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What does the Stark-Einstein law say about photons and photochemical reactions?
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photochemical reaction
Chain of fluorescent tunicates.
The Stark-Einstein law, also known as the photochemical equivalence law, states that for every quantum of radiation absorbed, one molecule of the substance undergoes a chemical reaction.1
Because a mole of a substance contains Avogadro’s number of molecules, the law is also expressed as: for every mole of a substance that reacts, one mole of quanta (photons) of light is absorbed.1
Key points regarding this principle include:
- Primary Process: The law applies specifically to the primary process of a photochemical reaction, which is the initial chemical change resulting directly from light absorption.1
- Secondary Processes: Many reactions involve subsequent secondary processes—normal chemical interactions that do not require light—which can make the overall reaction appear to deviate from the one-quantum-to-one-molecule ratio.1
- Limitations: The law is restricted to conventional processes using moderate light intensities. High-intensity sources, such as those used in lasers, can trigger biphotonic processes where a single molecule absorbs two photons.1