.022\times {10}^{23}$) of photons must be absorbed. This relationship specifically applies to the primary process—the initial change resulting directly from light absorption.1

In practice, many reactions do not appear to follow this one-to-one ratio because the primary process is often followed by secondary interactions that do not require further light absorption, or because high-intensity light sources can trigger biphotonic processes where a single molecule absorbs two photons. The efficiency of this process is measured by the quantum yield, which represents the number of molecules reacting per photon absorbed.12