The Stark-Einstein law of photochemical equivalence

The Stark-Einstein law of photochemical equivalence is in a sense simply a quantum-mechanical statement of the Grotthuss-Draper law. The Stark-Einstein law (1905) is another example of the break with classical physics. It states that each molecule which takes part in the photochemical reaction absorbs one quantum of the light which induces the reaction that is, one molecule absorbs the entire quantum the energy of the light beam is not spread continuously over a number of molecules. 

If we define the primary act of the photochemical reaction as the absorption of the quantum, then the quantum efficiency for the primary act is, by the Stark-Einstein law, equal to unity. For each quantum absorbed, one primary act occurs. For any substance X taking part in a photochemical reaction, the quantum efficiency or quantum yield for the formation (or decomposition) of X is 0x and is defined by 

More conveniently, if we measure the rate of formation of X in molecules per second, dNx/dt, then the quantum yield is 

The number of quanta absorbed per second is the absorbed intensity, so that 

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